# MySQL Reference Manual for version 4.0.6-gamma.

# 1 General Information

The `MySQL (TM)` software delivers a very fast, multi-threaded, multi-user, and robust `SQL` (`Structured Query Language`) database server. `MySQL Server` is intended for mission-critical, heavy-load production systems as well as for embedding into mass-deployed software. `MySQL` is a trademark of `MySQL AB`.

The `MySQL` software is `Dual Licensed`. Users can choose to use the `MySQL` software as an `Open Source`/`Free Software` product under the terms of the `GNU General Public License` (http://www.gnu.org/licenses/) or can purchase a standard commercial license from `MySQL AB`. See section 1.4 MySQL Support and Licensing.

The `MySQL` web site (http://www.mysql.com/) provides the latest information about the `MySQL` software.

The following list describes some sections of particular interest in this manual:

- For information about the company behind the `MySQL Database Server`, see section 1.3 What Is MySQL AB?.
- For a discussion about the capabilities of the `MySQL Database Server`, see section 1.2.2 The Main Features of MySQL.
- For installation instructions, see section 2 MySQL Installation.
- For tips on porting the `MySQL Database Software` to new architectures or operating systems, see section E Porting to Other Systems.
- For information about upgrading from a Version 3.23 release, see section 2.5.2 Upgrading From Version 3.23 to Version 4.0.
- For information about upgrading from a Version 3.22 release, see section 2.5.3 Upgrading From Version 3.22 to Version 3.23.
- For a tutorial introduction to the `MySQL Database Server`, see section 3 Tutorial Introduction.
- For examples of `SQL` and benchmarking information, see the benchmarking directory (``sql-bench'` in the distribution).
- For a history of new features and bug fixes, see section D MySQL Change History.
- For a list of currently known bugs and misfeatures, see section 1.7.5 Known Errors and Design Deficiencies in MySQL.
- For future plans, see section 1.8 MySQL and The Future (The TODO).
- For a list of all the contributors to this project, see section C Credits.

Important:

Reports of errors (often called bugs), as well as questions and comments, should be sent to the mailing list at mysql@lists.mysql.com. See section 1.6.2.3 How to Report Bugs or Problems.

The `mysqlbug` script should be used to generate bug reports. For source distributions, the `mysqlbug` script can be found in the ``scripts'` directory. For binary distributions, `mysqlbug` can be found in the ``bin'` directory. If you have found a sensitive security bug in `MySQL Server`, you should send an e-mail to security@mysql.com.

## 1.1 About This Manual

This is the `MySQL` reference manual; it documents `MySQL` upto Version 4.0.6-gamma. Functional changes are always indicated with reference to the version, so this manual is also suitable if you are using an older version of the `MySQL` software. Being a reference manual, it does not provide general instruction on `SQL` or relational database concepts.

As the `MySQL Database Software` is under constant development, the manual is also updated frequently. The most recent version of this manual is available at http://www.mysql.com/documentation/ in many different formats, including HTML, PDF, and Windows HLP versions.

The primary document is the Texinfo file. The HTML version is produced automatically using a modified version of `texi2html`. The plain text and Info versions are produced with `makeinfo`. The PostScript version is produced using `texi2dvi` and `dvips`. The PDF version is produced with `pdftex`.

If you have a hard time finding information in the manual, you can try our searchable version at http://www.mysql.com/doc/.

If you have any suggestions concerning additions or corrections to this manual, please send them to the documentation team at docs@mysql.com.

This manual was initially written by David Axmark and Michael (Monty) Widenius. It is currently maintained by Michael (Monty) Widenius, Arjen Lentz, and Paul DuBois. For other contributors, see section C Credits.

The copyright (2002) to this manual is owned by the Swedish company `MySQL AB`. See section 1.4.2 Copyrights and Licenses Used by MySQL.

### 1.1.1 Conventions Used in This Manual

This manual uses certain typographical conventions:

`constant`
  Constant-width font is used for command names and options; SQL statements; database, table, and column names; C and Perl code; and environment variables. Example: ``To see how `mysqladmin` works, invoke it with the `--help` option.''
``filename'`
  Constant-width font with surrounding quotes is used for filenames and pathnames. Example: ``The distribution is installed under the ``/usr/local/'` directory.''
``c'`
  Constant-width font with surrounding quotes is also used to indicate character sequences. Example: ``To specify a wildcard, use the ``%'` character.''
italic
  Italic font is used for emphasis, like this.
boldface
  Boldface font is used in table headings and to convey especially strong emphasis.

When commands are shown that are meant to be executed by a particular program, the program is indicated by a prompt shown before the command. For example, `shell>` indicates a command that you execute from your login shell, and `mysql>` indicates a command that you execute from the `mysql` client program:

    shell> type a shell command here
    mysql> type a mysql command here

Shell commands are shown using Bourne shell syntax. If you are using a `csh`-style shell, you may need to issue commands slightly differently. For example, the sequence to set an environment variable and run a command looks like this in Bourne shell syntax:

    shell> VARNAME=value some_command

For `csh`, you would execute the sequence like this:

    shell> setenv VARNAME value
    shell> some_command

Often database, table, and column names must be substituted into commands. To indicate that such substitution is necessary, this manual uses `db_name`, `tbl_name` and `col_name`. For example, you might see a statement like this:

    mysql> SELECT col_name FROM db_name.tbl_name;

This means that if you were to enter a similar statement, you would supply your own database, table, and column names, perhaps like this:

    mysql> SELECT author_name FROM biblio_db.author_list;

SQL keywords are not case-sensitive and may be written in uppercase or lowercase. This manual uses uppercase.

In syntax descriptions, square brackets (``['` and ``]'`) are used to indicate optional words or clauses. For example, in the following statement, `IF EXISTS` is optional:

    DROP TABLE [IF EXISTS] tbl_name

When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (``|'`). When one member from a set of choices may be chosen, the alternatives are listed within square brackets (``['` and ``]'`):

    TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)

When one member from a set of choices must be chosen, the alternatives are listed within braces (``{'` and ``}'`):

    {DESCRIBE | DESC} tbl_name {col_name | wild}

## 1.2 What Is MySQL?

`MySQL`, the most popular `Open Source` SQL database, is developed, distributed and supported by `MySQL AB`. `MySQL AB` is a commercial company founded by the MySQL developers that builds its business providing services around the `MySQL` database. See section 1.3 What Is MySQL AB?.

The `MySQL` web site (http://www.mysql.com/) provides the latest information about `MySQL` software and `MySQL AB`.

`MySQL` is a database management system.
  A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as `MySQL` Server. Since computers are very good at handling large amounts of data, database management plays a central role in computing, as stand-alone utilities, or as parts of other applications.
MySQL is a relational database management system.
  A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The tables are linked by defined relations making it possible to combine data from several tables on request. The `SQL` part of ```MySQL`'' stands for ```Structured Query Language`''@-the most common standardised language used to access databases.
MySQL software is `Open Source`.
  `Open Source` means that it is possible for anyone to use and modify. Anybody can download the `MySQL` software from the Internet and use it without paying anything. Anybody so inclined can study the source code and change it to fit their needs. The `MySQL` software uses the `GPL` (`GNU General Public License`), http://www.gnu.org/licenses/, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the `GPL` or need to embed `MySQL` code into a commercial application you can buy a commercially licensed version from us. See section 1.4.3 MySQL Licenses.
Why use the MySQL Database Server?
  The `MySQL Database Server` is very fast, reliable, and easy to use. If that is what you are looking for, you should give it a try. `MySQL Server` also has a practical set of features developed in close cooperation with our users. You can find a performance comparison of `MySQL Server` to some other database managers on our benchmark page. See section 5.1.4 The MySQL Benchmark Suite. `MySQL Server` was originally developed to handle large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Though under constant development, `MySQL Server` today offers a rich and useful set of functions. Its connectivity, speed, and security make `MySQL Server` highly suited for accessing databases on the Internet.
The technical features of MySQL Server
  For advanced technical information, see section 6 MySQL Language Reference. The `MySQL Database Software` is a client/server system that consists of a multi-threaded `SQL` server that supports different backends, several different client programs and libraries, administrative tools, and a wide range of programming interfaces (`API`s). We also provide `MySQL Server` as a multi-threaded library which you can link into your application to get a smaller, faster, easier-to-manage product.
There is a large amount of contributed MySQL software available.
  It is very likely that you will find that your favorite application or language already supports the `MySQL Database Server`.

The official way to pronounce `MySQL` is ``My Ess Que Ell'' (not ``my sequel''), but we don't mind if you pronounce it as ``my sequel'' or in some other localised way.

### 1.2.1 History of MySQL

We once started out with the intention of using `mSQL` to connect to our tables using our own fast low-level (ISAM) routines. However, after some testing we came to the conclusion that `mSQL` was not fast enough nor flexible enough for our needs. This resulted in a new SQL interface to our database but with almost the same API interface as `mSQL`. This API was chosen to ease porting of third-party code.

The derivation of the name `MySQL` is not perfectly clear. Our base directory and a large number of our libraries and tools have had the prefix ``my'' for well over 10 years. However, Monty's daughter (some years younger) is also named My. Which of the two gave its name to `MySQL` is still a mystery, even for us.

### 1.2.2 The Main Features of MySQL

The following list describes some of the important characteristics of the `MySQL Database Software`. See section 1.5 MySQL 4.x In A Nutshell.

Internals and Portability
  - Written in C and C++. Tested with a broad range of different compilers.
  - Works on many different platforms. See section 2.2.2 Operating Systems Supported by MySQL.
  - Uses GNU Automake, Autoconf, and Libtool for portability.
  - APIs for C, C++, Eiffel, Java, Perl, PHP, Python, Ruby, and Tcl. See section 8 MySQL APIs.
  - Fully multi-threaded using kernel threads. This means it can easily use multiple CPUs if available.
  - Very fast B-tree disk tables with index compression.
  - A very fast thread-based memory allocation system.
  - Very fast joins using an optimised one-sweep multi-join.
  - In-memory hash tables which are used as temporary tables.
  - SQL functions are implemented through a highly optimised class library and should be as fast as possible! Usually there isn't any memory allocation at all after query initialisation.
  - The `MySQL` code gets tested with Purify (a commercial memory leakage detector) as well as with Valgrind, a GPL tool (http://developer.kde.org/~sewardj/).
Column Types
  - Many column types: signed/unsigned integers 1, 2, 3, 4, and 8 bytes long, `FLOAT`, `DOUBLE`, `CHAR`, `VARCHAR`, `TEXT`, `BLOB`, `DATE`, `TIME`, `DATETIME`, `TIMESTAMP`, `YEAR`, `SET`, and `ENUM` types. See section 6.2 Column Types.
  - Fixed-length and variable-length records.
  - All columns have default values. You can use `INSERT` to insert a subset of a table's columns; those columns that are not explicitly given values are set to their default values.
Commands and Functions
  - Full operator and function support in the `SELECT` and `WHERE` parts of queries. For example:

        mysql> SELECT CONCAT(first_name, " ", last_name)
            -> FROM tbl_name
            -> WHERE income/dependents > 10000 AND age > 30;

  - Full support for SQL `GROUP BY` and `ORDER BY` clauses. Support for group functions (`COUNT()`, `COUNT(DISTINCT ...)`, `AVG()`, `STD()`, `SUM()`, `MAX()`, and `MIN()`).
  - Support for `LEFT OUTER JOIN` and `RIGHT OUTER JOIN` with ANSI SQL and ODBC syntax.
  - Aliases on tables and columns are allowed as in the SQL92 standard.
  - `DELETE`, `INSERT`, `REPLACE`, and `UPDATE` return the number of rows that were changed (affected). It is possible to return the number of rows matched instead by setting a flag when connecting to the server.
  - The `MySQL`-specific `SHOW` command can be used to retrieve information about databases, tables, and indexes. The `EXPLAIN` command can be used to determine how the optimiser resolves a query.
  - Function names do not clash with table or column names. For example, `ABS` is a valid column name. The only restriction is that for a function call, no spaces are allowed between the function name and the ``('` that follows it. See section 6.1.7 Is MySQL Picky About Reserved Words?.
  - You can mix tables from different databases in the same query (as of Version 3.22).
Security
  - A privilege and password system that is very flexible and secure, and allows host-based verification. Passwords are secure because all password traffic is encrypted when you connect to a server.
Scalability and Limits
  - Handles large databases. We are using `MySQL Server` with some databases that contain 50 million records and we know of users that use `MySQL Server` with 60,000 tables and about 5,000,000,000 rows.
  - Up to 32 indexes per table are allowed. Each index may consist of 1 to 16 columns or parts of columns. The maximum index width is 500 bytes (this may be changed when compiling `MySQL Server`). An index may use a prefix of a `CHAR` or `VARCHAR` field.
Connectivity
  - Clients may connect to the `MySQL` server using TCP/IP Sockets, Unix Sockets (Unix), or Named Pipes (NT).
  - `ODBC` (Open-DataBase-Connectivity) support for Win32 (with source). All ODBC 2.5 functions and many others. For example, you can use MS Access to connect to your `MySQL` server. See section 8.3 MySQL ODBC Support.
Localisation
  - The server can provide error messages to clients in many languages. See section 4.6.2 Non-English Error Messages.
  - Full support for several different character sets, including ISO-8859-1 (Latin1), german, big5, ujis, and more. For example, the Scandinavian characters 'å', 'ä' and 'ö' are allowed in table and column names.
  - All data is saved in the chosen character set. All comparisons for normal string columns are case-insensitive.
  - Sorting is done according to the chosen character set (the Swedish way by default). It is possible to change this when the `MySQL` server is started. To see an example of very advanced sorting, look at the Czech sorting code. `MySQL Server` supports many different character sets that can be specified at compile and runtime.
Clients and Tools
  - Includes `myisamchk`, a very fast utility for table checking, optimisation, and repair. All of the functionality of `myisamchk` is also available through the SQL interface as well. See section 4 Database Administration.
  - All `MySQL` programs can be invoked with the `--help` or `-?` options to obtain online assistance.

### 1.2.3 How Stable Is MySQL?

This section addresses the questions ``How stable is MySQL Server?'' and ``Can I depend on MySQL Server in this project?'' We will try to clarify these issues and answer some important questions that concern many potential users. The information in this section is based on data gathered from the mailing list, which is very active in identifying problems as well as reporting types of use.

Original code stems back from the early '80s, providing a stable code base, and the ISAM table format remains backward-compatible. At TcX, the predecessor of `MySQL AB`, `MySQL` code has worked in projects since mid-1996, without any problems. When the `MySQL Database Software` was released to a wider public, we noticed that there were some pieces of ``untested code'' that were quickly found by the new users who made different types of queries from us. Each new release has had fewer portability problems (even though each new release has had many new features).

Each release of the `MySQL Server` has been usable. There have only been problems when users try code from the ``gray zones.'' Naturally, new users don't know what the gray zones are; this section attempts to indicate those that are currently known. The descriptions mostly deal with Version 3.23 of `MySQL Server`. All known and reported bugs are fixed in the latest version, with the exception of those listed in the bugs section, which are things that are design-related. See section 1.7.5 Known Errors and Design Deficiencies in MySQL.

The `MySQL Server` design is multi-layered with independent modules. Some of the newer modules are listed here with an indication of how well-tested each of them is:

Replication -- Gamma
  Large server clusters using replication are in production use, with good results. Work on enhanced replication features is continuing in `MySQL` 4.x.
`InnoDB` tables -- Stable (in 3.23 from 3.23.49)
  The `InnoDB` transactional storage engine has now been declared stable in the `MySQL` 3.23 tree, starting from version 3.23.49. `InnoDB` is being used in large, heavy-load production systems.
`BDB` tables -- Gamma
  The `Berkeley DB` code is very stable, but we are still improving the `BDB` transactional storage engine interface in `MySQL Server`, so it will take some time before this is as well tested as the other table types.
`FULLTEXT` -- Beta
  Full-text search works but is not yet widely used. Important enhancements are being implemented for `MySQL` 4.0.
`MyODBC 2.50` (uses ODBC SDK 2.5) -- Gamma
  Increasingly in wide use. Some issues brought up appear to be application-related and independent of the ODBC driver or underlying database server.
Automatic recovery of `MyISAM` tables -- Gamma
  This status only regards the new code in the `MyISAM` storage engine that checks if the table was closed properly on open and executes an automatic check/repair of the table if it wasn't.
Bulk-insert -- Alpha
  New feature in `MyISAM` tables in `MySQL` 4.0 for faster insert of many rows.
Locking -- Gamma
  This is very system-dependent. On some systems there are big problems using standard OS locking (`fcntl()`). In these cases, you should run `mysqld` with the `--skip-external-locking` flag. Problems are known to occur on some Linux systems, and on SunOS when using NFS-mounted filesystems.

`MySQL AB` provides high-quality support for paying customers, but the `MySQL` mailing list usually provides answers to common questions. Bugs are usually fixed right away with a patch; for serious bugs, there is almost always a new release.

### 1.2.4 How Big Can MySQL Tables Be?

`MySQL` Version 3.22 has a 4G limit on table size. With the new `MyISAM` table type in `MySQL` Version 3.23, the maximum table size is pushed up to 8 million terabytes (2 ^ 63 bytes).

Note, however, that operating systems have their own file-size limits. Here are some examples:

Operating System | File-Size Limit
Linux-Intel 32 bit | 2G, 4G or more, depends on Linux version
Linux-Alpha | 8T (?)
Solaris 2.5.1 | 2G (possible 4G with patch)
Solaris 2.6 | 4G (can be changed with flag)
Solaris 2.7 Intel | 4G
Solaris 2.7 UltraSPARC | 512G

On Linux 2.2 you can get bigger tables than 2G by using the LFS patch for the ext2 filesystem. On Linux 2.4 patches also exist for ReiserFS to get support for big files.

This means that the table size for `MySQL` databases is normally limited by the operating system.

By default, `MySQL` tables have a maximum size of about 4G. You can check the maximum table size for a table with the `SHOW TABLE STATUS` command or with the `myisamchk -dv table_name`. See section 4.5.6 `SHOW` Syntax.

If you need bigger tables than 4G (and your operating system supports this), you should set the `AVG_ROW_LENGTH` and `MAX_ROWS` parameter when you create your table. See section 6.5.3 `CREATE TABLE` Syntax. You can also set these later with `ALTER TABLE`. See section 6.5.4 `ALTER TABLE` Syntax.

If your big table is going to be read-only, you could use `myisampack` to merge and compress many tables to one. `myisampack` usually compresses a table by at least 50%, so you can have, in effect, much bigger tables. See section 4.7.4 `myisampack`, The MySQL Compressed Read-only Table Generator.

You can go around the operating system file limit for `MyISAM` data files by using the `RAID` option. See section 6.5.3 `CREATE TABLE` Syntax.

Another solution can be the included `MERGE` library, which allows you to handle a collection of identical tables as one. See section 7.2 `MERGE` Tables.

### 1.2.5 Year 2000 Compliance

The `MySQL Server` itself has no problems with Year 2000 (Y2K) compliance:

- `MySQL Server` uses Unix time functions and has no problems with dates until `2069`; all 2-digit years are regarded to be in the range `1970` to `2069`, which means that if you store `01` in a `year` column, `MySQL Server` treats it as `2001`.
- All `MySQL` date functions are stored in one file, ``sql/time.cc'`, and are coded very carefully to be year 2000-safe.
- In `MySQL` Version 3.22 and later, the new `YEAR` column type can store years `0` and `1901` to `2155` in 1 byte and display them using 2 or 4 digits.

You may run into problems with applications that use `MySQL Server` in a way that is not Y2K-safe. For example, many old applications store or manipulate years using 2-digit values (which are ambiguous) rather than 4-digit values. This problem may be compounded by applications that use values such as `00` or `99` as ``missing'' value indicators.

Unfortunately, these problems may be difficult to fix because different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.

Here is a simple demonstration illustrating that `MySQL Server` doesn't have any problems with dates until the year 2030:

    mysql> DROP TABLE IF EXISTS y2k;
    Query OK, 0 rows affected (0.01 sec)

    mysql> CREATE TABLE y2k (date DATE,
        ->                   date_time DATETIME,
        ->                   time_stamp TIMESTAMP);
    Query OK, 0 rows affected (0.00 sec)

    mysql> INSERT INTO y2k VALUES
        -> ("1998-12-31","1998-12-31 23:59:59",19981231235959),
        -> ("1999-01-01","1999-01-01 00:00:00",19990101000000),
        -> ("1999-09-09","1999-09-09 23:59:59",19990909235959),
        -> ("2000-01-01","2000-01-01 00:00:00",20000101000000),
        -> ("2000-02-28","2000-02-28 00:00:00",20000228000000),
        -> ("2000-02-29","2000-02-29 00:00:00",20000229000000),
        -> ("2000-03-01","2000-03-01 00:00:00",20000301000000),
        -> ("2000-12-31","2000-12-31 23:59:59",20001231235959),
        -> ("2001-01-01","2001-01-01 00:00:00",20010101000000),
        -> ("2004-12-31","2004-12-31 23:59:59",20041231235959),
        -> ("2005-01-01","2005-01-01 00:00:00",20050101000000),
        -> ("2030-01-01","2030-01-01 00:00:00",20300101000000),
        -> ("2050-01-01","2050-01-01 00:00:00",20500101000000);
    Query OK, 13 rows affected (0.01 sec)
    Records: 13  Duplicates: 0  Warnings: 0

    mysql> SELECT * FROM y2k;
    +------------+---------------------+----------------+
    | date       | date_time           | time_stamp     |
    +------------+---------------------+----------------+
    | 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 |
    | 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 |
    | 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 |
    | 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 |
    | 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 |
    | 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 |
    | 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 |
    | 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 |
    | 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 |
    | 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 |
    | 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 |
    | 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 |
    | 2050-01-01 | 2050-01-01 00:00:00 | 00000000000000 |
    +------------+---------------------+----------------+
    13 rows in set (0.00 sec)

This shows that the `DATE` and `DATETIME` types will not give any problems with future dates (they handle dates until the year 9999).

The `TIMESTAMP` type, which is used to store the current time, has a range up to only `2030-01-01`. `TIMESTAMP` has a range of `1970` to `2030` on 32-bit machines (signed value). On 64-bit machines it handles times up to `2106` (unsigned value).

Even though `MySQL Server` is Y2K-compliant, it is your responsibility to provide unambiguous input. See section 6.2.2.1 Y2K Issues and Date Types for `MySQL Server`'s rules for dealing with ambiguous date input data (data containing 2-digit year values).

## 1.3 What Is MySQL AB?

`MySQL AB` is the company of the `MySQL` founders and main developers. `MySQL AB` was originally established in Sweden by David Axmark, Allan Larsson, and Michael `Monty` Widenius.

All the developers of the `MySQL` server are employed by the company. We are a virtual organisation with people in a dozen countries around the world. We communicate extensively over the Net every day with each other and with our users, supporters and partners.

We are dedicated to developing the `MySQL` software and spreading our database to new users. `MySQL AB` owns the copyright to the `MySQL` source code, the `MySQL` logo and trademark, and this manual. See section 1.2 What Is MySQL?.

The `MySQL` core values show our dedication to `MySQL` and `Open Source`.

We want the `MySQL Database Software` to be:

- The best and the most widely used database in the world.
- Available and affordable for all.
- Easy to use.
- Continuously improving while remaining fast and safe.
- Fun to use and improve.
- Free from bugs.

`MySQL AB` and the people at `MySQL AB`:

- Promote `Open Source` philosophy and support the `Open Source` community.
- Aim to be good citizens.
- Prefer partners that share our values and mind-set.
- Answer e-mail and provide support.
- Are a virtual company, networking with others.
- Work against software patents.

The `MySQL` web site (http://www.mysql.com/) provides the latest information about `MySQL` and `MySQL AB`.

### 1.3.1 The Business Model and Services of MySQL AB

One of the most common questions we encounter is: ``How can you make a living from something you give away for free?'' This is how.

`MySQL AB` makes money on support, services, commercial licenses, and royalties, and we use these revenues to fund product development and to expand the `MySQL` business.

The company has been profitable since its inception. In October 2001, we accepted venture financing from leading Scandinavian investors and a handful of business angels. This investment is used to solidify our business model and build a basis for sustainable growth.

#### 1.3.1.1 Support

`MySQL AB` is run and owned by the founders and main developers of the `MySQL` database. The developers are committed to giving support to customers and other users in order to stay in touch with their needs and problems. All our support is given by qualified developers. Really tricky questions are answered by Michael `Monty` Widenius, principal author of the `MySQL Server`. See section 1.4.1 Support Offered by MySQL AB.

For more information and ordering support at various levels, see http://www.mysql.com/support/ or contact our sales staff at sales@mysql.com.

#### 1.3.1.2 Training and Certification

`MySQL AB` delivers `MySQL` and related training worldwide. We offer both open courses and in-house courses tailored to the specific needs of your company. `MySQL Training` is also available through our partners, the `Authorised MySQL Training Centers`.

Our training material uses the same example databases as our documentation and our sample applications, and it is always updated to reflect the latest `MySQL` version. Our trainers are backed by the development team to guarantee the quality of the training and the continuous development of the course material. This also ensures that no questions raised during the courses remain unanswered.

Attending our training courses will enable you to achieve your goals related to your `MySQL` applications. You will also:

- Save time.
- Improve the performance of your application(s).
- Reduce or eliminate the need for additional hardware, decreasing cost.
- Enhance security.
- Increase customers' and co-workers' satisfaction.
- Prepare yourself for `MySQL Certification`.

If you are interested in our training as a potential participant or as a training partner, please visit the training section at http://www.mysql.com/training/ or contact us at: training@mysql.com.

The `MySQL Certification Program` is being released in the second half of 2002. For details please see http://www.mysql.com/certification/.

#### 1.3.1.3 Consulting

`MySQL AB` and its `Authorised Partners` offer consulting services to users of `MySQL Server` and to those who embed `MySQL Server` in their own software, all over the world.

Our consultants can help you design and tune your databases, construct efficient queries, tune your platform for optimal performance, resolve migration issues, set up replication, build robust transactional applications, and more. We also help customers embed `MySQL Server` in their products and applications for large-scale deployment.

Our consultants work in close collaboration with our development team, which ensures the technical quality of our professional services. Consulting assignments range from 2-day power-start sessions to projects that span weeks and months. Our expertise not only covers `MySQL Server`, but also extends into programming and scripting languages such as PHP, Perl, and more.

If you are interested in our consulting services or want to become a consulting partner, please visit the consulting section of our web site at http://www.mysql.com/consulting/ or contact our consulting staff at consulting@mysql.com.

#### 1.3.1.4 Commercial Licenses

The `MySQL` database is released under the `GNU General Public License` (`GPL`). This means that the `MySQL` software can be used free of charge under the `GPL`. If you do not want to be bound by the `GPL` terms (like the requirement that your own application becomes `GPL` as well), you may purchase a commercial license for the same product from `MySQL AB`. See http://www.mysql.com/support/arrangements/price.html. Since `MySQL AB` owns the copyright to the `MySQL` source code, we are able to employ `Dual Licensing` which means that the same product is available under `GPL` and under a commercial license. This does not in any way affect the `Open Source` commitment of `MySQL AB`. For details about when a commercial license is required, please see section 1.4.3 MySQL Licenses.

We also sell commercial licenses of third-party `Open Source GPL` software that adds value to `MySQL Server`. A good example is the `InnoDB` transactional storage engine that offers `ACID` support, row-level locking, crash recovery, multi-versioning, foreign key support, and more. See section 7.5 `InnoDB` Tables.

#### 1.3.1.5 Partnering

`MySQL AB` has a worldwide partner programme that covers training courses, consulting & support, publications plus reselling and distributing `MySQL` and related products. `MySQL AB Partners` get visibility on the http://www.mysql.com/ web site and the right to use special versions of the `MySQL` trademarks to identify their products and promote their business.

If you are interested in becoming a `MySQL AB Partner`, please e-mail partner@mysql.com.

The word `MySQL` and the `MySQL` dolphin logo are trademarks of `MySQL AB`. See section 1.4.4 MySQL AB Logos and Trademarks. These trademarks represent a significant value that the `MySQL` founders have built over the years.

#### 1.3.1.6 Advertising

The `MySQL` web site (http://www.mysql.com/) is popular among developers and users. In October 2001, we served 10 million page views. Our visitors represent a group that makes purchase decisions and recommendations for both software and hardware. Twelve percent of our visitors authorise purchase decisions, and only nine percent are not involved in purchase decisions at all. More than 65% have made one or more online business purchase within the last half-year, and 70% plan to make one in the next months.

### 1.3.2 Contact Information

The `MySQL` web site (http://www.mysql.com/) provides the latest information about `MySQL` and `MySQL AB`.

For press service and inquiries not covered in our News releases (http://www.mysql.com/news/), please send e-mail to press@mysql.com.

If you have a valid support contract with `MySQL AB`, you will get timely, precise answers to your technical questions about the `MySQL` software. For more information, see section 1.4.1 Support Offered by MySQL AB. On our website, see http://www.mysql.com/support/, or send an e-mail message to sales@mysql.com.

For information about `MySQL` training, please visit the training section at http://www.mysql.com/training/. If you have restricted access to the Internet, please contact the `MySQL AB` training staff at training@mysql.com. See section 1.3.1.2 Training and Certification.

For information on the `MySQL Certification Program`, please see http://www.mysql.com/certification/. See section 1.3.1.2 Training and Certification.

If you're interested in consulting, please visit the consulting section at http://www.mysql.com/consulting/. If you have restricted access to the Internet, please contact the `MySQL AB` consulting staff at consulting@mysql.com. See section 1.3.1.3 Consulting.

Commercial licenses may be purchased online at https://order.mysql.com/. There you will also find information on how to fax your purchase order to `MySQL AB`. More information about licensing can be found at http://www.mysql.com/support/arrangements/price.html. If you have questions regarding licensing or you want a quote for a high-volume license deal, please fill in the contact form on our web site (http://www.mysql.com/) or send an e-mail message to licensing@mysql.com (for licensing questions) or to sales@mysql.com (for sales inquiries). See section 1.4.3 MySQL Licenses.

If you represent a business that is interested in partnering with `MySQL AB`, please send e-mail to partner@mysql.com. See section 1.3.1.5 Partnering.

For more information on the `MySQL` trademark policy, refer to http://www.mysql.com/company/trademark.html or send e-mail to trademark@mysql.com. See section 1.4.4 MySQL AB Logos and Trademarks.

If you are interested in any of the `MySQL AB` jobs listed in our jobs section (http://www.mysql.com/company/jobs/), please send an e-mail message to jobs@mysql.com. Please do not send your CV as an attachment, but rather as plain text at the end of your e-mail message.

For general discussion among our many users, please direct your attention to the appropriate mailing list. See section 1.6.2 MySQL Mailing Lists.

Reports of errors (often called bugs), as well as questions and comments, should be sent to the mailing list at mysql@lists.mysql.com. If you have found a sensitive security bug in the `MySQL Server`, please send an e-mail to security@mysql.com. See section 1.6.2.3 How to Report Bugs or Problems.

If you have benchmark results that we can publish, please contact us at benchmarks@mysql.com.

If you have any suggestions concerning additions or corrections to this manual, please send them to the manual team at docs@mysql.com.

For questions or comments about the workings or content of the `MySQL` web site (http://www.mysql.com/), please send e-mail to webmaster@mysql.com.

Questions about the `MySQL Portals` (http://www.mysql.com/portal/) may be sent to portals@mysql.com.

`MySQL AB` has a privacy policy, which can be read at http://www.mysql.com/company/privacy.html. For any queries regarding this policy, please e-mail privacy@mysql.com.

For all other inquires, please send e-mail to info@mysql.com.

## 1.4 MySQL Support and Licensing

This section describes `MySQL` support and licensing arrangements.

### 1.4.1 Support Offered by MySQL AB

Technical support from `MySQL AB` means individualised answers to your unique problems direct from the software engineers who code the `MySQL` database engine.

We try to take a broad and inclusive view of technical support. Almost any problem involving `MySQL` software is important to us if it's important to you. Typically customers seek help on how to get different commands and utilities to work, remove performance bottlenecks, restore crashed systems, understand operating system or networking impacts on `MySQL`, set up best practices for backup and recovery, utilise `API`s, etc. Our support covers only the `MySQL` server and our own utilities, not third-party products that access the `MySQL` server, though we try to help with these where we can.

Detailed information about our various support options is given at http://www.mysql.com/support/, where support contracts can also be ordered online. If you have restricted access to the Internet, contact our sales staff at sales@mysql.com.

Technical support is like life insurance. You can live happily without it for years, but when your hour arrives it becomes critically important, yet it's too late to buy it! If you use `MySQL` Server for important applications and encounter sudden troubles, it might take too long to figure out all the answers yourself. You may need immediate access to the most experienced `MySQL` troubleshooters available, those employed by `MySQL AB`.

### 1.4.2 Copyrights and Licenses Used by MySQL

`MySQL AB` owns the copyright to the `MySQL` source code, the `MySQL` logos and trademarks and this manual. See section 1.3 What Is MySQL AB?. Several different licenses are relevant to the `MySQL` distribution:

1. All the `MySQL`-specific source in the server, the `mysqlclient` library and the client, as well as the `GNU` `readline` library is covered by the `GNU General Public License`. See section H GNU General Public License. The text of this license can also be found as the file ``COPYING'` in the distributions.
2. The `GNU` `getopt` library is covered by the `GNU Lesser General Public License`. See section I GNU Lesser General Public License.
3. Some parts of the source (the `regexp` library) are covered by a Berkeley-style copyright.
4. Older versions of `MySQL` (3.22 and earlier) are subject to a more strict license (http://www.mysql.com/support/arrangements/mypl.html). See the documentation of the specific version for information.
5. The manual is currently not distributed under a `GPL`-style license. Use of the manual is subject to the following terms:
   - Conversion to other formats is allowed, but the actual content may not be altered or edited in any way.
   - You may create a printed copy for your own personal use.
   - For all other uses, such as selling printed copies or using (parts of) the manual in another publication, prior written agreement from `MySQL AB` is required.
   Please e-mail docs@mysql.com for more information or if you are interested in doing a translation.

For information about how the `MySQL` licenses work in practice, please refer to section 1.4.3 MySQL Licenses. Also see section 1.4.4 MySQL AB Logos and Trademarks.

### 1.4.3 MySQL Licenses

The `MySQL` software is released under the `GNU General Public License` (`GPL`), which probably is the best known `Open Source` license. The formal terms of the `GPL` license can be found at http://www.gnu.org/licenses/. See also http://www.gnu.org/licenses/gpl-faq.html and http://www.gnu.org/philosophy/enforcing-gpl.html.

Since the `MySQL` software is released under the `GPL`, it may often be used for free, but for certain uses you may want or need to buy commercial licenses from `MySQL AB` at https://order.mysql.com/. See http://www.mysql.com/support/arrangements.html for more information.

Older versions of `MySQL` (3.22 and earlier) are subject to a more strict license (http://www.mysql.com/support/arrangements/mypl.html). See the documentation of the specific version for information.

Please note that the use of the `MySQL` software under commercial license, `GPL`, or the old `MySQL` license does not automatically give you the right to use `MySQL AB` trademarks. See section 1.4.4 MySQL AB Logos and Trademarks.

#### 1.4.3.1 Using the MySQL Software Under a Commercial License

The `GPL` license is contagious in the sense that when a program is linked to a `GPL` program all the source code for all the parts of the resulting product must also be released under the `GPL`. Otherwise you break the license terms and forfeit your right to use the `GPL` program altogether and also risk damages.

You need a commercial license:

- When you link a program with any `GPL` code from the `MySQL` software and don't want the resulting product to be `GPL`, maybe because you want to build a commercial product or keep the added non-`GPL` code closed source for other reasons. When purchasing commercial licenses, you are not using the `MySQL` software under `GPL` even though it's the same code.
- When you distribute a non-`GPL` application that only works with the `MySQL` software and ship it with the `MySQL` software. This type of solution is actually considered to be linking even if it's done over a network.
- When you distribute copies of the `MySQL` software without providing the source code as required under the `GPL` license.
- When you want to support the further development of the `MySQL` database even if you don't formally need a commercial license. Purchasing support directly from `MySQL AB` is another good way of contributing to the development of the `MySQL` software, with immediate advantages for you. See section 1.4.1 Support Offered by MySQL AB.

If you require a license, you will need one for each installation of the `MySQL` software. This covers any number of CPUs on a machine, and there is no artificial limit on the number of clients that connect to the server in any way.

For commercial licenses, please visit our website at http://www.mysql.com/support/arrangements/price.html. For support contracts, see http://www.mysql.com/support/. If you have special needs or you have restricted access to the Internet, please contact our sales staff at sales@mysql.com.

#### 1.4.3.2 Using the MySQL Software for Free Under GPL

You can use the `MySQL` software for free under the `GPL` if you adhere to the conditions of the `GPL`. For more complete coverage of the common questions about the `GPL` see the generic FAQ from the Free Software Foundation at http://www.gnu.org/licenses/gpl-faq.html. Some common cases:

- When you distribute both your own application as well as the `MySQL` source code under the `GPL` with your product.
- When you distribute the `MySQL` source code bundled with other programs that are not linked to or dependent on the `MySQL` system for their functionality even if you sell the distribution commercially. This is called mere aggregation in the `GPL` license.
- If you are not distributing any part of the `MySQL` system, you can use it for free.
- When you are an Internet Service Provider (ISP), offering web hosting with `MySQL` servers for your customers. However, we do encourage people to use ISPs that have MySQL support, as this will give them the confidence that if they have some problem with the `MySQL` installation, their ISP will in fact have the resources to solve the problem for them. Note that even if an ISP does not have a commercial license for `MySQL Server`, they should at least give their customers read access to the source of the `MySQL` installation so that the customers can verify that it is patched correctly.
- When you use the `MySQL` Database Software in conjunction with a web server, you do not need a commercial license (so long as it is not a product you distribute). This is true even if you run a commercial web server that uses `MySQL Server`, because you are not distributing any part of the `MySQL` system. However, in this case we would like you to purchase `MySQL` support because the `MySQL` software is helping your enterprise.

If your use of `MySQL` database software does not require a commercial license, we encourage you to purchase support from `MySQL AB` anyway. This way you contribute toward `MySQL` development and also gain immediate advantages for yourself. See section 1.4.1 Support Offered by MySQL AB.

If you use the `MySQL` database software in a commercial context such that you profit by its use, we ask that you further the development of the `MySQL` software by purchasing some level of support. We feel that if the `MySQL` database helps your business, it is reasonable to ask that you help `MySQL AB`. (Otherwise, if you ask us support questions, you are not only using for free something into which we've put a lot a work, you're asking us to provide free support, too.)

### 1.4.4 MySQL AB Logos and Trademarks

Many users of the `MySQL` database want to display the `MySQL AB` dolphin logo on their web sites, books, or boxed products. We welcome and encourage this, although it should be noted that the word `MySQL` and the `MySQL` dolphin logo are trademarks of `MySQL AB` and may only be used as stated in our trademark policy at http://www.mysql.com/company/trademark.html.

#### 1.4.4.1 The Original MySQL Logo

The `MySQL` dolphin logo was designed by the Finnish advertising agency Priority in 2001. The dolphin was chosen as a suitable symbol for the `MySQL` database since it is a smart, fast, and lean animal, effortlessly navigating oceans of data. We also happen to like dolphins.

The original `MySQL` logo may only be used by representatives of `MySQL AB` and by those having a written agreement allowing them to do so.

#### 1.4.4.2 MySQL Logos that may be Used Without Written Permission

We have designed a set of special Conditional Use logos that may be downloaded from our web site at http://www.mysql.com/press/logos.html and used on third-party web sites without written permission from `MySQL AB`. The use of these logos is not entirely unrestricted but, as the name implies, subject to our trademark policy that is also available on our web site. You should read through the trademark policy if you plan to use them. The requirements are basically:

- Use the logo you need as displayed on the http://www.mysql.com/ site. You may scale it to fit your needs, but not change colours or design, or alter the graphics in any way.
- Make it evident that you, and not `MySQL AB`, are the creator and owner of the site that displays the `MySQL` trademark.
- Don't use the trademark in a way that is detrimental to `MySQL AB` or to the value of `MySQL AB` trademarks. We reserve the right to revoke the right to use the `MySQL AB` trademark.
- If you use the trademark on a web site, make it clickable, leading directly to http://www.mysql.com/.
- If you are using the `MySQL` database under `GPL` in an application, your application must be `Open Source` and be able to connect to a `MySQL` server.

Contact us at trademark@mysql.com to inquire about special arrangements to fit your needs.

#### 1.4.4.3 When do you need a Written Permission to use MySQL Logos?

In the following cases you need a written permission from `MySQL AB` before using `MySQL` logos:

- When displaying any `MySQL AB` logo anywhere except on your web site.
- When displaying any `MySQL AB` logo except the Conditional Use logos mentioned previously on web sites or elsewhere.

Out of legal and commercial reasons we have to monitor the use of MySQL trademarks on products, books, etc. We will usually require a fee for displaying `MySQL AB` logos on commercial products, since we think it is reasonable that some of the revenue is returned to fund further development of the `MySQL` database.

#### 1.4.4.4 MySQL AB Partnership Logos

`MySQL` partnership logos may only be used by companies and persons having a written partnership agreement with `MySQL AB`. Partnerships include certification as a `MySQL` trainer or consultant. Please see section 1.3.1.5 Partnering.

#### 1.4.4.5 Using the word `MySQL` in Printed Text or Presentations

`MySQL AB` welcomes references to the `MySQL` database, but note that the word `MySQL` is a trademark of `MySQL AB`. Because of this, you should append the trademark symbol (`TM`) to the first or most prominent use of the word `MySQL` in a text and where appropriate, state that `MySQL` is a trademark of `MySQL AB`. Please refer to our trademark policy at http://www.mysql.com/company/trademark.html for details.

#### 1.4.4.6 Using the word `MySQL` in Company and Product Names

Use of the word `MySQL` in product or company names or in Internet domain names is not allowed without written permission from `MySQL AB`.

## 1.5 MySQL 4.x In A Nutshell

Long promised by `MySQL AB` and long awaited by our users, MySQL Server 4.0 is now available in beta version for download from http://www.mysql.com/ and our mirrors.

Main new features of MySQL Server 4.0 are geared toward our existing business and community users, enhancing the MySQL database software as the solution for mission-critical, heavy-load database systems. Other new features target the users of embedded databases.

### 1.5.1 Stepwise Rollout

The rollout of MySQL Server 4.x comes in several steps, with the first version labelled 4.0.0-alpha already containing most of the new features. Additional features have since been incorporated into MySQL 4.0.1, 4.0.2, and onward; MySQL 4.0.3 has been labelled beta. Further new features are being added in MySQL 4.1, targeted for alpha release in fourth quarter 2002.

### 1.5.2 Ready for Immediate Development Use

Users are not recommended to switch their production systems to an alpha version. However, all releases pass our extensive test suite without any errors on any of the platforms we test on. Due to the large number of new features, we thus recommend MySQL Server 4.x even in alpha form for development use, with the release schedule of MySQL Server 4.x being such that it will reach stable state before the deployment of user applications now under development. MySQL Server 4.x is now available in beta version (4.0.3 onward). See section 2.2.3 Which MySQL Version to Use.

### 1.5.3 Embedded MySQL

`libmysqld` makes MySQL Server suitable for a vastly expanded realm of applications. Using the embedded MySQL server library, one can embed MySQL Server into various applications and electronics devices, where the end user has no knowledge of there actually being an underlying database. Embedded MySQL Server is ideal for use behind the scenes in Internet appliances, public kiosks, turnkey hardware/software combination units, high performance Internet servers, self-contained databases distributed on CD-ROM, etc.

Many users of `libmysqld` will benefit from the MySQL Dual Licensing. For those not wishing to be bound by the GPL, the software is also made available under a commercial license. The embedded MySQL library uses the same interface as the normal client library, so it is convenient and easy to use. See section 8.4.9 libmysqld, the Embedded MySQL Server Library.

### 1.5.4 Other Features Available From MySQL 4.0

- Version 4.0 further increases the speed of MySQL Server in a number of areas, such as bulk `INSERT`s, searching on packed indexes, creation of `FULLTEXT` indexes, as well as `COUNT(DISTINCT)`.
- The `InnoDB` storage engine is now offered as a feature of the standard MySQL server, including full support for `transactions` and `row-level locking`.
- Our German, Austrian, and Swiss users will note that we have a new character set, `latin1_de`, which corrects the German sorting order, placing German umlauts in the same order as German telephone books.
- Features to simplify migration from other database systems to MySQL Server include `TRUNCATE TABLE` (like in Oracle) and `IDENTITY` as a synonym for automatically incremented keys (like in Sybase). Many users will also be happy to learn that MySQL Server now supports the `UNION` statement, a long-awaited standard SQL feature.
- In the process of building features for new users, we have not forgotten requests by the community of loyal users. We have multi-table `DELETE` and `UPDATE` statements. By adding support for `symbolic linking` to `MyISAM` on the table level (and not just the database level as before), as well as by enabling symlink handling by default on Windows, we hope to show that we take enhancement requests seriously. Functions like `SQL_CALC_FOUND_ROWS` and `FOUND_ROWS()` make it possible to know how many rows a query would have returned without a `LIMIT` clause.

### 1.5.5 Future MySQL 4.x Features

For the upcoming MySQL Server 4.x releases, expect the following features now still under development:

- Mission-critical, heavy-load users of MySQL Server will appreciate the additions to our replication system and our online hot backup. Later versions of 4.x will include `fail-safe replication`; already existing in 4.0, the `LOAD DATA FROM MASTER` command will soon automate slave setup. The `online backup` will make it easy to add a new replication slave without taking down the master, and have a very low performance penalty on update-heavy systems.
- A convenience feature for Database Administrators is that `mysqld` parameters (startup options) can soon be set without taking down the servers.
- The new `FULLTEXT` search properties of MySQL Server 4.0 enable the use of `FULLTEXT` indexing of large text masses with both binary and natural-language searching logic. Users can customise minimal word length and define their own stop word lists in any human language, enabling a new set of applications to be built on MySQL Server.
- Many read-heavy applications will benefit from further increased speed through the rewritten `key cache`.
- Many developers will also be happy to see the `MySQL command help` in the client.

### 1.5.6 MySQL 4.1, The Following Development Release

MySQL Server 4.0 lays the foundation for the new features of MySQL Server 4.1 and onward, such as `nested subqueries` (4.1), `stored procedures` (5.0), and `foreign key integrity rules` for `MyISAM` tables (5.0), which form the top of the wish list for many of our customers.

After those additions, critics of the MySQL Database Server have to be more imaginative than ever in pointing out deficiencies in the MySQL Database Management System. For long already known for its stability, speed, and ease of use, MySQL Server will then match the requirement checklist of very demanding buyers.

## 1.6 MySQL Information Sources

### 1.6.1 MySQL Portals

The `MySQL Portals` (http://www.mysql.com/portal/) represent the ultimate resource to find `MySQL AB Partners`, as well as books, or other `MySQL`-related solutions that you may be looking for. Items are categorised and rated in order to make it easy for you to locate information.

By registering as a user, you will have the ability to comment on and rate items presented in portals. You will also receive relevant newsletters according to your user profile that you may update at any time.

Some of the current `MySQL Portal` categories include:

Partners
  Find `MySQL AB` partners worldwide.
Books
  Comment on, vote for, and buy books related to `MySQL`.
Development
  Various links to different sites that are using `MySQL Server` for different purposes, with a description of each site. This information can give you an idea of who uses the `MySQL` database software and how `MySQL Server` can fulfill requirements. Let us know about your site or success story, too! Visit http://www.mysql.com/feedback/testimonial.php.
Software
  Find, buy, and download several applications and wrappers that make use of the `MySQL` server.
Distributions
  From here you can find the various Linux distributions and other software packages that contain the `MySQL` software.
Service Providers
  Companies providing `MySQL`-related services.

### 1.6.2 MySQL Mailing Lists

This section introduces you to the MySQL mailing lists, and gives some guidelines as to how to use them. By subscribing to a mailing list, you will receive as e-mail messages all other postings on the list, and you will be able to send in your own questions and answers.

#### 1.6.2.1 The MySQL Mailing Lists

To subscribe to the main MySQL mailing list, send a message to the electronic mail address mysql-subscribe@lists.mysql.com.

To unsubscribe from the main MySQL mailing list, send a message to the electronic mail address mysql-unsubscribe@lists.mysql.com.

Only the address to which you send your messages is significant. The subject line and the body of the message are ignored.

If your reply address is not valid, you can specify your address explicitly, by adding a hyphen to the subscribe or unsubscribe command word, followed by your address with the ``@'` character in your address replaced by a ``='`. For example, to subscribe `your_name@host.domain`, send a message to `mysql-subscribe-your_name=host.domain@lists.mysql.com`.

Mail to mysql-subscribe@lists.mysql.com or mysql-unsubscribe@lists.mysql.com is handled automatically by the ezmlm mailing list processor. Information about ezmlm is available at the ezmlm web site (http://www.ezmlm.org/).

To post a message to the list itself, send your message to `mysql@lists.mysql.com`. However, please do not send mail about subscribing or unsubscribing to mysql@lists.mysql.com because any mail sent to that address is distributed automatically to thousands of other users.

Your local site may have many subscribers to mysql@lists.mysql.com. If so, it may have a local mailing list, so messages sent from `lists.mysql.com` to your site are propagated to the local list. In such cases, please contact your system administrator to be added to or dropped from the local MySQL list.

If you wish to have traffic for a mailing list go to a separate mailbox in your mail program, set up a filter based on the message headers. You can use either the `List-ID:` or `Delivered-To:` headers to identify list messages.

The following MySQL mailing lists exist:

`announce-subscribe@lists.mysql.com announce`
  This is for announcement of new versions of MySQL and related programs. This is a low-volume list all MySQL users should subscribe to.
`mysql-subscribe@lists.mysql.com mysql`
  The main list for general MySQL discussion. Please note that some topics are better discussed on the more-specialised lists. If you post to the wrong list, you may not get an answer!
`mysql-digest-subscribe@lists.mysql.com mysql-digest`
  The `mysql` list in digest form. That means you get all individual messages, sent as one large mail message once a day.
`bugs-subscribe@lists.mysql.com bugs`
  On this list you should only post a full, repeatable bug report using the `mysqlbug` script (if you are running on Windows, you should include a description of the operating system and the MySQL version). Preferably, you should test the problem using the latest stable or development version of MySQL Server before posting! Anyone should be able to repeat the bug by just using `mysql test < script` on the included test case. All bugs posted on this list will be corrected or documented in the next MySQL release! If only small code changes are needed, we will also post a patch that fixes the problem.
`bugs-digest-subscribe@lists.mysql.com bugs-digest`
  The `bugs` list in digest form.
`internals-subscribe@lists.mysql.com internals`
  A list for people who work on the MySQL code. On this list one can also discuss MySQL development and post patches.
`internals-digest-subscribe@lists.mysql.com internals-digest`
  A digest version of the `internals` list.
`java-subscribe@lists.mysql.com java`
  Discussion about the MySQL server and Java. Mostly about the JDBC drivers including MySQL Connector/J.
`java-digest-subscribe@lists.mysql.com java-digest`
  A digest version of the `java` list.
`win32-subscribe@lists.mysql.com win32`
  All things concerning the MySQL software on Microsoft operating systems such as Windows 9x/Me/NT/2000/XP.
`win32-digest-subscribe@lists.mysql.com win32-digest`
  A digest version of the `win32` list.
`myodbc-subscribe@lists.mysql.com myodbc`
  All things about connecting to the MySQL server with ODBC.
`myodbc-digest-subscribe@lists.mysql.com myodbc-digest`
  A digest version of the `myodbc` list.
`mysqlcc-subscribe@lists.mysql.com mysqlcc`
  All things about the `MySQL Control Center` graphical client.
`mysqlcc-digest-subscribe@lists.mysql.com mysqlcc-digest`
  A digest version of the `mysqlcc` list.
`plusplus-subscribe@lists.mysql.com plusplus`
  All things concerning programming with the C++ API to MySQL.
`plusplus-digest-subscribe@lists.mysql.com plusplus-digest`
  A digest version of the `plusplus` list.
`msql-mysql-modules-subscribe@lists.mysql.com msql-mysql-modules`
  A list about the Perl support for MySQL with msql-mysql-modules.
`msql-mysql-modules-digest-subscribe@lists.mysql.com msql-mysql-modules-digest`
  A digest version of the `msql-mysql-modules` list.

You subscribe or unsubscribe to all lists in the same way as described previously. In your subscribe or unsubscribe message, just put the appropriate mailing list name rather than `mysql`. For example, to subscribe to or unsubscribe from the `myodbc` list, send a message to myodbc-subscribe@lists.mysql.com or myodbc-unsubscribe@lists.mysql.com.

If you can't get an answer for your questions from the mailing list, one option is to pay for support from MySQL AB, which will put you in direct contact with MySQL developers. See section 1.4.1 Support Offered by MySQL AB.

The following table shows some MySQL mailing in languages other than English. Note that these are not operated by MySQL AB, so we can't guarantee the quality on these.

`mysql-france-subscribe@yahoogroups.com A French mailing list`
`list@tinc.net A Korean mailing list`
  E-mail `subscribe mysql your@e-mail.address` to this list.
`mysql-de-request@lists.4t2.com A German mailing list`
  E-mail `subscribe mysql-de your@e-mail.address` to this list. You can find information about this mailing list at http://www.4t2.com/mysql/.
`mysql-br-request@listas.linkway.com.br A Portugese mailing list`
  E-mail `subscribe mysql-br your@e-mail.address` to this list.
`mysql-alta@elistas.net A Spanish mailing list`
  E-mail `subscribe mysql your@e-mail.address` to this list.

#### 1.6.2.2 Asking Questions or Reporting Bugs

Before posting a bug report or question, please do the following:

- Start by searching the MySQL online manual at:
  http://www.mysql.com/doc/
  We try to keep the manual up to date by updating it frequently with solutions to newly found problems! The change history appendix (http://www.mysql.com/doc/en/News.html) can be particularly useful since it is quite possible that a newer version already contains a solution to your problem.
- Search the MySQL mailing list archives:
  http://lists.mysql.com/
- You can also use http://www.mysql.com/search/ to search all the web pages (including the manual) that are located at http://www.mysql.com/.

If you can't find an answer in the manual or the archives, check with your local MySQL expert. If you still can't find an answer to your question, go ahead and read the next section about how to send mail to mysql@lists.mysql.com.

#### 1.6.2.3 How to Report Bugs or Problems

Writing a good bug report takes patience, but doing it right the first time saves time for us and for you. A good bug report containing a full test case for the bug will make it very likely that we will fix it in the next release. This section will help you write your report correctly so that you don't waste your time doing things that may not help us much or at all.

We encourage everyone to use the `mysqlbug` script to generate a bug report (or a report about any problem), if possible. `mysqlbug` can be found in the ``scripts'` directory in the source distribution, or for a binary distribution, in the ``bin'` directory under your MySQL installation directory. If you are unable to use `mysqlbug`, you should still include all the necessary information listed in this section.

The `mysqlbug` script helps you generate a report by determining much of the following information automatically, but if something important is missing, please include it with your message! Please read this section carefully and make sure that all the information described here is included in your report.

The normal place to report bugs and problems is mysql@lists.mysql.com. If you can make a test case that clearly demonstrates the bug, you should post it to the bugs@lists.mysql.com list. Note that on this list you should only post a full, repeatable bug report using the `mysqlbug` script. If you are running on Windows, you should include a description of the operating system and the MySQL version. Preferably, you should test the problem using the latest stable or development version of MySQL Server before posting! Anyone should be able to repeat the bug by just using ```mysql test < script`'' on the included test case or run the shell or Perl script that is included in the bug report. All bugs posted on the `bugs` list will be corrected or documented in the next MySQL release! If only small code changes are needed to correct this problem, we will also post a patch that fixes the problem.

If you have found a sensitive security bug in MySQL, you should send an e-mail to security@mysql.com.

Remember that it is possible to respond to a message containing too much information, but not to one containing too little. Often people omit facts because they think they know the cause of a problem and assume that some details don't matter. A good principle is: if you are in doubt about stating something, state it! It is a thousand times faster and less troublesome to write a couple of lines more in your report than to be forced to ask again and wait for the answer because you didn't include enough information the first time.

The most common errors are that people don't indicate the version number of the MySQL distribution they are using, or don't indicate what platform they have the MySQL server installed on (including the platform version number). This is highly relevant information, and in 99 cases out of 100 the bug report is useless without it! Very often we get questions like, ``Why doesn't this work for me?'' Then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has been fixed already in newer MySQL versions. Sometimes the error is platform-dependent; in such cases, it is next to impossible to fix anything without knowing the operating system and the version number of the platform.

Remember also to provide information about your compiler, if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL-related. Most compilers are under development all the time and become better version by version. To determine whether your problem depends on your compiler, we need to know what compiler is used. Note that every compiling problem should be regarded as a bug report and reported accordingly.

It is most helpful when a good description of the problem is included in the bug report. That is, a good example of all the things you did that led to the problem and the problem itself exactly described. The best reports are those that include a full example showing how to reproduce the bug or problem. See section E.1.6 Making a Test Case When You Experience Table Corruption.

If a program produces an error message, it is very important to include the message in your report! If we try to search for something from the archives using programs, it is better that the error message reported exactly matches the one that the program produces. (Even the case should be observed!) You should never try to remember what the error message was; instead, copy and paste the entire message into your report!

If you have a problem with MyODBC, you should try to generate a MyODBC trace file. See section 8.3.7 Reporting Problems with MyODBC.

Please remember that many of the people who will read your report will do so using an 80-column display. When generating reports or examples using the `mysql` command-line tool, you should therefore use the `--vertical` option (or the `\G` statement terminator) for output that would exceed the available width for such a display (for example, with the `EXPLAIN SELECT` statement; see the example later in this section).

Please include the following information in your report:

- The version number of the MySQL distribution you are using (for example, MySQL Version 3.22.22). You can find out which version you are running by executing `mysqladmin version`. `mysqladmin` can be found in the ``bin'` directory under your MySQL installation directory.
- The manufacturer and model of the machine you are working on.
- The operating system name and version. For most operating systems, you can get this information by executing the Unix command `uname -a`.
- Sometimes the amount of memory (real and virtual) is relevant. If in doubt, include these values.
- If you are using a source distribution of the MySQL software, the name and version number of the compiler used is needed. If you have a binary distribution, the distribution name is needed.
- If the problem occurs during compilation, include the exact error message(s) and also a few lines of context around the offending code in the file where the error occurred.
- If `mysqld` died, you should also report the query that crashed `mysqld`. You can usually find this out by running `mysqld` with logging enabled. See section E.1.5 Using Log Files to Find Cause of Errors in mysqld.
- If any database table is related to the problem, include the output from `mysqldump --no-data db_name tbl_name1 tbl_name2 ...`. This is very easy to do and is a powerful way to get information about any table in a database that will help us create a situation matching the one you have.
- For speed-related bugs or problems with `SELECT` statements, you should always include the output of `EXPLAIN SELECT ...`, and at least the number of rows that the `SELECT` statement produces. You should also include the output from `SHOW CREATE TABLE table_name` for each involved table. The more information you give about your situation, the more likely it is that someone can help you! For example, the following is an example of a very good bug report (it should of course be posted with the `mysqlbug` script): Example run using the `mysql` command-line tool (note the use of the `\G` statement terminator for statements whose output width would otherwise exceed that of an 80-column display device):

      mysql> SHOW VARIABLES;
      mysql> SHOW COLUMNS FROM ...\G
             <output from SHOW COLUMNS>
      mysql> EXPLAIN SELECT ...\G
             <output from EXPLAIN>
      mysql> FLUSH STATUS;
      mysql> SELECT ...;
             <A short version of the output from SELECT,
             including the time taken to run the query>
      mysql> SHOW STATUS;
             <output from SHOW STATUS>

- If a bug or problem occurs while running `mysqld`, try to provide an input script that will reproduce the anomaly. This script should include any necessary source files. The more closely the script can reproduce your situation, the better. If you can make a reproduceable test case, you should post this to bugs@lists.mysql.com for a high-priority treatment! If you can't provide a script, you should at least include the output from `mysqladmin variables extended-status processlist` in your mail to provide some information of how your system is performing!
- If you can't produce a test case in a few rows, or if the test table is too big to be mailed to the mailing list (more than 10 rows), you should dump your tables using `mysqldump` and create a ``README'` file that describes your problem. Create a compressed archive of your files using `tar` and `gzip` or `zip`, and use `ftp` to transfer the archive to ftp://support.mysql.com/pub/mysql/secret/. Then send a short description of the problem to bugs@lists.mysql.com.
- If you think that the MySQL server produces a strange result from a query, include not only the result, but also your opinion of what the result should be, and an account describing the basis for your opinion.
- When giving an example of the problem, it's better to use the variable names, table names, etc., that exist in your actual situation than to come up with new names. The problem could be related to the name of a variable or table! These cases are rare, perhaps, but it is better to be safe than sorry. After all, it should be easier for you to provide an example that uses your actual situation, and it is by all means better for us. In case you have data you don't want to show to others, you can use `ftp` to transfer it to ftp://support.mysql.com/pub/mysql/secret/. If the data is really top secret and you don't want to show it even to us, then go ahead and provide an example using other names, but please regard this as the last choice.
- Include all the options given to the relevant programs, if possible. For example, indicate the options that you use when you start the `mysqld` daemon and that you use to run any MySQL client programs. The options to programs like `mysqld` and `mysql`, and to the `configure` script, are often keys to answers and are very relevant! It is never a bad idea to include them anyway! If you use any modules, such as Perl or PHP, please include the version number(s) of those as well.
- If your question is related to the privilege system, please include the output of `mysqlaccess`, the output of `mysqladmin reload`, and all the error messages you get when trying to connect! When you test your privileges, you should first run `mysqlaccess`. After this, execute `mysqladmin reload version` and try to connect with the program that gives you trouble. `mysqlaccess` can be found in the ``bin'` directory under your MySQL installation directory.
- If you have a patch for a bug, that is good. But don't assume the patch is all we need, or that we will use it, if you don't provide some necessary information such as test cases showing the bug that your patch fixes. We might find problems with your patch or we might not understand it at all; if so, we can't use it. If we can't verify exactly what the patch is meant for, we won't use it. Test cases will help us here. Show that the patch will handle all the situations that may occur. If we find a borderline case (even a rare one) where the patch won't work, it may be useless.
- Guesses about what the bug is, why it occurs, or what it depends on are usually wrong. Even the MySQL team can't guess such things without first using a debugger to determine the real cause of a bug.
- Indicate in your mail message that you have checked the reference manual and mail archive so that others know you have tried to solve the problem yourself.
- If you get a `parse error`, please check your syntax closely! If you can't find something wrong with it, it's extremely likely that your current version of MySQL Server doesn't support the query you are using. If you are using the current version and the manual at http://www.mysql.com/doc/ doesn't cover the syntax you are using, MySQL Server doesn't support your query. In this case, your only options are to implement the syntax yourself or e-mail licensing@mysql.com and ask for an offer to implement it! If the manual covers the syntax you are using, but you have an older version of MySQL Server, you should check the MySQL change history to see when the syntax was implemented. In this case, you have the option of upgrading to a newer version of MySQL Server. See section D MySQL Change History.
- If you have a problem such that your data appears corrupt or you get errors when you access some particular table, you should first check and then try repairing your tables with `myisamchk` or `CHECK TABLE` and `REPAIR TABLE`. See section 4 Database Administration.
- If you often get corrupted tables you should try to find out when and why this happens. In this case, the ``mysql-data-directory/'hostname'.err'` file may contain some information about what happened. See section 4.9.1 The Error Log. Please include any relevant information from this file in your bug report. Normally `mysqld` should never crash a table if nothing killed it in the middle of an update! If you can find the cause of `mysqld` dying, it's much easier for us to provide you with a fix for the problem. See section A.1 How to Determine What Is Causing Problems.
- If possible, download and install the most recent version of MySQL Server and check whether it solves your problem. All versions of the MySQL software are thoroughly tested and should work without problems. We believe in making everything as backward-compatible as possible, and you should be able to switch MySQL versions without any hassle. See section 2.2.3 Which MySQL Version to Use.

If you are a support customer, please cross-post the bug report to mysql-support@mysql.com for higher-priority treatment, as well as to the appropriate mailing list to see if someone else has experienced (and perhaps solved) the problem.

For information on reporting bugs in `MyODBC`, see section 8.3.4 How to Report Problems with MyODBC.

For solutions to some common problems, see section A Problems and Common Errors.

When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarise the answers and send the summary to the mailing list so that others may have the benefit of responses you received that helped you solve your problem!

#### 1.6.2.4 Guidelines for Answering Questions on the Mailing List

If you consider your answer to have broad interest, you may want to post it to the mailing list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer.

Try to summarise the essential part of the question in your reply; don't feel obliged to quote the entire original message.

Please don't post mail messages from your browser with HTML mode turned on! Many users don't read mail with a browser!

## 1.7 How Standards-compatible Is MySQL?

This section describes how MySQL relates to the ANSI SQL standards. MySQL Server has many extensions to the ANSI SQL standards, and here you will find out what they are and how to use them. You will also find information about functionality missing from MySQL Server, and how to work around some differences.

Our goal is to not, without a very good reason, restrict MySQL Server usability for any usage. Even if we don't have the resources to do development for every possible use, we are always willing to help and offer suggestions to people who are trying to use MySQL Server in new territories.

One of our main goals with the product is to continue to work toward ANSI 99 compliancy, but without sacrificing speed or reliability. We are not afraid to add extensions to SQL or support for non-SQL features if this greatly increases the usability of MySQL Server for a big part of our users. (The new `HANDLER` interface in MySQL Server 4.0 is an example of this strategy. See section 6.4.2 `HANDLER` Syntax.)

We will continue to support transactional and non-transactional databases to satisfy both heavy web/logging usage and mission-critical 24/7 usage.

MySQL Server was designed from the start to work with medium size databases (10-100 million rows, or about 100 MB per table) on small computer systems. We will continue to extend MySQL Server to work even better with terabyte-size databases, as well as to make it possible to compile a reduced MySQL version that is more suitable for hand-held devices and embedded usage. The compact design of the MySQL server makes both of these directions possible without any conflicts in the source tree.

We are currently not targeting realtime support or clustered databases (even if you can already do a lot of things with our replication services).

We don't believe that one should have native XML support in the database, but will instead add the XML support our users request from us on the client side. We think it's better to keep the main server code as ``lean and clean'' as possible and instead develop libraries to deal with the complexity on the client side. This is part of the strategy mentioned previously of not sacrificing speed or reliability in the server.

### 1.7.1 What Standards Does MySQL Follow?

Entry-level SQL92. ODBC levels 0-3.51.

We are aiming toward supporting the full ANSI SQL99 standard, but without concessions to speed and quality of the code.

### 1.7.2 Running MySQL in ANSI Mode

If you start `mysqld` with the `--ansi` option, the following behaviour of MySQL Server changes:

- `||` is string concatenation instead of `OR`.
- You can have any number of spaces between a function name and the ``('`. This forces all function names to be treated as reserved words.
- ``"'` will be an identifier quote character (like the MySQL Server ```'` quote character) and not a string quote character.
- `REAL` will be a synonym for `FLOAT` instead of a synonym for `DOUBLE`.
- The default transaction isolation level is `SERIALIZABLE`. See section 6.7.3 `SET TRANSACTION` Syntax.

This is the same as using `--sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES, IGNORE_SPACE,SERIALIZE,ONLY_FULL_GROUP_BY`.

### 1.7.3 MySQL Extensions to ANSI SQL92

MySQL Server includes some extensions that you probably will not find in other SQL databases. Be warned that if you use them, your code will not be portable to other SQL servers. In some cases, you can write code that includes MySQL extensions, but is still portable, by using comments of the form `/*! ... */`. In this case, MySQL Server will parse and execute the code within the comment as it would any other MySQL statement, but other SQL servers will ignore the extensions. For example:

    SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ...

If you add a version number after the `'!'`, the syntax will be executed only if the MySQL version is equal to or newer than the used version number:

    CREATE /*!32302 TEMPORARY */ TABLE t (a int);

This means that if you have Version 3.23.02 or newer, MySQL Server will use the `TEMPORARY` keyword.

The following is a list of MySQL extensions:

- The field types `MEDIUMINT`, `SET`, `ENUM`, and the different `BLOB` and `TEXT` types.
- The field attributes `AUTO_INCREMENT`, `BINARY`, `NULL`, `UNSIGNED`, and `ZEROFILL`.
- All string comparisons are case-insensitive by default, with sort ordering determined by the current character set (ISO-8859-1 Latin1 by default). If you don't like this, you should declare your columns with the `BINARY` attribute or use the `BINARY` cast, which causes comparisons to be done according to the ASCII order used on the MySQL server host.
- MySQL Server maps each database to a directory under the MySQL data directory, and tables within a database to filenames in the database directory. This has a few implications:
  - Database names and table names are case-sensitive in MySQL Server on operating systems that have case-sensitive filenames (like most Unix systems). See section 6.1.3 Case Sensitivity in Names.
  - Database, table, index, column, or alias names may begin with a digit (but may not consist solely of digits).
  - You can use standard system commands to back up, rename, move, delete, and copy tables. For example, to rename a table, rename the ``.MYD'`, ``.MYI'`, and ``.frm'` files to which the table corresponds.
- In SQL statements, you can access tables from different databases with the `db_name.tbl_name` syntax. Some SQL servers provide the same functionality but call this `User space`. MySQL Server doesn't support tablespaces as in: `create table ralph.my_table...IN my_tablespace`.
- `LIKE` is allowed on numeric columns.
- Use of `INTO OUTFILE` and `STRAIGHT_JOIN` in a `SELECT` statement. See section 6.4.1 `SELECT` Syntax.
- The `SQL_SMALL_RESULT` option in a `SELECT` statement.
- `EXPLAIN SELECT` to get a description on how tables are joined.
- Use of index names, indexes on a prefix of a field, and use of `INDEX` or `KEY` in a `CREATE TABLE` statement. See section 6.5.3 `CREATE TABLE` Syntax.
- Use of `TEMPORARY` or `IF NOT EXISTS` with `CREATE TABLE`.
- Use of `COUNT(DISTINCT list)` where `list` is more than one element.
- Use of `CHANGE col_name`, `DROP col_name`, or `DROP INDEX`, `IGNORE` or `RENAME` in an `ALTER TABLE` statement. See section 6.5.4 `ALTER TABLE` Syntax.
- Use of `RENAME TABLE`. See section 6.5.5 `RENAME TABLE` Syntax.
- Use of multiple `ADD`, `ALTER`, `DROP`, or `CHANGE` clauses in an `ALTER TABLE` statement.
- Use of `DROP TABLE` with the keywords `IF EXISTS`.
- You can drop multiple tables with a single `DROP TABLE` statement.
- The `LIMIT` clause of the `DELETE` statement.
- The `DELAYED` clause of the `INSERT` and `REPLACE` statements.
- The `LOW_PRIORITY` clause of the `INSERT`, `REPLACE`, `DELETE`, and `UPDATE` statements.
- Use of `LOAD DATA INFILE`. In many cases, this syntax is compatible with Oracle's `LOAD DATA INFILE`. See section 6.4.9 `LOAD DATA INFILE` Syntax.
- The `ANALYZE TABLE`, `CHECK TABLE`, `OPTIMIZE TABLE`, and `REPAIR TABLE` statements.
- The `SHOW` statement. See section 4.5.6 `SHOW` Syntax.
- Strings may be enclosed by either ``"'` or ``''`, not just by ``''`.
- Use of the escape ``\'` character.
- The `SET` statement. See section 5.5.6 `SET` Syntax.
- You don't need to name all selected columns in the `GROUP BY` part. This gives better performance for some very specific, but quite normal queries. See section 6.3.7 Functions for Use with `GROUP BY` Clauses.
- One can specify `ASC` and `DESC` with `GROUP BY`.
- To make it easier for users who come from other SQL environments, MySQL Server supports aliases for many functions. For example, all string functions support both ANSI SQL syntax and ODBC syntax.
- MySQL Server understands the `||` and `&&` operators to mean logical OR and AND, as in the C programming language. In MySQL Server, `||` and `OR` are synonyms, as are `&&` and `AND`. Because of this nice syntax, MySQL Server doesn't support the ANSI SQL `||` operator for string concatenation; use `CONCAT()` instead. Because `CONCAT()` takes any number of arguments, it's easy to convert use of the `||` operator to MySQL Server.
- `CREATE DATABASE` or `DROP DATABASE`. See section 6.5.1 `CREATE DATABASE` Syntax.
- The `%` operator is a synonym for `MOD()`. That is, `N % M` is equivalent to `MOD(N,M)`. `%` is supported for C programmers and for compatibility with PostgreSQL.
- The `=`, `<>`, `<=` ,`<`, `>=`,`>`, `<<`, `>>`, `<=>`, `AND`, `OR`, or `LIKE` operators may be used in column comparisons to the left of the `FROM` in `SELECT` statements. For example:

      mysql> SELECT col1=1 AND col2=2 FROM tbl_name;

- The `LAST_INSERT_ID()` function. See section 8.4.3.126 `mysql_insert_id()`.
- The `REGEXP` and `NOT REGEXP` extended regular expression operators.
- `CONCAT()` or `CHAR()` with one argument or more than two arguments. (In MySQL Server, these functions can take any number of arguments.)
- The `BIT_COUNT()`, `CASE`, `ELT()`, `FROM_DAYS()`, `FORMAT()`, `IF()`, `PASSWORD()`, `ENCRYPT()`, `MD5()`, `ENCODE()`, `DECODE()`, `PERIOD_ADD()`, `PERIOD_DIFF()`, `TO_DAYS()`, or `WEEKDAY()` functions.
- Use of `TRIM()` to trim substrings. ANSI SQL only supports removal of single characters.
- The `GROUP BY` functions `STD()`, `BIT_OR()`, and `BIT_AND()`.
- Use of `REPLACE` instead of `DELETE` + `INSERT`. See section 6.4.8 `REPLACE` Syntax.
- The `FLUSH`, `RESET` and `DO` statements.
- The ability to set variables in a statement with `:=`:

      SELECT @a:=SUM(total),@b=COUNT(*),@a/@b AS avg FROM test_table;
      SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;

### 1.7.4 MySQL Differences Compared to ANSI SQL92

We try to make MySQL Server follow the ANSI SQL standard and the ODBC SQL standard, but in some cases MySQL Server does things differently:

- For `VARCHAR` columns, trailing spaces are removed when the value is stored. See section 1.7.5 Known Errors and Design Deficiencies in MySQL.
- In some cases, `CHAR` columns are silently changed to `VARCHAR` columns. See section 6.5.3.1 Silent Column Specification Changes.
- Privileges for a table are not automatically revoked when you delete a table. You must explicitly issue a `REVOKE` to revoke privileges for a table. See section 4.3.1 `GRANT` and `REVOKE` Syntax.
- `NULL AND FALSE` will evaluate to `NULL` and not to `FALSE`. This is because we don't think it's good to have to evaluate a lot of extra conditions in this case.

For a prioritised list indicating when new extensions will be added to MySQL Server, you should consult the online MySQL TODO list at http://www.mysql.com/doc/en/TODO.html. That is the latest version of the TODO list in this manual. See section 1.8 MySQL and The Future (The TODO).

#### 1.7.4.1 Sub`SELECT`s

MySQL Server until version 4.0 only supports nested queries of the form `INSERT ... SELECT ...` and `REPLACE ... SELECT ...`. You can, however, use the function `IN()` in other contexts. Subselects are being implemented in the 4.1 development tree.

Meanwhile, you can often rewrite the query without a subselect:

    SELECT * FROM table1 WHERE id IN (SELECT id FROM table2);

This can be rewritten as:

    SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id;

The queries:

    SELECT * FROM table1 WHERE id NOT IN (SELECT id FROM table2);
    SELECT * FROM table1 WHERE NOT EXISTS (SELECT id FROM table2
                                           WHERE table1.id=table2.id);

Can be rewritten as:

    SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id
                                           WHERE table2.id IS NULL;

For more complicated subqueries you can often create temporary tables to hold the subquery. In some cases, however, this option will not work. The most frequently encountered of these cases arises with `DELETE` statements, for which standard SQL does not support joins (except in subselects). For this situation there are two options available until subqueries are supported by MySQL Server.

The first option is to use a procedural programming language (such as Perl or PHP) to submit a `SELECT` query to obtain the primary keys for the records to be deleted, and then use these values to construct the `DELETE` statement (`DELETE FROM ... WHERE ... IN (key1, key2, ...)`).

The second option is to use interactive SQL to construct a set of `DELETE` statements automatically, using the MySQL extension `CONCAT()` (in lieu of the standard `||` operator). For example:

    SELECT CONCAT('DELETE FROM tab1 WHERE pkid = ', "'", tab1.pkid, "'", ';')
      FROM tab1, tab2
     WHERE tab1.col1 = tab2.col2;

You can place this query in a script file and redirect input from it to the `mysql` command-line interpreter, piping its output back to a second instance of the interpreter:

    shell> mysql --skip-column-names mydb < myscript.sql | mysql mydb

MySQL Server 4.0 supports multi-table deletes that can be used to efficiently delete rows based on information from one table or even from many tables at the same time.

#### 1.7.4.2 `SELECT INTO TABLE`

MySQL Server doesn't yet support the Oracle SQL extension: `SELECT ... INTO TABLE ...`. MySQL Server supports instead the ANSI SQL syntax `INSERT INTO ... SELECT ...`, which is basically the same thing. See section 6.4.3.1 `INSERT ... SELECT` Syntax.

    INSERT INTO tblTemp2 (fldID) SELECT tblTemp1.fldOrder_ID
           FROM tblTemp1 WHERE tblTemp1.fldOrder_ID > 100;

Alternatively, you can use `SELECT INTO OUTFILE...` or `CREATE TABLE ... SELECT`.

#### 1.7.4.3 Transactions and Atomic Operations

MySQL Server supports transactions with the `InnoDB` and `BDB` `Transactional table handlers`. See section 7 MySQL Table Types. `InnoDB` provides `ACID` compliancy.

However, the non-transactional table types in MySQL Server such as `MyISAM` follow another paradigm for data integrity called ```Atomic Operations`.'' Atomic operations often offer equal or even better integrity with much better performance. With MySQL Server supporting both paradigms, the user is able to decide if he needs the speed of atomic operations or if he need to use transactional features in his applications. This choice can be made on a per-table basis.

How does one use the features of MySQL Server to maintain rigorous integrity and how do these features compare with the transactional paradigm?

1. In the transactional paradigm, if your applications are written in a way that is dependent on the calling of `ROLLBACK` instead of `COMMIT` in critical situations, transactions are more convenient. Transactions also ensure that unfinished updates or corrupting activities are not committed to the database; the server is given the opportunity to do an automatic rollback and your database is saved. MySQL Server, in almost all cases, allows you to resolve potential problems by including simple checks before updates and by running simple scripts that check the databases for inconsistencies and automatically repair or warn if such an inconsistency occurs. Note that just by using the MySQL log or even adding one extra log, one can normally fix tables perfectly with no data integrity loss.
2. More often than not, fatal transactional updates can be rewritten to be atomic. Generally speaking, all integrity problems that transactions solve can be done with `LOCK TABLES` or atomic updates, ensuring that you never will get an automatic abort from the database, which is a common problem with transactional databases.
3. Even a transactional system can lose data if the server goes down. The difference between different systems lies in just how small the time-lap is where they could lose data. No system is 100% secure, only ``secure enough.'' Even Oracle, reputed to be the safest of transactional databases, is reported to sometimes lose data in such situations. To be safe with MySQL Server, whether using transactional tables or not, you only need to have backups and have the update logging turned on. With this you can recover from any situation that you could with any other transactional database. It is, of course, always good to have backups, independent of which database you use.

The transactional paradigm has its benefits and its drawbacks. Many users and application developers depend on the ease with which they can code around problems where an abort appears to be, or is necessary. However, even if you are new to the atomic operations paradigm, or more familiar with transactions, do consider the speed benefit that non-transactional tables can offer on the order of three to five times the speed of the fastest and most optimally tuned transactional tables.

In situations where integrity is of highest importance, MySQL Server offers transaction-level or better reliability and integrity even for non-transactional tables. If you lock tables with `LOCK TABLES`, all updates will stall until any integrity checks are made. If you only obtain a read lock (as opposed to a write lock), reads and inserts are still allowed to happen. The new inserted records will not be seen by any of the clients that have a read lock until they release their read locks. With `INSERT DELAYED` you can queue inserts into a local queue, until the locks are released, without having the client wait for the insert to complete. See section 6.4.4 `INSERT DELAYED` Syntax.

``Atomic,'' in the sense that we mean it, is nothing magical. It only means that you can be sure that while each specific update is running, no other user can interfere with it, and there will never be an automatic rollback (which can happen with transactional tables if you are not very careful). MySQL Server also guarantees that there will not be any dirty reads.

Following are some techniques for working with non-transactional tables:

- Loops that need transactions normally can be coded with the help of `LOCK TABLES`, and you don't need cursors when you can update records on the fly.
- To avoid using `ROLLBACK`, you can use the following strategy:
  1. Use `LOCK TABLES ...` to lock all the tables you want to access.
  2. Test conditions.
  3. Update if everything is okay.
  4. Use `UNLOCK TABLES` to release your locks.
  This is usually a much faster method than using transactions with possible `ROLLBACK`s, although not always. The only situation this solution doesn't handle is when someone kills the threads in the middle of an update. In this case, all locks will be released but some of the updates may not have been executed.
- You can also use functions to update records in a single operation. You can get a very efficient application by using the following techniques:
  - Modify fields relative to their current value.
  - Update only those fields that actually have changed.
  For example, when we are doing updates to some customer information, we update only the customer data that has changed and test only that none of the changed data, or data that depends on the changed data, has changed compared to the original row. The test for changed data is done with the `WHERE` clause in the `UPDATE` statement. If the record wasn't updated, we give the client a message: "Some of the data you have changed has been changed by another user." Then we show the old row versus the new row in a window, so the user can decide which version of the customer record he should use. This gives us something that is similar to column locking but is actually even better because we only update some of the columns, using values that are relative to their current values. This means that typical `UPDATE` statements look something like these:

      UPDATE tablename SET pay_back=pay_back+'relative change';

      UPDATE customer
        SET
          customer_date='current_date',
          address='new address',
          phone='new phone',
          money_he_owes_us=money_he_owes_us+'new_money'
        WHERE
          customer_id=id AND address='old address' AND phone='old phone';

  As you can see, this is very efficient and works even if another client has changed the values in the `pay_back` or `money_he_owes_us` columns.
- In many cases, users have wanted `ROLLBACK` and/or `LOCK TABLES` for the purpose of managing unique identifiers for some tables. This can be handled much more efficiently by using an `AUTO_INCREMENT` column and either the SQL function `LAST_INSERT_ID()` or the C API function `mysql_insert_id()`. See section 8.4.3.126 `mysql_insert_id()`. You can generally code around row-level locking. Some situations really need it, but they are very few. `InnoDB` tables support row-level locking. With MyISAM, you can use a flag column in the table and do something like the following:

      UPDATE tbl_name SET row_flag=1 WHERE id=ID;

  MySQL returns 1 for the number of affected rows if the row was found and `row_flag` wasn't already 1 in the original row. You can think of it as though MySQL Server changed the preceding query to:

      UPDATE tbl_name SET row_flag=1 WHERE id=ID AND row_flag <> 1;

#### 1.7.4.4 Stored Procedures and Triggers

A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don't need to keep re-issuing the entire query but can refer to the stored procedure. This provides better performance because the query has to be parsed only once, and less information needs to be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server.

A trigger is a stored procedure that is invoked when a particular event occurs. For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that automatically deletes the corresponding customer from a customer table when all his transactions are deleted.

The planned update language will be able to handle stored procedures. Our aim is to have stored procedures implemented in MySQL Server around version 5.0. We are also looking at triggers.

#### 1.7.4.5 Foreign Keys

Note that foreign keys in SQL are not used to join tables, but are used mostly for checking referential integrity (foreign key constraints). If you want to get results from multiple tables from a `SELECT` statement, you do this by joining tables:

    SELECT * FROM table1,table2 WHERE table1.id = table2.id;

See section 6.4.1.1 `JOIN` Syntax. See section 3.5.6 Using Foreign Keys.

In MySQL Server 3.23.44 and up, `InnoDB` tables support checking of foreign key constraints. See section 7.5 `InnoDB` Tables. For other table types, MySQL Server does parse the `FOREIGN KEY` syntax in `CREATE TABLE` commands, but without further action being taken.

The `FOREIGN KEY` syntax without `ON DELETE ...` is mostly used for documentation purposes. Some ODBC applications may use this to produce automatic `WHERE` clauses, but this is usually easy to override. `FOREIGN KEY` is sometimes used as a constraint check, but this check is unnecessary in practice if rows are inserted into the tables in the right order.

In MySQL Server, you can work around the problem of `ON DELETE ...` not being implemented by adding the appropriate `DELETE` statement to an application when you delete records from a table that has a foreign key. In practice this is as quick (in some cases quicker) and much more portable than using foreign keys.

In MySQL Server 4.0 you can use multi-table delete to delete rows from many tables with one command. See section 6.4.6 `DELETE` Syntax.

In the near future we will extend the `FOREIGN KEY` implementation so that the information will be saved in the table specification file and may be retrieved by `mysqldump` and ODBC. At a later stage we will implement the foreign key constraints for applications that can't easily be coded to avoid them.

Do keep in mind that foreign keys are often misused, which can cause severe problems. Even when used properly, it is not a magic solution for the referential integrity problem, although it does make things easier in some cases.

Some advantages of foreign key enforcement:

- Assuming proper design of the relations, foreign key constraints will make it more difficult for a programmer to introduce an inconsistency into the database.
- Using cascading updates and deletes can simplify the client code.
- Properly designed foreign key rules aid in documenting relations between tables.

Disadvantages:

- Mistakes, which are easy to make in designing key relations, can cause severe problems@-for example, circular rules, or the wrong combination of cascading deletes.
- A properly written application will make sure internally that it is not violating referential integrity constraints before proceding with a query. Thus, additional checks on the database level will only slow down performance for such an application.
- It is not uncommon for a DBA to make such a complex topology of relations that it becomes very difficult, and in some cases impossible, to back up or restore individual tables.

#### 1.7.4.6 Views

It is planned to implement views in MySQL Server around version 5.0.

Views are mostly useful for letting users access a set of relations as one table (in read-only mode). Many SQL databases don't allow one to update any rows in a view, but you have to do the updates in the separate tables.

As MySQL Server is mostly used in applications and on web systems where the application writer has full control on the database usage, most of our users haven't regarded views to be very important. (At least no one has been interested enough in this to be prepared to finance the implementation of views.)

One doesn't need views in MySQL Server to restrict access to columns, as MySQL Server has a very sophisticated privilege system. See section 4.2 General Security Issues and the MySQL Access Privilege System.

#### 1.7.4.7 ``--'` as the Start of a Comment

Some other SQL databases use ``--'` to start comments. MySQL Server has ``#'` as the start comment character. You can also use the C comment style `/* this is a comment */` with MySQL Server. See section 6.1.6 Comment Syntax.

MySQL Server Version 3.23.3 and above support the ``--'` comment style, provided the comment is followed by a space. This is because this comment style has caused many problems with automatically generated SQL queries that have used something like the following code, where we automatically insert the value of the payment for `!payment!`:

    UPDATE tbl_name SET credit=credit-!payment!

Think about what happens if the value of `payment` is negative. Because `1--1` is legal in SQL, the consequences of allowing comments to start with ``--'` are terrible.

Using our implementation of this method of commenting in MySQL Server Version 3.23.3 and up, `1-- This is a comment` is actually safe.

Another safe feature is that the `mysql` command-line client removes all lines that start with ``--'`.

The following information is relevant only if you are running a MySQL version earlier than 3.23.3:

If you have a SQL program in a text file that contains ``--'` comments you should use:

    shell> replace " --" " #" < text-file-with-funny-comments.sql \
             | mysql database

instead of the usual:

    shell> mysql database < text-file-with-funny-comments.sql

You can also edit the command file ``in place'' to change the ``--'` comments to ``#'` comments:

    shell> replace " --" " #" -- text-file-with-funny-comments.sql

Change them back with this command:

    shell> replace " #" " --" -- text-file-with-funny-comments.sql

### 1.7.5 Known Errors and Design Deficiencies in MySQL

The following problems are known and have a very high priority to get fixed:

- `ANALYZE TABLE` on a BDB table may in some case make the table unusable until one has restarted `mysqld`. When this happens you will see errors like the following in the MySQL error file:

      001207 22:07:56  bdb:  log_flush: LSN past current end-of-log

- Don't execute `ALTER TABLE` on a `BDB` table on which you are running multi-statement transactions until all those transactions complete. (The transaction will probably be ignored.)
- `ANALYZE TABLE`, `OPTIMIZE TABLE`, and `REPAIR TABLE` may cause problems on tables for which you are using `INSERT DELAYED`.
- Doing a `LOCK TABLE ...` and `FLUSH TABLES ...` doesn't guarantee that there isn't a half-finished transaction in progress on the table.
- BDB tables are a bit slow to open. If you have many BDB tables in a database, it will take a long time to use the `mysql` client on the database if you are not using the `-A` option or if you are using `rehash`. This is especially notable when you have a big table cache.

The following problems are known and will be fixed in due time:

- When using `RPAD` function, or any other string function that ends up adding blanks to the right, in a query that has to use temporary table to be resolved, then all resulting strings will be RTRIM'ed. This is an example of the query: `SELECT RPAD(t1.field1, 50, ' ') AS f2, RPAD(t2.field2, 50, ' ') AS f1 FROM table1 as t1 LEFT JOIN table2 AS t2 ON t1.record=t2.joinID ORDER BY t2.record;` Final result of this bug is that use will not be able to get blanks on the right side of the resulting field. The above behaviour exists in all versions of MySQL. The reason for this is due to the fact that HEAP tables, which are used first for temporary tables, are not capable of handling VARCHAR columns. This behaviour will be fixed in one of 4.1 releases.
- When using `SET CHARACTER SET`, one can't use translated characters in database, table, and column names.
- One can't use `_` or `%` with `ESCAPE` in `LIKE ... ESCAPE`.
- If you have a `DECIMAL` column with a number stored in different formats (+01.00, 1.00, 01.00), `GROUP BY` may regard each value as a different value.
- `DELETE FROM merge_table` used without a `WHERE` will only clear the mapping for the table, not delete everything in the mapped tables.
- You cannot build the server in another directory when using MIT-pthreads. Because this requires changes to MIT-pthreads, we are not likely to fix this. See section 2.3.6 MIT-pthreads Notes.
- `BLOB` values can't ``reliably'' be used in `GROUP BY` or `ORDER BY` or `DISTINCT`. Only the first `max_sort_length` bytes (default 1024) are used when comparing `BLOB`s in these cases. This can be changed with the `-O max_sort_length` option to `mysqld`. A workaround for most cases is to use a substring: `SELECT DISTINCT LEFT(blob,2048) FROM tbl_name`.
- Calculation is done with `BIGINT` or `DOUBLE` (both are normally 64 bits long). It depends on the function which precision one gets. The general rule is that bit functions are done with `BIGINT` precision, `IF`, and `ELT()` with `BIGINT` or `DOUBLE` precision and the rest with `DOUBLE` precision. One should try to avoid using unsigned long long values if they resolve to be bigger than 63 bits (9223372036854775807) for anything else than bit fields! MySQL Server 4.0 has better `BIGINT` handling than 3.23.
- All string columns, except `BLOB` and `TEXT` columns, automatically have all trailing spaces removed when retrieved. For `CHAR` types this is okay, and may be regarded as a feature according to ANSI SQL92. The bug is that in MySQL Server, `VARCHAR` columns are treated the same way.
- You can only have up to 255 `ENUM` and `SET` columns in one table.
- In `MIN()`, `MAX()` and other aggregate functions, MySQL currently compares `ENUM` and `SET` columns by their string value rather than by the string's relative position in the set.
- `safe_mysqld` redirects all messages from `mysqld` to the `mysqld` log. One problem with this is that if you execute `mysqladmin refresh` to close and reopen the log, `stdout` and `stderr` are still redirected to the old log. If you use `--log` extensively, you should edit `safe_mysqld` to log to ``'hostname'.err'` instead of ``'hostname'.log'` so you can easily reclaim the space for the old log by deleting the old one and executing `mysqladmin refresh`.
- In the `UPDATE` statement, columns are updated from left to right. If you refer to an updated column, you will get the updated value instead of the original value. For example:

      mysql> UPDATE tbl_name SET KEY=KEY+1,KEY=KEY+1;

  This will update `KEY` with `2` instead of with `1`.
- You can't use temporary tables more than once in the same query. For example, the following doesn't work:

      mysql> SELECT * FROM temporary_table, temporary_table AS t2;

- `RENAME` doesn't work with `TEMPORARY` tables or tables used in a `MERGE` table.
- The optimiser may handle `DISTINCT` differently if you are using 'hidden' columns in a join or not. In a join, hidden columns are counted as part of the result (even if they are not shown) while in normal queries hidden columns don't participate in the `DISTINCT` comparison. We will probably change this in the future to never compare the hidden columns when executing `DISTINCT`. An example of this is:

      SELECT DISTINCT mp3id FROM band_downloads
             WHERE userid = 9 ORDER BY id DESC;

  and

      SELECT DISTINCT band_downloads.mp3id
             FROM band_downloads,band_mp3
             WHERE band_downloads.userid = 9
             AND band_mp3.id = band_downloads.mp3id
             ORDER BY band_downloads.id DESC;

  In the second case you may in MySQL Server 3.23.x get two identical rows in the result set (because the hidden `id` column may differ). Note that this happens only for queries where you don't have the ORDER BY columns in the result, something that you are not allowed to do in ANSI SQL.
- Because MySQL Server allows you to work with table types that don't support transactions, and thus can't `rollback` data, some things behave a little differently in MySQL Server than in other SQL servers. This is just to ensure that MySQL Server never needs to do a rollback for a SQL command. This may be a little awkward at times as column values must be checked in the application, but this will actually give you a nice speed increase as it allows MySQL Server to do some optimisations that otherwise would be very hard to do. If you set a column to an incorrect value, MySQL Server will, instead of doing a rollback, store the `best possible value` in the column:
  - If you try to store a value outside the range in a numerical column, MySQL Server will instead store the smallest or biggest possible value in the column.
  - If you try to store a string that doesn't start with a number into a numerical column, MySQL Server will store 0 into it.
  - If you try to store `NULL` into a column that doesn't take `NULL` values, MySQL Server will store 0 or `''` (empty string) in it instead. (This behaviour can, however, be changed with the -DDONT_USE_DEFAULT_FIELDS compile option.)
  - MySQL allows you to store some wrong date values into `DATE` and `DATETIME` columns (like 2000-02-31 or 2000-02-00). The idea is that it's not the SQL server job to validate date. If MySQL can store a date and retrieve exactly the same date, then MySQL will store the date. If the date is totally wrong (outside the server's ability to store it), then the special date value 0000-00-00 will be stored in the column.
  - If you set an `ENUM` column to an unsupported value, it will be set to the error value `empty string`, with numeric value 0.
  - If you set a `SET` column to an unsupported value, the value will be ignored.
- If you execute a `PROCEDURE` on a query that returns an empty set, in some cases the `PROCEDURE` will not transform the columns.
- Creation of a table of type `MERGE` doesn't check if the underlying tables are of compatible types.
- MySQL Server can't yet handle `NaN`, `-Inf`, and `Inf` values in double. Using these will cause problems when trying to export and import data. We should as an intermediate solution change `NaN` to `NULL` (if possible) and `-Inf` and `Inf` to the minimum respective maximum possible `double` value.
- `LIMIT` on negative numbers are treated as big positive numbers.
- If you use `ALTER TABLE` to first add a `UNIQUE` index to a table used in a `MERGE` table and then use `ALTER TABLE` to add a normal index on the `MERGE` table, the key order will be different for the tables if there was an old key that was not unique in the table. This is because `ALTER TABLE` puts `UNIQUE` keys before normal keys to be able to detect duplicate keys as early as possible.

The following are known bugs in earlier versions of MySQL:

- You can get a hung thread if you do a `DROP TABLE` on a table that is one among many tables that is locked with `LOCK TABLES`.
- In the following case you can get a core dump:
  - Delayed insert handler has pending inserts to a table.
  - `LOCK table` with `WRITE`.
  - `FLUSH TABLES`.
- Before MySQL Server Version 3.23.2 an `UPDATE` that updated a key with a `WHERE` on the same key may have failed because the key was used to search for records and the same row may have been found multiple times:

      UPDATE tbl_name SET KEY=KEY+1 WHERE KEY > 100;

  A workaround is to use:

      mysql> UPDATE tbl_name SET KEY=KEY+1 WHERE KEY+0 > 100;

  This will work because MySQL Server will not use an index on expressions in the `WHERE` clause.
- Before MySQL Server Version 3.23, all numeric types where treated as fixed-point fields. That means you had to specify how many decimals a floating-point field shall have. All results were returned with the correct number of decimals.

For platform-specific bugs, see the sections about compiling and porting.

## 1.8 MySQL and The Future (The TODO)

This section lists the features that we plan to implement in MySQL Server.

Everything in this list is approximately in the order it will be done. If you want to affect the priority order, please register a license or support us and tell us what you want to have done more quickly. See section 1.4 MySQL Support and Licensing.

The plan is that we in the future will support the full ANSI SQL99 standard, but with a lot of useful extensions. The challenge is to do this without sacrificing the speed or compromising the code.

### 1.8.1 Things That Should be in 4.0

All done. We now only do bug fixes for MySQL 4.0. See section D.2 Changes in release 4.0.x (Gamma). Development has shifted to 4.1

### 1.8.2 Things That Should be in 4.1

The following features are planned for inclusion into MySQL 4.1. Note that because we have many developers that are working on different projects, there will also be many additional features. There is also a small chance that some of these features will be added to MySQL 4.0. Some of the work on MySQL 4.1 is already in progress.

- Subqueries.

      SELECT id FROM t WHERE grp IN (SELECT grp FROM g WHERE u > 100);

- New table definition file format (``.frm'` files). This will enable us to not run out of bits when adding more table options. One will still be able to use the old ``.frm'` file format with 4.0. All newly created tables will, however, use the new format. The new file format will enable us to add new column types, more options for keys, and possibly to store and retrieve `FOREIGN KEY` definitions.
- `SHOW COLUMNS FROM table_name` (used by `mysql` client to allow expansions of column names) should not open the table, only the definition file. This will require less memory and be much faster.
- Foreign keys for `MyISAM` tables, including cascading delete.
- Fail-safe replication.
- Replication should work with `RAND()` and user variables `@var`.
- Online backup with very low performance penalty. The online backup will make it easy to add a new replication slave without taking down the master.
- Derived tables:

      SELECT a.col1, b.col2
             FROM (SELECT MAX(col1) AS col1 FROM root_table) a,
             other_table b
             WHERE a.col1=b.col1;

  This could be done by automatically creating temporary tables for the derived tables for the duration of the query. Please, do note, that as per SQL-99, all derived tables have to be aliased. This is enforced in MySQL too ...
- `ROLLUP` and `CUBE` OLAP (Online Analytical Processing) grouping options for data warehousing applications.
- Allow `DELETE` on `MyISAM` tables to use the record cache. To do this, we need to update the threads record cache when we update the ``.MYD'` file.
- When using `SET CHARACTER SET` we should translate the whole query at once and not only strings. This will enable users to use the translated characters in database, table, and column names.
- Add `record_in_range()` method to `MERGE` tables to be able to choose the right index when there are many to choose from. We should also extend the info interface to get the key distribution for each index, if `analyze` is run on all subtables.
- Resolving the issue of `RENAME TABLE` on a table used in an active `MERGE` table possibly corrupting the table.
- A faster, smaller embedded MySQL library (compatible with the old one).
- Stable OpenSSL support (MySQL 4.0 supports rudimentary, not 100% tested, support for OpenSSL).
- Add support for sorting on `UNICODE`.
- Character set casts and syntax for handling multiple character sets.
- Help for all commands from the client.
- New faster client/server protocol which will support prepared statements, bound parameters, and bound result columns, binary transfer of data, warnings...
- Add database and real table name (in case of alias) to the MYSQL_FIELD structure.
- Add options to the client/server protocol to get progress notes for long running commands.
- Implement `RENAME DATABASE`. To make this safe for all storage engines, it should work as follows:
  - Create the new database.
  - For every table do a rename of the table to another database, as we do with the `RENAME` command.
  - Drop the old database.
- Add true `VARCHAR` support (there is already support for this in `MyISAM`).
- Optimise `BIT` type to take 1 bit (now `BIT` takes 1 char).
- New internal file interface change. This will make all file handling much more general and make it easier to add extensions like RAID. (the current implementation is a hack.)
- Better in-memory (`HEAP`) tables:
  - Support for B-tree indexes
  - Dynamic size rows
  - Faster row handling (less copying)

### 1.8.3 Things That Must be Done in the Near Future

- Don't allow more than a defined number of threads to run MyISAM recover at the same time.
- Change `INSERT ... SELECT` to optionally use concurrent inserts.
- Return the original field types() when doing `SELECT MIN(column) ... GROUP BY`.
- Multiple result sets.
- Make it possible to specify `long_query_time` with a granularity in microseconds.
- Link the `myisampack` code into the server.
- Port of the MySQL code to QNX.
- Port of the MySQL code to BeOS.
- Port of the MySQL clients to LynxOS.
- Add a temporary key buffer cache during `INSERT/DELETE/UPDATE` so that we can gracefully recover if the index file gets full.
- If you perform an `ALTER TABLE` on a table that is symlinked to another disk, create temporary tables on this disk.
- Implement a `DATE/DATETIME` type that handles time zone information properly so that dealing with dates in different time zones is easier.
- FreeBSD and MIT-pthreads; do sleeping threads take CPU time?
- Check if locked threads take any CPU time.
- Fix configure so that one can compile all libraries (like `MyISAM`) without threads.
- Add an option to periodically flush key pages for tables with delayed keys if they haven't been used in a while.
- Allow join on key parts (optimisation issue).
- `INSERT SQL_CONCURRENT` and `mysqld --concurrent-insert` to do a concurrent insert at the end of the file if the file is read-locked.
- Server-side cursors.
- Check if `lockd` works with modern Linux kernels; if not, we have to fix `lockd`! To test this, start `mysqld` with `--enable-locking` and run the different fork* test suits. They shouldn't give any errors if `lockd` works.
- Allow SQL variables in `LIMIT`, like in `LIMIT @a,@b`.
- Allow update of variables in `UPDATE` statements. For example: `UPDATE TABLE foo SET @a=a+b,a=@a, b=@a+c`.
- Change when user variables are updated so that one can use them with `GROUP BY`, as in the following example: `SELECT id, @a:=COUNT(*), SUM(sum_col)/@a FROM table_name GROUP BY id`.
- Don't add automatic `DEFAULT` values to columns. Give an error when using an `INSERT` that doesn't contain a column that doesn't have a `DEFAULT`.
- Fix ``libmysql.c'` to allow two `mysql_query()` commands in a row without reading results or give a nice error message when one does this.
- Check why MIT-pthreads `ctime()` doesn't work on some FreeBSD systems.
- Add an `IMAGE` option to `LOAD DATA INFILE` to not update `TIMESTAMP` and `AUTO_INCREMENT` fields.
- Added `LOAD DATE INFILE ... UPDATE` syntax.
  - For tables with primary keys, if the data contains the primary key, entries matching that primary key are updated from the remainder of the columns. However, columns missing from the incoming data feed are not touched.
  - For tables with primary keys that are missing some part of the key in the incoming data stream, or that have no primary key, the feed is treated as a `LOAD DATA INFILE ... REPLACE INTO` now.
- Make `LOAD DATA INFILE` understand syntax like:

      LOAD DATA INFILE 'file_name.txt' INTO TABLE tbl_name
           TEXT_FIELDS (text_field1, text_field2, text_field3)
           SET table_field1=CONCAT(text_field1, text_field2),
               table_field3=23
           IGNORE text_field3

  This can be used to skip over extra columns in the text file, or update columns based on expressions of the read data.
- `LOAD DATA INFILE 'file_name' INTO TABLE 'table_name' ERRORS TO err_table_name`. This would cause any errors and warnings to be logged into the `err_table_name` table. That table would have a structure like:

      line_number    - line number in datafile
      error_message  - the error/warning message
      and maybe
      data_line      - the line from the datafile

- Automatic output from `mysql` to Netscape.
- `LOCK DATABASES` (with various options.)
- Functions: ADD_TO_SET(value,set) and REMOVE_FROM_SET(value,set).
- Add use of `t1 JOIN t2 ON ...` and `t1 JOIN t2 USING ...` Currently, you can only use this syntax with `LEFT JOIN`.
- Many more variables for `show status`. Records reads and updates. Selects on 1 table and selects with joins. Mean number of tables in select. Number of `ORDER BY` and `GROUP BY` queries.
- If you abort `mysql` in the middle of a query, you should open another connection and kill the old running query. Alternatively, an attempt should be made to detect this in the server.
- Add a storage engine interface for table information so that you can use it as a system table. This would be a bit slow if you requested information about all tables, but very flexible. `SHOW INFO FROM tbl_name` for basic table information should be implemented.
- `NATURAL JOIN`.
- Allow `SELECT a FROM crash_me LEFT JOIN crash_me2 USING (a)`; in this case `a` is assumed to come from the `crash_me` table.
- Fix so that `ON` and `USING` works with the `JOIN` join type.
- Oracle-like `CONNECT BY PRIOR ...` to search hierarchy structures.
- `mysqladmin copy database new-database`; requires `COPY` command to be added to `mysqld`.
- Processlist should show number of queries/threads.
- `SHOW HOSTS` for printing information about the hostname cache.
- `DELETE` and `REPLACE` options to the `UPDATE` statement (this will delete rows when one gets a duplicate key error while updating).
- Change the format of `DATETIME` to store fractions of seconds.
- Add all missing ANSI92 and ODBC 3.0 types.
- Change table names from empty strings to `NULL` for calculated columns.
- Don't use `Item_copy_string` on numerical values to avoid number->string->number conversion in case of: `SELECT COUNT(*)*(id+0) FROM table_name GROUP BY id`
- Make it possible to use the new GNU regexp library instead of the current one (the GNU library should be much faster than the old one).
- Change so that `ALTER TABLE` doesn't abort clients that execute `INSERT DELAYED`.
- Fix so that when columns are referenced in an `UPDATE` clause, they contain the old values from before the update started.
- Add simulation of `pread()`/`pwrite()` on Windows to enable concurrent inserts.
- A logfile analyser that could parse out information about which tables are hit most often, how often multi-table joins are executed, etc. It should help users identify areas or table design that could be optimised to execute much more efficient queries.
- Add `SUM(DISTINCT)`.
- Add `ANY()`, `EVERY()`, and `SOME()` group functions. In ANSI SQL these work only on boolean columns, but we can extend these to work on any columns/expressions by applying: value == 0 -> FALSE and value <> 0 -> TRUE.
- Fix that the type for `MAX(column)` is the same as the column type:

      mysql> CREATE TABLE t1 (a DATE);
      mysql> INSERT INTO t1 VALUES (NOW());
      mysql> CREATE TABLE t2 SELECT MAX(a) FROM t1;
      mysql> SHOW COLUMNS FROM t2;

- Come up with a nice syntax for a statement that will `UPDATE` the row if it exists and `INSERT` a new row if the row didn't exist (like `REPLACE` works with `INSERT` / `DELETE`).

### 1.8.4 Things That Have to be Done Sometime

- Implement function: `get_changed_tables(timeout,table1,table2,...)`.
- Change reading through tables to use memmap when possible. Now only compressed tables use memmap.
- Make the automatic timestamp code nicer. Add timestamps to the update log with `SET TIMESTAMP=#;`.
- Use read/write mutex in some places to get more speed.
- Full foreign key support in for `MyISAM` tables, probably after the implementation of stored procedures with triggers.
- Simple views (first on one table, later on any expression).
- Automatically close some tables if a table, temporary table, or temporary files gets error 23 (not enough open files).
- When one finds a field=#, change all occurrences of field to #. Now this is only done for some simple cases.
- Change all const expressions with calculated expressions if possible.
- Optimise key = expression. At the moment only key = field or key = constant are optimised.
- Join some of the copy functions for nicer code.
- Change ``sql_yacc.yy'` to an inline parser to reduce its size and get better error messages (5 days).
- Change the parser to use only one rule per different number of arguments in function.
- Use of full calculation names in the order part (for ACCESS97).
- `MINUS`, `INTERSECT`, and `FULL OUTER JOIN`. (Currently `UNION` [in 4.0] and `LEFT OUTER JOIN` are supported.)
- `SQL_OPTION MAX_SELECT_TIME=#` to put a time limit on a query.
- Make the update log write to a database.
- Add to `LIMIT` to allow retrieval of data from the end of a result set.
- Alarm around client connect/read/write functions.
- Please note the changes to `safe_mysqld`: according to FSSTND (which Debian tries to follow) PID files should go into ``/var/run/<progname>.pid'` and log files into ``/var/log'`. It would be nice if you could put the "DATADIR" in the first declaration of "pidfile" and "log", so the placement of these files can be changed with a single statement.
- Allow a client to request logging.
- Add use of `zlib()` for `gzip`-ed files to `LOAD DATA INFILE`.
- Fix sorting and grouping of `BLOB` columns (partly solved now).
- Stored procedures. Triggers are also being looked at.
- A simple (atomic) update language that can be used to write loops and such in the MySQL server.
- Change to use semaphores when counting threads. One should first implement a semaphore library to MIT-pthreads.
- Don't assign a new `AUTO_INCREMENT` value when one sets a column to 0. Use `NULL` instead.
- Add full support for `JOIN` with parentheses.
- As an alternative for one thread/connection manage a pool of threads to handle the queries.
- Allow one to get more than one lock with `GET_LOCK`. When doing this, one must also handle the possible deadlocks this change will introduce.

Time is given according to amount of work, not real time.

### 1.8.5 Things We Don't Plan To Do

- Nothing; we aim toward full ANSI 92/ANSI 99 compliancy.

## 1.9 How MySQL Compares to Other Databases

Our users have successfully run their own benchmarks against a number of `Open Source` and traditional database servers. We are aware of tests against `Oracle` server, `DB/2` server, `Microsoft SQL Server`, and other commercial products. Due to legal reasons we are restricted from publishing some of those benchmarks in our reference manual.

This section includes a comparison with `mSQL` for historical reasons and with `PostgreSQL` as it is also an `Open Source` database. If you have benchmark results that we can publish, please contact us at benchmarks@mysql.com.

For comparative lists of all supported functions and types as well as measured operational limits of many different database systems, see the `crash-me` web page at http://www.mysql.com/information/crash-me.php.

### 1.9.1 How MySQL Compares to `mSQL`

Performance
  For a true comparison of speed, consult the growing MySQL benchmark suite. See section 5.1.4 The MySQL Benchmark Suite. Because there is no thread creation overhead, a small parser, few features, and simple security, `mSQL` should be quicker at:
  - Tests that perform repeated connects and disconnects, running a very simple query during each connection.
  - `INSERT` operations into very simple tables with few columns and keys.
  - `CREATE TABLE` and `DROP TABLE`.
  - `SELECT` on something that isn't an index. (A table scan is very easy.)
  Because these operations are so simple, it is hard to be better at them when you have a higher startup overhead. After the connection is established, MySQL Server should perform much better. On the other hand, MySQL Server is much faster than `mSQL` (and most other SQL implementations) on the following:
  - Complex `SELECT` operations.
  - Retrieving large results (MySQL Server has a better, faster, and safer protocol).
  - Tables with variable-length strings because MySQL Server has more efficient handling and can have indexes on `VARCHAR` columns.
  - Handling tables with many columns.
  - Handling tables with large record lengths.
  - `SELECT` with many expressions.
  - `SELECT` on large tables.
  - Handling many connections at the same time. MySQL Server is fully multi-threaded. Each connection has its own thread, which means that no thread has to wait for another (unless a thread is modifying a table another thread wants to access). In `mSQL`, once one connection is established, all others must wait until the first has finished, regardless of whether the connection is running a query that is short or long. When the first connection terminates, the next can be served, while all the others wait again, etc.
  - Joins. `mSQL` can become pathologically slow if you change the order of tables in a `SELECT`. In the benchmark suite, a time more than 15,000 times slower than MySQL Server was seen. This is due to `mSQL`'s lack of a join optimiser to order tables in the optimal order. However, if you put the tables in exactly the right order in `mSQL`2 and the `WHERE` is simple and uses index columns, the join will be relatively fast! See section 5.1.4 The MySQL Benchmark Suite.
  - `ORDER BY` and `GROUP BY`.
  - `DISTINCT`.
  - Using `TEXT` or `BLOB` columns.
SQL Features
  - `GROUP BY` and `HAVING`. `mSQL` does not support `GROUP BY` at all. MySQL Server supports a full `GROUP BY` with both `HAVING` and the following functions: `COUNT()`, `AVG()`, `MIN()`, `MAX()`, `SUM()`, and `STD()`. `COUNT(*)` is optimised to return very quickly if the `SELECT` retrieves from one table, no other columns are retrieved, and there is no `WHERE` clause. `MIN()` and `MAX()` may take string arguments.
  - `INSERT` and `UPDATE` with calculations. MySQL Server can do calculations in an `INSERT` or `UPDATE`. For example:

        mysql> UPDATE SET x=x*10+y WHERE x<20;

  - Aliasing. MySQL Server has column aliasing.
  - Qualifying column names. In MySQL Server, if a column name is unique among the tables used in a query, you do not have to use the full qualifier.
  - `SELECT` with functions. MySQL Server has many functions (too many to list here; see section 6.3 Functions for Use in `SELECT` and `WHERE` Clauses).
Disk Space Efficiency
  That is, how small can you make your tables? MySQL Server has very precise types, so you can create tables that take very little space. An example of a useful MySQL datatype is the `MEDIUMINT` that is 3 bytes long. If you have 100 million records, saving even 1 byte per record is very important. `mSQL2` has a more limited set of column types, so it is more difficult to get small tables.
Stability
  This is harder to judge objectively. For a discussion of MySQL Server stability, see section 1.2.3 How Stable Is MySQL?. We have no experience with `mSQL` stability, so we cannot say anything about that.
Price
  Another important issue is the license. MySQL Server has a more flexible license than `mSQL`, and is also less expensive than `mSQL`. Whichever product you choose to use, remember to at least consider paying for a license or e-mail support.
Perl Interfaces
  MySQL Server has basically the same interfaces to Perl as `mSQL` with some added features.
JDBC (Java)
  MySQL Server currently has a lot of different JDBC drivers:
  - MySQL Connector/J is a native Java driver. Version 3.x is released under dual licensing (GPL and commercial).
  - The Resin driver: this is a commercial JDBC driver released under open source. http://www.caucho.com/projects/jdbc-mysql/index.xtp
  - The gwe driver: a Java interface by GWE technologies (not supported anymore).
  - The jms driver: an improved gwe driver by Xiaokun Kelvin ZHU X.Zhu@brad.ac.uk (not supported anymore).
  - The twz driver: a type 4 JDBC driver by Terrence W. Zellers zellert@voicenet.com. This is commercial but is free for private and educational use (not supported anymore).
  The recommended driver is the mm driver. The Resin driver may also be good (at least the benchmarks look good), but we haven't received that much information about this yet. We know that `mSQL` has a JDBC driver, but we have too little experience with it to compare.
Rate of Development
  MySQL Server has a small core team of developers, but we are quite used to coding C and C++ very rapidly. Because threads, functions, `GROUP BY`, and so on are still not implemented in `mSQL`, it has a lot of catching up to do. To get some perspective on this, you can view the `mSQL` ``HISTORY'` file for the last year and compare it with the News section of the MySQL Reference Manual (see section D MySQL Change History). It should be pretty obvious which one has developed most rapidly.
Utility Programs
  Both `mSQL` and MySQL Server have many interesting third-party tools. Because it is very easy to port upward (from `mSQL` to MySQL Server), almost all the interesting applications that are available for `mSQL` are also available for MySQL Server. MySQL Server comes with a simple `msql2mysql` program that fixes differences in spelling between `mSQL` and MySQL Server for the most-used C API functions. For example, it changes instances of `msqlConnect()` to `mysql_connect()`. Converting a client program from `mSQL` to MySQL Server usually requires only minor effort.

#### 1.9.1.1 How to Convert `mSQL` Tools for MySQL

According to our experience, it doesn't take long to convert tools such as `msql-tcl` and `msqljava` that use the `mSQL` C API so that they work with the MySQL C API.

The conversion procedure is:

1. Run the shell script `msql2mysql` on the source. This requires the `replace` program, which is distributed with MySQL Server.
2. Compile.
3. Fix all compiler errors.

Differences between the `mSQL` C API and the MySQL C API are:

- MySQL Server uses a `MYSQL` structure as a connection type (`mSQL` uses an `int`).
- `mysql_connect()` takes a pointer to a `MYSQL` structure as a parameter. It is easy to define one globally or to use `malloc()` to get one. `mysql_connect()` also takes two parameters for specifying the user and password. You may set these to `NULL, NULL` for default use.
- `mysql_error()` takes the `MYSQL` structure as a parameter. Just add the parameter to your old `msql_error()` code if you are porting old code.
- MySQL Server returns an error number and a text error message for all errors. `mSQL` returns only a text error message.
- Some incompatibilities exist as a result of MySQL Server supporting multiple connections to the server from the same process.

#### 1.9.1.2 How `mSQL` and MySQL Client/Server Communications Protocols Differ

There are enough differences that it is impossible (or at least not easy) to support both.

The most significant ways in which the MySQL protocol differs from the `mSQL` protocol are listed here:

- A message buffer may contain many result rows.
- The message buffers are dynamically enlarged if the query or the result is bigger than the current buffer, up to a configurable server and client limit.
- All packets are numbered to catch duplicated or missing packets.
- All column values are sent in ASCII. The lengths of columns and rows are sent in packed binary coding (1, 2, or 3 bytes).
- MySQL can read in the result unbuffered (without having to store the full set in the client).
- If a single read/write takes more than 30 seconds, the server closes the connection.
- If a connection is idle for 8 hours, the server closes the connection.

#### 1.9.1.3 How `mSQL` 2.0 SQL Syntax Differs from MySQL

Column types

`MySQL Server`
  Has the following additional types (among others; see section 6.5.3 `CREATE TABLE` Syntax):
  - `ENUM` type for one of a set of strings.
  - `SET` type for many of a set of strings.
  - `BIGINT` type for 64-bit integers.
``
  MySQL Server also supports the following additional type attributes:
  - `UNSIGNED` option for integer and floating-point columns.
  - `ZEROFILL` option for integer columns.
  - `AUTO_INCREMENT` option for integer columns that are a `PRIMARY KEY`. See section 8.4.3.126 `mysql_insert_id()`.
  - `DEFAULT` value for all columns.
`mSQL2`
  `mSQL` column types correspond to the MySQL types shown in the following table:

  `mSQL` type | Corresponding MySQL type
  `CHAR(len)` | `CHAR(len)`
  `TEXT(len)` | `TEXT(len)`. `len` is the maximal length. And `LIKE` works.
  `INT` | `INT`. With many more options!
  `REAL` | `REAL`. Or `FLOAT`. Both 4- and 8-byte versions are available.
  `UINT` | `INT UNSIGNED`
  `DATE` | `DATE`. Uses ANSI SQL format rather than `mSQL`'s own format.
  `TIME` | `TIME`
  `MONEY` | `DECIMAL(12,2)`. A fixed-point value with two decimals.

Index Creation

`MySQL Server`
  Indexes may be specified at table creation time with the `CREATE TABLE` statement.
`mSQL`
  Indexes must be created after the table has been created, with separate `CREATE INDEX` statements.

To Insert a Unique Identifier into a Table

`MySQL Server`
  Use `AUTO_INCREMENT` as a column type specifier. See section 8.4.3.126 `mysql_insert_id()`.
`mSQL`
  Create a `SEQUENCE` on a table and select the `_seq` column.

To Obtain a Unique Identifier for a Row

`MySQL Server`
  Add a `PRIMARY KEY` or `UNIQUE` key to the table and use this. New in Version 3.23.11: If the `PRIMARY` or `UNIQUE` key consists of only one column and this is of type integer, one can also refer to it as `_rowid`.
`mSQL`
  Use the `_rowid` column. Observe that `_rowid` may change over time depending on many factors.

To Get the Time a Column Was Last Modified

`MySQL Server`
  Add a `TIMESTAMP` column to the table. This column is automatically set to the current date and time for `INSERT` or `UPDATE` statements if you don't give the column a value or if you give it a `NULL` value.
`mSQL`
  Use the `_timestamp` column.

`NULL` Value Comparisons

`MySQL Server`
  MySQL Server follows ANSI SQL, and a comparison with `NULL` is always `NULL`.
`mSQL`
  In `mSQL`, `NULL = NULL` is TRUE. You must change `=NULL` to `IS NULL` and `<>NULL` to `IS NOT NULL` when porting old code from `mSQL` to MySQL Server.

String Comparisons

`MySQL Server`
  Normally, string comparisons are performed in case-independent fashion with the sort order determined by the current character set (ISO-8859-1 Latin1 by default). If you don't like this, declare your columns with the `BINARY` attribute, which causes comparisons to be done according to the ASCII order used on the MySQL server host.
`mSQL`
  All string comparisons are performed in case-sensitive fashion with sorting in ASCII order.

Case-insensitive Searching

`MySQL Server`
  `LIKE` is a case-insensitive or case-sensitive operator, depending on the columns involved. If possible, MySQL uses indexes if the `LIKE` argument doesn't start with a wildcard character.
`mSQL`
  Use `CLIKE`.

Handling of Trailing Spaces

`MySQL Server`
  Strips all spaces at the end of `CHAR` and `VARCHAR` columns. Use a `TEXT` column if this behaviour is not desired.
`mSQL`
  Retains trailing space.

`WHERE` Clauses

`MySQL Server`
  MySQL correctly prioritises everything (`AND` is evaluated before `OR`). To get `mSQL` behaviour in MySQL Server, use parentheses (as shown in an example later in this section).
`mSQL`
  Evaluates everything from left to right. This means that some logical calculations with more than three arguments cannot be expressed in any way. It also means you must change some queries when you upgrade to MySQL Server. You do this easily by adding parentheses. Suppose you have the following `mSQL` query:

      mysql> SELECT * FROM table WHERE a=1 AND b=2 OR a=3 AND b=4;

  To make MySQL Server evaluate this the way that `mSQL` would, you must add parentheses:

      mysql> SELECT * FROM table WHERE (a=1 AND (b=2 OR (a=3 AND (b=4))));

Access Control

`MySQL Server`
  Has tables to store grant (permission) options per user, host, and database. See section 4.2.6 How the Privilege System Works.
`mSQL`
  Has a file ``mSQL.acl'` in which you can grant read/write privileges for users.

### 1.9.2 How MySQL Compares to `PostgreSQL`

When reading the following, please note that both products are continually evolving. We at MySQL AB and the PostgreSQL developers are both working on making our respective databases as good as possible, so we are both a serious alternative to any commercial database.

The following comparison is made by us at MySQL AB. We have tried to be as accurate and fair as possible, but although we know MySQL Server thoroughly, we don't have a full knowledge of all PostgreSQL features, so we may have got some things wrong. We will, however, correct these when they come to our attention.

We would first like to note that PostgreSQL and MySQL Server are both widely used products, but with different design goals, even if we are both striving toward ANSI SQL compliancy. This means that for some applications MySQL Server is more suited, while for others PostgreSQL is more suited. When choosing which database to use, you should first check if the database's feature set satisfies your application. If you need raw speed, MySQL Server is probably your best choice. If you need some of the extra features that only PostgreSQL can offer, you should use `PostgreSQL`.

#### 1.9.2.1 MySQL and PostgreSQL development strategies

When adding things to MySQL Server we take pride to do an optimal, definite solution. The code should be so good that we shouldn't have any need to change it in the foreseeable future. We also do not like to sacrifice speed for features but instead will do our utmost to find a solution that will give maximal throughput. This means that development will take a little longer, but the end result will be well worth this. This kind of development is only possible because all server code are checked by one of a few (currently two) persons before it's included in the MySQL server.

We at MySQL AB believe in frequent releases to be able to push out new features quickly to our users. Because of this we do a new small release about every three weeks, and a major branch every year. All releases are thoroughly tested with our testing tools on a lot of different platforms.

PostgreSQL is based on a kernel with lots of contributors. In this setup it makes sense to prioritise adding a lot of new features, instead of implementing them optimally, because one can always optimise things later if there arises a need for this.

Another big difference between MySQL Server and PostgreSQL is that nearly all of the code in the MySQL server is coded by developers that are employed by MySQL AB and are still working on the server code. The exceptions are the transaction engines and the regexp library.

This is in sharp contrast to the PostgreSQL code, the majority of which is coded by a big group of people with different backgrounds. It was only recently that the PostgreSQL developers announced that their current developer group had finally had time to take a look at all the code in the current PostgreSQL release.

Both of the aforementioned development methods have their own merits and drawbacks. We here at MySQL AB think, of course, that our model is better because our model gives better code consistency, more optimal and reusable code, and in our opinion, fewer bugs. Because we are the authors of the MySQL server code, we are better able to coordinate new features and releases.

#### 1.9.2.2 Featurewise Comparison of MySQL and PostgreSQL

On the `crash-me` page (http://www.mysql.com/information/crash-me.php) you can find a list of those database constructs and limits that one can detect automatically with a program. Note, however, that a lot of the numerical limits may be changed with startup options for their respective databases. This web page is, however, extremely useful when you want to ensure that your applications work with many different databases or when you want to convert your application from one database to another.

MySQL Server offers the following advantages over PostgreSQL:

- `MySQL` Server is generally much faster than PostgreSQL. MySQL 4.0.1 also has a query cache that can boost up the query speed for mostly-read-only sites many times.
- MySQL has a much larger user base than PostgreSQL. Therefore, the code is tested more and has historically proven more stable than PostgreSQL. MySQL Server is used more in production environments than PostgreSQL, mostly thanks to the fact that MySQL AB, formerly TCX DataKonsult AB, has provided top-quality commercial support for MySQL Server from the day it was released, whereas until recently PostgreSQL was unsupported.
- MySQL Server works better on Windows than PostgreSQL does. MySQL Server runs as a native Windows application (a service on NT/2000/XP), while PostgreSQL is run under the `Cygwin` emulation. We have heard that PostgreSQL is not yet that stable on Windows but we haven't been able to verify this ourselves.
- MySQL has more APIs to other languages and is supported by more existing programs than PostgreSQL. See section B Contributed Programs.
- MySQL Server works on 24/7 heavy-duty systems. In most circumstances you never have to run any cleanups on MySQL Server. PostgreSQL doesn't yet support 24/7 systems because you have to run `VACUUM` once in a while to reclaim space from `UPDATE` and `DELETE` commands and to perform statistics analyses that are critical to get good performance with PostgreSQL. `VACUUM` is also needed after adding a lot of new rows to a table. On a busy system with lots of changes, `VACUUM` must be run very frequently, in the worst cases even many times a day. During the `VACUUM` run, which may take hours if the database is big, the database is, from a production standpoint, practically dead. Please note: in PostgreSQL version 7.2, basic vacuuming no longer locks tables, thus allowing normal user access during the vacuum. A new `VACUUM FULL` command does old-style vacuum by locking the table and shrinking the on-disk copy of the table.
- MySQL replication has been thoroughly tested, and is used by sites like:
  - Yahoo Finance (http://finance.yahoo.com/)
  - Mobile.de (http://www.mobile.de/)
  - Slashdot (http://www.slashdot.org/)
- Included in the MySQL distribution are two different testing suites, ``mysql-test-run'` and `crash-me` (http://www.mysql.com/information/crash-me.php), as well as a benchmark suite. The test system is actively updated with code to test each new feature and almost all reproduceable bugs that have come to our attention. We test MySQL Server with these on a lot of platforms before every release. These tests are more sophisticated than anything we have seen from PostgreSQL, and they ensure that the MySQL Server is kept to a high standard.
- There are far more books in print about MySQL Server than about PostgreSQL. O'Reilly, SAMS, Que, and New Riders are all major publishers with books about MySQL. All MySQL features are also documented in the MySQL online manual because when a new feature is implemented, the MySQL developers are required to document it before it's included in the source.
- MySQL Server supports more of the standard ODBC functions than `PostgreSQL`.
- MySQL Server has a much more sophisticated `ALTER TABLE`.
- MySQL Server has support for tables without transactions for applications that need all the speed they can get. The tables may be memory-based, `HEAP` tables or disk based `MyISAM`. See section 7 MySQL Table Types.
- MySQL Server has support for two different storage engines that support transactions, `InnoDB`, and `BerkeleyDB`. Because every transaction engine performs differently under different conditions, this gives the application writer more options to find an optimal solution for his or her setup, if need be per individual table. See section 7 MySQL Table Types.
- `MERGE` tables gives you a unique way to instantly make a view over a set of identical tables and use these as one. This is perfect for systems where you have log files that you order, for example, by month. See section 7.2 `MERGE` Tables.
- The option to compress read-only tables, but still have direct access to the rows in the table, gives you better performance by minimising disk reads. This is very useful when you are archiving things. See section 4.7.4 `myisampack`, The MySQL Compressed Read-only Table Generator.
- MySQL Server has internal support for full-text search. See section 6.8 MySQL Full-text Search.
- You can access many databases from the same connection (depending, of course, on your privileges).
- MySQL Server is coded from the start to be multi-threaded, while PostgreSQL uses processes. Context switching and access to common storage areas is much faster between threads than between separate processes. This gives MySQL Server a big speed advantage in multi-user applications and also makes it easier for MySQL Server to take full advantage of symmetric multiprocessor (SMP) systems.
- MySQL Server has a much more sophisticated privilege system than PostgreSQL. While PostgreSQL only supports `INSERT`, `SELECT`, and `UPDATE/DELETE` grants per user on a database or a table, MySQL Server allows you to define a full set of different privileges on the database, table, and column level. MySQL Server also allows you to specify the privilege on host and user combinations. See section 4.3.1 `GRANT` and `REVOKE` Syntax.
- MySQL Server supports a compressed client/server protocol which improves performance over slow links.
- MySQL Server employs a ``storage engine'' concept, and is the only relational database we know of built around this concept. This allows different low-level table types to be called from the SQL engine, and each table type can be optimised for different performance characteristics.
- All MySQL table types (except `InnoDB`) are implemented as files (one table per file), which makes it really easy to back up, move, delete, and even symlink databases and tables, even when the server is down.
- Tools to repair and optimise `MyISAM` tables (the most common MySQL table type). A repair tool is only needed when a physical corruption of a datafile happens, usually from a hardware failure. It allows a majority of the data to be recovered.
- Upgrading MySQL Server is painless. When you are upgrading MySQL Server, you don't need to dump/restore your data, as you have to do with most PostgreSQL upgrades.

Drawbacks with MySQL Server compared to PostgreSQL:

- The transaction support in MySQL Server is not yet as well tested as PostgreSQL's system.
- Because MySQL Server uses threads, which are not yet flawless on many OSes, one must either use binaries from http://www.mysql.com/downloads/, or carefully follow our instructions in section 2.3 Installing a MySQL Source Distribution to get an optimal binary that works in all cases.
- Table locking, as used by the non-transactional `MyISAM` tables, is in many cases faster than page locks, row locks, or versioning. The drawback, however, is that if one doesn't take into account how table locks work, a single long-running query can block a table for updates for a long time. This can usually be avoided when designing the application. If not, one can always switch the trouble table to use one of the transactional table types. See section 5.3.2 Table Locking Issues.
- With UDF (user-defined functions) one can extend MySQL Server with both normal SQL functions and aggregates, but this is not yet as easy or as flexible as in PostgreSQL. See section 9.2 Adding New Functions to MySQL.
- Updates that run over multiple tables used to be harder to do in MySQL Server. However, this has been fixed in MySQL Server 4.0.2 with multi-table `UPDATE` and in MySQL Server 4.1 with subselects. In MySQL Server 4.0 one can use multi-table deletes to delete from many tables at the same time. See section 6.4.6 `DELETE` Syntax.

PostgreSQL currently offers the following advantages over MySQL Server:

Note that because we know the MySQL road map, we have included in the following table the version when MySQL Server should support this feature. Unfortunately we couldn't do this for previous comparisons, because we don't know the PostgreSQL roadmap.

Feature | MySQL version
Subselects | 4.1
Foreign keys | 5.0 (3.23 with InnoDB)
Views | 5.0
Stored procedures | 5.0
Triggers | 5.0
Unions | 4.0
Full join | 4.1
Constraints | 4.1 or 5.0
Cursors | 4.1 or 5.0
R-trees | 4.1 (for MyISAM tables)
Inherited tables | Not planned
Extensible type system | Not planned

Other reasons someone may consider using PostgreSQL:

- Standard usage in PostgreSQL is closer to ANSI SQL in some cases.
- One can speed up PostgreSQL by coding things as stored procedures.
- For geographical data, R-trees make PostgreSQL better than MySQL Server. (note: MySQL version 4.1 has R-trees for MyISAM tables).
- The PostgreSQL optimiser can do some optimisation that the current MySQL optimiser can't do. Most notable is doing joins when you don't have the proper keys in place and doing a join where you are using different keys combined with OR. The MySQL benchmark suite at http://www.mysql.com/information/benchmarks.html shows you what kind of constructs you should watch out for when using different databases.
- PostgreSQL has a bigger team of developers that contribute to the server.

Drawbacks with PostgreSQL compared to MySQL Server:

- `VACUUM` makes PostgreSQL hard to use in a 24/7 environment.
- Only transactional tables.
- Much slower `INSERT`, `DELETE`, and `UPDATE`.

For a complete list of drawbacks, you should also examine the first table in this section.

#### 1.9.2.3 Benchmarking MySQL and PostgreSQL

The only `Open Source` benchmark that we know of that can be used to benchmark MySQL Server and PostgreSQL (and other databases) is our own. It can be found at http://www.mysql.com/information/benchmarks.html.

We have many times asked the PostgreSQL developers and some PostgreSQL users to help us extend this benchmark to make it the definitive benchmark for databases, but unfortunately we haven't gotten any feedback for this.

We, the MySQL developers, have, because of this, spent a lot of hours to get maximum performance from PostgreSQL for the benchmarks, but because we don't know PostgreSQL intimately, we are sure that there are things that we have missed. We have on the benchmark page documented exactly how we did run the benchmark so that it should be easy for anyone to repeat and verify our results.

The benchmarks are usually run with and without the `--fast` option. When run with `--fast` we are trying to use every trick the server can do to get the code to execute as fast as possible. The idea is that the normal run should show how the server would work in a default setup and the `--fast` run shows how the server would do if the application developer would use extensions in the server to make his application run faster.

When running with PostgreSQL and `--fast` we do a `VACUUM` after every major table `UPDATE` and `DROP TABLE` to make the database in perfect shape for the following `SELECT`s. The time for `VACUUM` is measured separately.

When running with PostgreSQL 7.1.1 we could, however, not run with `--fast` because during the `INSERT` test, the postmaster (the PostgreSQL deamon) died and the database was so corrupted that it was impossible to restart postmaster. After this happened twice, we decided to postpone the `--fast` test until the next PostgreSQL release. The details about the machine we run the benchmark on can be found on the benchmark page.

Before going to the other benchmarks we know of, we would like to give some background on benchmarks.

It's very easy to write a test that shows any database to be the best database in the world, by just restricting the test to something the database is very good at and not testing anything that the database is not good at. If one, after doing this, summarises the result as a single figure, things are even easier.

This would be like us measuring the speed of MySQL Server compared to PostgreSQL by looking at the summary time of the MySQL benchmarks on our web page. Based on this MySQL Server would be more than 40 times faster than PostgreSQL, something that is, of course, not true. We could make things even worse by just taking the test where PostgreSQL performs worst and claim that MySQL Server is more than 2000 times faster than PostgreSQL.

The case is that MySQL does a lot of optimisations that PostgreSQL doesn't do. This is, of course, also true the other way around. An SQL optimiser is a very complex thing, and a company could spend years just making the optimiser faster and faster.

When looking at the benchmark results you should look for things that you do in your application and just use these results to decide which database would be best suited for your application. The benchmark results also show things a particular database is not good at and should give you a notion about things to avoid and what you may have to do in other ways.

We know of two benchmark tests that claim that PostgreSQL performs better than MySQL Server. These both where multi-user tests, a test that we here at MySQL AB haven't had time to write and include in the benchmark suite, mainly because it's a big task to do this in a manner that is fair to all databases.

One is the benchmark paid for by Great Bridge, the company that for 16 months attempted to build a business based on PostgreSQL but now has ceased operations. This is probably the worst benchmark we have ever seen anyone conduct. This was not only tuned to only test what PostgreSQL is absolutely best at, but it was also totally unfair to every other database involved in the test.

Note: We know that even some of the main PostgreSQL developers did not like the way Great Bridge conducted the benchmark, so we don't blame the PostgreSQL team for the way the benchmark was done.

This benchmark has been condemned in a lot of postings and newsgroups, so here we will just briefly repeat some things that were wrong with it.

- The tests were run with an expensive commercial tool that makes it impossible for an `Open Source` company like us to verify the benchmarks, or even check how the benchmarks were really done. The tool is not even a true benchmark tool, but an application/setup testing tool. To refer to this as a ``standard'' benchmark tool is to stretch the truth a long way.
- Great Bridge admitted that they had optimised the PostgreSQL database (with `VACUUM` before the test) and tuned the startup for the tests, something they hadn't done for any of the other databases involved. They say ``This process optimises indexes and frees up disk space a bit. The optimised indexes boost performance by some margin.'' Our benchmarks clearly indicate that the difference in running a lot of selects on a database with and without `VACUUM` can easily differ by a factor of 10.
- The test results were also strange. The AS3AP test documentation mentions that the test does ``selections, simple joins, projections, aggregates, one-tuple updates, and bulk updates.'' PostgreSQL is good at doing `SELECT`s and `JOIN`s (especially after a `VACUUM`), but doesn't perform as well on `INSERT`s or `UPDATE`s. The benchmarks seem to indicate that only `SELECT`s were done (or very few updates). This could easily explain the good results for PostgreSQL in this test. The bad results for MySQL will be obvious a bit down in this document.
- They did run the so-called benchmark from a Windows machine against a Linux machine over ODBC, a setup that no normal database user would ever do when running a heavy multi-user application. This tested more the ODBC driver and the Windows protocol used between the clients than the database itself.
- When running the database against Oracle and MS-SQL (Great Bridge has indirectly indicated the databases they used in the test), they didn't use the native protocol but instead ODBC. Anyone that has ever used Oracle knows that all real applications use the native interface instead of ODBC. Doing a test through ODBC and claiming that the results had anything to do with using the database in a real-world situation can't be regarded as fair. They should have done two tests with and without ODBC to provide the right facts (after having gotten experts to tune all involved databases, of course).
- They refer to the TPC-C tests, but they don't mention anywhere that the test they did was not a true TPC-C test and they were not even allowed to call it a TPC-C test. A TPC-C test can only be conducted by the rules approved by the TPC Council (http://www.tpc.org/). Great Bridge didn't do that. By doing this they have both violated the TPC trademark and miscredited their own benchmarks. The rules set by the TPC Council are very strict to ensure that no one can produce false results or make unprovable statements. Apparently Great Bridge wasn't interested in doing this.
- After the first test, we contacted Great Bridge and mentioned to them some of the obvious mistakes they had done with MySQL Server:
  - Running with a debug version of our ODBC driver
  - Running on a Linux system that wasn't optimised for threads
  - Using an old MySQL version when there was a recommended newer one available
  - Not starting MySQL Server with the right options for heavy multi-user use (the default installation of MySQL Server is tuned for minimal resource use)
  Great Bridge did run a new test, with our optimised ODBC driver and with better startup options for MySQL Server, but refused to either use our updated glibc library or our standard binary (used by 80% of our users), which was statically linked with a fixed glibc library. According to what we know, Great Bridge did nothing to ensure that the other databases were set up correctly to run well in their test environment. We are sure, however, that they didn't contact Oracle or Microsoft to ask for their advice in this matter. ;)
- The benchmark was paid for by Great Bridge, and they decided to publish only partial, chosen results (instead of publishing it all).

Tim Perdue, a long-time PostgreSQL fan and a reluctant MySQL user, published a comparison on PHPbuilder (http://www.phpbuilder.com/columns/tim20001112.php3).

When we became aware of the comparison, we phoned Tim Perdue about this because there were a lot of strange things in his results. For example, he claimed that MySQL Server had a problem with five users in his tests, when we know that there are users with similar machines as his that are using MySQL Server with 2000 simultaneous connections doing 400 queries per second. (In this case the limit was the web bandwidth, not the database.)

It sounded like he was using a Linux kernel that either had some problems with many threads, such as kernels before 2.4, which had a problem with many threads on multi-CPU machines. We have documented in this manual how to fix this and Tim should be aware of this problem.

The other possible problem could have been an old glibc library and that Tim didn't use a MySQL binary from our site, which is linked with a corrected glibc library, but had compiled a version of his own. In any of these cases, the symptom would have been exactly what Tim had measured.

We asked Tim if we could get access to his data so that we could repeat the benchmark and if he could check the MySQL version on the machine to find out what was wrong and he promised to come back to us about this. He has not done that yet.

Because of this we can't put any trust in this benchmark either. :(

Over time things also change and the preceding benchmarks are not that relevant anymore. MySQL Server now has a couple of different storage engines with different speed/concurrency tradeoffs. See section 7 MySQL Table Types. It would be interesting to see how the above tests would run with the different transactional table types in MySQL Server. PostgreSQL has, of course, also got new features since the test was made. As these tests are not publicly available there is no way for us to know how the database would perform in the same tests today.

Conclusion:

The only benchmarks that exist today that anyone can download and run against MySQL Server and PostgreSQL are the MySQL benchmarks. We here at MySQL AB believe that `Open Source` databases should be tested with `Open Source` tools! This is the only way to ensure that no one does tests that nobody can reproduce and use this to claim that one database is better than another. Without knowing all the facts it's impossible to answer the claims of the tester.

The thing we find strange is that every test we have seen about PostgreSQL, that is impossible to reproduce, claims that PostgreSQL is better in most cases while our tests, which anyone can reproduce, clearly show otherwise. With this we don't want to say that PostgreSQL isn't good at many things (it is!) or that it isn't faster than MySQL Server under certain conditions. We would just like to see a fair test where PostgreSQL performs very well, so that we could get some friendly competition going!

For more information about our benchmark suite, see section 5.1.4 The MySQL Benchmark Suite.

We are working on an even better benchmark suite, including multi-user tests, and a better documentation of what the individual tests really do and how to add more tests to the suite.

# 2 MySQL Installation

This chapter describes how to obtain and install MySQL:

- For a list of sites from which you can obtain MySQL, see section 2.2.1 How to Get MySQL.
- To see which platforms are supported, see section 2.2.2 Operating Systems Supported by MySQL. Please note that not all supported systems are equally good for running MySQL on them. On some it is much more robust and efficient than others@-see section 2.2.2 Operating Systems Supported by MySQL for details.
- Several versions of MySQL are available in both binary and source distributions. We also provide public access to our current source tree for those who want to see our most recent developments and help us test new code. To determine which version and type of distribution you should use, see section 2.2.3 Which MySQL Version to Use. When in doubt, use the binary distribution.
- Installation instructions for binary and source distributions are described in section 2.2.7 Installing a MySQL Binary Distribution, and section 2.3 Installing a MySQL Source Distribution. Each set of instructions includes a section on system-specific problems you may run into.
- For post-installation procedures, see section 2.4 Post-installation Setup and Testing. These procedures apply whether you install MySQL using a binary or source distribution.

## 2.1 Quick Standard Installation of MySQL

### 2.1.1 Installing MySQL on Linux

The recommended way to install MySQL on Linux is by using an RPM file. The MySQL RPMs are currently being built on a SuSE Linux 7.3 system but should work on most versions of Linux that support `rpm` and use `glibc`.

If you have problems with an RPM file, for example, if you receive the error ```Sorry, the host 'xxxx' could not be looked up`''@-see section 2.6.1.1 Linux Notes for Binary Distributions.

The RPM files you may want to use are:

- `MySQL-VERSION.i386.rpm` The MySQL server. You will need this unless you only want to connect to a MySQL server running on another machine.
- `MySQL-client-VERSION.i386.rpm` The standard MySQL client programs. You probably always want to install this package.
- `MySQL-bench-VERSION.i386.rpm` Tests and benchmarks. Requires Perl and msql-mysql-modules RPMs.
- `MySQL-devel-VERSION.i386.rpm` Libraries and include files needed if you want to compile other MySQL clients, such as the Perl modules.
- `MySQL-embedded-VERSION.i386.rpm` The embedded MySQL server library. (MySQL 4.x only)
- `MySQL-VERSION.src.rpm` This contains the source code for all of the previous packages. It can also be used to rebuild the RPMs on other architectures (for example, Alpha or SPARC).

To see all files in an RPM package, run:

    shell> rpm -qpl MySQL-VERSION.i386.rpm

To perform a standard minimal installation, run:

    shell> rpm -i MySQL-VERSION.i386.rpm MySQL-client-VERSION.i386.rpm

To install just the client package, run:

    shell> rpm -i MySQL-client-VERSION.i386.rpm

The RPM places data in ``/var/lib/mysql'`. The RPM also creates the appropriate entries in ``/etc/init.d/'` to start the server automatically at boot time. (This means that if you have performed a previous installation, you may want to make a copy of your previously installed MySQL startup file if you made any changes to it, so you don't lose your changes.)

If you want to install the MySQL RPM on older Linux distributions that do not support init scripts in ``/etc/init.d'` (directly or via a symlink), you should create a symbolic link pointing to the old location before installing the RPM:

    shell> cd /etc ; ln -s rc.d/init.d .

However, all current major Linux distributions should already support this new directory layout as it is required for LSB (Linux Standard Base) compliance.

After installing the RPM file(s), the `mysqld` daemon should be up and running and you should now be able to start using MySQL. See section 2.4 Post-installation Setup and Testing.

If something goes wrong, you can find more information in the binary installation chapter. See section 2.2.7 Installing a MySQL Binary Distribution.

### 2.1.2 Installing MySQL on Windows

The MySQL server for Windows is available in two distribution types:

1. The binary distribution contains a setup program which installs everything you need so that you can start the server immediately.
2. The source distribution contains all the code and support files for building the executables using the VC++ 6.0 compiler. See section 2.3.7 Windows Source Distribution.

Generally speaking, you should use the binary distribution.

You will need the following:

- A 32-bit Windows Operating System such as 9x, Me, NT, 2000, or XP. The NT family (NT, Windows 2000 and XP) permits running the MySQL server as a service. See section 2.6.2.2 Starting MySQL on Windows NT, 2000 or XP. If you want to use tables bigger than 4G, you should install MySQL on an NTFS or newer filesystem. Don't forget to use `MAX_ROWS` and `AVG_ROW_LENGTH` when you create the table. See section 6.5.3 `CREATE TABLE` Syntax.
- TCP/IP protocol support.
- A copy of the MySQL binary or distribution for Windows, which can be downloaded from http://www.mysql.com/downloads/. Note: The distribution files are supplied with a zipped format and we recommend the use of an adequate FTP client with resume feature to avoid corruption of files during the download process.
- A `ZIP` program to unpack the distribution file.
- Enough space on the hard drive to unpack, install, and create the databases in accorandance with your requirements.
- If you plan to connect to the MySQL server via `ODBC`, you will also need the `MyODBC` driver. See section 8.3 MySQL ODBC Support.

#### 2.1.2.1 Installing the Binaries

1. If you are working on an NT/2000/XP server, logon as a user with with administrator privileges.
2. If you are doing an upgrade of an earlier MySQL installation, it is necessary to stop the server. If you are running the server as a service, use:

       C:\> NET STOP MySQL

   Otherwise, use:

       C:\mysql\bin> mysqladmin -u root shutdown

3. On NT/2000/XP machines, if you want to change the server executable (e.g., -max or -nt), it is also necessary to remove the service:

       C:\mysql\bin> mysqld-max-nt --remove

4. Unzip the distribution file to a temporary directory.
5. Run the ``setup.exe'` file to begin the installation process. If you want to install into another directory than the default ``c:\mysql'`, use the `Browse` button to specify your preferred directory.
6. Finish the install process.

#### 2.1.2.2 Preparing the Windows MySQL Environment

Starting with MySQL 3.23.38, the Windows distribution includes both the normal and the MySQL-Max server binaries. Here is a list of the different MySQL servers you can use:

Binary | Description
`mysqld` | Compiled with full debugging and automatic memory allocation checking, symbolic links, InnoDB, and BDB tables.
`mysqld-opt` | Optimised binary with no support for transactional tables.
`mysqld-nt` | Optimised binary for NT/2000/XP with support for named pipes. You can run this version on Windows 9x/Me, but in this case no named pipes are created and you must have TCP/IP installed.
`mysqld-max` | Optimised binary with support for symbolic links, InnoDB and BDB tables.
`mysqld-max-nt` | Like `mysqld-max`, but compiled with support for named pipes.

Starting from 3.23.50, named pipes are only enabled if one starts mysqld with `--enable-named-pipe`.

All of the preceding binaries are optimised for the Pentium Pro processor but should work on any Intel processor >= i386.

You will need to use an option file to specify your MySQL configuration under the following circumstances:

- The installation or data directories are different from the default locations (``c:\mysql'` and ``c:\mysql\data'`).
- You want to use one of these servers:
  - mysqld.exe
  - mysqld-max.exe
  - mysqld-max-nt.exe
- You need to tune the server settings.

Normally you can use the `WinMySQLAdmin` tool to edit the option file `my.ini`. In this case you don't have to worry about the following section.

There are two option files with the same function: ``my.cnf'` and ``my.ini'`. However, to avoid confusion, it's best if you use only of one them. Both files are plain text. The ``my.cnf'` file, if used, should be created in the root directory of the C drive. The ``my.ini'` file, if used, should be created in the Windows system directory. (This directory is typically something like ``C:\WINDOWS'` or ``C:\WINNT'`. You can determine its exact location from the value of the `windir` environment variable.) MySQL looks first for the `my.ini` file, then for the ``my.cnf'` file.

If your PC uses a boot loader where the C drive isn't the boot drive, your only option is to use the ``my.ini'` file. Also note that if you use the `WinMySQLAdmin` tool, it uses only the ``my.ini'` file. The ``\mysql\bin'` directory contains a help file with instructions for using this tool.

Using `notepad.exe`, create the option file and edit the `[mysqld]` section to specify values for the `basedir` and `datadir` parameters:

    [mysqld]
    # set basedir to installation path, e.g., c:/mysql
    basedir=the_install_path
    # set datadir to location of data directory,
    # e.g., c:/mysql/data or d:/mydata/data
    datadir=the_data_path

Note that Windows pathnames should be specified in option files using forward slashes rather than backslashes. If you do use backslashes, you must double them.

If you would like to use a data directory different from the default of ``c:\mysql\data'`, you must copy the entire contents of the ``c:\mysql\data'` directory to the new location.

If you want to use the `InnoDB` transactional tables, you need to manually create two new directories to hold the InnoDB data and log files@-e.g., ``c:\ibdata'` and ``c:\iblogs'`. You will also need to add some extra lines to the option file. See section 7.5.2 InnoDB Startup Options.

If you don't want to use `InnoDB` tables, add the `skip-innodb` option to the option file.

Now you are ready to test starting the server.

#### 2.1.2.3 Starting the Server for the First Time

Testing from a DOS command prompt is the best thing to do because the server displays status messages that appear in the DOS window. If something is wrong with your configuration, these messages will make it easier for you to identify and fix any problems.

Make sure you are in the directory where the server is located, then enter this command:

    C:\mysql\bin> mysqld-max --standalone

You should see the following messages as the server starts up:

    InnoDB: The first specified datafile c:\ibdata\ibdata1 did not exist:
    InnoDB: a new database to be created!
    InnoDB: Setting file c:\ibdata\ibdata1 size to 209715200
    InnoDB: Database physically writes the file full: wait...
    InnoDB: Log file c:\iblogs\ib_logfile0 did not exist: new to be created
    InnoDB: Setting log file c:\iblogs\ib_logfile0 size to 31457280
    InnoDB: Log file c:\iblogs\ib_logfile1 did not exist: new to be created
    InnoDB: Setting log file c:\iblogs\ib_logfile1 size to 31457280
    InnoDB: Log file c:\iblogs\ib_logfile2 did not exist: new to be created
    InnoDB: Setting log file c:\iblogs\ib_logfile2 size to 31457280
    InnoDB: Doublewrite buffer not found: creating new
    InnoDB: Doublewrite buffer created
    InnoDB: creating foreign key constraint system tables
    InnoDB: foreign key constraint system tables created
    011024 10:58:25  InnoDB: Started

For further information about running MySQL on Windows, see section 2.6.2 Windows Notes.

## 2.2 General Installation Issues

### 2.2.1 How to Get MySQL

Check the MySQL homepage (http://www.mysql.com/) for information about the current version and for downloading instructions.

Our main mirror is located at http://mirrors.sunsite.dk/mysql/.

For a complete upto-date list of MySQL web/download mirrors, see http://www.mysql.com/downloads/mirrors.html. There you will also find information about becoming a MySQL mirror site and how to report a bad or out-of-date mirror.

### 2.2.2 Operating Systems Supported by MySQL

We use GNU Autoconf, so it is possible to port MySQL to all modern systems with working Posix threads and a C++ compiler. (To compile only the client code, a C++ compiler is required but not threads.) We use and develop the software ourselves primarily on Sun Solaris (Versions 2.5 - 2.7) and SuSE Linux Version 7.x.

Note that for many operating systems, the native thread support works only in the latest versions. MySQL has been reported to compile successfully on the following operating system/thread package combinations:

- AIX 4.x, 5.x with native threads. See section 2.6.6.4 IBM-AIX notes.
- Amiga.
- BSDI 2.x with the MIT-pthreads package. See section 2.6.4.5 BSD/OS Version 2.x Notes.
- BSDI 3.0, 3.1 and 4.x with native threads. See section 2.6.4.5 BSD/OS Version 2.x Notes.
- DEC Unix 4.x with native threads. See section 2.6.6.6 Alpha-DEC-UNIX Notes (Tru64).
- FreeBSD 2.x with the MIT-pthreads package. See section 2.6.4.1 FreeBSD Notes.
- FreeBSD 3.x and 4.x with native threads. See section 2.6.4.1 FreeBSD Notes.
- HP-UX 10.20 with the DCE threads or the MIT-pthreads package. See section 2.6.6.2 HP-UX Version 10.20 Notes.
- HP-UX 11.x with the native threads. See section 2.6.6.3 HP-UX Version 11.x Notes.
- Linux 2.0+ with LinuxThreads 0.7.1+ or `glibc` 2.0.7+. See section 2.6.1 Linux Notes (All Linux Versions).
- Mac OS X. See section 2.6.5 Mac OS X Notes.
- NetBSD 1.3/1.4 Intel and NetBSD 1.3 Alpha (Requires GNU make). See section 2.6.4.2 NetBSD notes.
- OpenBSD > 2.5 with native threads. OpenBSD < 2.5 with the MIT-pthreads package. See section 2.6.4.3 OpenBSD 2.5 Notes.
- OS/2 Warp 3, FixPack 29 and OS/2 Warp 4, FixPack 4. See section 2.6.7 OS/2 Notes.
- SGI Irix 6.x with native threads. See section 2.6.6.8 SGI Irix Notes.
- Solaris 2.5 and above with native threads on SPARC and x86. See section 2.6.3 Solaris Notes.
- SunOS 4.x with the MIT-pthreads package. See section 2.6.3 Solaris Notes.
- Caldera (SCO) OpenServer with a recent port of the FSU Pthreads package. See section 2.6.6.9 Caldera (SCO) Notes.
- Caldera (SCO) UnixWare 7.0.1. See section 2.6.6.10 Caldera (SCO) Unixware Version 7.0 Notes.
- Tru64 Unix
- Windows 9x, Me, NT, 2000 and XP. See section 2.6.2 Windows Notes.

Note that not all platforms are suited equally well for running MySQL. How well a certain platform is suited for a high-load mission-critical MySQL server is determined by the following factors:

- General stability of the thread library. A platform may have excellent reputation otherwise, but if the thread library is unstable in the code that is called by MySQL, even if everything else is perfect, MySQL will be only as stable as the thread library.
- The ability of the kernel and/or thread library to take advantage of SMP on multi-processor systems. In other words, when a process creates a thread, it should be possible for that thread to run on a different CPU than the original process.
- The ability of the kernel and/or the thread library to run many threads which acquire/release a mutex over a short critical region frequently without excessive context switches. In other words, if the implementation of `pthread_mutex_lock()` is too anxious to yield CPU time, this will hurt MySQL tremendously. If this issue is not taken care of, adding extra CPUs will actually make MySQL slower.
- General filesystem stability/performance.
- Ability of the filesystem to deal with large files at all and deal with them efficiently, if your tables are big.
- Our level of expertise here at MySQL AB with the platform. If we know a platform well, we introduce platform-specific optimisations/fixes enabled at compile time. We can also provide advice on configuring your system optimally for MySQL.
- The amount of testing of similar configurations we have done internally.
- The number of users that have successfully run MySQL on that platform in similar configurations. If this number is high, the chances of hitting some platform-specific surprises are much smaller.

Based on the preceding criteria, the best platforms for running MySQL at this point are x86 with SuSE Linux 7.1, 2.4 kernel, and ReiserFS (or any similar Linux distribution) and SPARC with Solaris 2.7 or 2.8. FreeBSD comes third, but we really hope it will join the top club once the thread library is improved. We also hope that at some point we will be able to include all other platforms on which MySQL compiles, runs okay, but not quite with the same level of stability and performance, into the top category. This will require some effort on our part in cooperation with the developers of the OS/library components MySQL depends upon. If you are interested in making one of those components better, are in a position to influence their development, and need more detailed instructions on what MySQL needs to run better, send an e-mail to internals@lists.mysql.com.

Please note that the preceding comparison is not to say that one OS is better or worse than the other in general. We are talking about choosing a particular OS for a dedicated purpose@-running MySQL, and compare platforms in that regard only. With this in mind, the result of this comparison would be different if we included more issues into it. And in some cases, the reason one OS is better than the other could simply be that we have put forth more effort into testing on and optimising for that particular platform. We are just stating our observations to help you decide on which platform to use MySQL on in your setup.

### 2.2.3 Which MySQL Version to Use

The first decision to make is whether you want to use the latest development release or the last stable release:

- Normally, if you are beginning to use MySQL for the first time or trying to port it to some system for which there is no binary distribution, we recommend going with the stable release (currently version 3.23). Note that all MySQL releases are checked with the MySQL benchmarks and an extensive test suite before each release (even the development releases).
- Otherwise, if you are running an old system and want to upgrade, but don't want to take chances with a non-seamless upgrade, you should upgrade to the latest in the same branch you are using (where only the last version number is newer than yours). We have tried to fix only fatal bugs and make small, relatively safe changes to that version.

The second decision to make is whether you want to use a source distribution or a binary distribution. In most cases you should probably use a binary distribution, if one exists for your platform, as this generally will be easier to install than a source distribution.

In the following cases you probably will be better off with a source installation:

- If you want to install MySQL at some explicit location. (The standard binary distributions are ``ready to run'' at any place, but you may want to get even more flexibility).
- To be able to satisfy different user requirements, we are providing two different binary versions: one compiled with the non-transactional storage engines (a small, fast binary), and one configured with the most important extended options like transaction-safe tables. Both versions are compiled from the same source distribution. All native `MySQL` clients can connect to both MySQL versions. The extended MySQL binary distribution is marked with the `-max` suffix and is configured with the same options as `mysqld-max`. See section 4.7.5 `mysqld-max`, An Extended `mysqld` Server. If you want to use the MySQL-Max RPM, you must first install the standard MySQL RPM.
- If you want to configure `mysqld` with some extra features that are not in the standard binary distributions. Here is a list of the most common extra options that you may want to use:
  - `--with-innodb`
  - `--with-berkeley-db`
  - `--with-raid`
  - `--with-libwrap`
  - `--with-named-z-lib (This is done for some of the binaries)`
  - `--with-debug[=full]`
- The default binary distribution is normally compiled with support for all character sets and should work on a variety of processors from the same processor family. If you want a faster MySQL server you may want to recompile it with support for only the character sets you need, use a better compiler (like `pgcc`), or use compiler options that are better optimised for your processor.
- If you have found a bug and reported it to the MySQL development team you will probably receive a patch that you need to apply to the source distribution to get the bug fixed.
- If you want to read (and/or modify) the C and C++ code that makes up MySQL, you should get a source distribution. The source code is always the ultimate manual. Source distributions also contain more tests and examples than binary distributions.

The MySQL naming scheme uses release numbers that consist of three numbers and a suffix. For example, a release name like `mysql-3.21.17-beta` is interpreted like this:

- The first number (`3`) describes the file format. All Version 3 releases have the same file format.
- The second number (`21`) is the release level. Normally there are two to choose from. One is the release/stable branch (currently `23`) and the other is the development branch (currently `4.0`). Normally both are stable, but the development version may have quirks, may be missing documentation on new features, or may fail to compile on some systems.
- The third number (`17`) is the version number within the release level. This is incremented for each new distribution. Usually you want the latest version for the release level you have chosen.
- The suffix (`beta`) indicates the stability level of the release. The possible suffixes are:
  - `alpha` indicates that the release contains some large section of new code that hasn't been 100% tested. Known bugs (usually there are none) should be documented in the News section. See section D MySQL Change History. There are also new commands and extensions in most alpha releases. Active development that may involve major code changes can occur on an alpha release, but everything will be tested before doing a release. There should be no known bugs in any MySQL release.
  - `beta` means that all new code has been tested. No major new features that could cause corruption on old code are added. There should be no known bugs. A version changes from alpha to beta when there haven't been any reported fatal bugs within an alpha version for at least a month and we don't plan to add any features that could make any old command more unreliable.
  - `gamma` is a beta that has been around a while and seems to work fine. Only minor fixes are added. This is what many other companies call a release.
  - If there is no suffix, it means that the version has been run for a while at many different sites with no reports of bugs other than platform-specific bugs. Only critical bug fixes are applied to the release. This is what we call a stable release.

All versions of MySQL are run through our standard tests and benchmarks to ensure that they are relatively safe to use. Because the standard tests are extended over time to check for all previously found bugs, the test suite keeps getting better.

Note that all releases have been tested at least with:

An internal test suite
  This is part of a production system for a customer. It has many tables with hundreds of megabytes of data.
The MySQL benchmark suite
  This runs a range of common queries. It is also a test to see whether the latest batch of optimisations actually made the code faster. See section 5.1.4 The MySQL Benchmark Suite.
The `crash-me` test
  This tries to determine what features the database supports and what its capabilities and limitations are. See section 5.1.4 The MySQL Benchmark Suite.

Another test is that we use the newest MySQL version in our internal production environment, on at least one machine. We have more than 100 gigabytes of data to work with.

### 2.2.4 Installation Layouts

This section describes the default layout of the directories created by installing binary and source distributions.

A binary distribution is installed by unpacking it at the installation location you choose (typically ``/usr/local/mysql'`) and creates the following directories in that location:

Directory | Contents of directory
``bin'` | Client programs and the `mysqld` server
``data'` | Log files, databases
``include'` | Include (header) files
``lib'` | Libraries
``scripts'` | `mysql_install_db`
``share/mysql'` | Error message files
``sql-bench'` | Benchmarks

A source distribution is installed after you configure and compile it. By default, the installation step installs files under ``/usr/local'`, in the following subdirectories:

Directory | Contents of directory
``bin'` | Client programs and scripts
``include/mysql'` | Include (header) files
``info'` | Documentation in Info format
``lib/mysql'` | Libraries
``libexec'` | The `mysqld` server
``share/mysql'` | Error message files
``sql-bench'` | Benchmarks and `crash-me` test
``var'` | Databases and log files

Within an installation directory, the layout of a source installation differs from that of a binary installation in the following ways:

- The `mysqld` server is installed in the ``libexec'` directory rather than in the ``bin'` directory.
- The data directory is ``var'` rather than ``data'`.
- `mysql_install_db` is installed in the ``/usr/local/bin'` directory rather than in ``/usr/local/mysql/scripts'`.
- The header file and library directories are ``include/mysql'` and ``lib/mysql'` rather than ``include'` and ``lib'`.

You can create your own binary installation from a compiled source distribution by executing the script ``scripts/make_binary_distribution'`.

### 2.2.5 How and When Updates Are Released

MySQL is evolving quite rapidly here at MySQL AB and we want to share this with other MySQL users. We try to make a release when we have very useful features that others seem to have a need for.

We also try to help out users who request features that are easy to implement. We take note of what our licensed users want to have, and we especially take note of what our extended e-mail supported customers want and try to help them out.

No one has to download a new release. The News section will tell you if the new release has something you really want. See section D MySQL Change History.

We use the following policy when updating MySQL:

- For each minor update, the last number in the version string is incremented. When there are major new features or minor incompatibilities with previous versions, the second number in the version string is incremented. When the file format changes, the first number is increased.
- Stable-tested releases are meant to appear about 1-2 times a year, but if small bugs are found, a release with only bug fixes will be released.
- Working releases/bug fixes to old releases are meant to appear about every 1-8 weeks.
- Binary distributions for some platforms will be made by us for major releases. Other people may make binary distributions for other systems but probably less frequently.
- We usually make patches available as soon as we have located and fixed small bugs. They are posted to bugs@lists.mysql.com and will be added to the next release.
- For non-critical but annoying bugs, we will add them the MySQL source repository and they will be fixed in the next release.
- If there is, by any chance, a fatal bug in a release we will make a new release as soon as possible. We would like other companies to do this, too.

The current stable release is Version 3.23; we have already moved active development to Version 4.0. Bugs will still be fixed in the stable version. We don't believe in a complete freeze, as this also leaves out bug fixes and things that ``must be done.'' ``Somewhat frozen'' means that we may add small things that ``almost surely will not affect anything that's already working.''

MySQL uses a slightly different naming scheme from most other products. In general it's relatively safe to use any version that has been out for a couple of weeks without being replaced with a new version. See section 2.2.3 Which MySQL Version to Use.

### 2.2.6 MySQL Binaries Compiled by MySQL AB

As a service, we at MySQL AB provide a set of binary distributions of MySQL that are compiled at our site or at sites where customers kindly have given us access to their machines.

These distributions are generated using the script `Build-tools/Do-compile` which compiles the source code and creates the binary `tar.gz` archive using `scripts/make_binary_distribution` and are configured with the following compilers and options:

SunOS 4.1.4 2 sun4c with `gcc` 2.7.2.1
  `CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors" ./configure --prefix=/usr/local/mysql --disable-shared --with-extra-charsets=complex --enable-assembler`
SunOS 5.5.1 (and above) sun4u with `egcs` 1.0.3a or 2.90.27 or gcc 2.95.2 and newer
  `CC=gcc CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex --enable-assembler`
SunOS 5.6 i86pc with `gcc` 2.8.1
  `CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex`
Solaris 2.8 sparc with `gcc 2.95.3`
  `CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql "--with-comment=Official MySQL binary" --with-extra-charsets=complex "--with-server-suffix=" --enable-thread-safe-client --enable-local-infile --enable-assembler --disable-shared`
Linux 2.0.33 i386 with `pgcc` 2.90.29 (`egcs` 1.0.3a)
  `CFLAGS="-O3 -mpentium -mstack-align-double" CXX=gcc CXXFLAGS="-O3 -mpentium -mstack-align-double -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --with-extra-charsets=complex`
Linux 2.2.x with x686 with `gcc` 2.95.2
  `CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-extra-charsets=complex`
SCO 3.2v5.0.4 i386 with `gcc` 2.7-95q4
  `CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex`
AIX 2 4 with `gcc` 2.7.2.2
  `CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex`
OSF/1 V4.0 564 alpha with `gcc` 2.8.1
  `CC=gcc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex`
Irix 6.3 IP32 with `gcc` 2.8.0
  `CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex`
BSDI BSD/OS 3.1 i386 with `gcc` 2.7.2.1
  `CC=gcc CXX=gcc CXXFLAGS=-O ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex`
BSDI BSD/OS 2.1 i386 with `gcc` 2.7.2
  `CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex`
FreeBSD 4.4-stable i386 with `gcc` 2.95.3
  `CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql "--with-comment=Official MySQL binary" --with-extra-charsets=complex "--with-server-suffix=" --enable-thread-safe-client --enable-local-infile --enable-assembler --with-named-z-libs=not-used --disable-shared`

Anyone who has more optimal options for any of the preceding configurations listed can always mail them to the developer's mailing list at internals@lists.mysql.com.

RPM distributions prior to MySQL Version 3.22 are user-contributed. Beginning with Version 3.22, the RPMs are generated by us at MySQL AB.

If you want to compile a debug version of MySQL, you should add `--with-debug` or `--with-debug=full` to the preceding configure lines and remove any `-fomit-frame-pointer` options.

For the Windows distribution, please see section 2.1.2 Installing MySQL on Windows.

### 2.2.7 Installing a MySQL Binary Distribution

See also section 2.1.2.1 Installing the Binaries, section 2.1.1 Installing MySQL on Linux, and section 8.4.7 Building Client Programs.

You need the following tools to install a MySQL binary distribution:

- GNU `gunzip` to uncompress the distribution.
- A reasonable `tar` to unpack the distribution. GNU `tar` is known to work. Sun `tar` is known to have problems.

An alternative installation method under Linux is to use RPM-based (RPM Package Manager) distributions. See section 2.1.1 Installing MySQL on Linux.

If you run into problems, please always use `mysqlbug` when posting questions to mysql@lists.mysql.com. Even if the problem isn't a bug, `mysqlbug` gathers system information that will help others solve your problem. By not using `mysqlbug`, you lessen the likelihood of getting a solution to your problem! You will find `mysqlbug` in the ``bin'` directory after you unpack the distribution. See section 1.6.2.3 How to Report Bugs or Problems.

The basic commands you must execute to install and use a MySQL binary distribution are:

    shell> groupadd mysql
    shell> useradd -g mysql mysql
    shell> cd /usr/local
    shell> gunzip < /path/to/mysql-VERSION-OS.tar.gz | tar xvf -
    shell> ln -s full-path-to-mysql-VERSION-OS mysql
    shell> cd mysql
    shell> scripts/mysql_install_db
    shell> chown -R root  .
    shell> chown -R mysql data
    shell> chgrp -R mysql .
    shell> bin/safe_mysqld --user=mysql &
    or
    shell> bin/mysqld_safe --user=mysql &
    if you are running MySQL 4.x

You can add new users using the `bin/mysql_setpermission` script if you install the `DBI` and `Msql-Mysql-modules` Perl modules.

A more detailed description follows.

To install a binary distribution, follow these steps, then proceed to section 2.4 Post-installation Setup and Testing, for post-installation setup and testing:

1. Pick the directory under which you want to unpack the distribution, and move into it. In the following example, we unpack the distribution under ``/usr/local'` and create a directory ``/usr/local/mysql'` into which MySQL is installed. (The following instructions, therefore, assume you have permission to create files in ``/usr/local'`. If that directory is protected, you will need to perform the installation as `root`.)
2. Obtain a distribution file from one of the sites listed in section 2.2.1 How to Get MySQL. MySQL binary distributions are provided as compressed `tar` archives and have names like ``mysql-VERSION-OS.tar.gz'`, where `VERSION` is a number (for example, `3.21.15`), and `OS` indicates the type of operating system for which the distribution is intended (for example, `pc-linux-gnu-i586`).
3. If you see a binary distribution marked with the `-max` suffix, this means that the binary has support for transaction-safe tables and other features. See section 4.7.5 `mysqld-max`, An Extended `mysqld` Server. Note that all binaries are built from the same MySQL source distribution.
4. Add a user and group for `mysqld` to run as:

       shell> groupadd mysql
       shell> useradd -g mysql mysql

   These commands add the `mysql` group and the `mysql` user. The syntax for `useradd` and `groupadd` may differ slightly on different versions of Unix. They may also be called `adduser` and `addgroup`. You may wish to call the user and group something else instead of `mysql`.
5. Change into the intended installation directory:

       shell> cd /usr/local

6. Unpack the distribution and create the installation directory:

       shell> gunzip < /path/to/mysql-VERSION-OS.tar.gz | tar xvf -
       shell> ln -s full-path-to-mysql-VERSION-OS mysql

   The first command creates a directory named ``mysql-VERSION-OS'`. The second command makes a symbolic link to that directory. This lets you refer more easily to the installation directory as ``/usr/local/mysql'`.
7. Change into the installation directory:

       shell> cd mysql

   You will find several files and subdirectories in the `mysql` directory. The most important for installation purposes are the ``bin'` and ``scripts'` subdirectories.
   ``bin'`
     This directory contains client programs and the server You should add the full pathname of this directory to your `PATH` environment variable so that your shell finds the MySQL programs properly. See section F Environment Variables.
   ``scripts'`
     This directory contains the `mysql_install_db` script used to initialise the `mysql` database containing the grant tables that store the server access permissions.
8. If you would like to use `mysqlaccess` and have the MySQL distribution in some non-standard place, you must change the location where `mysqlaccess` expects to find the `mysql` client. Edit the ``bin/mysqlaccess'` script at approximately line 18. Search for a line that looks like this:

       $MYSQL     = '/usr/local/bin/mysql';    # path to mysql executable

   Change the path to reflect the location where `mysql` actually is stored on your system. If you do not do this, you will get a `Broken pipe` error when you run `mysqlaccess`.
9. Create the MySQL grant tables (necessary only if you haven't installed MySQL before):

       shell> scripts/mysql_install_db

   Note that MySQL versions older than Version 3.22.10 started the MySQL server when you run `mysql_install_db`. This is no longer true!
10. Change ownership of binaries to `root` and ownership of the data directory to the user that you will run `mysqld` as:

        shell> chown -R root  /usr/local/mysql/.
        shell> chown -R mysql /usr/local/mysql/data
        shell> chgrp -R mysql /usr/local/mysql/.

    The first command changes the `owner` attribute of the files to the `root` user, the second one changes the `owner` attribute of the data directory to the `mysql` user, and the third one changes the `group` attribute to the `mysql` group.
11. If you want to install support for the Perl `DBI`/`DBD` interface, see section 2.7 Perl Installation Comments.
12. If you would like MySQL to start automatically when you boot your machine, you can copy `support-files/mysql.server` to the location where your system has its startup files. More information can be found in the `support-files/mysql.server` script itself and in section 2.4.3 Starting and Stopping MySQL Automatically.

After everything has been unpacked and installed, you should initialise and test your distribution.

You can start the MySQL server with the following command:

    shell> bin/safe_mysqld --user=mysql &

Now proceed to section 4.7.2 `safe_mysqld`, The Wrapper Around `mysqld`, and See section 2.4 Post-installation Setup and Testing.

## 2.3 Installing a MySQL Source Distribution

Before you proceed with the source installation, check first to see if our binary is available for your platform and if it will work for you. We put a lot of effort into making sure that our binaries are built with the best possible options.

You need the following tools to build and install MySQL from source:

- GNU `gunzip` to uncompress the distribution.
- A reasonable `tar` to unpack the distribution. GNU `tar` is known to work. Sun `tar` is known to have problems.
- A working ANSI C++ compiler. `gcc` >= 2.95.2, `egcs` >= 1.0.2 or `egcs 2.91.66`, SGI C++, and SunPro C++ are some of the compilers that are known to work. `libg++` is not needed when using `gcc`. `gcc` 2.7.x has a bug that makes it impossible to compile some perfectly legal C++ files, such as ``sql/sql_base.cc'`. If you only have `gcc` 2.7.x, you must upgrade your `gcc` to be able to compile MySQL. `gcc` 2.8.1 is also known to have problems on some platforms, so it should be avoided if a new compiler exists for the platform. `gcc` >= 2.95.2 is recommended when compiling MySQL Version 3.23.x.
- A good `make` program. GNU `make` is always recommended and is sometimes required. If you have problems, we recommend trying GNU `make` 3.75 or newer.

If you are using a recent version of `gcc`, recent enough to understand the `-fno-exceptions` option, it is very important that you use it. Otherwise, you may compile a binary that crashes randomly. We also recommend that you use `-felide-constructors` and `-fno-rtti` along with `-fno-exceptions`. When in doubt, do the following:

    CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions \
           -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler \
           --with-mysqld-ldflags=-all-static

On most systems this will give you a fast and stable binary.

If you run into problems, please always use `mysqlbug` when posting questions to mysql@lists.mysql.com. Even if the problem isn't a bug, `mysqlbug` gathers system information that will help others solve your problem. By not using `mysqlbug`, you lessen the likelihood of getting a solution to your problem! You will find `mysqlbug` in the ``scripts'` directory after you unpack the distribution. See section 1.6.2.3 How to Report Bugs or Problems.

### 2.3.1 Quick Installation Overview

The basic commands you must execute to install a MySQL source distribution are:

    shell> groupadd mysql
    shell> useradd -g mysql mysql
    shell> gunzip < mysql-VERSION.tar.gz | tar -xvf -
    shell> cd mysql-VERSION
    shell> ./configure --prefix=/usr/local/mysql
    shell> make
    shell> make install
    shell> scripts/mysql_install_db
    shell> chown -R root  /usr/local/mysql
    shell> chown -R mysql /usr/local/mysql/var
    shell> chgrp -R mysql /usr/local/mysql
    shell> cp support-files/my-medium.cnf /etc/my.cnf
    shell> /usr/local/mysql/bin/safe_mysqld --user=mysql &
    or
    shell> /usr/local/mysql/bin/mysqld_safe --user=mysql &
    if you are running MySQL 4.x.

If you want to have support for InnoDB tables, you should edit the `/etc/my.cnf` file and remove the `#` character before the parameter that starts with `innodb_...`. See section 4.1.2 ``my.cnf'` Option Files, and section 7.5.2 InnoDB Startup Options.

If you start from a source RPM, do the following:

    shell> rpm --rebuild MySQL-VERSION.src.rpm

This will make a binary RPM that you can install.

You can add new users using the `bin/mysql_setpermission` script if you install the `DBI` and `Msql-Mysql-modules` Perl modules.

A more detailed description follows.

To install a source distribution, follow these steps, then proceed to section 2.4 Post-installation Setup and Testing, for post-installation initialisation and testing:

1. Pick the directory under which you want to unpack the distribution, and move into it.
2. Obtain a distribution file from one of the sites listed in section 2.2.1 How to Get MySQL.
3. If you are interested in using Berkeley DB tables with MySQL, you will need to obtain a patched version of the Berkeley DB source code. Please read the chapter on Berkeley DB tables before proceeding. See section 7.6 `BDB` or `BerkeleyDB` Tables. MySQL source distributions are provided as compressed `tar` archives and have names like ``mysql-VERSION.tar.gz'`, where `VERSION` is a number like 4.0.6-gamma.
4. Add a user and group for `mysqld` to run as:

       shell> groupadd mysql
       shell> useradd -g mysql mysql

   These commands add the `mysql` group and the `mysql` user. The syntax for `useradd` and `groupadd` may differ slightly on different versions of Unix. They may also be called `adduser` and `addgroup`. You may wish to call the user and group something else instead of `mysql`.
5. Unpack the distribution into the current directory:

       shell> gunzip < /path/to/mysql-VERSION.tar.gz | tar xvf -

   This command creates a directory named ``mysql-VERSION'`.
6. Change into the top-level directory of the unpacked distribution:

       shell> cd mysql-VERSION

   Note that currently you must configure and build MySQL from this top-level directory. You cannot build it in a different directory.
7. Configure the release and compile everything:

       shell> ./configure --prefix=/usr/local/mysql
       shell> make

   When you run `configure`, you might want to specify some options. Run `./configure --help` for a list of options. section 2.3.3 Typical `configure` Options, discusses some of the more useful options. If `configure` fails, and you are going to send mail to mysql@lists.mysql.com to ask for assistance, please include any lines from ``config.log'` that you think can help solve the problem. Also include the last couple of lines of output from `configure` if `configure` aborts. Post the bug report using the `mysqlbug` script. See section 1.6.2.3 How to Report Bugs or Problems. If the compile fails, see section 2.3.5 Problems Compiling?, for help with a number of common problems.
8. Install everything:

       shell> make install

   You might need to run this command as `root`.
9. Create the MySQL grant tables (necessary only if you haven't installed MySQL before):

       shell> scripts/mysql_install_db

   Note that MySQL versions older than Version 3.22.10 started the MySQL server when you run `mysql_install_db`. This is no longer true!
10. Change ownership of binaries to `root` and ownership of the data directory to the user that you will run `mysqld` as:

        shell> chown -R root  /usr/local/mysql
        shell> chown -R mysql /usr/local/mysql/var
        shell> chgrp -R mysql /usr/local/mysql

    The first command changes the `owner` attribute of the files to the `root` user, the second one changes the `owner` attribute of the data directory to the `mysql` user, and the third one changes the `group` attribute to the `mysql` group.
11. If you want to install support for the Perl `DBI`/`DBD` interface, see section 2.7 Perl Installation Comments.
12. If you would like MySQL to start automatically when you boot your machine, you can copy `support-files/mysql.server` to the location where your system has its startup files. More information can be found in the `support-files/mysql.server` script itself and in section 2.4.3 Starting and Stopping MySQL Automatically.

After everything has been installed, you should initialise and test your distribution:

    shell> /usr/local/mysql/bin/safe_mysqld --user=mysql &

If that command fails immediately with `mysqld daemon ended`, you can find some information in the file ``mysql-data-directory/'hostname'.err'`. The likely reason is that you already have another `mysqld` server running. See section 4.1.4 Running Multiple MySQL Servers on the Same Machine.

Now proceed to section 2.4 Post-installation Setup and Testing.

### 2.3.2 Applying Patches

Sometimes patches appear on the mailing list or are placed in the patches area of the MySQL web site (http://www.mysql.com/Downloads/Patches/).

To apply a patch from the mailing list, save the message in which the patch appears in a file, change into the top-level directory of your MySQL source tree, and run these commands:

    shell> patch -p1 < patch-file-name
    shell> rm config.cache
    shell> make clean

Patches from the FTP site are distributed as plain text files or as files compressed with `gzip`. Apply a plain patch as shown previously for mailing list patches. To apply a compressed patch, change into the top-level directory of your MySQL source tree and run these commands:

    shell> gunzip < patch-file-name.gz | patch -p1
    shell> rm config.cache
    shell> make clean

After applying a patch, follow the instructions for a normal source install, beginning with the `./configure` step. After running the `make install` step, restart your MySQL server.

You may need to bring down any currently running server before you run `make install`. (Use `mysqladmin shutdown` to do this.) Some systems do not allow you to install a new version of a program if it replaces the version that is currently executing.

### 2.3.3 Typical `configure` Options

The `configure` script gives you a great deal of control over how you configure your MySQL distribution. Typically you do this using options on the `configure` command-line. You can also affect `configure` using certain environment variables. See section F Environment Variables. For a list of options supported by `configure`, run this command:

    shell> ./configure --help

Some of the more commonly-used `configure` options are described here:

- To compile just the MySQL client libraries and client programs and not the server, use the `--without-server` option:

      shell> ./configure --without-server

  If you don't have a C++ compiler, `mysql` will not compile (it is the one client program that requires C++). In this case, you can remove the code in `configure` that tests for the C++ compiler and then run `./configure` with the `--without-server` option. The compile step will still try to build `mysql`, but you can ignore any warnings about ``mysql.cc'`. (If `make` stops, try `make -k` to tell it to continue with the rest of the build even if errors occur.)
- If you want to get an embedded MySQL library (`libmysqld.a`) you should use the `--with-embedded-server` option.
- If you don't want your log files and database directories located under ``/usr/local/var'`, use a `configure` command, something like one of these:

      shell> ./configure --prefix=/usr/local/mysql
      shell> ./configure --prefix=/usr/local \
                 --localstatedir=/usr/local/mysql/data

  The first command changes the installation prefix so that everything is installed under ``/usr/local/mysql'` rather than the default of ``/usr/local'`. The second command preserves the default installation prefix, but overrides the default location for database directories (normally ``/usr/local/var'`) and changes it to `/usr/local/mysql/data`. After you have compiled MySQL, you can change these options with option files. See section 4.1.2 ``my.cnf'` Option Files.
- If you are using Unix and you want the MySQL socket located somewhere other than the default location (normally in the directory ``/tmp'` or ``/var/run'`) use a `configure` command like this:

      shell> ./configure --with-unix-socket-path=/usr/local/mysql/tmp/mysql.sock

  Note that the given file must be an absolute pathname! You can also later change the location ``mysql.sock'` by using the MySQL option files. See section A.4.5 How to Protect or Change the MySQL Socket File ``/tmp/mysql.sock'`.
- If you want to compile statically linked programs (for example, to make a binary distribution, to get more speed, or to work around problems with some RedHat Linux distributions), run `configure` like this:

      shell> ./configure --with-client-ldflags=-all-static \
                 --with-mysqld-ldflags=-all-static

- If you are using `gcc` and don't have `libg++` or `libstdc++` installed, you can tell `configure` to use `gcc` as your C++ compiler:

      shell> CC=gcc CXX=gcc ./configure

  When you use `gcc` as your C++ compiler, it will not attempt to link in `libg++` or `libstdc++`. This may be a good idea to do even if you have the above libraries installed, as some versions of these libraries have caused strange problems for MySQL users in the past. Here are some common environment variables to set depending on the compiler you are using:

  Compiler | Recommended options
  gcc 2.7.2.1 | CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors"
  egcs 1.0.3a | CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti"
  gcc 2.95.2 | CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro \ -felide-constructors -fno-exceptions -fno-rtti"
  pgcc 2.90.29 or newer | CFLAGS="-O3 -mpentiumpro -mstack-align-double" CXX=gcc \ CXXFLAGS="-O3 -mpentiumpro -mstack-align-double -felide-constructors \ -fno-exceptions -fno-rtti"

  In most cases you can get a reasonably optimal MySQL binary by using the options from the preceding table and adding the following options to the configure line:

      --prefix=/usr/local/mysql --enable-assembler \
      --with-mysqld-ldflags=-all-static

  The full configure line would, in other words, be something like the following for all recent gcc versions:

      CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro \
      -felide-constructors -fno-exceptions -fno-rtti" ./configure \
      --prefix=/usr/local/mysql --enable-assembler \
      --with-mysqld-ldflags=-all-static

  The binaries we provide on the MySQL web site at http://www.mysql.com/ are all compiled with full optimisation and should be perfect for most users. See section 2.2.6 MySQL Binaries Compiled by MySQL AB. There are some things you can tweak to make an even faster binary, but this is only for advanced users. See section 5.5.3 How Compiling and Linking Affects the Speed of MySQL. If the build fails and produces errors about your compiler or linker not being able to create the shared library ``libmysqlclient.so.#'` (``#'` is a version number), you can work around this problem by giving the `--disable-shared` option to `configure`. In this case, `configure` will not build a shared ``libmysqlclient.so.#'` library.
- You can configure MySQL not to use `DEFAULT` column values for non-`NULL` columns (that is, columns that are not allowed to be `NULL`). This causes `INSERT` statements to generate an error unless you explicitly specify values for all columns that require a non-`NULL` value. To suppress use of default values, run `configure` like this:

      shell> CXXFLAGS=-DDONT_USE_DEFAULT_FIELDS ./configure

- By default, MySQL uses the ISO-8859-1 (Latin1) character set. To change the default set, use the `--with-charset` option:

      shell> ./configure --with-charset=CHARSET

  `CHARSET` may be one of `big5`, `cp1251`, `cp1257`, `czech`, `danish`, `dec8`, `dos`, `euc_kr`, `gb2312`, `gbk`, `german1`, `hebrew`, `hp8`, `hungarian`, `koi8_ru`, `koi8_ukr`, `latin1`, `latin2`, `sjis`, `swe7`, `tis620`, `ujis`, `usa7`, or `win1251ukr`. See section 4.6.1 The Character Set Used for Data and Sorting. If you want to convert characters between the server and the client, you should take a look at the `SET CHARACTER SET` command. See section 5.5.6 `SET` Syntax. Warning: If you change character sets after having created any tables, you will have to run `myisamchk -r -q --set-character-set=charset` on every table. Your indexes may be sorted incorrectly otherwise. (This can happen if you install MySQL, create some tables, then reconfigure MySQL to use a different character set and reinstall it.) With the option `--with-extra-charsets=LIST` you can define which additional character sets should be compiled into the server. Here `LIST` is either a list of character sets separated with spaces, `complex` to include all characters that can't be dynamically loaded, or `all` to include all character sets into the binaries.
- To configure MySQL with debugging code, use the `--with-debug` option:

      shell> ./configure --with-debug

  This causes a safe memory allocator to be included that can find some errors and that provides output about what is happening. See section E.1 Debugging a MySQL server.
- If your client programs are using threads, you need to also compile a thread-safe version of the MySQL client library with the `--enable-thread-safe-client` configure options. This will create a `libmysqlclient_r` library with which you should link your threaded applications. See section 8.4.8 How to Make a Threaded Client.
- Options that pertain to particular systems can be found in the system-specific section of this manual. See section 2.6 Operating System Specific Notes.

### 2.3.4 Installing from the Development Source Tree

Caution: You should read this section only if you are interested in helping us test our new code. If you just want to get MySQL up and running on your system, you should use a standard release distribution (either a source or binary distribution will do).

To obtain our most recent development source tree, use these instructions:

1. Download `BitKeeper` from http://www.bitmover.com/cgi-bin/download.cgi. You will need `Bitkeeper` 3.0 or newer to access our repository.
2. Follow the instructions to install it.
3. After `BitKeeper` is installed, first go to the directory you want to work from, and then use one of the following commands to clone the MySQL version branch of your choice: To clone the 3.23 branch, use this command:

       shell> bk clone bk://work.mysql.com:7000 mysql-3.23

   To clone the 4.0 branch, use this command:

       shell> bk clone bk://work.mysql.com:7001 mysql-4.0

   To clone the 4.1 branch, use this command:

       shell> bk clone bk://work.mysql.com:7004 mysql-4.1

   In the preceding examples the source tree will be set up in the ``mysql-3.23/'`, ``mysql-4.0/'`, or ``mysql-4.1/'` subdirectory of your current directory. The initial download of the source tree may take a while, depending on the speed of your connection; be patient.
4. You will need GNU `make`, `autoconf 2.52`, `automake 1.5`, `libtool 1.4`, and `m4` to run the next set of commands.

       shell> cd mysql-4.0
       shell> bk -r get -Sq
       shell> aclocal; autoheader; autoconf; automake
       shell> (cd innobase ; aclocal; autoheader; autoconf; automake) # for InnoDB
       shell> (cd bdb/dist ; sh s_all ) # for Berkeley DB
       shell> ./configure  # Add your favorite options here
       shell> make

   If you get some strange error during this stage, check that you really have `libtool` installed! A collection of our standard configure scripts is located in the ``BUILD/'` subdirectory. If you are lazy, you can use ``BUILD/compile-pentium-debug'`. To compile on a different architecture, modify the script by removing flags that are Pentium-specific.
5. When the build is done, run `make install`. Be careful with this on a production machine; the command may overwrite your live release installation. If you have another installation of MySQL, we recommand that you run `./configure` with different values for the `prefix`, `with-tcp-port`, and `unix-socket-path` options than those used for your production server.
6. Play hard with your new installation and try to make the new features crash. Start by running `make test`. See section 9.1.2 MySQL Test Suite.
7. If you have gotten to the `make` stage and the distribution does not compile, please report it to bugs@lists.mysql.com. If you have installed the latest versions of the required GNU tools, and they crash trying to process our configuration files, please report that also. However, if you execute `aclocal` and get a `command not found` error or a similar problem, do not report it. Instead, make sure all the necessary tools are installed and that your `PATH` variable is set correctly so that your shell can find them.
8. After the initial `bk clone` operation to get the source tree, you should run `bk pull` periodically to get the updates.
9. You can examine the change history for the tree with all the diffs by using `bk sccstool`. If you see some funny diffs or code that you have a question about, do not hesitate to send e-mail to internals@lists.mysql.com. Also, if you think you have a better idea on how to do something, send an e-mail to the same address with a patch. `bk diffs` will produce a patch for you after you have made changes to the source. If you do not have the time to code your idea, just send a description.
10. `BitKeeper` has a nice help utility that you can access via `bk helptool`.
11. Please note that any commits (`bk ci` or `bk citool`) will trigger the posting of a message with the changeset to our internals mailing list, as well as the usual openlogging.org submission with just the changeset comments. Generally, you wouldn't need to use commit (since the public tree will not allow `bk push`), but rather use the `bk diffs` method described previously.

You can also browse changesets, comments and sourcecode online by browsing to e.g. http://work.mysql.com:7004 for MySQL 4.1.

The manual is in a separate tree which can be cloned with:

    shell> bk clone bk://work.mysql.com:7005 mysqldoc

### 2.3.5 Problems Compiling?

All MySQL programs compile cleanly for us with no warnings on Solaris using `gcc`. On other systems, warnings may occur due to differences in system include files. See section 2.3.6 MIT-pthreads Notes for warnings that may occur when using MIT-pthreads. For other problems, check the following list.

The solution to many problems involves reconfiguring. If you do need to reconfigure, take note of the following:

- If `configure` is run after it already has been run, it may use information that was gathered during its previous invocation. This information is stored in ``config.cache'`. When `configure` starts up, it looks for that file and reads its contents if it exists, on the assumption that the information is still correct. That assumption is invalid when you reconfigure.
- Each time you run `configure`, you must run `make` again to recompile. However, you may want to remove old object files from previous builds first because they were compiled using different configuration options.

To prevent old configuration information or object files from being used, run these commands before rerunning `configure`:

    shell> rm config.cache
    shell> make clean

Alternatively, you can run `make distclean`.

The following list describes some of the problems when compiling MySQL that have been found to occur most often:

- If you get errors when compiling ``sql_yacc.cc'`, such as the ones shown here, you have probably run out of memory or swap space:

      Internal compiler error: program cc1plus got fatal signal 11
        or
      Out of virtual memory
        or
      Virtual memory exhausted

  The problem is that `gcc` requires huge amounts of memory to compile ``sql_yacc.cc'` with inline functions. Try running `configure` with the `--with-low-memory` option:

      shell> ./configure --with-low-memory

  This option causes `-fno-inline` to be added to the compile line if you are using `gcc` and `-O0` if you are using something else. You should try the `--with-low-memory` option even if you have so much memory and swap space that you think you can't possibly have run out. This problem has been observed to occur even on systems with generous hardware configurations, and the `--with-low-memory` option usually fixes it.
- By default, `configure` picks `c++` as the compiler name and GNU `c++` links with `-lg++`. If you are using `gcc`, that behaviour can cause problems during configuration such as this:

      configure: error: installation or configuration problem:
      C++ compiler cannot create executables.

  You might also observe problems during compilation related to `g++`, `libg++`, or `libstdc++`. One cause of these problems is that you may not have `g++`, or you may have `g++` but not `libg++`, or `libstdc++`. Take a look at the ``config.log'` file. It should contain the exact reason why your c++ compiler didn't work! To work around these problems, you can use `gcc` as your C++ compiler. Try setting the environment variable `CXX` to `"gcc -O3"`. For example:

      shell> CXX="gcc -O3" ./configure

  This works because `gcc` compiles C++ sources as well as `g++` does, but does not link in `libg++` or `libstdc++` by default. Another way to fix these problems, of course, is to install `g++`, `libg++`, and `libstdc++`. We would however like to recommend you to not use `libg++` or `libstdc++` with MySQL as this will only increase the binary size of mysqld without giving you any benefits. Some versions of these libraries have also caused strange problems for MySQL users in the past.
- If your compile fails with errors, such as any of the following, you must upgrade your version of `make` to GNU `make`:

      making all in mit-pthreads
      make: Fatal error in reader: Makefile, line 18:
      Badly formed macro assignment
        or
      make: file `Makefile' line 18: Must be a separator (:
        or
      pthread.h: No such file or directory

  Solaris and FreeBSD are known to have troublesome `make` programs. GNU `make` Version 3.75 is known to work.
- If you want to define flags to be used by your C or C++ compilers, do so by adding the flags to the `CFLAGS` and `CXXFLAGS` environment variables. You can also specify the compiler names this way using `CC` and `CXX`. For example:

      shell> CC=gcc
      shell> CFLAGS=-O3
      shell> CXX=gcc
      shell> CXXFLAGS=-O3
      shell> export CC CFLAGS CXX CXXFLAGS

  See section 2.2.6 MySQL Binaries Compiled by MySQL AB, for a list of flag definitions that have been found to be useful on various systems.
- If you get an error message like this, you need to upgrade your `gcc` compiler:

      client/libmysql.c:273: parse error before `__attribute__'

  `gcc` 2.8.1 is known to work, but we recommend using `gcc` 2.95.2 or `egcs` 1.0.3a instead.
- If you get errors such as those shown here when compiling `mysqld`, `configure` didn't correctly detect the type of the last argument to `accept()`, `getsockname()`, or `getpeername()`:

      cxx: Error: mysqld.cc, line 645: In this statement, the referenced
           type of the pointer value "&length" is "unsigned long", which
           is not compatible with "int".
      new_sock = accept(sock, (struct sockaddr *)&cAddr, &length);

  To fix this, edit the ``config.h'` file (which is generated by `configure`). Look for these lines:

      /* Define as the base type of the last arg to accept */
      #define SOCKET_SIZE_TYPE XXX

  Change `XXX` to `size_t` or `int`, depending on your operating system. (Note that you will have to do this each time you run `configure` because `configure` regenerates ``config.h'`.)
- The ``sql_yacc.cc'` file is generated from ``sql_yacc.yy'`. Normally the build process doesn't need to create ``sql_yacc.cc'`, because MySQL comes with an already generated copy. However, if you do need to re-create it, you might encounter this error:

      "sql_yacc.yy", line xxx fatal: default action causes potential...

  This is a sign that your version of `yacc` is deficient. You probably need to install `bison` (the GNU version of `yacc`) and use that instead.
- If you need to debug `mysqld` or a MySQL client, run `configure` with the `--with-debug` option, then recompile and link your clients with the new client library. See section E.2 Debugging a MySQL client.

### 2.3.6 MIT-pthreads Notes

This section describes some of the issues involved in using MIT-pthreads.

Note that on Linux you should not use MIT-pthreads but install LinuxThreads! See section 2.6.1 Linux Notes (All Linux Versions).

If your system does not provide native thread support, you will need to build MySQL using the MIT-pthreads package. This includes older FreeBSD systems, SunOS 4.x, Solaris 2.4 and earlier, and some others. See section 2.2.2 Operating Systems Supported by MySQL.

Note, that beginning with MySQL 4.0.2 MIT-pthreads are no longer part of the source distribution! If you require this package, you need to download it separately from http://www.mysql.com/Downloads/Contrib/pthreads-1_60_beta6-mysql.tar.gz

After downloading, extract this source archive into the top level of the MySQL source directory. It will create a new subdirectory `mit-pthreads`.

- On most systems, you can force MIT-pthreads to be used by running `configure` with the `--with-mit-threads` option:

      shell> ./configure --with-mit-threads

  Building in a non-source directory is not supported when using MIT-pthreads because we want to minimise our changes to this code.
- The checks that determine whether to use MIT-pthreads occur only during the part of the configuration process that deals with the server code. If you have configured the distribution using `--without-server` to build only the client code, clients will not know whether MIT-pthreads is being used and will use Unix socket connections by default. Because Unix sockets do not work under MIT-pthreads on some platforms, this means you will need to use `-h` or `--host` when you run client programs.
- When MySQL is compiled using MIT-pthreads, system locking is disabled by default for performance reasons. You can tell the server to use system locking with the `--external-locking` option. This is only needed if you want to be able to run two MySQL servers against the same data files (not recommended).
- Sometimes the pthread `bind()` command fails to bind to a socket without any error message (at least on Solaris). The result is that all connections to the server fail. For example:

      shell> mysqladmin version
      mysqladmin: connect to server at '' failed;
      error: 'Can't connect to mysql server on localhost (146)'

  The solution to this is to kill the `mysqld` server and restart it. This has only happened to us when we have forced the server down and done a restart immediately.
- With MIT-pthreads, the `sleep()` system call isn't interruptible with `SIGINT` (break). This is only noticeable when you run `mysqladmin --sleep`. You must wait for the `sleep()` call to terminate before the interrupt is served and the process stops.
- When linking, you may receive warning messages like these (at least on Solaris); they can be ignored:

      ld: warning: symbol `_iob' has differing sizes:
          (file /my/local/pthreads/lib/libpthread.a(findfp.o) value=0x4;
      file /usr/lib/libc.so value=0x140);
          /my/local/pthreads/lib/libpthread.a(findfp.o) definition taken
      ld: warning: symbol `__iob' has differing sizes:
          (file /my/local/pthreads/lib/libpthread.a(findfp.o) value=0x4;
      file /usr/lib/libc.so value=0x140);
          /my/local/pthreads/lib/libpthread.a(findfp.o) definition taken

- Some other warnings also can be ignored:

      implicit declaration of function `int strtoll(...)'
      implicit declaration of function `int strtoul(...)'

- We haven't gotten `readline` to work with MIT-pthreads. (This isn't needed, but may be interesting for someone.)

### 2.3.7 Windows Source Distribution

You will need the following:

- VC++ 6.0 compiler (updated with 4 or 5 SP and Pre-processor package) The Pre-processor package is necessary for the macro assembler. More details at: http://msdn.microsoft.com/vstudio/sp/vs6sp5/faq.asp.
- The MySQL source distribution for Windows, which can be downloaded from http://www.mysql.com/downloads/.

Building MySQL

1. Create a work directory (e.g., workdir).
2. Unpack the source distribution in the aforementioned directory.
3. Start the VC++ 6.0 compiler.
4. In the `File` menu, select `Open Workspace`.
5. Open the ``mysql.dsw'` workspace you find on the work directory.
6. From the `Build` menu, select the `Set Active Configuration` menu.
7. Click over the screen selecting `mysqld - Win32 Debug` and click OK.
8. Press `F7` to begin the build of the debug server, libs, and some client applications.
9. When the compilation finishes, copy the libs and the executables to a separate directory.
10. Compile the release versions that you want, in the same way.
11. Create the directory for the MySQL stuff: e.g., ``c:\mysql'`
12. From the workdir directory copy for the c:\mysql directory the following directories:
    - Data
    - Docs
    - Share
13. Create the directory ``c:\mysql\bin'` and copy all the servers and clients that you compiled previously.
14. If you want, also create the ``lib'` directory and copy the libs that you compiled previously.
15. Do a clean using Visual Studio.

Set up and start the server in the same way as for the binary Windows distribution. See section 2.1.2.2 Preparing the Windows MySQL Environment.

## 2.4 Post-installation Setup and Testing

Once you've installed MySQL (from either a binary or source distribution), you need to initialise the grant tables, start the server, and make sure that the server works okay. You may also wish to arrange for the server to be started and stopped automatically when your system starts up and shuts down.

Normally you install the grant tables and start the server like this for installation from a source distribution:

    shell> ./scripts/mysql_install_db
    shell> cd mysql_installation_directory
    shell> ./bin/safe_mysqld --user=mysql &

For a binary distribution (not RPM or pkg packages), do this:

    shell> cd mysql_installation_directory
    shell> ./scripts/mysql_install_db
    shell> ./bin/safe_mysqld --user=mysql &
    or
    shell> ./bin/mysqld_safe --user=mysql &
    if you are running MySQL 4.x.

This creates the `mysql` database which will hold all database privileges, the `test` database which you can use to test MySQL, and also privilege entries for the user that run `mysql_install_db` and a `root` user (without any passwords). This also starts the `mysqld` server.

`mysql_install_db` will not overwrite any old privilege tables, so it should be safe to run in any circumstances. If you don't want to have the `test` database you can remove it with `mysqladmin -u root drop test`.

Testing is most easily done from the top-level directory of the MySQL distribution. For a binary distribution, this is your installation directory (typically something like ``/usr/local/mysql'`). For a source distribution, this is the main directory of your MySQL source tree.

In the commands shown in this section and in the following subsections, `BINDIR` is the path to the location in which programs like `mysqladmin` and `safe_mysqld` are installed. For a binary distribution, this is the ``bin'` directory within the distribution. For a source distribution, `BINDIR` is probably ``/usr/local/bin'`, unless you specified an installation directory other than ``/usr/local'` when you ran `configure`. `EXECDIR` is the location in which the `mysqld` server is installed. For a binary distribution, this is the same as `BINDIR`. For a source distribution, `EXECDIR` is probably ``/usr/local/libexec'`.

Testing is described in detail:

1. If necessary, start the `mysqld` server and set up the initial MySQL grant tables containing the privileges that determine how users are allowed to connect to the server. This is normally done with the `mysql_install_db` script:

       shell> scripts/mysql_install_db

   Typically, `mysql_install_db` needs to be run only the first time you install MySQL. Therefore, if you are upgrading an existing installation, you can skip this step. (However, `mysql_install_db` is quite safe to use and will not update any tables that already exist, so if you are unsure of what to do, you can always run `mysql_install_db`.) `mysql_install_db` creates six tables (`user`, `db`, `host`, `tables_priv`, `columns_priv`, and `func`) in the `mysql` database. A description of the initial privileges is given in section 4.3.4 Setting Up the Initial MySQL Privileges. Briefly, these privileges allow the MySQL `root` user to do anything, and allow anybody to create or use databases with a name of `test` or starting with `test_`. If you don't set up the grant tables, the following error will appear in the log file when you start the server:

       mysqld: Can't find file: 'host.frm'

   This may also happen with a binary MySQL distribution if you don't start MySQL by executing exactly `./bin/safe_mysqld`! See section 4.7.2 `safe_mysqld`, The Wrapper Around `mysqld`. You might need to run `mysql_install_db` as `root`. However, if you prefer, you can run the MySQL server as an unprivileged (non-`root`) user, provided that the user can read and write files in the database directory. Instructions for running MySQL as an unprivileged user are given in section A.3.2 How to Run MySQL As a Normal User. If you have problems with `mysql_install_db`, see section 2.4.1 Problems Running `mysql_install_db`. There are some alternatives to running the `mysql_install_db` script as it is provided in the MySQL distribution:
   - You may want to edit `mysql_install_db` before running it, to change the initial privileges that are installed into the grant tables. This is useful if you want to install MySQL on a lot of machines with the same privileges. In this case you probably should need only to add a few extra `INSERT` statements to the `mysql.user` and `mysql.db` tables!
   - If you want to change things in the grant tables after installing them, you can run `mysql_install_db`, then use `mysql -u root mysql` to connect to the grant tables as the MySQL `root` user and issue SQL statements to modify the grant tables directly.
   - It is possible to re-create the grant tables completely after they have already been created. You might want to do this if you've already installed the tables but then want to re-create them after editing `mysql_install_db`.
   For more information about these alternatives, see section 4.3.4 Setting Up the Initial MySQL Privileges.
2. Start the MySQL server like this:

       shell> cd mysql_installation_directory
       shell> bin/safe_mysqld &

   If you have problems starting the server, see section 2.4.2 Problems Starting the MySQL Server.
3. Use `mysqladmin` to verify that the server is running. The following commands provide a simple test to check that the server is up and responding to connections:

       shell> BINDIR/mysqladmin version
       shell> BINDIR/mysqladmin variables

   The output from `mysqladmin version` varies slightly depending on your platform and version of MySQL, but should be similar to that shown here:

       shell> BINDIR/mysqladmin version
       mysqladmin  Ver 8.14 Distrib 3.23.32, for linux on i586
       Copyright (C) 2000 MySQL AB & MySQL Finland AB & TCX DataKonsult AB
       This software comes with ABSOLUTELY NO WARRANTY. This is free software,
       and you are welcome to modify and redistribute it under the GPL license.

       Server version          3.23.32-debug
       Protocol version        10
       Connection              Localhost via Unix socket
       TCP port                3306
       UNIX socket             /tmp/mysql.sock
       Uptime:                 16 sec

       Threads: 1  Questions: 9  Slow queries: 0
       Opens: 7  Flush tables: 2  Open tables: 0
       Queries per second avg: 0.000
       Memory in use: 132K  Max memory used: 16773K

   To get a feeling for what else you can do with `BINDIR/mysqladmin`, invoke it with the `--help` option.
4. Verify that you can shut down the server:

       shell> BINDIR/mysqladmin -u root shutdown

5. Verify that you can restart the server. Do this using `safe_mysqld` or by invoking `mysqld` directly. For example:

       shell> BINDIR/safe_mysqld --log &

   If `safe_mysqld` fails, try running it from the MySQL installation directory (if you are not already there). If that doesn't work, see section 2.4.2 Problems Starting the MySQL Server.
6. Run some simple tests to verify that the server is working. The output should be similar to what is shown here:

       shell> BINDIR/mysqlshow
       +-----------+
       | Databases |
       +-----------+
       | mysql     |
       +-----------+

       shell> BINDIR/mysqlshow mysql
       Database: mysql
       +--------------+
       |    Tables    |
       +--------------+
       | columns_priv |
       | db           |
       | func         |
       | host         |
       | tables_priv  |
       | user         |
       +--------------+

       shell> BINDIR/mysql -e "SELECT host,db,user FROM db" mysql
       +------+--------+------+
       | host | db     | user |
       +------+--------+------+
       | %    | test   |      |
       | %    | test_% |      |
       +------+--------+------+

   There is also a benchmark suite in the ``sql-bench'` directory (under the MySQL installation directory) that you can use to compare how MySQL performs on different platforms. The ``sql-bench/Results'` directory contains the results from many runs against different databases and platforms. To run all tests, execute these commands:

       shell> cd sql-bench
       shell> run-all-tests

   If you don't have the ``sql-bench'` directory, you are probably using an RPM for a binary distribution. (Source distribution RPMs include the benchmark directory.) In this case, you must first install the benchmark suite before you can use it. Beginning with MySQL Version 3.22, there are benchmark RPM files named ``mysql-bench-VERSION-i386.rpm'` that contain benchmark code and data. If you have a source distribution, you can also run the tests in the ``tests'` subdirectory. For example, to run ``auto_increment.tst'`, do this:

       shell> BINDIR/mysql -vvf test < ./tests/auto_increment.tst

   The expected results are shown in the ``./tests/auto_increment.res'` file.

### 2.4.1 Problems Running `mysql_install_db`

The purpose of the `mysql_install_db` script is to generate new MySQL privilege tables. It will not affect any other data! It will also not do anything if you already have MySQL privilege tables installed!

If you want to re-create your privilege tables, you should take down the `mysqld` server, if it's running, and then do something like:

    mv mysql-data-directory/mysql mysql-data-directory/mysql-old
    mysql_install_db

This section lists problems you might encounter when you run `mysql_install_db`:

`mysql_install_db` doesn't install the grant tables
  You may find that `mysql_install_db` fails to install the grant tables and terminates after displaying the following messages:

      starting mysqld daemon with databases from XXXXXX
      mysql daemon ended

  In this case, you should examine the log file very carefully! The log should be located in the directory ``XXXXXX'` named by the error message, and should indicate why `mysqld` didn't start. If you don't understand what happened, include the log when you post a bug report using `mysqlbug`! See section 1.6.2.3 How to Report Bugs or Problems.
There is already a `mysqld` daemon running
  In this case, you probably don't have to run `mysql_install_db` at all. You have to run `mysql_install_db` only once, when you install MySQL the first time.
Installing a second `mysqld` daemon doesn't work when one daemon is running
  This can happen when you already have an existing MySQL installation, but want to put a new installation in a different place (for example, for testing, or perhaps you simply want to run two installations at the same time). Generally the problem that occurs when you try to run the second server is that it tries to use the same socket and port as the old one. In this case you will get the error message: `Can't start server: Bind on TCP/IP port: Address already in use` or `Can't start server: Bind on unix socket...`. See section 4.1.3 Installing Many Servers on the Same Machine.
You don't have write access to ``/tmp'`
  If you don't have write access to create a socket file at the default place (in ``/tmp'`) or permission to create temporary files in ``/tmp,'` you will get an error when running `mysql_install_db` or when starting or using `mysqld`. You can specify a different socket and temporary directory as follows:

      shell> TMPDIR=/some_tmp_dir/
      shell> MYSQL_UNIX_PORT=/some_tmp_dir/mysqld.sock
      shell> export TMPDIR MYSQL_UNIX_PORT

  See section A.4.5 How to Protect or Change the MySQL Socket File ``/tmp/mysql.sock'`. ``some_tmp_dir'` should be the path to some directory for which you have write permission. See section F Environment Variables. After this you should be able to run `mysql_install_db` and start the server with these commands:

      shell> scripts/mysql_install_db
      shell> BINDIR/safe_mysqld &

`mysqld` crashes immediately
  If you are running RedHat Version 5.0 with a version of `glibc` older than 2.0.7-5, you should make sure you have installed all `glibc` patches! There is a lot of information about this in the MySQL mail archives. Links to the mail archives are available online at http://lists.mysql.com/. Also, see section 2.6.1 Linux Notes (All Linux Versions). You can also start `mysqld` manually using the `--skip-grant-tables` option and add the privilege information yourself using `mysql`:

      shell> BINDIR/safe_mysqld --skip-grant-tables &
      shell> BINDIR/mysql -u root mysql

  From `mysql`, manually execute the SQL commands in `mysql_install_db`. Make sure you run `mysqladmin flush-privileges` or `mysqladmin reload` afterward to tell the server to reload the grant tables.

### 2.4.2 Problems Starting the MySQL Server

If you are going to use tables that support transactions (InnoDB, BDB), you should first create a ``my.cnf'` file and set startup options for the table types you plan to use. See section 7 MySQL Table Types.

Generally, you start the `mysqld` server in one of these ways:

- By invoking `mysql.server`. This script is used primarily at system startup and shutdown, and is described more fully in section 2.4.3 Starting and Stopping MySQL Automatically.
- By invoking `safe_mysqld`, which tries to determine the proper options for `mysqld` and then runs it with those options. See section 4.7.2 `safe_mysqld`, The Wrapper Around `mysqld`.
- For Windows NT/2000/XP, please see section 2.6.2.2 Starting MySQL on Windows NT, 2000 or XP.
- By invoking `mysqld` directly.

When the `mysqld` daemon starts up, it changes the directory to the data directory. This is where it expects to write log files and the pid (process ID) file, and where it expects to find databases.

The data directory location is hardwired in when the distribution is compiled. However, if `mysqld` expects to find the data directory somewhere other than where it really is on your system, it will not work properly. If you have problems with incorrect paths, you can find out what options `mysqld` allows and what the default path settings are by invoking `mysqld` with the `--help` option. You can override the defaults by specifying the correct pathnames as command-line arguments to `mysqld`. (These options can be used with `safe_mysqld` as well.)

Normally you should need to tell `mysqld` only the base directory under which MySQL is installed. You can do this with the `--basedir` option. You can also use `--help` to check the effect of changing path options (note that `--help` must be the final option of the `mysqld` command). For example:

    shell> EXECDIR/mysqld --basedir=/usr/local --help

Once you determine the path settings you want, start the server without the `--help` option.

Whichever method you use to start the server, if it fails to start up correctly, check the log file to see if you can find out why. Log files are located in the data directory (typically ``/usr/local/mysql/data'` for a binary distribution, ``/usr/local/var'` for a source distribution, and ``\mysql\data\mysql.err'` on Windows). Look in the data directory for files with names of the form ``host_name.err'` and ``host_name.log'` where `host_name` is the name of your server host. Then check the last few lines of these files:

    shell> tail host_name.err
    shell> tail host_name.log

Look for something like the following in the log file:

    000729 14:50:10  bdb:  Recovery function for LSN 1 27595 failed
    000729 14:50:10  bdb:  warning: ./test/t1.db: No such file or directory
    000729 14:50:10  Can't init databases

This means that you didn't start `mysqld` with `--bdb-no-recover` and Berkeley DB found something wrong with its log files when it tried to recover your databases. To be able to continue, you should move away the old Berkeley DB log file from the database directory to some other place, where you can later examine it. The log files are named ``log.0000000001'`, where the number will increase over time.

If you are running `mysqld` with BDB table support and `mysqld` core dumps at start this could be because of some problems with the BDB recover log. In this case you can try starting `mysqld` with `--bdb-no-recover`. If this helps, then you should remove all ``log.*'` files from the data directory and try starting `mysqld` again.

If you get the following error, it means that some other program (or another `mysqld` server) is already using the TCP/IP port or socket `mysqld` is trying to use:

    Can't start server: Bind on TCP/IP port: Address already in use
      or
    Can't start server : Bind on unix socket...

Use `ps` to make sure that you don't have another `mysqld` server running. If you can't find another server running, you can try to execute the command `telnet your-host-name tcp-ip-port-number` and press Enter a couple of times. If you don't get an error message like `telnet: Unable to connect to remote host: Connection refused`, something is using the TCP/IP port `mysqld` is trying to use. See section 2.4.1 Problems Running `mysql_install_db` and section 4.1.4 Running Multiple MySQL Servers on the Same Machine.

If `mysqld` is currently running, you can find out what path settings it is using by executing this command:

    shell> mysqladmin variables

or

    shell> mysqladmin -h 'your-host-name' variables

If you get `Errcode 13`, which means `Permission denied`, when starting `mysqld` this means that you didn't have the right to read/create files in the MySQL database or log directory. In this case you should either start `mysqld` as the root user or change the permissions for the involved files and directories so that you have the right to use them.

If `safe_mysqld` starts the server but you can't connect to it, you should make sure you have an entry in ``/etc/hosts'` that looks like this:

    127.0.0.1       localhost

This problem occurs only on systems that don't have a working thread library and for which MySQL must be configured to use MIT-pthreads.

If you can't get `mysqld` to start you can try to make a trace file to find the problem. See section E.1.2 Creating Trace Files.

If you are using InnoDB tables, refer to the InnoDB-specific startup options. See section 7.5.2 InnoDB Startup Options.

If you are using BDB (Berkeley DB) tables, you should familiarise yourself with the different BDB-specific startup options. See section 7.6.3 `BDB` startup options.

### 2.4.3 Starting and Stopping MySQL Automatically

The `mysql.server` and `safe_mysqld` scripts can be used to start the server automatically at system startup time. `mysql.server` can also be used to stop the server.

The `mysql.server` script can be used to start or stop the server by invoking it with `start` or `stop` arguments:

    shell> mysql.server start
    shell> mysql.server stop

`mysql.server` can be found in the ``share/mysql'` directory under the MySQL installation directory or in the ``support-files'` directory of the MySQL source tree.

Before `mysql.server` starts the server, it changes the directory to the MySQL installation directory, then invokes `safe_mysqld`. You might need to edit `mysql.server` if you have a binary distribution that you've installed in a non-standard location. Modify it to `cd` into the proper directory before it runs `safe_mysqld`. If you want the server to run as some specific user, add an appropriate `user` line to the ``/etc/my.cnf'` file, as shown later in this section.

`mysql.server stop` brings down the server by sending a signal to it. You can also take down the server manually by executing `mysqladmin shutdown`.

You need to add these start and stop commands to the appropriate places in your ``/etc/rc*'` files when you want to start up MySQL automatically on your server.

On most current Linux distributions, it is sufficient to copy the file `mysql.server` into the ``/etc/init.d'` directory (or ``/etc/rc.d/init.d'` on older Red Hat systems). Afterwards, run the the following command to enable the startup of MySQL on system bootup:

    shell> chkconfig --add mysql.server

As an alternative to the above, some operating systems also use ``/etc/rc.local'` or ``/etc/init.d/boot.local'` to start additional services on bootup. To start up MySQL using this method, you could append something like the following to it:

    /bin/sh -c 'cd /usr/local/mysql ; ./bin/safe_mysqld --user=mysql &'

You can also add options for `mysql.server` in a global ``/etc/my.cnf'` file. A typical ``/etc/my.cnf'` file might look like this:

    [mysqld]
    datadir=/usr/local/mysql/var
    socket=/var/tmp/mysql.sock
    port=3306
    user=mysql

    [mysql_server]
    basedir=/usr/local/mysql

The `mysql.server` script understands the following options: `datadir`, `basedir`, and `pid-file`.

The following table shows which option groups each of the startup scripts read from option files:

Script | Option groups
`mysqld` | `mysqld` and `server`
`mysql.server` | `mysql.server`, `mysqld`, and `server`
`safe_mysqld` | `mysql.server`, `mysqld`, and `server`

See section 4.1.2 ``my.cnf'` Option Files.

## 2.5 Upgrading/Downgrading MySQL

You can always move the MySQL form and datafiles between different versions on the same architecture as long as you have the same base version of MySQL. The current base version is 3. If you change the character set when running MySQL (which may also change the sort order), you must run `myisamchk -r -q --set-character-set=charset` on all tables. Otherwise, your indexes may not be ordered correctly.

If you are afraid of new versions, you can always rename your old `mysqld` to something like `mysqld-old-version-number`. If your new `mysqld` then does something unexpected, you can simply shut it down and restart with your old `mysqld`!

When you do an upgrade you should also back up your old databases, of course.

If after an upgrade, you experience problems with recompiled client programs, like `Commands out of sync` or unexpected core dumps, you probably have used an old header or library file when compiling your programs. In this case you should check the date for your ``mysql.h'` file and ``libmysqlclient.a'` library to verify that they are from the new MySQL distribution. If not, please recompile your programs!

If you get some problems that the new `mysqld` server doesn't want to start or that you can't connect without a password, check that you don't have some old ``my.cnf'` file from your old installation! You can check this with: `program-name --print-defaults`. If this outputs anything other than the program name, you have an active ``my.cnf'` file that will affect things!

It is a good idea to rebuild and reinstall the `Msql-Mysql-modules` distribution whenever you install a new release of MySQL, particularly if you notice symptoms such as all your `DBI` scripts dumping core after you upgrade MySQL.

### 2.5.1 Upgrading From Version 4.0 to to Version 4.1

In general what you have to do when upgrading to 4.1 from an earlier MySQL version:

- Run the `mysql_fix_privilege_tables` to get the new password field that is needed for secure handling of passwords.

The following is a more complete lists tell what you have to watch out for when upgrading to version 4.1;

- Functions that return a DATE, DATETIME or TIME result is now fixed up when returned to the client.

      mysql> SELECT cast("2001-1-1" as DATE)
          -> '2001-01-01'

- All column and tables now have a character set, which shows up in `SHOW CREATE TABLE` and `mysqldump`. (MySQL 4.0.6 and above can read the new dump files, but not previous MySQL versions).
- Timestamp is now returned as string of type `'YYYY-MM-DD HH:MM:DD'`. If you want to have this as a number you should add +0 to the timestamp column. Different timestamp lengths are not supported.

### 2.5.2 Upgrading From Version 3.23 to Version 4.0

In general what you have to do when upgrading to 4.0 from an earlier MySQL version:

- Run the `mysql_fix_privilege_tables` to add new privileges and features to the MySQL privilege tables.
- Edit any MySQL startup scripts or configure files to not use any of the deprecated options listed below.
- Convert your old ISAM files to MyISAM files with the command: `mysql_convert_table_format database`. Note that this should only be run if all tables in the given database is ISAM or MyISAM tables. If this is not the case you should run `ALTER TABLE table_name TYPE=MyISAM` on all ISAM tables.
- Ensure that you don't have any MySQL clients that uses shared libraries (like the perl Msql-Mysql-modules). If you have, you should recompile them as structures used in ``libmysqlclient.so'` have changed.

MySQL 4.0 will work even if you don't do the above, but you will not be able to use the new security privileges that MySQL 4.0 and you may run into problems when upgrading later to MySQL 4.1 or newer. The ISAM file format still works in MySQL 4.0 but it's deprecated and will be disabled in MySQL 5.0.

Old clients should work with a Version 4.0 server without any problems.

Even if you do the above, you can still downgrade to MySQL 3.23.52 or newer if you run into problems with the MySQL 4.0 series. In this case you have to do a `mysqldump` of any tables using a full-text index and restore these in 3.23 (because 4.0 uses a new format for full-text index).

The following is a more complete lists tell what you have to watch out for when upgrading to version 4.0;

- MySQL 4.0 has a lot of new privileges in the `mysql.user` table. See section 4.3.1 `GRANT` and `REVOKE` Syntax. To get these new privileges to work, one must run the `mysql_fix_privilege_tables` script. Until this script is run all users have the `SHOW DATABASES`, `CREATE TEMPORARY TABLES`, and `LOCK TABLES` privileges. `SUPER` and `EXECUTE` privileges take their value from `PROCESS`. `REPLICATION SLAVE` and `REPLICATION CLIENT` take their values from `FILE`. If you have any scripts that creates new users, you may want to change them to use the new privileges. If you are not using `GRANT` commands in the scripts, this is a good time to change your scripts. In version 4.0.2 the option `--safe-show-database` is deprecated (and no longer does anything). See section 4.2.3 Startup Options for `mysqld` Concerning Security. If you get access denied errors for new users in version 4.0.2, you should check if you need some of the new grants that you didn't need before. In particular, you will need `REPLICATION SLAVE` (instead of `FILE`) for new slaves.
- The startup parameters `myisam_max_extra_sort_file_size` and `myisam_max_extra_sort_file_size` are now given in bytes (was megabytes before 4.0.3). External system locking of MyISAM/ISAM files is now turned off by default. One can turn this on by doing `--external-locking`. (For most users this is never needed).
- The following startup variables/options have been renamed:

  From | to.
  `myisam_bulk_insert_tree_size` | `bulk_insert_buffer_size`
  `query_cache_startup_type` | `query_cache_type`
  `record_buffer` | `read_buffer_size`
  `record_rnd_buffer` | `read_rnd_buffer_size`
  `sort_buffer` | `sort_buffer_size`
  `warnings` | `log-warnings`

  The startup options `record_buffer`, `sort_buffer` and `warnings` will still work in MySQL 4.0 but are deprecated.
- The following SQL variables have changed name.

  From | To.
  `SQL_BIG_TABLES` | `BIG_TABLES`
  `SQL_LOW_PRIORITY_UPDATES` | `LOW_PRIORITY_UPDATES`
  `SQL_MAX_JOIN_SIZE` | `MAX_JOIN_SIZE`
  `SQL_QUERY_CACHE_TYPE` | `QUERY_CACHE_TYPE`

  The old names still work in MySQL 4.0 but are deprecated.
- You have to use `SET GLOBAL SQL_SLAVE_SKIP_COUNTER=#` instead of `SET SQL_SLAVE_SKIP_COUNTER=#`.
- Renamed mysqld startup options `--skip-locking` to `--skip-external-locking` and `--enable-locking` to `--external-locking`.
- `SHOW MASTER STATUS` now returns an empty set if binary log is not enabled.
- `SHOW SLAVE STATUS` now returns an empty set if slave is not initialised.
- mysqld now has the option `--temp-pool` enabled by default as this gives better performance with some OS (Most notable Linux).
- `DOUBLE` and `FLOAT` columns now honour the `UNSIGNED` flag on storage (before, `UNSIGNED` was ignored for these columns).
- `ORDER BY column DESC` now always sorts `NULL` values first; in 3.23 this was not always consistent.
- `SHOW INDEX` has 2 columns more (`Null` and `Index_type`) than it had in 3.23.
- `CHECK`, `SIGNED`, `LOCALTIME` and `LOCALTIMESTAMP` are now reserved words.
- The result of all bitwise operators `|`, `&`, `<<`, `>>`, and `~` is now unsigned. This may cause problems if you are using them in a context where you want a signed result. See section 6.3.5 Cast Functions.
- Note: when you use subtraction between integer values where one is of type `UNSIGNED`, the result will be unsigned! In other words, before upgrading to MySQL 4.0, you should check your application for cases where you are subtracting a value from an unsigned entity and want a negative answer or subtracting an unsigned value from an integer column. You can disable this behaviour by using the `--sql-mode=NO_UNSIGNED_SUBTRACTION` option when starting `mysqld`. See section 6.3.5 Cast Functions.
- To use `MATCH ... AGAINST (... IN BOOLEAN MODE)` with your tables, you need to rebuild them with `REPAIR TABLE table_name USE_FRM`.
- `LOCATE()` and `INSTR()` are case-sensitive if one of the arguments is a binary string. Otherwise they are case-insensitive.
- `STRCMP()` now uses the current character set when doing comparisons, which means that the default comparison behaviour now is case-insensitive.
- `HEX(string)` now returns the characters in string converted to hexadecimal. If you want to convert a number to hexadecimal, you should ensure that you call `HEX()` with a numeric argument.
- In 3.23, `INSERT INTO ... SELECT` always had `IGNORE` enabled. In 4.0.1, MySQL will stop (and possibly roll back) in case of an error if you don't specify `IGNORE`.
- ``safe_mysqld'` is renamed to ``mysqld_safe'`. For some time we will in our binary distributions include `safe_mysqld` as a symlink to `mysqld_safe`.
- The old C API functions `mysql_drop_db`, `mysql_create_db`, and `mysql_connect` are not supported anymore, unless you compile MySQL with `CFLAGS=-DUSE_OLD_FUNCTIONS`. Instead of doing this, it is preferable to change the client to use the new 4.0 API.
- In the `MYSQL_FIELD` structure, `length` and `max_length` have changed from `unsigned int` to `unsigned long`. This should not cause any problems, except that they may generate warning messages when used as arguments in the `printf()` class of functions.
- You should use `TRUNCATE TABLE` when you want to delete all rows from a table and you don't care how many rows were deleted. (Because `TRUNCATE TABLE` is faster than `DELETE FROM table_name`).
- You will get an error if you have an active `LOCK TABLES` or transaction when trying to execute `TRUNCATE TABLE` or `DROP DATABASE`.
- You should use integers to store values in BIGINT columns (instead of using strings, as you did in MySQL 3.23). Using strings will still work, but using integers is more efficient.
- Format of `SHOW OPEN TABLE` has changed.
- Multi-threaded clients should use `mysql_thread_init()` and `mysql_thread_end()`. See section 8.4.8 How to Make a Threaded Client.
- If you want to recompile the Perl DBD::mysql module, you must get Msql-Mysql-modules version 1.2218 or newer because the older DBD modules used the deprecated `drop_db()` call.
- `RAND(seed)` returns a different random number series in 4.0 than in 3.23; this was done to further differentiate `RAND(seed)` and `RAND(seed+1)`.
- The default type returned by `IFNULL(A,B)` is now set to be the more 'general' of the types of `A` and `B`. (The order is `STRING`, `REAL` or `INTEGER`).

### 2.5.3 Upgrading From Version 3.22 to Version 3.23

MySQL Version 3.23 supports tables of the new `MyISAM` type and the old `ISAM` type. You don't have to convert your old tables to use these with Version 3.23. By default, all new tables will be created with type `MyISAM` (unless you start `mysqld` with the `--default-table-type=isam` option). You can change an `ISAM` table to a `MyISAM` table with `ALTER TABLE table_name TYPE=MyISAM` or the Perl script `mysql_convert_table_format`.

Version 3.22 and 3.21 clients will work without any problems with a Version 3.23 server.

The following list tells what you have to watch out for when upgrading to Version 3.23:

- All tables that use the `tis620` character set must be fixed with `myisamchk -r` or `REPAIR TABLE`.
- If you do a `DROP DATABASE` on a symbolic linked database, both the link and the original database are deleted. (This didn't happen in 3.22 because configure didn't detect the `readlink` system call.)
- `OPTIMIZE TABLE` now works only for `MyISAM` tables. For other table types, you can use `ALTER TABLE` to optimise the table. During `OPTIMIZE TABLE` the table is now locked from other threads.
- The MySQL client `mysql` is now by default started with the option `--no-named-commands (-g)`. This option can be disabled with `--enable-named-commands (-G)`. This may cause incompatibility problems in some cases@-for example, in SQL scripts that use named commands without a semicolon! Long format commands still work from the first line.
- Date functions that work on parts of dates (like `MONTH()`) will now return 0 for `0000-00-00` dates. (MySQL 3.22 returned `NULL`.)
- If you are using the `german` character sort order, you must repair all your tables with `isamchk -r`, as we have made some changes in the sort order!
- The default return type of `IF` will now depend on both arguments and not only the first argument.
- `AUTO_INCREMENT` will not work with negative numbers. The reason for this is that negative numbers caused problems when wrapping from -1 to 0. `AUTO_INCREMENT` for MyISAM tables is now handled at a lower level and is much faster than before. For MyISAM tables old numbers are also not reused anymore, even if you delete some rows from the table.
- `CASE`, `DELAYED`, `ELSE`, `END`, `FULLTEXT`, `INNER`, `RIGHT`, `THEN`, and `WHEN` are now reserved words.
- `FLOAT(X)` is now a true floating-point type and not a value with a fixed number of decimals.
- When declaring `DECIMAL(length,dec)` the length argument no longer includes a place for the sign or the decimal point.
- A `TIME` string must now be of one of the following formats: `[[[DAYS] [H]H:]MM:]SS[.fraction]` or `[[[[[H]H]H]H]MM]SS[.fraction]`.
- `LIKE` now compares strings using the same character comparison rules as `=`. If you require the old behaviour, you can compile MySQL with the `CXXFLAGS=-DLIKE_CMP_TOUPPER` flag.
- `REGEXP` is now case-insensitive for normal (not binary) strings.
- When you check/repair tables you should use `CHECK TABLE` or `myisamchk` for `MyISAM` tables (``.MYI'`) and `isamchk` for ISAM (``.ISM'`) tables.
- If you want your `mysqldump` files to be compatible between MySQL Version 3.22 and Version 3.23, you should not use the `--opt` or `--all` option to `mysqldump`.
- Check all your calls to `DATE_FORMAT()` to make sure there is a ``%'` before each format character. (MySQL Version 3.22 and later already allowed this syntax.)
- `mysql_fetch_fields_direct` is now a function (it was a macro) and it returns a pointer to a `MYSQL_FIELD` instead of a `MYSQL_FIELD`.
- `mysql_num_fields()` can no longer be used on a `MYSQL*` object (it's now a function that takes `MYSQL_RES*` as an argument, so you should use `mysql_field_count()` instead).
- In MySQL Version 3.22, the output of `SELECT DISTINCT ...` was almost always sorted. In Version 3.23, you must use `GROUP BY` or `ORDER BY` to obtain sorted output.
- `SUM()` now returns `NULL`, instead of 0, if there are no matching rows. This is according to ANSI SQL.
- An `AND` or `OR` with `NULL` values will now return `NULL` instead of 0. This mostly affects queries that use `NOT` on an `AND/OR` expression as `NOT NULL` = `NULL`. `LPAD()` and `RPAD()` will shorten the result string if it's longer than the length argument.

### 2.5.4 Upgrading from Version 3.21 to Version 3.22

Nothing that affects compatibility has changed between versions 3.21 and 3.22. The only pitfall is that new tables that are created with `DATE` type columns will use the new way to store the date. You can't access these new fields from an old version of `mysqld`.

After installing MySQL Version 3.22, you should start the new server and then run the `mysql_fix_privilege_tables` script. This will add the new privileges that you need to use the `GRANT` command. If you forget this, you will get `Access denied` when you try to use `ALTER TABLE`, `CREATE INDEX`, or `DROP INDEX`. If your MySQL root user requires a password, you should give this as an argument to `mysql_fix_privilege_tables`.

The C API interface to `mysql_real_connect()` has changed. If you have an old client program that calls this function, you must place a `0` for the new `db` argument (or recode the client to send the `db` element for faster connections). You must also call `mysql_init()` before calling `mysql_real_connect()`! This change was done to allow the new `mysql_options()` function to save options in the `MYSQL` handler structure.

The `mysqld` variable `key_buffer` has changed names to `key_buffer_size`, but you can still use the old name in your startup files.

### 2.5.5 Upgrading from Version 3.20 to Version 3.21

If you are running a version older than Version 3.20.28 and want to switch to Version 3.21, you need to do the following:

You can start the `mysqld` Version 3.21 server with `safe_mysqld --old-protocol` to use it with clients from a Version 3.20 distribution. In this case, the new client function `mysql_errno()` will not return any server error, only `CR_UNKNOWN_ERROR` (but it works for client errors), and the server uses the old `password()` checking rather than the new one.

If you are not using the `--old-protocol` option to `mysqld`, you will need to make the following changes:

- All client code must be recompiled. If you are using ODBC, you must get the new `MyODBC` 2.x driver.
- The script `scripts/add_long_password` must be run to convert the `Password` field in the `mysql.user` table to `CHAR(16)`.
- All passwords must be reassigned in the `mysql.user` table (to get 62-bit rather than 31-bit passwords).
- The table format hasn't changed, so you don't have to convert any tables.

MySQL Version 3.20.28 and above can handle the new `user` table format without affecting clients. If you have a MySQL version earlier than Version 3.20.28, passwords will no longer work with it if you convert the `user` table. So to be safe, you should first upgrade to at least Version 3.20.28 and then upgrade to Version 3.21.

The new client code works with a 3.20.x `mysqld` server, so if you experience problems with 3.21.x, you can use the old 3.20.x server without having to recompile the clients again.

If you are not using the `--old-protocol` option to `mysqld`, old clients will issue the error message:

    ERROR: Protocol mismatch. Server Version = 10 Client Version = 9

The new Perl `DBI`/`DBD` interface also supports the old `mysqlperl` interface. The only change you have to make if you use `mysqlperl` is to change the arguments to the `connect()` function. The new arguments are: `host`, `database`, `user`, and `password` (the `user` and `password` arguments have changed places). See section 8.2.2 The `DBI` Interface.

The following changes may affect queries in old applications:

- `HAVING` must now be specified before any `ORDER BY` clause.
- The parameters to `LOCATE()` have been swapped.
- There are some new reserved words. The most notable are `DATE`, `TIME`, and `TIMESTAMP`.

### 2.5.6 Upgrading to Another Architecture

If you are using MySQL Version 3.23, you can copy the ``.frm'`, ``.MYI'`, and ``.MYD'` files between different architectures that support the same floating-point format. (MySQL takes care of any byte-swapping issues.)

The MySQL `ISAM` data and index files (``.ISD'` and ``*.ISM'`, respectively) are architecture-dependent and in some cases OS-dependent. If you want to move your applications to another machine that has a different architecture or OS than your current machine, you should not try to move a database by simply copying the files to the other machine. Use `mysqldump` instead.

By default, `mysqldump` will create a file full of SQL statements. You can then transfer the file to the other machine and feed it as input to the `mysql` client.

Try `mysqldump --help` to see what options are available. If you are moving the data to a newer version of MySQL, you should use `mysqldump --opt` with the newer version to get a fast, compact dump.

The easiest (although not the fastest) way to move a database between two machines is to run the following commands on the machine on which the database is located:

    shell> mysqladmin -h 'other hostname' create db_name
    shell> mysqldump --opt db_name \
            | mysql -h 'other hostname' db_name

If you want to copy a database from a remote machine over a slow network, you can use:

    shell> mysqladmin create db_name
    shell> mysqldump -h 'other hostname' --opt --compress db_name \
            | mysql db_name

You can also store the result in a file, then transfer the file to the target machine and load the file into the database there. For example, you can dump a database to a file on the source machine like this:

    shell> mysqldump --quick db_name | gzip > db_name.contents.gz

(The file created in this example is compressed.) Transfer the file containing the database contents to the target machine and run these commands there:

    shell> mysqladmin create db_name
    shell> gunzip < db_name.contents.gz | mysql db_name

You can also use `mysqldump` and `mysqlimport` to accomplish the database transfer. For big tables, this is much faster than simply using `mysqldump`. In the following commands, `DUMPDIR` represents the full pathname of the directory you use to store the output from `mysqldump`.

First, create the directory for the output files and dump the database:

    shell> mkdir DUMPDIR
    shell> mysqldump --tab=DUMPDIR db_name

Then transfer the files in the `DUMPDIR` directory to some corresponding directory on the target machine and load the files into MySQL there:

    shell> mysqladmin create db_name           # create database
    shell> cat DUMPDIR/*.sql | mysql db_name   # create tables in database
    shell> mysqlimport db_name DUMPDIR/*.txt   # load data into tables

Also, don't forget to copy the `mysql` database because that's where the grant tables (`user`, `db`, `host`) are stored. You may have to run commands as the MySQL `root` user on the new machine until you have the `mysql` database in place.

After you import the `mysql` database on the new machine, execute `mysqladmin flush-privileges` so that the server reloads the grant table information.

## 2.6 Operating System Specific Notes

### 2.6.1 Linux Notes (All Linux Versions)

The following notes regarding `glibc` apply only to the situation when you build MySQL yourself. If you are running Linux on an x86 machine, in most cases it is much better for you to just use our binary. We link our binaries against the best patched version of `glibc` we can come up with and with the best compiler options, in an attempt to make it suitable for a high-load server. So if you read the following text, and are in doubt about what you should do, try our binary first to see if it meets your needs, and worry about your own build only after you have discovered that our binary is not good enough. In that case, we would appreciate a note about it, so we can build a better binary next time. For a typical user, even for setups with a lot of concurrent connections and/or tables exceeding the 2G limit, our binary in most cases is the best choice.

MySQL uses LinuxThreads on Linux. If you are using an old Linux version that doesn't have `glibc2`, you must install LinuxThreads before trying to compile MySQL. You can get LinuxThreads at http://www.mysql.com/Downloads/Linux/.

Note: we have seen some strange problems with Linux 2.2.14 and MySQL on SMP systems. If you have a SMP system, we recommend you upgrade to Linux 2.4 as soon as possible! Your system will be faster and more stable by doing this!

Note that `glibc` versions before and including Version 2.1.1 have a fatal bug in `pthread_mutex_timedwait` handling, which is used when you do `INSERT DELAYED`. We recommend that you not use `INSERT DELAYED` before upgrading glibc.

If you plan to have 1000+ concurrent connections, you will need to make some changes to LinuxThreads, recompile it, and relink MySQL against the new ``libpthread.a'`. Increase `PTHREAD_THREADS_MAX` in ``sysdeps/unix/sysv/linux/bits/local_lim.h'` to 4096 and decrease `STACK_SIZE` in ``linuxthreads/internals.h'` to 256 KB. The paths are relative to the root of `glibc` Note that MySQL will not be stable with around 600-1000 connections if `STACK_SIZE` is the default of 2 MB.

If MySQL can't open enough files, or connections, it may be that you haven't configured Linux to handle enough files.

In Linux 2.2 and onward, you can check the number of allocated file handles by doing:

    cat /proc/sys/fs/file-max
    cat /proc/sys/fs/dquot-max
    cat /proc/sys/fs/super-max

If you have more than 16 MB of memory, you should add something like the following to your init scripts (e.g. ``/etc/init.d/boot.local'` on SuSE Linux):

    echo 65536 > /proc/sys/fs/file-max
    echo 8192 > /proc/sys/fs/dquot-max
    echo 1024 > /proc/sys/fs/super-max

You can also run the preceding commands from the command-line as root, but these settings will be lost the next time your computer reboots.

Alternatively, you can set these parameters on bootup by using the `sysctl` tool, which is used by many Linux distributions (SuSE has added it as well, beginning with SuSE Linux 8.0). Just put the following values into a file named ``/etc/sysctl.conf'`:

    # Increase some values for MySQL
    fs.file-max = 65536
    fs.dquot-max = 8192
    fs.super-max = 1024

You should also add the following to ``/etc/my.cnf'`:

    [safe_mysqld]
    open-files-limit=8192

This should allow MySQL to create up to 8192 connections + files.

The `STACK_SIZE` constant in LinuxThreads controls the spacing of thread stacks in the address space. It needs to be large enough so that there will be plenty of room for the stack of each individual thread, but small enough to keep the stack of some threads from running into the global `mysqld` data. Unfortunately, the Linux implementation of `mmap()`, as we have experimentally discovered, will successfully unmap an already mapped region if you ask it to map out an address already in use, zeroing out the data on the entire page, instead of returning an error. So, the safety of `mysqld` or any other threaded application depends on the "gentleman" behaviour of the code that creates threads. The user must take measures to make sure the number of running threads at any time is sufficiently low for thread stacks to stay away from the global heap. With `mysqld`, you should enforce this "gentleman" behaviour by setting a reasonable value for the `max_connections` variable.

If you build MySQL yourself and do not want to mess with patching LinuxThreads, you should set `max_connections` to a value no higher than 500. It should be even less if you have a large key buffer, large heap tables, or some other things that make `mysqld` allocate a lot of memory, or if you are running a 2.2 kernel with a 2G patch. If you are using our binary or RPM version 3.23.25 or later, you can safely set `max_connections` at 1500, assuming no large key buffer or heap tables with lots of data. The more you reduce `STACK_SIZE` in LinuxThreads the more threads you can safely create. We recommend the values between 128K and 256K.

If you use a lot of concurrent connections, you may suffer from a "feature" in the 2.2 kernel that penalises a process for forking or cloning a child in an attempt to prevent a fork bomb attack. This will cause MySQL not to scale well as you increase the number of concurrent clients. On single-CPU systems, we have seen this manifested in a very slow thread creation, which means it may take a long time to connect to MySQL (as long as 1 minute), and it may take just as long to shut it down. On multiple-CPU systems, we have observed a gradual drop in query speed as the number of clients increases. In the process of trying to find a solution, we have received a kernel patch from one of our users, who claimed it made a lot of difference for his site. The patch is available at http://www.mysql.com/Downloads/Patches/linux-fork.patch. We have now done rather extensive testing of this patch on both development and production systems. It has significantly improved `MySQL` performance without causing any problems and we now recommend it to our users who are still running high-load servers on 2.2 kernels. This issue has been fixed in the 2.4 kernel, so if you are not satisfied with the current performance of your system, rather than patching your 2.2 kernel, it might be easier to just upgrade to 2.4, which will also give you a nice SMP boost in addition to fixing this fairness bug.

We have tested MySQL on the 2.4 kernel on a 2-CPU machine and found MySQL scales much better@-there was virtually no slowdown on queries throughput all the way up to 1000 clients, and the MySQL scaling factor (computed as the ratio of maximum throughput to the throughput with one client) was 180%. We have observed similar results on a 4-CPU system@-virtually no slowdown as the number of clients was increased up to 1000, and 300% scaling factor. So for a high-load SMP server we would definitely recommend the 2.4 kernel at this point. We have discovered that it is essential to run `mysqld` process with the highest possible priority on the 2.4 kernel to achieve maximum performance. This can be done by adding `renice -20 $$` command to `safe_mysqld`. In our testing on a 4-CPU machine, increasing the priority gave 60% increase in throughput with 400 clients.

We are currently also trying to collect more info on how well `MySQL` performs on 2.4 kernel on 4-way and 8-way systems. If you have access such a system and have done some benchmarks, please send a mail to docs@mysql.com with the results - we will include them in the manual.

There is another issue that greatly hurts MySQL performance, especially on SMP systems. The implementation of mutex in LinuxThreads in `glibc-2.1` is very bad for programs with many threads that only hold the mutex for a short time. On an SMP system, ironic as it is, if you link MySQL against unmodified `LinuxThreads`, removing processors from the machine improves MySQL performance in many cases. We have made a patch available for `glibc 2.1.3` to correct this behaviour (http://www.mysql.com/Downloads/Linux/linuxthreads-2.1-patch).

With `glibc-2.2.2` MySQL version 3.23.36 will use the adaptive mutex, which is much better than even the patched one in `glibc-2.1.3`. Be warned, however, that under some conditions, the current mutex code in `glibc-2.2.2` overspins, which hurts MySQL performance. The chance of this condition can be reduced by renicing `mysqld` process to the highest priority. We have also been able to correct the overspin behaviour with a patch, available at http://www.mysql.com/Downloads/Linux/linuxthreads-2.2.2.patch. It combines the correction of overspin, maximum number of threads, and stack spacing all in one. You will need to apply it in the `linuxthreads` directory with `patch -p0 </tmp/linuxthreads-2.2.2.patch`. We hope it will be included in some form in to the future releases of `glibc-2.2`. In any case, if you link against `glibc-2.2.2` you still need to correct `STACK_SIZE` and `PTHREAD_THREADS_MAX`. We hope that the defaults will be corrected to some more acceptable values for high-load MySQL setup in the future, so that your own build can be reduced to `./configure; make; make install`.

We recommend that you use the above patches to build a special static version of `libpthread.a` and use it only for statically linking against `MySQL`. We know that the patches are safe for `MySQL` and significantly improve its performance, but we cannot say anything about other applications. If you link other applications against the patched version of the library, or build a patched shared version and install it on your system, you are doing it at your own risk with regard to other applications that depend on `LinuxThreads`.

If you experience any strange problems during the installation of MySQL, or with some common utilties hanging, it is very likely that they are either library or compiler related. If this is the case, using our binary will resolve them.

One known problem with the binary distribution is that with older Linux systems that use `libc` (like RedHat 4.x or Slackware), you will get some non-fatal problems with hostname resolution. See section 2.6.1.1 Linux Notes for Binary Distributions.

When using LinuxThreads you will see a minimum of three processes running. These are in fact threads. There will be one thread for the LinuxThreads manager, one thread to handle connections, and one thread to handle alarms and signals.

Note that the Linux kernel and the LinuxThread library can by default only have 1024 threads. This means that you can only have up to 1021 connections to MySQL on an unpatched system. The page http://www.volano.com/linuxnotes.html contains information how to go around this limit.

If you see a dead `mysqld` daemon process with `ps`, this usually means that you have found a bug in MySQL or you have a corrupted table. See section A.4.1 What To Do If MySQL Keeps Crashing.

To get a core dump on Linux if `mysqld` dies with a `SIGSEGV` signal, you can start `mysqld` with the `--core-file` option. Note that you also probably need to raise the `core file size` by adding `ulimit -c 1000000` to `safe_mysqld` or starting `safe_mysqld` with `--core-file-size=1000000`. See section 4.7.2 `safe_mysqld`, The Wrapper Around `mysqld`.

If you are linking your own MySQL client and get the error:

    ld.so.1: ./my: fatal: libmysqlclient.so.4:
    open failed: No such file or directory

When executing them, the problem can be avoided by one of the following methods:

- Link the client with the following flag (instead of `-Lpath`): `-Wl,r/path-libmysqlclient.so`.
- Copy `libmysqclient.so` to ``/usr/lib'`.
- Add the pathname of the directory where ``libmysqlclient.so'` is located to the `LD_RUN_PATH` environment variable before running your client.

If you are using the Fujitsu compiler `(fcc / FCC)` you will have some problems compiling MySQL because the Linux header files are very `gcc` oriented.

The following `configure` line should work with `fcc/FCC`:

    CC=fcc CFLAGS="-O -K fast -K lib -K omitfp -Kpreex -D_GNU_SOURCE \
    -DCONST=const -DNO_STRTOLL_PROTO" CXX=FCC CXXFLAGS="-O -K fast -K lib \
    -K omitfp -K preex --no_exceptions --no_rtti -D_GNU_SOURCE -DCONST=const \
    -Dalloca=__builtin_alloca -DNO_STRTOLL_PROTO \
    '-D_EXTERN_INLINE=static __inline'" ./configure --prefix=/usr/local/mysql \
    --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared \
    --with-low-memory

#### 2.6.1.1 Linux Notes for Binary Distributions

MySQL needs at least Linux Version 2.0.

Warning: We have reports from some MySQL users that they have got serious stability problems with MySQL with Linux kernel 2.2.14. If you are using this kernel you should upgrade to 2.2.19 (or newer) or to a 2.4 kernel. If you have a multi-cpu box, then you should seriously consider using 2.4 as this will give you a significant speed boost.

The binary release is linked with `-static`, which means you do not normally need to worry about which version of the system libraries you have. You need not install LinuxThreads, either. A program linked with `-static` is slightly bigger than a dynamically linked program but also slightly faster (3-5%). One problem, however, is that you can't use user-definable functions (UDFs) with a statically linked program. If you are going to write or use UDF functions (this is something only for C or C++ programmers), you must compile MySQL yourself, using dynamic linking.

If you are using a `libc`-based system (instead of a `glibc2` system), you will probably get some problems with hostname resolving and `getpwnam()` with the binary release. (This is because `glibc` unfortunately depends on some external libraries to resolve hostnames and `getpwent()`, even when compiled with `-static`). In this case you probably get the following error message when you run `mysql_install_db`:

    Sorry, the host 'xxxx' could not be looked up

or the following error when you try to run `mysqld` with the `--user` option:

    getpwnam: No such file or directory

You can solve this problem in one of the following ways:

- Get a MySQL source distribution (an RPM or the `tar.gz` distribution) and install this instead.
- Execute `mysql_install_db --force`; this will not execute the `resolveip` test in `mysql_install_db`. The downside is that you can't use host names in the grant tables; you must use IP numbers instead (except for `localhost`). If you are using an old MySQL release that doesn't support `--force`, you have to remove the `resolveip` test in `mysql_install` with an editor.
- Start `mysqld` with `su` instead of using `--user`.

The Linux-Intel binary and RPM releases of MySQL are configured for the highest possible speed. We are always trying to use the fastest stable compiler available.

MySQL Perl support requires Version Perl 5.004_03 or newer.

On some Linux 2.2 versions, you may get the error `Resource temporarily unavailable` when you do a lot of new connections to a `mysqld` server over TCP/IP.

The problem is that Linux has a delay between when you close a TCP/IP socket and until this is actually freed by the system. As there is only room for a finite number of TCP/IP slots, you will get the above error if you try to do too many new TCP/IP connections during a small time, like when you run the MySQL ``test-connect'` benchmark over TCP/IP.

We have mailed about this problem a couple of times to different Linux mailing lists but have never been able to resolve this properly.

The only known 'fix' to this problem is to use persistent connections in your clients or use sockets, if you are running the database server and clients on the same machine. We hope that the `Linux 2.4` kernel will fix this problem in the future.

#### 2.6.1.2 Linux x86 Notes

MySQL requires `libc` Version 5.4.12 or newer. It's known to work with `libc` 5.4.46. `glibc` Version 2.0.6 and later should also work. There have been some problems with the `glibc` RPMs from RedHat, so if you have problems, check whether there are any updates! The `glibc` 2.0.7-19 and 2.0.7-29 RPMs are known to work.

If you are using gcc 3.0 and above to compile MySQL, you must install the `libstdc++v3` library before compiling MySQL; if you don't do this you will get an error about a missing `__cxa_pure_virtual` symbol during linking!

On some older Linux distributions, `configure` may produce an error like this:

    Syntax error in sched.h. Change _P to __P in the /usr/include/sched.h file.
    See the Installation chapter in the Reference Manual.

Just do what the error message says and add an extra underscore to the `_P` macro that has only one underscore, then try again.

You may get some warnings when compiling; those shown here can be ignored:

    mysqld.cc -o objs-thread/mysqld.o
    mysqld.cc: In function `void init_signals()':
    mysqld.cc:315: warning: assignment of negative value `-1' to
    `long unsigned int'
    mysqld.cc: In function `void * signal_hand(void *)':
    mysqld.cc:346: warning: assignment of negative value `-1' to
    `long unsigned int'

In Debian GNU/Linux, if you want MySQL to start automatically when the system boots, do the following:

    shell> cp support-files/mysql.server /etc/init.d/mysql.server
    shell> /usr/sbin/update-rc.d mysql.server defaults 99

`mysql.server` can be found in the ``share/mysql'` directory under the MySQL installation directory or in the ``support-files'` directory of the MySQL source tree.

If `mysqld` always core dumps when it starts up, the problem may be that you have an old ``/lib/libc.a'`. Try renaming it, then remove ``sql/mysqld'` and do a new `make install` and try again. This problem has been reported on some Slackware installations.

If you get the following error when linking `mysqld`, it means that your ``libg++.a'` is not installed correctly:

    /usr/lib/libc.a(putc.o): In function `_IO_putc':
    putc.o(.text+0x0): multiple definition of `_IO_putc'

You can avoid using ``libg++.a'` by running `configure` like this:

    shell> CXX=gcc ./configure

#### 2.6.1.3 Linux SPARC Notes

In some implementations, `readdir_r()` is broken. The symptom is that `SHOW DATABASES` always returns an empty set. This can be fixed by removing `HAVE_READDIR_R` from ``config.h'` after configuring and before compiling.

Some problems will require patching your Linux installation. The patch can be found at http://www.mysql.com/Downloads/patches/Linux-sparc-2.0.30.diff. This patch is against the Linux distribution ``sparclinux-2.0.30.tar.gz'` that is available at `vger.rutgers.edu` (a version of Linux that was never merged with the official 2.0.30). You must also install LinuxThreads Version 0.6 or newer.

#### 2.6.1.4 Linux Alpha Notes

MySQL Version 3.23.12 is the first MySQL version that is tested on Linux-Alpha. If you plan to use MySQL on Linux-Alpha, you should ensure that you have this version or newer.

We have tested MySQL on Alpha with our benchmarks and test suite, and it appears to work nicely.

We currently build the MySQL binary packages on SuSE Linux 7.0 for AXP, kernel 2.4.4-SMP, Compaq C compiler (V6.2-505) and Compaq C++ compiler (V6.3-006) on a Compaq DS20 machine with an Alpha EV6 processor.

You can find the above compilers at http://www.support.compaq.com/alpha-tools/). By using these compilers, instead of gcc, we get about 9-14% better performance with MySQL.

Note that until MySQL version 3.23.52 and 4.0.2 we optimised the binary for the current CPU only (by using the `-fast` compile option); this meant that you could only use our binaries if you had an Alpha EV6 processor.

Starting with all following releases we added the `-arch generic` flag to our compile options, which makes sure the binary runs on all Alpha processors. We also compile statically to avoid library problems.

    CC=ccc CFLAGS="-fast -arch generic" CXX=cxx \
    CXXFLAGS="-fast -arch generic -noexceptions -nortti" \
    ./configure --prefix=/usr/local/mysql --disable-shared \
    --with-extra-charsets=complex --enable-thread-safe-client \
    --with-mysqld-ldflags=-non_shared --with-client-ldflags=-non_shared

If you want to use egcs the following configure line worked for us:

    CFLAGS="-O3 -fomit-frame-pointer" CXX=gcc \
    CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors \
    -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql \
    --disable-shared

Some known problems when running MySQL on Linux-Alpha:

- Debugging threaded applications like MySQL will not work with `gdb 4.18`. You should download and use gdb 5.1 instead!
- If you try linking `mysqld` statically when using `gcc`, the resulting image will core dump at start. In other words, don't use `--with-mysqld-ldflags=-all-static` with `gcc`.

#### 2.6.1.5 Linux PowerPC Notes

MySQL should work on MkLinux with the newest `glibc` package (tested with `glibc` 2.0.7).

#### 2.6.1.6 Linux MIPS Notes

To get MySQL to work on Qube2, (Linux Mips), you need the newest `glibc` libraries (`glibc-2.0.7-29C2` is known to work). You must also use the `egcs` C++ compiler (`egcs-1.0.2-9`, `gcc 2.95.2` or newer).

#### 2.6.1.7 Linux IA64 Notes

To get MySQL to compile on Linux IA64, we use the following compile line: Using `gcc-2.96`:

    CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc \
    CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors \
    -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql \
    "--with-comment=Official MySQL binary" --with-extra-charsets=complex

On IA64 the MySQL client binaries are using shared libraries. This means that if you install our binary distribution in some other place than ``/usr/local/mysql'` you need to either modify ``/etc/ld.so.conf'` or add the path to the directory where you have ``libmysqlclient.so'` to the `LD_LIBRARY_PATH` environment variable.

See section A.3.1 Problems When Linking with the MySQL Client Library.

### 2.6.2 Windows Notes

This section describes using MySQL on Windows. This information is also provided in the ``README'` file that comes with the MySQL Windows distribution. See section 2.1.2 Installing MySQL on Windows.

#### 2.6.2.1 Starting MySQL on Windows 95, 98 or Me

MySQL uses TCP/IP to connect a client to a server. (This will allow any machine on your network to connect to your MySQL server.) Because of this, you must install TCP/IP on your machine before starting MySQL. You can find TCP/IP on your Windows CD-ROM.

Note that if you are using an old Windows 95 release (for example OSR2), it's likely that you have an old Winsock package; MySQL requires Winsock 2! You can get the newest Winsock from http://www.microsoft.com/. Windows 98 has the new Winsock 2 library, so the above doesn't apply there.

To start the `mysqld` server, you should start an MS-DOS window and type:

    C:\> C:\mysql\bin\mysqld

This will start `mysqld` in the background without a window.

You can kill the MySQL server by executing:

    C:\> C:\mysql\bin\mysqladmin -u root shutdown

This calls the MySQL administation utility as user `root', which is the default Administrator in the MySQL grant system. Please note that the MySQL grant system is wholly independent from any login users under Windows.

Note that Windows 95/98/Me don't support creation of named pipes. So on those platforms, you can only use named pipes to connect to a remote MySQL server running on a Windows NT/2000/XP server host. (The MySQL server must also support named pipes, of course. For example, using `mysqld-opt` under NT/2000/XP will not allow named pipe connections. You should use either `mysqld-nt` or `mysqld-max-nt`.)

If `mysqld` doesn't start, please check the ``\mysql\data\mysql.err'` file to see if the server wrote any message there to indicate the cause of the problem. You can also try to start the server with `mysqld --standalone`; in this case, you may get some useful information on the screen that may help solve the problem.

The last option is to start `mysqld` with `--standalone --debug`. In this case `mysqld` will write a log file ``C:\mysqld.trace'` that should contain the reason why `mysqld` doesn't start. See section E.1.2 Creating Trace Files.

Use `mysqld --help` to display all the options that `mysqld` understands!

#### 2.6.2.2 Starting MySQL on Windows NT, 2000 or XP

To get MySQL to work with TCP/IP on Windows NT 4, you must install service pack 3 (or newer)!

Normally you should install MySQL as a service on Windows NT/2000/XP. In case the server was already running, first stop it using the following command:

    C:\mysql\bin> mysqladmin -u root shutdown

This calls the MySQL administation utility as user ``root`', which is the default `Administrator` in the MySQL grant system. Please note that the MySQL grant system is wholly independent from any login users under Windows.

Now install the server service:

    C:\mysql\bin> mysqld-max-nt --install

If any options are required, they must be specified as ```Start parameters`'' in the Windows `Services` utility before you start the MySQL service.

The `Services` utility (`Windows Service Control Manager`) can be found in the `Windows Control Panel` (under `Administrative Tools` on Windows 2000). It is advisable to close the Services utility while performing the `--install` or `--remove` operations, this prevents some odd errors.

For information about which server binary to run, see section 2.1.2.2 Preparing the Windows MySQL Environment.

Please note that from MySQL version 3.23.44, you have the choice of set up the service as `Manual` instead (if you don't wish the service to be started automatically during the boot process):

    C:\mysql\bin> mysqld-max-nt --install-manual

The service is installed with the name `MySQL`. Once installed, it can be immediately started from the `Services` utility, or by using the command `NET START MySQL`.

Once running, `mysqld-max-nt` can be stopped using `mysqladmin`, from the Services utility, or by using the command `NET STOP MySQL`.

When running as a service, the operating system will automatically stop the MySQL service on computer shutdown. In MySQL versions < 3.23.47, Windows only waited for a few seconds for the shutdown to complete, and killed the database server process if the time limit was exceeded (potentially causing problems). For instance, at the next startup the `InnoDB` storage engine had to do crash recovery. Starting from MySQL version 3.23.48, the Windows will wait longer for the MySQL server shutdown to complete. If you notice this is not enough for your intallation, it is safest to run the MySQL server not as a service, but from the Command prompt, and shut it down with `mysqladmin shutdown`.

There is a problem that Windows NT (but not Windows 2000/XP) by default only waits 20 seconds for a service to shut down, and after that kills the service process. You can increase this default by opening the Registry Editor ``\winnt\system32\regedt32.exe'` and editing the value of `WaitToKillServiceTimeout` at ``HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control'` in the Registry tree. Specify the new larger value in milliseconds, for example 120000 to have Windows NT wait upto 120 seconds.

Please note that when run as a service, `mysqld-max-nt` has no access to a console and so no messages can be seen. Errors can be checked in ``c:\mysql\data\mysql.err'`.

If you have problems installing `mysqld-max-nt` as a service, try starting it with the full path:

    C:\> C:\mysql\bin\mysqld-max-nt --install

If this doesn't work, you can get `mysqld-max-nt` to start properly by fixing the path in the registry!

If you don't want to start `mysqld-max-nt` as a service, you can start it as follows:

    C:\> C:\mysql\bin\mysqld-max-nt --standalone

or

    C:\> C:\mysql\bin\mysqld --standalone --debug

The last method gives you a debug trace in ``C:\mysqld.trace'`. See section E.1.2 Creating Trace Files.

#### 2.6.2.3 Running MySQL on Windows

MySQL supports TCP/IP on all Windows platforms and named pipes on NT/2000/XP. The default is to use named pipes for local connections on NT/2000/XP and TCP/IP for all other cases if the client has TCP/IP installed. The host name specifies which protocol is used:

Host name | Protocol
NULL (none) | On NT/2000/XP, try named pipes first; if that doesn't work, use TCP/IP. On 9x/Me, TCP/IP is used.
. | Named pipes
localhost | TCP/IP to current host
hostname | TCP/IP

You can force a MySQL client to use named pipes by specifying the `--pipe` option or by specifying `.` as the host name. Use the `--socket` option to specify the name of the pipe.

Note that starting from 3.23.50, named pipes are only enabled if mysqld is started with `--enable-named-pipe`. This is because some users have experienced problems shutting down the MySQL server when one uses named pipes.

You can test whether MySQL is working by executing the following commands:

    C:\> C:\mysql\bin\mysqlshow
    C:\> C:\mysql\bin\mysqlshow -u root mysql
    C:\> C:\mysql\bin\mysqladmin version status proc
    C:\> C:\mysql\bin\mysql test

If `mysqld` is slow to answer to connections on Windows 9x/Me, there is probably a problem with your DNS. In this case, start `mysqld` with `--skip-name-resolve` and use only `localhost` and IP numbers in the MySQL grant tables. You can also avoid DNS when connecting to a `mysqld-nt` MySQL server running on NT/2000/XP by using the `--pipe` argument to specify use of named pipes. This works for most MySQL clients.

There are two versions of the MySQL command-line tool:

Binary | Description
`mysql` | Compiled on native Windows, which offers very limited text editing capabilities.
`mysqlc` | Compiled with the Cygnus GNU compiler and libraries, which offers `readline` editing.

If you want to use `mysqlc.exe`, you must copy ``C:\mysql\lib\cygwinb19.dll'` to your Windows system directory (``\windows\system'` or similar place).

The default privileges on Windows give all local users full privileges to all databases without specifying a password. To make MySQL more secure, you should set a password for all users and remove the row in the `mysql.user` table that has `Host='localhost'` and `User=''`.

You should also add a password for the `root` user. The following example starts by removing the anonymous user that has all privileges, then sets a `root` user password:

    C:\> C:\mysql\bin\mysql mysql
    mysql> DELETE FROM user WHERE Host='localhost' AND User='';
    mysql> QUIT
    C:\> C:\mysql\bin\mysqladmin reload
    C:\> C:\mysql\bin\mysqladmin -u root password your_password

After you've set the password, if you want to take down the `mysqld` server, you can do so using this command:

    C:\> mysqladmin --user=root --password=your_password shutdown

If you are using the old shareware version of MySQL Version 3.21 under Windows, the above command will fail with an error: `parse error near 'SET password'`. The solution for this is to download and upgrade to the latest MySQL version, which is now freely available.

With the current MySQL versions you can easily add new users and change privileges with `GRANT` and `REVOKE` commands. See section 4.3.1 `GRANT` and `REVOKE` Syntax.

#### 2.6.2.4 Connecting to a Remote MySQL from Windows with SSH

Here is a note about how to connect to get a secure connection to remote MySQL server with SSH (by David Carlson dcarlson@mplcomm.com):

- Install an SSH client on your Windows machine. As a user, the best non-free one I've found is from `SecureCRT` from http://www.vandyke.com/. Another option is `f-secure` from http://www.f-secure.com/. You can also find some free ones on `Google` at http://directory.google.com/Top/Computers/Security/Products_and_Tools/Cryptography/SSH/Clients/Windows/.
- Start your Windows SSH client. Set `Host_Name = yourmysqlserver_URL_or_IP`. Set `userid=your_userid` to log in to your server (probably not the same as your MySQL login/password.
- Set up port forwarding. Either do a remote forward (Set `local_port: 3306`, `remote_host: yourmysqlservername_or_ip`, `remote_port: 3306` ) or a local forward (Set `port: 3306`, `host: localhost`, `remote port: 3306`).
- Save everything, otherwise you'll have to redo it the next time.
- Log in to your server with SSH session you just created.
- On your Windows machine, start some ODBC application (such as Access).
- Create a new file in Windows and link to MySQL using the ODBC driver the same way you normally do, except type in `localhost` for the MySQL host server@-not `yourmysqlservername`.

You should now have an ODBC connection to MySQL, encrypted using SSH.

#### 2.6.2.5 Splitting Data Across Different Disks on Windows

Beginning with MySQL Version 3.23.16, the `mysqld-max` and `mysql-max-nt` servers in the MySQL distribution are compiled with the `-DUSE_SYMDIR` option. This allows you to put a database on different disk by adding a symbolic link to it (in a manner similar to the way that symbolic links work on Unix).

On Windows, you make a symbolic link to a database by creating a file that contains the path to the destination directory and saving this in the ``mysql_data'` directory under the filename ``database.sym'`. Note that the symbolic link will be used only if the directory ``mysql_data_dir\database'` doesn't exist.

For example, if the MySQL data directory is ``C:\mysql\data'` and you want to have database `foo` located at ``D:\data\foo'`, you should create the file ``C:\mysql\data\foo.sym'` that contains the text `D:\data\foo\`. After that, all tables created in the database `foo` will be created in ``D:\data\foo'`.

Note that because of the speed penalty you get when opening every table, we have not enabled this by default even if you have compiled MySQL with support for this. To enable symlinks you should put in your ``my.cnf'` or ``my.ini'` file the following entry:

    [mysqld]
    use-symbolic-links

In MySQL 4.0 we will enable symlinks by default. Then you should instead use the `skip-symlink` option if you want to disable this.

#### 2.6.2.6 Compiling MySQL Clients on Windows

In your source files, you should include ``windows.h'` before you include ``mysql.h'`:

    #if defined(_WIN32) || defined(_WIN64)
    #include <windows.h>
    #endif
    #include <mysql.h>

You can either link your code with the dynamic ``libmysql.lib'` library, which is just a wrapper to load in ``libmysql.dll'` on demand, or link with the static ``mysqlclient.lib'` library.

Note that as the mysqlclient libraries are compiled as threaded libraries, you should also compile your code to be multi-threaded!

#### 2.6.2.7 MySQL-Windows Compared to Unix MySQL

MySQL-Windows has by now proven itself to be very stable. This version of MySQL has the same features as the corresponding Unix version with the following exceptions:

Windows 95 and threads
  Windows 95 leaks about 200 bytes of main memory for each thread creation. Each connection in MySQL creates a new thread, so you shouldn't run `mysqld` for an extended time on Windows 95 if your server handles many connections! Other versions of Windows don't suffer from this bug.
Concurrent reads
  MySQL depends on the `pread()` and `pwrite()` calls to be able to mix `INSERT` and `SELECT`. Currently we use mutexes to emulate `pread()`/`pwrite()`. We will, in the long run, replace the file level interface with a virtual interface so that we can use the `readfile()`/`writefile()` interface on NT/2000/XP to get more speed. The current implementation limits the number of open files MySQL can use to 1024, which means that you will not be able to run as many concurrent threads on NT/2000/XP as on Unix.
Blocking read
  MySQL uses a blocking read for each connection. This means that:
  - A connection will not be disconnected automatically after 8 hours, as happens with the Unix version of MySQL.
  - If a connection hangs, it's impossible to break it without killing MySQL.
  - `mysqladmin kill` will not work on a sleeping connection.
  - `mysqladmin shutdown` can't abort as long as there are sleeping connections.
  We plan to fix this problem when our Windows developers have figured out a nice workaround.
`DROP DATABASE`
  You can't drop a database that is in use by some thread.
Killing MySQL from the task manager
  You can't kill MySQL from the task manager or with the shutdown utility in Windows 95. You must take it down with `mysqladmin shutdown`.
Case-insensitive names
  Filenames are case-insensitive on Windows, so database and table names are also case-insensitive in MySQL for Windows. The only restriction is that database and table names must be specified using the same case throughout a given statement. See section 6.1.3 Case Sensitivity in Names.
The ``\'` directory character
  Pathname components in Windows 95 are separated by the ``\'` character, which is also the escape character in MySQL. If you are using `LOAD DATA INFILE` or `SELECT ... INTO OUTFILE`, you must double the ``\'` character:

      mysql> LOAD DATA INFILE "C:\\tmp\\skr.txt" INTO TABLE skr;
      mysql> SELECT * INTO OUTFILE 'C:\\tmp\\skr.txt' FROM skr;

  Alternatively, use Unix style filenames with ``/'` characters:

      mysql> LOAD DATA INFILE "C:/tmp/skr.txt" INTO TABLE skr;
      mysql> SELECT * INTO OUTFILE 'C:/tmp/skr.txt' FROM skr;

`Can't open named pipe` error
  If you use a MySQL 3.22 version on NT with the newest mysql-clients you will get the following error:

      error 2017: can't open named pipe to host: . pipe...

  This is because the release version of MySQL uses named pipes on NT by default. You can avoid this error by using the `--host=localhost` option to the new MySQL clients or create an option file ``C:\my.cnf'` that contains the following information:

      [client]
      host = localhost

  Starting from 3.23.50, named pipes are only enabled if `mysqld` is started with `--enable-named-pipe`.
`Access denied for user` error
  If you get the error `Access denied for user: 'some-user@unknown' to database 'mysql'` when accessing a MySQL server on the same machine, this means that MySQL can't resolve your host name properly. To fix this, you should create a file ``\windows\hosts'` with the following information:

      127.0.0.1       localhost

`ALTER TABLE`
  While you are executing an `ALTER TABLE` statement, the table is locked from usage by other threads. This has to do with the fact that on Windows, you can't delete a file that is in use by another threads. (In the future, we may find some way to work around this problem.)
  `DROP TABLE` on a table that is in use by a `MERGE` table will not work on Windows because the `MERGE` handler does the table mapping hidden from the upper layer of MySQL. Because Windows doesn't allow you to drop files that are open, you first must flush all `MERGE` tables (with `FLUSH TABLES`) or drop the `MERGE` table before dropping the table. We will fix this at the same time we introduce `VIEW`s.
  `DATA DIRECTORY` and `INDEX DIRECTORY` directives in `CREATE TABLE` is ignored on Windows, because Windows doesn't support symbolic links.

Here are some open issues for anyone who might want to help us with the Windows release:

- Make a single-user `MYSQL.DLL` server. This should include everything in a standard MySQL server, except thread creation. This will make MySQL much easier to use in applications that don't need a true client/server and don't need to access the server from other hosts.
- Add some nice start and shutdown icons to the MySQL installation.
- When registering `mysqld` as a service with `--install` (on NT) it would be nice if you could also add default options on the command-line. For the moment, the workaround is to list the parameters in the ``C:\my.cnf'` file instead.
- It would be really nice to be able to kill `mysqld` from the task manager. For the moment, you must use `mysqladmin shutdown`.
- Port `readline` to Windows for use in the `mysql` command-line tool.
- GUI versions of the standard MySQL clients (`mysql`, `mysqlshow`, `mysqladmin`, and `mysqldump`) would be nice.
- It would be nice if the socket read and write functions in ``net.c'` were interruptible. This would make it possible to kill open threads with `mysqladmin kill` on Windows.
- `mysqld` always starts in the "C" locale and not in the default locale. We would like to have `mysqld` use the current locale for the sort order.
- Add macros to use the faster thread-safe increment/decrement methods provided by Windows.

Other Windows-specific issues are described in the ``README'` file that comes with the MySQL-Windows distribution.

### 2.6.3 Solaris Notes

On Solaris, you may run into trouble even before you get the MySQL distribution unpacked! Solaris `tar` can't handle long file names, so you may see an error like this when you unpack MySQL:

    x mysql-3.22.12-beta/bench/Results/ATIS-mysql_odbc-NT_4.0-cmp-db2,\
    informix,ms-sql,mysql,oracle,solid,sybase, 0 bytes, 0 tape blocks
    tar: directory checksum error

In this case, you must use GNU `tar` (`gtar`) to unpack the distribution. You can find a precompiled copy for Solaris at http://www.mysql.com/Downloads/.

Sun native threads work only on Solaris 2.5 and higher. For Version 2.4 and earlier, MySQL will automatically use MIT-pthreads. See section 2.3.6 MIT-pthreads Notes.

If you get the following error from configure:

    checking for restartable system calls... configure: error can not run test
    programs while cross compiling

This means that you have something wrong with your compiler installation! In this case you should upgrade your compiler to a newer version. You may also be able to solve this problem by inserting the following row into the ``config.cache'` file:

    ac_cv_sys_restartable_syscalls=${ac_cv_sys_restartable_syscalls='no'}

If you are using Solaris on a SPARC, the recommended compiler is `gcc` 2.95.2. You can find this at http://gcc.gnu.org/. Note that `egcs` 1.1.1 and `gcc` 2.8.1 don't work reliably on SPARC!

The recommended `configure` line when using `gcc` 2.95.2 is:

    CC=gcc CFLAGS="-O3" \
    CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" \
    ./configure --prefix=/usr/local/mysql --with-low-memory --enable-assembler

If you have an UltraSPARC, you can get 4% more performance by adding "-mcpu=v8 -Wa,-xarch=v8plusa" to CFLAGS and CXXFLAGS.

If you have Sun's Forte 5.0 (or newer) compiler, you can run `configure` like this:

    CC=cc CFLAGS="-Xa -fast -native -xstrconst -mt" \
    CXX=CC CXXFLAGS="-noex -mt" \
    ./configure --prefix=/usr/local/mysql --enable-assembler

You can create a 64 bit binary with:

    CC=cc CFLAGS="-Xa -fast -native -xstrconst -mt -xarch=v9" \
    CXX=CC CXXFLAGS="-noex -mt -xarch=v9" ASFLAGS="-xarch=v9" \
    ./configure --prefix=/usr/local/mysql --enable-assembler

In the MySQL benchmarks, we got a 4% speedup on an UltraSPARC when using Forte 5.0 in 32 bit mode compared to using gcc 3.2 with -mcpu flags.

If you create a 64 bit binary, it's 4 % slower than the 32 bit binary, but mysqld can instead handle more treads and memory.

If you get a problem with `fdatasync` or `sched_yield`, you can fix this by adding `LIBS=-lrt` to the configure line

The following paragraph is only relevant for older compilers than WorkShop 5.3:

You may also have to edit the `configure` script to change this line:

    #if !defined(__STDC__) || __STDC__ != 1

to this:

    #if !defined(__STDC__)

If you turn on `__STDC__` with the `-Xc` option, the Sun compiler can't compile with the Solaris ``pthread.h'` header file. This is a Sun bug (broken compiler or broken include file).

If `mysqld` issues the error message shown here when you run it, you have tried to compile MySQL with the Sun compiler without enabling the multi-thread option (`-mt`):

    libc internal error: _rmutex_unlock: rmutex not held

Add `-mt` to `CFLAGS` and `CXXFLAGS` and try again.

If you are using the SFW version of gcc (which comes with Solaris 8), you must add ``/opt/sfw/lib'` to the environment variable `LD_LIBRARY_PATH` before running configure.

If you are using the gcc available from `sunfreeware.com`, you may have many problems. You should recompile gcc and GNU binutils on the machine you will be running them from to avoid any problems.

If you get the following error when compiling MySQL with `gcc`, it means that your `gcc` is not configured for your version of Solaris:

    shell> gcc -O3 -g -O2 -DDBUG_OFF  -o thr_alarm ...
    ./thr_alarm.c: In function `signal_hand':
    ./thr_alarm.c:556: too many arguments to function `sigwait'

The proper thing to do in this case is to get the newest version of `gcc` and compile it with your current `gcc` compiler! At least for Solaris 2.5, almost all binary versions of `gcc` have old, unusable include files that will break all programs that use threads (and possibly other programs)!

Solaris doesn't provide static versions of all system libraries (`libpthreads` and `libdl`), so you can't compile MySQL with `--static`. If you try to do so, you will get the error:

    ld: fatal: library -ldl: not found

    or

    undefined reference to `dlopen'

    or

    cannot find -lrt

If too many processes try to connect very rapidly to `mysqld`, you will see this error in the MySQL log:

    Error in accept: Protocol error

You might try starting the server with the `--set-variable back_log=50` option as a workaround for this. Please note that `--set-variable` is deprecated since MySQL 4.0, just use `--back_log=50` on its own. See section 4.1.1 `mysqld` Command-line Options.

If you are linking your own MySQL client, you might get the following error when you try to execute it:

    ld.so.1: ./my: fatal: libmysqlclient.so.#:
    open failed: No such file or directory

The problem can be avoided by one of the following methods:

- Link the client with the following flag (instead of `-Lpath`): `-Wl,r/full-path-to-libmysqlclient.so`.
- Copy ``libmysqclient.so'` to ``/usr/lib'`.
- Add the pathname of the directory where ``libmysqlclient.so'` is located to the `LD_RUN_PATH` environment variable before running your client.

If you have problems with configure trying to link with `-lz` and you don't have `zlib` installed, you have two options:

- If you want to be able to use the compressed communication protocol, you need to get and install zlib from ftp.gnu.org.
- Configure with `--with-named-z-libs=no`.

If you are using gcc and have problems with loading `UDF` functions into MySQL, try adding `-lgcc` to the link line for the `UDF` function.

If you would like MySQL to start automatically, you can copy ``support-files/mysql.server'` to ``/etc/init.d'` and create a symbolic link to it named ``/etc/rc3.d/S99mysql.server'`.

As Solaris doesn't support core files for `setuid()` applications, you can't get a core file from `mysqld` if you are using the `--user` option.

#### 2.6.3.1 Solaris 2.7/2.8 Notes

You can normally use a Solaris 2.6 binary on Solaris 2.7 and 2.8. Most of the Solaris 2.6 issues also apply for Solaris 2.7 and 2.8.

Note that MySQL Version 3.23.4 and above should be able to autodetect new versions of Solaris and enable workarounds for the following problems!

Solaris 2.7 / 2.8 has some bugs in the include files. You may see the following error when you use `gcc`:

    /usr/include/widec.h:42: warning: `getwc' redefined
    /usr/include/wchar.h:326: warning: this is the location of the previous
    definition

If this occurs, you can do the following to fix the problem:

Copy `/usr/include/widec.h` to `.../lib/gcc-lib/os/gcc-version/include` and change line 41 from:

    #if     !defined(lint) && !defined(__lint)

    to

    #if     !defined(lint) && !defined(__lint) && !defined(getwc)

Alternatively, you can edit ``/usr/include/widec.h'` directly. Either way, after you make the fix, you should remove ``config.cache'` and run `configure` again!

If you get errors like this when you run `make`, it's because `configure` didn't detect the ``curses.h'` file (probably because of the error in ``/usr/include/widec.h'`):

    In file included from mysql.cc:50:
    /usr/include/term.h:1060: syntax error before `,'
    /usr/include/term.h:1081: syntax error before `;'

The solution to this is to do one of the following:

- Configure with `CFLAGS=-DHAVE_CURSES_H CXXFLAGS=-DHAVE_CURSES_H ./configure`.
- Edit ``/usr/include/widec.h'` as indicted above and rerun configure.
- Remove the `#define HAVE_TERM` line from ``config.h'` file and run `make` again.

If you get a problem that your linker can't find `-lz` when linking your client program, the problem is probably that your ``libz.so'` file is installed in ``/usr/local/lib'`. You can fix this by one of the following methods:

- Add ``/usr/local/lib'` to `LD_LIBRARY_PATH`.
- Add a link to ``libz.so'` from ``/lib'`.
- If you are using Solaris 8, you can install the optional zlib from your Solaris 8 CD distribution.
- Configure MySQL with the `--with-named-z-libs=no` option.

#### 2.6.3.2 Solaris x86 Notes

On Solaris 2.8 on x86, `mysqld` will core dump if you run 'strip' in.

If you are using `gcc` or `egcs` on Solaris x86 and you experience problems with core dumps under load, you should use the following `configure` command:

    CC=gcc CFLAGS="-O3 -fomit-frame-pointer -DHAVE_CURSES_H" \
    CXX=gcc \
    CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors -fno-exceptions \
    -fno-rtti -DHAVE_CURSES_H" \
    ./configure --prefix=/usr/local/mysql

This will avoid problems with the `libstdc++` library and with C++ exceptions.

If this doesn't help, you should compile a debug version and run it with a trace file or under `gdb`. See section E.1.3 Debugging mysqld under gdb.

### 2.6.4 BSD Notes

This section provides information for the various BSD flavours, as well as specific versions within those.

#### 2.6.4.1 FreeBSD Notes

FreeBSD 3.x is recommended for running MySQL since the thread package is much more integrated.

The easiest and therefore the preferred way to install is to use the mysql-server and mysql-client ports available on http://www.freebsd.org/.

Using these gives you:

- A working MySQL with all optimisations known to work on your version of FreeBSD enabled.
- Automatic configuration and build.
- Startup scripts installed in /usr/local/etc/rc.d.
- Ability to see which files that are installed with pkg_info -L. And to remove them all with pkg_delete if you no longer want MySQL on that machine.

It is recommended you use MIT-pthreads on FreeBSD 2.x and native threads on Versions 3 and up. It is possible to run with native threads on some late 2.2.x versions but you may encounter problems shutting down `mysqld`.

The MySQL ``Makefile'`s require GNU make (`gmake`) to work. If you want to compile MySQL you need to install GNU `make` first.

Be sure to have your name resolver setup correct. Otherwise, you may experience resolver delays or failures when connecting to `mysqld`.

Make sure that the `localhost` entry in the ``/etc/hosts'` file is correct (otherwise, you will have problems connecting to the database). The ``/etc/hosts'` file should start with a line:

    127.0.0.1       localhost localhost.your.domain

The recommended way to compile and install MySQL on FreeBSD with gcc (2.95.2 and up) is:

    CC=gcc CFLAGS="-O2 -fno-strength-reduce" \
    CXX=gcc CXXFLAGS="-O2 -fno-rtti -fno-exceptions -felide-constructors \
    -fno-strength-reduce" \
    ./configure --prefix=/usr/local/mysql --enable-assembler
    gmake
    gmake install
    ./scripts/mysql_install_db
    cd /usr/local/mysql
    ./bin/mysqld_safe &

If you notice that `configure` will use MIT-pthreads, you should read the MIT-pthreads notes. See section 2.3.6 MIT-pthreads Notes.

If you get an error from `make install` that it can't find ``/usr/include/pthreads'`, `configure` didn't detect that you need MIT-pthreads. This is fixed by executing these commands:

    shell> rm config.cache
    shell> ./configure --with-mit-threads

FreeBSD is also known to have a very low default file handle limit. See section A.2.16 File Not Found. Uncomment the ulimit -n section in safe_mysqld or raise the limits for the `mysqld` user in /etc/login.conf (and rebuild it with cap_mkdb /etc/login.conf). Also be sure you set the appropriate class for this user in the password file if you are not using the default (use: chpass mysqld-user-name). See section 4.7.2 `safe_mysqld`, The Wrapper Around `mysqld`.

If you have a lot of memory you should consider rebuilding the kernel to allow MySQL to take more than 512M of RAM. Take a look at `option MAXDSIZ` in the LINT config file for more info.

If you get problems with the current date in MySQL, setting the `TZ` variable will probably help. See section F Environment Variables.

To get a secure and stable system you should only use FreeBSD kernels that are marked `-RELEASE`.

#### 2.6.4.2 NetBSD notes

To compile on NetBSD you need GNU `make`. Otherwise, the compile will crash when `make` tries to run `lint` on C++ files.

#### 2.6.4.3 OpenBSD 2.5 Notes

On OpenBSD Version 2.5, you can compile MySQL with native threads with the following options:

    CFLAGS=-pthread CXXFLAGS=-pthread ./configure --with-mit-threads=no

#### 2.6.4.4 OpenBSD 2.8 Notes

Our users have reported that OpenBSD 2.8 has a threading bug which causes problems with MySQL. The OpenBSD Developers have fixed the problem, but as of January 25th, 2001, it's only available in the ``-current'' branch. The symptoms of this threading bug are: slow response, high load, high CPU usage, and crashes.

If you get an error like `Error in accept:: Bad file descriptor` or error 9 when trying to open tables or directories, the problem is probably that you haven't allocated enough file descriptors for MySQL.

In this case try starting `safe_mysqld` as root with the following options:

`--user=mysql --open-files-limit=2048`

#### 2.6.4.5 BSD/OS Version 2.x Notes

If you get the following error when compiling MySQL, your `ulimit` value for virtual memory is too low:

    item_func.h: In method `Item_func_ge::Item_func_ge(const Item_func_ge &)':
    item_func.h:28: virtual memory exhausted
    make[2]: *** [item_func.o] Error 1

Try using `ulimit -v 80000` and run `make` again. If this doesn't work and you are using `bash`, try switching to `csh` or `sh`; some BSDI users have reported problems with `bash` and `ulimit`.

If you are using `gcc`, you may also use have to use the `--with-low-memory` flag for `configure` to be able to compile ``sql_yacc.cc'`.

If you get problems with the current date in MySQL, setting the `TZ` variable will probably help. See section F Environment Variables.

#### 2.6.4.6 BSD/OS Version 3.x Notes

Upgrade to BSD/OS Version 3.1. If that is not possible, install BSDIpatch M300-038.

Use the following command when configuring MySQL:

    shell> env CXX=shlicc++ CC=shlicc2 \
           ./configure \
               --prefix=/usr/local/mysql \
               --localstatedir=/var/mysql \
               --without-perl \
               --with-unix-socket-path=/var/mysql/mysql.sock

The following is also known to work:

    shell> env CC=gcc CXX=gcc CXXFLAGS=-O3 \
           ./configure \
               --prefix=/usr/local/mysql \
               --with-unix-socket-path=/var/mysql/mysql.sock

You can change the directory locations if you wish, or just use the defaults by not specifying any locations.

If you have problems with performance under heavy load, try using the `--skip-thread-priority` option to `mysqld`! This will run all threads with the same priority; on BSDI Version 3.1, this gives better performance (at least until BSDI fixes their thread scheduler).

If you get the error `virtual memory exhausted` while compiling, you should try using `ulimit -v 80000` and run `make` again. If this doesn't work and you are using `bash`, try switching to `csh` or `sh`; some BSDI users have reported problems with `bash` and `ulimit`.

#### 2.6.4.7 BSD/OS Version 4.x Notes

BSDI Version 4.x has some thread-related bugs. If you want to use MySQL on this, you should install all thread-related patches. At least M400-023 should be installed.

On some BSDI Version 4.x systems, you may get problems with shared libraries. The symptom is that you can't execute any client programs, for example, `mysqladmin`. In this case you need to reconfigure not to use shared libraries with the `--disable-shared` option to configure.

Some customers have had problems on BSDI 4.0.1 that the `mysqld` binary after a while can't open tables. This is because some library/system related bug causes `mysqld` to change current directory without asking for this!

The fix is to either upgrade to 3.23.34 or after running `configure` remove the line `#define HAVE_REALPATH` from `config.h` before running make.

Note that the above means that you can't symbolic link a database directories to another database directory or symbolic link a table to another database on BSDI! (Making a symbolic link to another disk is okay).

### 2.6.5 Mac OS X Notes

#### 2.6.5.1 Mac OS X Public Beta

MySQL should work without any problems on Mac OS X Public Beta (Darwin). You don't need the pthread patches for this OS!

#### 2.6.5.2 Mac OS X Server

Before trying to configure MySQL on Mac OS X server you must first install the pthread package from http://www.prnet.de/RegEx/mysql.html.

Our binary for Mac OS X is compiled on Rhapsody 5.5 with the following configure line:

    CC=gcc CFLAGS="-O2 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O2 \
    -fomit-frame-pointer" ./configure --prefix=/usr/local/mysql \
    "--with-comment=Official MySQL binary" --with-extra-charsets=complex \
    --disable-shared

You might want to also add aliases to your shell's resource file to access `mysql` and `mysqladmin` from the command-line:

    alias mysql '/usr/local/mysql/bin/mysql'
    alias mysqladmin '/usr/local/mysql/bin/mysqladmin'

### 2.6.6 Other Unix Notes

#### 2.6.6.1 HP-UX Notes for Binary Distributions

Some of the binary distributions of MySQL for HP-UX is distributed as an HP depot file and as a tar file. To use the depot file you must be running at least HP-UX 10.x to have access to HP's software depot tools.

The HP version of MySQL was compiled on an HP 9000/8xx server under HP-UX 10.20, and uses MIT-pthreads. It is known to work well under this configuration. MySQL Version 3.22.26 and newer can also be built with HP's native thread package.

Other configurations that may work:

- HP 9000/7xx running HP-UX 10.20+
- HP 9000/8xx running HP-UX 10.30

The following configurations almost definitely won't work:

- HP 9000/7xx or 8xx running HP-UX 10.x where x < 2
- HP 9000/7xx or 8xx running HP-UX 9.x

To install the distribution, use one of the commands here, where `/path/to/depot` is the full pathname of the depot file:

- To install everything, including the server, client and development tools:

      shell> /usr/sbin/swinstall -s /path/to/depot mysql.full

- To install only the server:

      shell> /usr/sbin/swinstall -s /path/to/depot mysql.server

- To install only the client package:

      shell> /usr/sbin/swinstall -s /path/to/depot mysql.client

- To install only the development tools:

      shell> /usr/sbin/swinstall -s /path/to/depot mysql.developer

The depot places binaries and libraries in ``/opt/mysql'` and data in ``/var/opt/mysql'`. The depot also creates the appropriate entries in ``/etc/init.d'` and ``/etc/rc2.d'` to start the server automatically at boot time. Obviously, this entails being `root` to install.

To install the HP-UX tar.gz distribution, you must have a copy of GNU `tar`.

#### 2.6.6.2 HP-UX Version 10.20 Notes

There are a couple of small problems when compiling MySQL on HP-UX. We recommend that you use `gcc` instead of the HP-UX native compiler, because `gcc` produces better code!

We recommend using gcc 2.95 on HP-UX. Don't use high optimisation flags (like -O6) as this may not be safe on HP-UX.

The following configure line should work with gcc 2.95:

    CFLAGS="-I/opt/dce/include -fpic" \
    CXXFLAGS="-I/opt/dce/include -felide-constructors -fno-exceptions \
    -fno-rtti" CXX=gcc ./configure --with-pthread \
    --with-named-thread-libs='-ldce' --prefix=/usr/local/mysql --disable-shared

The following configure line should work with gcc 3.1:

    CFLAGS="-DHPUX -I/opt/dce/include -O3 -fPIC" CXX=gcc \
    CXXFLAGS="-DHPUX -I/opt/dce/include -felide-constructors -fno-exceptions \
    -fno-rtti -O3 -fPIC" ./configure --prefix=/usr/local/mysql \
    --with-extra-charsets=complex --enable-thread-safe-client \
    --enable-local-infile  --with-pthread \
    --with-named-thread-libs=-ldce --with-lib-ccflags=-fPIC
    --disable-shared

#### 2.6.6.3 HP-UX Version 11.x Notes

For HP-UX Version 11.x we recommend MySQL Version 3.23.15 or later.

Because of some critical bugs in the standard HP-UX libraries, you should install the following patches before trying to run MySQL on HP-UX 11.0:

    PHKL_22840 Streams cumulative
    PHNE_22397 ARPA cumulative

This will solve the problem of getting `EWOULDBLOCK` from `recv()` and `EBADF` from `accept()` in threaded applications.

If you are using `gcc` 2.95.1 on an unpatched HP-UX 11.x system, you will get the error:

    In file included from /usr/include/unistd.h:11,
                     from ../include/global.h:125,
                     from mysql_priv.h:15,
                     from item.cc:19:
    /usr/include/sys/unistd.h:184: declaration of C function ...
    /usr/include/sys/pthread.h:440: previous declaration ...
    In file included from item.h:306,
                     from mysql_priv.h:158,
                     from item.cc:19:

The problem is that HP-UX doesn't define `pthreads_atfork()` consistently. It has conflicting prototypes in ``/usr/include/sys/unistd.h'`:184 and ``/usr/include/sys/pthread.h'`:440 (details below).

One solution is to copy ``/usr/include/sys/unistd.h'` into ``mysql/include'` and edit ``unistd.h'` and change it to match the definition in ``pthread.h'`. Here's the diff:

    183,184c183,184
    <      extern int pthread_atfork(void (*prepare)(), void (*parent)(),
    <                                                void (*child)());
    ---
    >      extern int pthread_atfork(void (*prepare)(void), void (*parent)(void),
    >                                                void (*child)(void));

After this, the following configure line should work:

    CFLAGS="-fomit-frame-pointer -O3 -fpic" CXX=gcc \
    CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -O3" \
    ./configure --prefix=/usr/local/mysql --disable-shared

If you are using MySQL 4.0.5 with the HP-UX compiler you can use: (tested with cc B.11.11.04):

    CC=cc CXX=aCC CFLAGS=+DD64 CXXFLAGS=+DD64 ./configure --with-extra-character-set=complex

You can ignore any errors of the following type:

    aCC: warning 901: unknown option: `-3': use +help for online documentation

If you get the following error from `configure`

    checking for cc option to accept ANSI C... no
    configure: error: MySQL requires a ANSI C compiler (and a C++ compiler).
    Try gcc. See the Installation chapter in the Reference Manual.

Check that you don't have the path to the K&R compiler before the path to the HP-UX C and C++ compiler.

Another reason for not beeing able to compile is that you didn't define the `+DD64` flags above.

#### 2.6.6.4 IBM-AIX notes

Automatic detection of `xlC` is missing from Autoconf, so a `configure` command something like this is needed when compiling MySQL (This example uses the IBM compiler):

    export CC="xlc_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192 "
    export CXX="xlC_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192"
    export CFLAGS="-I /usr/local/include"
    export LDFLAGS="-L /usr/local/lib"
    export CPPFLAGS=$CFLAGS
    export CXXFLAGS=$CFLAGS

    ./configure --prefix=/usr/local \
    		--localstatedir=/var/mysql \
    		--sysconfdir=/etc/mysql \
    		--sbindir='/usr/local/bin' \
    		--libexecdir='/usr/local/bin' \
    		--enable-thread-safe-client \
    		--enable-large-files

Above are the options used to compile the MySQL distribution that can be found at http://www-frec.bull.com/.

If you change the `-O3` to `-O2` in the above configure line, you must also remove the `-qstrict` option (this is a limitation in the IBM C compiler).

If you are using `gcc` or `egcs` to compile MySQL, you must use the `-fno-exceptions` flag, as the exception handling in `gcc`/`egcs` is not thread-safe! (This is tested with `egcs` 1.1.) There are also some known problems with IBM's assembler, which may cause it to generate bad code when used with gcc.

We recommend the following `configure` line with `egcs` and `gcc 2.95` on AIX:

    CC="gcc -pipe -mcpu=power -Wa,-many" \
    CXX="gcc -pipe -mcpu=power -Wa,-many" \
    CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti" \
    ./configure --prefix=/usr/local/mysql --with-low-memory

The `-Wa,-many` is necessary for the compile to be successful. IBM is aware of this problem but is in to hurry to fix it because of the workaround available. We don't know if the `-fno-exceptions` is required with `gcc 2.95`, but as MySQL doesn't use exceptions and the above option generates faster code, we recommend that you should always use this option with `egcs / gcc`.

If you get a problem with assembler code try changing the -mcpu=xxx to match your CPU. Typically power2, power, or powerpc may need to be used, alternatively you might need to use 604 or 604e. I'm not positive but I would think using "power" would likely be safe most of the time, even on a power2 machine.

If you don't know what your CPU is then do a "uname -m", this will give you back a string that looks like "000514676700", with a format of xxyyyyyymmss where xx and ss are always 0's, yyyyyy is a unique system id and mm is the id of the CPU Planar. A chart of these values can be found at http://publib.boulder.ibm.com/doc_link/en_US/a_doc_lib/cmds/aixcmds5/uname.htm. This will give you a machine type and a machine model you can use to determine what type of CPU you have.

If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:

    shell> CFLAGS=-DDONT_USE_THR_ALARM CXX=gcc \
           CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti \
           -DDONT_USE_THR_ALARM" \
           ./configure --prefix=/usr/local/mysql --with-debug --with-low-memory

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are ``sleeping'' on a connection with `mysqladmin kill` or `mysqladmin shutdown`. Instead, the client will die when it issues its next command.

On some versions of AIX, linking with `libbind.a` makes `getservbyname` core dump. This is an AIX bug and should be reported to IBM.

For AIX 4.2.1 and gcc you have to do the following changes.

After configuring, edit ``config.h'` and ``include/my_config.h'` and change the line that says

    #define HAVE_SNPRINTF 1

to

    #undef HAVE_SNPRINTF

And finally, in ``mysqld.cc'` you need to add a prototype for initgoups.

    #ifdef _AIX41
    extern "C" int initgroups(const char *,int);
    #endif

#### 2.6.6.5 SunOS 4 Notes

On SunOS 4, MIT-pthreads is needed to compile MySQL, which in turn means you will need GNU `make`.

Some SunOS 4 systems have problems with dynamic libraries and `libtool`. You can use the following `configure` line to avoid this problem:

    shell> ./configure --disable-shared --with-mysqld-ldflags=-all-static

When compiling `readline`, you may get warnings about duplicate defines. These may be ignored.

When compiling `mysqld`, there will be some `implicit declaration of function` warnings. These may be ignored.

#### 2.6.6.6 Alpha-DEC-UNIX Notes (Tru64)

If you are using egcs 1.1.2 on Digital Unix, you should upgrade to gcc 2.95.2, as egcs on DEC has some serious bugs!

When compiling threaded programs under Digital Unix, the documentation recommends using the `-pthread` option for `cc` and `cxx` and the libraries `-lmach -lexc` (in addition to `-lpthread`). You should run `configure` something like this:

    CC="cc -pthread" CXX="cxx -pthread -O" \
    ./configure --with-named-thread-libs="-lpthread -lmach -lexc -lc"

When compiling `mysqld`, you may see a couple of warnings like this:

    mysqld.cc: In function void handle_connections()':
    mysqld.cc:626: passing long unsigned int *' as argument 3 of
    accept(int,sockadddr *, int *)'

You can safely ignore these warnings. They occur because `configure` can detect only errors, not warnings.

If you start the server directly from the command-line, you may have problems with it dying when you log out. (When you log out, your outstanding processes receive a `SIGHUP` signal.) If so, try starting the server like this:

    shell> nohup mysqld [options] &

`nohup` causes the command following it to ignore any `SIGHUP` signal sent from the terminal. Alternatively, start the server by running `safe_mysqld`, which invokes `mysqld` using `nohup` for you. See section 4.7.2 `safe_mysqld`, The Wrapper Around `mysqld`.

If you get a problem when compiling mysys/get_opt.c, just remove the line #define _NO_PROTO from the start of that file!

If you are using Compac's CC compiler, the following configure line should work:

    CC="cc -pthread"
    CFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed all -arch host"
    CXX="cxx -pthread"
    CXXFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed all -arch host \
    -noexceptions -nortti"
    export CC CFLAGS CXX CXXFLAGS
    ./configure \
    --prefix=/usr/local/mysql \
    --with-low-memory \
    --enable-large-files \
    --enable-shared=yes \
    --with-named-thread-libs="-lpthread -lmach -lexc -lc"
    gnumake

If you get a problem with libtool, when compiling with shared libraries as above, when linking `mysql`, you should be able to get around this by issuing:

    cd mysql
    /bin/sh ../libtool --mode=link cxx -pthread  -O3 -DDBUG_OFF \
    -O4 -ansi_alias -ansi_args -fast -inline speed \
    -speculate all \ -arch host  -DUNDEF_HAVE_GETHOSTBYNAME_R \
    -o mysql  mysql.o readline.o sql_string.o completion_hash.o \
    ../readline/libreadline.a -lcurses \
    ../libmysql/.libs/libmysqlclient.so  -lm
    cd ..
    gnumake
    gnumake install
    scripts/mysql_install_db

#### 2.6.6.7 Alpha-DEC-OSF/1 Notes

If you have problems compiling and have DEC `CC` and `gcc` installed, try running `configure` like this:

    CC=cc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 \
    ./configure --prefix=/usr/local/mysql

If you get problems with the ``c_asm.h'` file, you can create and use a 'dummy' ``c_asm.h'` file with:

    touch include/c_asm.h
    CC=gcc CFLAGS=-I./include \
    CXX=gcc CXXFLAGS=-O3 \
    ./configure --prefix=/usr/local/mysql

Note that the following problems with the `ld` program can be fixed by downloading the latest DEC (Compaq) patch kit from: http://ftp.support.compaq.com/public/unix/.

On OSF/1 V4.0D and compiler "DEC C V5.6-071 on Digital Unix V4.0 (Rev. 878)" the compiler had some strange behaviour (undefined `asm` symbols). `/bin/ld` also appears to be broken (problems with `_exit undefined` errors occuring while linking `mysqld`). On this system, we have managed to compile MySQL with the following `configure` line, after replacing `/bin/ld` with the version from OSF 4.0C:

    CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql

With the Digital compiler "C++ V6.1-029", the following should work:

    CC=cc -pthread
    CFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all \
           -arch host
    CXX=cxx -pthread
    CXXFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all \
              -arch host -noexceptions -nortti
    export CC CFLAGS CXX CXXFLAGS
    ./configure --prefix=/usr/mysql/mysql --with-mysqld-ldflags=-all-static \
                --disable-shared --with-named-thread-libs="-lmach -lexc -lc"

In some versions of OSF/1, the `alloca()` function is broken. Fix this by removing the line in ``config.h'` that defines `'HAVE_ALLOCA'`.

The `alloca()` function also may have an incorrect prototype in `/usr/include/alloca.h`. This warning resulting from this can be ignored.

`configure` will use the following thread libraries automatically: `--with-named-thread-libs="-lpthread -lmach -lexc -lc"`.

When using `gcc`, you can also try running `configure` like this:

    shell> CFLAGS=-D_PTHREAD_USE_D4 CXX=gcc CXXFLAGS=-O3 ./configure ...

If you have problems with signals (MySQL dies unexpectedly under high load), you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:

    shell> CFLAGS=-DDONT_USE_THR_ALARM \
           CXXFLAGS=-DDONT_USE_THR_ALARM \
           ./configure ...

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are ``sleeping'' on a connection with `mysqladmin kill` or `mysqladmin shutdown`. Instead, the client will die when it issues its next command.

With `gcc` 2.95.2, you will probably run into the following compile error:

    sql_acl.cc:1456: Internal compiler error in `scan_region', at except.c:2566
    Please submit a full bug report.

To fix this you should change to the `sql` directory and do a ``cut and paste'' of the last `gcc` line, but change `-O3` to `-O0` (or add `-O0` immediately after `gcc` if you don't have any `-O` option on your compile line). After this is done you can just change back to the top-level directly and run `make` again.

#### 2.6.6.8 SGI Irix Notes

If you are using Irix Version 6.5.3 or newer `mysqld` will only be able to create threads if you run it as a user with `CAP_SCHED_MGT` privileges (like `root`) or give the `mysqld` server this privilege with the following shell command:

    shell> chcap "CAP_SCHED_MGT+epi" /opt/mysql/libexec/mysqld

You may have to undefine some things in ``config.h'` after running `configure` and before compiling.

In some Irix implementations, the `alloca()` function is broken. If the `mysqld` server dies on some `SELECT` statements, remove the lines from ``config.h'` that define `HAVE_ALLOC` and `HAVE_ALLOCA_H`. If `mysqladmin create` doesn't work, remove the line from ``config.h'` that defines `HAVE_READDIR_R`. You may have to remove the `HAVE_TERM_H` line as well.

SGI recommends that you install all of the patches on this page as a set: http://support.sgi.com/surfzone/patches/patchset/6.2_indigo.rps.html

At the very minimum, you should install the latest kernel rollup, the latest `rld` rollup, and the latest `libc` rollup.

You definitely need all the POSIX patches on this page, for pthreads support:

http://support.sgi.com/surfzone/patches/patchset/6.2_posix.rps.html

If you get the something like the following error when compiling ``mysql.cc'`:

    "/usr/include/curses.h", line 82: error(1084): invalid combination of type

Type the following in the top-level directory of your MySQL source tree:

    shell> extra/replace bool curses_bool < /usr/include/curses.h \
    > include/curses.h
    shell> make

There have also been reports of scheduling problems. If only one thread is running, things go slow. Avoid this by starting another client. This may lead to a 2-to-10-fold increase in execution speed thereafter for the other thread. This is a poorly understood problem with Irix threads; you may have to improvise to find solutions until this can be fixed.

If you are compiling with `gcc`, you can use the following `configure` command:

    CC=gcc CXX=gcc CXXFLAGS=-O3 \
    ./configure --prefix=/usr/local/mysql --enable-thread-safe-client \
    --with-named-thread-libs=-lpthread

On Irix 6.5.11 with native Irix C and C++ compilers ver. 7.3.1.2, the following is reported to work

    CC=cc CXX=CC CFLAGS='-O3 -n32 -TARG:platform=IP22 -I/usr/local/include \
    -L/usr/local/lib' CXXFLAGS='-O3 -n32 -TARG:platform=IP22 \
    -I/usr/local/include -L/usr/local/lib' ./configure \
    --prefix=/usr/local/mysql --with-innodb --with-berkeley-db \
    --with-libwrap=/usr/local \
    --with-named-curses-libs=/usr/local/lib/libncurses.a

#### 2.6.6.9 Caldera (SCO) Notes

The current port is tested only on a ``sco3.2v5.0.4'' and ``sco3.2v5.0.5'' system. There has also been a lot of progress on a port to ``sco 3.2v4.2''.

For the moment the recommended compiler on OpenServer is gcc 2.95.2. With this you should be able to compile MySQL with just:

    CC=gcc CXX=gcc ./configure ... (options)

1. For OpenServer 5.0.X you need to use gcc-2.95.2p1 or newer from the Skunkware. http://www.caldera.com/skunkware/ and choose browser OpenServer packages or by ftp to ftp2.caldera.com in the pub/skunkware/osr5/devtools/gcc directory.
2. You need the port of GCC 2.5.x for this product and the Development system. They are required on this version of Caldera (SCO) Unix. You cannot just use the GCC Dev system.
3. You should get the FSU Pthreads package and install it first. This can be found at http://www.cs.wustl.edu/~schmidt/ACE_wrappers/FSU-threads.tar.gz. You can also get a precompiled package from http://www.mysql.com/Downloads/SCO/FSU-threads-3.5c.tar.gz.
4. FSU Pthreads can be compiled with Caldera (SCO) Unix 4.2 with tcpip. Or OpenServer 3.0 or Open Desktop 3.0 (OS 3.0 ODT 3.0), with the Caldera (SCO) Development System installed using a good port of GCC 2.5.x ODT or OS 3.0 you will need a good port of GCC 2.5.x There are a lot of problems without a good port. The port for this product requires the SCO Unix Development system. Without it, you are missing the libraries and the linker that is needed.
5. To build FSU Pthreads on your system, do the following:
   1. Run `./configure` in the ``threads/src'` directory and select the SCO OpenServer option. This command copies ``Makefile.SCO5'` to ``Makefile'`.
   2. Run `make`.
   3. To install in the default ``/usr/include'` directory, login as root, then `cd` to the ``thread/src'` directory, and run `make install`.
6. Remember to use GNU `make` when making MySQL.
7. If you don't start `safe_mysqld` as root, you probably will get only the default 110 open files per process. `mysqld` will write a note about this in the log file.
8. With SCO 3.2V5.0.5, you should use FSU Pthreads version 3.5c or newer. You should also use gcc 2.95.2 or newer! The following `configure` command should work:

       shell> ./configure --prefix=/usr/local/mysql --disable-shared

9. With SCO 3.2V4.2, you should use FSU Pthreads version 3.5c or newer. The following `configure` command should work:

       shell> CFLAGS="-D_XOPEN_XPG4" CXX=gcc CXXFLAGS="-D_XOPEN_XPG4" \
              ./configure \
                  --prefix=/usr/local/mysql \
                  --with-named-thread-libs="-lgthreads -lsocket -lgen -lgthreads" \
                  --with-named-curses-libs="-lcurses"

   You may get some problems with some include files. In this case, you can find new SCO-specific include files at http://www.mysql.com/Downloads/SCO/SCO-3.2v4.2-includes.tar.gz. You should unpack this file in the ``include'` directory of your MySQL source tree.

Caldera (SCO) development notes:

- MySQL should automatically detect FSU Pthreads and link `mysqld` with `-lgthreads -lsocket -lgthreads`.
- The Caldera (SCO) development libraries are re-entrant in FSU Pthreads. Caldera claim sthat its libraries' functions are re-entrant, so they must be reentrant with FSU Pthreads. FSU Pthreads on OpenServer tries to use the SCO scheme to make re-entrant libraries.
- FSU Pthreads (at least the version at http://www.mysql.com/) comes linked with GNU `malloc`. If you encounter problems with memory usage, make sure that ``gmalloc.o'` is included in ``libgthreads.a'` and ``libgthreads.so'`.
- In FSU Pthreads, the following system calls are pthreads-aware: `read()`, `write()`, `getmsg()`, `connect()`, `accept()`, `select()`, and `wait()`.
- The CSSA-2001-SCO.35.2 (the patch is listed in custom as erg711905-dscr_remap security patch (version 2.0.0) breaks FSU threads and makes mysqld unstable. You have to remove this one if you want to run mysqld on an OpenServer 5.0.6 machine.

If you want to install DBI on Caldera (SCO), you have to edit the ``Makefile'` in DBI-xxx and each subdirectory.

Note that the following assumes gcc 2.95.2 or newer:

    OLD:                                  NEW:
    CC = cc                               CC = gcc
    CCCDLFLAGS = -KPIC -W1,-Bexport       CCCDLFLAGS = -fpic
    CCDLFLAGS = -wl,-Bexport              CCDLFLAGS =

    LD = ld                               LD = gcc -G -fpic
    LDDLFLAGS = -G -L/usr/local/lib       LDDLFLAGS = -L/usr/local/lib
    LDFLAGS = -belf -L/usr/local/lib      LDFLAGS = -L/usr/local/lib

    LD = ld                               LD = gcc -G -fpic
    OPTIMISE = -Od                        OPTIMISE = -O1

    OLD:
    CCCFLAGS = -belf -dy -w0 -U M_XENIX -DPERL_SCO5 -I/usr/local/include

    NEW:
    CCFLAGS = -U M_XENIX -DPERL_SCO5 -I/usr/local/include

This is because the Perl dynaloader will not load the `DBI` modules if they were compiled with `icc` or `cc`.

Perl works best when compiled with `cc`.

#### 2.6.6.10 Caldera (SCO) Unixware Version 7.0 Notes

You must use a version of MySQL at least as recent as Version 3.22.13 because that version fixes some portability problems under Unixware.

We have been able to compile MySQL with the following `configure` command on Unixware Version 7.0.1:

    CC=cc CXX=CC ./configure --prefix=/usr/local/mysql

If you want to use `gcc`, you must use `gcc` 2.95.2 or newer.

Caldera provides libsocket.so.2 at ftp://stage.caldera.com/pub/security/tools for pre-OSR506 security fixes. Also, the telnetd fix at ftp://stage.caldera.com/pub/security/openserver/CSSA-2001-SCO.10/ as both libsocket.so.2 and libresolv.so.1 with instructions for installing on pre-OSR506 systems.

It's probably a good idea to install the above patches before trying to compile/use MySQL.

### 2.6.7 OS/2 Notes

MySQL uses quite a few open files. Because of this, you should add something like the following to your ``CONFIG.SYS'` file:

    SET EMXOPT=-c -n -h1024

If you don't do this, you will probably run into the following error:

    File 'xxxx' not found (Errcode: 24)

When using MySQL with OS/2 Warp 3, FixPack 29 or above is required. With OS/2 Warp 4, FixPack 4 or above is required. This is a requirement of the Pthreads library. MySQL must be installed in a partition that supports long filenames such as HPFS, FAT32, etc.

The ``INSTALL.CMD'` script must be run from OS/2's own ``CMD.EXE'` and may not work with replacement shells such as ``4OS2.EXE'`.

The ``scripts/mysql-install-db'` script has been renamed. It is now called ``install.cmd'` and is a REXX script, which will set up the default MySQL security settings and create the WorkPlace Shell icons for MySQL.

Dynamic module support is compiled in but not fully tested. Dynamic modules should be compiled using the Pthreads run-time library.

    gcc -Zdll -Zmt -Zcrtdll=pthrdrtl -I../include -I../regex -I.. \
        -o example udf_example.cc -L../lib -lmysqlclient udf_example.def
    mv example.dll example.udf

Note: Due to limitations in OS/2, UDF module name stems must not exceed 8 characters. Modules are stored in the ``/mysql2/udf'` directory; the `safe-mysqld.cmd` script will put this directory in the `BEGINLIBPATH` environment variable. When using UDF modules, specified extensions are ignored@-it is assumed to be ``.udf'`. For example, in Unix, the shared module might be named ``example.so'` and you would load a function from it like this:

    mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "example.so";

Is OS/2, the module would be named ``example.udf'`, but you would not specify the module extension:

    mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "example";

### 2.6.8 BeOS Notes

We are really interested in getting MySQL to work on BeOS, but unfortunately we don't have any person who knows BeOS or has time to do a port.

We are interested in finding someone to do a port, and we will help them with any technical questions they may have while doing the port.

We have previously talked with some BeOS developers that have said that MySQL is 80% ported to BeOS, but we haven't heard from them in a while.

### 2.6.9 Novell NetWare Notes

We are really interested in getting MySQL to work on NetWare, but unfortunately we don't have any person who knows NetWare or has time to do a port.

We are interested in finding someone to do a port, and we will help them with any technical questions they may have while doing the port.

## 2.7 Perl Installation Comments

### 2.7.1 Installing Perl on Unix

Perl support for MySQL is provided by means of the `DBI`/`DBD` client interface. See section 8.2 MySQL Perl API. The Perl `DBD`/`DBI` client code requires Perl Version 5.004 or later. The interface will not work if you have an older version of Perl.

MySQL Perl support also requires that you've installed MySQL client programming support. If you installed MySQL from RPM files, client programs are in the client RPM, but client programming support is in the developer RPM. Make sure you've installed the latter RPM.

As of Version 3.22.8, Perl support is distributed separately from the main MySQL distribution. If you want to install Perl support, the files you will need can be obtained from http://www.mysql.com/Downloads/Contrib/.

The Perl distributions are provided as compressed `tar` archives and have names like ``MODULE-VERSION.tar.gz'`, where `MODULE` is the module name and `VERSION` is the version number. You should get the `Data-Dumper`, `DBI`, and `Msql-Mysql-modules` distributions and install them in that order. The installation procedure is shown here. The example shown is for the `Data-Dumper` module, but the procedure is the same for all three distributions:

1. Unpack the distribution into the current directory:

       shell> gunzip < Data-Dumper-VERSION.tar.gz | tar xvf -

   This command creates a directory named ``Data-Dumper-VERSION'`.
2. Change into the top-level directory of the unpacked distribution:

       shell> cd Data-Dumper-VERSION

3. Build the distribution and compile everything:

       shell> perl Makefile.PL
       shell> make
       shell> make test
       shell> make install

The `make test` command is important because it verifies that the module is working. Note that when you run that command during the `Msql-Mysql-modules` installation to exercise the interface code, the MySQL server must be running or the test will fail.

It is a good idea to rebuild and reinstall the `Msql-Mysql-modules` distribution whenever you install a new release of MySQL, particularly if you notice symptoms such as all your `DBI` scripts dumping core after you upgrade MySQL.

If you don't have the right to install Perl modules in the system directory or if you to install local Perl modules, the following reference may help you:

    http://www.iserver.com/support/contrib/perl5/modules.html

Look under the heading `Installing New Modules that Require Locally Installed Modules`.

### 2.7.2 Installing ActiveState Perl on Windows

To install the MySQL `DBD` module with ActiveState Perl on Windows, you should do the following:

- Get ActiveState Perl from http://www.activestate.com/Products/ActivePerl/ and install it.
- Open a DOS shell.
- If required, set the HTTP_proxy variable. For example, you might try:

      set HTTP_proxy=my.proxy.com:3128

- Start the PPM program:

      C:\> c:\perl\bin\ppm.pl

- If you have not already done so, install `DBI`:

      ppm> install DBI

- If this succeeds, run the following command:

      install \
      ftp://ftp.de.uu.net/pub/CPAN/authors/id/JWIED/DBD-mysql-1.2212.x86.ppd

The above should work at least with ActiveState Perl Version 5.6.

If you can't get the above to work, you should instead install the `MyODBC` driver and connect to MySQL server through ODBC:

    use DBI;
    $dbh= DBI->connect("DBI:ODBC:$dsn","$user","$password") ||
      die "Got error $DBI::errstr when connecting to $dsn\n";

### 2.7.3 Installing the MySQL Perl Distribution on Windows

The MySQL Perl distribution contains `DBI`, `DBD:MySQL` and `DBD:ODBC`.

- Get the Perl distribution for Windows from http://www.mysql.com/downloads/os-win32.html.
- Unzip the distribution in `C:` so that you get a ``C:\PERL'` directory.
- Add the directory ``C:\PERL\BIN'` to your path.
- Add the directory ``C:\PERL\BIN\MSWIN32-x86-thread'` or ``C:\PERL\BIN\MSWIN32-x86'` to your path.
- Test that `perl` works by executing `perl -v` in a DOS shell.

### 2.7.4 Problems Using the Perl `DBI`/`DBD` Interface

If Perl reports that it can't find the ``../mysql/mysql.so'` module, then the problem is probably that Perl can't locate the shared library ``libmysqlclient.so'`.

You can fix this by any of the following methods:

- Compile the `Msql-Mysql-modules` distribution with `perl Makefile.PL -static -config` rather than `perl Makefile.PL`.
- Copy ``libmysqlclient.so'` to the directory where your other shared libraries are located (probably ``/usr/lib'` or ``/lib'`).
- On Linux you can add the pathname of the directory where ``libmysqlclient.so'` is located to the ``/etc/ld.so.conf'` file.
- Add the pathname of the directory where ``libmysqlclient.so'` is located to the `LD_RUN_PATH` environment variable.

If you get the following errors from `DBD-mysql`, you are probably using `gcc` (or using an old binary compiled with `gcc`):

    /usr/bin/perl: can't resolve symbol '__moddi3'
    /usr/bin/perl: can't resolve symbol '__divdi3'

Add `-L/usr/lib/gcc-lib/... -lgcc` to the link command when the ``mysql.so'` library gets built (check the output from `make` for ``mysql.so'` when you compile the Perl client). The `-L` option should specify the pathname of the directory where ``libgcc.a'` is located on your system.

Another cause of this problem may be that Perl and MySQL aren't both compiled with `gcc`. In this case, you can solve the mismatch by compiling both with `gcc`.

If you get the following error from `Msql-Mysql-modules` when you run the tests:

    t/00base............install_driver(mysql) failed:
    Can't load '../blib/arch/auto/DBD/mysql/mysql.so' for module DBD::mysql:
    ../blib/arch/auto/DBD/mysql/mysql.so: undefined symbol:
    uncompress at /usr/lib/perl5/5.00503/i586-linux/DynaLoader.pm line 169.

it means that you need to include the compression library, -lz, to the link line. This can be doing the following change in the file ``lib/DBD/mysql/Install.pm'`:

    $sysliblist .= " -lm";

    to

    $sysliblist .= " -lm -lz";

After this, you must run 'make realclean' and then proceed with the installation from the beginning.

If you want to use the Perl module on a system that doesn't support dynamic linking (like Caldera/SCO) you can generate a static version of Perl that includes `DBI` and `DBD-mysql`. The way this works is that you generate a version of Perl with the `DBI` code linked in and install it on top of your current Perl. Then you use that to build a version of Perl that additionally has the `DBD` code linked in, and install that.

On Caldera (SCO), you must have the following environment variables set:

    shell> LD_LIBRARY_PATH=/lib:/usr/lib:/usr/local/lib:/usr/progressive/lib
    or
    shell> LD_LIBRARY_PATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:\
    /usr/progressive/lib:/usr/skunk/lib
    shell> LIBPATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:\
    /usr/progressive/lib:/usr/skunk/lib
    shell> MANPATH=scohelp:/usr/man:/usr/local1/man:/usr/local/man:\
    /usr/skunk/man:

First, create a Perl that includes a statically linked `DBI` by running these commands in the directory where your `DBI` distribution is located:

    shell> perl Makefile.PL -static -config
    shell> make
    shell> make install
    shell> make perl

Then you must install the new Perl. The output of `make perl` will indicate the exact `make` command you will need to execute to perform the installation. On Caldera (SCO), this is `make -f Makefile.aperl inst_perl MAP_TARGET=perl`.

Next, use the just-created Perl to create another Perl that also includes a statically-linked `DBD::mysql` by running these commands in the directory where your `Msql-Mysql-modules` distribution is located:

    shell> perl Makefile.PL -static -config
    shell> make
    shell> make install
    shell> make perl

Finally, you should install this new Perl. Again, the output of `make perl` indicates the command to use.

# 3 Tutorial Introduction

This chapter provides a tutorial introduction to MySQL by showing how to use the `mysql` client program to create and use a simple database. `mysql` (sometimes referred to as the ``terminal monitor'' or just ``monitor'') is an interactive program that allows you to connect to a MySQL server, run queries, and view the results. `mysql` may also be used in batch mode: you place your queries in a file beforehand, then tell `mysql` to execute the contents of the file. Both ways of using `mysql` are covered here.

To see a list of options provided by `mysql`, invoke it with the `--help` option:

    shell> mysql --help

This chapter assumes that `mysql` is installed on your machine and that a MySQL server is available to which you can connect. If this is not true, contact your MySQL administrator. (If you are the administrator, you will need to consult other sections of this manual.)

This chapter describes the entire process of setting up and using a database. If you are interested only in accessing an already-existing database, you may want to skip over the sections that describe how to create the database and the tables it contains.

Because this chapter is tutorial in nature, many details are necessarily left out. Consult the relevant sections of the manual for more information on the topics covered here.

## 3.1 Connecting to and Disconnecting from the Server

To connect to the server, you'll usually need to provide a MySQL user name when you invoke `mysql` and, most likely, a password. If the server runs on a machine other than the one where you log in, you'll also need to specify a hostname. Contact your administrator to find out what connection parameters you should use to connect (that is, what host, user name, and password to use). Once you know the proper parameters, you should be able to connect like this:

    shell> mysql -h host -u user -p
    Enter password: ********

The `********` represents your password; enter it when `mysql` displays the `Enter password:` prompt.

If that works, you should see some introductory information followed by a `mysql>` prompt:

    shell> mysql -h host -u user -p
    Enter password: ********
    Welcome to the MySQL monitor.  Commands end with ; or \g.
    Your MySQL connection id is 459 to server version: 3.22.20a-log

    Type 'help' for help.

    mysql>

The prompt tells you that `mysql` is ready for you to enter commands.

Some MySQL installations allow users to connect as the anonymous (unnamed) user to the server running on the local host. If this is the case on your machine, you should be able to connect to that server by invoking `mysql` without any options:

    shell> mysql

After you have connected successfully, you can disconnect any time by typing `QUIT` at the `mysql>` prompt:

    mysql> QUIT
    Bye

You can also disconnect by pressing Control-D.

Most examples in the following sections assume you are connected to the server. They indicate this by the `mysql>` prompt.

## 3.2 Entering Queries

Make sure you are connected to the server, as discussed in the previous section. Doing so will not in itself select any database to work with, but that's okay. At this point, it's more important to find out a little about how to issue queries than to jump right in creating tables, loading data into them, and retrieving data from them. This section describes the basic principles of entering commands, using several queries you can try out to familiarise yourself with how `mysql` works.

Here's a simple command that asks the server to tell you its version number and the current date. Type it in as shown here following the `mysql>` prompt and press Enter:

    mysql> SELECT VERSION(), CURRENT_DATE;
    +--------------+--------------+
    | VERSION()    | CURRENT_DATE |
    +--------------+--------------+
    | 3.22.20a-log | 1999-03-19   |
    +--------------+--------------+
    1 row in set (0.01 sec)
    mysql>

This query illustrates several things about `mysql`:

- A command normally consists of a SQL statement followed by a semicolon. (There are some exceptions where a semicolon is not needed. `QUIT`, mentioned earlier, is one of them. We'll get to others later.)
- When you issue a command, `mysql` sends it to the server for execution and displays the results, then prints another `mysql>` to indicate that it is ready for another command.
- `mysql` displays query output as a table (rows and columns). The first row contains labels for the columns. The rows following are the query results. Normally, column labels are the names of the columns you fetch from database tables. If you're retrieving the value of an expression rather than a table column (as in the example just shown), `mysql` labels the column using the expression itself.
- `mysql` shows how many rows were returned and how long the query took to execute, which gives you a rough idea of server performance. These values are imprecise because they represent wall clock time (not CPU or machine time), and because they are affected by factors such as server load and network latency. (For brevity, the ``rows in set'' line is not shown in the remaining examples in this chapter.)

Keywords may be entered in any lettercase. The following queries are equivalent:

    mysql> SELECT VERSION(), CURRENT_DATE;
    mysql> select version(), current_date;
    mysql> SeLeCt vErSiOn(), current_DATE;

Here's another query. It demonstrates that you can use `mysql` as a simple calculator:

    mysql> SELECT SIN(PI()/4), (4+1)*5;
    +-------------+---------+
    | SIN(PI()/4) | (4+1)*5 |
    +-------------+---------+
    |    0.707107 |      25 |
    +-------------+---------+

The commands shown thus far have been relatively short, single-line statements. You can even enter multiple statements on a single line. Just end each one with a semicolon:

    mysql> SELECT VERSION(); SELECT NOW();
    +--------------+
    | VERSION()    |
    +--------------+
    | 3.22.20a-log |
    +--------------+

    +---------------------+
    | NOW()               |
    +---------------------+
    | 1999-03-19 00:15:33 |
    +---------------------+

A command need not be given all on a single line, so lengthy commands that require several lines are not a problem. `mysql` determines where your statement ends by looking for the terminating semicolon, not by looking for the end of the input line. (In other words, `mysql` accepts free-format input: it collects input lines but does not execute them until it sees the semicolon.)

Here's a simple multiple-line statement:

    mysql> SELECT
        -> USER()
        -> ,
        -> CURRENT_DATE;
    +--------------------+--------------+
    | USER()             | CURRENT_DATE |
    +--------------------+--------------+
    | joesmith@localhost | 1999-03-18   |
    +--------------------+--------------+

In this example, notice how the prompt changes from `mysql>` to `->` after you enter the first line of a multiple-line query. This is how `mysql` indicates that it hasn't seen a complete statement and is waiting for the rest. The prompt is your friend, because it provides valuable feedback. If you use that feedback, you will always be aware of what `mysql` is waiting for.

If you decide you don't want to execute a command that you are in the process of entering, cancel it by typing `\c`:

    mysql> SELECT
        -> USER()
        -> \c
    mysql>

Here, too, notice the prompt. It switches back to `mysql>` after you type `\c`, providing feedback to indicate that `mysql` is ready for a new command.

The following table shows each of the prompts you may see and summarises what they mean about the state that `mysql` is in:

Prompt | Meaning
`mysql>` | Ready for new command.
`->` | Waiting for next line of multiple-line command.
`'>` | Waiting for next line, collecting a string that begins with a single quote (``''`).
`">` | Waiting for next line, collecting a string that begins with a double quote (``"'`).

Multiple-line statements commonly occur by accident when you intend to issue a command on a single line, but forget the terminating semicolon. In this case, `mysql` waits for more input:

    mysql> SELECT USER()
        ->

If this happens to you (you think you've entered a statement but the only response is a `->` prompt), most likely `mysql` is waiting for the semicolon. If you don't notice what the prompt is telling you, you might sit there for a while before realising what you need to do. Enter a semicolon to complete the statement, and `mysql` will execute it:

    mysql> SELECT USER()
        -> ;
    +--------------------+
    | USER()             |
    +--------------------+
    | joesmith@localhost |
    +--------------------+

The `'>` and `">` prompts occur during string collection. In MySQL, you can write strings surrounded by either ``''` or ``"'` characters (for example, `'hello'` or `"goodbye"`), and `mysql` lets you enter strings that span multiple lines. When you see a `'>` or `">` prompt, it means that you've entered a line containing a string that begins with a ``''` or ``"'` quote character, but have not yet entered the matching quote that terminates the string. That's fine if you really are entering a multiple-line string, but how likely is that? Not very. More often, the `'>` and `">` prompts indicate that you've inadvertantly left out a quote character. For example:

    mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30;
        ">

If you enter this `SELECT` statement, then press Enter and wait for the result, nothing will happen. Instead of wondering why this query takes so long, notice the clue provided by the `">` prompt. It tells you that `mysql` expects to see the rest of an unterminated string. (Do you see the error in the statement? The string `"Smith` is missing the second quote.)

At this point, what do you do? The simplest thing is to cancel the command. However, you cannot just type `\c` in this case, because `mysql` interprets it as part of the string that it is collecting! Instead, enter the closing quote character (so `mysql` knows you've finished the string), then type `\c`:

    mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30;
        "> "\c
    mysql>

The prompt changes back to `mysql>`, indicating that `mysql` is ready for a new command.

It's important to know what the `'>` and `">` prompts signify, because if you mistakenly enter an unterminated string, any further lines you type will appear to be ignored by `mysql`@-including a line containing `QUIT`! This can be quite confusing, especially if you don't know that you need to supply the terminating quote before you can cancel the current command.

## 3.3 Creating and Using a Database

Now that you know how to enter commands, it's time to access a database.

Suppose you have several pets in your home (your menagerie) and you'd like to keep track of various types of information about them. You can do so by creating tables to hold your data and loading them with the desired information. Then you can answer different sorts of questions about your animals by retrieving data from the tables. This section shows you how to:

- Create a database
- Create a table
- Load data into the table
- Retrieve data from the table in various ways
- Use multiple tables

The menagerie database will be simple (deliberately), but it is not difficult to think of real-world situations in which a similar type of database might be used. For example, a database like this could be used by a farmer to keep track of livestock, or by a veterinarian to keep track of patient records. A menagerie distribution containing some of the queries and sample data used in the following sections can be obtained from the MySQL web site. It's available in either compressed `tar` format (http://www.mysql.com/Downloads/Contrib/Examples/menagerie.tar.gz) or Zip format (http://www.mysql.com/Downloads/Contrib/Examples/menagerie.zip).

Use the `SHOW` statement to find out what databases currently exist on the server:

    mysql> SHOW DATABASES;
    +----------+
    | Database |
    +----------+
    | mysql    |
    | test     |
    | tmp      |
    +----------+

The list of databases is probably different on your machine, but the `mysql` and `test` databases are likely to be among them. The `mysql` database is required because it describes user access privileges. The `test` database is often provided as a workspace for users to try things out.

Note that you may not see all databases if you don't have the `SHOW DATABASES` privilege. See section 4.3.1 `GRANT` and `REVOKE` Syntax.

If the `test` database exists, try to access it:

    mysql> USE test
    Database changed

Note that `USE`, like `QUIT`, does not require a semicolon. (You can terminate such statements with a semicolon if you like; it does no harm.) The `USE` statement is special in another way, too: it must be given on a single line.

You can use the `test` database (if you have access to it) for the examples that follow, but anything you create in that database can be removed by anyone else with access to it. For this reason, you should probably ask your MySQL administrator for permission to use a database of your own. Suppose you want to call yours `menagerie`. The administrator needs to execute a command like this:

    mysql> GRANT ALL ON menagerie.* TO your_mysql_name;

where `your_mysql_name` is the MySQL user name assigned to you.

### 3.3.1 Creating and Selecting a Database

If the administrator creates your database for you when setting up your permissions, you can begin using it. Otherwise, you need to create it yourself:

    mysql> CREATE DATABASE menagerie;

Under Unix, database names are case-sensitive (unlike SQL keywords), so you must always refer to your database as `menagerie`, not as `Menagerie`, `MENAGERIE`, or some other variant. This is also true for table names. (Under Windows, this restriction does not apply, although you must refer to databases and tables using the same lettercase throughout a given query.)

Creating a database does not select it for use; you must do that explicitly. To make `menagerie` the current database, use this command:

    mysql> USE menagerie
    Database changed

Your database needs to be created only once, but you must select it for use each time you begin a `mysql` session. You can do this by issuing a `USE` statement as shown above. Alternatively, you can select the database on the command-line when you invoke `mysql`. Just specify its name after any connection parameters that you might need to provide. For example:

    shell> mysql -h host -u user -p menagerie
    Enter password: ********

Note that `menagerie` is not your password on the command just shown. If you want to supply your password on the command-line after the `-p` option, you must do so with no intervening space (for example, as `-pmypassword`, not as `-p mypassword`). However, putting your password on the command-line is not recommended, because doing so exposes it to snooping by other users logged in on your machine.

### 3.3.2 Creating a Table

Creating the database is the easy part, but at this point it's empty, as `SHOW TABLES` will tell you:

    mysql> SHOW TABLES;
    Empty set (0.00 sec)

The harder part is deciding what the structure of your database should be: what tables you will need and what columns will be in each of them.

You'll want a table that contains a record for each of your pets. This can be called the `pet` table, and it should contain, as a bare minimum, each animal's name. Because the name by itself is not very interesting, the table should contain other information. For example, if more than one person in your family keeps pets, you might want to list each animal's owner. You might also want to record some basic descriptive information such as species and sex.

How about age? That might be of interest, but it's not a good thing to store in a database. Age changes as time passes, which means you'd have to update your records often. Instead, it's better to store a fixed value such as date of birth. Then, whenever you need age, you can calculate it as the difference between the current date and the birth date. MySQL provides functions for doing date arithmetic, so this is not difficult. Storing birth date rather than age has other advantages, too:

- You can use the database for tasks such as generating reminders for upcoming pet birthdays. (If you think this type of query is somewhat silly, note that it is the same question you might ask in the context of a business database to identify clients to whom you'll soon need to send out birthday greetings, for that computer-assisted personal touch.)
- You can calculate age in relation to dates other than the current date. For example, if you store death date in the database, you can easily calculate how old a pet was when it died.

You can probably think of other types of information that would be useful in the `pet` table, but the ones identified so far are sufficient for now: name, owner, species, sex, birth, and death.

Use a `CREATE TABLE` statement to specify the layout of your table:

    mysql> CREATE TABLE pet (name VARCHAR(20), owner VARCHAR(20),
        -> species VARCHAR(20), sex CHAR(1), birth DATE, death DATE);

`VARCHAR` is a good choice for the `name`, `owner`, and `species` columns because the column values will vary in length. The lengths of those columns need not all be the same, and need not be `20`. You can pick any length from `1` to `255`, whatever seems most reasonable to you. (If you make a poor choice and it turns out later that you need a longer field, MySQL provides an `ALTER TABLE` statement.)

Several types of values can be chosen to represent sex in animal records, such as `"m"` and `"f"`, or perhaps `"male"` and `"female"`. It's simplest to use the single characters `"m"` and `"f"`.

The use of the `DATE` data type for the `birth` and `death` columns is a fairly obvious choice.

Now that you have created a table, `SHOW TABLES` should produce some output:

    mysql> SHOW TABLES;
    +---------------------+
    | Tables in menagerie |
    +---------------------+
    | pet                 |
    +---------------------+

To verify that your table was created the way you expected, use a `DESCRIBE` statement:

    mysql> DESCRIBE pet;
    +---------+-------------+------+-----+---------+-------+
    | Field   | Type        | Null | Key | Default | Extra |
    +---------+-------------+------+-----+---------+-------+
    | name    | varchar(20) | YES  |     | NULL    |       |
    | owner   | varchar(20) | YES  |     | NULL    |       |
    | species | varchar(20) | YES  |     | NULL    |       |
    | sex     | char(1)     | YES  |     | NULL    |       |
    | birth   | date        | YES  |     | NULL    |       |
    | death   | date        | YES  |     | NULL    |       |
    +---------+-------------+------+-----+---------+-------+

You can use `DESCRIBE` any time, for example, if you forget the names of the columns in your table or what types they are.

### 3.3.3 Loading Data into a Table

After creating your table, you need to populate it. The `LOAD DATA` and `INSERT` statements are useful for this.

Suppose your pet records can be described as shown here. (Observe that MySQL expects dates in `'YYYY-MM-DD'` format; this may be different from what you are used to.)

name | owner | species | sex | birth | death
Fluffy | Harold | cat | f | 1993-02-04 |
Claws | Gwen | cat | m | 1994-03-17 |
Buffy | Harold | dog | f | 1989-05-13 |
Fang | Benny | dog | m | 1990-08-27 |
Bowser | Diane | dog | m | 1998-08-31 | 1995-07-29
Chirpy | Gwen | bird | f | 1998-09-11 |
Whistler | Gwen | bird |  | 1997-12-09 |
Slim | Benny | snake | m | 1996-04-29 |

Because you are beginning with an empty table, an easy way to populate it is to create a text file containing a row for each of your animals, then load the contents of the file into the table with a single statement.

You could create a text file ``pet.txt'` containing one record per line, with values separated by tabs, and given in the order in which the columns were listed in the `CREATE TABLE` statement. For missing values (such as unknown sexes or death dates for animals that are still living), you can use `NULL` values. To represent these in your text file, use `\N`. For example, the record for Whistler the bird would look like this (where the whitespace between values is a single tab character):

name | owner | species | sex | birth | death
`Whistler` | `Gwen` | `bird` | `\N` | `1997-12-09` | `\N`

To load the text file ``pet.txt'` into the `pet` table, use this command:

    mysql> LOAD DATA LOCAL INFILE "pet.txt" INTO TABLE pet;

You can specify the column value separator and end of line marker explicitly in the `LOAD DATA` statement if you wish, but the defaults are tab and linefeed. These are sufficient for the statement to read the file ``pet.txt'` properly.

When you want to add new records one at a time, the `INSERT` statement is useful. In its simplest form, you supply values for each column, in the order in which the columns were listed in the `CREATE TABLE` statement. Suppose Diane gets a new hamster named Puffball. You could add a new record using an `INSERT` statement like this:

    mysql> INSERT INTO pet
        -> VALUES ('Puffball','Diane','hamster','f','1999-03-30',NULL);

Note that string and date values are specified as quoted strings here. Also, with `INSERT`, you can insert `NULL` directly to represent a missing value. You do not use `\N` like you do with `LOAD DATA`.

From this example, you should be able to see that there would be a lot more typing involved to load your records initially using several `INSERT` statements rather than a single `LOAD DATA` statement.

### 3.3.4 Retrieving Information from a Table

The `SELECT` statement is used to pull information from a table. The general form of the statement is:

    SELECT what_to_select
    FROM which_table
    WHERE conditions_to_satisfy

`what_to_select` indicates what you want to see. This can be a list of columns, or `*` to indicate ``all columns.'' `which_table` indicates the table from which you want to retrieve data. The `WHERE` clause is optional. If it's present, `conditions_to_satisfy` specifies conditions that rows must satisfy to qualify for retrieval.

#### 3.3.4.1 Selecting All Data

The simplest form of `SELECT` retrieves everything from a table:

    mysql> SELECT * FROM pet;
    +----------+--------+---------+------+------------+------------+
    | name     | owner  | species | sex  | birth      | death      |
    +----------+--------+---------+------+------------+------------+
    | Fluffy   | Harold | cat     | f    | 1993-02-04 | NULL       |
    | Claws    | Gwen   | cat     | m    | 1994-03-17 | NULL       |
    | Buffy    | Harold | dog     | f    | 1989-05-13 | NULL       |
    | Fang     | Benny  | dog     | m    | 1990-08-27 | NULL       |
    | Bowser   | Diane  | dog     | m    | 1998-08-31 | 1995-07-29 |
    | Chirpy   | Gwen   | bird    | f    | 1998-09-11 | NULL       |
    | Whistler | Gwen   | bird    | NULL | 1997-12-09 | NULL       |
    | Slim     | Benny  | snake   | m    | 1996-04-29 | NULL       |
    | Puffball | Diane  | hamster | f    | 1999-03-30 | NULL       |
    +----------+--------+---------+------+------------+------------+

This form of `SELECT` is useful if you want to review your entire table, for instance, after you've just loaded it with your initial dataset. As it happens, the output just shown reveals an error in your datafile: Bowser appears to have been born after he died! Consulting your original pedigree papers, you find that the correct birth year is 1989, not 1998.

There are are least a couple of ways to fix this:

- Edit the file ``pet.txt'` to correct the error, then empty the table and reload it using `DELETE` and `LOAD DATA`:

      mysql> SET AUTOCOMMIT=1;  # Used for quick re-create of the table
      mysql> DELETE FROM pet;
      mysql> LOAD DATA LOCAL INFILE "pet.txt" INTO TABLE pet;

  However, if you do this, you must also re-enter the record for Puffball.
- Fix only the erroneous record with an `UPDATE` statement:

      mysql> UPDATE pet SET birth = "1989-08-31" WHERE name = "Bowser";

As shown above, it is easy to retrieve an entire table. But typically you don't want to do that, particularly when the table becomes large. Instead, you're usually more interested in answering a particular question, in which case you specify some constraints on the information you want. Let's look at some selection queries in terms of questions about your pets that they answer.

#### 3.3.4.2 Selecting Particular Rows

You can select only particular rows from your table. For example, if you want to verify the change that you made to Bowser's birth date, select Bowser's record like this:

    mysql> SELECT * FROM pet WHERE name = "Bowser";
    +--------+-------+---------+------+------------+------------+
    | name   | owner | species | sex  | birth      | death      |
    +--------+-------+---------+------+------------+------------+
    | Bowser | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
    +--------+-------+---------+------+------------+------------+

The output confirms that the year is correctly recorded now as 1989, not 1998.

String comparisons are normally case-insensitive, so you can specify the name as `"bowser"`, `"BOWSER"`, etc. The query result will be the same.

You can specify conditions on any column, not just `name`. For example, if you want to know which animals were born after 1998, test the `birth` column:

    mysql> SELECT * FROM pet WHERE birth >= "1998-1-1";
    +----------+-------+---------+------+------------+-------+
    | name     | owner | species | sex  | birth      | death |
    +----------+-------+---------+------+------------+-------+
    | Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
    | Puffball | Diane | hamster | f    | 1999-03-30 | NULL  |
    +----------+-------+---------+------+------------+-------+

You can combine conditions, for example, to locate female dogs:

    mysql> SELECT * FROM pet WHERE species = "dog" AND sex = "f";
    +-------+--------+---------+------+------------+-------+
    | name  | owner  | species | sex  | birth      | death |
    +-------+--------+---------+------+------------+-------+
    | Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
    +-------+--------+---------+------+------------+-------+

The preceding query uses the `AND` logical operator. There is also an `OR` operator:

    mysql> SELECT * FROM pet WHERE species = "snake" OR species = "bird";
    +----------+-------+---------+------+------------+-------+
    | name     | owner | species | sex  | birth      | death |
    +----------+-------+---------+------+------------+-------+
    | Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
    | Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL  |
    | Slim     | Benny | snake   | m    | 1996-04-29 | NULL  |
    +----------+-------+---------+------+------------+-------+

`AND` and `OR` may be intermixed. If you do that, it's a good idea to use parentheses to indicate how conditions should be grouped:

    mysql> SELECT * FROM pet WHERE (species = "cat" AND sex = "m")
        -> OR (species = "dog" AND sex = "f");
    +-------+--------+---------+------+------------+-------+
    | name  | owner  | species | sex  | birth      | death |
    +-------+--------+---------+------+------------+-------+
    | Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
    | Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
    +-------+--------+---------+------+------------+-------+

#### 3.3.4.3 Selecting Particular Columns

If you don't want to see entire rows from your table, just name the columns in which you're interested, separated by commas. For example, if you want to know when your animals were born, select the `name` and `birth` columns:

    mysql> SELECT name, birth FROM pet;
    +----------+------------+
    | name     | birth      |
    +----------+------------+
    | Fluffy   | 1993-02-04 |
    | Claws    | 1994-03-17 |
    | Buffy    | 1989-05-13 |
    | Fang     | 1990-08-27 |
    | Bowser   | 1989-08-31 |
    | Chirpy   | 1998-09-11 |
    | Whistler | 1997-12-09 |
    | Slim     | 1996-04-29 |
    | Puffball | 1999-03-30 |
    +----------+------------+

To find out who owns pets, use this query:

    mysql> SELECT owner FROM pet;
    +--------+
    | owner  |
    +--------+
    | Harold |
    | Gwen   |
    | Harold |
    | Benny  |
    | Diane  |
    | Gwen   |
    | Gwen   |
    | Benny  |
    | Diane  |
    +--------+

However, notice that the query simply retrieves the `owner` field from each record, and some of them appear more than once. To minimise the output, retrieve each unique output record just once by adding the keyword `DISTINCT`:

    mysql> SELECT DISTINCT owner FROM pet;
    +--------+
    | owner  |
    +--------+
    | Benny  |
    | Diane  |
    | Gwen   |
    | Harold |
    +--------+

You can use a `WHERE` clause to combine row selection with column selection. For example, to get birth dates for dogs and cats only, use this query:

    mysql> SELECT name, species, birth FROM pet
        -> WHERE species = "dog" OR species = "cat";
    +--------+---------+------------+
    | name   | species | birth      |
    +--------+---------+------------+
    | Fluffy | cat     | 1993-02-04 |
    | Claws  | cat     | 1994-03-17 |
    | Buffy  | dog     | 1989-05-13 |
    | Fang   | dog     | 1990-08-27 |
    | Bowser | dog     | 1989-08-31 |
    +--------+---------+------------+

#### 3.3.4.4 Sorting Rows

You may have noticed in the preceding examples that the result rows are displayed in no particular order. However, it's often easier to examine query output when the rows are sorted in some meaningful way. To sort a result, use an `ORDER BY` clause.

Here are animal birthdays, sorted by date:

    mysql> SELECT name, birth FROM pet ORDER BY birth;
    +----------+------------+
    | name     | birth      |
    +----------+------------+
    | Buffy    | 1989-05-13 |
    | Bowser   | 1989-08-31 |
    | Fang     | 1990-08-27 |
    | Fluffy   | 1993-02-04 |
    | Claws    | 1994-03-17 |
    | Slim     | 1996-04-29 |
    | Whistler | 1997-12-09 |
    | Chirpy   | 1998-09-11 |
    | Puffball | 1999-03-30 |
    +----------+------------+

On character type columns, sorting@-like all other comparison operations@-is normally performed in a case-insensitive fashion. This means that the order will be undefined for columns that are identical except for their case. You can force a case-sensitive sort by using the BINARY cast: `ORDER BY BINARY(field)`.

To sort in reverse order, add the `DESC` (descending) keyword to the name of the column you are sorting by:

    mysql> SELECT name, birth FROM pet ORDER BY birth DESC;
    +----------+------------+
    | name     | birth      |
    +----------+------------+
    | Puffball | 1999-03-30 |
    | Chirpy   | 1998-09-11 |
    | Whistler | 1997-12-09 |
    | Slim     | 1996-04-29 |
    | Claws    | 1994-03-17 |
    | Fluffy   | 1993-02-04 |
    | Fang     | 1990-08-27 |
    | Bowser   | 1989-08-31 |
    | Buffy    | 1989-05-13 |
    +----------+------------+

You can sort on multiple columns. For example, to sort by type of animal, then by birth date within animal type with youngest animals first, use the following query:

    mysql> SELECT name, species, birth FROM pet ORDER BY species, birth DESC;
    +----------+---------+------------+
    | name     | species | birth      |
    +----------+---------+------------+
    | Chirpy   | bird    | 1998-09-11 |
    | Whistler | bird    | 1997-12-09 |
    | Claws    | cat     | 1994-03-17 |
    | Fluffy   | cat     | 1993-02-04 |
    | Fang     | dog     | 1990-08-27 |
    | Bowser   | dog     | 1989-08-31 |
    | Buffy    | dog     | 1989-05-13 |
    | Puffball | hamster | 1999-03-30 |
    | Slim     | snake   | 1996-04-29 |
    +----------+---------+------------+

Note that the `DESC` keyword applies only to the column name immediately preceding it (`birth`); `species` values are still sorted in ascending order.

#### 3.3.4.5 Date Calculations

MySQL provides several functions that you can use to perform calculations on dates, for example, to calculate ages or extract parts of dates.

To determine how many years old each of your pets is, compute the difference in the year part of the current date and the birth date, then subtract one if the current date occurs earlier in the calendar year than the birth date. The following query shows, for each pet, the birth date, the current date, and the age in years.

    mysql> SELECT name, birth, CURRENT_DATE,
        -> (YEAR(CURRENT_DATE)-YEAR(birth))
        -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5))
        -> AS age
        -> FROM pet;
    +----------+------------+--------------+------+
    | name     | birth      | CURRENT_DATE | age  |
    +----------+------------+--------------+------+
    | Fluffy   | 1993-02-04 | 2001-08-29   |    8 |
    | Claws    | 1994-03-17 | 2001-08-29   |    7 |
    | Buffy    | 1989-05-13 | 2001-08-29   |   12 |
    | Fang     | 1990-08-27 | 2001-08-29   |   11 |
    | Bowser   | 1989-08-31 | 2001-08-29   |   11 |
    | Chirpy   | 1998-09-11 | 2001-08-29   |    2 |
    | Whistler | 1997-12-09 | 2001-08-29   |    3 |
    | Slim     | 1996-04-29 | 2001-08-29   |    5 |
    | Puffball | 1999-03-30 | 2001-08-29   |    2 |
    +----------+------------+--------------+------+

Here, `YEAR()` pulls out the year part of a date and `RIGHT()` pulls off the rightmost five characters that represent the `MM-DD` (calendar year) part of the date. The part of the expression that compares the `MM-DD` values evaluates to 1 or 0, which adjusts the year difference down a year if `CURRENT_DATE` occurs earlier in the year than `birth`. The full expression is somewhat ungainly, so an alias (`age`) is used to make the output column label more meaningful.

The query works, but the result could be scanned more easily if the rows were presented in some order. This can be done by adding an `ORDER BY name` clause to sort the output by name:

    mysql> SELECT name, birth, CURRENT_DATE,
        -> (YEAR(CURRENT_DATE)-YEAR(birth))
        -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5))
        -> AS age
        -> FROM pet ORDER BY name;
    +----------+------------+--------------+------+
    | name     | birth      | CURRENT_DATE | age  |
    +----------+------------+--------------+------+
    | Bowser   | 1989-08-31 | 2001-08-29   |   11 |
    | Buffy    | 1989-05-13 | 2001-08-29   |   12 |
    | Chirpy   | 1998-09-11 | 2001-08-29   |    2 |
    | Claws    | 1994-03-17 | 2001-08-29   |    7 |
    | Fang     | 1990-08-27 | 2001-08-29   |   11 |
    | Fluffy   | 1993-02-04 | 2001-08-29   |    8 |
    | Puffball | 1999-03-30 | 2001-08-29   |    2 |
    | Slim     | 1996-04-29 | 2001-08-29   |    5 |
    | Whistler | 1997-12-09 | 2001-08-29   |    3 |
    +----------+------------+--------------+------+

To sort the output by `age` rather than `name`, just use a different `ORDER BY` clause:

    mysql> SELECT name, birth, CURRENT_DATE,
        -> (YEAR(CURRENT_DATE)-YEAR(birth))
        -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5))
        -> AS age
        -> FROM pet ORDER BY age;
    +----------+------------+--------------+------+
    | name     | birth      | CURRENT_DATE | age  |
    +----------+------------+--------------+------+
    | Chirpy   | 1998-09-11 | 2001-08-29   |    2 |
    | Puffball | 1999-03-30 | 2001-08-29   |    2 |
    | Whistler | 1997-12-09 | 2001-08-29   |    3 |
    | Slim     | 1996-04-29 | 2001-08-29   |    5 |
    | Claws    | 1994-03-17 | 2001-08-29   |    7 |
    | Fluffy   | 1993-02-04 | 2001-08-29   |    8 |
    | Fang     | 1990-08-27 | 2001-08-29   |   11 |
    | Bowser   | 1989-08-31 | 2001-08-29   |   11 |
    | Buffy    | 1989-05-13 | 2001-08-29   |   12 |
    +----------+------------+--------------+------+

A similar query can be used to determine age at death for animals that have died. You determine which animals these are by checking whether the `death` value is `NULL`. Then, for those with non-`NULL` values, compute the difference between the `death` and `birth` values:

    mysql> SELECT name, birth, death,
        -> (YEAR(death)-YEAR(birth)) - (RIGHT(death,5)<RIGHT(birth,5))
        -> AS age
        -> FROM pet WHERE death IS NOT NULL ORDER BY age;
    +--------+------------+------------+------+
    | name   | birth      | death      | age  |
    +--------+------------+------------+------+
    | Bowser | 1989-08-31 | 1995-07-29 |    5 |
    +--------+------------+------------+------+

The query uses `death IS NOT NULL` rather than `death <> NULL` because `NULL` is a special value. This is explained later. See section 3.3.4.6 Working with `NULL` Values.

What if you want to know which animals have birthdays next month? For this type of calculation, year and day are irrelevant; you simply want to extract the month part of the `birth` column. MySQL provides several date-part extraction functions, such as `YEAR()`, `MONTH()`, and `DAYOFMONTH()`. `MONTH()` is the appropriate function here. To see how it works, run a simple query that displays the value of both `birth` and `MONTH(birth)`:

    mysql> SELECT name, birth, MONTH(birth) FROM pet;
    +----------+------------+--------------+
    | name     | birth      | MONTH(birth) |
    +----------+------------+--------------+
    | Fluffy   | 1993-02-04 |            2 |
    | Claws    | 1994-03-17 |            3 |
    | Buffy    | 1989-05-13 |            5 |
    | Fang     | 1990-08-27 |            8 |
    | Bowser   | 1989-08-31 |            8 |
    | Chirpy   | 1998-09-11 |            9 |
    | Whistler | 1997-12-09 |           12 |
    | Slim     | 1996-04-29 |            4 |
    | Puffball | 1999-03-30 |            3 |
    +----------+------------+--------------+

Finding animals with birthdays in the upcoming month is easy, too. Suppose the current month is April. Then the month value is `4` and you look for animals born in May (month 5) like this:

    mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5;
    +-------+------------+
    | name  | birth      |
    +-------+------------+
    | Buffy | 1989-05-13 |
    +-------+------------+

There is a small complication if the current month is December, of course. You don't just add one to the month number (`12`) and look for animals born in month 13, because there is no such month. Instead, you look for animals born in January (month 1).

You can even write the query so that it works no matter what the current month is. That way you don't have to use a particular month number in the query. `DATE_ADD()` allows you to add a time interval to a given date. If you add a month to the value of `NOW()`, then extract the month part with `MONTH()`, the result produces the month in which to look for birthdays:

    mysql> SELECT name, birth FROM pet
        -> WHERE MONTH(birth) = MONTH(DATE_ADD(NOW(), INTERVAL 1 MONTH));

A different way to accomplish the same task is to add `1` to get the next month after the current one (after using the modulo function (`MOD`) to wrap around the month value to `0` if it is currently `12`):

    mysql> SELECT name, birth FROM pet
        -> WHERE MONTH(birth) = MOD(MONTH(NOW()), 12) + 1;

Note that `MONTH` returns a number between 1 and 12. And `MOD(something,12)` returns a number between 0 and 11. So the addition has to be after the `MOD()`, otherwise we would go from November (11) to January (1).

#### 3.3.4.6 Working with `NULL` Values

The `NULL` value can be surprising until you get used to it. Conceptually, `NULL` means missing value or unknown value and it is treated somewhat differently than other values. To test for `NULL`, you cannot use the arithmetic comparison operators such as `=`, `<`, or `<>`. To demonstrate this for yourself, try the following query:

    mysql> SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL;
    +----------+-----------+----------+----------+
    | 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL |
    +----------+-----------+----------+----------+
    |     NULL |      NULL |     NULL |     NULL |
    +----------+-----------+----------+----------+

Clearly you get no meaningful results from these comparisons. Use the `IS NULL` and `IS NOT NULL` operators instead:

    mysql> SELECT 1 IS NULL, 1 IS NOT NULL;
    +-----------+---------------+
    | 1 IS NULL | 1 IS NOT NULL |
    +-----------+---------------+
    |         0 |             1 |
    +-----------+---------------+

Note that two `NULL` are compared as equal is when you do an `GROUP BY`.

In MySQL, 0 or `NULL` means false and anything else means true. The default truth value from a boolean operation is 1.

When doing an `ORDER BY`, `NULL` values are always sorted first, even if you are using `DESC`.

This special treatment of `NULL` is why, in the previous section, it was necessary to determine which animals are no longer alive using `death IS NOT NULL` instead of `death <> NULL`.

#### 3.3.4.7 Pattern Matching

MySQL provides standard SQL pattern matching as well as a form of pattern matching based on extended regular expressions similar to those used by Unix utilities such as `vi`, `grep`, and `sed`.

SQL pattern matching allows you to use ``_'` to match any single character and ``%'` to match an arbitrary number of characters (including zero characters). In MySQL, SQL patterns are case-insensitive by default. Some examples are shown here. Note that you do not use `=` or `<>` when you use SQL patterns; use the `LIKE` or `NOT LIKE` comparison operators instead.

To find names beginning with ``b'`:

    mysql> SELECT * FROM pet WHERE name LIKE "b%";
    +--------+--------+---------+------+------------+------------+
    | name   | owner  | species | sex  | birth      | death      |
    +--------+--------+---------+------+------------+------------+
    | Buffy  | Harold | dog     | f    | 1989-05-13 | NULL       |
    | Bowser | Diane  | dog     | m    | 1989-08-31 | 1995-07-29 |
    +--------+--------+---------+------+------------+------------+

To find names ending with ``fy'`:

    mysql> SELECT * FROM pet WHERE name LIKE "%fy";
    +--------+--------+---------+------+------------+-------+
    | name   | owner  | species | sex  | birth      | death |
    +--------+--------+---------+------+------------+-------+
    | Fluffy | Harold | cat     | f    | 1993-02-04 | NULL  |
    | Buffy  | Harold | dog     | f    | 1989-05-13 | NULL  |
    +--------+--------+---------+------+------------+-------+

To find names containing a ``w'`:

    mysql> SELECT * FROM pet WHERE name LIKE "%w%";
    +----------+-------+---------+------+------------+------------+
    | name     | owner | species | sex  | birth      | death      |
    +----------+-------+---------+------+------------+------------+
    | Claws    | Gwen  | cat     | m    | 1994-03-17 | NULL       |
    | Bowser   | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
    | Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL       |
    +----------+-------+---------+------+------------+------------+

To find names containing exactly five characters, use the ``_'` pattern character:

    mysql> SELECT * FROM pet WHERE name LIKE "_____";
    +-------+--------+---------+------+------------+-------+
    | name  | owner  | species | sex  | birth      | death |
    +-------+--------+---------+------+------------+-------+
    | Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
    | Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
    +-------+--------+---------+------+------------+-------+

The other type of pattern matching provided by MySQL uses extended regular expressions. When you test for a match for this type of pattern, use the `REGEXP` and `NOT REGEXP` operators (or `RLIKE` and `NOT RLIKE`, which are synonyms).

Some characteristics of extended regular expressions are:

- ``.'` matches any single character.
- A character class ``[...]'` matches any character within the brackets. For example, ``[abc]'` matches ``a'`, ``b'`, or ``c'`. To name a range of characters, use a dash. ``[a-z]'` matches any lowercase letter, whereas ``[0-9]'` matches any digit.
- ``*'` matches zero or more instances of the thing preceding it. For example, ``x*'` matches any number of ``x'` characters, ``[0-9]*'` matches any number of digits, and ``.*'` matches any number of anything.
- The pattern matches if it occurs anywhere in the value being tested. (SQL patterns match only if they match the entire value.)
- To anchor a pattern so that it must match the beginning or end of the value being tested, use ``^'` at the beginning or ``$'` at the end of the pattern.

To demonstrate how extended regular expressions work, the `LIKE` queries shown previously are rewritten here to use `REGEXP`.

To find names beginning with ``b'`, use ``^'` to match the beginning of the name:

    mysql> SELECT * FROM pet WHERE name REGEXP "^b";
    +--------+--------+---------+------+------------+------------+
    | name   | owner  | species | sex  | birth      | death      |
    +--------+--------+---------+------+------------+------------+
    | Buffy  | Harold | dog     | f    | 1989-05-13 | NULL       |
    | Bowser | Diane  | dog     | m    | 1989-08-31 | 1995-07-29 |
    +--------+--------+---------+------+------------+------------+

Prior to MySQL Version 3.23.4, `REGEXP` is case-sensitive, and the previous query will return no rows. To match either lowercase or uppercase ``b'`, use this query instead:

    mysql> SELECT * FROM pet WHERE name REGEXP "^[bB]";

From MySQL 3.23.4 on, to force a `REGEXP` comparison to be case-sensitive, use the `BINARY` keyword to make one of the strings a binary string. This query will match only lowercase ``b'` at the beginning of a name:

    mysql> SELECT * FROM pet WHERE name REGEXP BINARY "^b";

To find names ending with ``fy'`, use ``$'` to match the end of the name:

    mysql> SELECT * FROM pet WHERE name REGEXP "fy$";
    +--------+--------+---------+------+------------+-------+
    | name   | owner  | species | sex  | birth      | death |
    +--------+--------+---------+------+------------+-------+
    | Fluffy | Harold | cat     | f    | 1993-02-04 | NULL  |
    | Buffy  | Harold | dog     | f    | 1989-05-13 | NULL  |
    +--------+--------+---------+------+------------+-------+

To find names containing a lowercase or uppercase ``w'`, use this query:

    mysql> SELECT * FROM pet WHERE name REGEXP "w";
    +----------+-------+---------+------+------------+------------+
    | name     | owner | species | sex  | birth      | death      |
    +----------+-------+---------+------+------------+------------+
    | Claws    | Gwen  | cat     | m    | 1994-03-17 | NULL       |
    | Bowser   | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
    | Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL       |
    +----------+-------+---------+------+------------+------------+

Because a regular expression pattern matches if it occurs anywhere in the value, it is not necessary in the previous query to put a wildcard on either side of the pattern to get it to match the entire value like it would be if you used a SQL pattern.

To find names containing exactly five characters, use ``^'` and ``$'` to match the beginning and end of the name, and five instances of ``.'` in between:

    mysql> SELECT * FROM pet WHERE name REGEXP "^.....$";
    +-------+--------+---------+------+------------+-------+
    | name  | owner  | species | sex  | birth      | death |
    +-------+--------+---------+------+------------+-------+
    | Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
    | Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
    +-------+--------+---------+------+------------+-------+

You could also write the previous query using the ``{n}'` ``repeat-`n`-times'' operator:

    mysql> SELECT * FROM pet WHERE name REGEXP "^.{5}$";
    +-------+--------+---------+------+------------+-------+
    | name  | owner  | species | sex  | birth      | death |
    +-------+--------+---------+------+------------+-------+
    | Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
    | Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
    +-------+--------+---------+------+------------+-------+

#### 3.3.4.8 Counting Rows

Databases are often used to answer the question, ``How often does a certain type of data occur in a table?'' For example, you might want to know how many pets you have, or how many pets each owner has, or you might want to perform various kinds of censuses on your animals.

Counting the total number of animals you have is the same question as ``How many rows are in the `pet` table?'' because there is one record per pet. The `COUNT()` function counts the number of non-`NULL` results, so the query to count your animals looks like this:

    mysql> SELECT COUNT(*) FROM pet;
    +----------+
    | COUNT(*) |
    +----------+
    |        9 |
    +----------+

Earlier, you retrieved the names of the people who owned pets. You can use `COUNT()` if you want to find out how many pets each owner has:

    mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner;
    +--------+----------+
    | owner  | COUNT(*) |
    +--------+----------+
    | Benny  |        2 |
    | Diane  |        2 |
    | Gwen   |        3 |
    | Harold |        2 |
    +--------+----------+

Note the use of `GROUP BY` to group together all records for each `owner`. Without it, all you get is an error message:

    mysql> SELECT owner, COUNT(owner) FROM pet;
    ERROR 1140 at line 1: Mixing of GROUP columns (MIN(),MAX(),COUNT()...)
    with no GROUP columns is illegal if there is no GROUP BY clause

`COUNT()` and `GROUP BY` are useful for characterising your data in various ways. The following examples show different ways to perform animal census operations.

Number of animals per species:

    mysql> SELECT species, COUNT(*) FROM pet GROUP BY species;
    +---------+----------+
    | species | COUNT(*) |
    +---------+----------+
    | bird    |        2 |
    | cat     |        2 |
    | dog     |        3 |
    | hamster |        1 |
    | snake   |        1 |
    +---------+----------+

Number of animals per sex:

    mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex;
    +------+----------+
    | sex  | COUNT(*) |
    +------+----------+
    | NULL |        1 |
    | f    |        4 |
    | m    |        4 |
    +------+----------+

(In this output, `NULL` indicates sex unknown.)

Number of animals per combination of species and sex:

    mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex;
    +---------+------+----------+
    | species | sex  | COUNT(*) |
    +---------+------+----------+
    | bird    | NULL |        1 |
    | bird    | f    |        1 |
    | cat     | f    |        1 |
    | cat     | m    |        1 |
    | dog     | f    |        1 |
    | dog     | m    |        2 |
    | hamster | f    |        1 |
    | snake   | m    |        1 |
    +---------+------+----------+

You need not retrieve an entire table when you use `COUNT()`. For example, the previous query, when performed just on dogs and cats, looks like this:

    mysql> SELECT species, sex, COUNT(*) FROM pet
        -> WHERE species = "dog" OR species = "cat"
        -> GROUP BY species, sex;
    +---------+------+----------+
    | species | sex  | COUNT(*) |
    +---------+------+----------+
    | cat     | f    |        1 |
    | cat     | m    |        1 |
    | dog     | f    |        1 |
    | dog     | m    |        2 |
    +---------+------+----------+

Or, if you wanted the number of animals per sex only for known-sex animals:

    mysql> SELECT species, sex, COUNT(*) FROM pet
        -> WHERE sex IS NOT NULL
        -> GROUP BY species, sex;
    +---------+------+----------+
    | species | sex  | COUNT(*) |
    +---------+------+----------+
    | bird    | f    |        1 |
    | cat     | f    |        1 |
    | cat     | m    |        1 |
    | dog     | f    |        1 |
    | dog     | m    |        2 |
    | hamster | f    |        1 |
    | snake   | m    |        1 |
    +---------+------+----------+

#### 3.3.4.9 Using More Than one Table

The `pet` table keeps track of which pets you have. If you want to record other information about them, such as events in their lives like visits to the vet or when litters are born, you need another table. What should this table look like? It needs:

- To contain the pet name so you know which animal each event pertains to.
- A date so you know when the event occurred.
- A field to describe the event.
- An event type field, if you want to be able to categorise events.

Given these considerations, the `CREATE TABLE` statement for the `event` table might look like this:

    mysql> CREATE TABLE event (name VARCHAR(20), date DATE,
        -> type VARCHAR(15), remark VARCHAR(255));

As with the `pet` table, it's easiest to load the initial records by creating a tab-delimited text file containing the information:

name | date | type | remark
Fluffy | 1995-05-15 | litter | 4 kittens, 3 female, 1 male
Buffy | 1993-06-23 | litter | 5 puppies, 2 female, 3 male
Buffy | 1994-06-19 | litter | 3 puppies, 3 female
Chirpy | 1999-03-21 | vet | needed beak straightened
Slim | 1997-08-03 | vet | broken rib
Bowser | 1991-10-12 | kennel |
Fang | 1991-10-12 | kennel |
Fang | 1998-08-28 | birthday | Gave him a new chew toy
Claws | 1998-03-17 | birthday | Gave him a new flea collar
Whistler | 1998-12-09 | birthday | First birthday

Load the records like this:

    mysql> LOAD DATA LOCAL INFILE "event.txt" INTO TABLE event;

Based on what you've learned from the queries you've run on the `pet` table, you should be able to perform retrievals on the records in the `event` table; the principles are the same. But when is the `event` table by itself insufficient to answer questions you might ask?

Suppose you want to find out the ages of each pet when they had their litters. The `event` table indicates when this occurred, but to calculate the age of the mother, you need her birth date. Because that is stored in the `pet` table, you need both tables for the query:

    mysql> SELECT pet.name,
        -> (TO_DAYS(date) - TO_DAYS(birth))/365 AS age,
        -> remark
        -> FROM pet, event
        -> WHERE pet.name = event.name AND type = "litter";
    +--------+------+-----------------------------+
    | name   | age  | remark                      |
    +--------+------+-----------------------------+
    | Fluffy | 2.27 | 4 kittens, 3 female, 1 male |
    | Buffy  | 4.12 | 5 puppies, 2 female, 3 male |
    | Buffy  | 5.10 | 3 puppies, 3 female         |
    +--------+------+-----------------------------+

There are several things to note about this query:

- The `FROM` clause lists two tables because the query needs to pull information from both of them.
- When combining (joining) information from multiple tables, you need to specify how records in one table can be matched to records in the other. This is easy because they both have a `name` column. The query uses `WHERE` clause to match up records in the two tables based on the `name` values.
- Because the `name` column occurs in both tables, you must be specific about which table you mean when referring to the column. This is done by prepending the table name to the column name.

You need not have two different tables to perform a join. Sometimes it is useful to join a table to itself, if you want to compare records in a table to other records in that same table. For example, to find breeding pairs among your pets, you can join the `pet` table with itself to pair up males and females of like species:

    mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species
        -> FROM pet AS p1, pet AS p2
        -> WHERE p1.species = p2.species AND p1.sex = "f" AND p2.sex = "m";
    +--------+------+--------+------+---------+
    | name   | sex  | name   | sex  | species |
    +--------+------+--------+------+---------+
    | Fluffy | f    | Claws  | m    | cat     |
    | Buffy  | f    | Fang   | m    | dog     |
    | Buffy  | f    | Bowser | m    | dog     |
    +--------+------+--------+------+---------+

In this query, we specify aliases for the table name in order to refer to the columns and keep straight which instance of the table each column reference is associated with.

## 3.4 Getting Information About Databases and Tables

What if you forget the name of a database or table, or what the structure of a given table is (for example, what its columns are called)? MySQL addresses this problem through several statements that provide information about the databases and tables it supports.

You have already seen `SHOW DATABASES`, which lists the databases managed by the server. To find out which database is currently selected, use the `DATABASE()` function:

    mysql> SELECT DATABASE();
    +------------+
    | DATABASE() |
    +------------+
    | menagerie  |
    +------------+

If you haven't selected any database yet, the result is blank.

To find out what tables the current database contains (for example, when you're not sure about the name of a table), use this command:

    mysql> SHOW TABLES;
    +---------------------+
    | Tables in menagerie |
    +---------------------+
    | event               |
    | pet                 |
    +---------------------+

If you want to find out about the structure of a table, the `DESCRIBE` command is useful; it displays information about each of a table's columns:

    mysql> DESCRIBE pet;
    +---------+-------------+------+-----+---------+-------+
    | Field   | Type        | Null | Key | Default | Extra |
    +---------+-------------+------+-----+---------+-------+
    | name    | varchar(20) | YES  |     | NULL    |       |
    | owner   | varchar(20) | YES  |     | NULL    |       |
    | species | varchar(20) | YES  |     | NULL    |       |
    | sex     | char(1)     | YES  |     | NULL    |       |
    | birth   | date        | YES  |     | NULL    |       |
    | death   | date        | YES  |     | NULL    |       |
    +---------+-------------+------+-----+---------+-------+

`Field` indicates the column name, `Type` is the data type for the column, `NULL` indicates whether the column can contain `NULL` values, `Key` indicates whether the column is indexed, and `Default` specifies the column's default value.

If you have indexes on a table, `SHOW INDEX FROM tbl_name` produces information about them.

## 3.5 Examples of Common Queries

Here are examples of how to solve some common problems with MySQL.

Some of the examples use the table `shop` to hold the price of each article (item number) for certain traders (dealers). Supposing that each trader has a single fixed price per article, then (`article`, `dealer`) is a primary key for the records.

Start the command-line tool `mysql` and select a database:

    mysql your-database-name

(In most MySQL installations, you can use the database-name 'test').

You can create the example table as:

    CREATE TABLE shop (
     article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
     dealer  CHAR(20)                 DEFAULT ''     NOT NULL,
     price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL,
     PRIMARY KEY(article, dealer));

    INSERT INTO shop VALUES
    (1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),(3,'C',1.69),
    (3,'D',1.25),(4,'D',19.95);

Okay, so the example data is:

    mysql> SELECT * FROM shop;

    +---------+--------+-------+
    | article | dealer | price |
    +---------+--------+-------+
    |    0001 | A      |  3.45 |
    |    0001 | B      |  3.99 |
    |    0002 | A      | 10.99 |
    |    0003 | B      |  1.45 |
    |    0003 | C      |  1.69 |
    |    0003 | D      |  1.25 |
    |    0004 | D      | 19.95 |
    +---------+--------+-------+

### 3.5.1 The Maximum Value for a Column

``What's the highest item number?''

    SELECT MAX(article) AS article FROM shop

    +---------+
    | article |
    +---------+
    |       4 |
    +---------+

### 3.5.2 The Row Holding the Maximum of a Certain Column

``Find number, dealer, and price of the most expensive article.''

In ANSI SQL this is easily done with a subquery:

    SELECT article, dealer, price
    FROM   shop
    WHERE  price=(SELECT MAX(price) FROM shop)

In MySQL (which does not yet have subselects), just do it in two steps:

1. Get the maximum price value from the table with a `SELECT` statement.
2. Using this value compile the actual query:

       SELECT article, dealer, price
       FROM   shop
       WHERE  price=19.95

Another solution is to sort all rows descending by price and only get the first row using the MySQL-specific `LIMIT` clause:

    SELECT article, dealer, price
    FROM   shop
    ORDER BY price DESC
    LIMIT 1

NOTE: If there are several most expensive articles (for example, each 19.95) the `LIMIT` solution shows only one of them!

### 3.5.3 Maximum of Column per Group

``What's the highest price per article?''

    SELECT article, MAX(price) AS price
    FROM   shop
    GROUP BY article

    +---------+-------+
    | article | price |
    +---------+-------+
    |    0001 |  3.99 |
    |    0002 | 10.99 |
    |    0003 |  1.69 |
    |    0004 | 19.95 |
    +---------+-------+

### 3.5.4 The Rows Holding the Group-wise Maximum of a Certain Field

``For each article, find the dealer(s) with the most expensive price.''

In ANSI SQL, I'd do it with a subquery like this:

    SELECT article, dealer, price
    FROM   shop s1
    WHERE  price=(SELECT MAX(s2.price)
                  FROM shop s2
                  WHERE s1.article = s2.article);

In MySQL it's best do it in several steps:

1. Get the list of (article,maxprice).
2. For each article get the corresponding rows that have the stored maximum price.

This can easily be done with a temporary table:

    CREATE TEMPORARY TABLE tmp (
            article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
            price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL);

    LOCK TABLES shop read;

    INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article;

    SELECT shop.article, dealer, shop.price FROM shop, tmp
    WHERE shop.article=tmp.article AND shop.price=tmp.price;

    UNLOCK TABLES;

    DROP TABLE tmp;

If you don't use a `TEMPORARY` table, you must also lock the 'tmp' table.

``Can it be done with a single query?''

Yes, but only by using a quite inefficient trick that I call the ``MAX-CONCAT trick'':

    SELECT article,
           SUBSTRING( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 7) AS dealer,
      0.00+LEFT(      MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 6) AS price
    FROM   shop
    GROUP BY article;

    +---------+--------+-------+
    | article | dealer | price |
    +---------+--------+-------+
    |    0001 | B      |  3.99 |
    |    0002 | A      | 10.99 |
    |    0003 | C      |  1.69 |
    |    0004 | D      | 19.95 |
    +---------+--------+-------+

The last example can, of course, be made a bit more efficient by doing the splitting of the concatenated column in the client.

### 3.5.5 Using user variables

You can use MySQL user variables to remember results without having to store them in temporary variables in the client. See section 6.1.4 User Variables.

For example, to find the articles with the highest and lowest price you can do:

    mysql> SELECT @min_price:=MIN(price),@max_price:=MAX(price) FROM shop;
    mysql> SELECT * FROM shop WHERE price=@min_price OR price=@max_price;
    +---------+--------+-------+
    | article | dealer | price |
    +---------+--------+-------+
    |    0003 | D      |  1.25 |
    |    0004 | D      | 19.95 |
    +---------+--------+-------+

### 3.5.6 Using Foreign Keys

In MySQL 3.23.44 and up, `InnoDB` tables supports checking of foreign key constraints. See section 7.5 `InnoDB` Tables. See also section 1.7.4.5 Foreign Keys.

You don't actually need foreign keys to join 2 tables. The only thing MySQL currently doesn't do (in table types other than `InnoDB`), is `CHECK` to make sure that the keys you use really exist in the table(s) you're referencing and it doesn't automatically delete rows from a table with a foreign key definition. If you use your keys like normal, it'll work just fine:

    CREATE TABLE person (
        id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
        name CHAR(60) NOT NULL,
        PRIMARY KEY (id)
    );

    CREATE TABLE shirt (
        id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
        style ENUM('t-shirt', 'polo', 'dress') NOT NULL,
        color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL,
        owner SMALLINT UNSIGNED NOT NULL REFERENCES person(id),
        PRIMARY KEY (id)
    );

    INSERT INTO person VALUES (NULL, 'Antonio Paz');

    INSERT INTO shirt VALUES
    (NULL, 'polo', 'blue', LAST_INSERT_ID()),
    (NULL, 'dress', 'white', LAST_INSERT_ID()),
    (NULL, 't-shirt', 'blue', LAST_INSERT_ID());

    INSERT INTO person VALUES (NULL, 'Lilliana Angelovska');

    INSERT INTO shirt VALUES
    (NULL, 'dress', 'orange', LAST_INSERT_ID()),
    (NULL, 'polo', 'red', LAST_INSERT_ID()),
    (NULL, 'dress', 'blue', LAST_INSERT_ID()),
    (NULL, 't-shirt', 'white', LAST_INSERT_ID());

    SELECT * FROM person;
    +----+---------------------+
    | id | name                |
    +----+---------------------+
    |  1 | Antonio Paz         |
    |  2 | Lilliana Angelovska |
    +----+---------------------+

    SELECT * FROM shirt;
    +----+---------+--------+-------+
    | id | style   | color  | owner |
    +----+---------+--------+-------+
    |  1 | polo    | blue   |     1 |
    |  2 | dress   | white  |     1 |
    |  3 | t-shirt | blue   |     1 |
    |  4 | dress   | orange |     2 |
    |  5 | polo    | red    |     2 |
    |  6 | dress   | blue   |     2 |
    |  7 | t-shirt | white  |     2 |
    +----+---------+--------+-------+

    SELECT s.* FROM person p, shirt s
     WHERE p.name LIKE 'Lilliana%'
       AND s.owner = p.id
       AND s.color <> 'white';

    +----+-------+--------+-------+
    | id | style | color  | owner |
    +----+-------+--------+-------+
    |  4 | dress | orange |     2 |
    |  5 | polo  | red    |     2 |
    |  6 | dress | blue   |     2 |
    +----+-------+--------+-------+

### 3.5.7 Searching on Two Keys

MySQL doesn't yet optimise when you search on two different keys combined with `OR` (searching on one key with different `OR` parts is optimised quite well):

    SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1'
    OR  field2_index = '1'

The reason is that we haven't yet had time to come up with an efficient way to handle this in the general case. (The `AND` handling is, in comparison, now completely general and works very well.)

For the moment you can solve this very efficiently by using a `TEMPORARY` table. This type of optimisation is also very good if you are using very complicated queries where the SQL server does the optimisations in the wrong order.

    CREATE TEMPORARY TABLE tmp
    SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1';
    INSERT INTO tmp
    SELECT field1_index, field2_index FROM test_table WHERE field2_index = '1';
    SELECT * from tmp;
    DROP TABLE tmp;

The above way to solve this query is in effect a `UNION` of two queries. See section 6.4.1.2 `UNION` Syntax.

### 3.5.8 Calculating Visits Per Day

The following shows an idea of how you can use the bit group functions to calculate the number of days per month a user has visited a web page.

    CREATE TABLE t1 (year YEAR(4), month INT(2) UNSIGNED ZEROFILL,
                 day INT(2) UNSIGNED ZEROFILL);
    INSERT INTO t1 VALUES(2000,1,1),(2000,1,20),(2000,1,30),(2000,2,2),
                (2000,2,23),(2000,2,23);
    SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1
           GROUP BY year,month;

    Which returns:

    +------+-------+------+
    | year | month | days |
    +------+-------+------+
    | 2000 |    01 |    3 |
    | 2000 |    02 |    2 |
    +------+-------+------+

The above calculates how many different days was used for a given year/month combination, with automatic removal of duplicate entries.

### 3.5.9 Using `AUTO_INCREMENT`

The `AUTO_INCREMENT` attribute can be used to generate a unique identity for new rows:

    CREATE TABLE animals (
                 id MEDIUMINT NOT NULL AUTO_INCREMENT,
                 name CHAR(30) NOT NULL,
                 PRIMARY KEY (id)
                 );
    INSERT INTO animals (name) VALUES ("dog"),("cat"),("penguin"),
                                      ("lax"),("whale");
    SELECT * FROM animals;

    Which returns:

    +----+---------+
    | id | name    |
    +----+---------+
    |  1 | dog     |
    |  2 | cat     |
    |  3 | penguin |
    |  4 | lax     |
    |  5 | whale   |
    +----+---------+

You can retrieve the used `AUTO_INCREMENT` key with the `LAST_INSERT_ID()` SQL function or the `mysql_insert_id()` API function. Note: for a multi-row insert, `LAST_INSERT_ID()`/`mysql_insert_id()` will actually return the `AUTO_INCREMENT` key from the first inserted row. This allows multi-row inserts to be reproduced on other servers.

For MyISAM and BDB tables you can specify `AUTO_INCREMENT` on secondary column in a multi-column key. In this case the generated value for the autoincrement column is calculated as `MAX(auto_increment_column)+1) WHERE prefix=given-prefix`. This is useful when you want to put data into ordered groups.

    CREATE TABLE animals (
                 grp ENUM('fish','mammal','bird') NOT NULL,
                 id MEDIUMINT NOT NULL AUTO_INCREMENT,
                 name CHAR(30) NOT NULL,
                 PRIMARY KEY (grp,id)
                 );
    INSERT INTO animals (grp,name) VALUES("mammal","dog"),("mammal","cat"),
                      ("bird","penguin"),("fish","lax"),("mammal","whale");
    SELECT * FROM animals ORDER BY grp,id;

    Which returns:

    +--------+----+---------+
    | grp    | id | name    |
    +--------+----+---------+
    | fish   |  1 | lax     |
    | mammal |  1 | dog     |
    | mammal |  2 | cat     |
    | mammal |  3 | whale   |
    | bird   |  1 | penguin |
    +--------+----+---------+

Note that in this case, the `AUTO_INCREMENT` value will be reused if you delete the row with the biggest `AUTO_INCREMENT` value in any group.

## 3.6 Using `mysql` in Batch Mode

In the previous sections, you used `mysql` interactively to enter queries and view the results. You can also run `mysql` in batch mode. To do this, put the commands you want to run in a file, then tell `mysql` to read its input from the file:

    shell> mysql < batch-file

If you are running `mysql` under windows and have some special characters in the file that causes problems, you can do:

    dos> mysql -e "source batch-file"

If you need to specify connection parameters on the command-line, the command might look like this:

    shell> mysql -h host -u user -p < batch-file
    Enter password: ********

When you use `mysql` this way, you are creating a script file, then executing the script.

If you want the script to continue even if you have errors, you should use the `--force` command-line option.

Why use a script? Here are a few reasons:

- If you run a query repeatedly (say, every day or every week), making it a script allows you to avoid retyping it each time you execute it.
- You can generate new queries from existing ones that are similar by copying and editing script files.
- Batch mode can also be useful while you're developing a query, particularly for multiple-line commands or multiple-statement sequences of commands. If you make a mistake, you don't have to retype everything. Just edit your script to correct the error, then tell `mysql` to execute it again.
- If you have a query that produces a lot of output, you can run the output through a pager rather than watching it scroll off the top of your screen:

      shell> mysql < batch-file | more

- You can catch the output in a file for further processing:

      shell> mysql < batch-file > mysql.out

- You can distribute your script to other people so they can run the commands, too.
- Some situations do not allow for interactive use, for example, when you run a query from a `cron` job. In this case, you must use batch mode.

The default output format is different (more concise) when you run `mysql` in batch mode than when you use it interactively. For example, the output of `SELECT DISTINCT species FROM pet` looks like this when run interactively:

    +---------+
    | species |
    +---------+
    | bird    |
    | cat     |
    | dog     |
    | hamster |
    | snake   |
    +---------+

But like this when run in batch mode:

    species
    bird
    cat
    dog
    hamster
    snake

If you want to get the interactive output format in batch mode, use `mysql -t`. To echo to the output the commands that are executed, use `mysql -vvv`.

You can also use scripts in the `mysql` command-line prompt by using the `source` command:

    mysql> source filename;

## 3.7 Queries from Twin Project

At Analytikerna and Lentus, we have been doing the systems and field work for a big research project. This project is a collaboration between the Institute of Environmental Medicine at Karolinska Institutet Stockholm and the Section on Clinical Research in Aging and Psychology at the University of Southern California.

The project involves a screening part where all twins in Sweden older than 65 years are interviewed by telephone. Twins who meet certain criteria are passed on to the next stage. In this latter stage, twins who want to participate are visited by a doctor/nurse team. Some of the examinations include physical and neuropsychological examination, laboratory testing, neuroimaging, psychological status assessment, and family history collection. In addition, data are collected on medical and environmental risk factors.

More information about Twin studies can be found at: http://www.imm.ki.se/TWIN/TWINUKW.HTM

The latter part of the project is administered with a web interface written using Perl and MySQL.

Each night all data from the interviews are moved into a MySQL database.

### 3.7.1 Find all Non-distributed Twins

The following query is used to determine who goes into the second part of the project:

    SELECT
            CONCAT(p1.id, p1.tvab) + 0 AS tvid,
            CONCAT(p1.christian_name, " ", p1.surname) AS Name,
            p1.postal_code AS Code,
            p1.city AS City,
            pg.abrev AS Area,
            IF(td.participation = "Aborted", "A", " ") AS A,
            p1.dead AS dead1,
            l.event AS event1,
            td.suspect AS tsuspect1,
            id.suspect AS isuspect1,
            td.severe AS tsevere1,
            id.severe AS isevere1,
            p2.dead AS dead2,
            l2.event AS event2,
            h2.nurse AS nurse2,
            h2.doctor AS doctor2,
            td2.suspect AS tsuspect2,
            id2.suspect AS isuspect2,
            td2.severe AS tsevere2,
            id2.severe AS isevere2,
            l.finish_date
    FROM
            twin_project AS tp
            /* For Twin 1 */
            LEFT JOIN twin_data AS td ON tp.id = td.id
                      AND tp.tvab = td.tvab
            LEFT JOIN informant_data AS id ON tp.id = id.id
                      AND tp.tvab = id.tvab
            LEFT JOIN harmony AS h ON tp.id = h.id
                      AND tp.tvab = h.tvab
            LEFT JOIN lentus AS l ON tp.id = l.id
                      AND tp.tvab = l.tvab
            /* For Twin 2 */
            LEFT JOIN twin_data AS td2 ON p2.id = td2.id
                      AND p2.tvab = td2.tvab
            LEFT JOIN informant_data AS id2 ON p2.id = id2.id
                      AND p2.tvab = id2.tvab
            LEFT JOIN harmony AS h2 ON p2.id = h2.id
                      AND p2.tvab = h2.tvab
            LEFT JOIN lentus AS l2 ON p2.id = l2.id
                      AND p2.tvab = l2.tvab,
            person_data AS p1,
            person_data AS p2,
            postal_groups AS pg
    WHERE
            /* p1 gets main twin and p2 gets his/her twin. */
            /* ptvab is a field inverted from tvab */
            p1.id = tp.id AND p1.tvab = tp.tvab AND
            p2.id = p1.id AND p2.ptvab = p1.tvab AND
            /* Just the sceening survey */
            tp.survey_no = 5 AND
            /* Skip if partner died before 65 but allow emigration (dead=9) */
            (p2.dead = 0 OR p2.dead = 9 OR
             (p2.dead = 1 AND
              (p2.death_date = 0 OR
               (((TO_DAYS(p2.death_date) - TO_DAYS(p2.birthday)) / 365)
                >= 65))))
            AND
            (
            /* Twin is suspect */
            (td.future_contact = 'Yes' AND td.suspect = 2) OR
            /* Twin is suspect - Informant is Blessed */
            (td.future_contact = 'Yes' AND td.suspect = 1
                                       AND id.suspect = 1) OR
            /* No twin - Informant is Blessed */
            (ISNULL(td.suspect) AND id.suspect = 1
                                AND id.future_contact = 'Yes') OR
            /* Twin broken off - Informant is Blessed */
            (td.participation = 'Aborted'
             AND id.suspect = 1 AND id.future_contact = 'Yes') OR
            /* Twin broken off - No inform - Have partner */
            (td.participation = 'Aborted' AND ISNULL(id.suspect)
                                          AND p2.dead = 0))
            AND
            l.event = 'Finished'
            /* Get at area code */
            AND SUBSTRING(p1.postal_code, 1, 2) = pg.code
            /* Not already distributed */
            AND (h.nurse IS NULL OR h.nurse=00 OR h.doctor=00)
            /* Has not refused or been aborted */
            AND NOT (h.status = 'Refused' OR h.status = 'Aborted'
            OR h.status = 'Died' OR h.status = 'Other')
    ORDER BY
            tvid;

Some explanations:

`CONCAT(p1.id, p1.tvab) + 0 AS tvid`
  We want to sort on the concatenated `id` and `tvab` in numerical order. Adding `0` to the result causes MySQL to treat the result as a number.
column `id`
  This identifies a pair of twins. It is a key in all tables.
column `tvab`
  This identifies a twin in a pair. It has a value of `1` or `2`.
column `ptvab`
  This is an inverse of `tvab`. When `tvab` is `1` this is `2`, and vice versa. It exists to save typing and to make it easier for MySQL to optimise the query.

This query demonstrates, among other things, how to do lookups on a table from the same table with a join (`p1` and `p2`). In the example, this is used to check whether a twin's partner died before the age of 65. If so, the row is not returned.

All of the above exist in all tables with twin-related information. We have a key on both `id,tvab` (all tables), and `id,ptvab` (`person_data`) to make queries faster.

On our production machine (A 200MHz UltraSPARC), this query returns about 150-200 rows and takes less than one second.

The current number of records in the tables used above:

Table | Rows
`person_data` | 71074
`lentus` | 5291
`twin_project` | 5286
`twin_data` | 2012
`informant_data` | 663
`harmony` | 381
`postal_groups` | 100

### 3.7.2 Show a Table on Twin Pair Status

Each interview ends with a status code called `event`. The query shown here is used to display a table over all twin pairs combined by event. This indicates in how many pairs both twins are finished, in how many pairs one twin is finished and the other refused, and so on.

    SELECT
            t1.event,
            t2.event,
            COUNT(*)
    FROM
            lentus AS t1,
            lentus AS t2,
            twin_project AS tp
    WHERE
            /* We are looking at one pair at a time */
            t1.id = tp.id
            AND t1.tvab=tp.tvab
            AND t1.id = t2.id
            /* Just the sceening survey */
            AND tp.survey_no = 5
            /* This makes each pair only appear once */
            AND t1.tvab='1' AND t2.tvab='2'
    GROUP BY
            t1.event, t2.event;

## 3.8 Using MySQL with Apache

There are programs that let you authenticate your users from a MySQL database and also let you log your log files into a MySQL table. See section 1.6.1 MySQL Portals.

You can change the Apache logging format to be easily readable by MySQL by putting the following into the Apache configuration file:

    LogFormat \
            "\"%h\",%{%Y%m%d%H%M%S}t,%>s,\"%b\",\"%{Content-Type}o\",  \
            \"%U\",\"%{Referer}i\",\"%{User-Agent}i\""

In MySQL you can do something like this:

    LOAD DATA INFILE '/local/access_log' INTO TABLE table_name
    FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' ESCAPED BY '\\'

# 4 Database Administration

## 4.1 Configuring MySQL

### 4.1.1 `mysqld` Command-line Options

In most cases you should manage mysqld options through option files. See section 4.1.2 ``my.cnf'` Option Files.

`mysqld` and `mysqld.server` reads options from the `mysqld` and `server` groups. `mysqld_safe` read options from the `mysqld`, `server`, `mysqld_safe` and `safe_mysqld` groups. An embedded MySQL server usually reads options from the `server`, `embedded` and `xxxxx_SERVER`, where `xxxxx` is the name of the application.

`mysqld` accepts the following command-line options:

`--ansi`
  Use ANSI SQL syntax instead of MySQL syntax. See section 1.7.2 Running MySQL in ANSI Mode.
`-b, --basedir=path`
  Path to installation directory. All paths are usually resolved relative to this.
`--big-tables`
  Allow big result sets by saving all temporary sets on file. It solves most 'table full' errors, but also slows down the queries where in-memory tables would suffice. Since Version 3.23.2, MySQL is able to solve it automatically by using memory for small temporary tables and switching to disk tables where necessary.
`--bind-address=IP`
  IP address to bind to.
`--character-sets-dir=path`
  Directory where character sets are. See section 4.6.1 The Character Set Used for Data and Sorting.
`--chroot=path`
  Put `mysqld` daemon in chroot environment at startup. Recommended security measure. It somewhat limits `LOAD DATA INFILE` and `SELECT ... INTO OUTFILE` though.
`--core-file`
  Write a core file if `mysqld` dies. For some systems you must also specify `--core-file-size` to `safe_mysqld`. See section 4.7.2 `safe_mysqld`, The Wrapper Around `mysqld`. Note that on some systems, like Solaris, you will not get a core file if you are also using the `--user` option.
`-h, --datadir=path`
  Path to the database root.
`--debug[...]=`
  If MySQL is configured with `--with-debug`, you can use this option to get a trace file of what `mysqld` is doing. See section E.1.2 Creating Trace Files.
`--default-character-set=charset`
  Set the default character set. See section 4.6.1 The Character Set Used for Data and Sorting.
`--default-table-type=type`
  Set the default table type for tables. See section 7 MySQL Table Types.
`--delay-key-write[= OFF | ON | ALL]`
  How MyISAM `DELAYED KEYS` should be used. See section 5.5.2 Tuning Server Parameters.
`--delay-key-write-for-all-tables; In MySQL 4.0.3 you should use --delay-key-write=ALL instead.`
  Don't flush key buffers between writes for any `MyISAM` table. See section 5.5.2 Tuning Server Parameters.
`--des-key-file=filename`
  Read the default keys used by `DES_ENCRYPT()` and `DES_DECRYPT()` from this file.
`--enable-external-locking (was --enable-locking)`
  Enable system locking. Note that if you use this option on a system on which `lockd` does not fully work (as on Linux), you will easily get mysqld to deadlock.
`--enable-named-pipe`
  Enable support for named pipes (only on NT/Win2000/XP).
`-T, --exit-info`
  This is a bit mask of different flags one can use for debugging the mysqld server; one should not use this option if one doesn't know exactly what it does!
`--flush`
  Flush all changes to disk after each SQL command. Normally MySQL only does a write of all changes to disk after each SQL command and lets the operating system handle the syncing to disk. See section A.4.1 What To Do If MySQL Keeps Crashing.
`-?, --help`
  Display short help and exit.
`--init-file=file`
  Read SQL commands from this file at startup.
`-L, --language=...`
  Client error messages in given language. May be given as a full path. See section 4.6.2 Non-English Error Messages.
`-l, --log[=file]`
  Log connections and queries to file. See section 4.9.2 The General Query Log.
`--log-isam[=file]`
  Log all ISAM/MyISAM changes to file (only used when debugging ISAM/MyISAM).
`--log-slow-queries[=file]`
  Log all queries that have taken more than `long_query_time` seconds to execute to file. See section 4.9.5 The Slow Query Log.
`--log-update[=file]`
  Log updates to `file.#` where `#` is a unique number if not given. See section 4.9.3 The Update Log.
`--log-long-format`
  Log some extra information to update log. If you are using `--log-slow-queries` then queries that are not using indexes are logged to the slow query log.
`--low-priority-updates`
  Table-modifying operations (`INSERT`/`DELETE`/`UPDATE`) will have lower priority than selects. It can also be done via `{INSERT | REPLACE | UPDATE | DELETE} LOW_PRIORITY ...` to lower the priority of only one query, or by `SET LOW_PRIORITY_UPDATES=1` to change the priority in one thread. See section 5.3.2 Table Locking Issues.
`--memlock`
  Lock the `mysqld` process in memory. This works only if your system supports the `mlockall()` system call (like Solaris). This may help if you have a problem where the operating system is causing `mysqld` to swap on disk.
`--myisam-recover [=option[,option...]]]`
  Option is any combination of `DEFAULT`, `BACKUP`, `FORCE` or `QUICK`. You can also set this explicitly to `""` if you want to disable this option. If this option is used, `mysqld` will on open check if the table is marked as crashed or if the table wasn't closed properly. (The last option only works if you are running with `--skip-external-locking`.) If this is the case `mysqld` will run check on the table. If the table was corrupted, `mysqld` will attempt to repair it. The following options affects how the repair works.

  Option | Description
  DEFAULT | The same as not giving any option to `--myisam-recover`.
  BACKUP | If the data table was changed during recover, save a backup of the ``table_name.MYD'` datafile as ``table_name-datetime.BAK'`.
  FORCE | Run recover even if we will lose more than one row from the .MYD file.
  QUICK | Don't check the rows in the table if there aren't any delete blocks.

  Before a table is automatically repaired, MySQL will add a note about this in the error log. If you want to be able to recover from most things without user intervention, you should use the options `BACKUP,FORCE`. This will force a repair of a table even if some rows would be deleted, but it will keep the old datafile as a backup so that you can later examine what happened.
`--pid-file=path`
  Path to pid file used by `safe_mysqld`.
`-P, --port=...`
  Port number to listen for TCP/IP connections.
`-o, --old-protocol`
  Use the 3.20 protocol for compatibility with some very old clients. See section 2.5.5 Upgrading from Version 3.20 to Version 3.21.
`--one-thread`
  Only use one thread (for debugging under Linux). See section E.1 Debugging a MySQL server.
`-O, --set-variable var=option`
  Give a variable a value. `--help` lists variables. You can find a full description for all variables in the `SHOW VARIABLES` section in this manual. See section 4.5.6.4 `SHOW VARIABLES`. The tuning server parameters section includes information of how to optimise these. Please note that `--set-variable` is deprecated since MySQL 4.0, just use `--var=option` on its own. See section 5.5.2 Tuning Server Parameters. In MySQL 4.0.2 one can set a variable directly with `--variable-name=option` and `set-variable` is not anymore needed in option files. If you want to restrict the maximum value a startup option can be set to with `SET`, you can define this by using the `--maximum-variable-name` command line option. See section 5.5.6 `SET` Syntax. Note that when setting a variable to a value, MySQL may automatically correct it to stay within a given range and also adjusts the value a little to fix for the used algorithm.
`--safe-mode`
  Skip some optimise stages.
`--safe-show-database`
  With this option, the `SHOW DATABASES` command returns only those databases for which the user has some kind of privilege. From version 4.0.2 this option is deprecated and doesn't do anything (the option is enabled by default) as we now have the `SHOW DATABASES` privilege. See section 4.3.1 `GRANT` and `REVOKE` Syntax.
`--safe-user-create`
  If this is enabled, a user can't create new users with the GRANT command, if the user doesn't have `INSERT` privilege to the `mysql.user` table or any column in this table.
`--skip-bdb`
  Disable usage of BDB tables. This will save memory and may speed up some things.
`--skip-concurrent-insert`
  Turn off the ability to select and insert at the same time on `MyISAM` tables. (This is only to be used if you think you have found a bug in this feature.)
`--skip-delay-key-write; In MySQL 4.0.3 you should use --delay-key-write=OFF instead.`
  Ignore the `DELAY_KEY_WRITE` option for all tables. See section 5.5.2 Tuning Server Parameters.
`--skip-grant-tables`
  This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing `mysqladmin flush-privileges` or `mysqladmin reload`.)
`--skip-host-cache`
  Never use host name cache for faster name-ip resolution, but query DNS server on every connect instead. See section 5.5.5 How MySQL uses DNS.
`--skip-innodb`
  Disable usage of Innodb tables. This will save memory and disk space and speed up some things.
`--skip-external-locking (was --skip-locking)`
  Don't use system locking. To use `isamchk` or `myisamchk` you must shut down the server. See section 1.2.3 How Stable Is MySQL?. Note that in MySQL Version 3.23 you can use `REPAIR` and `CHECK` to repair/check `MyISAM` tables.
`--skip-name-resolve`
  Hostnames are not resolved. All `Host` column values in the grant tables must be IP numbers or `localhost`. See section 5.5.5 How MySQL uses DNS.
`--skip-networking`
  Don't listen for TCP/IP connections at all. All interaction with `mysqld` must be made via Unix sockets. This option is highly recommended for systems where only local requests are allowed. See section 5.5.5 How MySQL uses DNS.
`--skip-new`
  Don't use new, possible wrong routines.
`--skip-symlink`
  Don't delete or rename files that a symlinked file in the data directory points to.
`--skip-safemalloc`
  If MySQL is configured with `--with-debug=full`, all programs will check the memory for overruns for every memory allocation and memory freeing. As this checking is very slow, you can avoid this, when you don't need memory checking, by using this option.
`--skip-show-database`
  Don't allow `SHOW DATABASES` command, unless the user has the `SHOW DATABASES` privilege. From version 4.0.2 you should no longer need this option, since access can now be granted specifically with the `SHOW DATABASES` privilege.
`--skip-stack-trace`
  Don't write stack traces. This option is useful when you are running `mysqld` under a debugger. On some systems you also have to use this option to get a core file. See section E.1 Debugging a MySQL server.
`--skip-thread-priority`
  Disable using thread priorities for faster response time.
`--socket=path`
  Socket file to use for local connections instead of default `/tmp/mysql.sock`.
`--sql-mode=option[,option[,option...]]`
  Option can be any combination of: `REAL_AS_FLOAT`, `PIPES_AS_CONCAT`, `ANSI_QUOTES`, `IGNORE_SPACE`, `SERIALIZE`, `ONLY_FULL_GROUP_BY`. It can also be empty (`""`) if you want to reset this. By specifying all of the above options is same as using --ansi. With this option one can turn on only needed SQL modes. See section 1.7.2 Running MySQL in ANSI Mode.
`--temp-pool`
  Using this option will cause most temporary files created to use a small set of names, rather than a unique name for each new file. This is to work around a problem in the Linux kernel dealing with creating a bunch of new files with different names. With the old behaviour, Linux seems to 'leak' memory, as it's being allocated to the directory entry cache instead of the disk cache.
`--transaction-isolation= { READ-UNCOMMITTED | READ-COMMITTED | REPEATABLE-READ | SERIALIZABLE }`
  Sets the default transaction isolation level. See section 6.7.3 `SET TRANSACTION` Syntax.
`-t, --tmpdir=path`
  Path for temporary files. It may be useful if your default `/tmp` directory resides on a partition too small to hold temporary tables.
`-u, --user= [user_name | userid]`
  Run `mysqld` daemon as user `user_name` or `userid` (numeric). This option is mandatory when starting `mysqld` as root.
`-V, --version`
  Output version information and exit.
`-W, --log-warnings (Was --warnings)`
  Print out warnings like `Aborted connection...` to the ``.err'` file. See section A.2.9 Communication Errors / Aborted Connection.

One can change most values for a running server with the `SET` command. See section 5.5.6 `SET` Syntax.

### 4.1.2 ``my.cnf'` Option Files

MySQL can, since Version 3.22, read default startup options for the server and for clients from option files.

MySQL reads default options from the following files on Unix:

Filename | Purpose
`/etc/my.cnf` | Global options
`DATADIR/my.cnf` | Server-specific options
`defaults-extra-file` | The file specified with --defaults-extra-file=#
`~/.my.cnf` | User-specific options

`DATADIR` is the MySQL data directory (typically ``/usr/local/mysql/data'` for a binary installation or ``/usr/local/var'` for a source installation). Note that this is the directory that was specified at configuration time, not the one specified with `--datadir` when `mysqld` starts up! (`--datadir` has no effect on where the server looks for option files, because it looks for them before it processes any command-line arguments.)

MySQL reads default options from the following files on Windows:

Filename | Purpose
`windows-system-directory\my.ini` | Global options
`C:\my.cnf` | Global options

Note that on Windows, you should specify all paths with `/` instead of `\`. If you use `\`, you need to specify this twice, as `\` is the escape character in MySQL.

MySQL tries to read option files in the order listed above. If multiple option files exist, an option specified in a file read later takes precedence over the same option specified in a file read earlier. Options specified on the command-line take precedence over options specified in any option file. Some options can be specified using environment variables. Options specified on the command-line or in option files take precedence over environment variable values. See section F Environment Variables.

The following programs support option files: `mysql`, `mysqladmin`, `mysqld`, `mysqld_safe`, `mysql.server`, `mysqldump`, `mysqlimport`, `mysqlshow`, `mysqlcheck`, `myisamchk`, and `myisampack`.

Any long option that may be given on the command-line when running a MySQL program can be given in an option file as well (without the leading double dash). Run the program with `--help` to get a list of available options.

An option file can contain lines of the following forms:

`#comment`
  Comment lines start with ``#'` or ``;'`. Empty lines are ignored.
`[group]`
  `group` is the name of the program or group for which you want to set options. After a group line, any `option` or `set-variable` lines apply to the named group until the end of the option file or another group line is given.
`option`
  This is equivalent to `--option` on the command-line.
`option=value`
  This is equivalent to `--option=value` on the command-line.
`set-variable = variable=value`
  This is equivalent to `--set-variable variable=value` on the command-line. This syntax must be used to set a `mysqld` variable. Please note that `--set-variable` is deprecated since MySQL 4.0, just use `--variable=value` on its own.

The `client` group allows you to specify options that apply to all MySQL clients (not `mysqld`). This is the perfect group to use to specify the password you use to connect to the server. (But make sure the option file is readable and writable only by yourself.)

Note that for options and values, all leading and trailing blanks are automatically deleted. You may use the escape sequences ``\b'`, ``\t'`, ``\n'`, ``\r'`, ``\\'`, and ``\s'` in your value string (``\s'` == blank).

Here is a typical global option file:

    [client]
    port=3306
    socket=/tmp/mysql.sock

    [mysqld]
    port=3306
    socket=/tmp/mysql.sock
    set-variable = key_buffer_size=16M
    set-variable = max_allowed_packet=1M

    [mysqldump]
    quick

Here is typical user option file:

    [client]
    # The following password will be sent to all standard MySQL clients
    password=my_password

    [mysql]
    no-auto-rehash
    set-variable = connect_timeout=2

    [mysqlhotcopy]
    interactive-timeout

If you have a source distribution, you will find sample configuration files named ``my-xxxx.cnf'` in the ``support-files'` directory. If you have a binary distribution, look in the ``DIR/support-files'` directory, where `DIR` is the pathname to the MySQL installation directory (typically ``/usr/local/mysql'`). Currently there are sample configuration files for small, medium, large, and very large systems. You can copy ``my-xxxx.cnf'` to your home directory (rename the copy to ``.my.cnf'`) to experiment with this.

All MySQL clients that support option files support the following options:

Option | Description
--no-defaults | Don't read any option files.
--print-defaults | Print the program name and all options that it will get.
--defaults-file=full-path-to-default-file | Only use the given configuration file.
--defaults-extra-file=full-path-to-default-file | Read this configuration file after the global configuration file but before the user configuration file.

Note that the above options must be first on the command-line to work! `--print-defaults` may however be used directly after the `--defaults-xxx-file` commands.

Note for developers: Option file handling is implemented simply by processing all matching options (that is, options in the appropriate group) before any command-line arguments. This works nicely for programs that use the last instance of an option that is specified multiple times. If you have an old program that handles multiply-specified options this way but doesn't read option files, you need add only two lines to give it that capability. Check the source code of any of the standard MySQL clients to see how to do this.

In shell scripts you can use the ``my_print_defaults'` command to parse the config files:

    shell> my_print_defaults client mysql
    --port=3306
    --socket=/tmp/mysql.sock
    --no-auto-rehash

The above output contains all options for the groups 'client' and 'mysql'.

### 4.1.3 Installing Many Servers on the Same Machine

In some cases you may want to have many different `mysqld` daemons (servers) running on the same machine. You may for example want to run a new version of MySQL for testing together with an old version that is in production. Another case is when you want to give different users access to different `mysqld` servers that they manage themselves.

One way to get a new server running is by starting it with a different socket and port as follows:

    shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock
    shell> MYSQL_TCP_PORT=3307
    shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT
    shell> scripts/mysql_install_db
    shell> bin/safe_mysqld &

The environment variables appendix includes a list of other environment variables you can use to affect `mysqld`. See section F Environment Variables.

The above is the quick and dirty way that one commonly uses for testing. The nice thing with this is that all connections you do in the above shell will automatically be directed to the new running server!

If you need to do this more permanently, you should create an option file for each server. See section 4.1.2 ``my.cnf'` Option Files. In your startup script that is executed at boot time you should specify for both servers:

`safe_mysqld --defaults-file=path-to-option-file`

At least the following options should be different per server:

- port=#
- socket=path
- pid-file=path

The following options should be different, if they are used:

- log=path
- log-bin=path
- log-update=path
- log-isam=path
- bdb-logdir=path

If you want more performance, you can also specify the following differently:

- tmpdir=path
- bdb-tmpdir=path

See section 4.1.1 `mysqld` Command-line Options.

If you are installing binary MySQL versions (.tar files) and start them with `./bin/safe_mysqld` then in most cases the only option you need to add/change is the `socket` and `port` argument to `safe_mysqld`.

See section 4.1.4 Running Multiple MySQL Servers on the Same Machine.

### 4.1.4 Running Multiple MySQL Servers on the Same Machine

There are circumstances when you might want to run multiple servers on the same machine. For example, you might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you might be an Internet service provider that wants to provide independent MySQL installations for different customers.

If you want to run multiple servers, the easiest way is to compile the servers with different TCP/IP ports and socket files so they are not both listening to the same TCP/IP port or socket file. See section 4.7.3 `mysqld_multi`, A Program for Managing Multiple MySQL Servers.

Assume an existing server is configured for the default port number and socket file. Then configure the new server with a `configure` command something like this:

    shell> ./configure  --with-tcp-port=port_number \
                 --with-unix-socket-path=file_name \
                 --prefix=/usr/local/mysql-3.22.9

Here `port_number` and `file_name` should be different from the default port number and socket file pathname, and the `--prefix` value should specify an installation directory different from the one under which the existing MySQL installation is located.

You can check the socket used by any currently executing MySQL server with this command:

    shell> mysqladmin -h hostname --port=port_number variables

Note that if you specify ```localhost`'' as a hostname, `mysqladmin` will default to using Unix sockets instead of TCP/IP.

If you have a MySQL server running on the port you used, you will get a list of some of the most important configurable variables in MySQL, including the socket name.

You don't have to recompile a new MySQL server just to start with a different port and socket. You can change the port and socket to be used by specifying them at runtime as options to `safe_mysqld`:

    shell> /path/to/safe_mysqld --socket=file_name --port=port_number

`mysqld_multi` can also take `safe_mysqld` (or `mysqld`) as an argument and pass the options from a configuration file to `safe_mysqld` and further to `mysqld`.

If you run the new server on the same database directory as another server with logging enabled, you should also specify the name of the log files to `safe_mysqld` with `--log`, `--log-update`, or `--log-slow-queries`. Otherwise, both servers may be trying to write to the same log file.

Warning: normally you should never have two servers that update data in the same database! If your OS doesn't support fault-free system locking, this may lead to unpleasant surprises!

If you want to use another database directory for the second server, you can use the `--datadir=path` option to `safe_mysqld`.

Note also that starting several MySQL servers (`mysqlds`) in different machines and letting them access one data directory over `NFS` is generally a bad idea! The problem is that the `NFS` will become the bottleneck with the speed. It is not meant for such use. And last but not least, you would still have to come up with a solution how to make sure that two or more `mysqlds` are not interfering with each other. At the moment there is no platform that would 100% reliable do the file locking (`lockd` daemon usually) in every situation. Yet there would be one more possible risk with `NFS`; it would make the work even more complicated for `lockd` daemon to handle. So make it easy for your self and forget about the idea. The working solution is to have one computer with an operating system that efficiently handles threads and have several CPUs in it.

When you want to connect to a MySQL server that is running with a different port than the port that is compiled into your client, you can use one of the following methods:

- Start the client with `--host 'hostname' --port=port_number` to connect with TCP/IP, or `[--host localhost] --socket=file_name` to connect via a Unix socket.
- In your C or Perl programs, you can give the port or socket arguments when connecting to the MySQL server.
- If your are using the Perl `DBD::mysql` module you can read the options from the MySQL option files. See section 4.1.2 ``my.cnf'` Option Files.

      $dsn = "DBI:mysql:test;mysql_read_default_group=client;
              mysql_read_default_file=/usr/local/mysql/data/my.cnf"
      $dbh = DBI->connect($dsn, $user, $password);

- Set the `MYSQL_UNIX_PORT` and `MYSQL_TCP_PORT` environment variables to point to the Unix socket and TCP/IP port before you start your clients. If you normally use a specific socket or port, you should place commands to set these environment variables in your ``.login'` file. See section F Environment Variables.
- Specify the default socket and TCP/IP port in the ``.my.cnf'` file in your home directory. See section 4.1.2 ``my.cnf'` Option Files.

## 4.2 General Security Issues and the MySQL Access Privilege System

MySQL has an advanced but non-standard security/privilege system. This section describes how it works.

### 4.2.1 General Security Guidelines

Anyone using MySQL on a computer connected to the Internet should read this section to avoid the most common security mistakes.

In discussing security, we emphasise the necessity of fully protecting the entire server host (not simply the MySQL server) against all types of applicable attacks: eavesdropping, altering, playback, and denial of service. We do not cover all aspects of availability and fault tolerance here.

MySQL uses security based on Access Control Lists (ACLs) for all connections, queries, and other operations that a user may attempt to perform. There is also some support for SSL-encrypted connections between MySQL clients and servers. Many of the concepts discussed here are not specific to MySQL at all; the same general ideas apply to almost all applications.

When running MySQL, follow these guidelines whenever possible:

- Do not ever give anyone (except the mysql root user) access to the `user` table in the `mysql` database! This is critical. The encrypted password is the real password in MySQL. Anyone who knows the password which is listed in the `user` table and has access to the host listed for the account can easily log in as that user.
- Learn the MySQL access privilege system. The `GRANT` and `REVOKE` commands are used for controlling access to MySQL. Do not grant any more privileges than necessary. Never grant privileges to all hosts. Checklist:
  - Try `mysql -u root`. If you are able to connect successfully to the server without being asked for a password, you have problems. Anyone can connect to your MySQL server as the MySQL `root` user with full privileges! Review the MySQL installation instructions, paying particular attention to the item about setting a `root` password.
  - Use the command `SHOW GRANTS` and check to see who has access to what. Remove those privileges that are not necessary using the `REVOKE` command.
- Do not keep any plain-text passwords in your database. When your computer becomes compromised, the intruder can take the full list of passwords and use them. Instead use `MD5()`, `SHA1()` or another one-way hashing function.
- Do not choose passwords from dictionaries. There are special programs to break them. Even passwords like ``xfish98'' are very bad. Much better is ``duag98'' which contains the same word ``fish'' but typed one key to the left on a standard QWERTY keyboard. Another method is to use ``Mhall'' which is taken from the first characters of each word in the sentence ``Mary had a little lamb.'' This is easy to remember and type, but difficult to guess for someone who does not know it.
- Invest in a firewall. This protects you from at least 50% of all types of exploits in any software. Put MySQL behind the firewall or in a demilitarised zone (DMZ). Checklist:
  - Try to scan your ports from the Internet using a tool such as `nmap`. MySQL uses port 3306 by default. This port should be inaccessible from untrusted hosts. Another simple way to check whether or not your MySQL port is open is to try the following command from some remote machine, where `server_host` is the hostname of your MySQL server:

        shell> telnet server_host 3306

    If you get a connection and some garbage characters, the port is open, and should be closed on your firewall or router, unless you really have a good reason to keep it open. If `telnet` just hangs or the connection is refused, everything is OK; the port is blocked.
- Do not trust any data entered by your users. They can try to trick your code by entering special or escaped character sequences in web forms, URLs, or whatever application you have built. Be sure that your application remains secure if a user enters something like ```; DROP DATABASE mysql;`''. This is an extreme example, but large security leaks and data loss may occur as a result of hackers using similar techniques, if you do not prepare for them. Also remember to check numeric data. A common mistake is to protect only strings. Sometimes people think that if a database contains only publicly available data that it need not be protected. This is incorrect. At least denial-of-service type attacks can be performed on such databases. The simplest way to protect from this type of attack is to use apostrophes around the numeric constants: `SELECT * FROM table WHERE ID='234'` rather than `SELECT * FROM table WHERE ID=234`. MySQL automatically converts this string to a number and strips all non-numeric symbols from it. Checklist:
  - All web applications:
    - Try to enter ``''` and ``"'` in all your web forms. If you get any kind of MySQL error, investigate the problem right away.
    - Try to modify any dynamic URLs by adding `%22` (``"'`), `%23` (``#'`), and `%27` (``''`) in the URL.
    - Try to modify datatypes in dynamic URLs from numeric ones to character ones containing characters from previous examples. Your application should be safe against this and similar attacks.
    - Try to enter characters, spaces, and special symbols instead of numbers in numeric fields. Your application should remove them before passing them to MySQL or your application should generate an error. Passing unchecked values to MySQL is very dangerous!
    - Check data sizes before passing them to MySQL.
    - Consider having your application connect to the database using a different user name than the one you use for administrative purposes. Do not give your applications any more access privileges than they need.
  - Users of PHP:
    - Check out the `addslashes()` function. As of PHP 4.0.3, a `mysql_escape_string()` function is available that is based on the function of the same name in the MySQL C API.
  - Users of MySQL C API:
    - Check out the `mysql_real_escape_string()` API call.
  - Users of MySQL++:
    - Check out the `escape` and `quote` modifiers for query streams.
  - Users of Perl DBI:
    - Check out the `quote()` method or use placeholders.
  - Users of Java JDBC:
    - Use a `PreparedStatement` object and placeholders.
- Do not transmit plain (unencrypted) data over the Internet. These data are accessible to everyone who has the time and ability to intercept it and use it for their own purposes. Instead, use an encrypted protocol such as SSL or SSH. MySQL supports internal SSL connections as of Version 4.0.0. SSH port-forwarding can be used to create an encrypted (and compressed) tunnel for the communication.
- Learn to use the `tcpdump` and `strings` utilities. For most cases, you can check whether MySQL data streams are unencrypted by issuing a command like the following:

      shell> tcpdump -l -i eth0 -w - src or dst port 3306 | strings

  (This works under Linux and should work with small modifications under other systems.) Warning: If you do not see data this doesn't always actually mean that it is encrypted. If you need high security, you should consult with a security expert.

### 4.2.2 How to Make MySQL Secure Against Crackers

When you connect to a MySQL server, you normally should use a password. The password is not transmitted in clear text over the connection, however the encryption algorithm is not very strong, and with some effort a clever attacker can crack the password if he is able to sniff the traffic between the client and the server. If the connection between the client and the server goes through an untrusted network, you should use an SSH tunnel to encrypt the communication.

All other information is transferred as text that can be read by anyone who is able to watch the connection. If you are concerned about this, you can use the compressed protocol (in MySQL Version 3.22 and above) to make things much harder. To make things even more secure you should use `ssh`. You can find an `Open Source` `ssh` client at http://www.openssh.org/, and a commercial `ssh` client at http://www.ssh.com/. With this, you can get an encrypted TCP/IP connection between a MySQL server and a MySQL client.

If you are using MySQL 4.0, you can also use internal OpenSSL support. See section 4.3.9 Using Secure Connections.

To make a MySQL system secure, you should strongly consider the following suggestions:

- Use passwords for all MySQL users. Remember that anyone can log in as any other person as simply as `mysql -u other_user db_name` if `other_user` has no password. It is common behaviour with client/server applications that the client may specify any user name. You can change the password of all users by editing the `mysql_install_db` script before you run it, or only the password for the MySQL `root` user like this:

      shell> mysql -u root mysql
      mysql> UPDATE user SET Password=PASSWORD('new_password')
          ->             WHERE user='root';
      mysql> FLUSH PRIVILEGES;

- Don't run the MySQL daemon as the Unix `root` user. This is very dangerous, because any user with the `FILE` privilege will be able to create files as `root` (for example, `~root/.bashrc`). To prevent this, `mysqld` will refuse to run as `root` unless it is specified directly using a `--user=root` option. `mysqld` can be run as an ordinary unprivileged user instead. You can also create a new Unix user `mysql` to make everything even more secure. If you run `mysqld` as another Unix user, you don't need to change the `root` user name in the `user` table, because MySQL user names have nothing to do with Unix user names. To start `mysqld` as another Unix user, add a `user` line that specifies the user name to the `[mysqld]` group of the ``/etc/my.cnf'` option file or the ``my.cnf'` option file in the server's data directory. For example:

      [mysqld]
      user=mysql

  This will cause the server to start as the designated user whether you start it manually or by using `safe_mysqld` or `mysql.server`. For more details, see section A.3.2 How to Run MySQL As a Normal User.
- Don't support symlinks to tables (this can be disabled with the `--skip-symlink` option). This is especially important if you run `mysqld` as root as anyone that has write access to the mysqld data directories could then delete any file in the system! See section 5.6.1.2 Using Symbolic Links for Tables.
- Check that the Unix user that `mysqld` runs as is the only user with read/write privileges in the database directories.
- Don't give the `PROCESS` privilege to all users. The output of `mysqladmin processlist` shows the text of the currently executing queries, so any user who is allowed to execute that command might be able to see if another user issues an `UPDATE user SET password=PASSWORD('not_secure')` query. `mysqld` reserves an extra connection for users who have the `PROCESS` privilege, so that a MySQL `root` user can log in and check things even if all normal connections are in use.
- Don't give the `FILE` privilege to all users. Any user that has this privilege can write a file anywhere in the filesystem with the privileges of the `mysqld` daemon! To make this a bit safer, all files generated with `SELECT ... INTO OUTFILE` are readable to everyone, and you cannot overwrite existing files. The `FILE` privilege may also be used to read any file accessible to the Unix user that the server runs as. This could be abused, for example, by using `LOAD DATA` to load ``/etc/passwd'` into a table, which can then be read with `SELECT`.
- If you don't trust your DNS, you should use IP numbers instead of hostnames in the grant tables. In any case, you should be very careful about creating grant table entries using hostname values that contain wildcards!
- If you want to restrict the number of connections for a single user, you can do this by setting the `max_user_connections` variable in `mysqld`.

### 4.2.3 Startup Options for `mysqld` Concerning Security

The following `mysqld` options affect security:

`--local-infile[=(0|1)]`
  If one uses `--local-infile=0` then one can't use `LOAD DATA LOCAL INFILE`.
`--safe-show-database`
  With this option, the `SHOW DATABASES` command returns only those databases for which the user has some kind of privilege. From version 4.0.2 this option is deprecated and doesn't do anything (the option is enabled by default) as we now have the `SHOW DATABASES` privilege. See section 4.3.1 `GRANT` and `REVOKE` Syntax.
`--safe-user-create`
  If this is enabled, an user can't create new users with the `GRANT` command, if the user doesn't have the `INSERT` privilege for the `mysql.user` table. If you want to give a user access to just create new users with those privileges that the user has right to grant, you should give the user the following privilege:

      mysql> GRANT INSERT(user) ON mysql.user TO 'user'@'hostname';

  This will ensure that the user can't change any privilege columns directly, but has to use the `GRANT` command to give privileges to other users.
`--skip-grant-tables`
  This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing `mysqladmin flush-privileges` or `mysqladmin reload`.)
`--skip-name-resolve`
  Hostnames are not resolved. All `Host` column values in the grant tables must be IP numbers or `localhost`.
`--skip-networking`
  Don't allow TCP/IP connections over the network. All connections to `mysqld` must be made via Unix sockets. This option is unsuitable when using a MySQL version prior to 3.23.27 with the MIT-pthreads package, because Unix sockets were not supported by MIT-pthreads at that time.
`--skip-show-database`
  Don't allow `SHOW DATABASES` command, unless the user has the `SHOW DATABASES` privilege. From version 4.0.2 you should no longer need this option, since access can now be granted specifically with the `SHOW DATABASES` privilege.

### 4.2.4 Security issues with LOAD DATA LOCAL

In MySQL 3.23.49 and MySQL 4.0.2, we added some new options to deal with possible security issues when it comes to `LOAD DATA LOCAL`.

There are two possible problems with supporting this command:

As the reading of the file is initiated from the server, one could theoretically create a patched MySQL server that could read any file on the client machine that the current user has read access to, when the client issues a query against the table.

In a web environment where the clients are connecting from a web server, a user could use `LOAD DATA LOCAL` to read any files that the web server process has read access to (assuming a user could run any command against the SQL server).

There are two separate fixes for this:

If you don't configure MySQL with `--enable-local-infile`, then `LOAD DATA LOCAL` will be disabled by all clients, unless one calls `mysql_options(... MYSQL_OPT_LOCAL_INFILE, 0)` in the client. See section 8.4.3.159 `mysql_options()`.

For the `mysql` command-line client, `LOAD DATA LOCAL` can be enabled by specifying the option `--local-infile[=1]`, or disabled with `--local-infile=0`.

By default, all MySQL clients and libraries are compiled with `--enable-local-infile`, to be compatible with MySQL 3.23.48 and before.

One can disable all `LOAD DATA LOCAL` commands in the MySQL server by starting `mysqld` with `--local-infile=0`.

In the case that `LOAD DATA LOCAL INFILE` is disabled in the server or the client, you will get the error message (1148):

    The used command is not allowed with this MySQL version

### 4.2.5 What the Privilege System Does

The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as `SELECT`, `INSERT`, `UPDATE` and `DELETE`.

Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as `LOAD DATA INFILE` and administrative operations.

### 4.2.6 How the Privilege System Works

The MySQL privilege system ensures that all users may do exactly the things that they are supposed to be allowed to do. When you connect to a MySQL server, your identity is determined by the host from which you connect and the user name you specify. The system grants privileges according to your identity and what you want to do.

MySQL considers both your hostname and user name in identifying you because there is little reason to assume that a given user name belongs to the same person everywhere on the Internet. For example, the user `joe` who connects from `office.com` need not be the same person as the user `joe` who connects from `elsewhere.com`. MySQL handles this by allowing you to distinguish users on different hosts that happen to have the same name: you can grant `joe` one set of privileges for connections from `office.com`, and a different set of privileges for connections from `elsewhere.com`.

MySQL access control involves two stages:

- Stage 1: The server checks whether you are even allowed to connect.
- Stage 2: Assuming you can connect, the server checks each request you issue to see whether you have sufficient privileges to perform it. For example, if you try to select rows from a table in a database or drop a table from the database, the server makes sure you have the `SELECT` privilege for the table or the `DROP` privilege for the database.

The server uses the `user`, `db`, and `host` tables in the `mysql` database at both stages of access control. The fields in these grant tables are shown here:

Table name | `user` | `db` | `host`
Scope fields | `Host` | `Host` | `Host`
| `User` | `Db` | `Db`
| `Password` | `User` |
Privilege fields | `Select_priv` | `Select_priv` | `Select_priv`
| `Insert_priv` | `Insert_priv` | `Insert_priv`
| `Update_priv` | `Update_priv` | `Update_priv`
| `Delete_priv` | `Delete_priv` | `Delete_priv`
| `Index_priv` | `Index_priv` | `Index_priv`
| `Alter_priv` | `Alter_priv` | `Alter_priv`
| `Create_priv` | `Create_priv` | `Create_priv`
| `Drop_priv` | `Drop_priv` | `Drop_priv`
| `Grant_priv` | `Grant_priv` | `Grant_priv`
| `References_priv` |  |
| `Reload_priv` |  |
| `Shutdown_priv` |  |
| `Process_priv` |  |
| `File_priv` |  |

For the second stage of access control (request verification), the server may, if the request involves tables, additionally consult the `tables_priv` and `columns_priv` tables. The fields in these tables are shown here:

Table name | `tables_priv` | `columns_priv`
Scope fields | `Host` | `Host`
| `Db` | `Db`
| `User` | `User`
| `Table_name` | `Table_name`
|  | `Column_name`
Privilege fields | `Table_priv` | `Column_priv`
| `Column_priv` |
Other fields | `Timestamp` | `Timestamp`
| `Grantor` |

Each grant table contains scope fields and privilege fields.

Scope fields determine the scope of each entry in the tables, that is, the context in which the entry applies. For example, a `user` table entry with `Host` and `User` values of `'thomas.loc.gov'` and `'bob'` would be used for authenticating connections made to the server by `bob` from the host `thomas.loc.gov`. Similarly, a `db` table entry with `Host`, `User`, and `Db` fields of `'thomas.loc.gov'`, `'bob'` and `'reports'` would be used when `bob` connects from the host `thomas.loc.gov` to access the `reports` database. The `tables_priv` and `columns_priv` tables contain scope fields indicating tables or table/column combinations to which each entry applies.

For access-checking purposes, comparisons of `Host` values are case-insensitive. `User`, `Password`, `Db`, and `Table_name` values are case-sensitive. `Column_name` values are case-insensitive in MySQL Version 3.22.12 or later.

Privilege fields indicate the privileges granted by a table entry, that is, what operations can be performed. The server combines the information in the various grant tables to form a complete description of a user's privileges. The rules used to do this are described in section 4.2.10 Access Control, Stage 2: Request Verification.

Scope fields are strings, declared as shown here; the default value for each is the empty string:

Field name | Type | Notes
`Host` | `CHAR(60)` |
`User` | `CHAR(16)` |
`Password` | `CHAR(16)` |
`Db` | `CHAR(64)` | (`CHAR(60)` for the `tables_priv` and `columns_priv` tables)
`Table_name` | `CHAR(60)` |
`Column_name` | `CHAR(60)` |

In the `user`, `db` and `host` tables, all privilege fields are declared as `ENUM('N','Y')`@-each can have a value of `'N'` or `'Y'`, and the default value is `'N'`.

In the `tables_priv` and `columns_priv` tables, the privilege fields are declared as `SET` fields:

Table name | Field name | Possible set elements
`tables_priv` | `Table_priv` | `'Select', 'Insert', 'Update', 'Delete', 'Create', 'Drop', 'Grant', 'References', 'Index', 'Alter'`
`tables_priv` | `Column_priv` | `'Select', 'Insert', 'Update', 'References'`
`columns_priv` | `Column_priv` | `'Select', 'Insert', 'Update', 'References'`

Briefly, the server uses the grant tables like this:

- The `user` table scope fields determine whether to allow or reject incoming connections. For allowed connections, any privileges granted in the `user` table indicate the user's global (superuser) privileges. These privileges apply to all databases on the server.
- The `db` and `host` tables are used together:
  - The `db` table scope fields determine which users can access which databases from which hosts. The privilege fields determine which operations are allowed.
  - The `host` table is used as an extension of the `db` table when you want a given `db` table entry to apply to several hosts. For example, if you want a user to be able to use a database from several hosts in your network, leave the `Host` value empty in the user's `db` table entry, then populate the `host` table with an entry for each of those hosts. This mechanism is described more detail in section 4.2.10 Access Control, Stage 2: Request Verification.
- The `tables_priv` and `columns_priv` tables are similar to the `db` table, but are more fine-grained: they apply at the table and column levels rather than at the database level.

Note that administrative privileges (`RELOAD`, `SHUTDOWN`, etc.) are specified only in the `user` table. This is because administrative operations are operations on the server itself and are not database-specific, so there is no reason to list such privileges in the other grant tables. In fact, only the `user` table need be consulted to determine whether you can perform an administrative operation.

The `FILE` privilege is specified only in the `user` table, too. It is not an administrative privilege as such, but your ability to read or write files on the server host is independent of the database you are accessing.

The `mysqld` server reads the contents of the grant tables once, when it starts up. Changes to the grant tables take effect as indicated in section 4.3.3 When Privilege Changes Take Effect.

When you modify the contents of the grant tables, it is a good idea to make sure that your changes set up privileges the way you want. For help in diagnosing problems, see section 4.2.11 Causes of `Access denied` Errors. For advice on security issues, see section 4.2.2 How to Make MySQL Secure Against Crackers.

A useful diagnostic tool is the `mysqlaccess` script, which Yves Carlier has provided for the MySQL distribution. Invoke `mysqlaccess` with the `--help` option to find out how it works. Note that `mysqlaccess` checks access using only the `user`, `db` and `host` tables. It does not check table- or column-level privileges.

### 4.2.7 Privileges Provided by MySQL

Information about user privileges is stored in the `user`, `db`, `host`, `tables_priv`, and `columns_priv` tables in the `mysql` database (that is, in the database named `mysql`). The MySQL server reads the contents of these tables when it starts up and under the circumstances indicated in section 4.3.3 When Privilege Changes Take Effect.

The names used in this manual to refer to the privileges provided by MySQL version 4.0.2 are shown here, along with the table column name associated with each privilege in the grant tables and the context in which the privilege applies:

Privilege | Column | Context
`ALTER` | `Alter_priv` | tables
`DELETE` | `Delete_priv` | tables
`INDEX` | `Index_priv` | tables
`INSERT` | `Insert_priv` | tables
`SELECT` | `Select_priv` | tables
`UPDATE` | `Update_priv` | tables
`CREATE` | `Create_priv` | databases, tables, or indexes
`DROP` | `Drop_priv` | databases or tables
`GRANT` | `Grant_priv` | databases or tables
`REFERENCES` | `References_priv` | databases or tables
`CREATE TEMPORARY TABLES` | `Create_tmp_table_priv` | server administration
`EXECUTE` | `Execute_priv` | server administration
`FILE` | `File_priv` | file access on server
`LOCK TABLES` | `Lock_tables_priv` | server administration
`PROCESS` | `Process_priv` | server administration
`RELOAD` | `Reload_priv` | server administration
`REPLICATION CLIENT` | `Repl_client_priv` | server administration
`REPLICATION SLAVE` | `Repl_slave_priv` | server administration
`SHOW DATABASES` | `Show_db_priv` | server administration
`SHUTDOWN` | `Shutdown_priv` | server administration
`SUPER` | `Super_priv` | server administration

The `SELECT`, `INSERT`, `UPDATE`, and `DELETE` privileges allow you to perform operations on rows in existing tables in a database.

`SELECT` statements require the `SELECT` privilege only if they actually retrieve rows from a table. You can execute certain `SELECT` statements even without permission to access any of the databases on the server. For example, you could use the `mysql` client as a simple calculator:

    mysql> SELECT 1+1;
    mysql> SELECT PI()*2;

The `INDEX` privilege allows you to create or drop (remove) indexes.

The `ALTER` privilege allows you to use `ALTER TABLE`.

The `CREATE` and `DROP` privileges allow you to create new databases and tables, or to drop (remove) existing databases and tables.

Note that if you grant the `DROP` privilege for the `mysql` database to a user, that user can drop the database in which the MySQL access privileges are stored!

The `GRANT` privilege allows you to give to other users those privileges you yourself possess.

The `FILE` privilege gives you permission to read and write files on the server using the `LOAD DATA INFILE` and `SELECT ... INTO OUTFILE` statements. Any user to whom this privilege is granted can read or write any file that the MySQL server can read or write.

The remaining privileges are used for administrative operations, which are performed using the `mysqladmin` program. The table here shows which `mysqladmin` commands each administrative privilege allows you to execute:

Privilege | Commands permitted to privilege holders
`RELOAD` | `reload`, `refresh`, `flush-privileges`, `flush-hosts`, `flush-logs`, and `flush-tables`
`SHUTDOWN` | `shutdown`
`PROCESS` | `processlist`
`SUPER` | `kill`

The `reload` command tells the server to re-read the grant tables. The `refresh` command flushes all tables and opens and closes the log files. `flush-privileges` is a synonym for `reload`. The other `flush-*` commands perform functions similar to `refresh` but are more limited in scope, and may be preferable in some instances. For example, if you want to flush just the log files, `flush-logs` is a better choice than `refresh`.

The `shutdown` command shuts down the server.

The `processlist` command displays information about the threads executing within the server. The `kill` command kills server threads. You can always display or kill your own threads, but you need the `PROCESS` privilege to display and `SUPER` privilege to kill threads initiated by other users. See section 4.5.5 `KILL` Syntax.

It is a good idea in general to grant privileges only to those users who need them, but you should exercise particular caution in granting certain privileges:

- The `GRANT` privilege allows users to give away their privileges to other users. Two users with different privileges and with the `GRANT` privilege are able to combine privileges.
- The `ALTER` privilege may be used to subvert the privilege system by renaming tables.
- The `FILE` privilege can be abused to read any world-readable file on the server into a database table, the contents of which can then be accessed using `SELECT`. This includes the contents of all databases hosted by the server!
- The `SHUTDOWN` privilege can be abused to deny service to other users entirely, by terminating the server.
- The `PROCESS` privilege can be used to view the plain text of currently executing queries, including queries that set or change passwords.
- Privileges on the `mysql` database can be used to change passwords and other access privilege information. (Passwords are stored encrypted, so a malicious user cannot simply read them to know the plain text password.) If they can access the `mysql.user` password column, they can use it to log into the MySQL server for the given user. (With sufficient privileges, the same user can replace a password with a different one.)

There are some things that you cannot do with the MySQL privilege system:

- You cannot explicitly specify that a given user should be denied access. That is, you cannot explicitly match a user and then refuse the connection.
- You cannot specify that a user has privileges to create or drop tables in a database but not to create or drop the database itself.

### 4.2.8 Connecting to the MySQL Server

MySQL client programs generally require that you specify connection parameters when you want to access a MySQL server: the host you want to connect to, your user name, and your password. For example, the `mysql` client can be started like this (optional arguments are enclosed between ``['` and ``]'`):

    shell> mysql [-h host_name] [-u user_name] [-pyour_pass]

Alternate forms of the `-h`, `-u`, and `-p` options are `--host=host_name`, `--user=user_name`, and `--password=your_pass`. Note that there is no space between `-p` or `--password=` and the password following it.

Note: Specifying a password on the command-line is not secure! Any user on your system may then find out your password by typing a command like: `ps auxww`. See section 4.1.2 ``my.cnf'` Option Files.

`mysql` uses default values for connection parameters that are missing from the command-line:

- The default hostname is `localhost`.
- The default user name is your Unix login name.
- No password is supplied if `-p` is missing.

Thus, for a Unix user `joe`, the following commands are equivalent:

    shell> mysql -h localhost -u joe
    shell> mysql -h localhost
    shell> mysql -u joe
    shell> mysql

Other MySQL clients behave similarly.

On Unix systems, you can specify different default values to be used when you make a connection, so that you need not enter them on the command-line each time you invoke a client program. This can be done in a couple of ways:

- You can specify connection parameters in the `[client]` section of the ``.my.cnf'` configuration file in your home directory. The relevant section of the file might look like this:

      [client]
      host=host_name
      user=user_name
      password=your_pass

  See section 4.1.2 ``my.cnf'` Option Files.
- You can specify connection parameters using environment variables. The host can be specified for `mysql` using `MYSQL_HOST`. The MySQL user name can be specified using `USER` (this is for Windows only). The password can be specified using `MYSQL_PWD` (but this is insecure; see the next section). See section F Environment Variables.

### 4.2.9 Access Control, Stage 1: Connection Verification

When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, then enters Stage 2 and waits for requests.

Your identity is based on two pieces of information:

- The host from which you connect
- Your MySQL user name

Identity checking is performed using the three `user` table scope fields (`Host`, `User`, and `Password`). The server accepts the connection only if a `user` table entry matches your hostname and user name, and you supply the correct password.

Values in the `user` table scope fields may be specified as follows:

- A `Host` value may be a hostname or an IP number, or `'localhost'` to indicate the local host.
- You can use the wildcard characters ``%'` and ``_'` in the `Host` field.
- A `Host` value of `'%'` matches any hostname.
- A blank `Host` value means that the privilege should be anded with the entry in the `host` table that matches the given host name. You can find more information about this in the next chapter.
- As of MySQL Version 3.23, for `Host` values specified as IP numbers, you can specify a netmask indicating how many address bits to use for the network number. For example:

      mysql> GRANT ALL PRIVILEGES ON db.*
          -> TO david@'192.58.197.0/255.255.255.0';

  This will allow everyone to connect from an IP where the following is true:

      user_ip & netmask = host_ip.

  In the above example all IP:s in the interval 192.58.197.0 - 192.58.197.255 can connect to the MySQL server.
- Wildcard characters are not allowed in the `User` field, but you can specify a blank value, which matches any name. If the `user` table entry that matches an incoming connection has a blank user name, the user is considered to be the anonymous user (the user with no name), rather than the name that the client actually specified. This means that a blank user name is used for all further access checking for the duration of the connection (that is, during Stage 2).
- The `Password` field can be blank. This does not mean that any password matches, it means the user must connect without specifying a password.

Non-blank `Password` values represent encrypted passwords. MySQL does not store passwords in plaintext form for anyone to see. Rather, the password supplied by a user who is attempting to connect is encrypted (using the `PASSWORD()` function). The encrypted password is then used when the client/server is checking if the password is correct. (This is done without the encrypted password ever traveling over the connection.) Note that from MySQL's point of view the encrypted password is the REAL password, so you should not give anyone access to it! In particular, don't give normal users read access to the tables in the `mysql` database!

The examples here show how various combinations of `Host` and `User` values in `user` table entries apply to incoming connections:

`Host` value | `User` value | Connections matched by entry
`'thomas.loc.gov'` | `'fred'` | `fred`, connecting from `thomas.loc.gov`
`'thomas.loc.gov'` | `''` | Any user, connecting from `thomas.loc.gov`
`'%'` | `'fred'` | `fred`, connecting from any host
`'%'` | `''` | Any user, connecting from any host
`'%.loc.gov'` | `'fred'` | `fred`, connecting from any host in the `loc.gov` domain
`'x.y.%'` | `'fred'` | `fred`, connecting from `x.y.net`, `x.y.com`,`x.y.edu`, etc. (this is probably not useful)
`'144.155.166.177'` | `'fred'` | `fred`, connecting from the host with IP address `144.155.166.177`
`'144.155.166.%'` | `'fred'` | `fred`, connecting from any host in the `144.155.166` class C subnet
`'144.155.166.0/255.255.255.0'` | `'fred'` | Same as previous example

Because you can use IP wildcard values in the `Host` field (for example, `'144.155.166.%'` to match every host on a subnet), there is the possibility that someone might try to exploit this capability by naming a host `144.155.166.somewhere.com`. To foil such attempts, MySQL disallows matching on hostnames that start with digits and a dot. Thus, if you have a host named something like `1.2.foo.com`, its name will never match the `Host` column of the grant tables. Only an IP number can match an IP wildcard value.

An incoming connection may be matched by more than one entry in the `user` table. For example, a connection from `thomas.loc.gov` by `fred` would be matched by several of the entries just shown above. How does the server choose which entry to use if more than one matches? The server resolves this question by sorting the `user` table after reading it at startup time, then looking through the entries in sorted order when a user attempts to connect. The first matching entry is the one that is used.

`user` table sorting works as follows. Suppose the `user` table looks like this:

    +-----------+----------+-
    | Host      | User     | ...
    +-----------+----------+-
    | %         | root     | ...
    | %         | jeffrey  | ...
    | localhost | root     | ...
    | localhost |          | ...
    +-----------+----------+-

When the server reads in the table, it orders the entries with the most-specific `Host` values first (`'%'` in the `Host` column means ``any host'' and is least specific). Entries with the same `Host` value are ordered with the most-specific `User` values first (a blank `User` value means ``any user'' and is least specific). The resulting sorted `user` table looks like this:

    +-----------+----------+-
    | Host      | User     | ...
    +-----------+----------+-
    | localhost | root     | ...
    | localhost |          | ...
    | %         | jeffrey  | ...
    | %         | root     | ...
    +-----------+----------+-

When a connection is attempted, the server looks through the sorted entries and uses the first match found. For a connection from `localhost` by `jeffrey`, the entries with `'localhost'` in the `Host` column match first. Of those, the entry with the blank user name matches both the connecting hostname and user name. (The `'%'/'jeffrey'` entry would have matched, too, but it is not the first match in the table.)

Here is another example. Suppose the `user` table looks like this:

    +----------------+----------+-
    | Host           | User     | ...
    +----------------+----------+-
    | %              | jeffrey  | ...
    | thomas.loc.gov |          | ...
    +----------------+----------+-

The sorted table looks like this:

    +----------------+----------+-
    | Host           | User     | ...
    +----------------+----------+-
    | thomas.loc.gov |          | ...
    | %              | jeffrey  | ...
    +----------------+----------+-

A connection from `thomas.loc.gov` by `jeffrey` is matched by the first entry, whereas a connection from `whitehouse.gov` by `jeffrey` is matched by the second.

A common misconception is to think that for a given user name, all entries that explicitly name that user will be used first when the server attempts to find a match for the connection. This is simply not true. The previous example illustrates this, where a connection from `thomas.loc.gov` by `jeffrey` is first matched not by the entry containing `'jeffrey'` as the `User` field value, but by the entry with no user name!

If you have problems connecting to the server, print out the `user` table and sort it by hand to see where the first match is being made. If connection was successful, but your privileges are not what you expected you may use `CURRENT_USER()` function (new in version 4.0.6) to see what user/host combination your connection actually matched. See section 6.3.6.2 Miscellaneous Functions.

### 4.2.10 Access Control, Stage 2: Request Verification

Once you establish a connection, the server enters Stage 2. For each request that comes in on the connection, the server checks whether you have sufficient privileges to perform it, based on the type of operation you wish to perform. This is where the privilege fields in the grant tables come into play. These privileges can come from any of the `user`, `db`, `host`, `tables_priv`, or `columns_priv` tables. The grant tables are manipulated with `GRANT` and `REVOKE` commands. See section 4.3.1 `GRANT` and `REVOKE` Syntax. (You may find it helpful to refer to section 4.2.6 How the Privilege System Works, which lists the fields present in each of the grant tables.)

The `user` table grants privileges that are assigned to you on a global basis and that apply no matter what the current database is. For example, if the `user` table grants you the `DELETE` privilege, you can delete rows from any database on the server host! In other words, `user` table privileges are superuser privileges. It is wise to grant privileges in the `user` table only to superusers such as server or database administrators. For other users, you should leave the privileges in the `user` table set to `'N'` and grant privileges on a database-specific basis only, using the `db` and `host` tables.

The `db` and `host` tables grant database-specific privileges. Values in the scope fields may be specified as follows:

- The wildcard characters ``%'` and ``_'` can be used in the `Host` and `Db` fields of either table. If you wish to use for instance a ``_'` character as part of a database name, specify it as ``\_'` in the `GRANT` command.
- A `'%'` `Host` value in the `db` table means ``any host.'' A blank `Host` value in the `db` table means ``consult the `host` table for further information.''
- A `'%'` or blank `Host` value in the `host` table means ``any host.''
- A `'%'` or blank `Db` value in either table means ``any database.''
- A blank `User` value in either table matches the anonymous user.

The `db` and `host` tables are read in and sorted when the server starts up (at the same time that it reads the `user` table). The `db` table is sorted on the `Host`, `Db`, and `User` scope fields, and the `host` table is sorted on the `Host` and `Db` scope fields. As with the `user` table, sorting puts the most-specific values first and least-specific values last, and when the server looks for matching entries, it uses the first match that it finds.

The `tables_priv` and `columns_priv` tables grant table- and column-specific privileges. Values in the scope fields may be specified as follows:

- The wildcard characters ``%'` and ``_'` can be used in the `Host` field of either table.
- A `'%'` or blank `Host` value in either table means ``any host.''
- The `Db`, `Table_name` and `Column_name` fields cannot contain wildcards or be blank in either table.

The `tables_priv` and `columns_priv` tables are sorted on the `Host`, `Db`, and `User` fields. This is similar to `db` table sorting, although the sorting is simpler because only the `Host` field may contain wildcards.

The request verification process is described here. (If you are familiar with the access-checking source code, you will notice that the description here differs slightly from the algorithm used in the code. The description is equivalent to what the code actually does; it differs only to make the explanation simpler.)

For administrative requests (`SHUTDOWN`, `RELOAD`, etc.), the server checks only the `user` table entry, because that is the only table that specifies administrative privileges. Access is granted if the entry allows the requested operation and denied otherwise. For example, if you want to execute `mysqladmin shutdown` but your `user` table entry doesn't grant the `SHUTDOWN` privilege to you, access is denied without even checking the `db` or `host` tables. (They contain no `Shutdown_priv` column, so there is no need to do so.)

For database-related requests (`INSERT`, `UPDATE`, etc.), the server first checks the user's global (superuser) privileges by looking in the `user` table entry. If the entry allows the requested operation, access is granted. If the global privileges in the `user` table are insufficient, the server determines the user's database-specific privileges by checking the `db` and `host` tables:

1. The server looks in the `db` table for a match on the `Host`, `Db`, and `User` fields. The `Host` and `User` fields are matched to the connecting user's hostname and MySQL user name. The `Db` field is matched to the database the user wants to access. If there is no entry for the `Host` and `User`, access is denied.
2. If there is a matching `db` table entry and its `Host` field is not blank, that entry defines the user's database-specific privileges.
3. If the matching `db` table entry's `Host` field is blank, it signifies that the `host` table enumerates which hosts should be allowed access to the database. In this case, a further lookup is done in the `host` table to find a match on the `Host` and `Db` fields. If no `host` table entry matches, access is denied. If there is a match, the user's database-specific privileges are computed as the intersection (not the union!) of the privileges in the `db` and `host` table entries, that is, the privileges that are `'Y'` in both entries. (This way you can grant general privileges in the `db` table entry and then selectively restrict them on a host-by-host basis using the `host` table entries.)

After determining the database-specific privileges granted by the `db` and `host` table entries, the server adds them to the global privileges granted by the `user` table. If the result allows the requested operation, access is granted. Otherwise, the server checks the user's table and column privileges in the `tables_priv` and `columns_priv` tables and adds those to the user's privileges. Access is allowed or denied based on the result.

Expressed in boolean terms, the preceding description of how a user's privileges are calculated may be summarised like this:

    global privileges
    OR (database privileges AND host privileges)
    OR table privileges
    OR column privileges

It may not be apparent why, if the global `user` entry privileges are initially found to be insufficient for the requested operation, the server adds those privileges to the database-, table-, and column-specific privileges later. The reason is that a request might require more than one type of privilege. For example, if you execute an `INSERT ... SELECT` statement, you need both `INSERT` and `SELECT` privileges. Your privileges might be such that the `user` table entry grants one privilege and the `db` table entry grants the other. In this case, you have the necessary privileges to perform the request, but the server cannot tell that from either table by itself; the privileges granted by the entries in both tables must be combined.

The `host` table can be used to maintain a list of secure servers.

At TcX, the `host` table contains a list of all machines on the local network. These are granted all privileges.

You can also use the `host` table to indicate hosts that are not secure. Suppose you have a machine `public.your.domain` that is located in a public area that you do not consider secure. You can allow access to all hosts on your network except that machine by using `host` table entries like this:

    +--------------------+----+-
    | Host               | Db | ...
    +--------------------+----+-
    | public.your.domain | %  | ... (all privileges set to 'N')
    | %.your.domain      | %  | ... (all privileges set to 'Y')
    +--------------------+----+-

Naturally, you should always test your entries in the grant tables (for example, using `mysqlaccess`) to make sure your access privileges are actually set up the way you think they are.

### 4.2.11 Causes of `Access denied` Errors

If you encounter `Access denied` errors when you try to connect to the MySQL server, the following list indicates some courses of action you can take to correct the problem:

- After installing MySQL, did you run the `mysql_install_db` script to set up the initial grant table contents? If not, do so. See section 4.3.4 Setting Up the Initial MySQL Privileges. Test the initial privileges by executing this command:

      shell> mysql -u root test

  The server should let you connect without error. You should also make sure you have a file ``user.MYD'` in the MySQL database directory. Ordinarily, this is ``PATH/var/mysql/user.MYD'`, where `PATH` is the pathname to the MySQL installation root.
- After a fresh installation, you should connect to the server and set up your users and their access permissions:

      shell> mysql -u root mysql

  The server should let you connect because the MySQL `root` user has no password initially. That is also a security risk, so setting the `root` password is something you should do while you're setting up your other MySQL users. If you try to connect as `root` and get this error:

      Access denied for user: '@unknown' to database mysql

  this means that you don't have an entry in the `user` table with a `User` column value of `'root'` and that `mysqld` cannot resolve the hostname for your client. In this case, you must restart the server with the `--skip-grant-tables` option and edit your ``/etc/hosts'` or ``\windows\hosts'` file to add an entry for your host.
- If you get an error like the following:

      shell> mysqladmin -u root -pxxxx ver
      Access denied for user: 'root@localhost' (Using password: YES)

  It means that you are using a wrong password. See section 4.3.7 Setting Up Passwords. If you have forgot the root password, you can restart `mysqld` with `--skip-grant-tables` to change the password. See section A.4.2 How to Reset a Forgotten Root Password. If you get the above error even if you haven't specified a password, this means that you a wrong password in some `my.ini` file. See section 4.1.2 ``my.cnf'` Option Files. You can avoid using option files with the `--no-defaults` option, as follows:

      shell> mysqladmin --no-defaults -u root ver

- If you updated an existing MySQL installation from a version earlier than Version 3.22.11 to Version 3.22.11 or later, did you run the `mysql_fix_privilege_tables` script? If not, do so. The structure of the grant tables changed with MySQL Version 3.22.11 when the `GRANT` statement became functional.
- If your privileges seem to have changed in the middle of a session, it may be that a superuser has changed them. Reloading the grant tables affects new client connections, but it also affects existing connections as indicated in section 4.3.3 When Privilege Changes Take Effect.
- If you can't get your password to work, remember that you must use the `PASSWORD()` function if you set the password with the `INSERT`, `UPDATE`, or `SET PASSWORD` statements. The `PASSWORD()` function is unnecessary if you specify the password using the `GRANT ... INDENTIFIED BY` statement or the `mysqladmin password` command. See section 4.3.7 Setting Up Passwords.
- `localhost` is a synonym for your local hostname, and is also the default host to which clients try to connect if you specify no host explicitly. However, connections to `localhost` do not work if you are using a MySQL version prior to 3.23.27 that uses MIT-pthreads (`localhost` connections are made using Unix sockets, which were not supported by MIT-pthreads at that time). To avoid this problem on such systems, you should use the `--host` option to name the server host explicitly. This will make a TCP/IP connection to the `mysqld` server. In this case, you must have your real hostname in `user` table entries on the server host. (This is true even if you are running a client program on the same host as the server.)
- If you get an `Access denied` error when trying to connect to the database with `mysql -u user_name db_name`, you may have a problem with the `user` table. Check this by executing `mysql -u root mysql` and issuing this SQL statement:

      mysql> SELECT * FROM user;

  The result should include an entry with the `Host` and `User` columns matching your computer's hostname and your MySQL user name.
- The `Access denied` error message will tell you who you are trying to log in as, the host from which you are trying to connect, and whether or not you were using a password. Normally, you should have one entry in the `user` table that exactly matches the hostname and user name that were given in the error message. For example if you get an error message that contains `Using password: NO`, this means that you tried to login without an password.
- If you get the following error when you try to connect from a different host than the one on which the MySQL server is running, then there is no row in the `user` table that matches that host:

      Host ... is not allowed to connect to this MySQL server

  You can fix this by using the command-line tool `mysql` (on the server host!) to add a row to the `user`, `db`, or `host` table for the user/hostname combination from which you are trying to connect and then execute `mysqladmin flush-privileges`. If you are not running MySQL Version 3.22 and you don't know the IP number or hostname of the machine from which you are connecting, you should put an entry with `'%'` as the `Host` column value in the `user` table and restart `mysqld` with the `--log` option on the server machine. After trying to connect from the client machine, the information in the MySQL log will indicate how you really did connect. (Then replace the `'%'` in the `user` table entry with the actual hostname that shows up in the log. Otherwise, you'll have a system that is insecure.) Another reason for this error on Linux is that you are using a binary MySQL version that is compiled with a different glibc version than the one you are using. In this case you should either upgrade your OS/glibc or download the source MySQL version and compile this yourself. A source RPM is normally trivial to compile and install, so this isn't a big problem.
- If you get an error message where the hostname is not shown or where the hostname is an IP, even if you try to connect with a hostname:

      shell> mysqladmin -u root -pxxxx -h some-hostname ver
      Access denied for user: 'root@' (Using password: YES)

  This means that MySQL got some error when trying to resolve the IP to a hostname. In this case you can execute `mysqladmin flush-hosts` to reset the internal DNS cache. See section 5.5.5 How MySQL uses DNS. Some permanent solutions are:
  - Try to find out what is wrong with your DNS server and fix this.
  - Specify IPs instead of hostnames in the MySQL privilege tables.
  - Start `mysqld` with `--skip-name-resolve`.
  - Start `mysqld` with `--skip-host-cache`.
  - Connect to `localhost` if you are running the server and the client on the same machine.
  - Put the client machine names in `/etc/hosts`.
- If `mysql -u root test` works but `mysql -h your_hostname -u root test` results in `Access denied`, then you may not have the correct name for your host in the `user` table. A common problem here is that the `Host` value in the user table entry specifies an unqualified hostname, but your system's name resolution routines return a fully qualified domain name (or vice-versa). For example, if you have an entry with host `'tcx'` in the `user` table, but your DNS tells MySQL that your hostname is `'tcx.subnet.se'`, the entry will not work. Try adding an entry to the `user` table that contains the IP number of your host as the `Host` column value. (Alternatively, you could add an entry to the `user` table with a `Host` value that contains a wildcard--for example, `'tcx.%'`. However, use of hostnames ending with ``%'` is insecure and is not recommended!)
- If `mysql -u user_name test` works but `mysql -u user_name other_db_name` doesn't work, you don't have an entry for `other_db_name` listed in the `db` table.
- If `mysql -u user_name db_name` works when executed on the server machine, but `mysql -u host_name -u user_name db_name` doesn't work when executed on another client machine, you don't have the client machine listed in the `user` table or the `db` table.
- If you can't figure out why you get `Access denied`, remove from the `user` table all entries that have `Host` values containing wildcards (entries that contain ``%'` or ``_'`). A very common error is to insert a new entry with `Host`=`'%'` and `User`=`'some user'`, thinking that this will allow you to specify `localhost` to connect from the same machine. The reason that this doesn't work is that the default privileges include an entry with `Host`=`'localhost'` and `User`=`''`. Because that entry has a `Host` value `'localhost'` that is more specific than `'%'`, it is used in preference to the new entry when connecting from `localhost`! The correct procedure is to insert a second entry with `Host`=`'localhost'` and `User`=`'some_user'`, or to remove the entry with `Host`=`'localhost'` and `User`=`''`.
- If you get the following error, you may have a problem with the `db` or `host` table:

      Access to database denied

  If the entry selected from the `db` table has an empty value in the `Host` column, make sure there are one or more corresponding entries in the `host` table specifying which hosts the `db` table entry applies to. If you get the error when using the SQL commands `SELECT ... INTO OUTFILE` or `LOAD DATA INFILE`, your entry in the `user` table probably doesn't have the `FILE` privilege enabled.
- Remember that client programs will use connection parameters specified in configuration files or environment variables. See section F Environment Variables. If a client seems to be sending the wrong default connection parameters when you don't specify them on the command-line, check your environment and the ``.my.cnf'` file in your home directory. You might also check the system-wide MySQL configuration files, though it is far less likely that client connection parameters will be specified there. See section 4.1.2 ``my.cnf'` Option Files. If you get `Access denied` when you run a client without any options, make sure you haven't specified an old password in any of your option files! See section 4.1.2 ``my.cnf'` Option Files.
- If you make changes to the grant tables directly (using an `INSERT` or `UPDATE` statement) and your changes seem to be ignored, remember that you must issue a `FLUSH PRIVILEGES` statement or execute a `mysqladmin flush-privileges` command to cause the server to re-read the privilege tables. Otherwise, your changes have no effect until the next time the server is restarted. Remember that after you set the `root` password with an `UPDATE` command, you won't need to specify it until after you flush the privileges, because the server won't know you've changed the password yet!
- If you have access problems with a Perl, PHP, Python, or ODBC program, try to connect to the server with `mysql -u user_name db_name` or `mysql -u user_name -pyour_pass db_name`. If you are able to connect using the `mysql` client, there is a problem with your program and not with the access privileges. (Note that there is no space between `-p` and the password; you can also use the `--password=your_pass` syntax to specify the password. If you use the `-p` option alone, MySQL will prompt you for the password.)
- For testing, start the `mysqld` daemon with the `--skip-grant-tables` option. Then you can change the MySQL grant tables and use the `mysqlaccess` script to check whether your modifications have the desired effect. When you are satisfied with your changes, execute `mysqladmin flush-privileges` to tell the `mysqld` server to start using the new grant tables. Note: reloading the grant tables overrides the `--skip-grant-tables` option. This allows you to tell the server to begin using the grant tables again without bringing it down and restarting it.
- If everything else fails, start the `mysqld` daemon with a debugging option (for example, `--debug=d,general,query`). This will print host and user information about attempted connections, as well as information about each command issued. See section E.1.2 Creating Trace Files.
- If you have any other problems with the MySQL grant tables and feel you must post the problem to the mailing list, always provide a dump of the MySQL grant tables. You can dump the tables with the `mysqldump mysql` command. As always, post your problem using the `mysqlbug` script. See section 1.6.2.3 How to Report Bugs or Problems. In some cases you may need to restart `mysqld` with `--skip-grant-tables` to run `mysqldump`.

## 4.3 MySQL User Account Management

### 4.3.1 `GRANT` and `REVOKE` Syntax

    GRANT priv_type [(column_list)] [, priv_type [(column_list)] ...]
        ON {tbl_name | * | *.* | db_name.*}
        TO user_name [IDENTIFIED BY [PASSWORD] 'password']
            [, user_name [IDENTIFIED BY 'password'] ...]
        [REQUIRE
            NONE |
        	[{SSL| X509}]
    	[CIPHER cipher [AND]]
    	[ISSUER issuer [AND]]
    	[SUBJECT subject]]
        [WITH [GRANT OPTION | MAX_QUERIES_PER_HOUR # |
                              MAX_UPDATES_PER_HOUR # |
                              MAX_CONNECTIONS_PER_HOUR #]]

    REVOKE priv_type [(column_list)] [, priv_type [(column_list)] ...]
        ON {tbl_name | * | *.* | db_name.*}
        FROM user_name [, user_name ...]

`GRANT` is implemented in MySQL Version 3.22.11 or later. For earlier MySQL versions, the `GRANT` statement does nothing.

The `GRANT` and `REVOKE` commands allow system administrators to create users and grant and revoke rights to MySQL users at four privilege levels:

Global level
  Global privileges apply to all databases on a given server. These privileges are stored in the `mysql.user` table.
Database level
  Database privileges apply to all tables in a given database. These privileges are stored in the `mysql.db` and `mysql.host` tables.
Table level
  Table privileges apply to all columns in a given table. These privileges are stored in the `mysql.tables_priv` table.
Column level
  Column privileges apply to single columns in a given table. These privileges are stored in the `mysql.columns_priv` table.

If you give a grant for a users that doesn't exists, that user is created. For examples of how `GRANT` works, see section 4.3.5 Adding New Users to MySQL.

For the `GRANT` and `REVOKE` statements, `priv_type` may be specified as any of the following:

`ALL [PRIVILEGES]` | Sets all simple privileges except `WITH GRANT OPTION`
`ALTER` | Allows usage of `ALTER TABLE`
`CREATE` | Allows usage of `CREATE TABLE`
`CREATE TEMPORARY TABLES` | Allows usage of `CREATE TEMPORARY TABLE`
`DELETE` | Allows usage of `DELETE`
`DROP` | Allows usage of `DROP TABLE`.
`EXECUTE` | Allows the user to run stored procedures (for MySQL 5.0)
`FILE` | Allows usage of `SELECT ... INTO OUTFILE` and `LOAD DATA INFILE`.
`INDEX` | Allows usage of `CREATE INDEX` and `DROP INDEX`
`INSERT` | Allows usage of `INSERT`
`LOCK TABLES` | Allows usage of `LOCK TABLES` on tables for which one has the `SELECT` privilege.
`PROCESS` | Allows usage of `SHOW FULL PROCESSLIST`
`REFERENCES` | For the future
`RELOAD` | Allows usage of `FLUSH`
`REPLICATION CLIENT` | Gives the right to the user to ask where the slaves/masters are.
`REPLICATION SLAVE` | Needed for the replication slaves (to read binlogs from master).
`SELECT` | Allows usage of `SELECT`
`SHOW DATABASES` | `SHOW DATABASES` shows all databases.
`SHUTDOWN` | Allows usage of `mysqladmin shutdown`
`SUPER` | Allows one connect (once) even if max_connections is reached and execute commands `CHANGE MASTER`, `KILL thread`, `mysqladmin debug`, `PURGE MASTER LOGS` and `SET GLOBAL`
`UPDATE` | Allows usage of `UPDATE`
`USAGE` | Synonym for ``no privileges.''

`USAGE` can be used when you want to create a user that has no privileges.

The privileges `CREATE TEMPORARY TABLES`, `EXECUTE`, `LOCK TABLES`, `REPLICATION ...`, `SHOW DATABASES` and `SUPER` are new for in version 4.0.2. To use these new privileges after upgrading to 4.0.2, you have to run the `mysql_fix_privilege_tables` script.

In older MySQL versions, the `PROCESS` privilege gives the same rights as the new `SUPER` privilege.

To revoke the `GRANT` privilege from a user, use a `priv_type` value of `GRANT OPTION`:

    mysql> REVOKE GRANT OPTION ON ... FROM ...;

The only `priv_type` values you can specify for a table are `SELECT`, `INSERT`, `UPDATE`, `DELETE`, `CREATE`, `DROP`, `GRANT`, `INDEX`, and `ALTER`.

The only `priv_type` values you can specify for a column (that is, when you use a `column_list` clause) are `SELECT`, `INSERT`, and `UPDATE`.

You can set global privileges by using `ON *.*` syntax. You can set database privileges by using `ON db_name.*` syntax. If you specify `ON *` and you have a current database, you will set the privileges for that database. (Warning: if you specify `ON *` and you don't have a current database, you will affect the global privileges!)

Please note: the ``_'` and ``%'` wildcards are allowed when specifying database names in `GRANT` commands. This means that if you wish to use for instance a ``_'` character as part of a database name, you should specify it as ``\_'` in the `GRANT` command, to prevent the user from being able to access additional databases matching the wildcard pattern, e.g., `GRANT ... ON `foo\_bar`.* TO ...`.

In order to accommodate granting rights to users from arbitrary hosts, MySQL supports specifying the `user_name` value in the form `user@host`. If you want to specify a `user` string containing special characters (such as ``-'`), or a `host` string containing special characters or wildcard characters (such as ``%'`), you can quote the user or host name (for example, `'test-user'@'test-hostname'`).

You can specify wildcards in the hostname. For example, `user@'%.loc.gov'` applies to `user` for any host in the `loc.gov` domain, and `user@'144.155.166.%'` applies to `user` for any host in the `144.155.166` class C subnet.

The simple form `user` is a synonym for `user@"%"`.

MySQL doesn't support wildcards in user names. Anonymous users are defined by inserting entries with `User=''` into the `mysql.user` table or creating an user with an empty name with the `GRANT` command.

Note: if you allow anonymous users to connect to the MySQL server, you should also grant privileges to all local users as `user@localhost` because otherwise the anonymous user entry for the local host in the `mysql.user` table will be used when the user tries to log into the MySQL server from the local machine!

You can verify if this applies to you by executing this query:

    mysql> SELECT Host,User FROM mysql.user WHERE User='';

For the moment, `GRANT` only supports host, table, database, and column names up to 60 characters long. A user name can be up to 16 characters.

The privileges for a table or column are formed from the logical OR of the privileges at each of the four privilege levels. For example, if the `mysql.user` table specifies that a user has a global `SELECT` privilege, this can't be denied by an entry at the database, table, or column level.

The privileges for a column can be calculated as follows:

    global privileges
    OR (database privileges AND host privileges)
    OR table privileges
    OR column privileges

In most cases, you grant rights to a user at only one of the privilege levels, so life isn't normally as complicated as above. The details of the privilege-checking procedure are presented in section 4.2 General Security Issues and the MySQL Access Privilege System.

If you grant privileges for a user/hostname combination that does not exist in the `mysql.user` table, an entry is added and remains there until deleted with a `DELETE` command. In other words, `GRANT` may create `user` table entries, but `REVOKE` will not remove them; you must do that explicitly using `DELETE`.

In MySQL Version 3.22.12 or later, if a new user is created or if you have global grant privileges, the user's password will be set to the password specified by the `IDENTIFIED BY` clause, if one is given. If the user already had a password, it is replaced by the new one.

If you don't want to send the password in clear text you can use the `PASSWORD` option followed by a scrambled password from SQL function `PASSWORD()` or the C API function `make_scrambled_password(char *to, const char *password)`.

Warning: if you create a new user but do not specify an `IDENTIFIED BY` clause, the user has no password. This is insecure.

Passwords can also be set with the `SET PASSWORD` command. See section 5.5.6 `SET` Syntax.

If you grant privileges for a database, an entry in the `mysql.db` table is created if needed. When all privileges for the database have been removed with `REVOKE`, this entry is deleted.

If a user doesn't have any privileges on a table, the table is not displayed when the user requests a list of tables (for example, with a `SHOW TABLES` statement).

The `WITH GRANT OPTION` clause gives the user the ability to give to other users any privileges the user has at the specified privilege level. You should be careful to whom you give the `GRANT` privilege, as two users with different privileges may be able to join privileges!

`MAX_QUERIES_PER_HOUR #`, `MAX_UPDATES_PER_HOUR #` and `MAX_CONNECTIONS_PER_HOUR #` are new in MySQL version 4.0.2. These options limit the number of queries/updates and logins the user can do during one hour. If `#` is 0 (default), then this means that there are no limitations for that user. See section 4.3.6 Limiting user resources.

You cannot grant another user a privilege you don't have yourself; the `GRANT` privilege allows you to give away only those privileges you possess.

Be aware that when you grant a user the `GRANT` privilege at a particular privilege level, any privileges the user already possesses (or is given in the future!) at that level are also grantable by that user. Suppose you grant a user the `INSERT` privilege on a database. If you then grant the `SELECT` privilege on the database and specify `WITH GRANT OPTION`, the user can give away not only the `SELECT` privilege, but also `INSERT`. If you then grant the `UPDATE` privilege to the user on the database, the user can give away the `INSERT`, `SELECT` and `UPDATE`.

You should not grant `ALTER` privileges to a normal user. If you do that, the user can try to subvert the privilege system by renaming tables!

Note that if you are using table or column privileges for even one user, the server examines table and column privileges for all users and this will slow down MySQL a bit.

When `mysqld` starts, all privileges are read into memory. Database, table, and column privileges take effect at once, and user-level privileges take effect the next time the user connects. Modifications to the grant tables that you perform using `GRANT` or `REVOKE` are noticed by the server immediately. If you modify the grant tables manually (using `INSERT`, `UPDATE`, etc.), you should execute a `FLUSH PRIVILEGES` statement or run `mysqladmin flush-privileges` to tell the server to reload the grant tables. See section 4.3.3 When Privilege Changes Take Effect.

The biggest differences between the ANSI SQL and MySQL versions of `GRANT` are:

- In MySQL privileges are given for an username + hostname combination and not only for an username.
- ANSI SQL doesn't have global or database-level privileges, and ANSI SQL doesn't support all privilege types that MySQL supports. MySQL doesn't support the ANSI SQL `TRIGGER` or `UNDER` privileges.
- ANSI SQL privileges are structured in a hierarchal manner. If you remove an user, all privileges the user has granted are revoked. In MySQL the granted privileges are not automatically revoked, but you have to revoke these yourself if needed.
- In MySQL, if you have the `INSERT` privilege on only some of the columns in a table, you can execute `INSERT` statements on the table; the columns for which you don't have the `INSERT` privilege will be set to their default values. ANSI SQL requires you to have the `INSERT` privilege on all columns.
- When you drop a table in ANSI SQL, all privileges for the table are revoked. If you revoke a privilege in ANSI SQL, all privileges that were granted based on this privilege are also revoked. In MySQL, privileges can be dropped only with explicit `REVOKE` commands or by manipulating the MySQL grant tables.

For a description of using `REQUIRE`, see section 4.3.9 Using Secure Connections.

### 4.3.2 MySQL User Names and Passwords

There are several distinctions between the way user names and passwords are used by MySQL and the way they are used by Unix or Windows:

- User names, as used by MySQL for authentication purposes, have nothing to do with Unix user names (login names) or Windows user names. Most MySQL clients by default try to log in using the current Unix user name as the MySQL user name, but that is for convenience only. Client programs allow a different name to be specified with the `-u` or `--user` options. This means that you can't make a database secure in any way unless all MySQL user names have passwords. Anyone may attempt to connect to the server using any name, and they will succeed if they specify any name that doesn't have a password.
- MySQL user names can be up to 16 characters long; Unix user names typically are limited to 8 characters.
- MySQL passwords have nothing to do with Unix passwords. There is no necessary connection between the password you use to log in to a Unix machine and the password you use to access a database on that machine.
- MySQL encrypts passwords using a different algorithm than the one used during the Unix login process. See the descriptions of the `PASSWORD()` and `ENCRYPT()` functions in section 6.3.6.2 Miscellaneous Functions. Note that even if the password is stored 'scrambled', and knowing your 'scrambled' password is enough to be able to connect to the MySQL server!

MySQL users and their privileges are normally created with the `GRANT` command. See section 4.3.1 `GRANT` and `REVOKE` Syntax.

When you login to a MySQL server with a command-line client you should specify the password with `--password=your-password`. See section 4.2.8 Connecting to the MySQL Server.

    mysql --user=monty --password=guess database_name

If you want the client to prompt for a password, you should use `--password` without any argument

    mysql --user=monty --password database_name

or the short form:

    mysql -u monty -p database_name

Note that in the last example the password is not 'database_name'.

If you want to use the `-p` option to supply a password you should do so like this:

    mysql -u monty -pguess database_name

On some systems, the library call that MySQL uses to prompt for a password will automatically cut the password to 8 characters. Internally MySQL doesn't have any limit for the length of the password.

### 4.3.3 When Privilege Changes Take Effect

When `mysqld` starts, all grant table contents are read into memory and become effective at that point.

Modifications to the grant tables that you perform using `GRANT`, `REVOKE`, or `SET PASSWORD` are noticed by the server immediately.

If you modify the grant tables manually (using `INSERT`, `UPDATE`, etc.), you should execute a `FLUSH PRIVILEGES` statement or run `mysqladmin flush-privileges` or `mysqladmin reload` to tell the server to reload the grant tables. Otherwise, your changes will have no effect until you restart the server. If you change the grant tables manually but forget to reload the privileges, you will be wondering why your changes don't seem to make any difference!

When the server notices that the grant tables have been changed, existing client connections are affected as follows:

- Table and column privilege changes take effect with the client's next request.
- Database privilege changes take effect at the next `USE db_name` command.
- Global privilege changes and password changes take effect the next time the client connects.

### 4.3.4 Setting Up the Initial MySQL Privileges

After installing MySQL, you set up the initial access privileges by running `scripts/mysql_install_db`. See section 2.3.1 Quick Installation Overview. The `mysql_install_db` script starts up the `mysqld` server, then initialises the grant tables to contain the following set of privileges:

- The MySQL `root` user is created as a superuser who can do anything. Connections must be made from the local host. Note: The initial `root` password is empty, so anyone can connect as `root` without a password and be granted all privileges.
- An anonymous user is created that can do anything with databases that have a name of `'test'` or starting with `'test_'`. Connections must be made from the local host. This means any local user can connect without a password and be treated as the anonymous user.
- Other privileges are denied. For example, normal users can't use `mysqladmin shutdown` or `mysqladmin processlist`.

Note: the default privileges are different for Windows. See section 2.6.2.3 Running MySQL on Windows.

Because your installation is initially wide open, one of the first things you should do is specify a password for the MySQL `root` user. You can do this as follows (note that you specify the password using the `PASSWORD()` function):

    shell> mysql -u root mysql
    mysql> SET PASSWORD FOR root@localhost=PASSWORD('new_password');

If you know what you are doing, you can also directly manipulate the privilege tables:

    shell> mysql -u root mysql
    mysql> UPDATE user SET Password=PASSWORD('new_password')
        ->     WHERE user='root';
    mysql> FLUSH PRIVILEGES;

Another way to set the password is by using the `mysqladmin` command:

    shell> mysqladmin -u root password new_password

Only users with write/update access to the `mysql` database can change the password for others users. All normal users (not anonymous ones) can only change their own password with either of the above commands or with `SET PASSWORD=PASSWORD('new password')`.

Note that if you update the password in the `user` table directly using the first method, you must tell the server to re-read the grant tables (with `FLUSH PRIVILEGES`), because the change will go unnoticed otherwise.

Once the `root` password has been set, thereafter you must supply that password when you connect to the server as `root`.

You may wish to leave the `root` password blank so that you don't need to specify it while you perform additional setup or testing. However, be sure to set it before using your installation for any real production work.

See the `scripts/mysql_install_db` script to see how it sets up the default privileges. You can use this as a basis to see how to add other users.

If you want the initial privileges to be different from those just described above, you can modify `mysql_install_db` before you run it.

To re-create the grant tables completely, remove all the ``.frm'`, ``.MYI'`, and ``.MYD'` files in the directory containing the `mysql` database. (This is the directory named ``mysql'` under the database directory, which is listed when you run `mysqld --help`.) Then run the `mysql_install_db` script, possibly after editing it first to have the privileges you want.

Note: for MySQL versions older than Version 3.22.10, you should not delete the ``.frm'` files. If you accidentally do this, you should copy them back from your MySQL distribution before running `mysql_install_db`.

### 4.3.5 Adding New Users to MySQL

You can add users two different ways: by using `GRANT` statements or by manipulating the MySQL grant tables directly. The preferred method is to use `GRANT` statements, because they are more concise and less error-prone. See section 4.3.1 `GRANT` and `REVOKE` Syntax.

There are also a lot of contributed programs like `phpmyadmin` that can be used to create and administrate users. See section 1.6.1 MySQL Portals.

The following examples show how to use the `mysql` client to set up new users. These examples assume that privileges are set up according to the defaults described in the previous section. This means that to make changes, you must be on the same machine where `mysqld` is running, you must connect as the MySQL `root` user, and the `root` user must have the `INSERT` privilege for the `mysql` database and the `RELOAD` administrative privilege. Also, if you have changed the `root` user password, you must specify it for the `mysql` commands here.

You can add new users by issuing `GRANT` statements:

    shell> mysql --user=root mysql
    mysql> GRANT ALL PRIVILEGES ON *.* TO monty@localhost
        ->     IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
    mysql> GRANT ALL PRIVILEGES ON *.* TO monty@"%"
        ->     IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
    mysql> GRANT RELOAD,PROCESS ON *.* TO admin@localhost;
    mysql> GRANT USAGE ON *.* TO dummy@localhost;

These `GRANT` statements set up three new users:

`monty`
  A full superuser who can connect to the server from anywhere, but who must use a password `'some_pass'` to do so. Note that we must issue `GRANT` statements for both `monty@localhost` and `monty@"%"`. If we don't add the entry with `localhost`, the anonymous user entry for `localhost` that is created by `mysql_install_db` will take precedence when we connect from the local host, because it has a more specific `Host` field value and thus comes earlier in the `user` table sort order.
`admin`
  A user who can connect from `localhost` without a password and who is granted the `RELOAD` and `PROCESS` administrative privileges. This allows the user to execute the `mysqladmin reload`, `mysqladmin refresh`, and `mysqladmin flush-*` commands, as well as `mysqladmin processlist` . No database-related privileges are granted. (They can be granted later by issuing additional `GRANT` statements.)
`dummy`
  A user who can connect without a password, but only from the local host. The global privileges are all set to `'N'`@-the `USAGE` privilege type allows you to create a user with no privileges. It is assumed that you will grant database-specific privileges later.

You can also add the same user access information directly by issuing `INSERT` statements and then telling the server to reload the grant tables:

    shell> mysql --user=root mysql
    mysql> INSERT INTO user VALUES('localhost','monty',PASSWORD('some_pass'),
        ->          'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
    mysql> INSERT INTO user VALUES('%','monty',PASSWORD('some_pass'),
        ->          'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
    mysql> INSERT INTO user SET Host='localhost',User='admin',
        ->           Reload_priv='Y', Process_priv='Y';
    mysql> INSERT INTO user (Host,User,Password)
        ->                  VALUES('localhost','dummy','');
    mysql> FLUSH PRIVILEGES;

Depending on your MySQL version, you may have to use a different number of `'Y'` values above (versions prior to Version 3.22.11 had fewer privilege columns). For the `admin` user, the more readable extended `INSERT` syntax that is available starting with Version 3.22.11 is used.

Note that to set up a superuser, you need only create a `user` table entry with the privilege fields set to `'Y'`. No `db` or `host` table entries are necessary.

The privilege columns in the `user` table were not set explicitly in the last `INSERT` statement (for the `dummy` user), so those columns are assigned the default value of `'N'`. This is the same thing that `GRANT USAGE` does.

The following example adds a user `custom` who can connect from hosts `localhost`, `server.domain`, and `whitehouse.gov`. He wants to access the `bankaccount` database only from `localhost`, the `expenses` database only from `whitehouse.gov`, and the `customer` database from all three hosts. He wants to use the password `stupid` from all three hosts.

To set up this user's privileges using `GRANT` statements, run these commands:

    shell> mysql --user=root mysql
    mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
        ->     ON bankaccount.*
        ->     TO custom@localhost
        ->     IDENTIFIED BY 'stupid';
    mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
        ->     ON expenses.*
        ->     TO custom@whitehouse.gov
        ->     IDENTIFIED BY 'stupid';
    mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
        ->     ON customer.*
        ->     TO custom@'%'
        ->     IDENTIFIED BY 'stupid';

The reason that we do to grant statements for the user 'custom' is that we want the give the user access to MySQL both from the local machine with Unix sockets and from the remote machine 'whitehouse.gov' over TCP/IP.

To set up the user's privileges by modifying the grant tables directly, run these commands (note the `FLUSH PRIVILEGES` at the end):

    shell> mysql --user=root mysql
    mysql> INSERT INTO user (Host,User,Password)
        -> VALUES('localhost','custom',PASSWORD('stupid'));
    mysql> INSERT INTO user (Host,User,Password)
        -> VALUES('server.domain','custom',PASSWORD('stupid'));
    mysql> INSERT INTO user (Host,User,Password)
        -> VALUES('whitehouse.gov','custom',PASSWORD('stupid'));
    mysql> INSERT INTO db
        -> (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        ->  Create_priv,Drop_priv)
        -> VALUES
        -> ('localhost','bankaccount','custom','Y','Y','Y','Y','Y','Y');
    mysql> INSERT INTO db
        -> (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        ->  Create_priv,Drop_priv)
        -> VALUES
        -> ('whitehouse.gov','expenses','custom','Y','Y','Y','Y','Y','Y');
    mysql> INSERT INTO db
        -> (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        ->  Create_priv,Drop_priv)
        -> VALUES('%','customer','custom','Y','Y','Y','Y','Y','Y');
    mysql> FLUSH PRIVILEGES;

The first three `INSERT` statements add `user` table entries that allow user `custom` to connect from the various hosts with the given password, but grant no permissions to him (all privileges are set to the default value of `'N'`). The next three `INSERT` statements add `db` table entries that grant privileges to `custom` for the `bankaccount`, `expenses`, and `customer` databases, but only when accessed from the proper hosts. As usual, when the grant tables are modified directly, the server must be told to reload them (with `FLUSH PRIVILEGES`) so that the privilege changes take effect.

If you want to give a specific user access from any machine in a given domain, you can issue a `GRANT` statement like the following:

    mysql> GRANT ...
        ->     ON *.*
        ->     TO myusername@"%.mydomainname.com"
        ->     IDENTIFIED BY 'mypassword';

To do the same thing by modifying the grant tables directly, do this:

    mysql> INSERT INTO user VALUES ('%.mydomainname.com', 'myusername',
        ->             PASSWORD('mypassword'),...);
    mysql> FLUSH PRIVILEGES;

You can also use `xmysqladmin`, `mysql_webadmin`, and even `xmysql` to insert, change, and update values in the grant tables. You can find these utilities in the Contrib directory of the MySQL web site (http://www.mysql.com/Downloads/Contrib/).

### 4.3.6 Limiting user resources

Starting from MySQL 4.0.2 one can limit certain resources per user.

So far, the only available method of limiting usage of MySQL server resources has been setting the `max_user_connections` startup variable to a non-zero value. But this method is strictly global and does not allow for management of individual users, which could be of particular interest to Internet Service Providers.

Therefore, management of three resources is introduced on the individual user level:

- Number of all queries per hour: All commands that could be run by a user.
- Number of all updates per hour: Any command that changes any table or database.
- Number of connections made per hour: New connections opened per hour.

A user in the aforementioned context is a single entry in the `user` table, which is uniquely identified by its `user` and `host` columns.

All users are by default not limited in using the above resources, unless the limits are granted to them. These limits can be granted only via global `GRANT (*.*)`, using this syntax:

    GRANT ... WITH MAX_QUERIES_PER_HOUR N1
                   MAX_UPDATES_PER_HOUR N2
                   MAX_CONNECTIONS_PER_HOUR N3;

One can specify any combination of the above resources. N1, N2 and N3 are integers and stands for count / hour.

If user reaches any of the above limits withing one hour, his connection will be terminated or refused and the appropriate error message shall be issued.

Current usage values for a particular user can be flushed (set to zero) by issuing a `GRANT` statement with any of the above clauses, including a `GRANT` statement with the current values.

Also, current values for all users will be flushed if privileges are reloaded (in the server or using `mysqladmin reload`) or if the `FLUSH USER_RESOURCES` command is issued.

The feature is enabled as soon as a single user is granted with any of the limiting `GRANT` clauses.

As a prerequisite for enabling this feature, the `user` table in the `mysql` database must contain the additional columns, as defined in the table creation scripts `mysql_install_db` and `mysql_install_db.sh` in ``scripts'` subdirectory.

### 4.3.7 Setting Up Passwords

In most cases you should use `GRANT` to set up your users/passwords, so the following only applies for advanced users. See section 4.3.1 `GRANT` and `REVOKE` Syntax.

The examples in the preceding sections illustrate an important principle: when you store a non-empty password using `INSERT` or `UPDATE` statements, you must use the `PASSWORD()` function to encrypt it. This is because the `user` table stores passwords in encrypted form, not as plaintext. If you forget that fact, you are likely to attempt to set passwords like this:

    shell> mysql -u root mysql
    mysql> INSERT INTO user (Host,User,Password)
        -> VALUES('%','jeffrey','biscuit');
    mysql> FLUSH PRIVILEGES;

The result is that the plaintext value `'biscuit'` is stored as the password in the `user` table. When the user `jeffrey` attempts to connect to the server using this password, the `mysql` client encrypts it with `PASSWORD()`, generates an authentification vector based on encrypted password and a random number, obtained from server, and sends the result to the server. The server uses the `password` value in the `user` table (that is not encrypted value `'biscuit'`) to perform the same calculations, and compares results. The comparison fails and the server rejects the connection:

    shell> mysql -u jeffrey -pbiscuit test
    Access denied

Passwords must be encrypted when they are inserted in the `user` table, so the `INSERT` statement should have been specified like this instead:

    mysql> INSERT INTO user (Host,User,Password)
        -> VALUES('%','jeffrey',PASSWORD('biscuit'));

You must also use the `PASSWORD()` function when you use `SET PASSWORD` statements:

    mysql> SET PASSWORD FOR jeffrey@"%" = PASSWORD('biscuit');

If you set passwords using the `GRANT ... IDENTIFIED BY` statement or the `mysqladmin password` command, the `PASSWORD()` function is unnecessary. They both take care of encrypting the password for you, so you would specify a password of `'biscuit'` like this:

    mysql> GRANT USAGE ON *.* TO jeffrey@"%" IDENTIFIED BY 'biscuit';

or

    shell> mysqladmin -u jeffrey password biscuit

Note: `PASSWORD()` does not perform password encryption in the same way that Unix passwords are encrypted. You should not assume that if your Unix password and your MySQL password are the same, that `PASSWORD()` will result in the same encrypted value as is stored in the Unix password file. See section 4.3.2 MySQL User Names and Passwords.

### 4.3.8 Keeping Your Password Secure

It is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed here, along with an assessment of the risks of each method:

- Never give a normal user access to the `mysql.user` table. Knowing the encrypted password for a user makes it possible to login as this user. The passwords are only scrambled so that one shouldn't be able to see the real password you used (if you happen to use a similar password with your other applications).
- Use a `-pyour_pass` or `--password=your_pass` option on the command line. This is convenient but insecure, because your password becomes visible to system status programs (such as `ps`) that may be invoked by other users to display command-lines. (MySQL clients typically overwrite the command-line argument with zeroes during their initialisation sequence, but there is still a brief interval during which the value is visible.)
- Use a `-p` or `--password` option (with no `your_pass` value specified). In this case, the client program solicits the password from the terminal:

      shell> mysql -u user_name -p
      Enter password: ********

  The ``*'` characters represent your password. It is more secure to enter your password this way than to specify it on the command-line because it is not visible to other users. However, this method of entering a password is suitable only for programs that you run interactively. If you want to invoke a client from a script that runs non-interactively, there is no opportunity to enter the password from the terminal. On some systems, you may even find that the first line of your script is read and interpreted (incorrectly) as your password!
- Store your password in a configuration file. For example, you can list your password in the `[client]` section of the ``.my.cnf'` file in your home directory:

      [client]
      password=your_pass

  If you store your password in ``.my.cnf'`, the file should not be group or world readable or writable. Make sure the file's access mode is `400` or `600`. See section 4.1.2 ``my.cnf'` Option Files.
- You can store your password in the `MYSQL_PWD` environment variable, but this method must be considered extremely insecure and should not be used. Some versions of `ps` include an option to display the environment of running processes; your password will be in plain sight for all to see if you set `MYSQL_PWD`. Even on systems without such a version of `ps`, it is unwise to assume there is no other method to observe process environments. See section F Environment Variables.

All in all, the safest methods are to have the client program prompt for the password or to specify the password in a properly protected ``.my.cnf'` file.

### 4.3.9 Using Secure Connections

#### 4.3.9.1 Basics

Beginning with version 4.0.0, MySQL has support for SSL encrypted connections. To understand how MySQL uses SSL, it's necessary to explain some basic SSL and X509 concepts. People who are already familiar with them can skip this part.

By default, MySQL uses unencrypted connections between the client and the server. This means that someone could watch all your traffic and look at the data being sent or received. They could even change the data while it is in transit between client and server. Sometimes you need to move information over public networks in a secure fashion; in such cases, using an unencrypted connection is unacceptable.

SSL is a protocol that uses different encryption algorithms to ensure that data received over a public network can be trusted. It has mechanisms to detect any change, loss or replay of data. SSL also incorporates algorithms to recognise and provide identity verification using the X509 standard.

Encryption is the way to make any kind of data unreadable. In fact, today's practice requires many additional security elements from encryption algorithms. They should resist many kind of known attacks like just messing with the order of encrypted messages or replaying data twice.

X509 is a standard that makes it possible to identify someone on the Internet. It is most commonly used in e-commerce applications. In basic terms, there should be some company (called a ``Certificate Authority'') that assigns electronic certificates to anyone who needs them. Certificates rely on asymmetric encryption algorithms that have two encryption keys (a public key and a secret key). A certificate owner can prove his identity by showing his certificate to other party. A certificate consists of its owner's public key. Any data encrypted with this public key can be decrypted only using the corresponding secret key, which is held by the owner of the certificate.

MySQL doesn't use encrypted connections by default, because doing so would make the client/server protocol much slower. Any kind of additional functionality requires the computer to do additional work and encrypting data is a CPU-intensive operation that requires time and can delay MySQL main tasks. By default MySQL is tuned to be fast as possible.

If you need more information about SSL, X509, or encryption, you should use your favourite Internet search engine and search for keywords in which you are interested.

#### 4.3.9.2 Requirements

To get secure connections to work with MySQL you must do the following:

1. Install the OpenSSL library. We have tested MySQL with OpenSSL 0.9.6. http://www.openssl.org/.
2. Configure MySQL with `--with-vio --with-openssl`.
3. If you are using an old MySQL installation, you have to update your `mysql.user` table with some new SSL-related columns. You can do this by running the `mysql_fix_privilege_tables.sh` script. This is necessary if your grant tables date from a version prior to MySQL 4.0.0.
4. You can check if a running `mysqld` server supports OpenSSL by examining if `SHOW VARIABLES LIKE 'have_openssl'` returns `YES`.

#### 4.3.9.3 `GRANT` Options

MySQL can check X509 certificate attributes in addition to the normal username/password scheme. All the usual options are still required (username, password, IP address mask, database/table name).

There are different possibilities to limit connections:

- Without any SSL or X509 options, all kind of encrypted/unencrypted connections are allowed if the username and password are valid.
- `REQUIRE SSL` option limits the server to allow only SSL encrypted connections. Note that this option can be omitted if there are any ACL records which allow non-SSL connections.

      mysql> GRANT ALL PRIVILEGES ON test.* TO root@localhost
          -> IDENTIFIED BY "goodsecret" REQUIRE SSL;

- `REQUIRE X509` means that the client should have a valid certificate but we do not care about the exact certificate, issuer or subject. The only restriction is that it should be possible to verify its signature with one of the CA certificates.

      mysql> GRANT ALL PRIVILEGES ON test.* TO root@localhost
          -> IDENTIFIED BY "goodsecret" REQUIRE X509;

- `REQUIRE ISSUER "issuer"` places a restriction on connection attempts: The client must present a valid X509 certificate issued by CA `"issuer"`. Using X509 certificates always implies encryption, so the `SSL` option is unneccessary.

      mysql> GRANT ALL PRIVILEGES ON test.* TO root@localhost
          -> IDENTIFIED BY "goodsecret"
          -> REQUIRE ISSUER "C=FI, ST=Some-State, L=Helsinki,
          "> O=MySQL Finland AB, CN=Tonu Samuel/Email=tonu@mysql.com";

- `REQUIRE SUBJECT "subject"` requires clients to have valid X509 certificate with subject `"subject"` on it. If the client presents a certificate that is valid but has a different `"subject"`, the connection is disallowed.

      mysql> GRANT ALL PRIVILEGES ON test.* TO root@localhost
          -> IDENTIFIED BY "goodsecret"
          -> REQUIRE SUBJECT "C=EE, ST=Some-State, L=Tallinn,
          "> O=MySQL demo client certificate,
          "> CN=Tonu Samuel/Email=tonu@mysql.com";

- `REQUIRE CIPHER "cipher"` is needed to assure enough strong ciphers and keylengths will be used. SSL itself can be weak if old algorithms with short encryption keys are used. Using this option, we can ask for some exact cipher method to allow a connection.

      mysql> GRANT ALL PRIVILEGES ON test.* TO root@localhost
          -> IDENTIFIED BY "goodsecret"
          -> REQUIRE CIPHER "EDH-RSA-DES-CBC3-SHA";

  The `SUBJECT`, `ISSUER`, and `CIPHER` options can be combined in the `REQUIRE` clause like this:

      mysql> GRANT ALL PRIVILEGES ON test.* TO root@localhost
          -> IDENTIFIED BY "goodsecret"
          -> REQUIRE SUBJECT "C=EE, ST=Some-State, L=Tallinn,
          "> O=MySQL demo client certificate,
          "> CN=Tonu Samuel/Email=tonu@mysql.com"
          -> AND ISSUER "C=FI, ST=Some-State, L=Helsinki,
          "> O=MySQL Finland AB, CN=Tonu Samuel/Email=tonu@mysql.com"
          -> AND CIPHER "EDH-RSA-DES-CBC3-SHA";

  Starting from MySQL 4.0.4 the `AND` keyword is optional between `REQUIRE` options. The order of the options does not matter, but no option can be specified twice.

## 4.4 Disaster Prevention and Recovery

### 4.4.1 Database Backups

Because MySQL tables are stored as files, it is easy to do a backup. To get a consistent backup, do a `LOCK TABLES` on the relevant tables followed by `FLUSH TABLES` for the tables. See section 6.7.2 `LOCK TABLES/UNLOCK TABLES` Syntax. See section 4.5.3 `FLUSH` Syntax. You only need a read lock; this allows other threads to continue to query the tables while you are making a copy of the files in the database directory. The `FLUSH TABLE` is needed to ensure that the all active index pages is written to disk before you start the backup.

If you want to make a SQL level backup of a table, you can use `SELECT INTO OUTFILE` or `BACKUP TABLE`. See section 6.4.1 `SELECT` Syntax. See section 4.4.2 `BACKUP TABLE` Syntax.

Another way to back up a database is to use the `mysqldump` program or the `mysqlhotcopy script`. See section 4.8.5 `mysqldump`, Dumping Table Structure and Data. See section 4.8.6 `mysqlhotcopy`, Copying MySQL Databases and Tables.

1. Do a full backup of your databases:

       shell> mysqldump --tab=/path/to/some/dir --opt --all

       or

       shell> mysqlhotcopy database /path/to/some/dir

   You can also simply copy all table files (``*.frm'`, ``*.MYD'`, and ``*.MYI'` files) as long as the server isn't updating anything. The script `mysqlhotcopy` does use this method.
2. Stop `mysqld` if it's running, then start it with the `--log-update[=file_name]` option. See section 4.9.3 The Update Log. The update log file(s) provide you with the information you need to replicate changes to the database that are made subsequent to the point at which you executed `mysqldump`.

If you have to restore something, try to recover your tables using `REPAIR TABLE` or `myisamchk -r` first. That should work in 99.9% of all cases. If `myisamchk` fails, try the following procedure (this will only work if you have started MySQL with `--log-update`, see section 4.9.3 The Update Log):

1. Restore the original `mysqldump` backup.
2. Execute the following command to re-run the updates in the binary log:

       shell> mysqlbinlog hostname-bin.[0-9]* | mysql

   If you are using the update log you can use:

       shell> ls -1 -t -r hostname.[0-9]* | xargs cat | mysql

`ls` is used to get all the update log files in the right order.

You can also do selective backups with `SELECT * INTO OUTFILE 'file_name' FROM tbl_name` and restore with `LOAD DATA INFILE 'file_name' REPLACE ...` To avoid duplicate records, you need a `PRIMARY KEY` or a `UNIQUE` key in the table. The `REPLACE` keyword causes old records to be replaced with new ones when a new record duplicates an old record on a unique key value.

If you get performance problems in making backups on your system, you can solve this by setting up replication and do the backups on the slave instead of on the master. See section 4.10.1 Introduction.

If you are using a Veritas filesystem, you can do:

1. From a client (or Perl), execute: `FLUSH TABLES WITH READ LOCK`.
2. From another shell, execute: `mount vxfs snapshot`.
3. From the first client, execute: `UNLOCK TABLES`.
4. Copy files from snapshot.
5. Unmount snapshot.

### 4.4.2 `BACKUP TABLE` Syntax

    BACKUP TABLE tbl_name[,tbl_name...] TO '/path/to/backup/directory'

Copies to the backup directory the minimum number of table files needed to restore the table, after flushing any buffered changes to disk. Currently works only for `MyISAM` tables. For `MyISAM` tables, copies ``.frm'` (definition) and ``.MYD'` (data) files. The index file can be rebuilt from those two.

Before using this command, please see section 4.4.1 Database Backups.

During the backup, a read lock will be held for each table, one at time, as they are being backed up. If you want to back up several tables as a snapshot, you must first issue `LOCK TABLES` obtaining a read lock for each table in the group.

The command returns a table with the following columns:

Column | Value
Table | Table name
Op | Always ``backup''
Msg_type | One of `status`, `error`, `info` or `warning`.
Msg_text | The message.

Note that `BACKUP TABLE` is only available in MySQL version 3.23.25 and later.

### 4.4.3 `RESTORE TABLE` Syntax

    RESTORE TABLE tbl_name[,tbl_name...] FROM '/path/to/backup/directory'

Restores the table(s) from the backup that was made with `BACKUP TABLE`. Existing tables will not be overwritten; if you try to restore over an existing table, you will get an error. Restoring will take longer than backing up due to the need to rebuild the index. The more keys you have, the longer it will take. Just as `BACKUP TABLE`, `RESTORE TABLE` currently works only for `MyISAM` tables.

The command returns a table with the following columns:

Column | Value
Table | Table name
Op | Always ``restore''
Msg_type | One of `status`, `error`, `info` or `warning`.
Msg_text | The message.

### 4.4.4 `CHECK TABLE` Syntax

    CHECK TABLE tbl_name[,tbl_name...] [option [option...]]

    option = QUICK | FAST | MEDIUM | EXTENDED | CHANGED

`CHECK TABLE` works only on `MyISAM` and `InnoDB` tables. On `MyISAM` tables it's the same thing as running `myisamchk -m table_name` on the table.

If you don't specify any option `MEDIUM` is used.

Checks the table(s) for errors. For `MyISAM` tables the key statistics are updated. The command returns a table with the following columns:

Column | Value
Table | Table name.
Op | Always ``check''.
Msg_type | One of `status`, `error`, `info`, or `warning`.
Msg_text | The message.

Note that you can get many rows of information for each checked table. The last row will be of `Msg_type status` and should normally be `OK`. If you don't get `OK`, or `Not checked` you should normally run a repair of the table. See section 4.4.6 Using `myisamchk` for Table Maintenance and Crash Recovery. `Not checked` means that the table the given `TYPE` told MySQL that there wasn't any need to check the table.

The different check types stand for the following:

Type | Meaning
`QUICK` | Don't scan the rows to check for wrong links.
`FAST` | Only check tables which haven't been closed properly.
`CHANGED` | Only check tables which have been changed since last check or haven't been closed properly.
`MEDIUM` | Scan rows to verify that deleted links are okay. This also calculates a key checksum for the rows and verifies this with a calculated checksum for the keys.
`EXTENDED` | Do a full key lookup for all keys for each row. This ensures that the table is 100% consistent, but will take a long time!

For dynamically sized `MyISAM` tables a started check will always do a `MEDIUM` check. For statically sized rows we skip the row scan for `QUICK` and `FAST` as the rows are very seldom corrupted.

You can combine check options as in:

    CHECK TABLE test_table FAST QUICK;

Which would simply do a quick check on the table to see whether it was closed properly.

Note: that in some case `CHECK TABLE` will change the table! This happens if the table is marked as 'corrupted' or 'not closed properly' but `CHECK TABLE` didn't find any problems in the table. In this case `CHECK TABLE` will mark the table as okay.

If a table is corrupted, then it's most likely that the problem is in the indexes and not in the data part. All of the above check types checks the indexes thoroughly and should thus find most errors.

If you just want to check a table that you assume is okay, you should use no check options or the `QUICK` option. The latter should be used when you are in a hurry and can take the very small risk that `QUICK` didn't find an error in the datafile. (In most cases MySQL should find, under normal usage, any error in the data file. If this happens then the table will be marked as 'corrupted', in which case the table can't be used until it's repaired.)

`FAST` and `CHANGED` are mostly intended to be used from a script (for example to be executed from `cron`) if you want to check your table from time to time. In most cases you `FAST` is to be prefered over `CHANGED`. (The only case when it isn't is when you suspect a bug you have found a bug in the `MyISAM` code.)

`EXTENDED` is only to be used after you have run a normal check but still get strange errors from a table when MySQL tries to update a row or find a row by key (this is very unlikely if a normal check has succeeded!).

Some things reported by check table, can't be corrected automatically:

- `Found row where the auto_increment column has the value 0`. This means that you have in the table a row where the `AUTO_INCREMENT` index column contains the value 0. (It's possible to create a row where the `AUTO_INCREMENT` column is 0 by explicitly setting the column to 0 with an `UPDATE` statement) This isn't an error in itself, but could cause trouble if you decide to dump the table and restore it or do an `ALTER TABLE` on the table. In this case the `AUTO_INCREMENT` column will change value, according to the rules of `AUTO_INCREMENT` columns, which could cause problems like a duplicate key error. To get rid of the warning, just execute an `UPDATE` statement to set the column to some other value than 0.

### 4.4.5 `REPAIR TABLE` Syntax

    REPAIR TABLE tbl_name[,tbl_name...] [QUICK] [EXTENDED] [USE_FRM]

`REPAIR TABLE` works only on `MyISAM` tables and is the same as running `myisamchk -r table_name` on the table.

Normally you should never have to run this command, but if disaster strikes you are very likely to get back all your data from a MyISAM table with `REPAIR TABLE`. If your tables get corrupted a lot you should try to find the reason for this! See section A.4.1 What To Do If MySQL Keeps Crashing. See section 7.1.3 `MyISAM` Table Problems.

`REPAIR TABLE` repairs a possible corrupted table. The command returns a table with the following columns:

Column | Value
Table | Table name
Op | Always ``repair''
Msg_type | One of `status`, `error`, `info` or `warning`.
Msg_text | The message.

Note that you can get many rows of information for each repaired table. The last one row will be of `Msg_type status` and should normally be `OK`. If you don't get `OK`, you should try repairing the table with `myisamchk -o`, as `REPAIR TABLE` does not yet implement all the options of `myisamchk`. In the near future, we will make it more flexible.

If `QUICK` is given then MySQL will try to do a `REPAIR` of only the index tree.

If you use `EXTENDED` then MySQL will create the index row by row instead of creating one index at a time with sorting; this may be better than sorting on fixed-length keys if you have long `CHAR` keys that compress very well. This type of repair is like that done by `myisamchk --safe-recover`.

As of `MySQL` 4.0.2, there is a `USE_FRM` mode for `REPAIR`. Use it if the ``.MYI'` file is missing or if its header is corrupted. In this mode MySQL will recreate the table, using information from the ``.frm'` file. This kind of repair cannot be done with `myisamchk`.

### 4.4.6 Using `myisamchk` for Table Maintenance and Crash Recovery

Starting with MySQL Version 3.23.13, you can check MyISAM tables with the `CHECK TABLE` command. See section 4.4.4 `CHECK TABLE` Syntax. You can repair tables with the `REPAIR TABLE` command. See section 4.4.5 `REPAIR TABLE` Syntax.

To check/repair MyISAM tables (``.MYI'` and ``.MYD'`) you should use the `myisamchk` utility. To check/repair ISAM tables (``.ISM'` and ``.ISD'`) you should use the `isamchk` utility. See section 7 MySQL Table Types.

In the following text we will talk about `myisamchk`, but everything also applies to the old `isamchk`.

You can use the `myisamchk` utility to get information about your database tables, check and repair them, or optimise them. The following sections describe how to invoke `myisamchk` (including a description of its options), how to set up a table maintenance schedule, and how to use `myisamchk` to perform its various functions.

You can, in most cases, also use the command `OPTIMIZE TABLES` to optimise and repair tables, but this is not as fast or reliable (in case of real fatal errors) as `myisamchk`. On the other hand, `OPTIMIZE TABLE` is easier to use and you don't have to worry about flushing tables. See section 4.5.1 `OPTIMIZE TABLE` Syntax.

Even that the repair in `myisamchk` is quite secure, it's always a good idea to make a backup before doing a repair (or anything that could make a lot of changes to a table)

#### 4.4.6.1 `myisamchk` Invocation Syntax

`myisamchk` is invoked like this:

    shell> myisamchk [options] tbl_name

The `options` specify what you want `myisamchk` to do. They are described here. (You can also get a list of options by invoking `myisamchk --help`.) With no options, `myisamchk` simply checks your table. To get more information or to tell `myisamchk` to take corrective action, specify options as described here and in the following sections.

`tbl_name` is the database table you want to check/repair. If you run `myisamchk` somewhere other than in the database directory, you must specify the path to the file, because `myisamchk` has no idea where your database is located. Actually, `myisamchk` doesn't care whether the files you are working on are located in a database directory; you can copy the files that correspond to a database table into another location and perform recovery operations on them there.

You can name several tables on the `myisamchk` command-line if you wish. You can also specify a name as an index file name (with the ``.MYI'` suffix), which allows you to specify all tables in a directory by using the pattern ``*.MYI'`. For example, if you are in a database directory, you can check all the tables in the directory like this:

    shell> myisamchk *.MYI

If you are not in the database directory, you can check all the tables there by specifying the path to the directory:

    shell> myisamchk /path/to/database_dir/*.MYI

You can even check all tables in all databases by specifying a wildcard with the path to the MySQL data directory:

    shell> myisamchk /path/to/datadir/*/*.MYI

The recommended way to quickly check all tables is:

    myisamchk --silent --fast /path/to/datadir/*/*.MYI
    isamchk --silent /path/to/datadir/*/*.ISM

If you want to check all tables and repair all tables that are corrupted, you can use the following line:

    myisamchk --silent --force --fast --update-state -O key_buffer=64M \
              -O sort_buffer=64M -O read_buffer=1M -O write_buffer=1M \
              /path/to/datadir/*/*.MYI
    isamchk --silent --force -O key_buffer=64M -O sort_buffer=64M \
            -O read_buffer=1M -O write_buffer=1M /path/to/datadir/*/*.ISM

The above assumes that you have more than 64 M free.

Note that if you get an error like:

    myisamchk: warning: 1 clients is using or hasn't closed the table properly

This means that you are trying to check a table that has been updated by the another program (like the `mysqld` server) that hasn't yet closed the file or that has died without closing the file properly.

If you `mysqld` is running, you must force a sync/close of all tables with `FLUSH TABLES` and ensure that no one is using the tables while you are running `myisamchk`. In MySQL Version 3.23 the easiest way to avoid this problem is to use `CHECK TABLE` instead of `myisamchk` to check tables.

#### 4.4.6.2 General Options for `myisamchk`

`myisamchk` supports the following options.

`-# or --debug=debug_options`
  Output debug log. The `debug_options` string often is `'d:t:o,filename'`.
`-? or --help`
  Display a help message and exit.
`-O var=option, --set-variable var=option`
  Set the value of a variable. Please note that `--set-variable` is deprecated since MySQL 4.0, just use `--var=option` on its own. The possible variables and their default values for myisamchk can be examined with `myisamchk --help`:

  Variable | Value
  key_buffer_size | 523264
  read_buffer_size | 262136
  write_buffer_size | 262136
  sort_buffer_size | 2097144
  sort_key_blocks | 16
  decode_bits | 9

  `sort_buffer_size` is used when the keys are repaired by sorting keys, which is the normal case when you use `--recover`. `key_buffer_size` is used when you are checking the table with `--extended-check` or when the keys are repaired by inserting key row by row in to the table (like when doing normal inserts). Repairing through the key buffer is used in the following cases:
  - If you use `--safe-recover`.
  - If the temporary files needed to sort the keys would be more than twice as big as when creating the key file directly. This is often the case when you have big `CHAR`, `VARCHAR` or `TEXT` keys as the sort needs to store the whole keys during sorting. If you have lots of temporary space and you can force `myisamchk` to repair by sorting you can use the `--sort-recover` option.
  Reparing through the key buffer takes much less disk space than using sorting, but is also much slower. If you want a faster repair, set the above variables to about 1/4 of your available memory. You can set both variables to big values, as only one of the above buffers will be used at a time.
`-s or --silent`
  Silent mode. Write output only when errors occur. You can use `-s` twice (`-ss`) to make `myisamchk` very silent.
`-v or --verbose`
  Verbose mode. Print more information. This can be used with `-d` and `-e`. Use `-v` multiple times (`-vv`, `-vvv`) for more verbosity!
`-V or --version`
  Print the `myisamchk` version and exit.
`-w or, --wait`
  Instead of giving an error if the table is locked, wait until the table is unlocked before continuing. Note that if you are running `mysqld` on the table with `--skip-external-locking`, the table can only be locked by another `myisamchk` command.

#### 4.4.6.3 Check Options for `myisamchk`

`-c or --check`
  Check table for errors. This is the default operation if you are not giving `myisamchk` any options that override this.
`-e or --extend-check`
  Check the table very thoroughly (which is quite slow if you have many indexes). This option should only be used in extreme cases. Normally, `myisamchk` or `myisamchk --medium-check` should, in most cases, be able to find out if there are any errors in the table. If you are using `--extended-check` and have much memory, you should increase the value of `key_buffer_size` a lot!
`-F or --fast`
  Check only tables that haven't been closed properly.
`-C or --check-only-changed`
  Check only tables that have changed since the last check.
`-f or --force`
  Restart `myisamchk` with `-r` (repair) on the table, if `myisamchk` finds any errors in the table.
`-i or --information`
  Print informational statistics about the table that is checked.
`-m or --medium-check`
  Faster than extended-check, but only finds 99.99% of all errors. Should, however, be good enough for most cases.
`-U or --update-state`
  Store in the ``.MYI'` file when the table was checked and if the table crashed. This should be used to get full benefit of the `--check-only-changed` option, but you shouldn't use this option if the `mysqld` server is using the table and you are running `mysqld` with `--skip-external-locking`.
`-T or --read-only`
  Don't mark table as checked. This is useful if you use `myisamchk` to check a table that is in use by some other application that doesn't use locking (like `mysqld --skip-external-locking`).

#### 4.4.6.4 Repair Options for myisamchk

The following options are used if you start `myisamchk` with `-r` or `-o`:

`-D # or --data-file-length=#`
  Max length of datafile (when re-creating datafile when it's 'full').
`-e or --extend-check`
  Try to recover every possible row from the datafile. Normally this will also find a lot of garbage rows. Don't use this option if you are not totally desperate.
`-f or --force`
  Overwrite old temporary files (`table_name.TMD`) instead of aborting.
`-k # or keys-used=#`
  If you are using ISAM, tells the ISAM storage engine to update only the first `#` indexes. If you are using `MyISAM`, tells which keys to use, where each binary bit stands for one key (first key is bit 0). This can be used to get faster inserts! Deactivated indexes can be reactivated by using `myisamchk -r`. keys.
`-l or --no-symlinks`
  Do not follow symbolic links. Normally `myisamchk` repairs the table a symlink points at. This option doesn't exist in MySQL 4.0, as MySQL 4.0 will not remove symlinks during repair.
`-r or --recover`
  Can fix almost anything except unique keys that aren't unique (which is an extremely unlikely error with ISAM/MyISAM tables). If you want to recover a table, this is the option to try first. Only if myisamchk reports that the table can't be recovered by `-r`, you should then try `-o`. (Note that in the unlikely case that `-r` fails, the datafile is still intact.) If you have lots of memory, you should increase the size of `sort_buffer_size`!
`-o or --safe-recover`
  Uses an old recovery method (reads through all rows in order and updates all index trees based on the found rows); this is an order of magnitude slower than `-r`, but can handle a couple of very unlikely cases that `-r` cannot handle. This recovery method also uses much less disk space than `-r`. Normally one should always first repair with `-r`, and only if this fails use `-o`. If you have lots of memory, you should increase the size of `key_buffer_size`!
`-n or --sort-recover`
  Force `myisamchk` to use sorting to resolve the keys even if the temporary files should be very big.
`--character-sets-dir=...`
  Directory where character sets are stored.
`--set-character-set=name`
  Change the character set used by the index
`-t or --tmpdir=path`
  Path for storing temporary files. If this is not set, `myisamchk` will use the environment variable `TMPDIR` for this.
`-q or --quick`
  Faster repair by not modifying the datafile. One can give a second `-q` to force `myisamchk` to modify the original datafile in case of duplicate keys
`-u or --unpack`
  Unpack file packed with myisampack.

#### 4.4.6.5 Other Options for `myisamchk`

Other actions that `myisamchk` can do, besides repair and check tables:

`-a or --analyze`
  Analyse the distribution of keys. This improves join performance by enabling the join optimiser to better choose in which order it should join the tables and which keys it should use: `myisamchk --describe --verbose table_name'` or using `SHOW KEYS` in MySQL.
`-d or --description`
  Prints some information about table.
`-A or --set-auto-increment[=value]`
  Force `AUTO_INCREMENT` to start at this or higher value. If no value is given, then sets the next `AUTO_INCREMENT` value to the highest used value for the auto key + 1.
`-S or --sort-index`
  Sort the index tree blocks in high-low order. This will optimise seeks and will make table scanning by key faster.
`-R or --sort-records=#`
  Sorts records according to an index. This makes your data much more localised and may speed up ranged `SELECT` and `ORDER BY` operations on this index. (It may be very slow to do a sort the first time!) To find out a table's index numbers, use `SHOW INDEX`, which shows a table's indexes in the same order that `myisamchk` sees them. Indexes are numbered beginning with 1.

#### 4.4.6.6 `myisamchk` Memory Usage

Memory allocation is important when you run `myisamchk`. `myisamchk` uses no more memory than you specify with the `-O` options. If you are going to use `myisamchk` on very large files, you should first decide how much memory you want it to use. The default is to use only about 3M to fix things. By using larger values, you can get `myisamchk` to operate faster. For example, if you have more than 32M RAM, you could use options such as these (in addition to any other options you might specify):

    shell> myisamchk -O sort=16M -O key=16M -O read=1M -O write=1M ...

Using `-O sort=16M` should probably be enough for most cases.

Be aware that `myisamchk` uses temporary files in `TMPDIR`. If `TMPDIR` points to a memory filesystem, you may easily get out of memory errors. If this happens, set `TMPDIR` to point at some directory with more space and restart `myisamchk`.

When repairing, `myisamchk` will also need a lot of disk space:

- Double the size of the record file (the original one and a copy). This space is not needed if one does a repair with `--quick`, as in this case only the index file will be re-created. This space is needed on the same disk as the original record file!
- Space for the new index file that replaces the old one. The old index file is truncated at start, so one usually ignore this space. This space is needed on the same disk as the original index file!
- When using `--recover` or `--sort-recover` (but not when using `--safe-recover`), you will need space for a sort buffer for: `(largest_key + row_pointer_length)*number_of_rows * 2`. You can check the length of the keys and the row_pointer_length with `myisamchk -dv table`. This space is allocated on the temporary disk (specified by `TMPDIR` or `--tmpdir=#`).

If you have a problem with disk space during repair, you can try to use `--safe-recover` instead of `--recover`.

#### 4.4.6.7 Using `myisamchk` for Crash Recovery

If you run `mysqld` with `--skip-external-locking` (which is the default on some systems, like Linux), you can't reliably use `myisamchk` to check a table when `mysqld` is using the same table. If you can be sure that no one is accessing the tables through `mysqld` while you run `myisamchk`, you only have to do `mysqladmin flush-tables` before you start checking the tables. If you can't guarantee the above, then you must take down `mysqld` while you check the tables. If you run `myisamchk` while `mysqld` is updating the tables, you may get a warning that a table is corrupt even if it isn't.

If you are not using `--skip-external-locking`, you can use `myisamchk` to check tables at any time. While you do this, all clients that try to update the table will wait until `myisamchk` is ready before continuing.

If you use `myisamchk` to repair or optimise tables, you must always ensure that the `mysqld` server is not using the table (this also applies if you are using `--skip-external-locking`). If you don't take down `mysqld` you should at least do a `mysqladmin flush-tables` before you run `myisamchk`. Your tables may be corrupted if the server and `myisamchk` access the tables simultaneously.

This chapter describes how to check for and deal with data corruption in MySQL databases. If your tables get corrupted frequently you should try to find the reason for this! See section A.4.1 What To Do If MySQL Keeps Crashing.

The `MyISAM` table section contains reason for why a table could be corrupted. See section 7.1.3 `MyISAM` Table Problems.

When performing crash recovery, it is important to understand that each table `tbl_name` in a database corresponds to three files in the database directory:

File | Purpose
``tbl_name.frm'` | Table definition (form) file
``tbl_name.MYD'` | Datafile
``tbl_name.MYI'` | Index file

Each of these three file types is subject to corruption in various ways, but problems occur most often in datafiles and index files.

`myisamchk` works by creating a copy of the ``.MYD'` (data) file row by row. It ends the repair stage by removing the old ``.MYD'` file and renaming the new file to the original file name. If you use `--quick`, `myisamchk` does not create a temporary ``.MYD'` file, but instead assumes that the ``.MYD'` file is correct and only generates a new index file without touching the ``.MYD'` file. This is safe, because `myisamchk` automatically detects if the ``.MYD'` file is corrupt and aborts the repair in this case. You can also give two `--quick` options to `myisamchk`. In this case, `myisamchk` does not abort on some errors (like duplicate key) but instead tries to resolve them by modifying the ``.MYD'` file. Normally the use of two `--quick` options is useful only if you have too little free disk space to perform a normal repair. In this case you should at least make a backup before running `myisamchk`.

#### 4.4.6.8 How to Check Tables for Errors

To check a MyISAM table, use the following commands:

`myisamchk tbl_name`
  This finds 99.99% of all errors. What it can't find is corruption that involves only the datafile (which is very unusual). If you want to check a table, you should normally run `myisamchk` without options or with either the `-s` or `--silent` option.
`myisamchk -m tbl_name`
  This finds 99.999% of all errors. It checks first all index entries for errors and then it reads through all rows. It calculates a checksum for all keys in the rows and verifies that they checksum matches the checksum for the keys in the index tree.
`myisamchk -e tbl_name`
  This does a complete and thorough check of all data (`-e` means ``extended check''). It does a check-read of every key for each row to verify that they indeed point to the correct row. This may take a long time on a big table with many keys. `myisamchk` will normally stop after the first error it finds. If you want to obtain more information, you can add the `--verbose` (`-v`) option. This causes `myisamchk` to keep going, up through a maximum of 20 errors. In normal usage, a simple `myisamchk` (with no arguments other than the table name) is sufficient.
`myisamchk -e -i tbl_name`
  Like the previous command, but the `-i` option tells `myisamchk` to print some informational statistics, too.

#### 4.4.6.9 How to Repair Tables

In the following section we only talk about using `myisamchk` on `MyISAM` tables (extensions ``.MYI'` and ``.MYD'`). If you are using `ISAM` tables (extensions ``.ISM'` and ``.ISD'`), you should use `isamchk` instead.

Starting with MySQL Version 3.23.14, you can repair MyISAM tables with the `REPAIR TABLE` command. See section 4.4.5 `REPAIR TABLE` Syntax.

The symptoms of a corrupted table include queries that abort unexpectedly and observable errors such as these:

- ``tbl_name.frm'` is locked against change
- Can't find file ``tbl_name.MYI'` (Errcode: ###)
- Unexpected end of file
- Record file is crashed
- Got error ### from table handler To get more information about the error you can run `perror ###`. Here is the most common errors that indicates a problem with the table:

      shell> perror 126 127 132 134 135 136 141 144 145
      126 = Index file is crashed / Wrong file format
      127 = Record-file is crashed
      132 = Old database file
      134 = Record was already deleted (or record file crashed)
      135 = No more room in record file
      136 = No more room in index file
      141 = Duplicate unique key or constraint on write or update
      144 = Table is crashed and last repair failed
      145 = Table was marked as crashed and should be repaired

  Note that error 135, no more room in record file, is not an error that can be fixed by a simple repair. In this case you have to do:

      ALTER TABLE table MAX_ROWS=xxx AVG_ROW_LENGTH=yyy;

In the other cases, you must repair your tables. `myisamchk` can usually detect and fix most things that go wrong.

The repair process involves up to four stages, described here. Before you begin, you should `cd` to the database directory and check the permissions of the table files. Make sure they are readable by the Unix user that `mysqld` runs as (and to you, because you need to access the files you are checking). If it turns out you need to modify files, they must also be writable by you.

If you are using MySQL Version 3.23.16 and above, you can (and should) use the `CHECK` and `REPAIR` commands to check and repair `MyISAM` tables. See section 4.4.4 `CHECK TABLE` Syntax. See section 4.4.5 `REPAIR TABLE` Syntax.

The manual section about table maintenance includes the options to `isamchk`/`myisamchk`. See section 4.4.6 Using `myisamchk` for Table Maintenance and Crash Recovery.

The following section is for the cases where the above command fails or if you want to use the extended features that `isamchk`/`myisamchk` provides.

If you are going to repair a table from the command-line, you must first take down the `mysqld` server. Note that when you do `mysqladmin shutdown` on a remote server, the `mysqld` server will still be alive for a while after `mysqladmin` returns, until all queries are stopped and all keys have been flushed to disk.

Stage 1: Checking your tables

Run `myisamchk *.MYI` or `myisamchk -e *.MYI` if you have more time. Use the `-s` (silent) option to suppress unnecessary information.

If the `mysqld` server is done you should use the --update option to tell `myisamchk` to mark the table as 'checked'.

You have to repair only those tables for which `myisamchk` announces an error. For such tables, proceed to Stage 2.

If you get weird errors when checking (such as `out of memory` errors), or if `myisamchk` crashes, go to Stage 3.

Stage 2: Easy safe repair

Note: If you want repairing to go much faster, you should add: `-O sort_buffer=# -O key_buffer=#` (where # is about 1/4 of the available memory) to all `isamchk/myisamchk` commands.

First, try `myisamchk -r -q tbl_name` (`-r -q` means ``quick recovery mode''). This will attempt to repair the index file without touching the datafile. If the datafile contains everything that it should and the delete links point at the correct locations within the datafile, this should work, and the table is fixed. Start repairing the next table. Otherwise, use the following procedure:

1. Make a backup of the datafile before continuing.
2. Use `myisamchk -r tbl_name` (`-r` means ``recovery mode''). This will remove incorrect records and deleted records from the datafile and reconstruct the index file.
3. If the preceding step fails, use `myisamchk --safe-recover tbl_name`. Safe recovery mode uses an old recovery method that handles a few cases that regular recovery mode doesn't (but is slower).

If you get weird errors when repairing (such as `out of memory` errors), or if `myisamchk` crashes, go to Stage 3.

Stage 3: Difficult repair

You should only reach this stage if the first 16K block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it's necessary to create a new index file. Do so as follows:

1. Move the datafile to some safe place.
2. Use the table description file to create new (empty) data and index files:

       shell> mysql db_name
       mysql> SET AUTOCOMMIT=1;
       mysql> TRUNCATE TABLE table_name;
       mysql> quit

   If your SQL version doesn't have `TRUNCATE TABLE`, use `DELETE FROM table_name` instead.
3. Copy the old datafile back onto the newly created datafile. (Don't just move the old file back onto the new file; you want to retain a copy in case something goes wrong.)

Go back to Stage 2. `myisamchk -r -q` should work now. (This shouldn't be an endless loop.)

As of `MySQL` 4.0.2 you can also use `REPAIR ... USE_FRM` which performs the whole procedure automatically.

Stage 4: Very difficult repair

You should reach this stage only if the description file has also crashed. That should never happen, because the description file isn't changed after the table is created:

1. Restore the description file from a backup and go back to Stage 3. You can also restore the index file and go back to Stage 2. In the latter case, you should start with `myisamchk -r`.
2. If you don't have a backup but know exactly how the table was created, create a copy of the table in another database. Remove the new datafile, then move the description and index files from the other database to your crashed database. This gives you new description and index files, but leaves the datafile alone. Go back to Stage 2 and attempt to reconstruct the index file.

#### 4.4.6.10 Table Optimisation

To coalesce fragmented records and eliminate wasted space resulting from deleting or updating records, run `myisamchk` in recovery mode:

    shell> myisamchk -r tbl_name

You can optimise a table in the same way using the SQL `OPTIMIZE TABLE` statement. `OPTIMIZE TABLE` does a repair of the table and a key analysis, and also sorts the index tree to give faster key lookups. There is also no possibility of unwanted interaction between a utility and the server, because the server does all the work when you use `OPTIMIZE TABLE`. See section 4.5.1 `OPTIMIZE TABLE` Syntax.

`myisamchk` also has a number of other options you can use to improve the performance of a table:

- `-S`, `--sort-index`
- `-R index_num`, `--sort-records=index_num`
- `-a`, `--analyze`

For a full description of the option. See section 4.4.6.1 `myisamchk` Invocation Syntax.

### 4.4.7 Setting Up a Table Maintenance Regimen

Starting with MySQL Version 3.23.13, you can check MyISAM tables with the `CHECK TABLE` command. See section 4.4.4 `CHECK TABLE` Syntax. You can repair tables with the `REPAIR TABLE` command. See section 4.4.5 `REPAIR TABLE` Syntax.

It is a good idea to perform table checks on a regular basis rather than waiting for problems to occur. For maintenance purposes, you can use `myisamchk -s` to check tables. The `-s` option (short for `--silent`) causes `myisamchk` to run in silent mode, printing messages only when errors occur.

It's also a good idea to check tables when the server starts up. For example, whenever the machine has done a reboot in the middle of an update, you usually need to check all the tables that could have been affected. (This is an ``expected crashed table''.) You could add a test to `safe_mysqld` that runs `myisamchk` to check all tables that have been modified during the last 24 hours if there is an old ``.pid'` (process ID) file left after a reboot. (The ``.pid'` file is created by `mysqld` when it starts up and removed when it terminates normally. The presence of a ``.pid'` file at system startup time indicates that `mysqld` terminated abnormally.)

An even better test would be to check any table whose last-modified time is more recent than that of the ``.pid'` file.

You should also check your tables regularly during normal system operation. At MySQL AB, we run a `cron` job to check all our important tables once a week, using a line like this in a ``crontab'` file:

    35 0 * * 0 /path/to/myisamchk --fast --silent /path/to/datadir/*/*.MYI

This prints out information about crashed tables so we can examine and repair them when needed.

As we haven't had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for a couple of years now (this is really true), once a week is more than enough for us.

We recommend that to start with, you execute `myisamchk -s` each night on all tables that have been updated during the last 24 hours, until you come to trust MySQL as much as we do.

Normally you don't need to maintain MySQL tables that much. If you are changing tables with dynamic size rows (tables with `VARCHAR`, `BLOB` or `TEXT` columns) or have tables with many deleted rows you may want to from time to time (once a month?) defragment/reclaim space from the tables.

You can do this by using `OPTIMIZE TABLE` on the tables in question or if you can take the `mysqld` server down for a while do:

    isamchk -r --silent --sort-index -O sort_buffer_size=16M */*.ISM
    myisamchk -r --silent --sort-index  -O sort_buffer_size=16M */*.MYI

### 4.4.8 Getting Information About a Table

To get a description of a table or statistics about it, use the commands shown here. We explain some of the information in more detail later:

- myisamchk -d tbl_name Runs `myisamchk` in ``describe mode'' to produce a description of your table. If you start the MySQL server using the `--skip-external-locking` option, `myisamchk` may report an error for a table that is updated while it runs. However, because `myisamchk` doesn't change the table in describe mode, there isn't any risk of destroying data.
- myisamchk -d -v tbl_name To produce more information about what `myisamchk` is doing, add `-v` to tell it to run in verbose mode.
- myisamchk -eis tbl_name Shows only the most important information from a table. It is slow because it must read the whole table.
- myisamchk -eiv tbl_name This is like `-eis`, but tells you what is being done.

Example of `myisamchk -d` output:

    MyISAM file:     company.MYI
    Record format:   Fixed length
    Data records:    1403698  Deleted blocks:         0
    Recordlength:    226

    table description:
    Key Start Len Index   Type
    1   2     8   unique  double
    2   15    10  multip. text packed stripped
    3   219   8   multip. double
    4   63    10  multip. text packed stripped
    5   167   2   multip. unsigned short
    6   177   4   multip. unsigned long
    7   155   4   multip. text
    8   138   4   multip. unsigned long
    9   177   4   multip. unsigned long
        193   1           text

Example of `myisamchk -d -v` output:

    MyISAM file:         company
    Record format:       Fixed length
    File-version:        1
    Creation time:       1999-10-30 12:12:51
    Recover time:        1999-10-31 19:13:01
    Status:              checked
    Data records:           1403698  Deleted blocks:              0
    Datafile parts:         1403698  Deleted data:                0
    Datafilepointer (bytes):      3  Keyfile pointer (bytes):     3
    Max datafile length: 3791650815  Max keyfile length: 4294967294
    Recordlength:               226

    table description:
    Key Start Len Index   Type                  Rec/key     Root Blocksize
    1   2     8   unique  double                      1 15845376      1024
    2   15    10  multip. text packed stripped        2 25062400      1024
    3   219   8   multip. double                     73 40907776      1024
    4   63    10  multip. text packed stripped        5 48097280      1024
    5   167   2   multip. unsigned short           4840 55200768      1024
    6   177   4   multip. unsigned long            1346 65145856      1024
    7   155   4   multip. text                     4995 75090944      1024
    8   138   4   multip. unsigned long              87 85036032      1024
    9   177   4   multip. unsigned long             178 96481280      1024
        193   1           text

Example of `myisamchk -eis` output:

    Checking MyISAM file: company
    Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
    Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
    Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
    Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
    Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
    Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
    Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
    Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
    Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
    Total:    Keyblocks used:  98%  Packed:   17%

    Records:          1403698    M.recordlength:     226
    Packed:             0%
    Recordspace used:     100%   Empty space:          0%
    Blocks/Record:   1.00
    Record blocks:    1403698    Delete blocks:        0
    Recorddata:     317235748    Deleted data:         0
    Lost space:             0    Linkdata:             0

    User time 1626.51, System time 232.36
    Maximum resident set size 0, Integral resident set size 0
    Non physical pagefaults 0, Physical pagefaults 627, Swaps 0
    Blocks in 0 out 0, Messages in 0 out 0, Signals 0
    Voluntary context switches 639, Involuntary context switches 28966

Example of `myisamchk -eiv` output:

    Checking MyISAM file: company
    Data records: 1403698   Deleted blocks:       0
    - check file-size
    - check delete-chain
    block_size 1024:
    index  1:
    index  2:
    index  3:
    index  4:
    index  5:
    index  6:
    index  7:
    index  8:
    index  9:
    No recordlinks
    - check index reference
    - check data record references index: 1
    Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
    - check data record references index: 2
    Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
    - check data record references index: 3
    Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
    - check data record references index: 4
    Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
    - check data record references index: 5
    Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
    - check data record references index: 6
    Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
    - check data record references index: 7
    Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
    - check data record references index: 8
    Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
    - check data record references index: 9
    Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
    Total:    Keyblocks used:   9%  Packed:   17%

    - check records and index references
    [LOTS OF ROW NUMBERS DELETED]

    Records:          1403698    M.recordlength:     226   Packed:             0%
    Recordspace used:     100%   Empty space:          0%  Blocks/Record:   1.00
    Record blocks:    1403698    Delete blocks:        0
    Recorddata:     317235748    Deleted data:         0
    Lost space:             0    Linkdata:             0

    User time 1639.63, System time 251.61
    Maximum resident set size 0, Integral resident set size 0
    Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0
    Blocks in 4 out 0, Messages in 0 out 0, Signals 0
    Voluntary context switches 10604, Involuntary context switches 122798

Here are the sizes of the data and index files for the table used in the preceding examples:

    -rw-rw-r--   1 monty    tcx     317235748 Jan 12 17:30 company.MYD
    -rw-rw-r--   1 davida   tcx      96482304 Jan 12 18:35 company.MYM

Explanations for the types of information `myisamchk` produces are given here. The ``keyfile'' is the index file. ``Record'' and ``row'' are synonymous:

- ISAM file Name of the ISAM (index) file.
- Isam-version Version of ISAM format. Currently always 2.
- Creation time When the datafile was created.
- Recover time When the index/datafile was last reconstructed.
- Data records How many records are in the table.
- Deleted blocks How many deleted blocks still have reserved space. You can optimise your table to minimise this space. See section 4.4.6.10 Table Optimisation.
- Data file: Parts For dynamic record format, this indicates how many data blocks there are. For an optimised table without fragmented records, this is the same as `Data records`.
- Deleted data How many bytes of non-reclaimed deleted data there are. You can optimise your table to minimise this space. See section 4.4.6.10 Table Optimisation.
- Data file pointer The size of the datafile pointer, in bytes. It is usually 2, 3, 4, or 5 bytes. Most tables manage with 2 bytes, but this cannot be controlled from MySQL yet. For fixed tables, this is a record address. For dynamic tables, this is a byte address.
- Keyfile pointer The size of the index file pointer, in bytes. It is usually 1, 2, or 3 bytes. Most tables manage with 2 bytes, but this is calculated automatically by MySQL. It is always a block address.
- Max datafile length How long the table's datafile (``.MYD'` file) can become, in bytes.
- Max keyfile length How long the table's key file (``.MYI'` file) can become, in bytes.
- Recordlength How much space each record takes, in bytes.
- Record format The format used to store table rows. The examples shown above use `Fixed length`. Other possible values are `Compressed` and `Packed`.
- table description A list of all keys in the table. For each key, some low-level information is presented:
  - Key This key's number.
  - Start Where in the record this index part starts.
  - Len How long this index part is. For packed numbers, this should always be the full length of the column. For strings, it may be shorter than the full length of the indexed column, because you can index a prefix of a string column.
  - Index `unique` or `multip.` (multiple). Indicates whether one value can exist multiple times in this index.
  - Type What data-type this index part has. This is an ISAM data-type with the options `packed`, `stripped` or `empty`.
  - Root Address of the root index block.
  - Blocksize The size of each index block. By default this is 1024, but the value may be changed at compile time.
  - Rec/key This is a statistical value used by the optimiser. It tells how many records there are per value for this key. A unique key always has a value of 1. This may be updated after a table is loaded (or greatly changed) with `myisamchk -a`. If this is not updated at all, a default value of 30 is given.
- In the first example above, the 9th key is a multi-part key with two parts.
- Keyblocks used What percentage of the keyblocks are used. Because the table used in the examples had just been reorganised with `myisamchk`, the values are very high (very near the theoretical maximum).
- Packed MySQL tries to pack keys with a common suffix. This can only be used for `CHAR`/`VARCHAR`/`DECIMAL` keys. For long strings like names, this can significantly reduce the space used. In the third example above, the 4th key is 10 characters long and a 60% reduction in space is achieved.
- Max levels How deep the B-tree for this key is. Large tables with long keys get high values.
- Records How many rows are in the table.
- M.recordlength The average record length. For tables with fixed-length records, this is the exact record length.
- Packed MySQL strips spaces from the end of strings. The `Packed` value indicates the percentage of savings achieved by doing this.
- Recordspace used What percentage of the datafile is used.
- Empty space What percentage of the datafile is unused.
- Blocks/Record Average number of blocks per record (that is, how many links a fragmented record is composed of). This is always 1.0 for fixed-format tables. This value should stay as close to 1.0 as possible. If it gets too big, you can reorganise the table with `myisamchk`. See section 4.4.6.10 Table Optimisation.
- Recordblocks How many blocks (links) are used. For fixed format, this is the same as the number of records.
- Deleteblocks How many blocks (links) are deleted.
- Recorddata How many bytes in the datafile are used.
- Deleted data How many bytes in the datafile are deleted (unused).
- Lost space If a record is updated to a shorter length, some space is lost. This is the sum of all such losses, in bytes.
- Linkdata When the dynamic table format is used, record fragments are linked with pointers (4 to 7 bytes each). `Linkdata` is the sum of the amount of storage used by all such pointers.

If a table has been compressed with `myisampack`, `myisamchk -d` prints additional information about each table column. See section 4.7.4 `myisampack`, The MySQL Compressed Read-only Table Generator, for an example of this information and a description of what it means.

## 4.5 Database Administration Language Reference

### 4.5.1 `OPTIMIZE TABLE` Syntax

    OPTIMIZE TABLE tbl_name[,tbl_name]...

`OPTIMIZE TABLE` should be used if you have deleted a large part of a table or if you have made many changes to a table with variable-length rows (tables that have `VARCHAR`, `BLOB`, or `TEXT` columns). Deleted records are maintained in a linked list and subsequent `INSERT` operations reuse old record positions. You can use `OPTIMIZE TABLE` to reclaim the unused space and to defragment the datafile.

For the moment, `OPTIMIZE TABLE` works only on `MyISAM` and `BDB` tables. For `BDB` tables, `OPTIMIZE TABLE` is currently mapped to `ANALYZE TABLE`. See section 4.5.2 `ANALYZE TABLE` Syntax.

You can get `OPTIMIZE TABLE` to work on other table types by starting `mysqld` with `--skip-new` or `--safe-mode`, but in this case `OPTIMIZE TABLE` is just mapped to `ALTER TABLE`.

`OPTIMIZE TABLE` works the following way:

- If the table has deleted or split rows, repair the table.
- If the index pages are not sorted, sort them.
- If the statistics are not up to date (and the repair couldn't be done by sorting the index), update them.

`OPTIMIZE TABLE` for a `MyISAM` table is equivalent to running `myisamchk --quick --check-only-changed --sort-index --analyze` on the table.

Note that the table is locked during the time `OPTIMIZE TABLE` is running!

### 4.5.2 `ANALYZE TABLE` Syntax

    ANALYZE TABLE tbl_name[,tbl_name...]

Analyse and store the key distribution for the table. During the analysis, the table is locked with a read lock. This works on `MyISAM` and `BDB` tables.

This is equivalent to running `myisamchk -a` on the table.

MySQL uses the stored key distribution to decide in which order tables should be joined when one does a join on something else than a constant.

The command returns a table with the following columns:

Column | Value
Table | Table name
Op | Always ``analyze''
Msg_type | One of `status`, `error`, `info` or `warning`.
Msg_text | The message.

You can check the stored key distribution with the `SHOW INDEX` command. See section 4.5.6.1 Retrieving information about Database, Tables, Columns, and Indexes.

If the table hasn't changed since the last `ANALYZE TABLE` command, the table will not be analysed again.

### 4.5.3 `FLUSH` Syntax

    FLUSH flush_option [,flush_option] ...

You should use the `FLUSH` command if you want to clear some of the internal caches MySQL uses. To execute `FLUSH`, you must have the `RELOAD` privilege.

`flush_option` can be any of the following:

Option | Description
`HOSTS` | Empties the host cache tables. You should flush the host tables if some of your hosts change IP number or if you get the error message `Host ... is blocked`. When more than `max_connect_errors` errors occur in a row for a given host while connection to the MySQL server, MySQL assumes something is wrong and blocks the host from further connection requests. Flushing the host tables allows the host to attempt to connect again. See section A.2.4 `Host '...' is blocked` Error. You can start `mysqld` with `-O max_connect_errors=999999999` to avoid this error message.
`DES_KEY_FILE` | Reloads the DES keys from the file that was specified with the `--des-key-file` option at server startup time.
`LOGS` | Closes and reopens all log files. If you have specified the update log file or a binary log file without an extension, the extension number of the log file will be incremented by one relative to the previous file. If you have used an extension in the file name, MySQL will close and reopen the update log file. See section 4.9.3 The Update Log. This is the same thing as sending the `SIGHUP` signal to the `mysqld` server.
`PRIVILEGES` | Reloads the privileges from the grant tables in the `mysql` database.
`QUERY CACHE` | Defragment the query cache to better utilise its memory. This command will not remove any queries from the cache, unlike `RESET QUERY CACHE`.
`TABLES` | Closes all open tables and force all tables in use to be closed. This also flushes the query cache.
`[TABLE | TABLES] tbl_name [,tbl_name...]` | Flushes only the given tables.
`TABLES WITH READ LOCK` | Closes all open tables and locks all tables for all databases with a read lock until you execute `UNLOCK TABLES`. This is very convenient way to get backups if you have a filesystem, like Veritas, that can take snapshots in time.
`STATUS` | Resets most status variables to zero. This is something one should only use when debugging a query.
`USER_RESOURCES` | Resets all user resources to zero. This will enable blocked users to login again. See section 4.3.6 Limiting user resources.

You can also access each of the commands shown above with the `mysqladmin` utility, using the `flush-hosts`, `flush-logs`, `reload`, or `flush-tables` commands.

Take also a look at the `RESET` command used with replication. See section 4.5.4 `RESET` Syntax.

### 4.5.4 `RESET` Syntax

    RESET reset_option [,reset_option] ...

The `RESET` command is used to clear things. It also acts as an stronger version of the `FLUSH` command. See section 4.5.3 `FLUSH` Syntax.

To execute `RESET`, you must have the `RELOAD` privilege.

Option | Description
`MASTER` | Deletes all binary logs listed in the index file, resetting the binlog index file to be empty. In pre-3.23.26 versions, `FLUSH MASTER` (Master)
`SLAVE` | Makes the slave forget its replication position in the master logs. In pre 3.23.26 versions the command was called `FLUSH SLAVE`(Slave)
`QUERY CACHE` | Removes all query results from the query cache.

### 4.5.5 `KILL` Syntax

    KILL thread_id

Each connection to `mysqld` runs in a separate thread. You can see which threads are running with the `SHOW PROCESSLIST` command and kill a thread with the `KILL thread_id` command.

If you have the `PROCESS` privilege, you can see all threads. If you have the `SUPER` privilege, you can kill all threads. Otherwise, you can only see and kill your own threads.

You can also use the `mysqladmin processlist` and `mysqladmin kill` commands to examine and kill threads.

When you do a `KILL`, a thread-specific `kill flag` is set for the thread.

In most cases it may take some time for the thread to die as the kill flag is only checked at specific intervals.

- In `SELECT`, `ORDER BY` and `GROUP BY` loops, the flag is checked after reading a block of rows. If the kill flag is set the statement is aborted
- When doing an `ALTER TABLE` the kill flag is checked before each block of rows are read from the original table. If the kill flag was set the command is aborted and the temporary table is deleted.
- When doing an `UPDATE TABLE` and `DELETE TABLE`, the kill flag is checked after each block read and after each updated or delete row. If the kill flag is set the statement is aborted. Note that if you are not using transactions, the changes will not be rolled back!
- `GET_LOCK()` will abort with `NULL`.
- An `INSERT DELAYED` thread will quickly flush all rows it has in memory and die.
- If the thread is in the table lock handler (state: `Locked`), the table lock will be quickly aborted.
- If the thread is waiting for free disk space in a `write` call, the write is aborted with an disk full error message.

### 4.5.6 `SHOW` Syntax

       SHOW DATABASES [LIKE wild]
    or SHOW [OPEN] TABLES [FROM db_name] [LIKE wild]
    or SHOW [FULL] COLUMNS FROM tbl_name [FROM db_name] [LIKE wild]
    or SHOW INDEX FROM tbl_name [FROM db_name]
    or SHOW TABLE STATUS [FROM db_name] [LIKE wild]
    or SHOW STATUS [LIKE wild]
    or SHOW VARIABLES [LIKE wild]
    or SHOW LOGS
    or SHOW [FULL] PROCESSLIST
    or SHOW GRANTS FOR user
    or SHOW CREATE TABLE table_name
    or SHOW MASTER STATUS
    or SHOW MASTER LOGS
    or SHOW SLAVE STATUS

`SHOW` provides information about databases, tables, columns, or status information about the server. If the `LIKE wild` part is used, the `wild` string can be a string that uses the SQL ``%'` and ``_'` wildcard characters.

#### 4.5.6.1 Retrieving information about Database, Tables, Columns, and Indexes

You can use `db_name.tbl_name` as an alternative to the `tbl_name FROM db_name` syntax. These two statements are equivalent:

    mysql> SHOW INDEX FROM mytable FROM mydb;
    mysql> SHOW INDEX FROM mydb.mytable;

`SHOW DATABASES` lists the databases on the MySQL server host. You can also get this list using the `mysqlshow` command line tool. In version 4.0.2 you will only see those databases for which you have some kind of privilege, if you don't have the global `SHOW DATABASES` privilege.

`SHOW TABLES` lists the tables in a given database. You can also get this list using the `mysqlshow db_name` command.

Note: if a user doesn't have any privileges for a table, the table will not show up in the output from `SHOW TABLES` or `mysqlshow db_name`.

`SHOW OPEN TABLES` lists the tables that are currently open in the table cache. See section 5.4.7 How MySQL Opens and Closes Tables. The `Comment` field tells how many times the table is `cached` and `in_use`.

`SHOW COLUMNS` lists the columns in a given table. If you specify the `FULL` option, you will also get the privileges you have for each column. If the column types are different from what you expect them to be based on a `CREATE TABLE` statement, note that MySQL sometimes changes column types. See section 6.5.3.1 Silent Column Specification Changes.

The `DESCRIBE` statement provides information similar to `SHOW COLUMNS`. See section 6.6.2 `DESCRIBE` Syntax (Get Information About Columns).

`SHOW FIELDS` is a synonym for `SHOW COLUMNS`, and `SHOW KEYS` is a synonym for `SHOW INDEX`. You can also list a table's columns or indexes with `mysqlshow db_name tbl_name` or `mysqlshow -k db_name tbl_name`.

`SHOW INDEX` returns the index information in a format that closely resembles the `SQLStatistics` call in ODBC. The following columns are returned:

Column | Meaning
`Table` | Name of the table.
`Non_unique` | 0 if the index can't contain duplicates.
`Key_name` | Name of the index.
`Seq_in_index` | Column sequence number in index, starting with 1.
`Column_name` | Column name.
`Collation` | How the column is sorted in the index. In MySQL, this can have values ``A'` (Ascending) or `NULL` (Not sorted).
`Cardinality` | Number of unique values in the index. This is updated by running `isamchk -a`.
`Sub_part` | Number of indexed characters if the column is only partly indexed. `NULL` if the entire key is indexed.
`Null` | Contains 'YES' if the column may contain `NULL`.
`Index_type` | Index method used.
`Comment` | Various remarks. For now, it tells in MySQL < 4.0.2 whether index is `FULLTEXT` or not.

Note that as the `Cardinality` is counted based on statistics stored as integers, it's not necessarily accurate for small tables.

The `Null` and `Index_type` columns were added in MySQL 4.0.2.

#### 4.5.6.2 `SHOW TABLE STATUS`

    SHOW TABLE STATUS [FROM db_name] [LIKE wild]

`SHOW TABLE STATUS` (new in Version 3.23) works likes `SHOW STATUS`, but provides a lot of information about each table. You can also get this list using the `mysqlshow --status db_name` command. The following columns are returned:

Column | Meaning
`Name` | Name of the table.
`Type` | Type of table. See section 7 MySQL Table Types.
`Row_format` | The row storage format (Fixed, Dynamic, or Compressed).
`Rows` | Number of rows.
`Avg_row_length` | Average row length.
`Data_length` | Length of the datafile.
`Max_data_length` | Max length of the datafile.
`Index_length` | Length of the index file.
`Data_free` | Number of allocated but not used bytes.
`Auto_increment` | Next autoincrement value.
`Create_time` | When the table was created.
`Update_time` | When the datafile was last updated.
`Check_time` | When the table was last checked.
`Create_options` | Extra options used with `CREATE TABLE`.
`Comment` | The comment used when creating the table (or some information why MySQL couldn't access the table information).

`InnoDB` tables will report the free space in the tablespace in the table comment.

#### 4.5.6.3 `SHOW STATUS`

`SHOW STATUS` provides server status information (like `mysqladmin extended-status`). The output resembles that shown here, though the format and numbers probably differ:

    +--------------------------+------------+
    | Variable_name            | Value      |
    +--------------------------+------------+
    | Aborted_clients          | 0          |
    | Aborted_connects         | 0          |
    | Bytes_received           | 155372598  |
    | Bytes_sent               | 1176560426 |
    | Connections              | 30023      |
    | Created_tmp_disk_tables  | 0          |
    | Created_tmp_tables       | 8340       |
    | Created_tmp_files        | 60         |
    | Delayed_insert_threads   | 0          |
    | Delayed_writes           | 0          |
    | Delayed_errors           | 0          |
    | Flush_commands           | 1          |
    | Handler_delete           | 462604     |
    | Handler_read_first       | 105881     |
    | Handler_read_key         | 27820558   |
    | Handler_read_next        | 390681754  |
    | Handler_read_prev        | 6022500    |
    | Handler_read_rnd         | 30546748   |
    | Handler_read_rnd_next    | 246216530  |
    | Handler_update           | 16945404   |
    | Handler_write            | 60356676   |
    | Key_blocks_used          | 14955      |
    | Key_read_requests        | 96854827   |
    | Key_reads                | 162040     |
    | Key_write_requests       | 7589728    |
    | Key_writes               | 3813196    |
    | Max_used_connections     | 0          |
    | Not_flushed_key_blocks   | 0          |
    | Not_flushed_delayed_rows | 0          |
    | Open_tables              | 1          |
    | Open_files               | 2          |
    | Open_streams             | 0          |
    | Opened_tables            | 44600      |
    | Questions                | 2026873    |
    | Select_full_join         | 0          |
    | Select_full_range_join   | 0          |
    | Select_range             | 99646      |
    | Select_range_check       | 0          |
    | Select_scan              | 30802      |
    | Slave_running            | OFF        |
    | Slave_open_temp_tables   | 0          |
    | Slow_launch_threads      | 0          |
    | Slow_queries             | 0          |
    | Sort_merge_passes        | 30         |
    | Sort_range               | 500        |
    | Sort_rows                | 30296250   |
    | Sort_scan                | 4650       |
    | Table_locks_immediate    | 1920382    |
    | Table_locks_waited       | 0          |
    | Threads_cached           | 0          |
    | Threads_created          | 30022      |
    | Threads_connected        | 1          |
    | Threads_running          | 1          |
    | Uptime                   | 80380      |
    +--------------------------+------------+

The status variables listed above have the following meaning:

Variable | Meaning
`Aborted_clients` | Number of connections aborted because the client died without closing the connection properly. See section A.2.9 Communication Errors / Aborted Connection.
`Aborted_connects` | Number of tries to connect to the MySQL server that failed. See section A.2.9 Communication Errors / Aborted Connection.
`Bytes_received` | Number of bytes received from all clients.
`Bytes_sent` | Number of bytes sent to all clients.
`Com_xxx` | Number of times each xxx command has been executed.
`Connections` | Number of connection attempts to the MySQL server.
`Created_tmp_disk_tables` | Number of implicit temporary tables on disk created while executing statements.
`Created_tmp_tables` | Number of implicit temporary tables in memory created while executing statements.
`Created_tmp_files` | How many temporary files `mysqld` has created.
`Delayed_insert_threads` | Number of delayed insert handler threads in use.
`Delayed_writes` | Number of rows written with `INSERT DELAYED`.
`Delayed_errors` | Number of rows written with `INSERT DELAYED` for which some error occurred (probably `duplicate key`).
`Flush_commands` | Number of executed `FLUSH` commands.
`Handler_commit` | Number of internal `COMMIT` commands.
`Handler_delete` | Number of times a row was deleted from a table.
`Handler_read_first` | Number of times the first entry was read from an index. If this is high, it suggests that the server is doing a lot of full index scans, for example, `SELECT col1 FROM foo`, assuming that col1 is indexed.
`Handler_read_key` | Number of requests to read a row based on a key. If this is high, it is a good indication that your queries and tables are properly indexed.
`Handler_read_next` | Number of requests to read next row in key order. This will be incremented if you are querying an index column with a range constraint. This also will be incremented if you are doing an index scan.
`Handler_read_prev` | Number of requests to read previous row in key order. This is mainly used to optimise `ORDER BY ... DESC`.
`Handler_read_rnd` | Number of requests to read a row based on a fixed position. This will be high if you are doing a lot of queries that require sorting of the result.
`Handler_read_rnd_next` | Number of requests to read the next row in the datafile. This will be high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your queries are not written to take advantage of the indexes you have.
`Handler_rollback` | Number of internal `ROLLBACK` commands.
`Handler_update` | Number of requests to update a row in a table.
`Handler_write` | Number of requests to insert a row in a table.
`Key_blocks_used` | The number of used blocks in the key cache.
`Key_read_requests` | The number of requests to read a key block from the cache.
`Key_reads` | The number of physical reads of a key block from disk.
`Key_write_requests` | The number of requests to write a key block to the cache.
`Key_writes` | The number of physical writes of a key block to disk.
`Max_used_connections` | The maximum number of connections in use simultaneously.
`Not_flushed_key_blocks` | Keys blocks in the key cache that has changed but hasn't yet been flushed to disk.
`Not_flushed_delayed_rows` | Number of rows waiting to be written in `INSERT DELAY` queues.
`Open_tables` | Number of tables that are open.
`Open_files` | Number of files that are open.
`Open_streams` | Number of streams that are open (used mainly for logging).
`Opened_tables` | Number of tables that have been opened.
`Rpl_status` | Status of failsafe replication. (Not yet in use).
`Select_full_join` | Number of joins without keys (If this is 0, you should carefully check the index of your tables).
`Select_full_range_join` | Number of joins where we used a range search on reference table.
`Select_range` | Number of joins where we used ranges on the first table. (It's normally not critical even if this is big.)
`Select_scan` | Number of joins where we did a full scan of the first table.
`Select_range_check` | Number of joins without keys where we check for key usage after each row (If this is 0, you should carefully check the index of your tables).
`Questions` | Number of queries sent to the server.
`Slave_open_temp_tables` | Number of temporary tables currently open by the slave thread
`Slave_running` | Is `ON` if this is a slave that is connected to a master.
`Slow_launch_threads` | Number of threads that have taken more than `slow_launch_time` to create.
`Slow_queries` | Number of queries that have taken more than `long_query_time`. See section 4.9.5 The Slow Query Log.
`Sort_merge_passes` | Number of merges passes the sort algoritm have had to do. If this value is large you should consider increasing `sort_buffer`.
`Sort_range` | Number of sorts that where done with ranges.
`Sort_rows` | Number of sorted rows.
`Sort_scan` | Number of sorts that where done by scanning the table.
`ssl_xxx` | Variables used by SSL; Not yet implemented.
`Table_locks_immediate` | Number of times a table lock was acquired immediately. Available after 3.23.33.
`Table_locks_waited` | Number of times a table lock could not be acquired immediately and a wait was needed. If this is high, and you have performance problems, you should first optimise your queries, and then either split your table(s) or use replication. Available after 3.23.33.
`Threads_cached` | Number of threads in the thread cache.
`Threads_connected` | Number of currently open connections.
`Threads_created` | Number of threads created to handle connections.
`Threads_running` | Number of threads that are not sleeping.
`Uptime` | How many seconds the server has been up.

Some comments about the above:

- If `Opened_tables` is big, then your `table_cache` variable is probably too small.
- If `Key_reads` is big, then your `key_buffer_size` variable is probably too small. The cache hit rate can be calculated with `Key_reads`/`Key_read_requests`.
- If `Handler_read_rnd` is big, then you probably have a lot of queries that require MySQL to scan whole tables or you have joins that don't use keys properly.
- If `Threads_created` is big, you may want to increase the `thread_cache_size` variable. The cache hit rate can be calculated with `Threads_created`/`Connections`.
- If `Created_tmp_disk_tables` is big, you may want to increase the `tmp_table_size` variable to get the temporary tables memory-based instead of disk based.

#### 4.5.6.4 `SHOW VARIABLES`

    SHOW [GLOBAL | SESSION] VARIABLES [LIKE wild]

`SHOW VARIABLES` shows the values of some MySQL system variables. You can also get this information using the `mysqladmin variables` command. If the default values are unsuitable, you can set most of these variables using command-line options when `mysqld` starts up. See section 4.1.1 `mysqld` Command-line Options.

The options `GLOBAL` and `SESSION` are new in MySQL 4.0.3. With `GLOBAL` you will get the variables that will be used for new connections to MySQL. With `SESSION` you will get the values that are in effect for the current connection. If you are not using either option, `SESSION` is used.

You can change most options with the `SET` command. See section 5.5.6 `SET` Syntax.

The output resembles that shown here, though the format and numbers may differ somewhat:

    +---------------------------------+------------------------------+
    | Variable_name                   | Value                        |
    +---------------------------------+------------------------------|
    | back_log                        | 50                           |
    | basedir                         | /usr/local/mysql             |
    | bdb_cache_size                  | 8388572                      |
    | bdb_log_buffer_size             | 32768                        |
    | bdb_home                        | /usr/local/mysql             |
    | bdb_max_lock                    | 10000                        |
    | bdb_logdir                      |                              |
    | bdb_shared_data                 | OFF                          |
    | bdb_tmpdir                      | /tmp/                        |
    | bdb_version                     | Sleepycat Software: ...	 |
    | binlog_cache_size               | 32768                        |
    | bulk_insert_buffer_size         | 8388608                      |
    | character_set                   | latin1                       |
    | character_sets                  | latin1 big5 czech euc_kr	 |
    | concurrent_insert               | ON                           |
    | connect_timeout                 | 5                            |
    | convert_character_set           |                              |
    | datadir                         | /usr/local/mysql/data/       |
    | delay_key_write                 | ON                           |
    | delayed_insert_limit            | 100                          |
    | delayed_insert_timeout          | 300                          |
    | delayed_queue_size              | 1000                         |
    | flush                           | OFF                          |
    | flush_time                      | 0                            |
    | ft_min_word_len                 | 4                            |
    | ft_max_word_len                 | 254                          |
    | ft_max_word_len_for_sort        | 20                           |
    | ft_boolean_syntax               | + -><()~*:""&|               |
    | have_bdb                        | YES                          |
    | have_innodb                     | YES                          |
    | have_isam                       | YES                          |
    | have_raid                       | NO                           |
    | have_symlink                    | DISABLED                     |
    | have_openssl                    | YES                          |
    | have_query_cache                | YES                          |
    | init_file                       |                              |
    | innodb_additional_mem_pool_size | 1048576                      |
    | innodb_buffer_pool_size         | 8388608                      |
    | innodb_data_file_path           | ibdata1:10M:autoextend       |
    | innodb_data_home_dir            |                              |
    | innodb_file_io_threads          | 4                            |
    | innodb_force_recovery           | 0                            |
    | innodb_thread_concurrency       | 8                            |
    | innodb_flush_log_at_trx_commit  | 0                            |
    | innodb_fast_shutdown            | ON                           |
    | innodb_flush_method             |                              |
    | innodb_lock_wait_timeout        | 50                           |
    | innodb_log_arch_dir             |                              |
    | innodb_log_archive              | OFF                          |
    | innodb_log_buffer_size          | 1048576                      |
    | innodb_log_file_size            | 5242880                      |
    | innodb_log_files_in_group       | 2                            |
    | innodb_log_group_home_dir       | ./                           |
    | innodb_mirrored_log_groups      | 1                            |
    | interactive_timeout             | 28800                        |
    | join_buffer_size                | 131072                       |
    | key_buffer_size                 | 16773120                     |
    | language                        | /usr/local/mysql/share/...   |
    | large_files_support             | ON                           |
    | local_infile                    | ON                           |
    | locked_in_memory                | OFF                          |
    | log                             | OFF                          |
    | log_update                      | OFF                          |
    | log_bin                         | OFF                          |
    | log_slave_updates               | OFF                          |
    | log_slow_queries                | OFF                          |
    | log_warnings                    | OFF                          |
    | long_query_time                 | 10                           |
    | low_priority_updates            | OFF                          |
    | lower_case_table_names          | OFF                          |
    | max_allowed_packet              | 1047552                      |
    | max_binlog_cache_size           | 4294967295                   |
    | max_binlog_size                 | 1073741824                   |
    | max_connections                 | 100                          |
    | max_connect_errors              | 10                           |
    | max_delayed_threads             | 20                           |
    | max_heap_table_size             | 16777216                     |
    | max_join_size                   | 4294967295                   |
    | max_sort_length                 | 1024                         |
    | max_user_connections            | 0                            |
    | max_tmp_tables                  | 32                           |
    | max_write_lock_count            | 4294967295                   |
    | myisam_max_extra_sort_file_size | 268435456                    |
    | myisam_max_sort_file_size       | 2147483647                   |
    | myisam_recover_options          | force                        |
    | myisam_sort_buffer_size         | 8388608                      |
    | net_buffer_length               | 16384                        |
    | net_read_timeout                | 30                           |
    | net_retry_count                 | 10                           |
    | net_write_timeout               | 60                           |
    | open_files_limit                | 0                            |
    | pid_file                        | /usr/local/mysql/name.pid    |
    | port                            | 3306                         |
    | protocol_version                | 10                           |
    | read_buffer_size                | 131072                       |
    | read_rnd_buffer_size            | 262144                       |
    | rpl_recovery_rank               | 0                            |
    | query_cache_limit               | 1048576                      |
    | query_cache_size                | 0                            |
    | query_cache_type                | ON                           |
    | safe_show_database              | OFF                          |
    | server_id                       | 0                            |
    | slave_net_timeout               | 3600                         |
    | skip_external_locking           | ON                           |
    | skip_networking                 | OFF                          |
    | skip_show_database              | OFF                          |
    | slow_launch_time                | 2                            |
    | socket                          | /tmp/mysql.sock              |
    | sort_buffer_size                | 2097116                      |
    | sql_mode                        | 0                            |
    | table_cache                     | 64                           |
    | table_type                      | MYISAM                       |
    | thread_cache_size               | 3                            |
    | thread_stack                    | 131072                       |
    | tx_isolation                    | READ-COMMITTED               |
    | timezone                        | EEST                         |
    | tmp_table_size                  | 33554432                     |
    | tmpdir                          | /tmp/                        |
    | version                         | 4.0.4-beta                   |
    | wait_timeout                    | 28800                        |
    +---------------------------------+------------------------------+

Each option is described here. Values for buffer sizes, lengths, and stack sizes are given in bytes. You can specify values with a suffix of ``K'` or ``M'` to indicate kilobytes or megabytes. For example, `16M` indicates 16 megabytes. The case of suffix letters does not matter; `16M` and `16m` are equivalent:

- `ansi_mode`. Is `ON` if `mysqld` was started with `--ansi`. See section 1.7.2 Running MySQL in ANSI Mode.
- `back_log` The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets very many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The `back_log` value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time. In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix `listen(2)` system call should have more details. Check your OS documentation for the maximum value for this variable. Attempting to set `back_log` higher than your operating system limit will be ineffective.
- `basedir` The value of the `--basedir` option.
- `bdb_cache_size` The buffer that is allocated to cache index and rows for `BDB` tables. If you don't use `BDB` tables, you should start `mysqld` with `--skip-bdb` to not waste memory for this cache.
- `bdb_log_buffer_size` The buffer that is allocated to cache index and rows for `BDB` tables. If you don't use `BDB` tables, you should set this to 0 or start `mysqld` with `--skip-bdb` to not waste memory for this cache.
- `bdb_home` The value of the `--bdb-home` option.
- `bdb_max_lock` The maximum number of locks (10,000 by default) you can have active on a BDB table. You should increase this if you get errors of type `bdb: Lock table is out of available locks` or `Got error 12 from ...` when you have do long transactions or when `mysqld` has to examine a lot of rows to calculate the query.
- `bdb_logdir` The value of the `--bdb-logdir` option.
- `bdb_shared_data` Is `ON` if you are using `--bdb-shared-data`.
- `bdb_tmpdir` The value of the `--bdb-tmpdir` option.
- `binlog_cache_size`. The size of the cache to hold the SQL statements for the binary log during a transaction. If you often use big, multi-statement transactions you can increase this to get more performance. See section 6.7.1 `BEGIN/COMMIT/ROLLBACK` Syntax.
- `bulk_insert_buffer_size` (was `myisam_bulk_insert_tree_size`) MyISAM uses special tree-like cache to make bulk inserts (that is, `INSERT ... SELECT`, `INSERT ... VALUES (...), (...), ...`, and `LOAD DATA INFILE`) faster. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 will disable this optimisation. Note: this cache is only used when adding data to non-empty table. Default value is 8 MB.
- `character_set` The default character set.
- `character_sets` The supported character sets.
- `concurrent_inserts` If `ON` (the default), MySQL will allow you to use `INSERT` on `MyISAM` tables at the same time as you run `SELECT` queries on them. You can turn this option off by starting `mysqld` with `--safe` or `--skip-new`.
- `connect_timeout` The number of seconds the `mysqld` server is waiting for a connect packet before responding with `Bad handshake`.
- `datadir` The value of the `--datadir` option.
- `delay_key_write` Option for MyISAM tables. Can have one of the following values:

  OFF | All CREATE TABLE ... DELAYED_KEY_WRITES are ignored.
  ON | (default) MySQL will honor the `DELAY_KEY_WRITE` option for `CREATE TABLE`.
  ALL | All new opened tables are treated as if they were created with the `DELAY_KEY_WRITE` option.

  If `DELAY_KEY_WRITE` is enabled this means that the key buffer for tables with this option will not get flushed on every index update, but only when a table is closed. This will speed up writes on keys a lot, but you should add automatic checking of all tables with `myisamchk --fast --force` if you use this.
- `delayed_insert_limit` After inserting `delayed_insert_limit` rows, the `INSERT DELAYED` handler will check if there are any `SELECT` statements pending. If so, it allows these to execute before continuing.
- `delayed_insert_timeout` How long a `INSERT DELAYED` thread should wait for `INSERT` statements before terminating.
- `delayed_queue_size` What size queue (in rows) should be allocated for handling `INSERT DELAYED`. If the queue becomes full, any client that does `INSERT DELAYED` will wait until there is room in the queue again.
- `flush` This is `ON` if you have started MySQL with the `--flush` option.
- `flush_time` If this is set to a non-zero value, then every `flush_time` seconds all tables will be closed (to free up resources and sync things to disk). We only recommend this option on Windows 9x/Me, or on systems where you have very little resources.
- `ft_min_word_len` The minimum length of the word to be included in a `FULLTEXT` index. Note: `FULLTEXT` indexes must be rebuilt after changing this variable. (This option is new for MySQL 4.0.)
- `ft_max_word_len` The maximum length of the word to be included in a `FULLTEXT` index. Note: `FULLTEXT` indexes must be rebuilt after changing this variable. (This option is new for MySQL 4.0.)
- `ft_max_word_len_for_sort` The maximum length of the word in a `FULLTEXT` index to be used in fast index recreation method in `REPAIR`, `CREATE INDEX`, or `ALTER TABLE`. Longer words are inserted the slow way. The rule of the thumb is as follows: with `ft_max_word_len_for_sort` increasing, MySQL will create bigger temporary files (thus slowing the process down, due to disk I/O), and will put fewer keys in one sort block (again, decreasing the efficiency). When `ft_max_word_len_for_sort` is too small, instead, MySQL will insert a lot of words into index the slow way, but short words will be inserted very quickly.
- `ft_boolean_syntax` List of operators supported by `MATCH ... AGAINST(... IN BOOLEAN MODE)`. See section 6.8 MySQL Full-text Search.
- `have_innodb` `YES` if `mysqld` supports InnoDB tables. `DISABLED` if `--skip-innodb` is used.
- `have_bdb` `YES` if `mysqld` supports Berkeley DB tables. `DISABLED` if `--skip-bdb` is used.
- `have_raid` `YES` if `mysqld` supports the `RAID` option.
- `have_openssl` `YES` if `mysqld` supports SSL (encryption) on the client/server protocol.
- `init_file` The name of the file specified with the `--init-file` option when you start the server. This is a file of SQL statements you want the server to execute when it starts.
- `interactive_timeout` The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the `CLIENT_INTERACTIVE` option to `mysql_real_connect()`. See also `wait_timeout`.
- `join_buffer_size` The size of the buffer that is used for full joins (joins that do not use indexes). The buffer is allocated one time for each full join between two tables. Increase this value to get a faster full join when adding indexes is not possible. (Normally the best way to get fast joins is to add indexes.)
- `key_buffer_size` Index blocks are buffered and are shared by all threads. `key_buffer_size` is the size of the buffer used for index blocks. Increase this to get better index handling (for all reads and multiple writes) to as much as you can afford; 64M on a 256M machine that mainly runs MySQL is quite common. If you, however, make this too big (for instance more than 50% of your total memory) your system may start to page and become extremely slow. Remember that because MySQL does not cache data reads, you will have to leave some room for the OS filesystem cache. You can check the performance of the key buffer by doing `SHOW STATUS` and examine the variables `Key_read_requests`, `Key_reads`, `Key_write_requests`, and `Key_writes`. The `Key_reads/Key_read_request` ratio should normally be < 0.01. The `Key_write/Key_write_requests` is usually near 1 if you are using mostly updates/deletes but may be much smaller if you tend to do updates that affect many at the same time or if you are using `DELAY_KEY_WRITE`. See section 4.5.6 `SHOW` Syntax. To get even more speed when writing many rows at the same time, use `LOCK TABLES`. See section 6.7.2 `LOCK TABLES/UNLOCK TABLES` Syntax.
- `language` The language used for error messages.
- `large_file_support` If `mysqld` was compiled with options for big file support.
- `locked_in_memory` If `mysqld` was locked in memory with `--memlock`
- `log` If logging of all queries is enabled.
- `log_update` If the update log is enabled.
- `log_bin` If the binary log is enabled.
- `log_slave_updates` If the updates from the slave should be logged.
- `long_query_time` If a query takes longer than this (in seconds), the `Slow_queries` counter will be incremented. If you are using `--log-slow-queries`, the query will be logged to the slow query logfile. This value is measured in real time, not CPU time, so a query that may be under the threshold on a lightly loaded system may be above the threshold on a heavily loaded one. See section 4.9.5 The Slow Query Log.
- `lower_case_table_names` If set to 1 table names are stored in lowercase on disk and table name comparisons will be case-insensitive. From version 4.0.2, this option also applies to database names. See section 6.1.3 Case Sensitivity in Names.
- `max_allowed_packet` The maximum size of one packet. The message buffer is initialised to `net_buffer_length` bytes, but can grow up to `max_allowed_packet` bytes when needed. This value by default is small, to catch big (possibly wrong) packets. You must increase this value if you are using big `BLOB` columns. It should be as big as the biggest `BLOB` you want to use. The protocol limits for `max_allowed_packet` is 16M in MySQL 3.23 and 1G in MySQL 4.0.
- `max_binlog_cache_size` If a multi-statement transaction requires more than this amount of memory, one will get the error "Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage".
- `max_binlog_size` Available after 3.23.33. If a write to the binary (replication) log exceeds the given value, rotate the logs. You cannot set it to less than 1024 bytes, or more than 1 GB. Default is 1 GB.
- `max_connections` The number of simultaneous clients allowed. Increasing this value increases the number of file descriptors that `mysqld` requires. See below for comments on file descriptor limits. See section A.2.5 `Too many connections` Error.
- `max_connect_errors` If there is more than this number of interrupted connections from a host this host will be blocked from further connections. You can unblock a host with the command `FLUSH HOSTS`.
- `max_delayed_threads` Don't start more than this number of threads to handle `INSERT DELAYED` statements. If you try to insert data into a new table after all `INSERT DELAYED` threads are in use, the row will be inserted as if the `DELAYED` attribute wasn't specified.
- `max_heap_table_size` Don't allow creation of heap tables bigger than this.
- `max_join_size` Joins that are probably going to read more than `max_join_size` records return an error. Set this value if your users tend to perform joins that lack a `WHERE` clause, that take a long time, and that return millions of rows.
- `max_sort_length` The number of bytes to use when sorting `BLOB` or `TEXT` values (only the first `max_sort_length` bytes of each value are used; the rest are ignored).
- `max_user_connections` The maximum number of active connections for a single user (0 = no limit).
- `max_tmp_tables` (This option doesn't yet do anything.) Maximum number of temporary tables a client can keep open at the same time.
- `max_write_lock_count` After this many write locks, allow some read locks to run in between.
- `myisam_recover_options` The value of the `--myisam-recover` option.
- `myisam_sort_buffer_size` The buffer that is allocated when sorting the index when doing a `REPAIR` or when creating indexes with `CREATE INDEX` or `ALTER TABLE`.
- `myisam_max_extra_sort_file_size`. If the temporary file used for fast index creation would be bigger than using the key cache by the amount specified here, then prefer the key cache method. This is mainly used to force long character keys in large tables to use the slower key cache method to create the index. Note that this parameter is given in megabytes before 4.0.3 and in bytes starting from this version.
- `myisam_max_sort_file_size` The maximum size of the temporary file MySQL is allowed to use while recreating the index (during `REPAIR`, `ALTER TABLE` or `LOAD DATA INFILE`. If the file-size would be bigger than this, the index will be created through the key cache (which is slower). Note that this parameter is given in megabytes before 4.0.3 and in bytes starting from this version.
- `net_buffer_length` The communication buffer is reset to this size between queries. This should not normally be changed, but if you have very little memory, you can set it to the expected size of a query. (That is, the expected length of SQL statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to `max_allowed_packet` bytes.)
- `net_read_timeout` Number of seconds to wait for more data from a connection before aborting the read. Note that when we don't expect data from a connection, the timeout is defined by `write_timeout`. See also `slave_net_timeout`.
- `net_retry_count` If a read on a communication port is interrupted, retry this many times before giving up. This value should be quite high on `FreeBSD` as internal interrupts are sent to all threads.
- `net_write_timeout` Number of seconds to wait for a block to be written to a connection before aborting the write.
- `open_files_limit` If this is not 0, then `mysqld` will use this value to reserve file descriptors to use with `setrlimit()`. If this value is 0 then `mysqld` will reserve `max_connections*5` or `max_connections + table_cache*2` (whichever is larger) number of files. You should try increasing this if `mysqld` gives you the error 'Too many open files'.
- `pid_file` The value of the `--pid-file` option.
- `port` The value of the `--port` option.
- `protocol_version` The protocol version used by the MySQL server.
- `read_buffer_size` (was `record_buffer`) Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you may want to increase this value.
- `record_rnd_buffer_size` When reading rows in sorted order after a sort, the rows are read through this buffer to avoid a disk seeks. Can improve `ORDER BY` by a lot if set to a high value. As this is a thread-specific variable, one should not set this big globally, but just change this when running some specific big queries.
- `query_cache_limit` Don't cache results that are bigger than this. (Default 1M).
- `query_cache_size` The memory allocated to store results from old queries. If this is 0, the query cache is disabled (default).
- `query_cache_type` This may be set (only numeric) to

  Value | Alias | Comment
  0 | OFF | Don't cache or retrieve results.
  1 | ON | Cache all results except `SELECT SQL_NO_CACHE ...` queries.
  2 | DEMAND | Cache only `SELECT SQL_CACHE ...` queries.

- `safe_show_database` Don't show databases for which the user doesn't have any database or table privileges. This can improve security if you're concerned about people being able to see what databases other users have. See also `skip_show_database`.
- `server_id` The value of the `--server-id` option.
- `skip_locking` Is OFF if `mysqld` uses external locking.
- `skip_networking` Is ON if we only allow local (socket) connections.
- `skip_show_database` This prevents people from doing `SHOW DATABASES` if they don't have the `PROCESS` privilege. This can improve security if you're concerned about people being able to see what databases other users have. See also `safe_show_database`.
- `slave_net_timeout` Number of seconds to wait for more data from a master/slave connection before aborting the read.
- `slow_launch_time` If creating the thread takes longer than this value (in seconds), the `Slow_launch_threads` counter will be incremented.
- `socket` The Unix socket used by the server.
- `sort_buffer` Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster `ORDER BY` or `GROUP BY` operations. See section A.4.4 Where MySQL Stores Temporary Files.
- `table_cache` The number of open tables for all threads. Increasing this value increases the number of file descriptors that `mysqld` requires. You can check if you need to increase the table cache by checking the `Opened_tables` variable. See section 4.5.6.3 `SHOW STATUS`. If this variable is big and you don't do `FLUSH TABLES` a lot (which just forces all tables to be closed and reopenend), then you should increase the value of this variable. For more information about the table cache, see section 5.4.7 How MySQL Opens and Closes Tables.
- `table_type` The default table type.
- `thread_cache_size` How many threads we should keep in a cache for reuse. When a client disconnects, the client's threads are put in the cache if there aren't more than `thread_cache_size` threads from before. All new threads are first taken from the cache, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. (Normally this doesn't give a notable performance improvement if you have a good thread implementation.) By examing the difference between the `Connections` and `Threads_created` status variables (see section 4.5.6.3 `SHOW STATUS` for details) you can see how efficient thread cache is.
- `thread_concurrency` On Solaris, `mysqld` will call `thr_setconcurrency()` with this value. `thr_setconcurrency()` permits the application to give the threads system a hint for the desired number of threads that should be run at the same time.
- `thread_stack` The stack size for each thread. Many of the limits detected by the `crash-me` test are dependent on this value. The default is large enough for normal operation. See section 5.1.4 The MySQL Benchmark Suite.
- `timezone` The timezone for the server.
- `tmp_table_size` If an in-memory temporary table exceeds this size, MySQL will automatically convert it to an on-disk `MyISAM` table. Increase the value of `tmp_table_size` if you do many advanced `GROUP BY` queries and you have lots of memory.
- `tmpdir` The directory used for temporary files and temporary tables.
- `version` The version number for the server.
- `wait_timeout` The number of seconds the server waits for activity on a not interactive connection before closing it. On thread startup `SESSION.WAIT_TIMEOUT` is initialised from `GLOBAL.WAIT_TIMEOUT` or `GLOBAL.INTERACTIVE_TIMEOUT` depending on the type of client (as defined by the `CLIENT_INTERACTIVE` connect option). See also `interactive_timeout`.

The manual section that describes tuning MySQL contains some information of how to tune the above variables. See section 5.5.2 Tuning Server Parameters.

#### 4.5.6.5 `SHOW LOGS`

`SHOW LOGS` shows you status information about existing log files. It currently only displays information about Berkeley DB log files.

- `File` shows the full path to the log file
- `Type` shows the type of the log file (`BDB` for Berkeley DB log files)
- `Status` shows the status of the log file (`FREE` if the file can be removed, or `IN USE` if the file is needed by the transaction subsystem)

#### 4.5.6.6 `SHOW PROCESSLIST`

`SHOW [FULL] PROCESSLIST` shows you which threads are running. You can also get this information using the `mysqladmin processlist` command. If you have the `SUPER` privilege, you can see all threads. Otherwise, you can see only your own threads. See section 4.5.5 `KILL` Syntax. If you don't use the `FULL` option, then only the first 100 characters of each query will be shown.

This command is very useful if you get the 'too many connections' error message and want to find out what's going on. MySQL reserves one extra connection for a client with the `SUPER` privilege to ensure that you should always be able to login and check the system (assuming you are not giving this privilege to all your users).

Some states commonly seen in `mysqladmin processlist`

- `Checking table` The thread is performing [automatic] checking of the table.
- `Closing tables` Means that the thread is flushing the changed table data to disk and closing the used tables. This should be a fast operation. If not, then you should check that you don't have a full disk or that the disk is not in very heavy use.
- `Connect Out` Slave connecting to master.
- `Copying to tmp table on disk` The temporary result set was larger than `tmp_table_size` and the thread is now changing the in memory-based temporary table to a disk based one to save memory.
- `Creating tmp table` The thread is creating a temporary table to hold a part of the result for the query.
- `deleting from main table` When executing the first part of a multi-table delete and we are only deleting from the first table.
- `deleting from reference tables` When executing the second part of a multi-table delete and we are deleting the matched rows from the other tables.
- `Flushing tables` The thread is executing `FLUSH TABLES` and is waiting for all threads to close their tables.
- `Killed` Someone has sent a kill to the thread and it should abort next time it checks the kill flag. The flag is checked in each major loop in MySQL, but in some cases it may still take a short time for the thread to die. If the thread is locked by some other thread, the kill will take affect as soon as the other thread releases it's lock.
- `Sending data` The thread is processing rows for a `SELECT` statement and is also sending data to the client.
- `Sorting for group` The thread is doing a sort to satsify a `GROUP BY`.
- `Sorting for order` The thread is doing a sort to satsify a `ORDER BY`.
- `Opening tables` This simply means that the thread is trying to open a table. This is should be very fast procedure, unless something prevents opening. For example an `ALTER TABLE` or a `LOCK TABLE` can prevent opening a table until the command is finished.
- `Removing duplicates` The query was using `SELECT DISTINCT` in such a way that MySQL couldn't optimise that distinct away at an early stage. Because of this MySQL has to do an extra stage to remove all duplicated rows before sending the result to the client.
- `Reopen table` The thread got a lock for the table, but noticed after getting the lock that the underlying table structure changed. It has freed the lock, closed the table and is now trying to reopen it.
- `Repair by sorting` The repair code is using sorting to create indexes.
- `Repair with keycache` The repair code is using creating keys one by one through the key cache. This is much slower than `Repair by sorting`.
- `Searching rows for update` The thread is doing a first phase to find all matching rows before updating them. This has to be done if the `UPDATE` is changing the index that is used to find the involved rows.
- `Sleeping` The thread is wating for the client to send a new command to it.
- `System lock` The thread is waiting for getting to get a external system lock for the table. If you are not using multiple mysqld servers that are accessing the same tables, you can disable system locks with the `--skip-external-locking` option.
- `Upgrading lock` The `INSERT DELAYED` handler is trying to get a lock for the table to insert rows.
- `Updating` The thread is searching for rows to update and updating them.
- `User Lock` The thread is waiting on a `GET_LOCK()`.
- `Waiting for tables` The thread got a notification that the underlying structure for a table has changed and it needs to reopen the table to get the new structure. To be able to reopen the table it must however wait until all other threads have closed the table in question. This notification happens if another thread has used `FLUSH TABLES` or one of the following commands on the table in question: `FLUSH TABLES table_name`, `ALTER TABLE`, `RENAME TABLE`, `REPAIR TABLE`, `ANALYZE TABLE` or `OPTIMIZE TABLE`.
- `waiting for handler insert` The `INSERT DELAYED` handler has processed all inserts and are waiting to get new ones.

Most states are very quick operations. If threads last in any of these states for many seconds, there may be a problem around that needs to be investigated.

There are some other states that are not mentioned previously, but most of these are only useful to find bugs in `mysqld`.

#### 4.5.6.7 `SHOW GRANTS`

`SHOW GRANTS FOR user` lists the grant commands that must be issued to duplicate the grants for a user.

    mysql> SHOW GRANTS FOR root@localhost;
    +---------------------------------------------------------------------+
    | Grants for root@localhost                                           |
    +---------------------------------------------------------------------+
    | GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' WITH GRANT OPTION |
    +---------------------------------------------------------------------+

To list grants for the current session one may use `CURRENT_USER()` function (new in version 4.0.6) to find out what user the session was authentificated as. See section 6.3.6.2 Miscellaneous Functions.

#### 4.5.6.8 `SHOW CREATE TABLE`

Shows a `CREATE TABLE` statement that will create the given table:

    mysql> SHOW CREATE TABLE t\G
    *************************** 1. row ***************************
           Table: t
    Create Table: CREATE TABLE t (
      id int(11) default NULL auto_increment,
      s char(60) default NULL,
      PRIMARY KEY (id)
    ) TYPE=MyISAM

`SHOW CREATE TABLE` will quote table and column names according to `SQL_QUOTE_SHOW_CREATE` option. section 5.5.6 `SET` Syntax.

## 4.6 MySQL Localisation and International Usage

### 4.6.1 The Character Set Used for Data and Sorting

By default, MySQL uses the ISO-8859-1 (Latin1) character set with sorting according to Swedish/Finnish. This is the character set suitable in the USA and western Europe.

All standard MySQL binaries are compiled with `--with-extra-charsets=complex`. This will add code to all standard programs to be able to handle `latin1` and all multi-byte character sets within the binary. Other character sets will be loaded from a character-set definition file when needed.

The character set determines what characters are allowed in names and how things are sorted by the `ORDER BY` and `GROUP BY` clauses of the `SELECT` statement.

You can change the character set with the `--default-character-set` option when you start the server. The character sets available depend on the `--with-charset=charset` and `--with-extra-charsets= list-of-charset | complex | all` options to `configure`, and the character set configuration files listed in ``SHAREDIR/charsets/Index'`. See section 2.3.3 Typical `configure` Options.

If you change the character set when running MySQL (which may also change the sort order), you must run `myisamchk -r -q --set-character-set=charset` on all tables. Otherwise, your indexes may not be ordered correctly.

When a client connects to a MySQL server, the server sends the default character set in use to the client. The client will switch to use this character set for this connection.

One should use `mysql_real_escape_string()` when escaping strings for a SQL query. `mysql_real_escape_string()` is identical to the old `mysql_escape_string()` function, except that it takes the `MYSQL` connection handle as the first parameter.

If the client is compiled with different paths than where the server is installed and the user who configured MySQL didn't include all character sets in the MySQL binary, one must specify for the client where it can find the additional character sets it will need if the server runs with a different character set than the client.

One can specify this by putting in a MySQL option file:

    [client]
    character-sets-dir=/usr/local/mysql/share/mysql/charsets

where the path points to the directory in which the dynamic MySQL character sets are stored.

One can force the client to use specific character set by specifying:

    [client]
    default-character-set=character-set-name

but normally this is never needed.

#### 4.6.1.1 German character set

To get German sorting order, you should start `mysqld` with `--default-character-set=latin1_de`. This will give you the following characteristics.

When sorting and comparing string's the following mapping is done on the strings before doing the comparison:

    ä  ->  ae
    ö  ->  oe
    ü  ->  ue
    ß  ->  ss

All accented characters, are converted to their un-accented uppercase counterpart. All letters are converted to uppercase.

When comparing strings with `LIKE` the one -> two character mapping is not done. All letters are converted to uppercase. Accent are removed from all letters except: `Ü`, `ü`, `Ö`, `ö`, `Ä` and `ä`.

### 4.6.2 Non-English Error Messages

`mysqld` can issue error messages in the following languages: Czech, Danish, Dutch, English (the default), Estonian, French, German, Greek, Hungarian, Italian, Japanese, Korean, Norwegian, Norwegian-ny, Polish, Portuguese, Romanian, Russian, Slovak, Spanish, and Swedish.

To start `mysqld` with a particular language, use either the `--language=lang` or `-L lang` options. For example:

    shell> mysqld --language=swedish

or:

    shell> mysqld --language=/usr/local/share/swedish

Note that all language names are specified in lowercase.

The language files are located (by default) in ``mysql_base_dir/share/LANGUAGE/'`.

To update the error message file, you should edit the ``errmsg.txt'` file and execute the following command to generate the ``errmsg.sys'` file:

    shell> comp_err errmsg.txt errmsg.sys

If you upgrade to a newer version of MySQL, remember to repeat your changes with the new ``errmsg.txt'` file.

### 4.6.3 Adding a New Character Set

To add another character set to MySQL, use the following procedure.

Decide if the set is simple or complex. If the character set does not need to use special string collating routines for sorting and does not need multi-byte character support, it is simple. If it needs either of those features, it is complex.

For example, `latin1` and `danish` are simple charactersets while `big5` or `czech` are complex character sets.

In the following section, we have assumed that you name your character set `MYSET`.

For a simple character set do the following:

1. Add MYSET to the end of the ``sql/share/charsets/Index'` file Assign a unique number to it.
2. Create the file ``sql/share/charsets/MYSET.conf'`. (You can use ``sql/share/charsets/latin1.conf'` as a base for this.) The syntax for the file very simple:
   - Comments start with a '#' character and proceed to the end of the line.
   - Words are separated by arbitrary amounts of whitespace.
   - When defining the character set, every word must be a number in hexadecimal format
   - The `ctype` array takes up the first 257 words. The `to_lower[]`, `to_upper[]` and `sort_order[]` arrays take up 256 words each after that.
   See section 4.6.4 The Character Definition Arrays.
3. Add the character set name to the `CHARSETS_AVAILABLE` and `COMPILED_CHARSETS` lists in `configure.in`.
4. Reconfigure, recompile, and test.

For a complex character set do the following:

1. Create the file ``strings/ctype-MYSET.c'` in the MySQL source distribution.
2. Add MYSET to the end of the ``sql/share/charsets/Index'` file. Assign a unique number to it.
3. Look at one of the existing ``ctype-*.c'` files to see what needs to be defined, for example ``strings/ctype-big5.c'`. Note that the arrays in your file must have names like `ctype_MYSET`, `to_lower_MYSET`, and so on. This corresponds to the arrays in the simple character set. See section 4.6.4 The Character Definition Arrays. For a complex character set
4. Near the top of the file, place a special comment like this:

       /*
        * This comment is parsed by configure to create ctype.c,
        * so don't change it unless you know what you are doing.
        *
        * .configure. number_MYSET=MYNUMBER
        * .configure. strxfrm_multiply_MYSET=N
        * .configure. mbmaxlen_MYSET=N
        */

   The `configure` program uses this comment to include the character set into the MySQL library automatically. The strxfrm_multiply and mbmaxlen lines will be explained in the following sections. Only include these if you need the string collating functions or the multi-byte character set functions, respectively.
5. You should then create some of the following functions:
   - `my_strncoll_MYSET()`
   - `my_strcoll_MYSET()`
   - `my_strxfrm_MYSET()`
   - `my_like_range_MYSET()`
   See section 4.6.5 String Collating Support.
6. Add the character set name to the `CHARSETS_AVAILABLE` and `COMPILED_CHARSETS` lists in `configure.in`.
7. Reconfigure, recompile, and test.

The file ``sql/share/charsets/README'` includes some more instructions.

If you want to have the character set included in the MySQL distribution, mail a patch to internals@lists.mysql.com.

### 4.6.4 The Character Definition Arrays

`to_lower[]` and `to_upper[]` are simple arrays that hold the lowercase and uppercase characters corresponding to each member of the character set. For example:

    to_lower['A'] should contain 'a'
    to_upper['a'] should contain 'A'

`sort_order[]` is a map indicating how characters should be ordered for comparison and sorting purposes. For many character sets, this is the same as `to_upper[]` (which means sorting will be case-insensitive). MySQL will sort characters based on the value of `sort_order[character]`. For more complicated sorting rules, see the discussion of string collating below. See section 4.6.5 String Collating Support.

`ctype[]` is an array of bit values, with one element for one character. (Note that `to_lower[]`, `to_upper[]`, and `sort_order[]` are indexed by character value, but `ctype[]` is indexed by character value + 1. This is an old legacy to be able to handle `EOF`.)

You can find the following bitmask definitions in ``m_ctype.h'`:

    #define _U      01      /* Uppercase */
    #define _L      02      /* Lowercase */
    #define _N      04      /* Numeral (digit) */
    #define _S      010     /* Spacing character */
    #define _P      020     /* Punctuation */
    #define _C      040     /* Control character */
    #define _B      0100    /* Blank */
    #define _X      0200    /* heXadecimal digit */

The `ctype[]` entry for each character should be the union of the applicable bitmask values that describe the character. For example, `'A'` is an uppercase character (`_U`) as well as a hexadecimal digit (`_X`), so `ctype['A'+1]` should contain the value:

    _U + _X = 01 + 0200 = 0201

### 4.6.5 String Collating Support

If the sorting rules for your language are too complex to be handled with the simple `sort_order[]` table, you need to use the string collating functions.

Right now the best documentation on this is the character sets that are already implemented. Look at the `big5`, `czech`, `gbk`, `sjis`, and `tis160` character sets for examples.

You must specify the `strxfrm_multiply_MYSET=N` value in the special comment at the top of the file. `N` should be set to the maximum ratio the strings may grow during `my_strxfrm_MYSET` (it must be a positive integer).

### 4.6.6 Multi-byte Character Support

If your want to add support for a new character set that includes multi-byte characters, you need to use the multi-byte character functions.

Right now the best documentation on this is the character sets that are already implemented. Look at the `euc_kr`, `gb2312`, `gbk`, `sjis`, and `ujis` character sets for examples. These are implemented in the ``ctype-'charset'.c'` files in the ``strings'` directory.

You must specify the `mbmaxlen_MYSET=N` value in the special comment at the top of the source file. `N` should be set to the size in bytes of the largest character in the set.

### 4.6.7 Problems With Character Sets

If you try to use a character set that is not compiled into your binary, you can run into a couple of different problems:

- Your program has a wrong path to where the character sets are stored. (Default ``/usr/local/mysql/share/mysql/charsets'`). This can be fixed by using the `--character-sets-dir` option to the program in question.
- The character set is a multi-byte character set that can't be loaded dynamically. In this case you have to recompile the program with the support for the character set.
- The character set is a dynamic character set, but you don't have a configure file for it. In this case you should install the configure file for the character set from a new MySQL distribution.
- Your ``Index'` file doesn't contain the name for the character set.

      ERROR 1105: File '/usr/local/share/mysql/charsets/?.conf' not found
      (Errcode: 2)

  In this case you should either get a new `Index` file or add by hand the name of any missing character sets.

For `MyISAM` tables, you can check the character set name and number for a table with `myisamchk -dvv table_name`.

## 4.7 MySQL Server-Side Scripts and Utilities

### 4.7.1 Overview of the Server-Side Scripts and Utilities

All MySQL programs take many different options. However, every MySQL program provides a `--help` option that you can use to get a full description of the program's different options. For example, try `mysql --help`.

You can override default options for all standard programs with an option file. section 4.1.2 ``my.cnf'` Option Files.

The following list briefly describes the server-side MySQL programs:

`myisamchk`
  Utility to describe, check, optimise, and repair MySQL tables. Because `myisamchk` has many functions, it is described in its own chapter. See section 4 Database Administration.
`make_binary_distribution`
  Makes a binary release of a compiled MySQL. This could be sent by FTP to ``/pub/mysql/Incoming'` on `support.mysql.com` for the convenience of other MySQL users.
`mysqlbug`
  The MySQL bug report script. This script should always be used when filing a bug report to the MySQL list.
`mysqld`
  The SQL daemon. This should always be running.
`mysql_install_db`
  Creates the MySQL grant tables with default privileges. This is usually executed only once, when first installing MySQL on a system.

### 4.7.2 `safe_mysqld`, The Wrapper Around `mysqld`

`safe_mysqld` is the recommended way to start a `mysqld` daemon on Unix. `safe_mysqld` adds some safety features such as restarting the server when an error occurs and logging run-time information to a log file.

If you don't use `--mysqld=#` or `--mysqld-version=#` `safe_mysqld` will use an executable named `mysqld-max` if it exists. If not, `safe_mysqld` will start `mysqld`. This makes it very easy to test to use `mysqld-max` instead of `mysqld`; just copy `mysqld-max` to where you have `mysqld` and it will be used.

Normally one should never edit the `safe_mysqld` script, but instead put the options to `safe_mysqld` in the `[safe_mysqld]` section in the ``my.cnf'` file. `safe_mysqld` will read all options from the `[mysqld]`, `[server]` and `[safe_mysqld]` sections from the option files. See section 4.1.2 ``my.cnf'` Option Files.

Note that all options on the command-line to `safe_mysqld` are passed to `mysqld`. If you wants to use any options in `safe_mysqld` that `mysqld` doesn't support, you must specify these in the option file.

Most of the options to `safe_mysqld` are the same as the options to `mysqld`. See section 4.1.1 `mysqld` Command-line Options.

`safe_mysqld` supports the following options:

`--basedir=path`
`--core-file-size=#`
  Size of the core file `mysqld` should be able to create. Passed to `ulimit -c`.
`--datadir=path`
`--defaults-extra-file=path`
`--defaults-file=path`
`--err-log=path`
`--ledir=path`
  Path to `mysqld`
`--log=path`
`--mysqld=mysqld-version`
  Name of the `mysqld` version in the `ledir` directory you want to start.
`--mysqld-version=version`
  Similar to `--mysqld=` but here you only give the suffix for `mysqld`. For example if you use `--mysqld-version=max`, `safe_mysqld` will start the `ledir/mysqld-max` version. If the argument to `--mysqld-version` is empty, `ledir/mysqld` will be used.
`--no-defaults`
`--open-files-limit=#`
  Number of files `mysqld` should be able to open. Passed to `ulimit -n`. Note that you need to start `safe_mysqld` as root for this to work properly!
`--pid-file=path`
`--port=#`
`--socket=path`
`--timezone=#`
  Set the timezone (the `TZ`) variable to the value of this parameter.
`--user=#`

The `safe_mysqld` script is written so that it normally is able to start a server that was installed from either a source or a binary version of MySQL, even if these install the server in slightly different locations. `safe_mysqld` expects one of these conditions to be true:

- The server and databases can be found relative to the directory from which `safe_mysqld` is invoked. `safe_mysqld` looks under its working directory for ``bin'` and ``data'` directories (for binary distributions) or for ``libexec'` and ``var'` directories (for source distributions). This condition should be met if you execute `safe_mysqld` from your MySQL installation directory (for example, ``/usr/local/mysql'` for a binary distribution).
- If the server and databases cannot be found relative to the working directory, `safe_mysqld` attempts to locate them by absolute pathnames. Typical locations are ``/usr/local/libexec'` and ``/usr/local/var'`. The actual locations are determined when the distribution was built from which `safe_mysqld` comes. They should be correct if MySQL was installed in a standard location.

Because `safe_mysqld` will try to find the server and databases relative to its own working directory, you can install a binary distribution of MySQL anywhere, as long as you start `safe_mysqld` from the MySQL installation directory:

    shell> cd mysql_installation_directory
    shell> bin/safe_mysqld &

If `safe_mysqld` fails, even when invoked from the MySQL installation directory, you can modify it to use the path to `mysqld` and the pathname options that are correct for your system. Note that if you upgrade MySQL in the future, your modified version of `safe_mysqld` will be overwritten, so you should make a copy of your edited version that you can reinstall.

### 4.7.3 `mysqld_multi`, A Program for Managing Multiple MySQL Servers

`mysqld_multi` is meant for managing several `mysqld` processes that listen for connections on different Unix sockets and TCP/IP ports.

The program will search for group(s) named `[mysqld#]` from ``my.cnf'` (or the file named by the `--config-file=...` option), where `#` can be any positive number starting from 1. This number is referred to in the following discussion as the option group number, or GNR. Group numbers distinquish option groups from one another and are used as arguments to `mysqld_multi` to specify which servers you want to start, stop, or obtain status for. Options listed in these groups should be the same as you would use in the the usual `[mysqld]` group used for starting `mysqld`. (See, for example, section 2.4.3 Starting and Stopping MySQL Automatically.) However, for `mysqld_multi`, be sure that each group includes options for values such as the port, socket, etc., to be used for each individual `mysqld` process.

`mysqld_multi` is invoked using the following syntax:

    Usage: mysqld_multi [OPTIONS] {start|stop|report} [GNR,GNR,GNR...]
    or     mysqld_multi [OPTIONS] {start|stop|report} [GNR-GNR,GNR,GNR-GNR,...]

Each GNR represents an option group number. You can start, stop or report any GNR, or several of them at the same time. For an example of how you might set up an option file, use this command:

    shell> mysqld_multi --example

The GNR values in the list can be comma-separated or combined with a dash; in the latter case, all the GNRs between GNR1-GNR2 will be affected. With no GNR argument, all groups listed in the option file will be either started, stopped, or reported. Note that you must not have any white spaces in the GNR list. Anything after a white space is ignored.

`mysqld_multi` supports the following options:

`--config-file=...`
  Alternative config file. Note: This will not affect this program's own options (group `[mysqld_multi]`), but only groups `[mysqld#]`. Without this option, everything will be searched from the ordinary ``my.cnf'` file.
`--example`
  Display an example option file.
`--help`
  Print this help and exit.
`--log=...`
  Log file. Full path to and the name for the log file. Note: If the file exists, everything will be appended.
`--mysqladmin=...`
  `mysqladmin` binary to be used for a server shutdown.
`--mysqld=...`
  `mysqld` binary to be used. Note that you can give `safe_mysqld` to this option also. The options are passed to `mysqld`. Just make sure you have `mysqld` in your environment variable `PATH` or fix `safe_mysqld`.
`--no-log`
  Print to stdout instead of the log file. By default the log file is turned on.
`--password=...`
  Password for user for `mysqladmin`.
`--tcp-ip`
  Connect to the MySQL server(s) via the TCP/IP port instead of the Unix socket. This affects stopping and reporting. If a socket file is missing, the server may still be running, but can be accessed only via the TCP/IP port. By default, connections are made using the Unix socket.
`--user=...`
  MySQL user for `mysqladmin`.
`--version`
  Print the version number and exit.

Some notes about `mysqld_multi`:

- Make sure that the MySQL user, who is stopping the `mysqld` services (e.g using the `mysqladmin` program) have the same password and username for all the data directories accessed (to the `mysql` database) And make sure that the user has the `SHUTDOWN` privilege! If you have many data directories and many different `mysql` databases with different passwords for the MySQL `root` user, you may want to create a common `multi_admin` user for each using the same password (see below). Example how to do it:

      shell> mysql -u root -S /tmp/mysql.sock -proot_password -e
      "GRANT SHUTDOWN ON *.* TO multi_admin@localhost IDENTIFIED BY 'multipass'"

  See section 4.2.6 How the Privilege System Works. You will have to do the above for each `mysqld` running in each data directory, that you have (just change the socket, `-S=...`).
- `pid-file` is very important, if you are using `safe_mysqld` to start `mysqld` (e.g., `--mysqld=safe_mysqld`) Every `mysqld` should have its own `pid-file`. The advantage using `safe_mysqld` instead of `mysqld` directly here is, that `safe_mysqld` ``guards'' every `mysqld` process and will restart it, if a `mysqld` process terminates due to a signal sent using `kill -9`, or for other reasons such as a segmentation fault (which MySQL should never do, of course ;). Please note that the `safe_mysqld` script may require that you start it from a certain place. This means that you may have to `cd` to a certain directory, before you start the `mysqld_multi`. If you have problems starting, please see the `safe_mysqld` script. Check especially the lines:

      --------------------------------------------------------------------------
      MY_PWD=`pwd` Check if we are starting this relative (for the binary
      release) if test -d /data/mysql -a -f ./share/mysql/english/errmsg.sys
      -a -x ./bin/mysqld
      --------------------------------------------------------------------------

  See section 4.7.2 `safe_mysqld`, The Wrapper Around `mysqld`. The above test should be successful, or you may encounter problems.
- Beware of the dangers starting multiple `mysqld`s in the same data directory. Use separate data directories, unless you know what you are doing!
- The socket file and the TCP/IP port must be different for every `mysqld`.
- The first and fifth `mysqld` group were intentionally left out from the example. You may have 'gaps' in the config file. This gives you more flexibility. The order in which the `mysqlds` are started or stopped depends on the order in which they appear in the config file.
- When you want to refer to a certain group using GNR with this program, just use the number in the end of the group name. For example, the GNR for a group named `[mysqld17]` is 17.
- You may want to use option `--user` for `mysqld`, but in order to do this you need to run the `mysqld_multi` script as the Unix `root` user. Having the option in the config file doesn't matter; you will just get a warning, if you are not the superuser and the `mysqlds` are started under your Unix account. Important: Make sure that the `pid-file` and the data directory are read+write(+execute for the latter one) accessible for that Unix user, who the specific `mysqld` process is started as. Do not use the Unix root account for this, unless you know what you are doing!
- Most important: Make sure that you understand the meanings of the options that are passed to the `mysqld`s and why one would want to have separate `mysqld` processes. Starting multiple `mysqld`s in one data directory will not give you extra performance in a threaded system!

See section 4.1.4 Running Multiple MySQL Servers on the Same Machine.

This is an example of the config file on behalf of `mysqld_multi`.

    # This file should probably be in your home dir (~/.my.cnf) or /etc/my.cnf
    # Version 2.1 by Jani Tolonen

    [mysqld_multi]
    mysqld     = /usr/local/bin/safe_mysqld
    mysqladmin = /usr/local/bin/mysqladmin
    user       = multi_admin
    password   = multipass

    [mysqld2]
    socket     = /tmp/mysql.sock2
    port       = 3307
    pid-file   = /usr/local/mysql/var2/hostname.pid2
    datadir    = /usr/local/mysql/var2
    language   = /usr/local/share/mysql/english
    user       = john

    [mysqld3]
    socket     = /tmp/mysql.sock3
    port       = 3308
    pid-file   = /usr/local/mysql/var3/hostname.pid3
    datadir    = /usr/local/mysql/var3
    language   = /usr/local/share/mysql/swedish
    user       = monty

    [mysqld4]
    socket     = /tmp/mysql.sock4
    port       = 3309
    pid-file   = /usr/local/mysql/var4/hostname.pid4
    datadir    = /usr/local/mysql/var4
    language   = /usr/local/share/mysql/estonia
    user       = tonu

    [mysqld6]
    socket     = /tmp/mysql.sock6
    port       = 3311
    pid-file   = /usr/local/mysql/var6/hostname.pid6
    datadir    = /usr/local/mysql/var6
    language   = /usr/local/share/mysql/japanese
    user       = jani

See section 4.1.2 ``my.cnf'` Option Files.

### 4.7.4 `myisampack`, The MySQL Compressed Read-only Table Generator

`myisampack` is used to compress MyISAM tables, and `pack_isam` is used to compress ISAM tables. Because ISAM tables are deprecated, we will only discuss `myisampack` here, but everything said about `myisampack` should also be true for `pack_isam`.

`myisampack` works by compressing each column in the table separately. The information needed to decompress columns is read into memory when the table is opened. This results in much better performance when accessing individual records, because you only have to uncompress exactly one record, not a much larger disk block as when using Stacker on MS-DOS. Usually, `myisampack` packs the datafile 40%-70%.

MySQL uses memory mapping (`mmap()`) on compressed tables and falls back to normal read/write file usage if `mmap()` doesn't work.

Please note the following:

- After packing, the table is read-only. This is generally intended (such as when accessing packed tables on a CD). Also allowing writes to a packed table is on our TODO list but with low priority.
- `myisampack` can also pack `BLOB` or `TEXT` columns. The older `pack_isam` (for `ISAM` tables) can not do this.

`myisampack` is invoked like this:

    shell> myisampack [options] filename ...

Each filename should be the name of an index (``.MYI'`) file. If you are not in the database directory, you should specify the pathname to the file. It is permissible to omit the ``.MYI'` extension.

`myisampack` supports the following options:

`-b, --backup`
  Make a backup of the table as `tbl_name.OLD`.
`-#, --debug=debug_options`
  Output debug log. The `debug_options` string often is `'d:t:o,filename'`.
`-f, --force`
  Force packing of the table even if it becomes bigger or if the temporary file exists. `myisampack` creates a temporary file named ``tbl_name.TMD'` while it compresses the table. If you kill `myisampack`, the ``.TMD'` file may not be deleted. Normally, `myisampack` exits with an error if it finds that ``tbl_name.TMD'` exists. With `--force`, `myisampack` packs the table anyway.
`-?, --help`
  Display a help message and exit.
`-j big_tbl_name, --join=big_tbl_name`
  Join all tables named on the command-line into a single table `big_tbl_name`. All tables that are to be combined must be identical (same column names and types, same indexes, etc.).
`-p #, --packlength=#`
  Specify the record length storage size, in bytes. The value should be 1, 2, or 3. (`myisampack` stores all rows with length pointers of 1, 2, or 3 bytes. In most normal cases, `myisampack` can determine the right length value before it begins packing the file, but it may notice during the packing process that it could have used a shorter length. In this case, `myisampack` will print a note that the next time you pack the same file, you could use a shorter record length.)
`-s, --silent`
  Silent mode. Write output only when errors occur.
`-t, --test`
  Don't actually pack table, just test packing it.
`-T dir_name, --tmp_dir=dir_name`
  Use the named directory as the location in which to write the temporary table.
`-v, --verbose`
  Verbose mode. Write information about progress and packing result.
`-V, --version`
  Display version information and exit.
`-w, --wait`
  Wait and retry if table is in use. If the `mysqld` server was invoked with the `--skip-external-locking` option, it is not a good idea to invoke `myisampack` if the table might be updated during the packing process.

The sequence of commands shown here illustrates a typical table compression session:

    shell> ls -l station.*
    -rw-rw-r--   1 monty    my         994128 Apr 17 19:00 station.MYD
    -rw-rw-r--   1 monty    my          53248 Apr 17 19:00 station.MYI
    -rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

    shell> myisamchk -dvv station

    MyISAM file:     station
    Isam-version:  2
    Creation time: 1996-03-13 10:08:58
    Recover time:  1997-02-02  3:06:43
    Data records:              1192  Deleted blocks:              0
    Datafile: Parts:           1192  Deleted data:                0
    Datafile pointer (bytes):     2  Keyfile pointer (bytes):     2
    Max datafile length:   54657023  Max keyfile length:   33554431
    Recordlength:               834
    Record format: Fixed length

    table description:
    Key Start Len Index   Type                       Root  Blocksize    Rec/key
    1   2     4   unique  unsigned long              1024       1024          1
    2   32    30  multip. text                      10240       1024          1

    Field Start Length Type
    1     1     1
    2     2     4
    3     6     4
    4     10    1
    5     11    20
    6     31    1
    7     32    30
    8     62    35
    9     97    35
    10    132   35
    11    167   4
    12    171   16
    13    187   35
    14    222   4
    15    226   16
    16    242   20
    17    262   20
    18    282   20
    19    302   30
    20    332   4
    21    336   4
    22    340   1
    23    341   8
    24    349   8
    25    357   8
    26    365   2
    27    367   2
    28    369   4
    29    373   4
    30    377   1
    31    378   2
    32    380   8
    33    388   4
    34    392   4
    35    396   4
    36    400   4
    37    404   1
    38    405   4
    39    409   4
    40    413   4
    41    417   4
    42    421   4
    43    425   4
    44    429   20
    45    449   30
    46    479   1
    47    480   1
    48    481   79
    49    560   79
    50    639   79
    51    718   79
    52    797   8
    53    805   1
    54    806   1
    55    807   20
    56    827   4
    57    831   4

    shell> myisampack station.MYI
    Compressing station.MYI: (1192 records)
    - Calculating statistics

    normal:     20  empty-space:      16  empty-zero:        12  empty-fill:  11
    pre-space:   0  end-space:        12  table-lookups:      5  zero:         7
    Original trees:  57  After join: 17
    - Compressing file
    87.14%

    shell> ls -l station.*
    -rw-rw-r--   1 monty    my         127874 Apr 17 19:00 station.MYD
    -rw-rw-r--   1 monty    my          55296 Apr 17 19:04 station.MYI
    -rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

    shell> myisamchk -dvv station

    MyISAM file:     station
    Isam-version:  2
    Creation time: 1996-03-13 10:08:58
    Recover time:  1997-04-17 19:04:26
    Data records:              1192  Deleted blocks:              0
    Datafile: Parts:           1192  Deleted data:                0
    Datafilepointer (bytes):      3  Keyfile pointer (bytes):     1
    Max datafile length:   16777215  Max keyfile length:     131071
    Recordlength:               834
    Record format: Compressed

    table description:
    Key Start Len Index   Type                       Root  Blocksize    Rec/key
    1   2     4   unique  unsigned long             10240       1024          1
    2   32    30  multip. text                      54272       1024          1

    Field Start Length Type                         Huff tree  Bits
    1     1     1      constant                             1     0
    2     2     4      zerofill(1)                          2     9
    3     6     4      no zeros, zerofill(1)                2     9
    4     10    1                                           3     9
    5     11    20     table-lookup                         4     0
    6     31    1                                           3     9
    7     32    30     no endspace, not_always              5     9
    8     62    35     no endspace, not_always, no empty    6     9
    9     97    35     no empty                             7     9
    10    132   35     no endspace, not_always, no empty    6     9
    11    167   4      zerofill(1)                          2     9
    12    171   16     no endspace, not_always, no empty    5     9
    13    187   35     no endspace, not_always, no empty    6     9
    14    222   4      zerofill(1)                          2     9
    15    226   16     no endspace, not_always, no empty    5     9
    16    242   20     no endspace, not_always              8     9
    17    262   20     no endspace, no empty                8     9
    18    282   20     no endspace, no empty                5     9
    19    302   30     no endspace, no empty                6     9
    20    332   4      always zero                          2     9
    21    336   4      always zero                          2     9
    22    340   1                                           3     9
    23    341   8      table-lookup                         9     0
    24    349   8      table-lookup                        10     0
    25    357   8      always zero                          2     9
    26    365   2                                           2     9
    27    367   2      no zeros, zerofill(1)                2     9
    28    369   4      no zeros, zerofill(1)                2     9
    29    373   4      table-lookup                        11     0
    30    377   1                                           3     9
    31    378   2      no zeros, zerofill(1)                2     9
    32    380   8      no zeros                             2     9
    33    388   4      always zero                          2     9
    34    392   4      table-lookup                        12     0
    35    396   4      no zeros, zerofill(1)               13     9
    36    400   4      no zeros, zerofill(1)                2     9
    37    404   1                                           2     9
    38    405   4      no zeros                             2     9
    39    409   4      always zero                          2     9
    40    413   4      no zeros                             2     9
    41    417   4      always zero                          2     9
    42    421   4      no zeros                             2     9
    43    425   4      always zero                          2     9
    44    429   20     no empty                             3     9
    45    449   30     no empty                             3     9
    46    479   1                                          14     4
    47    480   1                                          14     4
    48    481   79     no endspace, no empty               15     9
    49    560   79     no empty                             2     9
    50    639   79     no empty                             2     9
    51    718   79     no endspace                         16     9
    52    797   8      no empty                             2     9
    53    805   1                                          17     1
    54    806   1                                           3     9
    55    807   20     no empty                             3     9
    56    827   4      no zeros, zerofill(2)                2     9
    57    831   4      no zeros, zerofill(1)                2     9

The information printed by `myisampack` is described here:

`normal`
  The number of columns for which no extra packing is used.
`empty-space`
  The number of columns containing values that are only spaces; these will occupy 1 bit.
`empty-zero`
  The number of columns containing values that are only binary 0's; these will occupy 1 bit.
`empty-fill`
  The number of integer columns that don't occupy the full byte range of their type; these are changed to a smaller type (for example, an `INTEGER` column may be changed to `MEDIUMINT`).
`pre-space`
  The number of decimal columns that are stored with leading spaces. In this case, each value will contain a count for the number of leading spaces.
`end-space`
  The number of columns that have a lot of trailing spaces. In this case, each value will contain a count for the number of trailing spaces.
`table-lookup`
  The column had only a small number of different values, which were converted to an `ENUM` before Huffman compression.
`zero`
  The number of columns for which all values are zero.
`Original trees`
  The initial number of Huffman trees.
`After join`
  The number of distinct Huffman trees left after joining trees to save some header space.

After a table has been compressed, `myisamchk -dvv` prints additional information about each field:

`Type`
  The field type may contain the following descriptors:
  `constant`
    All rows have the same value.
  `no endspace`
    Don't store endspace.
  `no endspace, not_always`
    Don't store endspace and don't do end space compression for all values.
  `no endspace, no empty`
    Don't store endspace. Don't store empty values.
  `table-lookup`
    The column was converted to an `ENUM`.
  `zerofill(n)`
    The most significant `n` bytes in the value are always 0 and are not stored.
  `no zeros`
    Don't store zeros.
  `always zero`
    0 values are stored in 1 bit.
`Huff tree`
  The Huffman tree associated with the field.
`Bits`
  The number of bits used in the Huffman tree.

After you have run `pack_isam`/`myisampack` you must run `isamchk`/`myisamchk` to re-create the index. At this time you can also sort the index blocks and create statistics needed for the MySQL optimiser to work more efficiently:

    myisamchk -rq --analyze --sort-index table_name.MYI
    isamchk   -rq --analyze --sort-index table_name.ISM

After you have installed the packed table into the MySQL database directory you should do `mysqladmin flush-tables` to force `mysqld` to start using the new table.

If you want to unpack a packed table, you can do this with the `--unpack` option to `isamchk` or `myisamchk`.

### 4.7.5 `mysqld-max`, An Extended `mysqld` Server

`mysqld-max` is the MySQL server (`mysqld`) configured with the following configure options:

Option | Comment
--with-server-suffix=-max | Add a suffix to the `mysqld` version string.
--with-innodb | Support for InnoDB tables.
--with-bdb | Support for Berkeley DB (BDB) tables
CFLAGS=-DUSE_SYMDIR | Symbolic links support for Windows.

You can find the MySQL-max binaries at http://www.mysql.com/downloads/mysql-max-3.23.html.

The Windows MySQL binary distributions includes both the standard `mysqld.exe` binary and the `mysqld-max.exe` binary. http://www.mysql.com/downloads/mysql-3.23.html. See section 2.1.2 Installing MySQL on Windows.

Note that as InnoDB and Berkeley DB are not available for all platforms, some of the `Max` binaries may not have support for both of these. You can check which table types are supported by doing the following query:

    mysql> SHOW VARIABLES LIKE "have_%";
    +---------------+-------+
    | Variable_name | Value |
    +---------------+-------+
    | have_bdb      | YES   |
    | have_innodb   | NO    |
    | have_isam     | YES   |
    | have_raid     | NO    |
    | have_openssl  | NO    |
    +---------------+-------+

The meaning of the values are:

Value | Meaning
`YES` | The option is activated and usable.
`NO` | MySQL is not compiled with support for this option.
`DISABLED` | The xxxx option is disabled because one started `mysqld` with `--skip-xxxx` or because one didn't start `mysqld` with all needed options to enable the option. In this case the `hostname.err` file should contain a reason for why the option is disabled.

Note: To be able to create InnoDB tables you must edit your startup options to include at least the `innodb_data_file_path` option. See section 7.5.2 InnoDB Startup Options.

To get better performance for BDB tables, you should add some configuration options for these too. See section 7.6.3 `BDB` startup options.

`safe_mysqld` will automatically try to start any `mysqld` binary with the `-max` suffix. This makes it very easy to test out a another `mysqld` binary in an existing installation. Just run `configure` with the options you want and then install the new `mysqld` binary as `mysqld-max` in the same directory where your old `mysqld` binary is. See section 4.7.2 `safe_mysqld`, The Wrapper Around `mysqld`.

The `mysqld-max` RPM uses the above mentioned `safe_mysqld` feature. It just installs the `mysqld-max` executable and `safe_mysqld` will automatically use this executable when `safe_mysqld` is restarted.

The following table shows which table types our standard MySQL-Max binaries includes:

System | `BDB` | `InnoDB`
AIX 4.3 | N | Y
HP-UX 11.0 | N | Y
Linux-Alpha | N | Y
Linux-Intel | Y | Y
Linux-IA64 | N | Y
Solaris-Intel | N | Y
Solaris-SPARC | Y | Y
Caldera (SCO) OSR5 | Y | Y
UnixWare | Y | Y
Windows/NT | Y | Y

## 4.8 MySQL Client-Side Scripts and Utilities

### 4.8.1 Overview of the Client-Side Scripts and Utilities

All MySQL clients that communicate with the server using the `mysqlclient` library use the following environment variables:

Name | Description
`MYSQL_UNIX_PORT` | The default socket; used for connections to `localhost`
`MYSQL_TCP_PORT` | The default TCP/IP port
`MYSQL_PWD` | The default password
`MYSQL_DEBUG` | Debug-trace options when debugging
`TMPDIR` | The directory where temporary tables/files are created

Use of `MYSQL_PWD` is insecure. See section 4.2.8 Connecting to the MySQL Server.

The ``mysql'` client uses the file named in the `MYSQL_HISTFILE` environment variable to save the command-line history. The default value for the history file is ``$HOME/.mysql_history'`, where `$HOME` is the value of the `HOME` environment variable. See section F Environment Variables.

All MySQL programs take many different options. However, every MySQL program provides a `--help` option that you can use to get a full description of the program's different options. For example, try `mysql --help`.

You can override default options for all standard client programs with an option file. section 4.1.2 ``my.cnf'` Option Files.

The following list briefly describes the client-side MySQL programs:

`msql2mysql`
  A shell script that converts `mSQL` programs to MySQL. It doesn't handle all cases, but it gives a good start when converting.
`mysqlaccess`
  A script that checks the access privileges for a host, user, and database combination.
`mysqladmin`
  Utility for performing administrative operations, such as creating or dropping databases, reloading the grant tables, flushing tables to disk, and reopening log files. `mysqladmin` can also be used to retrieve version, process, and status information from the server. See section 4.8.3 `mysqladmin`, Administrating a MySQL Server.
`mysqldump`
  Dumps a MySQL database into a file as SQL statements or as tab-separated text files. Enhanced freeware originally by Igor Romanenko. See section 4.8.5 `mysqldump`, Dumping Table Structure and Data.
`mysqlimport`
  Imports text files into their respective tables using `LOAD DATA INFILE`. See section 4.8.7 `mysqlimport`, Importing Data from Text Files.
`mysqlshow`
  Displays information about databases, tables, columns, and indexes.
`replace`
  A utility program that is used by `msql2mysql`, but that has more general applicability as well. `replace` changes strings in place in files or on the standard input. Uses a finite state machine to match longer strings first. Can be used to swap strings. For example, this command swaps `a` and `b` in the given files:

      shell> replace a b b a -- file1 file2 ...

### 4.8.2 `mysql`, The Command-line Tool

`mysql` is a simple SQL shell (with GNU `readline` capabilities). It supports interactive and non-interactive use. When used interactively, query results are presented in an ASCII-table format. When used non-interactively (for example, as a filter), the result is presented in tab-separated format. (The output format can be changed using command-line options.) You can run scripts simply like this:

    shell> mysql database < script.sql > output.tab

If you have problems due to insufficient memory in the client, use the `--quick` option! This forces `mysql` to use `mysql_use_result()` rather than `mysql_store_result()` to retrieve the result set.

Using `mysql` is very easy. Just start it as follows: `mysql database` or `mysql --user=user_name --password=your_password database`. Type a SQL statement, end it with ``;'`, ``\g'`, or ``\G'` and press Enter.

`mysql` supports the following options:

`-?, --help`
  Display this help and exit.
`-A, --no-auto-rehash`
  No automatic rehashing. One has to use 'rehash' to get table and field completion. This gives a quicker start of mysql.
`--prompt=...`
  Set the mysql prompt to specified format.
`-b, --no-beep`
  Turn off beep-on-error.
`-B, --batch`
  Print results with a tab as separator, each row on a new line. Doesn't use history file.
`--character-sets-dir=...`
  Directory where character sets are located.
`-C, --compress`
  Use compression in server/client protocol.
`-#, --debug[=...]`
  Debug log. Default is 'd:t:o,/tmp/mysql.trace'.
`-D, --database=...`
  Database to use. This is mainly useful in the ``my.cnf'` file.
`--default-character-set=...`
  Set the default character set.
`-e, --execute=...`
  Execute command and quit. (Output like with --batch)
`-E, --vertical`
  Print the output of a query (rows) vertically. Without this option you can also force this output by ending your statements with `\G`.
`-f, --force`
  Continue even if we get a SQL error.
`-g, --no-named-commands`
  Named commands are disabled. Use \* form only, or use named commands only in the beginning of a line ending with a semicolon (``;'`). Since Version 10.9, the client now starts with this option enabled by default! With the -g option, long format commands will still work from the first line, however.
`-G, --enable-named-commands`
  Named commands are enabled. Long format commands are allowed as well as shortened \* commands.
`-i, --ignore-space`
  Ignore space after function names.
`-h, --host=...`
  Connect to the given host.
`-H, --html`
  Produce HTML output.
`-X, --xml`
  Produce XML output.
`-L, --skip-line-numbers`
  Don't write line number for errors. Useful when one wants to compare result files that includes error messages
`--no-pager`
  Disable pager and print to stdout. See interactive help (\h) also.
`--no-tee`
  Disable outfile. See interactive help (\h) also.
`-n, --unbuffered`
  Flush buffer after each query.
`-N, --skip-column-names`
  Don't write column names in results.
`-O, --set-variable var=option`
  Give a variable a value. `--help` lists variables. Please note that `--set-variable` is deprecated since MySQL 4.0, just use `--var=option` on its own.
`-o, --one-database`
  Only update the default database. This is useful for skipping updates to other database in the update log.
``--pager[=...]``
  Output type. Default is your `ENV` variable `PAGER`. Valid pagers are less, more, cat [> filename], etc. See interactive help (\h) also. This option does not work in batch mode. Pager works only in Unix.
`-p[password], --password[=...]`
  Password to use when connecting to server. If a password is not given on the command-line, you will be prompted for it. Note that if you use the short form `-p` you can't have a space between the option and the password.
`-P --port=...`
  TCP/IP port number to use for connection.
`-q, --quick`
  Don't cache result, print it row-by-row. This may slow down the server if the output is suspended. Doesn't use history file.
`-r, --raw`
  Write column values without escape conversion. Used with `--batch`
`-s, --silent`
  Be more silent.
`-S --socket=...`
  Socket file to use for connection.
`-t --table`
  Output in table format. This is default in non-batch mode.
`-T, --debug-info`
  Print some debug information at exit.
`--tee=...`
  Append everything into outfile. See interactive help (\h) also. Does not work in batch mode.
`-u, --user=#`
  User for login if not current user.
`-U, --safe-updates[=#], --i-am-a-dummy[=#]`
  Only allow `UPDATE` and `DELETE` that uses keys. See below for more information about this option. You can reset this option if you have it in your ``my.cnf'` file by using `--safe-updates=0`.
`-v, --verbose`
  More verbose output (-v -v -v gives the table output format).
`-V, --version`
  Output version information and exit.
`-w, --wait`
  Wait and retry if connection is down instead of aborting.

You can also set the following variables with `-O` or `--set-variable`; please note that `--set-variable` is deprecated since MySQL 4.0, just use `--var=option` on its own:

Variable Name | Default | Description
connect_timeout | 0 | Number of seconds before timeout connection.
max_allowed_packet | 16777216 | Max packetlength to send/receive from to server
net_buffer_length | 16384 | Buffer for TCP/IP and socket communication
select_limit | 1000 | Automatic limit for SELECT when using --i-am-a-dummy
max_join_size | 1000000 | Automatic limit for rows in a join when using --i-am-a-dummy.

If you type 'help' on the command-line, `mysql` will print out the commands that it supports:

    mysql> help

    MySQL commands:
    help    (\h)    Display this text.
    ?       (\h)    Synonym for `help'.
    clear   (\c)    Clear command.
    connect (\r)    Reconnect to the server.
                    Optional arguments are db and host.
    edit    (\e)    Edit command with $EDITOR.
    ego     (\G)    Send command to mysql server,
                    display result vertically.
    exit    (\q)    Exit mysql. Same as quit.
    go      (\g)    Send command to mysql server.
    nopager (\n)    Disable pager, print to stdout.
    notee   (\t)    Don't write into outfile.
    pager   (\P)    Set PAGER [to_pager].
                    Print the query results via PAGER.
    print   (\p)    Print current command.
    prompt  (\R)    Change your mysql prompt.
    quit    (\q)    Quit mysql.
    rehash  (\#)    Rebuild completion hash.
    source  (\.)    Execute a SQL script file.
                    Takes a file name as an argument.
    status  (\s)    Get status information from the server.
    tee     (\T)    Set outfile [to_outfile].
                    Append everything into given outfile.
    use     (\u)    Use another database.
                    Takes database name as argument.

The `pager` command works only in Unix.

The `status` command gives you some information about the connection and the server you are using. If you are running in the `--safe-updates` mode, `status` will also print the values for the `mysql` variables that affect your queries.

A useful startup option for beginners (introduced in MySQL Version 3.23.11) is `--safe-updates` (or `--i-am-a-dummy` for users that has at some time done a `DELETE FROM table_name` but forgot the `WHERE` clause). When using this option, `mysql` sends the following command to the MySQL server when opening the connection:

    SET SQL_SAFE_UPDATES=1,SQL_SELECT_LIMIT=#select_limit#,
        SQL_MAX_JOIN_SIZE=#max_join_size#"

where `#select_limit#` and `#max_join_size#` are variables that can be set from the `mysql` command-line. See section 5.5.6 `SET` Syntax.

The effect of the above is:

- You are not allowed to do an `UPDATE` or `DELETE` statement if you don't have a key constraint in the `WHERE` part. One can, however, force an `UPDATE/DELETE` by using `LIMIT`:

      UPDATE table_name SET not_key_column=# WHERE not_key_column=# LIMIT 1;

- All big results are automatically limited to `#select_limit#` rows.
- `SELECT`s that will probably need to examine more than `#max_join_size` row combinations will be aborted.

Some useful hints about the `mysql` client:

Some data is much more readable when displayed vertically, instead of the usual horizontal box type output. For example longer text, which includes new lines, is often much easier to be read with vertical output.

    mysql> SELECT * FROM mails WHERE LENGTH(txt) < 300 lIMIT 300,1\G
    *************************** 1. row ***************************
      msg_nro: 3068
         date: 2000-03-01 23:29:50
    time_zone: +0200
    mail_from: Monty
        reply: monty@no.spam.com
      mail_to: "Thimble Smith" <tim@no.spam.com>
          sbj: UTF-8
          txt: >>>>> "Thimble" == Thimble Smith writes:

    Thimble> Hi.  I think this is a good idea.  Is anyone familiar with UTF-8
    Thimble> or Unicode? Otherwise, I'll put this on my TODO list and see what
    Thimble> happens.

    Yes, please do that.

    Regards,
    Monty
         file: inbox-jani-1
         hash: 190402944
    1 row in set (0.09 sec)

For logging, you can use the `tee` option. The `tee` can be started with option `--tee=...`, or from the command-line interactively with command `tee`. All the data displayed on the screen will also be appended into a given file. This can be very useful for debugging purposes also. The `tee` can be disabled from the command-line with command `notee`. Executing `tee` again starts logging again. Without a parameter the previous file will be used. Note that `tee` will flush the results into the file after each command, just before the command-line appears again waiting for the next command.

Browsing, or searching the results in the interactive mode in Unix less, more, or any other similar program, is now possible with option `--pager[=...]`. Without argument, `mysql` client will look for environment variable PAGER and set `pager` to that. `pager` can be started from the interactive command-line with command `pager` and disabled with command `nopager`. The command takes an argument optionally and the `pager` will be set to that. Command `pager` can be called without an argument, but this requires that the option `--pager` was used, or the `pager` will default to stdout. `pager` works only in Unix, since it uses the popen() function, which doesn't exist in Windows. In Windows, the `tee` option can be used instead, although it may not be as handy as `pager` can be in some situations.

A few tips about `pager`:

- You can use it to write to a file:

      mysql> pager cat > /tmp/log.txt

  and the results will only go to a file. You can also pass any options for the programs that you want to use with the `pager`:

      mysql> pager less -n -i -S

- From the above do note the option '-S'. You may find it very useful when browsing the results; try the option with horizontal output (end commands with '\g', or ';') and with vertical output (end commands with '\G'). Sometimes a very wide result set is hard to be read from the screen, with option -S to less you can browse the results within the interactive less from left to right, preventing lines longer than your screen from being continued to the next line. This can make the result set much more readable. You can swith the mode between on and off within the interactive less with '-S'. See the 'h' for more help about less.
- You can combine very complex ways to handle the results, for example the following would send the results to two files in two different directories, on two different hard-disks mounted on /dr1 and /dr2, yet let the results still be seen on the screen via less:

      mysql> pager cat | tee /dr1/tmp/res.txt | \
      tee /dr2/tmp/res2.txt | less -n -i -S

You can also combine the two functions above; have the `tee` enabled, `pager` set to 'less' and you will be able to browse the results in unix 'less' and still have everything appended into a file the same time. The difference between Unix `tee` used with the `pager` and the `mysql` client in-built `tee`, is that the in-built `tee` works even if you don't have the Unix `tee` available. The in-built `tee` also logs everything that is printed on the screen, where the Unix `tee` used with `pager` doesn't log quite that much. Last, but not least, the interactive `tee` is more handy to switch on and off, when you want to log something into a file, but want to be able to turn the feature off sometimes.

From MySQL version 4.0.2 it is possible to change the prompt in the `mysql` command-line client.

You can use the following prompt options:

Option | Description
\v | mysqld version
\d | database in use
\h | host connected to
\p | port connected on
\u | username
\U | full username@host
\\ | ``\'`
\n | new line break
\t | tab
\ | space
\_ | space
\R | military hour time (0-23)
\r | standard hour time (1-12)
\m | minutes
\y | two digit year
\Y | four digit year
\D | full date format
\s | seconds
\w | day of the week in three letter format (Mon, Tue, ...)
\P | am/pm
\o | month in number format
\O | month in three letter format (Jan, Feb, ...)
\c | counter that counts up for each command you do

``\'` followed by any other letter just becomes that letter.

You may set the prompt in the following places:

Environment Variable
  You may set the `MYSQL_PS1` environment variable to a prompt string. For example:

      shell> export MYSQL_PS1="(\u@\h) [\d]> "

``my.cnf'`
``.my.cnf'`
  You may set the `prompt` option in any MySQL configuration file, in the `mysql` group. For example:

      [mysql]
      prompt=(\u@\h) [\d]>\_

Command Line
  You may set the `--prompt` option on the command line to `mysql`. For example:

      shell> mysql --prompt="(\u@\h) [\d]> "

      (user@host) [database]>

Interactively
  You may also use the `prompt` (or `\R`) command to change your prompt interactively. For example:

      mysql> prompt (\u@\h) [\d]>\_
      PROMPT set to '(\u@\h) [\d]>\_'
      (user@host) [database]>
      (user@host) [database]> prompt
      Returning to default PROMPT of mysql>
      mysql>

### 4.8.3 `mysqladmin`, Administrating a MySQL Server

A utility for performing administrative operations. The syntax is:

    shell> mysqladmin [OPTIONS] command [command-option] command ...

You can get a list of the options your version of `mysqladmin` supports by executing `mysqladmin --help`.

The current `mysqladmin` supports the following commands:

`create databasename`
  Create a new database.
`drop databasename`
  Delete a database and all its tables.
`extended-status`
  Gives an extended status message from the server.
`flush-hosts`
  Flush all cached hosts.
`flush-logs`
  Flush all logs.
`flush-tables`
  Flush all tables.
`flush-privileges`
  Reload grant tables (same as reload).
`kill id,id,...`
  Kill mysql threads.
`password`
  Set a new password. Change old password to new-password.
`ping`
  Check if mysqld is alive.
`processlist`
  Show list of active threads in server.
`reload`
  Reload grant tables.
`refresh`
  Flush all tables and close and open logfiles.
`shutdown`
  Take server down.
`slave-start`
  Start slave replication thread.
`slave-stop`
  Stop slave replication thread.
`status`
  Gives a short status message from the server.
`variables`
  Prints variables available.
`version`
  Get version info from server.

All commands can be shortened to their unique prefix. For example:

    shell> mysqladmin proc stat
    +----+-------+-----------+----+-------------+------+-------+------+
    | Id | User  | Host      | db | Command     | Time | State | Info |
    +----+-------+-----------+----+-------------+------+-------+------+
    | 6  | monty | localhost |    | Processlist | 0    |       |      |
    +----+-------+-----------+----+-------------+------+-------+------+
    Uptime: 10077  Threads: 1  Questions: 9  Slow queries: 0
    Opens: 6 Flush tables: 1  Open tables: 2
    Memory in use: 1092K  Max memory used: 1116K

The `mysqladmin status` command result has the following columns:

Column | Description
Uptime | Number of seconds the MySQL server has been up.
Threads | Number of active threads (clients).
Questions | Number of questions from clients since `mysqld` was started.
Slow queries | Queries that have taken more than `long_query_time` seconds. See section 4.9.5 The Slow Query Log.
Opens | How many tables `mysqld` has opened.
Flush tables | Number of `flush ...`, `refresh`, and `reload` commands.
Open tables | Number of tables that are open now.
Memory in use | Memory allocated directly by the `mysqld` code (only available when MySQL is compiled with --with-debug=full).
Max memory used | Maximum memory allocated directly by the `mysqld` code (only available when MySQL is compiled with --with-debug=full).

If you do `mysqladmin shutdown` on a socket (in other words, on a the computer where `mysqld` is running), `mysqladmin` will wait until the MySQL `pid-file` is removed to ensure that the `mysqld` server has stopped properly.

### 4.8.4 Using `mysqlcheck` for Table Maintenance and Crash Recovery

Since MySQL version 3.23.38 you will be able to use a new checking and repairing tool for `MyISAM` tables. The difference to `myisamchk` is that `mysqlcheck` should be used when the `mysqld` server is running, where as `myisamchk` should be used when it is not. The benefit is that you no longer have to take the server down for checking or repairing your tables.

`mysqlcheck` uses MySQL server commands `CHECK`, `REPAIR`, `ANALYZE` and `OPTIMIZE` in a convenient way for the user.

There are three alternative ways to invoke `mysqlcheck`:

    shell> mysqlcheck [OPTIONS] database [tables]
    shell> mysqlcheck [OPTIONS] --databases DB1 [DB2 DB3...]
    shell> mysqlcheck [OPTIONS] --all-databases

So it can be used in a similar way as `mysqldump` when it comes to what databases and tables you want to choose.

`mysqlcheck` does have a special feature compared to the other clients; the default behaviour, checking tables (-c), can be changed by renaming the binary. So if you want to have a tool that repairs tables by default, you should just copy `mysqlcheck` to your harddrive with a new name, `mysqlrepair`, or alternatively make a symbolic link to `mysqlrepair` and name the symbolic link as `mysqlrepair`. If you invoke `mysqlrepair` now, it will repair tables by default.

The names that you can use to change `mysqlcheck` default behaviour are here:

    mysqlrepair:   The default option will be -r
    mysqlanalyze:  The default option will be -a
    mysqloptimize: The default option will be -o

The options available for `mysqlcheck` are listed here, please check what your version supports with `mysqlcheck --help`.

`-A, --all-databases`
  Check all the databases. This will be same as --databases with all databases selected
`-1, --all-in-1`
  Instead of making one query for each table, execute all queries in 1 query separately for each database. Table names will be in a comma separated list.
`-a, --analyze`
  Analyse given tables.
`--auto-repair`
  If a checked table is corrupted, automatically fix it. Repairing will be done after all tables have been checked, if corrupted ones were found.
`-#, --debug=...`
  Output debug log. Often this is 'd:t:o,filename'
`--character-sets-dir=...`
  Directory where character sets are
`-c, --check`
  Check table for errors
`-C, --check-only-changed`
  Check only tables that have changed since last check or haven't been closed properly.
`--compress`
  Use compression in server/client protocol.
`-?, --help`
  Display this help message and exit.
`-B, --databases`
  To check several databases. Note the difference in usage; in this case no tables are given. All name arguments are regarded as database names.
`--default-character-set=...`
  Set the default character set
`-F, --fast`
  Check only tables that hasn't been closed properly
`-f, --force`
  Continue even if we get an sql-error.
`-e, --extended`
  If you are using this option with CHECK TABLE, it will ensure that the table is 100 percent consistent, but will take a long time. If you are using this option with REPAIR TABLE, it will run an extended repair on the table, which may not only take a long time to execute, but may produce a lot of garbage rows also!
`-h, --host=...`
  Connect to host.
`-m, --medium-check`
  Faster than extended-check, but only finds 99.99 percent of all errors. Should be good enough for most cases.
`-o, --optimize`
  Optimise table
`-p, --password[=...]`
  Password to use when connecting to server. If password is not given it's solicited on the tty.
`-P, --port=...`
  Port number to use for connection.
`-q, --quick`
  If you are using this option with CHECK TABLE, it prevents the check from scanning the rows to check for wrong links. This is the fastest check. If you are using this option with REPAIR TABLE, it will try to repair only the index tree. This is the fastest repair method for a table.
`-r, --repair`
  Can fix almost anything except unique keys that aren't unique.
`-s, --silent`
  Print only error messages.
`-S, --socket=...`
  Socket file to use for connection.
`--tables`
  Overrides option --databases (-B).
`-u, --user=#`
  User for login if not current user.
`-v, --verbose`
  Print info about the various stages.
`-V, --version`
  Output version information and exit.

### 4.8.5 `mysqldump`, Dumping Table Structure and Data

Utility to dump a database or a collection of database for backup or for transferring the data to another SQL server (not necessarily a MySQL server). The dump will contain SQL statements to create the table and/or populate the table.

If you are doing a backup on the server, you should consider using the `mysqlhotcopy` instead. See section 4.8.6 `mysqlhotcopy`, Copying MySQL Databases and Tables.

    shell> mysqldump [OPTIONS] database [tables]
    OR     mysqldump [OPTIONS] --databases [OPTIONS] DB1 [DB2 DB3...]
    OR     mysqldump [OPTIONS] --all-databases [OPTIONS]

If you don't give any tables or use the `--databases` or `--all-databases`, the whole database(s) will be dumped.

You can get a list of the options your version of `mysqldump` supports by executing `mysqldump --help`.

Note that if you run `mysqldump` without `--quick` or `--opt`, `mysqldump` will load the whole result set into memory before dumping the result. This will probably be a problem if you are dumping a big database.

Note that if you are using a new copy of the `mysqldump` program and you are going to do a dump that will be read into a very old MySQL server, you should not use the `--opt` or `-e` options.

`mysqldump` supports the following options:

`--add-locks`
  Add `LOCK TABLES` before and `UNLOCK TABLE` after each table dump. (To get faster inserts into MySQL.)
`--add-drop-table`
  Add a `drop table` before each create statement.
`-A, --all-databases`
  Dump all the databases. This will be same as `--databases` with all databases selected.
`-a, --all`
  Include all MySQL-specific create options.
`--allow-keywords`
  Allow creation of column names that are keywords. This works by prefixing each column name with the table name.
`-c, --complete-insert`
  Use complete insert statements (with column names).
`-C, --compress`
  Compress all information between the client and the server if both support compression.
`-B, --databases`
  To dump several databases. Note the difference in usage. In this case no tables are given. All name arguments are regarded as database names. `USE db_name;` will be included in the output before each new database.
`--delayed`
  Insert rows with the `INSERT DELAYED` command.
`-e, --extended-insert`
  Use the new multiline `INSERT` syntax. (Gives more compact and faster inserts statements.)
`-#, --debug[=option_string]`
  Trace usage of the program (for debugging).
`--help`
  Display a help message and exit.
`--fields-terminated-by=...`
`--fields-enclosed-by=...`
`--fields-optionally-enclosed-by=...`
`--fields-escaped-by=...`
`--lines-terminated-by=...`
  These options are used with the `-T` option and have the same meaning as the corresponding clauses for `LOAD DATA INFILE`. See section 6.4.9 `LOAD DATA INFILE` Syntax.
`-F, --flush-logs`
  Flush log file in the MySQL server before starting the dump.
`-f, --force,`
  Continue even if we get a SQL error during a table dump.
`-h, --host=..`
  Dump data from the MySQL server on the named host. The default host is `localhost`.
`-l, --lock-tables.`
  Lock all tables before starting the dump. The tables are locked with `READ LOCAL` to allow concurrent inserts in the case of `MyISAM` tables. Please note that when dumping multiple databases, `--lock-tables` will lock tables for each database separately. So using this option will not guarantee your tables will be logically consistent between databases. Tables in different databases may be dumped in completely different states.
`-K, --disable-keys`
  `/*!40000 ALTER TABLE tb_name DISABLE KEYS */;` and `/*!40000 ALTER TABLE tb_name ENABLE KEYS */;` will be put in the output. This will make loading the data into a MySQL 4.0 server faster as the indexes are created after all data are inserted.
`-n, --no-create-db`
  `CREATE DATABASE /*!32312 IF NOT EXISTS*/ db_name;` will not be put in the output. The above line will be added otherwise, if a `--databases` or `--all-databases` option was given.
`-t, --no-create-info`
  Don't write table creation information (the `CREATE TABLE` statement).
`-d, --no-data`
  Don't write any row information for the table. This is very useful if you just want to get a dump of the structure for a table!
`--opt`
  Same as `--quick --add-drop-table --add-locks --extended-insert --lock-tables`. Should give you the fastest possible dump for reading into a MySQL server.
`-pyour_pass, --password[=your_pass]`
  The password to use when connecting to the server. If you specify no ``=your_pass'` part, `mysqldump` you will be prompted for a password.
`-P port_num, --port=port_num`
  The TCP/IP port number to use for connecting to a host. (This is used for connections to hosts other than `localhost`, for which Unix sockets are used.)
`-q, --quick`
  Don't buffer query, dump directly to stdout. Uses `mysql_use_result()` to do this.
`-Q, --quote-names`
  Quote table and column names within ```'` characters.
`-r, --result-file=...`
  Direct output to a given file. This option should be used in MSDOS, because it prevents new line ``\n'` from being converted to ``\n\r'` (new line + carriage return).
`--single-transaction`
  This option issues a `BEGIN` SQL command before dumping data from server. It is mostly useful with `InnoDB` tables and `READ_COMMITTED` transaction isolation level, as in this mode it will dump the consistent state of the database at the time then `BEGIN` was issued without blocking any applications. When using this option you should keep in mind that only transactional tables will be dumped in a consistent state, e.g., any `MyISAM` or `HEAP` tables dumped while using this option may still change state. The `--single-transaction` option was added in version 4.0.2. This option is mutually exclusive with the `--lock-tables` option as `LOCK TABLES` already commits a previous transaction internally.
`-S /path/to/socket, --socket=/path/to/socket`
  The socket file to use when connecting to `localhost` (which is the default host).
`--tables`
  Overrides option --databases (-B).
`-T, --tab=path-to-some-directory`
  Creates a `table_name.sql` file, that contains the SQL CREATE commands, and a `table_name.txt` file, that contains the data, for each give table. The format of the ``.txt'` file is made according to the `--fields-xxx` and `--lines--xxx` options. Note: This option only works if `mysqldump` is run on the same machine as the `mysqld` daemon, and the user/group that `mysqld` is running as (normally user `mysql`, group `mysql`) needs to have permission to create/write a file at the location you specify.
`-u user_name, --user=user_name`
  The MySQL user name to use when connecting to the server. The default value is your Unix login name.
`-O var=option, --set-variable var=option`
  Set the value of a variable. The possible variables are listed below. Please note that `--set-variable` is deprecated since MySQL 4.0, just use `--var=option` on its own.
`-v, --verbose`
  Verbose mode. Print out more information on what the program does.
`-V, --version`
  Print version information and exit.
`-w, --where='where-condition'`
  Dump only selected records. Note that quotes are mandatory:
`-X, --xml`
  Dumps a database as well formed XML
`-x, --first-slave`
  Locks all tables across all databases.

      "--where=user='jimf'" "-wuserid>1" "-wuserid<1"

`-O net_buffer_length=#, where # < 16M`
  When creating multi-row-insert statements (as with option `--extended-insert` or `--opt`), `mysqldump` will create rows up to `net_buffer_length` length. If you increase this variable, you should also ensure that the `max_allowed_packet` variable in the MySQL server is bigger than the `net_buffer_length`.

The most normal use of `mysqldump` is probably for making a backup of whole databases. See section 4.4.1 Database Backups.

    mysqldump --opt database > backup-file.sql

You can read this back into MySQL with:

    mysql database < backup-file.sql

or

    mysql -e "source /patch-to-backup/backup-file.sql" database

However, it's also very useful to populate another MySQL server with information from a database:

    mysqldump --opt database | mysql --host=remote-host -C database

It is possible to dump several databases with one command:

    mysqldump --databases database1 [database2 ...] > my_databases.sql

If all the databases are wanted, one can use:

    mysqldump --all-databases > all_databases.sql

### 4.8.6 `mysqlhotcopy`, Copying MySQL Databases and Tables

`mysqlhotcopy` is a Perl script that uses `LOCK TABLES`, `FLUSH TABLES` and `cp` or `scp` to quickly make a backup of a database. It's the fastest way to make a backup of the database or single tables, but it can only be run on the same machine where the database directories are.

    mysqlhotcopy db_name [/path/to/new_directory]

    mysqlhotcopy db_name_1 ... db_name_n /path/to/new_directory

    mysqlhotcopy db_name./regex/

`mysqlhotcopy` supports the following options:

`-?, --help`
  Display a help screen and exit
`-u, --user=#`
  User for database login
`-p, --password=#`
  Password to use when connecting to server
`-P, --port=#`
  Port to use when connecting to local server
`-S, --socket=#`
  Socket to use when connecting to local server
`--allowold`
  Don't abort if target already exists (rename it _old)
`--keepold`
  Don't delete previous (now renamed) target when done
`--noindices`
  Don't include full index files in copy to make the backup smaller and faster The indexes can later be reconstructed with `myisamchk -rq.`.
`--method=#`
  Method for copy (`cp` or `scp`).
`-q, --quiet`
  Be silent except for errors
`--debug`
  Enable debug
`-n, --dryrun`
  Report actions without doing them
`--regexp=#`
  Copy all databases with names matching regexp
`--suffix=#`
  Suffix for names of copied databases
`--checkpoint=#`
  Insert checkpoint entry into specified db.table
`--flushlog`
  Flush logs once all tables are locked.
`--tmpdir=#`
  Temporary directory (instead of /tmp).

You can use `perldoc mysqlhotcopy` to get more complete documentation for `mysqlhotcopy`.

`mysqlhotcopy` reads the groups `[client]` and `[mysqlhotcopy]` from the option files.

To be able to execute `mysqlhotcopy` you need write access to the backup directory, the `SELECT` privilege for the tables you are about to copy and the MySQL `RELOAD` privilege (to be able to execute `FLUSH TABLES`).

### 4.8.7 `mysqlimport`, Importing Data from Text Files

`mysqlimport` provides a command-line interface to the `LOAD DATA INFILE` SQL statement. Most options to `mysqlimport` correspond directly to the same options to `LOAD DATA INFILE`. See section 6.4.9 `LOAD DATA INFILE` Syntax.

`mysqlimport` is invoked like this:

    shell> mysqlimport [options] database textfile1 [textfile2 ...]

For each text file named on the command-line, `mysqlimport` strips any extension from the filename and uses the result to determine which table to import the file's contents into. For example, files named ``patient.txt'`, ``patient.text'`, and ``patient'` would all be imported into a table named `patient`.

`mysqlimport` supports the following options:

`-c, --columns=...`
  This option takes a comma-separated list of field names as an argument. The field list is used to create a proper `LOAD DATA INFILE` command, which is then passed to MySQL. See section 6.4.9 `LOAD DATA INFILE` Syntax.
`-C, --compress`
  Compress all information between the client and the server if both support compression.
`-#, --debug[=option_string]`
  Trace usage of the program (for debugging).
`-d, --delete`
  Empty the table before importing the text file.
`--fields-terminated-by=...`
`--fields-enclosed-by=...`
`--fields-optionally-enclosed-by=...`
`--fields-escaped-by=...`
`--lines-terminated-by=...`
  These options have the same meaning as the corresponding clauses for `LOAD DATA INFILE`. See section 6.4.9 `LOAD DATA INFILE` Syntax.
`-f, --force`
  Ignore errors. For example, if a table for a text file doesn't exist, continue processing any remaining files. Without `--force`, `mysqlimport` exits if a table doesn't exist.
`--help`
  Display a help message and exit.
`-h host_name, --host=host_name`
  Import data to the MySQL server on the named host. The default host is `localhost`.
`-i, --ignore`
  See the description for the `--replace` option.
`-l, --lock-tables`
  Lock all tables for writing before processing any text files. This ensures that all tables are synchronised on the server.
`-L, --local`
  Read input files from the client. By default, text files are assumed to be on the server if you connect to `localhost` (which is the default host).
`-pyour_pass, --password[=your_pass]`
  The password to use when connecting to the server. If you specify no ``=your_pass'` part, `mysqlimport` you will be prompted for a password.
`-P port_num, --port=port_num`
  The TCP/IP port number to use for connecting to a host. (This is used for connections to hosts other than `localhost`, for which Unix sockets are used.)
`-r, --replace`
  The `--replace` and `--ignore` options control handling of input records that duplicate existing records on unique key values. If you specify `--replace`, new rows replace existing rows that have the same unique key value. If you specify `--ignore`, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored.
`-s, --silent`
  Silent mode. Write output only when errors occur.
`-S /path/to/socket, --socket=/path/to/socket`
  The socket file to use when connecting to `localhost` (which is the default host).
`-u user_name, --user=user_name`
  The MySQL user name to use when connecting to the server. The default value is your Unix login name.
`-v, --verbose`
  Verbose mode. Print out more information what the program does.
`-V, --version`
  Print version information and exit.

Here is a sample run using `mysqlimport`:

    $ mysql --version
    mysql  Ver 9.33 Distrib 3.22.25, for pc-linux-gnu (i686)
    $ uname -a
    Linux xxx.com 2.2.5-15 #1 Mon Apr 19 22:21:09 EDT 1999 i586 unknown
    $ mysql -e 'CREATE TABLE imptest(id INT, n VARCHAR(30))' test
    $ ed
    a
    100     Max Sydow
    101     Count Dracula
    .
    w imptest.txt
    32
    q
    $ od -c imptest.txt
    0000000   1   0   0  \t   M   a   x       S   y   d   o   w  \n   1   0
    0000020   1  \t   C   o   u   n   t       D   r   a   c   u   l   a  \n
    0000040
    $ mysqlimport --local test imptest.txt
    test.imptest: Records: 2  Deleted: 0  Skipped: 0  Warnings: 0
    $ mysql -e 'SELECT * FROM imptest' test
    +------+---------------+
    | id   | n             |
    +------+---------------+
    |  100 | Max Sydow     |
    |  101 | Count Dracula |
    +------+---------------+

### 4.8.8 Showing Databases, Tables, and Columns

`mysqlshow` can be used to quickly look at which databases exist, their tables, and the table's columns.

With the `mysql` program you can get the same information with the `SHOW` commands. See section 4.5.6 `SHOW` Syntax.

`mysqlshow` is invoked like this:

    shell> mysqlshow [OPTIONS] [database [table [column]]]

- If no database is given, all matching databases are shown.
- If no table is given, all matching tables in the database are shown.
- If no column is given, all matching columns and column types in the table are shown.

Note that in newer MySQL versions, you only see those database/tables/columns for which you have some privileges.

If the last argument contains a shell or SQL wildcard (`*`, `?`, `%` or `_`) then only what's matched by the wildcard is shown. If a database contains underscore(s), those should be escaped with backslash (some Unix shells will require two), in order to get tables / columns properly. '*' are converted into SQL '%' wildcard and '?' into SQL '_' wildcard. This may cause some confusion when you try to display the columns for a table with a `_` as in this case `mysqlshow` only shows you the table names that match the pattern. This is easily fixed by adding an extra `%` last on the command-line (as a separate argument).

### 4.8.9 perror, Explaining Error Codes

For most system errors MySQL will, in addition to a internal text message, also print the system error code in one of the following styles: `message ... (errno: #)` or `message ... (Errcode: #)`.

You can find out what the error code means by either examining the documentation for your system or use the `perror` utility.

`perror` prints a description for a system error code, or an MyISAM/ISAM storage engine (table handler) error code.

`perror` is invoked like this:

    shell> perror [OPTIONS] [ERRORCODE [ERRORCODE...]]

    Example:

    shell> perror 13 64
    Error code  13:  Permission denied
    Error code  64:  Machine is not on the network

Note that the error messages are mostly system dependent!

### 4.8.10 How to Run SQL Commands from a Text File

The `mysql` client typically is used interactively, like this:

    shell> mysql database

However, it's also possible to put your SQL commands in a file and tell `mysql` to read its input from that file. To do so, create a text file ``text_file'` that contains the commands you wish to execute. Then invoke `mysql` as shown here:

    shell> mysql database < text_file

You can also start your text file with a `USE db_name` statement. In this case, it is unnecessary to specify the database name on the command line:

    shell> mysql < text_file

If you are already running `mysql`, you can execute a SQL script file using the `source` command:

    mysql> source filename;

For more information about batch mode, section 3.6 Using `mysql` in Batch Mode.

## 4.9 The MySQL Log Files

MySQL has several different log files that can help you find out what's going on inside `mysqld`:

Log file | Description
The error log | Problems encountering starting, running or stopping `mysqld`.
The isam log | Logs all changes to the ISAM tables. Used only for debugging the isam code.
The query log | Established connections and executed queries.
The update log | Deprecated: Stores all statements that changes data
The binary log | Stores all statements that changes something. Used also for replication
The slow log | Stores all queries that took more than `long_query_time` to execute or didn't use indexes.

All logs can be found in the `mysqld` data directory. You can force `mysqld` to reopen the log files (or in some cases switch to a new log) by executing `FLUSH LOGS`. See section 4.5.3 `FLUSH` Syntax.

### 4.9.1 The Error Log

`mysqld` writes all errors to the stderr, which the `safe_mysqld` script redirects to a file called `'hostname'.err`. (On Windows, `mysqld` writes this directly to ``\mysql\data\mysql.err'`.)

This contains information indicating when `mysqld` was started and stopped and also any critical errors found when running. If `mysqld` dies unexpectedly and `safe_mysqld` needs to restart `mysqld`, `safe_mysqld` will write a `restarted mysqld` row in this file. This log also holds a warning if `mysqld` notices a table that needs to be automatically checked or repaired.

On some operating systems, the error log will contain a stack trace for where `mysqld` died. This can be used to find out where `mysqld` died. See section E.1.4 Using a Stack Trace.

### 4.9.2 The General Query Log

If you want to know what happens within `mysqld`, you should start it with `--log[=file]`. This will log all connections and queries to the log file (by default named ``'hostname'.log'`). This log can be very useful when you suspect an error in a client and want to know exactly what `mysqld` thought the client sent to it.

Older versions of the `mysql.server` script (from MySQL 3.23.4 to 3.23.8) pass `safe_mysqld` a `--log` option (enable general query log). If you need better performance when you start using MySQL in a production environment, you can remove the `--log` option from `mysql.server` or change it to `--log-bin`. See section 4.9.4 The Binary Update Log.

The entries in this log are written as `mysqld` receives the questions. This may be different from the order in which the statements are executed. This is in contrast to the update log and the binary log which are written after the query is executed, but before any locks are released.

### 4.9.3 The Update Log

Note: the update log is replaced by the binary log. See section 4.9.4 The Binary Update Log. With this you can do anything that you can do with the update log.

When started with the `--log-update[=file_name]` option, `mysqld` writes a log file containing all SQL commands that update data. If no filename is given, it defaults to the name of the host machine. If a filename is given, but it doesn't contain a path, the file is written in the data directory. If ``file_name'` doesn't have an extension, `mysqld` will create log file names like so: ``file_name.###'`, where `###` is a number that is incremented each time you execute `mysqladmin refresh`, execute `mysqladmin flush-logs`, execute the `FLUSH LOGS` statement, or restart the server.

Note: for the above scheme to work, you must not create your own files with the same filename as the update log + some extensions that may be regarded as a number, in the directory used by the update log!

If you use the `--log` or `-l` options, `mysqld` writes a general log with a filename of ``hostname.log'`, and restarts and refreshes do not cause a new log file to be generated (although it is closed and reopened). In this case you can copy it (on Unix) by doing:

    mv hostname.log hostname-old.log
    mysqladmin flush-logs
    cp hostname-old.log to-backup-directory
    rm hostname-old.log

Update logging is smart because it logs only statements that really update data. So an `UPDATE` or a `DELETE` with a `WHERE` that finds no rows is not written to the log. It even skips `UPDATE` statements that set a column to the value it already has.

The update logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order.

If you want to update a database from update log files, you could do the following (assuming your update logs have names of the form ``file_name.###'`):

    shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql

`ls` is used to get all the log files in the right order.

This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash.

### 4.9.4 The Binary Update Log

The intention is that the binary log should replace the update log, so we recommend you to switch to this log format as soon as possible!

The binary log contains all information that is available in the update log in a more efficient format. It also contains information about how long each query took that updated the database. It doesn't contain queries that don't modify any data. If you want to log all queries (for example to find a problem query) you should use the general query log. See section 4.9.2 The General Query Log.

The binary log is also used when you are replicating a slave from a master. See section 4.10 Replication in MySQL.

When started with the `--log-bin[=file_name]` option, `mysqld` writes a log file containing all SQL commands that update data. If no file name is given, it defaults to the name of the host machine followed by `-bin`. If file name is given, but it doesn't contain a path, the file is written in the data directory.

If you supply an extension to `--log-bin=filename.extension`, the extension will be silenty removed.

To the binary log filename `mysqld` will append an extension that is a number that is incremented each time you execute `mysqladmin refresh`, execute `mysqladmin flush-logs`, execute the `FLUSH LOGS` statement or restart the server. A new binary log will also automatically be created when it reaches `max_binlog_size`. You can delete all not active binary log files with the `RESET MASTER` command. See section 4.5.4 `RESET` Syntax.

You can use the following options to `mysqld` to affect what is logged to the binary log:

Option | Description
`binlog-do-db=database_name` | Tells the master it should log updates for the specified database, and exclude all others not explicitly mentioned. (Example: `binlog-do-db=some_database`)
`binlog-ignore-db=database_name` | Tells the master that updates to the given database should not be logged to the binary log (Example: `binlog-ignore-db=some_database`)

To be able to know which different binary log files have been used, `mysqld` will also create a binary log index file that contains the name of all used binary log files. By default this has the same name as the binary log file, with the extension `'.index'`. You can change the name of the binary log index file with the `--log-bin-index=[filename]` option.

If you are using replication, you should not delete old binary log files until you are sure that no slave will ever need to use them. One way to do this is to do `mysqladmin flush-logs` once a day and then remove any logs that are more than 3 days old.

You can examine the binary log file with the `mysqlbinlog` command. For example, you can update a MySQL server from the binary log as follows:

    shell> mysqlbinlog log-file | mysql -h server_name

You can also use the `mysqlbinlog` program to read the binary log directly from a remote MySQL server!

`mysqlbinlog --help` will give you more information of how to use this program!

If you are using `BEGIN [WORK]` or `SET AUTOCOMMIT=0`, you must use the MySQL binary log for backups instead of the old update log.

The binary logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order.

Updates to non-transactional tables are stored in the binary log immediately after execution. For transactional tables such as `BDB` or `InnoDB` tables, all updates (`UPDATE`, `DELETE` or `INSERT`) that change tables are cached until a `COMMIT` command is sent to the server. At this point `mysqld` writes the whole transaction to the binary log before the `COMMIT` is executed. very thread will, on start, allocate a buffer of `binlog_cache_size` to buffer queries. If a query is bigger than this, the thread will open a temporary file to store the transaction. The temporary file will be deleted when the thread ends.

The `max_binlog_cache_size` (default 4G) can be used to restrict the total size used to cache a multi-query transaction. If a transaction is bigger than this it will fail and roll back.

If you are using the update or binary log, concurrent inserts will be converted to normal inserts when using `CREATE ... SELECT` or `INSERT ... SELECT`. This is to ensure that you can recreate an exact copy of your tables by applying the log on a backup.

### 4.9.5 The Slow Query Log

When started with the `--log-slow-queries[=file_name]` option, `mysqld` writes a log file containing all SQL commands that took more than `long_query_time` to execute. The time to get the initial table locks are not counted as execution time.

The slow query log is logged after the query is executed and after all locks has been released. This may be different from the order in which the statements are executed.

If no file name is given, it defaults to the name of the host machine suffixed with `-slow.log`. If a filename is given, but doesn't contain a path, the file is written in the data directory.

The slow query log can be used to find queries that take a long time to execute and are thus candidates for optimisation. With a large log, that can become a difficult task. You can pipe the slow query log through the `mysqldumpslow` command to get a summary of the queries which appear in the log.

You are using `--log-long-format` then also queries that are not using indexes are printed. See section 4.1.1 `mysqld` Command-line Options.

### 4.9.6 Log File Maintenance

The MySQL Server can create a number of different log files, which make it easy to see what is going on. See section 4.9 The MySQL Log Files. One must however regularly clean up these files, to ensure that the logs don't take up too much disk space.

When using MySQL with log files, you will, from time to time, want to remove/backup old log files and tell MySQL to start logging on new files. See section 4.4.1 Database Backups.

On a Linux (`Redhat`) installation, you can use the `mysql-log-rotate` script for this. If you installed MySQL from an RPM distribution, the script should have been installed automatically. Note that you should be careful with this if you are using the log for replication!

On other systems you must install a short script yourself that you start from `cron` to handle log files.

You can force MySQL to start using new log files by using `mysqladmin flush-logs` or by using the SQL command `FLUSH LOGS`. If you are using MySQL Version 3.21 you must use `mysqladmin refresh`.

The above command does the following:

- If standard logging (`--log`) or slow query logging (`--log-slow-queries`) is used, closes and reopens the log file (``mysql.log'` and ```hostname`-slow.log'` as default).
- If update logging (`--log-update`) is used, closes the update log and opens a new log file with a higher sequence number.

If you are using only an update log, you only have to flush the logs and then move away the old update log files to a backup. If you are using the normal logging, you can do something like:

    shell> cd mysql-data-directory
    shell> mv mysql.log mysql.old
    shell> mysqladmin flush-logs

and then take a backup and remove ``mysql.old'`.

## 4.10 Replication in MySQL

This section describes the various replication features in MySQL. It serves as a reference to the options available with replication. You will be introduced to replication and learn how to implement it. Toward the end, there are some frequently asked questions and descriptions of problems and how to solve them.

We suggest that you visit our website at http://www.mysql.com/ often and read updates to this section. Replication is constantly being improved, and we update the manual frequently with the most current information.

### 4.10.1 Introduction

One way replication can be used is to increase both robustness and speed. For robustness you can have two systems and can switch to the backup if you have problems with the master. The extra speed is achieved by sending a part of the non-updating queries to the replica server. Of course this only works if non-updating queries dominate, but that is the normal case.

Starting in Version 3.23.15, MySQL supports one-way replication internally. One server acts as the master, while the other acts as the slave. Note that one server could play the roles of master in one pair and slave in the other. The master server keeps a binary log of updates (see section 4.9.4 The Binary Update Log) and an index file to binary logs to keep track of log rotation. The slave, upon connecting, informs the master where it left off since the last successfully propagated update, catches up on the updates, and then blocks and waits for the master to notify it of the new updates.

Note that if you are replicating a database, all updates to this database should be done through the master!

Another benefit of using replication is that one can get live backups of the system by doing a backup on a slave instead of doing it on the master. See section 4.4.1 Database Backups.

### 4.10.2 Replication Implementation Overview

MySQL replication is based on the server keeping track of all changes to your database (updates, deletes, etc) in the binary log (see section 4.9.4 The Binary Update Log) and the slave server(s) reading the saved queries from the master server's binary log so that the slave can execute the same queries on its copy of the data.

It is very important to realise that the binary log is simply a record starting from a fixed point in time (the moment you enable binary logging). Any slaves which you set up will need copies of all the data from your master as it existed the moment that you enabled binary logging on the master. If you start your slaves with data that doesn't agree with what was on the master when the binary log was started, your slaves may fail.

Please see the following table for an indication of master-slave compatibility between different versions. With regard to version 4.0, we recommend using same version on both sides.

|  | Master | Master | Master | Master
|  | 3.23.33 and up | 4.0.0 | 4.0.1 | 4.0.3 and up
Slave | 3.23.33 and up | yes | no | no | no
Slave | 4.0.0 | no | yes | no | no
Slave | 4.0.1 | yes | no | yes | no
Slave | 4.0.3 and up | yes | no | no | yes

Note: MySQL Version 4.0.2 is not recommended for replication.

Starting from 4.0.0, one can use `LOAD DATA FROM MASTER` to set up a slave. Be aware that `LOAD DATA FROM MASTER` currently works only if all the tables on the master are `MyISAM` type, and will acquire a global read lock, so no writes are possible while the tables are being transferred from the master. This limitation is of a temporary nature, and is due to the fact that we have not yet implemented hot lock-free table backup. It will be removed in the future 4.0 branch versions once we implement hot backup enabling `LOAD DATA FROM MASTER` to work without blocking master updates.

Due to the above limitation, we recommend that at this point you use `LOAD DATA FROM MASTER` only if the dataset on the master is relatively small, or if a prolonged read lock on the master is acceptable. While the actual speed of `LOAD DATA FROM MASTER` may vary from system to system, a good rule for a rough estimate of how long it is going to take is 1 second per 1 MB of the datafile. You will get close to the estimate if both master and slave are equivalent to 700 MHz Pentium, are connected through 100 MBit/s network, and your index file is about half the size of your data file. Of course, your mileage will vary from system to system, the above rule just gives you a rough order of magnitude estimate.

Once a slave is properly configured and running, it will simply connect to the master and wait for updates to process. If the master goes away or the slave loses connectivity with your master, it will keep trying to connect every `master-connect-retry` seconds until it is able to reconnect and resume listening for updates.

Each slave keeps track of where it left off. The master server has no knowledge of how many slaves there are or which ones are up-to-date at any given time.

The next section explains the master/slave setup process in more detail.

### 4.10.3 How To Set Up Replication

Here is a quick description of how to set up complete replication on your current MySQL server. It assumes you want to replicate all your databases and have not configured replication before. You will need to shutdown your master server briefly to complete the steps outlined here.

While this method is the most straightforward way to set up a slave, it is not the only one. For example, if you already have a snapshot of the master, and the master already has server id set and binary logging enabled, you can set up a slave without shutting the master down or even blocking the updates. For more details, please see section 4.10.7 Replication FAQ.

If you want to become a real MySQL replication guru, we suggest that you begin by studying, pondering, and trying all commands mentioned in section 4.10.6 SQL Commands Related to Replication. You should also familiarise yourself with replication startup options in ``my.cnf'` in section 4.10.5 Replication Options in ``my.cnf'`.

1. Make sure you have a recent version of MySQL installed on the master and slave(s). Use Version 3.23.29 or higher. Previous releases used a different binary log format and had bugs which have been fixed in newer releases. Please, do not report bugs until you have verified that the problem is present in the latest release.
2. Set up special a replication user on the master with the `FILE` (in MySQL versions older than 4.0.2) or `REPLICATION SLAVE` privilege in newer MySQL versions. You must also have given permission to connect from all the slaves. If the user is only doing replication (which is recommended), you don't need to grant any additional privileges. For example, to create a user named `repl` which can access your master from any host, you might use this command:

       mysql> GRANT FILE ON *.* TO repl@"%" IDENTIFIED BY '<password>';

3. If you are using MyISAM tables, flush all the tables and block write queries by executing `FLUSH TABLES WITH READ LOCK` command.

       mysql> FLUSH TABLES WITH READ LOCK;

   and then take a snapshot of the data on your master server. The easiest way to do this (on Unix) is to simply use tar to produce an archive of your entire data directory. The exact data directory location depends on your installation.

       tar -cvf /tmp/mysql-snapshot.tar /path/to/data-dir

   Windows users can use `WinZIP` or similar software to create an archive of the data directory. After or during the process of taking a snapshot, read the value of the current binary log name and the offset on the master:

       mysql > SHOW MASTER STATUS;
       +---------------+----------+--------------+-------------------------------+
       | File          | Position | Binlog_do_db | Binlog_ignore_db              |
       +---------------+----------+--------------+-------------------------------+
       | mysql-bin.003 | 73       | test,bar     | foo,manual,sasha_likes_to_run |
       +---------------+----------+--------------+-------------------------------+
       1 row in set (0.06 sec)

   The `File` column shows the name of the log, while `Position` shows the offset. n the above example, the binary log value is `mysql-bin.003` and the offset is 73. Record the values - you will need to use them later when you are setting up the slave. Once you have taken the snapshot and recorded the log name and offset, you can re-enable write activity on the master:

       mysql> UNLOCK TABLES;

   If you are using InnoDB tables, ideally you should use the InnoDB Hot Backup tool that is available to those who purchase MySQL commercial licenses, support, or the backup tool itself. It will take a consistent snapshot without acquiring any locks on the master server, and record the log name and offset corresponding to the snapshot to be later used on the slave. More information about the tool is avaliable at http://www.innodb.com/hotbackup.html. Without the hot backup tool, the quickest way to take a snapshot of InnoDB tables is to shut the master server down and copy the data files, the logs, and the table definition files (`.frm`). To record the current log file name and offset, you should do the following before you shut down the server:

       mysql> FLUSH TABLES WITH READ LOCK;
       mysql> SHOW MASTER STATUS;

   And then record the log name and the offset from the output of `SHOW MASTER STATUS` as was shown earlier. Once you have recorded the log name and the offset, shut the server down without unlocking the tables to make sure it goes down with the snapshot corresponding to the current log file and offset:

       shell> mysqladmin -uroot shutdown

   If the master has been previously running without `log-bin` enabled, the values of log name and position will be empty when you run `SHOW MASTER STATUS`. In that case, record empty string ('') for the log name, and 4 for the offset.
4. Make sure that ``my.cnf'` on the master has `log-bin` if it is not there already and `server-id=unique number` in the `[mysqld]` section. If those options are not present, add them and restart the server. It is very important that the id of the slave is different from the id of the master. Think of `server-id` as something similar to the IP address - it uniquely identifies the server instance in the community of replication partners.

       [mysqld]
       log-bin
       server-id=1

5. Add the following to ``my.cnf'` on the slave(s):

       server-id=<some unique number between 1 and 2^32-1 that is different from
        that of the master>

   replacing the values in <> with what is relevant to your system. `server-id` must be different for each server participating in replication. If you don't specify a server-id, it will be set to 1 if you have not defined `master-host`, else it will be set to 2. Note that in the case of `server-id` omission the master will refuse connections from all slaves, and the slave will refuse to connect to a master. Thus, omitting `server-id` is only good for backup with a binary log.
6. While the slave is running, make it forget about the old replication configuration if it has been replicating previously:

       mysql> RESET SLAVE;

7. Copy the snapshot data into your data directory on your slave(s). Make sure that the privileges on the files and directories are correct. The user which MySQL runs as needs to be able to read and write to them, just as on the master.
8. Restart the slave(s).
9. Once the slave comes up, execute the following command:

       mysql> CHANGE MASTER TO MASTER_HOST='<master host name>',
        MASTER_USER='<replication user name>',
        MASTER_PASSWORD='<replication password>',
        MASTER_LOG_FILE='<recorded log file name>',
        MASTER_LOG_POS=<recorded log offset>;

   replacing the values in <> with the actual values relevant to your system.
10. Start the slave thread:

        mysql> SLAVE START;

After you have done the above, the slave(s) should connect to the master and catch up on any updates which happened since the snapshot was taken.

If you have forgotten to set `server-id` for the slave you will get the following error in the error log file:

    Warning: one should set server_id to a non-0 value if master_host is set.
    The server will not act as a slave.

If you have forgotten to do this for the master, the slaves will not be able to connect to the master.

If a slave is not able to replicate for any reason, you will find error messages in the error log on the slave.

Once a slave is replicating, you will find a file called ``master.info'` in the same directory as your error log. The ``master.info'` file is used by the slave to keep track of how much of the master's binary log it has processed. Do not remove or edit the file, unless you really know what you are doing. Even in that case, it is preferred that you use `CHANGE MASTER TO` command.

Now that you have a snapshot, you can use it to set up other slaves. To do so, follow the slave portion of the procedure described above. You do not need to take another snapshot of the master.

### 4.10.4 Replication Features and Known Problems

Here is an explanation of what is supported and what is not:

- Replication will be done correctly with `AUTO_INCREMENT`, `LAST_INSERT_ID()`, and `TIMESTAMP` values.
- `RAND()` in updates does not replicate properly. Use `RAND(some_non_rand_expr)` if you are replicating updates with `RAND()`. You can, for example, use `UNIX_TIMESTAMP()` for the argument to `RAND()`.
- You have to use the same character set (`--default-character-set`) on the master and the slave. If not, you may get duplicate key errors on the slave, because a key that is regarded as unique in the master character set may not be unique in the slave character set.
- In 3.23, `LOAD DATA INFILE` will be handled properly as long as the file still resides on the master server at the time of update propagation. `LOAD LOCAL DATA INFILE` will be skipped. In 4.0, this limitation is not present - all forms of `LOAD DATA INFILE` are properly replicated.
- Update queries that use user variables are not replication-safe (yet).
- `FLUSH` commands are not stored in the binary log and are because of this not replicated to the slaves. This is not normally a problem as `FLUSH` doesn't change anything. This does however mean that if you update the MySQL privilege tables directly without using the `GRANT` statement and you replicate the `mysql` privilege database, you must do a `FLUSH PRIVILEGES` on your slaves to put the new privileges into effect.
- Temporary tables starting in 3.23.29 are replicated properly with the exception of the case when you shut down slave server ( not just slave thread), you have some temporary tables open, and they are used in subsequent updates. To deal with this problem shutting down the slave, do `SLAVE STOP`, check `Slave_open_temp_tables` variable to see if it is 0, then issue `mysqladmin shutdown`. If the number is not 0, restart the slave thread with `SLAVE START` and see if you have better luck next time. There will be a cleaner solution, but it has to wait until version 4.0. In earlier versions temporary tables are not replicated properly - we recommend that you either upgrade, or execute `SET SQL_LOG_BIN=0` on your clients before all queries with temp tables.
- MySQL only supports one master and many slaves. In 4.x, we will add a voting algorithm to automatically change master if something goes wrong with the current master. We will also introduce 'agent' processes to help do load balancing by sending select queries to different slaves.
- Starting in Version 3.23.26, it is safe to connect servers in a circular master-slave relationship with `log-slave-updates` enabled. Note, however, that many queries will not work right in this kind of setup unless your client code is written to take care of the potential problems that can happen from updates that occur in different sequence on different servers. This means that you can do a setup like the following:

      A -> B -> C -> A

  This setup will only works if you only do non conflicting updates between the tables. In other words, if you insert data in A and C, you should never insert a row in A that may have a conflicting key with a row insert in C. You should also not update the sam rows on two servers if the order in which the updates are applied matters. Note that the log format has changed in Version 3.23.26 so that pre-3.23.26 slaves will not be able to read it.
- If the query on the slave gets an error, the slave thread will terminate, and a message will appear in the ``.err'` file. You should then connect to the slave manually, fix the cause of the error (for example, non-existent table), and then run the `SLAVE START` SQL command (available starting in Version 3.23.16). In Version 3.23.15, you will have to restart the server.
- If connection to the master is lost, the slave will retry immediately, and then in case of failure every `master-connect-retry` (default 60) seconds. Because of this, it is safe to shut down the master, and then restart it after a while. The slave will also be able to deal with network connectivity outages.
- Shutting down the slave (cleanly) is also safe, as it keeps track of where it left off. Unclean shutdowns might produce problems, especially if disk cache was not synced before the system died. Your system fault tolerance will be greatly increased if you have a good UPS.
- If the master is listening on a non-standard port, you will also need to specify this with `master-port` parameter in ``my.cnf'` .
- In Version 3.23.15, all of the tables and databases will be replicated. Starting in Version 3.23.16, you can restrict replication to a set of databases with `replicate-do-db` directives in ``my.cnf'` or just exclude a set of databases with `replicate-ignore-db`. Note that up until Version 3.23.23, there was a bug that did not properly deal with `LOAD DATA INFILE` if you did it in a database that was excluded from replication.
- Starting in Version 3.23.16, `SET SQL_LOG_BIN = 0` will turn off replication (binary) logging on the master, and `SET SQL_LOG_BIN = 1` will turn it back on -- you must have the `SUPER` (in MySQL 4.0.2 and above) or `PROCESS` (in older MySQL versions) privilege to do this.
- Starting in Version 3.23.19, you can clean up stale replication leftovers when something goes wrong and you want a clean start with `FLUSH MASTER` and `FLUSH SLAVE` commands. In Version 3.23.26 we have renamed them to `RESET MASTER` and `RESET SLAVE` respectively to clarify what they do. The old `FLUSH` variants still work, though, for compatibility.
- Starting in Version 3.23.23, you can change masters and adjust log position with `CHANGE MASTER TO`.
- Starting in Version 3.23.23, you tell the master that updates in certain databases should not be logged to the binary log with `binlog-ignore-db`.
- Starting in Version 3.23.26, you can use `replicate-rewrite-db` to tell the slave to apply updates from one database on the master to the one with a different name on the slave.
- Starting in Version 3.23.28, you can use `PURGE MASTER LOGS TO 'log-name'` to get rid of old logs while the slave is running. This will remove all old logs before, but not including `'log-name'`.
- Due to the non-transactional nature of MyISAM tables, it is possible to have a query that will only partially update a table and return an error code. This can happen, for example, on a multi-row insert that has one row violating a key constraint, or if a long update query is killed after updating some of the rows. If that happens on the master, the slave thread will exit and wait for the DBA to decide what to do about it unless the error code is legitimate and the query execution results in the same error code. If this error code validation behaviour is not desirable, some ( or all) errors could be masked out with `slave-skip-errors` option starting in Version 3.23.47.
- While individual tables can be excluded from replication with `replicate-do-table`/`replicate-ignore-table` or `replicate-wild-do-table`/`replicate-wild-ignore-table`, there are currently some design deficiencies that in some rather rare cases produce unexpected results. The replication protocol does not inform the slave explicitly which tables are going to be modified by the query -- so the slave has to parse the query to know this. To avoid redundant parsing for queries that will end up actually being executed, table exclusion is currently implemented by sending the query to the standard MySQL parser, which will short-circuit the query and report success if it detects that the table should be ignored. In addition to several inefficiencies, this approach is also more bug prone, and there are two known bugs as of Version 3.23.49 -- because the parser automatically opens the table when parsing some queries the ignored table has to exist on the slave. The other bug is that if the ignored table gets partially updated, the slave thread will not notice that the table actually should have been ignored and will suspend the replication process. While the above bugs are conceptually very simple to fix, we have not yet found a way to do this without a significant code change that would compromise the stability status of 3.23 branch. There exists a workaround for both if in the rare case it happens to affect your application -- use `slave-skip-errors`.

### 4.10.5 Replication Options in ``my.cnf'`

If you are using replication, we recommend that you use MySQL Version 3.23.33 or later. Older versions work, but they do have some bugs and are missing some features. Some of the options mentioned here may not be available in your version if it is not the most recent one. For all options specific to the 4.0 branch, there is a note indicating so. Otherwise, if you discover that the option you are interested in is not available in your 3.23 version, and you really need it, please upgrade to the most recent 3.23 branch.

Please be aware that 4.0 branch is still in alpha, so some things may not be working as smoothly as you would like. If you really would like to try the new features of 4.0, we recommend you do it in such a way that in case there is a problem your mission critical applications will not be disrupted.

On both master and slave you need to use the `server-id` option. This sets a unique replication id. You should pick a unique value in the range between 1 to 2^32-1 for each master and slave. Example: `server-id=3`

The following table describes the options you can use for the `MASTER`:

Option | Description
`log-bin=filename` | Write to a binary update log to the specified location. Note that if you give it a parameter with an extension (for example, `log-bin=/mysql/logs/replication.log` ) versions up to 3.23.24 will not work right during replication if you do `FLUSH LOGS` . The problem is fixed in Version 3.23.25. If you are using this kind of log name, `FLUSH LOGS` will be ignored on binlog. To clear the log, run `FLUSH MASTER`, and do not forget to run `FLUSH SLAVE` on all slaves. In Versions 3.23.26 and later, you should use `RESET MASTER` and `RESET SLAVE`
`log-bin-index=filename` | Because the user could issue the `FLUSH LOGS` command, we need to know which log is currently active and which ones have been rotated out and in what sequence. This information is stored in the binary log index file. The default is ```hostname`.index'`. You should not need to change this. Example: `log-bin-index=db.index`
`sql-bin-update-same` | If set, setting `SQL_LOG_BIN` to a value will automatically set `SQL_LOG_UPDATE` to the same value and vice versa.
`binlog-do-db=database_name` | Tells the master that it should log updates to the binary log if the current database is `database_name`. All other databases are ignored. Note that if you use this, you should ensure that you do updates only in the current database. Example: `binlog-do-db=sales`
`binlog-ignore-db=database_name` | Tells the master that updates where the current database is `database_name` should not be stored in the binary log. Note that if you use this, you should ensure that you do updates only in the current database. Example: `binlog-ignore-db=accounting`

The following table describes the options you can use for the `SLAVE`:

Option | Description
`master-host=host` | Master hostname or IP address for replication. If not set, the slave thread will not be started. Note that the setting of `master-host` will be ignored if there exists a valid ``master.info'` file. Probably a better name for this options would have been something like `bootstrap-master-host`, but it is too late to change now. Example: `master-host=db-master.mycompany.com`
`master-user=username` | The username the slave thread will use for authentication when connecting to the master. The user must have the `FILE` privilege. If the master user is not set, user `test` is assumed. The value in ``master.info'` will take precedence if it can be read. Example: `master-user=scott`
`master-password=password` | The password the slave thread will authenticate with when connecting to the master. If not set, an empty password is assumed.The value in ``master.info'` will take precedence if it can be read. Example: `master-password=tiger`
`master-port=portnumber` | The port the master is listening on. If not set, the compiled setting of `MYSQL_PORT` is assumed. If you have not tinkered with `configure` options, this should be 3306. The value in ``master.info'` will take precedence if it can be read. Example: `master-port=3306`
`master-connect-retry=seconds` | The number of seconds the slave thread will sleep before retrying to connect to the master in case the master goes down or the connection is lost. Default is 60. Example: `master-connect-retry=60`
`master-ssl` | Available after 4.0.0. Turn SSL on for replication. Be warned that is this is a relatively new feature. Example: `master-ssl`
`master-ssl-key` | Available after 4.0.0. Master SSL keyfile name. Only applies if you have enabled `master-ssl`. Example: `master-ssl-key=SSL/master-key.pem`
`master-ssl-cert` | Available after 4.0.0. Master SSL certificate file name. Only applies if you have enabled `master-ssl`. Example: `master-ssl-key=SSL/master-cert.pem`
`master-info-file=filename` | The location of the file that remembers where we left off on the master during the replication process. The default is ``master.info'` in the data directory. You should not need to change this. Example: `master-info-file=master.info`
`report-host` | Available after 4.0.0. Hostname or IP of the slave to be reported to to the master during slave registration. Will appear in the output of `SHOW SLAVE HOSTS`. Leave unset if you do not want the slave to register itself with the master. Note that it is not sufficient for the master to simply read the IP of the slave off the socket once the slave connects. Due to `NAT` and other routing issues, that IP may not be valid for connecting to the slave from the master or other hosts. Example: `report-host=slave1.mycompany.com`
`report-port` | Available after 4.0.0. Port for connecting to slave reported to the master during slave registration. Set it only if the slave is listening on a non-default port or if you have a special tunnel from the master or other clients to the slave. If not sure, leave this option unset.
`replicate-do-table=db_name.table_name` | Tells the slave thread to restrict replication to the specified table. To specify more than one table, use the directive multiple times, once for each table. This will work for cross-database updates, in contrast to `replicate-do-db`. Example: `replicate-do-table=some_db.some_table`
`replicate-ignore-table=db_name.table_name` | Tells the slave thread to not replicate any command that updates the specified table (even if any other tables may be update by the same command). To specify more than one table to ignore, use the directive multiple times, once for each table. This will work for cross-datbase updates, in contrast to `replicate-ignore-db`. Example: `replicate-ignore-table=db_name.some_table`
`replicate-wild-do-table=db_name.table_name` | Tells the slave thread to restrict replication to queries where any of the updated tables match the specified wildcard pattern. To specify more than one table, use the directive multiple times, once for each table. This will work for cross-database updates. Example: `replicate-wild-do-table=foo%.bar%` will replicate only updates that uses a table in any databases that start with `foo` and whose table names start with `bar`.
`replicate-wild-ignore-table=db_name.table_name` | Tells the slave thread to not replicate a query where any table matches the given wildcard pattern. To specify more than one table to ignore, use the directive multiple times, once for each table. This will work for cross-database updates. Example: `replicate-wild-ignore-table=foo%.bar%` will not do updates to tables in databases that start with `foo` and whose table names start with `bar`.
`replicate-ignore-db=database_name` | Tells the slave thread to not replicate any command where the current database is `database_name`. To specify more than one database to ignore, use the directive multiple times, once for each database. You should not use this directive if you are using cross table updates and you don't want these update to be replicated. The main reason for this behaviour is that it's hard from the command alone know if a query should be replicated or not; For example if you are using multi-table-delete or multi-table-update commands in MySQL 4.x that goes across multiple databases. It's also very fast to just check the current database, as this only has to be done once at connect time or when the database changes. If you need cross database updates to work, make sure you have 3.23.28 or later, and use `replicate-wild-ignore-table=db_name.%`. Example: `replicate-ignore-db=some_db`
`replicate-do-db=database_name` | Tells the slave thread to restrict replication to commands where the current database is `database_name`. To specify more than one database, use the directive multiple times, once for each database. Note that this will not replicate cross-database queries such as `UPDATE some_db.some_table SET foo='bar'` while having selected a different or no database. If you need cross database updates to work, make sure you have 3.23.28 or later, and use `replicate-wild-do-table=db_name.%`. Example: `replicate-do-db=some_db`
`log-slave-updates` | Tells the slave to log the updates from the slave thread to the binary log. Off by default. You will need to turn it on if you plan to daisy-chain the slaves.
`replicate-rewrite-db=from_name->to_name` | Updates to a database with a different name than the original. Example: `replicate-rewrite-db=master_db_name->slave_db_name`
`slave-skip-errors= [err_code1,err_code2,... | all]` | Available only in 3.23.47 and later. Tells the slave thread to continue replication when a query returns an error from the provided list. Normally, replication will discontinue when an error is encountered, giving the user a chance to resolve the inconsistency in the data manually. Do not use this option unless you fully understand why you are getting the errors. If there are no bugs in your replication setup and client programs, and no bugs in MySQL itself, you should never get an abort with error. Indiscriminate use of this option will result in slaves being hopelessly out of sync with the master and you having no idea how the problem happened. For error codes, you should use the numbers provided by the error message in your slave error log and in the output of `SHOW SLAVE STATUS`. Full list of error messages can be found in the source distribution in ``Docs/mysqld_error.txt'`. You can (but should not) also use a very non-recommended value of `all` which will ignore all error messages and keep barging along regardless. Needless to say, if you use it, we make no promises regarding your data integrity. Please do not complain if your data on the slave is not anywhere close to what it is on the master in this case -- you have been warned. Example: `slave-skip-errors=1062,1053` or `slave-skip-errors=all`
`skip-slave-start` | Tells the slave server not to start the slave on the startup. The user can start it later with `SLAVE START`.
`slave_compressed_protocol=#` | If 1, then use compression on the slave/client protocol if both slave and master support this.
`slave_net_timeout=#` | Number of seconds to wait for more data from the master before aborting the read.

### 4.10.6 SQL Commands Related to Replication

Replication can be controlled through the SQL interface. Here is the summary of commands:

Command | Description
`SLAVE START` | Starts the slave thread. As of MySQL 4.0.2, you can add `IO_THREAD` or `SQL_THREAD` options to the statement to start the I/O thread or the SQL thread. The I/O thread reads queries from the master server and stores them in the relay log. The SQL thread reads the relay log and executes the queries. (Slave)
`SLAVE STOP` | Stops the slave thread. Like `SLAVE START`, this statement may be used with `IO_THREAD` and `SQL_THREAD` options. (Slave)
`SET SQL_LOG_BIN=0` | Disables update logging if the user has the `SUPER` privilege. Ignored otherwise. (Master)
`SET SQL_LOG_BIN=1` | Re-enables update logging if the user has the `SUPER` privilege. Ignored otherwise. (Master)
`SET GLOBAL SQL_SLAVE_SKIP_COUNTER=n` | Skip the next `n` events from the master. Only valid when the slave thread is not running, otherwise, gives an error. Useful for recovering from replication glitches.
`RESET MASTER` | Deletes all binary logs listed in the index file, resetting the binlog index file to be empty. In pre-3.23.26 versions, use `FLUSH MASTER`. (Master)
`RESET SLAVE` | Makes the slave forget its replication position in the master logs. In pre 3.23.26 versions the command was called `FLUSH SLAVE`. (Slave)
`LOAD TABLE tblname FROM MASTER` | Downloads a copy of the table from master to the slave. Implemented mainly for debugging of `LOAD DATA FROM MASTER`, but some ``gourmet'' users might find it useful for other things. Do not use it if you consider yourself the average ``non-hacker'' type user. (Slave)
`LOAD DATA FROM MASTER` | Available starting in 4.0.0. Takes a snapshot of the master and copies it to the slave. Updates the values of `MASTER_LOG_FILE` and `MASTER_LOG_POS` so that the slave will start replicating from the correct position. Will honor table and database exclusion rules specified with `replicate-*` options. So far works only with `MyISAM` tables and acquires a global read lock on the master while taking the snapshot. In the future it is planned to make it work with `InnoDB` tables and to remove the need for global read lock using the non-blocking online backup feature. Note that `LOAD DATA FROM MASTER` does NOT copy of any tables in the `mysql` database. This is to make it easy to have different users and privileges on the master and and the slave.

`CHANGE MASTER TO master_def_list`

Changes the master parameters to the values specified in `master_def_list` and restarts the slave thread. `master_def_list` is a comma-separated list of `master_def` where `master_def` is one of the following: `MASTER_HOST`, `MASTER_USER`, `MASTER_PASSWORD`, `MASTER_PORT`, `MASTER_CONNECT_RETRY`, `MASTER_LOG_FILE`, `MASTER_LOG_POS`. For example:

    CHANGE MASTER TO
      MASTER_HOST='master2.mycompany.com',
      MASTER_USER='replication',
      MASTER_PASSWORD='bigs3cret',
      MASTER_PORT=3306,
      MASTER_LOG_FILE='master2-bin.001',
      MASTER_LOG_POS=4;

You only need to specify the values that need to be changed. The values that you omit will stay the same with the exception of when you change the host or the port. In that case, the slave will assume that since you are connecting to a different host or a different port, the master is different. Therefore, the old values of log and position are not applicable anymore, and will automatically be reset to an empty string and 0, respectively (the start values). Note that if you restart the slave, it will remember its last master. If this is not desirable, you should delete the ``master.info'` file before restarting, and the slave will read its master from ``my.cnf'` or the command-line. This command is useful for setting up a slave when you have the snapshot of the master and have recorded the log and the offset on the master that the snapshot corresponds to. You can run `CHANGE MASTER TO MASTER_LOG_FILE='log_name_on_master', MASTER_LOG_POS=log_offset_on_master` on the slave after restoring the snapshot. (Slave)

`SHOW MASTER STATUS` | Provides status information on the binlog of the master. (Master)
`SHOW SLAVE HOSTS` | Available after 4.0.0. Gives a listing of slaves currently registered with the master. (Master)
`SHOW SLAVE STATUS` | Provides status information on essential parameters of the slave thread. (Slave)
`SHOW MASTER LOGS` | Only available starting in Version 3.23.28. Lists the binary logs on the master. You should use this command prior to `PURGE MASTER LOGS TO` to find out how far you should go. (Master)
`SHOW BINLOG EVENTS [ IN 'logname' ] [ FROM pos ] [ LIMIT [offset,] rows ]` | Shows the events in the binary update log. Primarily used for testing/debugging, but can also be used by regular clients that for some reason need to read the binary log contents. (Master)
`SHOW NEW MASTER FOR SLAVE WITH MASTER_LOG_FILE='logfile' AND MASTER_LOG_POS=pos AND MASTER_LOG_SEQ=log_seq AND MASTER_SERVER_ID=server_id` | This command is used when a slave of a possibly dead/unavailable master needs to be switched to replicate off another slave that has been replicating the same master. The command will return recalculated replication coordinates (the slave's current binary log file name and position within that file). The output can be used in a subsequent `CHANGE MASTER TO` command. Normal users should never need to run this command. It is primarily reserved for internal use by the fail-safe replication code. We may later change the syntax if we find a more intuitive way to describe this operation.

`PURGE MASTER LOGS TO 'logname'`

Available starting in Version 3.23.28. Deletes all the replication logs that are listed in the log index as being prior to the specified log, and removes them from the log index, so that the given log now becomes the first. Example:

    PURGE MASTER LOGS TO 'mysql-bin.010'

This command will do nothing and fail with an error if you have an active slave that is currently reading one of the logs you are trying to delete. However, if you have a dormant slave, and happen to purge one of the logs it wants to read, the slave will be unable to replicate once it comes up. The command is safe to run while slaves are replicating -- you do not need to stop them. You must first check all the slaves with `SHOW SLAVE STATUS` to see which log they are on, then do a listing of the logs on the master with `SHOW MASTER LOGS`, find the earliest log among all the slaves (if all the slaves are up to date, this will be the last log on the list), backup all the logs you are about to delete (optional) and purge up to the target log.

### 4.10.7 Replication FAQ

Q: How do I configure a slave if the master is already running and I do not want to stop it?

A: There are several options. If you have taken a backup of the master at some point and recorded the binlog name and offset ( from the output of `SHOW MASTER STATUS` ) corresponding to the snapshot, do the following:

- Make sure unique server id is assigned to the slave.
- Execute `CHANGE MASTER TO MASTER_HOST='master-host-name', MASTER_USER='master-user-name', MASTER_PASSWORD='master-pass', MASTER_LOG_FILE='recorded-log-name', MASTER_LOG_POS=recorded_log_pos`
- Execute `SLAVE START`

If you do not have a backup of the master already, here is a quick way to do it consistently:

- `FLUSH TABLES WITH READ LOCK`
- `gtar zcf /tmp/backup.tar.gz /var/lib/mysql` ( or a variation of this)
- `SHOW MASTER STATUS` - make sure to record the output - you will need it later
- `UNLOCK TABLES`

Afterwards, follow the instructions for the case when you have a snapshot and have recorded the log name and offset. You can use the same snapshot to set up several slaves. As long as the binary logs of the master are left intact, you can wait as long as several days or in some cases maybe a month to set up a slave once you have the snapshot of the master. In theory the waiting gap can be infinite. The two practical limitations is the diskspace of the master getting filled with old logs, and the amount of time it will take the slave to catch up.

In version 4.0.0 and newer, you can also use `LOAD DATA FROM MASTER`. This is a convenient command that will take a snapshot, restore it to the slave, and adjust the log name and offset on the slave all at once. In the future, `LOAD DATA FROM MASTER` will be the recommended way to set up a slave. Be warned, howerver, that the read lock may be held for a long time if you use this command. It is not yet implemented as efficiently as we would like to have it. If you have large tables, the preferred method at this time is still with a local `tar` snapshot after executing `FLUSH TABLES WITH READ LOCK`.

Q: Does the slave need to be connected to the master all the time?

A: No, it does not. You can have the slave go down or stay disconnected for hours or even days, then reconnect, catch up on the updates, and then disconnect or go down for a while again. So you can, for example, use master-slave setup over a dial-up link that is up only for short periods of time. The implications of that are that at any given time the slave is not guaranteed to be in sync with the master unless you take some special measures. In the future, we will have the option to block the master until at least one slave is in sync.

Q: How do I force the master to block updates until the slave catches up?

A: Execute the following commands:

- Master: `FLUSH TABLES WITH READ LOCK`
- Master: `SHOW MASTER STATUS` - record the log name and the offset
- Slave: `SELECT MASTER_POS_WAIT('recorded_log_name', recorded_log_offset)` When the select returns, the slave is currently in sync with the master
- Master: `UNLOCK TABLES` - now the master will continue updates.

Q: Why do I sometimes see more than one `Binlog_Dump` thread on the master after I have restarted the slave?

A: `Binlog_Dump` is a continuous process that is handled by the server in the following way:

- Catch up on the updates.
- Once there are no more updates left, go into `pthread_cond_wait()`, from which we can be awakened either by an update or a kill.
- On wake up, check the reason. If we are not supposed to die, continue the `Binlog_dump` loop.
- If there is some fatal error, such as detecting a dead client, terminate the loop.

So if the slave thread stops on the slave, the corresponding `Binlog_Dump` thread on the master will not notice it until after at least one update to the master (or a kill), which is needed to wake it up from `pthread_cond_wait()`. In the meantime, the slave could have opened another connection, which resulted in another `Binlog_Dump` thread.

The above problem should not be present in Version 3.23.26 and later versions. In Version 3.23.26 we added `server-id` to each replication server, and now all the old zombie threads are killed on the master when a new replication thread connects from the same slave

Q: How do I rotate replication logs?

A: In Version 3.23.28 you should use `PURGE MASTER LOGS TO` command after determining which logs can be deleted, and optionally backing them up first. In earlier versions the process is much more painful, and cannot be safely done without stopping all the slaves in the case that you plan to re-use log names. You will need to stop the slave threads, edit the binary log index file, delete all the old logs, restart the master, start slave threads, and then remove the old log files.

Q: How do I upgrade on a hot replication setup?

A: If you are upgrading pre-3.23.26 versions, you should just lock the master tables, let the slave catch up, then run `FLUSH MASTER` on the master, and `FLUSH SLAVE` on the slave to reset the logs, then restart new versions of the master and the slave. Note that the slave can stay down for some time -- since the master is logging all the updates, the slave will be able to catch up once it is up and can connect.

After 3.23.26, we have locked the replication protocol for modifications, so you can upgrade masters and slave on the fly to a newer 3.23 version and you can have different versions of MySQL running on the slave and the master, as long as they are both newer than 3.23.26.

Q: What issues should I be aware of when setting up two-way replication?

A: MySQL replication currently does not support any locking protocol between master and slave to guarantee the atomicity of a distributed (cross-server) update. In in other words, it is possible for client A to make an update to co-master 1, and in the meantime, before it propagates to co-master 2, client B could make an update to co-master 2 that will make the update of client A work differently than it did on co-master 1. Thus when the update of client A will make it to co-master 2, it will produce tables that will be different from what you have on co-master 1, even after all the updates from co-master 2 have also propagated. So you should not co-chain two servers in a two-way replication relationship, unless you are sure that you updates can safely happen in any order, or unless you take care of mis-ordered updates somehow in the client code.

You must also realise that two-way replication actually does not improve performance very much, if at all, as far as updates are concerned. Both servers need to do the same amount of updates each, as you would have one server do. The only difference is that there will be a little less lock contention, because the updates originating on another server will be serialised in one slave thread. This benefit, though, might be offset by network delays.

Q: How can I use replication to improve performance of my system?

A: You should set up one server as the master, and direct all writes to it, and configure as many slaves as you have the money and rackspace for, distributing the reads among the master and the slaves. You can also start the slaves with `--skip-bdb`, `--low-priority-updates` and `--delay-key-write=ALL` to get speed improvements for the slave. In this case the slave will use non-transactional `MyISAM` tables instead of `BDB` tables to get more speed.

Q: What should I do to prepare my client code to use performance-enhancing replication?

A: If the part of your code that is responsible for database access has been properly abstracted/modularised, converting it to run with the replicated setup should be very smooth and easy -- just change the implementation of your database access to read from some slave or the master, and to always write to the master. If your code does not have this level of abstraction, setting up a replicated system will give you an opportunity/motivation to it clean up. You should start by creating a wrapper library /module with the following functions:

- `safe_writer_connect()`
- `safe_reader_connect()`
- `safe_reader_query()`
- `safe_writer_query()`

`safe_` means that the function will take care of handling all the error conditions.

You should then convert your client code to use the wrapper library. It may be a painful and scary process at first, but it will pay off in the long run. All applications that follow the above pattern will be able to take advantage of one-master/many slaves solution. The code will be a lot easier to maintain, and adding troubleshooting options will be trivial. You will just need to modify one or two functions, for example, to log how long each query took, or which query, among your many thousands, gave you an error. If you have written a lot of code already, you may want to automate the conversion task by using Monty's `replace` utility, which comes with the standard distribution of MySQL, or just write your own Perl script. Hopefully, your code follows some recognisable pattern. If not, then you are probably better off rewriting it anyway, or at least going through and manually beating it into a pattern.

Note that, of course, you can use different names for the functions. What is important is having unified interface for connecting for reads, connecting for writes, doing a read, and doing a write.

Q: When and how much can MySQL replication improve the performance of my system?

A: MySQL replication is most beneficial for a system with frequent reads and not so frequent writes. In theory, by using a one master/many slaves setup you can scale by adding more slaves until you either run out of network bandwidth, or your update load grows to the point that the master cannot handle it.

In order to determine how many slaves you can get before the added benefits begin to level out, and how much you can improve performance of your site, you need to know your query patterns, and empirically (by benchmarking) determine the relationship between the throughput on reads (reads per second, or `max_reads`) and on writes `max_writes`) on a typical master and a typical slave. The example here will show you a rather simplified calculation of what you can get with replication for our imagined system.

Let's say our system load consists of 10% writes and 90% reads, and we have determined that `max_reads` = 1200 - 2 * `max_writes`, or in other words, our system can do 1200 reads per second with no writes, our average write is twice as slow as average read, and the relationship is linear. Let us suppose that our master and slave are of the same capacity, and we have N slaves and 1 master. Then we have for each server (master or slave):

`reads = 1200 - 2 * writes` (from bencmarks)

`reads = 9* writes / (N + 1)` (reads split, but writes go to all servers)

`9*writes/(N+1) + 2 * writes = 1200`

`writes = 1200/(2 + 9/(N+1)`

So if N = 0, which means we have no replication, our system can handle 1200/11, about 109 writes per second (which means we will have 9 times as many reads due to the nature of our application).

If N = 1, we can get up to 184 writes per second.

If N = 8, we get up to 400.

If N = 17, 480 writes.

Eventually as N approaches infinity (and our budget negative infinity), we can get very close to 600 writes per second, increasing system throughput about 5.5 times. However, with only 8 servers, we increased it almost 4 times already.

Note that our computations assumed infinite network bandwidth, and neglected several other factors that could turn out to be significant on your system. In many cases, you may not be able to make a computation similar to the one above that will accurately predict what will happen on your system if you add N replication slaves. However, answering the following questions should help you decided whether and how much, if at all, the replication will improve the performance of your system:

- What is the read/write ratio on your system?
- How much more write load can one server handle if you reduce the reads?
- How many slaves do you have bandwidth for on your network?

Q: How can I use replication to provide redundancy/high availability?

A: With the currently available features, you would have to set up a master and a slave (or several slaves), and write a script that will monitor the master to see if it is up, and instruct your applications and the slaves of the master change in case of failure. Some suggestions:

- To tell a slave to change the master use the `CHANGE MASTER TO` command.
- A good way to keep your applications informed as to the location of the master is by having a dynamic DNS entry for the master. With `bind` you can use ``nsupdate'` to dynamically update your DNS.
- You should run your slaves with the `log-bin` option and without `log-slave-updates`. This way the slave will be ready to become a master as soon as you issue `STOP SLAVE`; `RESET MASTER`, and `CHANGE MASTER TO` on the other slaves. It will also help you catch spurious updates that may happen because of misconfiguration of the slave (ideally, you want to configure access rights so that no client can update the slave, except for the slave thread) combined with the bugs in your client programs (they should never update the slave directly).

We are currently working on integrating an automatic master election system into MySQL, but until it is ready, you will have to create your own monitoring tools.

### 4.10.8 Troubleshooting Replication

If you have followed the instructions, and your replication setup is not working, first eliminate the user error factor by checking the following:

- Is the master logging to the binary log? Check with `SHOW MASTER STATUS`. If it is, `Position` will be non-zero. If not, verify that you have given the master `log-bin` option and have set `server-id`.
- Is the slave running? Check with `SHOW SLAVE STATUS`. The answer is found in `Slave_running` column. If not, verify slave options and check the error log for messages.
- If the slave is running, did it establish connection with the master? Do `SHOW PROCESSLIST`, find the thread with `system user` value in `User` column and `none` in the `Host` column, and check the `State` column. If it says `connecting to master`, verify the privileges for the replication user on the master, master host name, your DNS setup, whether the master is actually running, whether it is reachable from the slave, and if all that seems okay, read the error logs.
- If the slave was running, but then stopped, look at SHOW SLAVE STATUS output and check the error logs. It usually happens when some query that succeeded on the master fails on the slave. This should never happen if you have taken a proper snapshot of the master, and never modify the data on the slave outside of the slave thread. If it does, it is a bug, read below on how to report it.
- If a query on that succeeded on the master refuses to run on the slave, and a full database resync ( the proper thing to do ) does not seem feasible, try the following:
  - First see if there is some stray record in the way. Understand how it got there, then delete it and run `SLAVE START`
  - If the above does not work or does not apply, try to understand if it would be safe to make the update manually ( if needed) and then ignore the next query from the master.
  - If you have decided you can skip the next query, do `SET GLOBAL SQL_SLAVE_SKIP_COUNTER=1; SLAVE START;` to skip a query that does not use `AUTO_INCREMENT` or `LAST_INSERT_ID()`, or `SET GLOBAL SQL_SLAVE_SKIP_COUNTER=2; SLAVE START;` otherwise. The reason queries that use `AUTO_INCREMENT` or `LAST_INSERT_ID()` are different is that they take two events in the binary log of the master.
  - If you are sure the slave started out perfectly in sync with the master, and no one has updated the tables involved outside of slave thread, report the bug, so you will not have to do the above tricks again.
- Make sure you are not running into an old bug by upgrading to the most recent version.
- If all else fails, read the error logs. If they are big, `grep -i slave /path/to/your-log.err` on the slave. There is no generic pattern to search for on the master, as the only errors it logs are general system errors -- if it can, it will send the error to the slave when things go wrong.

When you have determined that there is no user error involved, and replication still either does not work at all or is unstable, it is time to start working on a bug report. We need to get as much info as possible from you to be able to track down the bug. Please do spend some time and effort preparing a good bug report. Ideally, we would like to have a test case in the format found in `mysql-test/t/rpl*` directory of the source tree. If you submit a test case like that, you can expect a patch within a day or two in most cases, although, of course, you mileage may vary depending on a number of factors.

The second best option is to write a simple program with easily configurable connection arguments for the master and the slave that will demonstrate the problem on our systems. You can write one in Perl or in C, depending on which language you know better.

If you have one of the above ways to demonstrate the bug, use `mysqlbug` to prepare a bug report and send it to bugs@lists.mysql.com. If you have a phantom -- a problem that does occur but you cannot duplicate "at will":

- Verify that there is no user error involved. For example, if you update the slave outside of the slave thread, the data will be out of sync, and you can have unique key violations on updates, in which case the slave thread will stop and wait for you to clean up the tables manually to bring them in sync.
- Run slave with `log-slave-updates` and `log-bin` -- this will keep a log of all updates on the slave.
- Save all evidence before resetting the replication. If we have no or only sketchy information, it would take us a while to track down the problem. The evidence you should collect is:
  - All binary logs on the master
  - All binary log on the slave
  - The output of `SHOW MASTER STATUS` on the master at the time you have discovered the problem
  - The output of `SHOW SLAVE STATUS` on the master at the time you have discovered the problem
  - Error logs on the master and on the slave
- Use `mysqlbinlog` to examine the binary logs. The following should be helpful to find the trouble query, for example:

      mysqlbinlog -j pos_from_slave_status /path/to/log_from_slave_status | head

Once you have collected the evidence on the phantom problem, try hard to isolate it into a separate test case first. Then report the problem to bugs@lists.mysql.com with as much info as possible.

# 5 MySQL Optimisation

Optimisation is a complicated task because it ultimately requires understanding of the whole system. While it may be possible to do some local optimisations with small knowledge of your system or application, the more optimal you want your system to become the more you will have to know about it.

This chapter will try to explain and give some examples of different ways to optimise MySQL. Remember, however, that there are always some (increasingly harder) additional ways to make the system even faster.

## 5.1 Optimisation Overview

The most important part for getting a system fast is of course the basic design. You also need to know what kinds of things your system will be doing, and what your bottlenecks are.

The most common bottlenecks are:

- Disk seeks. It takes time for the disk to find a piece of data. With modern disks in 1999, the mean time for this is usually lower than 10ms, so we can in theory do about 100 seeks a second. This time improves slowly with new disks and is very hard to optimise for a single table. The way to optimise this is to spread the data on more than one disk.
- Disk reading/writing. When the disk is at the correct position we need to read the data. With modern disks in 1999, one disk delivers something like 10-20MB/s. This is easier to optimise than seeks because you can read in parallel from multiple disks.
- CPU cycles. When we have the data in main memory (or if it already were there) we need to process it to get to our result. Having small tables compared to the memory is the most common limiting factor. But then, with small tables speed is usually not the problem.
- Memory bandwidth. When the CPU needs more data than can fit in the CPU cache the main memory bandwidth becomes a bottleneck. This is an uncommon bottleneck for most systems, but one should be aware of it.

### 5.1.1 MySQL Design Limitations/Tradeoffs

When using the MyISAM storage engine, MySQL uses extremely fast table locking (multiple readers / single writers). The biggest problem with this table type is a if you have a mix of a steady stream of updates and slow selects on the same table. If this is a problem with some tables, you can use another table type for these. See section 7 MySQL Table Types.

MySQL can work with both transactional and not transactional tables. To be able to work smoothly with not transactional tables (which can't rollback if something goes wrong), MySQL has the following rules:

- All columns has default values.
- If you insert a 'wrong' value in a column like a `NULL` in a `NOT NULL` column or a too big numerical value in a numerical column, MySQL will instead of giving an error instead set the column to the 'best possible value'. For numerical values this is 0, the smallest possible values or the largest possible value. For strings this is either the empty string or the longest possible string that can be in the column.
- All calculated expressions returns a value that can be used instead of signaling an error condition. For example 1/0 returns `NULL`

The reason for the above rules is that we can't check these conditions before the query starts to execute. If we encounter a problem after updating a few rows, we can't just rollback as the table type may not support this. We can't stop because in that case the update would be 'half done' which is probably the worst possible scenario. In this case it's better to 'do the best you can' and then continue as if nothing happened.

The above means that one should not use MySQL to check fields content, but one should do this in the application.

### 5.1.2 Portability

Because all SQL servers implement different parts of SQL, it takes work to write portable SQL applications. For very simple selects/inserts it is very easy, but the more you need the harder it gets. If you want an application that is fast with many databases it becomes even harder!

To make a complex application portable you need to choose a number of SQL servers that it should work with.

You can use the MySQL `crash-me` program/web-page http://www.mysql.com/information/crash-me.php to find functions, types, and limits you can use with a selection of database servers. Crash-me now tests far from everything possible, but it is still comprehensive with about 450 things tested.

For example, you shouldn't have column names longer than 18 characters if you want to be able to use Informix or DB2.

Both the MySQL benchmarks and `crash-me` programs are very database-independent. By taking a look at how we have handled this, you can get a feeling for what you have to do to write your application database-independent. The benchmarks themselves can be found in the ``sql-bench'` directory in the MySQL source distribution. They are written in Perl with DBI database interface (which solves the access part of the problem).

See http://www.mysql.com/information/benchmarks.html for the results from this benchmark.

As you can see in these results, all databases have some weak points. That is, they have different design compromises that lead to different behaviour.

If you strive for database independence, you need to get a good feeling for each SQL server's bottlenecks. MySQL is very fast in retrieving and updating things, but will have a problem in mixing slow readers/writers on the same table. Oracle, on the other hand, has a big problem when you try to access rows that you have recently updated (until they are flushed to disk). Transaction databases in general are not very good at generating summary tables from log tables, as in this case row locking is almost useless.

To get your application really database-independent, you need to define an easy extendable interface through which you manipulate your data. As C++ is available on most systems, it makes sense to use a C++ classes interface to the databases.

If you use some specific feature for some database (like the `REPLACE` command in MySQL), you should code a method for the other SQL servers to implement the same feature (but slower). With MySQL you can use the `/*! */` syntax to add MySQL-specific keywords to a query. The code inside `/**/` will be treated as a comment (ignored) by most other SQL servers.

If high performance is more important than exactness, as in some web applications, it is possibile to create an application layer that caches all results to give you even higher performance. By letting old results 'expire' after a while, you can keep the cache reasonably fresh. This provides a method to handle high load spikes, in which case you can dynamically increase the cache and set the expire timeout higher until things get back to normal.

In this case the table creation information should contain information of the initial size of the cache and how often the table should normally be refreshed.

### 5.1.3 What Have We Used MySQL For?

During MySQL initial development, the features of MySQL were made to fit our largest customer. They handle data warehousing for a couple of the biggest retailers in Sweden.

From all stores, we get weekly summaries of all bonus card transactions, and we are expected to provide useful information for the store owners to help them find how their advertisement campaigns are affecting their customers.

The data is quite huge (about 7 million summary transactions per month), and we have data for 4-10 years that we need to present to the users. We got weekly requests from the customers that they want to get 'instant' access to new reports from this data.

We solved this by storing all information per month in compressed 'transaction' tables. We have a set of simple macros (script) that generates summary tables grouped by different criteria (product group, customer id, store ...) from the transaction tables. The reports are web pages that are dynamically generated by a small Perl script that parses a web page, executes the SQL statements in it, and inserts the results. We would have used PHP or mod_perl instead but they were not available at that time.

For graphical data we wrote a simple tool in `C` that can produce GIFs based on the result of a SQL query (with some processing of the result). This is also dynamically executed from the Perl script that parses the `HTML` files.

In most cases a new report can simply be done by copying an existing script and modifying the SQL query in it. In some cases, we will need to add more fields to an existing summary table or generate a new one, but this is also quite simple, as we keep all transactions tables on disk. (Currently we have at least 50G of transactions tables and 200G of other customer data.)

We also let our customers access the summary tables directly with ODBC so that the advanced users can themselves experiment with the data.

We haven't had any problems handling this with quite modest Sun Ultra SPARCstation (2x200 Mhz). We recently upgraded one of our servers to a 2 CPU 400 Mhz UltraSPARC, and we are now planning to start handling transactions on the product level, which would mean a ten-fold increase of data. We think we can keep up with this by just adding more disk to our systems.

We are also experimenting with Intel-Linux to be able to get more CPU power cheaper. Now that we have the binary portable database format (new in Version 3.23), we will start to use this for some parts of the application.

Our initial feelings are that Linux will perform much better on low-to-medium load and Solaris will perform better when you start to get a high load because of extreme disk IO, but we don't yet have anything conclusive about this. After some discussion with a Linux Kernel developer, this might be a side effect of Linux giving so much resources to the batch job that the interactive performance gets very low. This makes the machine feel very slow and unresponsive while big batches are going. Hopefully this will be better handled in future Linux Kernels.

### 5.1.4 The MySQL Benchmark Suite

This should contain a technical description of the MySQL benchmark suite (and `crash-me`), but that description is not written yet. Currently, you can get a good idea of the benchmark by looking at the code and results in the ``sql-bench'` directory in any MySQL source distributions.

This benchmark suite is meant to be a benchmark that will tell any user what things a given SQL implementation performs well or poorly at.

Note that this benchmark is single threaded, so it measures the minimum time for the operations. We plan to in the future add a lot of multi-threaded tests to the benchmark suite.

For example, (run on the same NT 4.0 machine):

Reading 2000000 rows by index | Seconds | Seconds
mysql | 367 | 249
mysql_odbc | 464 |
db2_odbc | 1206 |
informix_odbc | 121126 |
ms-sql_odbc | 1634 |
oracle_odbc | 20800 |
solid_odbc | 877 |
sybase_odbc | 17614 |

Inserting (350768) rows | Seconds | Seconds
mysql | 381 | 206
mysql_odbc | 619 |
db2_odbc | 3460 |
informix_odbc | 2692 |
ms-sql_odbc | 4012 |
oracle_odbc | 11291 |
solid_odbc | 1801 |
sybase_odbc | 4802 |

In the above test MySQL was run with a 8M index cache.

We have gathered some more benchmark results at http://www.mysql.com/information/benchmarks.html.

Note that Oracle is not included because they asked to be removed. All Oracle benchmarks have to be passed by Oracle! We believe that makes Oracle benchmarks very biased because the above benchmarks are supposed to show what a standard installation can do for a single client.

To run the benchmark suite, you have to download a MySQL source distribution, install the perl DBI driver, the perl DBD driver for the database you want to test and then do:

    cd sql-bench
    perl run-all-tests --server=#

where # is one of supported servers. You can get a list of all options and supported servers by doing `run-all-tests --help`.

`crash-me` tries to determine what features a database supports and what its capabilities and limitations are by actually running queries. For example, it determines:

- What column types are supported
- How many indexes are supported
- What functions are supported
- How big a query can be
- How big a `VARCHAR` column can be

We can find the result from `crash-me` on a lot of different databases at http://www.mysql.com/information/crash-me.php.

### 5.1.5 Using Your Own Benchmarks

You should definitely benchmark your application and database to find out where the bottlenecks are. By fixing it (or by replacing the bottleneck with a 'dummy module') you can then easily identify the next bottleneck (and so on). Even if the overall performance for your application is sufficient, you should at least make a plan for each bottleneck, and decide how to solve it if someday you really need the extra performance.

For an example of portable benchmark programs, look at the MySQL benchmark suite. See section 5.1.4 The MySQL Benchmark Suite. You can take any program from this suite and modify it for your needs. By doing this, you can try different solutions to your problem and test which is really the fastest solution for you.

It is very common that some problems only occur when the system is very heavily loaded. We have had many customers who contact us when they have a (tested) system in production and have encountered load problems. In every one of these cases so far, it has been problems with basic design (table scans are not good at high load) or OS/Library issues. Most of this would be a lot easier to fix if the systems were not already in production.

To avoid problems like this, you should put some effort into benchmarking your whole application under the worst possible load! You can use Super Smack for this, and it is available at: http://www.mysql.com/Downloads/super-smack/super-smack-1.0.tar.gz. As the name suggests, it can bring your system down to its knees if you ask it, so make sure to use it only on your development systems.

## 5.2 Optimising `SELECT`s and Other Queries

First, one thing that affects all queries: The more complex permission system setup you have, the more overhead you get.

If you do not have any `GRANT` statements done, MySQL will optimise the permission checking somewhat. So if you have a very high volume it may be worth the time to avoid grants. Otherwise, more permission check results in a larger overhead.

If your problem is with some explicit MySQL function, you can always time this in the MySQL client:

    mysql> SELECT BENCHMARK(1000000,1+1);
    +------------------------+
    | BENCHMARK(1000000,1+1) |
    +------------------------+
    |                      0 |
    +------------------------+
    1 row in set (0.32 sec)

The above shows that MySQL can execute 1,000,000 `+` expressions in 0.32 seconds on a `PentiumII 400MHz`.

All MySQL functions should be very optimised, but there may be some exceptions, and the `BENCHMARK(loop_count,expression)` is a great tool to find out if this is a problem with your query.

### 5.2.1 `EXPLAIN` Syntax (Get Information About a `SELECT`)

        EXPLAIN tbl_name
    or  EXPLAIN SELECT select_options

`EXPLAIN tbl_name` is a synonym for `DESCRIBE tbl_name` or `SHOW COLUMNS FROM tbl_name`.

When you precede a `SELECT` statement with the keyword `EXPLAIN`, MySQL explains how it would process the `SELECT`, providing information about how tables are joined and in which order.

With the help of `EXPLAIN`, you can see when you must add indexes to tables to get a faster `SELECT` that uses indexes to find the records.

You should frequently run `ANALYZE TABLE` to update table statistics such as cardinality of keys which can affect the choices the optimiser makes. See section 4.5.2 `ANALYZE TABLE` Syntax.

You can also see if the optimiser joins the tables in an optimal order. To force the optimiser to use a specific join order for a `SELECT` statement, add a `STRAIGHT_JOIN` clause.

For non-simple joins, `EXPLAIN` returns a row of information for each table used in the `SELECT` statement. The tables are listed in the order they would be read. MySQL resolves all joins using a single-sweep multi-join method. This means that MySQL reads a row from the first table, then finds a matching row in the second table, then in the third table and so on. When all tables are processed, it outputs the selected columns and backtracks through the table list until a table is found for which there are more matching rows. The next row is read from this table and the process continues with the next table.

In MySQL version 4.1 the `EXPLAIN` output was changed to work better with constructs like `UNION`s, subqueries and derived tables. Most notable is the addition of two new columns: `id` and `select_type`.

Output from `EXPLAIN` consists of the following columns:

`id`
  `SELECT` identifier, the sequential number of this `SELECT` within the query.
`select_type`
  Type of `SELECT` clause, which can be any of the following:
  `SIMPLE`
    Simple `SELECT` (without `UNION`s or subqueries).
  `PRIMARY`
    Outermost `SELECT`.
  `UNION`
    Second and further `UNION` `SELECT`s.
  `DEPENDENT UNION`
    Second and further `UNION` `SELECTS`s, dependent on outer subquery.
  `SUBSELECT`
    First `SELECT` in subquery.
  `DEPENDENT SUBSELECT`
    First `SELECT`, dependent on outer subquery.
  `DERIVED`
    Derived table `SELECT`.
`table`
  The table to which the row of output refers.
`type`
  The join type. The different join types are listed here, ordered from best to worst type:
  `system`
    The table has only one row (= system table). This is a special case of the `const` join type.
  `const`
    The table has at most one matching row, which will be read at the start of the query. Because there is only one row, values from the column in this row can be regarded as constants by the rest of the optimiser. `const` tables are very fast as they are read only once!
  `eq_ref`
    One row will be read from this table for each combination of rows from the previous tables. This is the best possible join type, other than the `const` types. It is used when all parts of an index are used by the join and the index is `UNIQUE` or a `PRIMARY KEY`.
  `ref`
    All rows with matching index values will be read from this table for each combination of rows from the previous tables. `ref` is used if the join uses only a leftmost prefix of the key, or if the key is not `UNIQUE` or a `PRIMARY KEY` (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this join type is good.
  `range`
    Only rows that are in a given range will be retrieved, using an index to select the rows. The `key` column indicates which index is used. The `key_len` contains the longest key part that was used. The `ref` column will be `NULL` for this type.
  `index`
    This is the same as `ALL`, except that only the index tree is scanned. This is usually faster than `ALL`, as the index file is usually smaller than the datafile.
  `ALL`
    A full table scan will be done for each combination of rows from the previous tables. This is normally not good if the table is the first table not marked `const`, and usually very bad in all other cases. You normally can avoid `ALL` by adding more indexes, so that the row can be retrieved based on constant values or column values from earlier tables.
`possible_keys`
  The `possible_keys` column indicates which indexes MySQL could use to find the rows in this table. Note that this column is totally independent of the order of the tables. That means that some of the keys in possible_keys may not be usable in practice with the generated table order. If this column is empty, there are no relevant indexes. In this case, you may be able to improve the performance of your query by examining the `WHERE` clause to see if it refers to some column or columns that would be suitable for indexing. If so, create an appropriate index and check the query with `EXPLAIN` again. See section 6.5.4 `ALTER TABLE` Syntax. To see what indexes a table has, use `SHOW INDEX FROM tbl_name`.
`key`
  The `key` column indicates the key (index) that MySQL actually decided to use. The key is `NULL` if no index was chosen. To force MySQL to use an key listed in the `possible_keys` column, use `USE KEY/IGNORE KEY` in your query. See section 6.4.1 `SELECT` Syntax. Also, running `myisamchk --analyze` (see section 4.4.6.1 `myisamchk` Invocation Syntax) or `ANALYZE TABLE` (see section 4.5.2 `ANALYZE TABLE` Syntax) on the table will help the optimiser choose better indexes.
`key_len`
  The `key_len` column indicates the length of the key that MySQL decided to use. The length is `NULL` if the `key` is `NULL`. Note that this tells us how many parts of a multi-part key MySQL will actually use.
`ref`
  The `ref` column shows which columns or constants are used with the `key` to select rows from the table.
`rows`
  The `rows` column indicates the number of rows MySQL believes it must examine to execute the query.
`Extra`
  This column contains additional information of how MySQL will resolve the query. Here is an explanation of the different text strings that can be found in this column:
  `Distinct`
    MySQL will not continue searching for more rows for the current row combination after it has found the first matching row.
  `Not exists`
    MySQL was able to do a `LEFT JOIN` optimisation on the query and will not examine more rows in this table for the previous row combination after it finds one row that matches the `LEFT JOIN` criteria. Here is an example for this:

        SELECT * FROM t1 LEFT JOIN t2 ON t1.id=t2.id WHERE t2.id IS NULL;

    Assume that `t2.id` is defined with `NOT NULL`. In this case MySQL will scan `t1` and look up the rows in `t2` through `t1.id`. If MySQL finds a matching row in `t2`, it knows that `t2.id` can never be `NULL`, and will not scan through the rest of the rows in `t2` that has the same `id`. In other words, for each row in `t1`, MySQL only needs to do a single lookup in `t2`, independent of how many matching rows there are in `t2`.
  ``range checked for each record (index map: #)``
    MySQL didn't find a real good index to use. It will, instead, for each row combination in the preceding tables, do a check on which index to use (if any), and use this index to retrieve the rows from the table. This isn't very fast but is faster than having to do a join without an index.
  `Using filesort`
    MySQL will need to do an extra pass to find out how to retrieve the rows in sorted order. The sort is done by going through all rows according to the `join type` and storing the sort key + pointer to the row for all rows that match the `WHERE`. Then the keys are sorted. Finally the rows are retrieved in sorted order.
  `Using index`
    The column information is retrieved from the table using only information in the index tree without having to do an additional seek to read the actual row. This can be done when all the used columns for the table are part of the same index.
  `Using temporary`
    To resolve the query MySQL will need to create a temporary table to hold the result. This typically happens if you do an `ORDER BY` on a different column set than you did a `GROUP BY` on.
  `Using where`
    A `WHERE` clause will be used to restrict which rows will be matched against the next table or sent to the client. If you don't have this information and the table is of type `ALL` or `index`, you may have something wrong in your query (if you don't intend to fetch/examine all rows from the table).
  If you want to get your queries as fast as possible, you should look out for `Using filesort` and `Using temporary`.

You can get a good indication of how good a join is by multiplying all values in the `rows` column of the `EXPLAIN` output. This should tell you roughly how many rows MySQL must examine to execute the query. This number is also used when you restrict queries with the `max_join_size` variable. See section 5.5.2 Tuning Server Parameters.

The following example shows how a `JOIN` can be optimised progressively using the information provided by `EXPLAIN`.

Suppose you have the `SELECT` statement shown here, that you examine using `EXPLAIN`:

    EXPLAIN SELECT tt.TicketNumber, tt.TimeIn,
                tt.ProjectReference, tt.EstimatedShipDate,
                tt.ActualShipDate, tt.ClientID,
                tt.ServiceCodes, tt.RepetitiveID,
                tt.CurrentProcess, tt.CurrentDPPerson,
                tt.RecordVolume, tt.DPPrinted, et.COUNTRY,
                et_1.COUNTRY, do.CUSTNAME
            FROM tt, et, et AS et_1, do
            WHERE tt.SubmitTime IS NULL
                AND tt.ActualPC = et.EMPLOYID
                AND tt.AssignedPC = et_1.EMPLOYID
                AND tt.ClientID = do.CUSTNMBR;

For this example, assume that:

- The columns being compared have been declared as follows:

  Table | Column | Column type
  `tt` | `ActualPC` | `CHAR(10)`
  `tt` | `AssignedPC` | `CHAR(10)`
  `tt` | `ClientID` | `CHAR(10)`
  `et` | `EMPLOYID` | `CHAR(15)`
  `do` | `CUSTNMBR` | `CHAR(15)`

- The tables have the indexes shown here:

  Table | Index
  `tt` | `ActualPC`
  `tt` | `AssignedPC`
  `tt` | `ClientID`
  `et` | `EMPLOYID` (primary key)
  `do` | `CUSTNMBR` (primary key)

- The `tt.ActualPC` values aren't evenly distributed.

Initially, before any optimisations have been performed, the `EXPLAIN` statement produces the following information:

    table type possible_keys                key  key_len ref  rows  Extra
    et    ALL  PRIMARY                      NULL NULL    NULL 74
    do    ALL  PRIMARY                      NULL NULL    NULL 2135
    et_1  ALL  PRIMARY                      NULL NULL    NULL 74
    tt    ALL  AssignedPC,ClientID,ActualPC NULL NULL    NULL 3872
          range checked for each record (key map: 35)

Because `type` is `ALL` for each table, this output indicates that MySQL is doing a full join for all tables! This will take quite a long time, as the product of the number of rows in each table must be examined! For the case at hand, this is `74 * 2135 * 74 * 3872 = 45,268,558,720` rows. If the tables were bigger, you can only imagine how long it would take.

One problem here is that MySQL can't (yet) use indexes on columns efficiently if they are declared differently. In this context, `VARCHAR` and `CHAR` are the same unless they are declared as different lengths. Because `tt.ActualPC` is declared as `CHAR(10)` and `et.EMPLOYID` is declared as `CHAR(15)`, there is a length mismatch.

To fix this disparity between column lengths, use `ALTER TABLE` to lengthen `ActualPC` from 10 characters to 15 characters:

    mysql> ALTER TABLE tt MODIFY ActualPC VARCHAR(15);

Now `tt.ActualPC` and `et.EMPLOYID` are both `VARCHAR(15)`. Executing the `EXPLAIN` statement again produces this result:

    table type   possible_keys   key     key_len ref         rows    Extra
    tt    ALL    AssignedPC,ClientID,ActualPC NULL NULL NULL 3872    Using where
    do    ALL    PRIMARY         NULL    NULL    NULL        2135
          range checked for each record (key map: 1)
    et_1  ALL    PRIMARY         NULL    NULL    NULL        74
          range checked for each record (key map: 1)
    et    eq_ref PRIMARY         PRIMARY 15      tt.ActualPC 1

This is not perfect, but is much better (the product of the `rows` values is now less by a factor of 74). This version is executed in a couple of seconds.

A second alteration can be made to eliminate the column length mismatches for the `tt.AssignedPC = et_1.EMPLOYID` and `tt.ClientID = do.CUSTNMBR` comparisons:

    mysql> ALTER TABLE tt MODIFY AssignedPC VARCHAR(15),
        ->                MODIFY ClientID   VARCHAR(15);

Now `EXPLAIN` produces the output shown here:

    table type   possible_keys   key      key_len ref           rows Extra
    et    ALL    PRIMARY         NULL     NULL    NULL          74
    tt    ref    AssignedPC,     ActualPC 15      et.EMPLOYID   52   where used
                 ClientID,
                 ActualPC
    et_1  eq_ref PRIMARY         PRIMARY  15      tt.AssignedPC 1
    do    eq_ref PRIMARY         PRIMARY  15      tt.ClientID   1

This is almost as good as it can get.

The remaining problem is that, by default, MySQL assumes that values in the `tt.ActualPC` column are evenly distributed, and that isn't the case for the `tt` table. Fortunately, it is easy to tell MySQL about this:

    shell> myisamchk --analyze PATH_TO_MYSQL_DATABASE/tt
    shell> mysqladmin refresh

Now the join is perfect, and `EXPLAIN` produces this result:

    table type   possible_keys key     key_len ref           rows Extra
    tt    ALL    AssignedPC    NULL    NULL    NULL          3872 where used
                 ClientID,
                 ActualPC
    et    eq_ref PRIMARY       PRIMARY 15      tt.ActualPC   1
    et_1  eq_ref PRIMARY       PRIMARY 15      tt.AssignedPC 1
    do    eq_ref PRIMARY       PRIMARY 15      tt.ClientID   1

Note that the `rows` column in the output from `EXPLAIN` is an educated guess from the MySQL join optimiser. To optimise a query, you should check if the numbers are even close to the truth. If not, you may get better performance by using `STRAIGHT_JOIN` in your `SELECT` statement and trying to list the tables in a different order in the `FROM` clause.

### 5.2.2 Estimating Query Performance

In most cases you can estimate the performance by counting disk seeks. For small tables, you can usually find the row in 1 disk seek (as the index is probably cached). For bigger tables, you can estimate that (using B++ tree indexes) you will need: `log(row_count) / log(index_block_length / 3 * 2 / (index_length + data_pointer_length)) + 1` seeks to find a row.

In MySQL an index block is usually 1024 bytes and the data pointer is usually 4 bytes. A 500,000 row table with an index length of 3 (medium integer) gives you: `log(500,000)/log(1024/3*2/(3+4)) + 1` = 4 seeks.

As the above index would require about 500,000 * 7 * 3/2 = 5.2M, (assuming that the index buffers are filled to 2/3, which is typical) you will probably have much of the index in memory and you will probably only need 1-2 calls to read data from the OS to find the row.

For writes, however, you will need 4 seek requests (as above) to find where to place the new index and normally 2 seeks to update the index and write the row.

Note that the above doesn't mean that your application will slowly degenerate by log N! As long as everything is cached by the OS or SQL server things will only go marginally slower while the table gets bigger. After the data gets too big to be cached, things will start to go much slower until your applications is only bound by disk-seeks (which increase by log N). To avoid this, increase the index cache as the data grows. See section 5.5.2 Tuning Server Parameters.

### 5.2.3 Speed of `SELECT` Queries

In general, when you want to make a slow `SELECT ... WHERE` faster, the first thing to check is whether you can add an index. See section 5.4.3 How MySQL Uses Indexes. All references between different tables should usually be done with indexes. You can use the `EXPLAIN` command to determine which indexes are used for a `SELECT`. See section 5.2.1 `EXPLAIN` Syntax (Get Information About a `SELECT`).

Some general tips:

- To help MySQL optimise queries better, run `myisamchk --analyze` on a table after it has been loaded with relevant data. This updates a value for each index part that indicates the average number of rows that have the same value. (For unique indexes, this is always 1, of course.) MySQL will use this to decide which index to choose when you connect two tables with 'a non-constant expression'. You can check the result from the `analyze` run by doing `SHOW INDEX FROM table_name` and examining the `Cardinality` column.
- To sort an index and data according to an index, use `myisamchk --sort-index --sort-records=1` (if you want to sort on index 1). If you have a unique index from which you want to read all records in order according to that index, this is a good way to make that faster. Note, however, that this sorting isn't written optimally and will take a long time for a large table!

### 5.2.4 How MySQL Optimises `WHERE` Clauses

The `WHERE` optimisations are put in the `SELECT` part here because they are mostly used with `SELECT`, but the same optimisations apply for `WHERE` in `DELETE` and `UPDATE` statements.

Also note that this section is incomplete. MySQL does many optimisations, and we have not had time to document them all.

Some of the optimisations performed by MySQL are listed here:

- Removal of unnecessary parentheses:

         ((a AND b) AND c OR (((a AND b) AND (c AND d))))
      -> (a AND b AND c) OR (a AND b AND c AND d)

- Constant folding:

         (a<b AND b=c) AND a=5
      -> b>5 AND b=c AND a=5

- Constant condition removal (needed because of constant folding):

         (B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6)
      -> B=5 OR B=6

- Constant expressions used by indexes are evaluated only once.
- `COUNT(*)` on a single table without a `WHERE` is retrieved directly from the table information for `MyISAM` and `HEAP` tables. This is also done for any `NOT NULL` expression when used with only one table.
- Early detection of invalid constant expressions. MySQL quickly detects that some `SELECT` statements are impossible and returns no rows.
- `HAVING` is merged with `WHERE` if you don't use `GROUP BY` or group functions (`COUNT()`, `MIN()`...).
- For each sub-join, a simpler `WHERE` is constructed to get a fast `WHERE` evaluation for each sub-join and also to skip records as soon as possible.
- All constant tables are read first, before any other tables in the query. A constant table is:
  - An empty table or a table with 1 row.
  - A table that is used with a `WHERE` clause on a `UNIQUE` index, or a `PRIMARY KEY`, where all index parts are used with constant expressions and the index parts are defined as `NOT NULL`.
  All the following tables are used as constant tables:

      mysql> SELECT * FROM t WHERE primary_key=1;
      mysql> SELECT * FROM t1,t2
          ->          WHERE t1.primary_key=1 AND t2.primary_key=t1.id;

- The best join combination to join the tables is found by trying all possibilities. If all columns in `ORDER BY` and in `GROUP BY` come from the same table, then this table is preferred first when joining.
- If there is an `ORDER BY` clause and a different `GROUP BY` clause, or if the `ORDER BY` or `GROUP BY` contains columns from tables other than the first table in the join queue, a temporary table is created.
- If you use `SQL_SMALL_RESULT`, MySQL will use an in-memory temporary table.
- Each table index is queried, and the best index that spans fewer than 30% of the rows is used. If no such index can be found, a quick table scan is used.
- In some cases, MySQL can read rows from the index without even consulting the datafile. If all columns used from the index are numeric, then only the index tree is used to resolve the query.
- Before each record is output, those that do not match the `HAVING` clause are skipped.

Some examples of queries that are very fast:

    mysql> SELECT COUNT(*) FROM tbl_name;
    mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name;
    mysql> SELECT MAX(key_part2) FROM tbl_name
        ->        WHERE key_part_1=constant;
    mysql> SELECT ... FROM tbl_name
        ->        ORDER BY key_part1,key_part2,... LIMIT 10;
    mysql> SELECT ... FROM tbl_name
        ->        ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;

The following queries are resolved using only the index tree (assuming the indexed columns are numeric):

    mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val;
    mysql> SELECT COUNT(*) FROM tbl_name
        ->        WHERE key_part1=val1 AND key_part2=val2;
    mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;

The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass:

    mysql> SELECT ... FROM tbl_name
        ->            ORDER BY key_part1,key_part2,... ;
    mysql> SELECT ... FROM tbl_name
        ->            ORDER BY key_part1 DESC,key_part2 DESC,... ;

### 5.2.5 How MySQL Optimises `DISTINCT`

`DISTINCT` is converted to a `GROUP BY` on all columns, `DISTINCT` combined with `ORDER BY` will in many cases also need a temporary table.

When combining `LIMIT #` with `DISTINCT`, MySQL will stop as soon as it finds `#` unique rows.

If you don't use columns from all used tables, MySQL will stop the scanning of the not used tables as soon as it has found the first match.

    SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;

In the case, assuming `t1` is used before `t2` (check with `EXPLAIN`), then MySQL will stop reading from `t2` (for that particular row in `t1`) when the first row in `t2` is found.

### 5.2.6 How MySQL Optimises `LEFT JOIN` and `RIGHT JOIN`

`A LEFT JOIN B` in MySQL is implemented as follows:

- The table `B` is set to be dependent on table `A` and all tables that `A` is dependent on.
- The table `A` is set to be dependent on all tables (except `B`) that are used in the `LEFT JOIN` condition.
- All `LEFT JOIN` conditions are moved to the `WHERE` clause.
- All standard join optimisations are done, with the exception that a table is always read after all tables it is dependent on. If there is a circular dependence then MySQL will issue an error.
- All standard `WHERE` optimisations are done.
- If there is a row in `A` that matches the `WHERE` clause, but there wasn't any row in `B` that matched the `LEFT JOIN` condition, then an extra `B` row is generated with all columns set to `NULL`.
- If you use `LEFT JOIN` to find rows that don't exist in some table and you have the following test: `column_name IS NULL` in the `WHERE` part, where column_name is a column that is declared as `NOT NULL`, then MySQL will stop searching after more rows (for a particular key combination) after it has found one row that matches the `LEFT JOIN` condition.

`RIGHT JOIN` is implemented analogously as `LEFT JOIN`.

The table read order forced by `LEFT JOIN` and `STRAIGHT JOIN` will help the join optimiser (which calculates in which order tables should be joined) to do its work much more quickly, as there are fewer table permutations to check.

Note that the above means that if you do a query of type:

    SELECT * FROM a,b LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key)
             WHERE b.key=d.key

MySQL will do a full scan on `b` as the `LEFT JOIN` will force it to be read before `d`.

The fix in this case is to change the query to:

    SELECT * FROM b,a LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key)
             WHERE b.key=d.key

### 5.2.7 How MySQL Optimises `ORDER BY`

In some cases MySQL can uses index to satisfy an `ORDER BY` or `GROUP BY` request without doing any extra sorting.

The index can also be used even if the `ORDER BY` doesn't match the index exactly, as long as all the unused index parts and all the extra are `ORDER BY` columns are constants in the `WHERE` clause. The following queries will use the index to resolve the `ORDER BY` / `GROUP BY` part:

    SELECT * FROM t1 ORDER BY key_part1,key_part2,...
    SELECT * FROM t1 WHERE key_part1=constant ORDER BY key_part2
    SELECT * FROM t1 WHERE key_part1=constant GROUP BY key_part2
    SELECT * FROM t1 ORDER BY key_part1 DESC,key_part2 DESC
    SELECT * FROM t1 WHERE key_part1=1 ORDER BY key_part1 DESC,key_part2 DESC

Some cases where MySQL can not use indexes to resolve the `ORDER BY`: (Note that MySQL will still use indexes to find the rows that matches the `WHERE` clause):

- You are doing an `ORDER BY` on different keys: `SELECT * FROM t1 ORDER BY key1,key2`
- You are doing an `ORDER BY` using non-consecutive key parts. `SELECT * FROM t1 WHERE key2=constant ORDER BY key_part2`
- You are mixing `ASC` and `DESC`. `SELECT * FROM t1 ORDER BY key_part1 DESC,key_part2 ASC`
- The key used to fetch the rows are not the same one that is used to do the `ORDER BY`: `SELECT * FROM t1 WHERE key2=constant ORDER BY key1`
- You are joining many tables and the columns you are doing an `ORDER BY` on are not all from the first not-`const` table that is used to retrieve rows (This is the first table in the `EXPLAIN` output which doesn't use a `const` row fetch method).
- You have different `ORDER BY` and `GROUP BY` expressions.
- The used table index is an index type that doesn't store rows in order. (Like the `HASH` index in `HEAP` tables).
- The index colum may contain `NULL` values and one is using `ORDER BY ... DESC`. This is because in SQL `NULL` values is always sorted before normal values, independent of you are using `DESC` or not.

In the cases where MySQL have to sort the result, it uses the following algorithm:

- Read all rows according to key or by table scanning. Rows that don't match the `WHERE` clause are skipped.
- Store the sort-key in a buffer (of size `sort_buffer`).
- When the buffer gets full, run a qsort on it and store the result in a temporary file. Save a pointer to the sorted block. (In the case where all rows fits into the sort buffer, no temporary file is created)
- Repeat the above until all rows have been read.
- Do a multi-merge of up to `MERGEBUFF` (7) regions to one block in another temporary file. Repeat until all blocks from the first file are in the second file.
- Repeat the following until there is less than `MERGEBUFF2` (15) blocks left.
- On the last multi-merge, only the pointer to the row (last part of the sort-key) is written to a result file.
- Now the code in ``sql/records.cc'` will be used to read through them in sorted order by using the row pointers in the result file. To optimise this, we read in a big block of row pointers, sort these and then we read the rows in the sorted order into a row buffer (`record_rnd_buffer`) .

You can with `EXPLAIN SELECT ... ORDER BY` check if MySQL can use indexes to resolve the query. If you get `Using filesort` in the `extra` column, then MySQL can't use indexes to resolve the `ORDER BY`. See section 5.2.1 `EXPLAIN` Syntax (Get Information About a `SELECT`).

If you want to have a higher `ORDER BY` speed, you should first see if you can get MySQL to use indexes instead of having to do an extra sorting phase. If this is not possible, then you can do:

- Increase the size of the `sort_buffer` variable.
- Increase the size of the `record_rnd_buffer` variable.
- Change `tmpdir` to point to a dedicated disk with lots of empty space.

### 5.2.8 How MySQL Optimises `LIMIT`

In some cases MySQL will handle the query differently when you are using `LIMIT #` and not using `HAVING`:

- If you are selecting only a few rows with `LIMIT`, MySQL will use indexes in some cases when it normally would prefer to do a full table scan.
- If you use `LIMIT #` with `ORDER BY`, MySQL will end the sorting as soon as it has found the first `#` lines instead of sorting the whole table.
- When combining `LIMIT #` with `DISTINCT`, MySQL will stop as soon as it finds `#` unique rows.
- In some cases a `GROUP BY` can be resolved by reading the key in order (or do a sort on the key) and then calculate summaries until the key value changes. In this case `LIMIT #` will not calculate any unnecessary `GROUP BY`s.
- As soon as MySQL has sent the first `#` rows to the client, it will abort the query (if you are not using `SQL_CALC_FOUND_ROWS`).
- `LIMIT 0` will always quickly return an empty set. This is useful to check the query and to get the column types of the result columns.
- When the server uses temporary tables to resolve the query, the `LIMIT #` is used to calculate how much space is required.

### 5.2.9 Speed of `INSERT` Queries

The time to insert a record consists approximately of:

- Connect: (3)
- Sending query to server: (2)
- Parsing query: (2)
- Inserting record: (1 x size of record)
- Inserting indexes: (1 x number of indexes)
- Close: (1)

where the numbers are somewhat proportional to the overall time. This does not take into consideration the initial overhead to open tables (which is done once for each concurrently running query).

The size of the table slows down the insertion of indexes by log N (B-trees).

Some ways to speed up inserts:

- If you are inserting many rows from the same client at the same time, use multiple value lists `INSERT` statements. This is much faster (many times in some cases) than using separate `INSERT` statements. If you are adding data to non-empty table, you may tune up `bulk_insert_buffer_size`u`bulk_insert_buffer_size` variable to make it even faster. See section 4.5.6.4 `SHOW VARIABLES`.
- If you are inserting a lot of rows from different clients, you can get higher speed by using the `INSERT DELAYED` statement. See section 6.4.3 `INSERT` Syntax.
- Note that with `MyISAM` tables you can insert rows at the same time `SELECT`s are running if there are no deleted rows in the tables.
- When loading a table from a text file, use `LOAD DATA INFILE`. This is usually 20 times faster than using a lot of `INSERT` statements. See section 6.4.9 `LOAD DATA INFILE` Syntax.
- It is possible with some extra work to make `LOAD DATA INFILE` run even faster when the table has many indexes. Use the following procedure:
  1. Optionally create the table with `CREATE TABLE`. For example, using `mysql` or Perl-DBI.
  2. Execute a `FLUSH TABLES` statement or the shell command `mysqladmin flush-tables`.
  3. Use `myisamchk --keys-used=0 -rq /path/to/db/tbl_name`. This will remove all usage of all indexes from the table.
  4. Insert data into the table with `LOAD DATA INFILE`. This will not update any indexes and will therefore be very fast.
  5. If you are going to only read the table in the future, run `myisampack` on it to make it smaller. See section 7.1.2.3 Compressed Table Characteristics.
  6. Re-create the indexes with `myisamchk -r -q /path/to/db/tbl_name`. This will create the index tree in memory before writing it to disk, which is much faster because it avoids lots of disk seeks. The resulting index tree is also perfectly balanced.
  7. Execute a `FLUSH TABLES` statement or the shell command `mysqladmin flush-tables`.
  Note that `LOAD DATA INFILE` also does the above optimisation if you insert into an empty table; the main difference with the above procedure is that you can let `myisamchk` allocate much more temporary memory for the index creation that you may want MySQL to allocate for every index recreation. Since MySQL 4.0 you can also use `ALTER TABLE tbl_name DISABLE KEYS` instead of `myisamchk --keys-used=0 -rq /path/to/db/tbl_name` and `ALTER TABLE tbl_name ENABLE KEYS` instead of `myisamchk -r -q /path/to/db/tbl_name`. This way you can also skip `FLUSH TABLES` steps.
- You can speed up insertions that is done over multiple statements by locking your tables:

      mysql> LOCK TABLES a WRITE;
      mysql> INSERT INTO a VALUES (1,23),(2,34),(4,33);
      mysql> INSERT INTO a VALUES (8,26),(6,29);
      mysql> UNLOCK TABLES;

  The main speed difference is that the index buffer is flushed to disk only once, after all `INSERT` statements have completed. Normally there would be as many index buffer flushes as there are different `INSERT` statements. Locking is not needed if you can insert all rows with a single statement. For transactional tables, you should use `BEGIN/COMMIT` instead of `LOCK TABLES` to get a speedup. Locking will also lower the total time of multi-connection tests, but the maximum wait time for some threads will go up (because they wait for locks). For example:

      thread 1 does 1000 inserts
      thread 2, 3, and 4 does 1 insert
      thread 5 does 1000 inserts

  If you don't use locking, 2, 3, and 4 will finish before 1 and 5. If you use locking, 2, 3, and 4 probably will not finish before 1 or 5, but the total time should be about 40% faster. As `INSERT`, `UPDATE`, and `DELETE` operations are very fast in MySQL, you will obtain better overall performance by adding locks around everything that does more than about 5 inserts or updates in a row. If you do very many inserts in a row, you could do a `LOCK TABLES` followed by an `UNLOCK TABLES` once in a while (about each 1000 rows) to allow other threads access to the table. This would still result in a nice performance gain. Of course, `LOAD DATA INFILE` is much faster for loading data.

To get some more speed for both `LOAD DATA INFILE` and `INSERT`, enlarge the key buffer. See section 5.5.2 Tuning Server Parameters.

### 5.2.10 Speed of `UPDATE` Queries

Update queries are optimised as a `SELECT` query with the additional overhead of a write. The speed of the write is dependent on the size of the data that is being updated and the number of indexes that are updated. Indexes that are not changed will not be updated.

Also, another way to get fast updates is to delay updates and then do many updates in a row later. Doing many updates in a row is much quicker than doing one at a time if you lock the table.

Note that, with dynamic record format, updating a record to a longer total length may split the record. So if you do this often, it is very important to `OPTIMIZE TABLE` sometimes. See section 4.5.1 `OPTIMIZE TABLE` Syntax.

### 5.2.11 Speed of `DELETE` Queries

If you want to delete all rows in the table, you should use `TRUNCATE TABLE table_name`. See section 6.4.7 `TRUNCATE` Syntax.

The time to delete a record is exactly proportional to the number of indexes. To delete records more quickly, you can increase the size of the index cache. See section 5.5.2 Tuning Server Parameters.

### 5.2.12 Other Optimisation Tips

Unsorted tips for faster systems:

- Use persistent connections to the database to avoid the connection overhead. If you can't use persistent connections and you are doing a lot of new connections to the database, you may want to change the value of the `thread_cache_size` variable. See section 5.5.2 Tuning Server Parameters.
- Always check that all your queries really use the indexes you have created in the tables. In MySQL you can do this with the `EXPLAIN` command. See section 5.2.1 `EXPLAIN` Syntax (Get Information About a `SELECT`).
- Try to avoid complex `SELECT` queries on `MyISAM` tables that are updated a lot. This is to avoid problems with table locking.
- The new `MyISAM` tables can insert rows in a table without deleted rows at the same time another table is reading from it. If this is important for you, you should consider methods where you don't have to delete rows or run `OPTIMIZE TABLE` after you have deleted a lot of rows.
- Use `ALTER TABLE ... ORDER BY expr1,expr2...` if you mostly retrieve rows in `expr1,expr2...` order. By using this option after big changes to the table, you may be able to get higher performance.
- In some cases it may make sense to introduce a column that is 'hashed' based on information from other columns. If this column is short and reasonably unique it may be much faster than a big index on many columns. In MySQL it's very easy to use this extra column: `SELECT * FROM table_name WHERE hash=MD5(CONCAT(col1,col2)) AND col_1='constant' AND col_2='constant'`
- For tables that change a lot you should try to avoid all `VARCHAR` or `BLOB` columns. You will get dynamic row length as soon as you are using a single `VARCHAR` or `BLOB` column. See section 7 MySQL Table Types.
- It's not normally useful to split a table into different tables just because the rows gets 'big'. To access a row, the biggest performance hit is the disk seek to find the first byte of the row. After finding the data most new disks can read the whole row fast enough for most applications. The only cases where it really matters to split up a table is if it's a dynamic row size table (see above) that you can change to a fixed row size, or if you very often need to scan the table and don't need most of the columns. See section 7 MySQL Table Types.
- If you very often need to calculate things based on information from a lot of rows (like counts of things), it's probably much better to introduce a new table and update the counter in real time. An update of type `UPDATE table set count=count+1 where index_column=constant` is very fast! This is really important when you use MySQL table types like MyISAM and ISAM that only have table locking (multiple readers / single writers). This will also give better performance with most databases, as the row locking manager in this case will have less to do.
- If you need to collect statistics from big log tables, use summary tables instead of scanning the whole table. Maintaining the summaries should be much faster than trying to do statistics 'live'. It's much faster to regenerate new summary tables from the logs when things change (depending on business decisions) than to have to change the running application!
- If possible, one should classify reports as 'live' or 'statistical', where data needed for statistical reports are only generated based on summary tables that are generated from the actual data.
- Take advantage of the fact that columns have default values. Insert values explicitly only when the value to be inserted differs from the default. This reduces the parsing that MySQL need to do and improves the insert speed.
- In some cases it's convenient to pack and store data into a blob. In this case you have to add some extra code in your application to pack/unpack things in the blob, but this may save a lot of accesses at some stage. This is practical when you have data that doesn't conform to a static table structure.
- Normally you should try to keep all data non-redundant (what is called 3rd normal form in database theory), but you should not be afraid of duplicating things or creating summary tables if you need these to gain more speed.
- Stored procedures or UDF (user-defined functions) may be a good way to get more performance. In this case you should, however, always have a way to do this some other (slower) way if you use some database that doesn't support this.
- You can always gain something by caching queries/answers in your application and trying to do many inserts/updates at the same time. If your database supports lock tables (like MySQL and Oracle), this should help to ensure that the index cache is only flushed once after all updates.
- Use `INSERT /*! DELAYED */` when you do not need to know when your data is written. This speeds things up because many records can be written with a single disk write.
- Use `INSERT /*! LOW_PRIORITY */` when you want your selects to be more important.
- Use `SELECT /*! HIGH_PRIORITY */` to get selects that jump the queue. That is, the select is done even if there is somebody waiting to do a write.
- Use the multi-line `INSERT` statement to store many rows with one SQL command (many SQL servers supports this).
- Use `LOAD DATA INFILE` to load bigger amounts of data. This is faster than normal inserts and will be even faster when `myisamchk` is integrated in `mysqld`.
- Use `AUTO_INCREMENT` columns to make unique values.
- Use `OPTIMIZE TABLE` once in a while to avoid fragmentation when using a dynamic table format. See section 4.5.1 `OPTIMIZE TABLE` Syntax.
- Use `HEAP` tables to get more speed when possible. See section 7 MySQL Table Types.
- When using a normal web server setup, images should be stored as files. That is, store only a file reference in the database. The main reason for this is that a normal web server is much better at caching files than database contents. So it it's much easier to get a fast system if you are using files.
- Use in memory tables for non-critical data that are accessed often (like information about the last shown banner for users that don't have cookies).
- Columns with identical information in different tables should be declared identical and have identical names. Before Version 3.23 you got slow joins otherwise. Try to keep the names simple (use `name` instead of `customer_name` in the customer table). To make your names portable to other SQL servers you should keep them shorter than 18 characters.
- If you need really high speed, you should take a look at the low-level interfaces for data storage that the different SQL servers support! For example, by accessing the MySQL `MyISAM` directly, you could get a speed increase of 2-5 times compared to using the SQL interface. To be able to do this the data must be on the same server as the application, and usually it should only be accessed by one process (because external file locking is really slow). One could eliminate the above problems by introducing low-level `MyISAM` commands in the MySQL server (this could be one easy way to get more performance if needed). By carefully designing the database interface, it should be quite easy to support this types of optimisation.
- In many cases it's faster to access data from a database (using a live connection) than accessing a text file, just because the database is likely to be more compact than the text file (if you are using numerical data), and this will involve fewer disk accesses. You will also save code because you don't have to parse your text files to find line and column boundaries.
- You can also use replication to speed things up. See section 4.10 Replication in MySQL.
- Declaring a table with `DELAY_KEY_WRITE=1` will make the updating of indexes faster, as these are not logged to disk until the file is closed. The downside is that you should run `myisamchk` on these tables before you start `mysqld` to ensure that they are okay if something killed `mysqld` in the middle. As the key information can always be generated from the data, you should not lose anything by using `DELAY_KEY_WRITE`.

## 5.3 Locking Issues

### 5.3.1 How MySQL Locks Tables

You can find a discussion about different locking methods in the appendix. See section E.4 Locking methods.

All locking in MySQL is deadlock-free, except for `InnoDB` and `BDB` type tables. This is managed by always requesting all needed locks at once at the beginning of a query and always locking the tables in the same order.

`InnoDB` type tables automatically acquire their row locks and `BDB` type tables their page locks during the processing of SQL statements, not at the start of the transaction.

The locking method MySQL uses for `WRITE` locks works as follows:

- If there are no locks on the table, put a write lock on it.
- Otherwise, put the lock request in the write lock queue.

The locking method MySQL uses for `READ` locks works as follows:

- If there are no write locks on the table, put a read lock on it.
- Otherwise, put the lock request in the read lock queue.

When a lock is released, the lock is made available to the threads in the write lock queue, then to the threads in the read lock queue.

This means that if you have many updates on a table, `SELECT` statements will wait until there are no more updates.

To work around this for the case where you want to do many `INSERT` and `SELECT` operations on a table, you can insert rows in a temporary table and update the real table with the records from the temporary table once in a while.

This can be done with the following code:

    mysql> LOCK TABLES real_table WRITE, insert_table WRITE;
    mysql> INSERT INTO real_table SELECT * FROM insert_table;
    mysql> TRUNCATE TABLE insert_table;
    mysql> UNLOCK TABLES;

You can use the `LOW_PRIORITY` options with `INSERT`, `UPDATE` or `DELETE` or `HIGH_PRIORITY` with `SELECT` if you want to prioritise retrieval in some specific cases. You can also start `mysqld` with `--low-priority-updates` to get the same behaveour.

Using `SQL_BUFFER_RESULT` can also help making table locks shorter. See section 6.4.1 `SELECT` Syntax.

You could also change the locking code in ``mysys/thr_lock.c'` to use a single queue. In this case, write locks and read locks would have the same priority, which might help some applications.

### 5.3.2 Table Locking Issues

The table locking code in MySQL is deadlock free.

MySQL uses table locking (instead of row locking or column locking) on all table types, except `InnoDB` and `BDB` tables, to achieve a very high lock speed. For large tables, table locking is much better than row locking for most applications, but there are, of course, some pitfalls.

For `InnoDB` and `BDB` tables, MySQL only uses table locking if you explicitly lock the table with `LOCK TABLES`. For these table types we recommend you to not use `LOCK TABLES` at all, because `InnoDB` uses automatic row level locking and `BDB` uses page level locking to ensure transaction isolation.

In MySQL Version 3.23.7 and above, you can insert rows into `MyISAM` tables at the same time other threads are reading from the table. Note that currently this only works if there are no holes after deleted rows in the table at the time the insert is made. When all holes has been filled with new data, concurrent inserts will automatically be enabled again.

Table locking enables many threads to read from a table at the same time, but if a thread wants to write to a table, it must first get exclusive access. During the update, all other threads that want to access this particular table will wait until the update is ready.

As updates on tables normally are considered to be more important than `SELECT`, all statements that update a table have higher priority than statements that retrieve information from a table. This should ensure that updates are not 'starved' because one issues a lot of heavy queries against a specific table. (You can change this by using `LOW_PRIORITY` with the statement that does the update or `HIGH_PRIORITY` with the `SELECT` statement.)

Starting from MySQL Version 3.23.7 one can use the `max_write_lock_count` variable to force MySQL to temporary give all `SELECT` statements, that wait for a table, a higher priority after a specific number of inserts on a table.

Table locking is, however, not very good under the following senario:

- A client issues a `SELECT` that takes a long time to run.
- Another client then issues an `UPDATE` on a used table. This client will wait until the `SELECT` is finished.
- Another client issues another `SELECT` statement on the same table. As `UPDATE` has higher priority than `SELECT`, this `SELECT` will wait for the `UPDATE` to finish. It will also wait for the first `SELECT` to finish!
- A thread is waiting for something like `full disk`, in which case all threads that wants to access the problem table will also be put in a waiting state until more disk space is made available.

Some possible solutions to this problem are:

- Try to get the `SELECT` statements to run faster. You may have to create some summary tables to do this.
- Start `mysqld` with `--low-priority-updates`. This will give all statements that update (modify) a table lower priority than a `SELECT` statement. In this case the last `SELECT` statement in the previous scenario would execute before the `INSERT` statement.
- You can give a specific `INSERT`, `UPDATE`, or `DELETE` statement lower priority with the `LOW_PRIORITY` attribute.
- Start `mysqld` with a low value for `max_write_lock_count` to give `READ` locks after a certain number of `WRITE` locks.
- You can specify that all updates from a specific thread should be done with low priority by using the SQL command: `SET LOW_PRIORITY_UPDATES=1`. See section 5.5.6 `SET` Syntax.
- You can specify that a specific `SELECT` is very important with the `HIGH_PRIORITY` attribute. See section 6.4.1 `SELECT` Syntax.
- If you have problems with `INSERT` combined with `SELECT`, switch to use the new `MyISAM` tables as these support concurrent `SELECT`s and `INSERT`s.
- If you mainly mix `INSERT` and `SELECT` statements, the `DELAYED` attribute to `INSERT` will probably solve your problems. See section 6.4.3 `INSERT` Syntax.
- If you have problems with `SELECT` and `DELETE`, the `LIMIT` option to `DELETE` may help. See section 6.4.6 `DELETE` Syntax.

## 5.4 Optimising Database Structure

### 5.4.1 Design Choices

MySQL keeps row data and index data in separate files. Many (almost all) other databases mix row and index data in the same file. We believe that the MySQL choice is better for a very wide range of modern systems.

Another way to store the row data is to keep the information for each column in a separate area (examples are SDBM and Focus). This will cause a performance hit for every query that accesses more than one column. Because this degenerates so quickly when more than one column is accessed, we believe that this model is not good for general purpose databases.

The more common case is that the index and data are stored together (like in Oracle/Sybase et al). In this case you will find the row information at the leaf page of the index. The good thing with this layout is that it, in many cases, depending on how well the index is cached, saves a disk read. The bad things with this layout are:

- Table scanning is much slower because you have to read through the indexes to get at the data.
- You can't use only the index table to retrieve data for a query.
- You lose a lot of space, as you must duplicate indexes from the nodes (as you can't store the row in the nodes).
- Deletes will degenerate the table over time (as indexes in nodes are usually not updated on delete).
- It's harder to cache only the index data.

### 5.4.2 Get Your Data as Small as Possible

One of the most basic optimisation is to get your data (and indexes) to take as little space on the disk (and in memory) as possible. This can give huge improvements because disk reads are faster and normally less main memory will be used. Indexing also takes less resources if done on smaller columns.

MySQL supports a lot of different table types and row formats. Choosing the right table format may give you a big performance gain. See section 7 MySQL Table Types.

You can get better performance on a table and minimise storage space using the techniques listed here:

- Use the most efficient (smallest) types possible. MySQL has many specialised types that save disk space and memory.
- Use the smaller integer types if possible to get smaller tables. For example, `MEDIUMINT` is often better than `INT`.
- Declare columns to be `NOT NULL` if possible. It makes everything faster and you save one bit per column. Note that if you really need `NULL` in your application you should definitely use it. Just avoid having it on all columns by default.
- If you don't have any variable-length columns (`VARCHAR`, `TEXT`, or `BLOB` columns), a fixed-size record format is used. This is faster but unfortunately may waste some space. See section 7.1.2 `MyISAM` Table Formats.
- The primary index of a table should be as short as possible. This makes identification of one row easy and efficient.
- For each table, you have to decide which storage/index method to use. See section 7 MySQL Table Types.
- Only create the indexes that you really need. Indexes are good for retrieval but bad when you need to store things fast. If you mostly access a table by searching on a combination of columns, make an index on them. The first index part should be the most used column. If you are always using many columns, you should use the column with more duplicates first to get better compression of the index.
- If it's very likely that a column has a unique prefix on the first number of characters, it's better to only index this prefix. MySQL supports an index on a part of a character column. Shorter indexes are faster not only because they take less disk space but also because they will give you more hits in the index cache and thus fewer disk seeks. See section 5.5.2 Tuning Server Parameters.
- In some circumstances it can be beneficial to split into two a table that is scanned very often. This is especially true if it is a dynamic format table and it is possible to use a smaller static format table that can be used to find the relevant rows when scanning the table.

### 5.4.3 How MySQL Uses Indexes

Indexes are used to find rows with a specific value of one column fast. Without an index MySQL has to start with the first record and then read through the whole table until it finds the relevant rows. The bigger the table, the more this costs. If the table has an index for the columns in question, MySQL can quickly get a position to seek to in the middle of the datafile without having to look at all the data. If a table has 1000 rows, this is at least 100 times faster than reading sequentially. Note that if you need to access almost all 1000 rows it is faster to read sequentially because we then avoid disk seeks.

All MySQL indexes (`PRIMARY`, `UNIQUE`, and `INDEX`) are stored in B-trees. Strings are automatically prefix- and end-space compressed. See section 6.5.7 `CREATE INDEX` Syntax.

Indexes are used to:

- Quickly find the rows that match a `WHERE` clause.
- Retrieve rows from other tables when performing joins.
- Find the `MAX()` or `MIN()` value for a specific indexed column. This is optimised by a preprocessor that checks if you are using `WHERE` key_part_# = constant on all key parts < N. In this case MySQL will do a single key lookup and replace the `MIN()` expression with a constant. If all expressions are replaced with constants, the query will return at once:

      SELECT MIN(key_part2),MAX(key_part2) FROM table_name where key_part1=10

- Sort or group a table if the sorting or grouping is done on a leftmost prefix of a usable key (for example, `ORDER BY key_part_1,key_part_2` ). The key is read in reverse order if all key parts are followed by `DESC`. See section 5.2.7 How MySQL Optimises `ORDER BY`.
- In some cases a query can be optimised to retrieve values without consulting the datafile. If all used columns for some table are numeric and form a leftmost prefix for some key, the values may be retrieved from the index tree for greater speed:

      SELECT key_part3 FROM table_name WHERE key_part1=1

Suppose you issue the following `SELECT` statement:

    mysql> SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2;

If a multiple-column index exists on `col1` and `col2`, the appropriate rows can be fetched directly. If separate single-column indexes exist on `col1` and `col2`, the optimiser tries to find the most restrictive index by deciding which index will find fewer rows and using that index to fetch the rows.

If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimiser to find rows. For example, if you have a three-column index on `(col1,col2,col3)`, you have indexed search capabilities on `(col1)`, `(col1,col2)`, and `(col1,col2,col3)`.

MySQL can't use a partial index if the columns don't form a leftmost prefix of the index. Suppose you have the `SELECT` statements shown here:

    mysql> SELECT * FROM tbl_name WHERE col1=val1;
    mysql> SELECT * FROM tbl_name WHERE col2=val2;
    mysql> SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3;

If an index exists on `(col1,col2,col3)`, only the first query shown above uses the index. The second and third queries do involve indexed columns, but `(col2)` and `(col2,col3)` are not leftmost prefixes of `(col1,col2,col3)`.

MySQL also uses indexes for `LIKE` comparisons if the argument to `LIKE` is a constant string that doesn't start with a wildcard character. For example, the following `SELECT` statements use indexes:

    mysql> SELECT * FROM tbl_name WHERE key_col LIKE "Patrick%";
    mysql> SELECT * FROM tbl_name WHERE key_col LIKE "Pat%_ck%";

In the first statement, only rows with `"Patrick" <= key_col < "Patricl"` are considered. In the second statement, only rows with `"Pat" <= key_col < "Pau"` are considered.

The following `SELECT` statements will not use indexes:

    mysql> SELECT * FROM tbl_name WHERE key_col LIKE "%Patrick%";
    mysql> SELECT * FROM tbl_name WHERE key_col LIKE other_col;

In the first statement, the `LIKE` value begins with a wildcard character. In the second statement, the `LIKE` value is not a constant.

MySQL 4.0 does another optimisation on `LIKE`. If you use `... LIKE "%string%"` and `string` is longer than 3 characters, MySQL will use the `Turbo Boyer-Moore` algorithm to initialise the pattern for the string and then use this pattern to perform the search quicker.

Searching using `column_name IS NULL` will use indexes if column_name is an index.

MySQL normally uses the index that finds the least number of rows. An index is used for columns that you compare with the following operators: `=`, `>`, `>=`, `<`, `<=`, `BETWEEN`, and a `LIKE` with a non-wildcard prefix like `'something%'`.

Any index that doesn't span all `AND` levels in the `WHERE` clause is not used to optimise the query. In other words: To be able to use an index, a prefix of the index must be used in every `AND` group.

The following `WHERE` clauses use indexes:

    ... WHERE index_part1=1 AND index_part2=2 AND other_column=3
    ... WHERE index=1 OR A=10 AND index=2      /* index = 1 OR index = 2 */
    ... WHERE index_part1='hello' AND index_part_3=5
              /* optimised like "index_part1='hello'" */
    ... WHERE index1=1 and index2=2 or index1=3 and index3=3;
              /* Can use index on index1 but not on index2 or index 3 */

These `WHERE` clauses do NOT use indexes:

    ... WHERE index_part2=1 AND index_part3=2  /* index_part_1 is not used */
    ... WHERE index=1 OR A=10                  /* Index is not used in
                                                            both AND parts */
    ... WHERE index_part1=1 OR index_part2=10  /* No index spans all rows  */

Note that in some cases MySQL will not use an index, even if one would be available. Some of the cases where this happens are:

- If the use of the index would require MySQL to access more than 30% of the rows in the table. (In this case a table scan is probably much faster, as this will require us to do much fewer seeks.) Note that if such a query uses `LIMIT` to only retrieve part of the rows, MySQL will use an index anyway, as it can much more quickly find the few rows to return in the result.
- If the index range may contain `NULL` values and you are using `ORDER BY ... DESC`

### 5.4.4 Column Indexes

All MySQL column types can be indexed. Use of indexes on the relevant columns is the best way to improve the performance of `SELECT` operations.

The maximum number of keys and the maximum index length is defined per storage engine. See section 7 MySQL Table Types. You can with all storage engines have at least 16 keys and a total index length of at least 256 bytes.

For `CHAR` and `VARCHAR` columns, you can index a prefix of a column. This is much faster and requires less disk space than indexing the whole column. The syntax to use in the `CREATE TABLE` statement to index a column prefix looks like this:

    KEY index_name (col_name(length))

The example here creates an index for the first 10 characters of the `name` column:

    mysql> CREATE TABLE test (
        ->        name CHAR(200) NOT NULL,
        ->        KEY index_name (name(10)));

For `BLOB` and `TEXT` columns, you must index a prefix of the column. You cannot index the entire column.

In MySQL Version 3.23.23 or later, you can also create special `FULLTEXT` indexes. They are used for full-text search. Only the `MyISAM` table type supports `FULLTEXT` indexes. They can be created only from `CHAR`, `VARCHAR`, and `TEXT` columns. Indexing always happens over the entire column and partial indexing is not supported. See section 6.8 MySQL Full-text Search for details.

### 5.4.5 Multiple-Column Indexes

MySQL can create indexes on multiple columns. An index may consist of up to 15 columns. (On `CHAR` and `VARCHAR` columns you can also use a prefix of the column as a part of an index.)

A multiple-column index can be considered a sorted array containing values that are created by concatenating the values of the indexed columns.

MySQL uses multiple-column indexes in such a way that queries are fast when you specify a known quantity for the first column of the index in a `WHERE` clause, even if you don't specify values for the other columns.

Suppose a table is created using the following specification:

    mysql> CREATE TABLE test (
        ->       id INT NOT NULL,
        ->       last_name CHAR(30) NOT NULL,
        ->       first_name CHAR(30) NOT NULL,
        ->       PRIMARY KEY (id),
        ->       INDEX name (last_name,first_name));

Then the index `name` is an index over `last_name` and `first_name`. The index will be used for queries that specify values in a known range for `last_name`, or for both `last_name` and `first_name`. Therefore, the `name` index will be used in the following queries:

    mysql> SELECT * FROM test WHERE last_name="Widenius";

    mysql> SELECT * FROM test WHERE last_name="Widenius"
        ->                    AND first_name="Michael";

    mysql> SELECT * FROM test WHERE last_name="Widenius"
        ->                    AND (first_name="Michael" OR first_name="Monty");

    mysql> SELECT * FROM test WHERE last_name="Widenius"
        ->                    AND first_name >="M" AND first_name < "N";

However, the `name` index will NOT be used in the following queries:

    mysql> SELECT * FROM test WHERE first_name="Michael";

    mysql> SELECT * FROM test WHERE last_name="Widenius"
        ->                    OR first_name="Michael";

For more information on the manner in which MySQL uses indexes to improve query performance, see section 5.4.3 How MySQL Uses Indexes.

### 5.4.6 Why So Many Open tables?

When you run `mysqladmin status`, you'll see something like this:

    Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12

This can be somewhat perplexing if you only have 6 tables.

MySQL is multi-threaded, so it may have many queries on the same table simultaneously. To minimise the problem with two threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory but will normaly increase performance. Wth `ISAM` and `MyISAM` tables this also requires one extra file descriptor for the datafile. With these table types the index file descriptor is shared between all threads.

You can read more about this topic in the next section. See section 5.4.7 How MySQL Opens and Closes Tables.

### 5.4.7 How MySQL Opens and Closes Tables

`table_cache`, `max_connections`, and `max_tmp_tables` affect the maximum number of files the server keeps open. If you increase one or both of these values, you may run up against a limit imposed by your operating system on the per-process number of open file descriptors. However, you can increase the limit on many systems. Consult your OS documentation to find out how to do this, because the method for changing the limit varies widely from system to system.

`table_cache` is related to `max_connections`. For example, for 200 concurrent running connections, you should have a table cache of at least `200 * n`, where `n` is the maximum number of tables in a join. You also need to reserve some extra file descriptors for temporary tables and files.

Make sure that your operating system can handle the number of open file descriptors implied by the `table_cache` setting. If `table_cache` is set too high, MySQL may run out of file descriptors and refuse connections, fail to perform queries, and be very unreliable. You also have to take into account that the `MyISAM` storage engine needs two file descriptors for each unique open table. You can in increase the number of file descriptors available for MySQL with the `--open-files-limit=#` startup option. See section A.2.16 File Not Found.

The cache of open tables will be keept at a level of `table_cache` entries (default 64; this can be changed with the `-O table_cache=#` option to `mysqld`). Note that in MySQL may temporarly open even more tables to be able to execute queries.

A not used table is closed and removed from the table cache under the following circumstances:

- When the cache is full and a thread tries to open a table that is not in the cache.
- When the cache contains more than `table_cache` entries and a thread is no longer using a table.
- When someone executes `mysqladmin refresh` or `mysqladmin flush-tables`.
- When someone executes a `FLUSH TABLES` statement.

When the table cache fills up, the server uses the following procedure to locate a cache entry to use:

- Tables that are not currently in use are released, in least-recently-used order.
- If the cache is full and no tables can be released, but a new table needs to be opened, the cache is temporarily extended as necessary.
- If the cache is in a temporarily-extended state and a table goes from in-use to not-in-use state, the table is closed and released from the cache.

A table is opened for each concurrent access. This means that if you have two threads accessing the same table or access the table twice in the same query (with `AS`) the table needs to be opened twice. The first open of any table takes two file descriptors; each additional use of the table takes only one file descriptor. The extra descriptor for the first open is used for the index file; this descriptor is shared among all threads.

If you are opening a table with the `HANDLER table_name OPEN` statement, a dedicated table object is allocated for the thread. This table object is not shared by other threads an will not be closed until the thread calls `HANDLER table_name CLOSE` or the thread dies. See section 6.4.2 `HANDLER` Syntax. When this happens, the table is put back in the table cache (if it isn't full).

You can check if your table cache is too small by checking the `mysqld` variable `Opened_tables`. If this is quite big, even if you haven't done a lot of `FLUSH TABLES`, you should increase your table cache. See section 4.5.6.3 `SHOW STATUS`.

### 5.4.8 Drawbacks to Creating Large Numbers of Tables in the Same Database

If you have many files in a directory, open, close, and create operations will be slow. If you execute `SELECT` statements on many different tables, there will be a little overhead when the table cache is full, because for every table that has to be opened, another must be closed. You can reduce this overhead by making the table cache larger.

## 5.5 Optimising the MySQL Server

### 5.5.1 System/Compile Time and Startup Parameter Tuning

We start with the system level things since some of these decisions have to be made very early. In other cases a fast look at this part may suffice because it not that important for the big gains. However, it is always nice to have a feeling about how much one could gain by changing things at this level.

The default OS to use is really important! To get the most use of multiple-CPU machines one should use Solaris (because the threads works really nice) or Linux (because the 2.2 kernel has really good SMP support). Also on 32-bit machines Linux has a 2G file-size limit by default. Hopefully this will be fixed soon when new filesystems are released (XFS/Reiserfs). If you have a desperate need for files bigger than 2G on Linux-intel 32 bit, you should get the LFS patch for the ext2 filesystem.

Because we have not run MySQL in production on that many platforms, we advice you to test your intended platform before choosing it, if possible.

Other tips:

- If you have enough RAM, you could remove all swap devices. Some operating systems will use a swap device in some contexts even if you have free memory.
- Use the `--skip-external-locking` MySQL option to avoid external locking. Note that this will not impact MySQL's functionality as long as you only run one server. Just remember to take down the server (or lock relevant parts) before you run `myisamchk`. On some system this switch is mandatory because the external locking does not work in any case. The `--skip-external-locking` option is on by default when compiling with MIT-pthreads, because `flock()` isn't fully supported by MIT-pthreads on all platforms. It's also on default for Linux as Linux file locking are not yet safe. The only case when you can't use `--skip-external-locking` is if you run multiple MySQL servers (not clients) on the same data, or run `myisamchk` on the table without first flushing and locking the `mysqld` server tables first. You can still use `LOCK TABLES`/`UNLOCK TABLES` even if you are using `--skip-external-locking`

### 5.5.2 Tuning Server Parameters

You can get the default buffer sizes used by the `mysqld` server with this command:

    shell> mysqld --help

This command produces a list of all `mysqld` options and configurable variables. The output includes the default values and looks something like this:

    Possible variables for option --set-variable (-O) are:
    back_log                 current value: 5
    bdb_cache_size           current value: 1048540
    binlog_cache_size        current value: 32768
    connect_timeout          current value: 5
    delayed_insert_timeout   current value: 300
    delayed_insert_limit     current value: 100
    delayed_queue_size       current value: 1000
    flush_time               current value: 0
    interactive_timeout      current value: 28800
    join_buffer_size         current value: 131072
    key_buffer_size          current value: 1048540
    lower_case_table_names   current value: 0
    long_query_time          current value: 10
    max_allowed_packet       current value: 1048576
    max_binlog_cache_size    current value: 4294967295
    max_connections          current value: 100
    max_connect_errors       current value: 10
    max_delayed_threads      current value: 20
    max_heap_table_size      current value: 16777216
    max_join_size            current value: 4294967295
    max_sort_length          current value: 1024
    max_tmp_tables           current value: 32
    max_write_lock_count     current value: 4294967295
    myisam_sort_buffer_size  current value: 8388608
    net_buffer_length        current value: 16384
    net_retry_count          current value: 10
    net_read_timeout         current value: 30
    net_write_timeout        current value: 60
    read_buffer_size         current value: 131072
    record_rnd_buffer_size   current value: 131072
    slow_launch_time         current value: 2
    sort_buffer              current value: 2097116
    table_cache              current value: 64
    thread_concurrency       current value: 10
    tmp_table_size           current value: 1048576
    thread_stack             current value: 131072
    wait_timeout             current value: 28800

Please note that `--set-variable` is deprecated since MySQL 4.0, just use `--var=option` on its own.

If there is a `mysqld` server currently running, you can see what values it actually is using for the variables by executing this command:

    shell> mysqladmin variables

You can find a full description for all variables in the `SHOW VARIABLES` section in this manual. See section 4.5.6.4 `SHOW VARIABLES`.

You can also see some statistics from a running server by issuing the command `SHOW STATUS`. See section 4.5.6.3 `SHOW STATUS`.

MySQL uses algorithms that are very scalable, so you can usually run with very little memory. If you, however, give MySQL more memory, you will normally also get better performance.

When tuning a MySQL server, the two most important variables to use are `key_buffer_size` and `table_cache`. You should first feel confident that you have these right before trying to change any of the other variables.

If you have much memory (>=256M) and many tables and want maximum performance with a moderate number of clients, you should use something like this:

    shell> safe_mysqld -O key_buffer=64M -O table_cache=256 \
               -O sort_buffer=4M -O read_buffer_size=1M &

If you have only 128M and only a few tables, but you still do a lot of sorting, you can use something like:

    shell> safe_mysqld -O key_buffer=16M -O sort_buffer=1M

If you have little memory and lots of connections, use something like this:

    shell> safe_mysqld -O key_buffer=512k -O sort_buffer=100k \
               -O read_buffer_size=100k &

or even:

    shell> safe_mysqld -O key_buffer=512k -O sort_buffer=16k \
               -O table_cache=32 -O read_buffer_size=8k -O net_buffer_length=1K &

If you are doing a `GROUP BY` or `ORDER BY` on files that are much bigger than your available memory you should increase the value of `record_rnd_buffer` to speed up the reading of rows after the sorting is done.

When you have installed MySQL, the ``support-files'` directory will contain some different ``my.cnf'` example files, ``my-huge.cnf'`, ``my-large.cnf'`, ``my-medium.cnf'`, and ``my-small.cnf'`, you can use as a base to optimise your system.

If there are very many connections, ``swapping problems'' may occur unless `mysqld` has been configured to use very little memory for each connection. `mysqld` performs better if you have enough memory for all connections, of course.

Note that if you change an option to `mysqld`, it remains in effect only for that instance of the server.

To see the effects of a parameter change, do something like this:

    shell> mysqld -O key_buffer=32m --help

Make sure that the `--help` option is last; otherwise, the effect of any options listed after it on the command-line will not be reflected in the output.

### 5.5.3 How Compiling and Linking Affects the Speed of MySQL

Most of the following tests are done on Linux with the MySQL benchmarks, but they should give some indication for other operating systems and workloads.

You get the fastest executable when you link with `-static`.

On Linux, you will get the fastest code when compiling with `pgcc` and `-O3`. To compile ``sql_yacc.cc'` with these options, you need about 200M memory because `gcc/pgcc` needs a lot of memory to make all functions inline. You should also set `CXX=gcc` when configuring MySQL to avoid inclusion of the `libstdc++` library (it is not needed). Note that with some versions of `pgcc`, the resulting code will only run on true Pentium processors, even if you use the compiler option that you want the resulting code to be working on all x586 type processors (like AMD).

By just using a better compiler and/or better compiler options you can get a 10-30% speed increase in your application. This is particularly important if you compile the SQL server yourself!

We have tested both the Cygnus CodeFusion and Fujitsu compilers, but when we tested them, neither was sufficiently bug free to allow MySQL to be compiled with optimisations on.

When you compile MySQL you should only include support for the character sets that you are going to use. (Option `--with-charset=xxx`.) The standard MySQL binary distributions are compiled with support for all character sets.

Here is a list of some measurements that we have done:

- If you use `pgcc` and compile everything with `-O6`, the `mysqld` server is 1% faster than with `gcc` 2.95.2.
- If you link dynamically (without `-static`), the result is 13% slower on Linux. Note that you still can use a dynamic linked MySQL library. It is only the server that is critical for performance.
- If you strip your `mysqld` binary with `strip libexec/mysqld`, the resulting binary can be up to 4% faster.
- If you connect using TCP/IP rather than Unix sockets, the result is 7.5% slower on the same computer. (If you are connection to `localhost`, MySQL will, by default, use sockets.)
- If you connect using TCP/IP from another computer over a 100M Ethernet, things will be 8-11% slower.
- When running our benchmark tests using secure connections (all data encrypted with internal SSL support) things were 55% slower.
- If you compile with `--with-debug=full`, then you will lose 20% for most queries, but some queries may take substantially longer (The MySQL benchmarks ran 35% slower) If you use `--with-debug`, then you will only lose 15%. By starting a `mysqld` version compiled with `--with-debug=full` with `--skip-safemalloc` the end result should be close to when configuring with `--with-debug`.
- On a Sun UltraSPARC-IIe, Forte 5.0 is 4% faster than `gcc` 3.2
- On a Sun UltraSPARC-IIe, Forte 5.0 is 4% faster in 32 bit mode than in 64 bit mode.
- Compiling with `gcc` 2.95.2 for UltraSPARC with the option `-mcpu=v8 -Wa,-xarch=v8plusa` gives 4% more performance.
- On Solaris 2.5.1, MIT-pthreads is 8-12% slower than Solaris native threads on a single processor. With more load/CPUs the difference should get bigger.
- Running with `--log-bin` makes mysqld 1% slower.
- Compiling on Linux-x86 using gcc without frame pointers `-fomit-frame-pointer` or `-fomit-frame-pointer -ffixed-ebp` makes `mysqld` 1-4% faster.

The MySQL-Linux distribution provided by MySQL AB used to be compiled with `pgcc`, but we had to go back to regular gcc because of a bug in `pgcc` that would generate the code that does not run on AMD. We will continue using gcc until that bug is resolved. In the meantime, if you have a non-AMD machine, you can get a faster binary by compiling with `pgcc`. The standard MySQL Linux binary is linked statically to get it faster and more portable.

### 5.5.4 How MySQL Uses Memory

The following list indicates some of the ways that the `mysqld` server uses memory. Where applicable, the name of the server variable relevant to the memory use is given:

- The key buffer (variable `key_buffer_size`) is shared by all threads; other buffers used by the server are allocated as needed. See section 5.5.2 Tuning Server Parameters.
- Each connection uses some thread-specific space: A stack (default 64K, variable `thread_stack`), a connection buffer (variable `net_buffer_length`), and a result buffer (variable `net_buffer_length`). The connection buffer and result buffer are dynamically enlarged up to `max_allowed_packet` when needed. When a query is running, a copy of the current query string is also allocated.
- All threads share the same base memory.
- Only the compressed `ISAM` / `MyISAM` tables are memory mapped. This is because the 32-bit memory space of 4GB is not large enough for most big tables. When systems with a 64-bit address space become more common we may add general support for memory mapping.
- Each request doing a sequential scan over a table allocates a read buffer (variable `record_buffer`).
- When reading rows in 'random' order (for example after a sort) a random-read buffer is allocated to avoid disk seeks. (variable `record_rnd_buffer`).
- All joins are done in one pass, and most joins can be done without even using a temporary table. Most temporary tables are memory-based (`HEAP`) tables. Temporary tables with a big record length (calculated as the sum of all column lengths) or that contain `BLOB` columns are stored on disk. One problem in MySQL versions before Version 3.23.2 is that if a `HEAP` table exceeds the size of `tmp_table_size`, you get the error `The table tbl_name is full`. In newer versions this is handled by automatically changing the in-memory (`HEAP`) table to a disk-based (`MyISAM`) table as necessary. To work around this problem, you can increase the temporary table size by setting the `tmp_table_size` option to `mysqld`, or by setting the SQL option `BIG_TABLES` in the client program. See section 5.5.6 `SET` Syntax. In MySQL Version 3.20, the maximum size of the temporary table was `record_buffer*16`, so if you are using this version, you have to increase the value of `record_buffer`. You can also start `mysqld` with the `--big-tables` option to always store temporary tables on disk. However, this will affect the speed of many complicated queries.
- Most requests doing a sort allocates a sort buffer and 0-2 temporary files depending on the result set size. See section A.4.4 Where MySQL Stores Temporary Files.
- Almost all parsing and calculating is done in a local memory store. No memory overhead is needed for small items and the normal slow memory allocation and freeing is avoided. Memory is allocated only for unexpectedly large strings (this is done with `malloc()` and `free()`).
- Each index file is opened once and the datafile is opened once for each concurrently running thread. For each concurrent thread, a table structure, column structures for each column, and a buffer of size `3 * n` is allocated (where `n` is the maximum row length, not counting `BLOB` columns). A `BLOB` uses 5 to 8 bytes plus the length of the `BLOB` data. The `ISAM`/`MyISAM` storage engines will use one extra row buffer for internal usage.
- For each table having `BLOB` columns, a buffer is enlarged dynamically to read in larger `BLOB` values. If you scan a table, a buffer as large as the largest `BLOB` value is allocated.
- Storage engines for all in-use tables are saved in a cache and managed as a FIFO. Normally the cache has 64 entries. If a table has been used by two running threads at the same time, the cache contains two entries for the table. See section 5.4.7 How MySQL Opens and Closes Tables.
- A `mysqladmin flush-tables` command closes all tables that are not in use and marks all in-use tables to be closed when the currently executing thread finishes. This will effectively free most in-use memory.

`ps` and other system status programs may report that `mysqld` uses a lot of memory. This may be caused by thread-stacks on different memory addresses. For example, the Solaris version of `ps` counts the unused memory between stacks as used memory. You can verify this by checking available swap with `swap -s`. We have tested `mysqld` with commercial memory-leakage detectors, so there should be no memory leaks.

### 5.5.5 How MySQL uses DNS

When a new thread connects to `mysqld`, `mysqld` will span a new thread to handle the request. This thread will first check if the hostname is in the hostname cache. If not the thread will call `gethostbyaddr_r()` and `gethostbyname_r()` to resolve the hostname.

If the operating system doesn't support the above thread-safe calls, the thread will lock a mutex and call `gethostbyaddr()` and `gethostbyname()` instead. Note that in this case no other thread can resolve other hostnames that is not in the hostname cache until the first thread is ready.

You can disable DNS host lookup by starting `mysqld` with `--skip-name-resolve`. In this case you can however only use IP names in the MySQL privilege tables.

If you have a very slow DNS and many hosts, you can get more performance by either disabling DNS lookop with `--skip-name-resolve` or by increasing the `HOST_CACHE_SIZE` define (default: 128) and recompile `mysqld`.

You can disable the hostname cache with `--skip-host-cache`. You can clear the hostname cache with `FLUSH HOSTS` or `mysqladmin flush-hosts`.

If you don't want to allow connections over `TCP/IP`, you can do this by starting `mysqld` with `--skip-networking`.

### 5.5.6 `SET` Syntax

    SET [GLOBAL | SESSION] sql_variable=expression, [[GLOBAL | SESSION] sql_variable=expression...]

`SET` sets various options that affect the operation of the server or your client.

The following examples shows the different syntaxes one can use to set variables:

In old MySQL versions we allowed the use of the `SET OPTION` syntax, but this syntax is now deprecated.

In MySQL 4.0.3 we added the `GLOBAL` and `SESSION` options and access to most important startup variables.

`LOCAL` can be used as a synonym for `SESSION`.

If you set several variables on the same command line, the last used `GLOBAL | SESSION` mode is used.

    SET sort_buffer_size=10000;
    SET @@local.sort_buffer_size=10000;
    SET GLOBAL sort_buffer_size=1000000, SESSION sort_buffer_size=1000000;
    SET @@sort_buffer_size=1000000;
    SET @@global.sort_buffer_size=1000000, @@local.sort_buffer_size=1000000;

The `@@variable_name` syntax is supported to make MySQL syntax compatible with some other databases.

The different system variables one can set are described in the system variable section of this manual. See section 6.1.5 System Variables.

If you are using `SESSION` (the default) the option you set remains in effect until the current session ends, or until you set the option to a different value. If you use `GLOBAL`, which require the `SUPER` privilege, the option is remembered and used for new connections until the server restarts. If you want to make an option permanent, you should set it in one of the MySQL option files. See section 4.1.2 ``my.cnf'` Option Files.

To avoid wrong usage MySQL will give an error if you use `SET GLOBAL` with a variable that can only be used with `SET SESSION` or if you are not using `SET GLOBAL` with a global variable.

If you want to set a `SESSION` variable to the `GLOBAL` value or a `GLOBAL` value to the MySQL default value, you can set it to `DEFAULT`.

    SET max_join_size=DEFAULT;

This is identical to:

    SET @@session.max_join_size=@@global.max_join_size;

If you want to restrict the maximum value a startup option can be set to with the `SET` command, you can specify this by using the `--maximum-variable-name` command line option. See section 4.1.1 `mysqld` Command-line Options.

You can get a list of most variables with `SHOW VARIABLES`. See section 4.5.6.4 `SHOW VARIABLES`. You can get the value for a specific value with the `@@[global.|local.]variable_name` syntax:

    SHOW VARIABLES like "max_join_size";
    SHOW GLOBAL VARIABLES like "max_join_size";
    SELECT @@max_join_size, @@global.max_join_size;

Here follows a description of the variables that uses a the variables that uses a non-standard `SET` syntax and some of the other variables. The other variable definitions can be found in the system variable section, among the startup options or in the description of `SHOW VARIABLES`. See section 6.1.5 System Variables. See section 4.1.1 `mysqld` Command-line Options. See section 4.5.6.4 `SHOW VARIABLES`.

`CHARACTER SET character_set_name | DEFAULT`
  This maps all strings from and to the client with the given mapping. Currently the only option for `character_set_name` is `cp1251_koi8`, but you can easily add new mappings by editing the ``sql/convert.cc'` file in the MySQL source distribution. The default mapping can be restored by using a `character_set_name` value of `DEFAULT`. Note that the syntax for setting the `CHARACTER SET` option differs from the syntax for setting the other options.
`PASSWORD = PASSWORD('some password')`
  Set the password for the current user. Any non-anonymous user can change his own password!
`PASSWORD FOR user = PASSWORD('some password')`
  Set the password for a specific user on the current server host. Only a user with access to the `mysql` database can do this. The user should be given in `user@hostname` format, where `user` and `hostname` are exactly as they are listed in the `User` and `Host` columns of the `mysql.user` table entry. For example, if you had an entry with `User` and `Host` fields of `'bob'` and `'%.loc.gov'`, you would write:

      mysql> SET PASSWORD FOR bob@"%.loc.gov" = PASSWORD("newpass");

  Which is equivalent to:

      mysql> UPDATE mysql.user SET password=PASSWORD("newpass")
          ->                   WHERE user="bob' AND host="%.loc.gov";

`SQL_AUTO_IS_NULL = 0 | 1`
  If set to `1` (default) then one can find the last inserted row for a table with an `AUTO_INCREMENT` column with the following construct: `WHERE auto_increment_column IS NULL`. This is used by some ODBC programs like Access.
`AUTOCOMMIT= 0 | 1`
  If set to `1` all changes to a table will be done at once. To start a multi-command transaction, you have to use the `BEGIN` statement. See section 6.7.1 `BEGIN/COMMIT/ROLLBACK` Syntax. If set to `0` you have to use `COMMIT` / `ROLLBACK` to accept/revoke that transaction. See section 6.7.1 `BEGIN/COMMIT/ROLLBACK` Syntax. Note that when you change from not `AUTOCOMMIT` mode to `AUTOCOMMIT` mode, MySQL will do an automatic `COMMIT` on any open transactions.
`BIG_TABLES = 0 | 1`
  If set to `1`, all temporary tables are stored on disk rather than in memory. This will be a little slower, but you will not get the error `The table tbl_name is full` for big `SELECT` operations that require a large temporary table. The default value for a new connection is `0` (that is, use in-memory temporary tables). This option was before named `SQL_BIG_TABLES`.
`SQL_BIG_SELECTS = 0 | 1`
  If set to `0`, MySQL will abort if a `SELECT` is attempted that probably will take a very long time. This is useful when an inadvisable `WHERE` statement has been issued. A big query is defined as a `SELECT` that probably will have to examine more than `max_join_size` rows. The default value for a new connection is `1` (which will allow all `SELECT` statements).
`SQL_BUFFER_RESULT = 0 | 1`
  `SQL_BUFFER_RESULT` will force the result from `SELECT`s to be put into a temporary table. This will help MySQL free the table locks early and will help in cases where it takes a long time to send the result set to the client.
`LOW_PRIORITY_UPDATES = 0 | 1`
  If set to `1`, all `INSERT`, `UPDATE`, `DELETE`, and and `LOCK TABLE WRITE` statements wait until there is no pending `SELECT` or `LOCK TABLE READ` on the affected table. This option was before named `SQL_LOW_PRIORITY_UPDATES`.
`MAX_JOIN_SIZE = value | DEFAULT`
  Don't allow `SELECT`s that will probably need to examine more than `value` row combinations. By setting this value, you can catch `SELECT`s where keys are not used properly and that would probably take a long time. Setting this to a value other than `DEFAULT` will reset the `SQL_BIG_SELECTS` flag. If you set the `SQL_BIG_SELECTS` flag again, the `SQL_MAX_JOIN_SIZE` variable will be ignored. You can set a default value for this variable by starting `mysqld` with `-O max_join_size=#`. This option was before named `SQL_MAX_JOIN_SIZE`. Note that if the result of the query is already in the query cache, the above check will not be made. Instead, MySQL will send the result to the client. Since the query result is already computed and it will not burden the server to send the result to the client.
`QUERY_CACHE_TYPE = OFF | ON | DEMAND`
`QUERY_CACHE_TYPE = 0 | 1 | 2`
  Set query cache setting for this thread.

  Option | Description
  0 or OFF | Don't cache or retrieve results.
  1 or ON | Cache all results except `SELECT SQL_NO_CACHE ...` queries.
  2 or DEMAND | Cache only `SELECT SQL_CACHE ...` queries.

`SQL_SAFE_UPDATES = 0 | 1`
  If set to `1`, MySQL will abort if an `UPDATE` or `DELETE` is attempted that doesn't use a key or `LIMIT` in the `WHERE` clause. This makes it possible to catch wrong updates when creating SQL commands by hand.
`SQL_SELECT_LIMIT = value | DEFAULT`
  The maximum number of records to return from `SELECT` statements. If a `SELECT` has a `LIMIT` clause, the `LIMIT` takes precedence over the value of `SQL_SELECT_LIMIT`. The default value for a new connection is ``unlimited.'' If you have changed the limit, the default value can be restored by using a `SQL_SELECT_LIMIT` value of `DEFAULT`.
`SQL_LOG_OFF = 0 | 1`
  If set to `1`, no logging will be done to the standard log for this client, if the client has the `SUPER` privilege. This does not affect the update log!
`SQL_LOG_UPDATE = 0 | 1`
  If set to `0`, no logging will be done to the update log for the client, if the client has the `SUPER` privilege. This does not affect the standard log!
`SQL_QUOTE_SHOW_CREATE = 0 | 1`
  If set to `1`, `SHOW CREATE TABLE` will quote table and column names. This is on by default, for replication of tables with fancy column names to work. section 4.5.6.8 `SHOW CREATE TABLE`.
`TIMESTAMP = timestamp_value | DEFAULT`
  Set the time for this client. This is used to get the original timestamp if you use the update log to restore rows. `timestamp_value` should be a Unix epoch timestamp, not a MySQL timestamp.
`LAST_INSERT_ID = #`
  Set the value to be returned from `LAST_INSERT_ID()`. This is stored in the update log when you use `LAST_INSERT_ID()` in a command that updates a table.
`INSERT_ID = #`
  Set the value to be used by the following `INSERT` or `ALTER TABLE` command when inserting an `AUTO_INCREMENT` value. This is mainly used with the update log.

## 5.6 Disk Issues

- As mentioned before, disks seeks are a big performance bottleneck. This problems gets more and more apparent when the data starts to grow so large that effective caching becomes impossible. For large databases, where you access data more or less randomly, you can be sure that you will need at least one disk seek to read and a couple of disk seeks to write things. To minimise this problem, use disks with low seek times.
- Increase the number of available disk spindles (and thereby reduce the seek overhead) by either symlink files to different disks or striping the disks.
  Using symbolic links
    This means that you symlink the index and/or datafile(s) from the normal data directory to another disk (that may also be striped). This makes both the seek and read times better (if the disks are not used for other things). See section 5.6.1 Using Symbolic Links.
  Striping
    Striping means that you have many disks and put the first block on the first disk, the second block on the second disk, and the Nth on the (N mod number_of_disks) disk, and so on. This means if your normal data size is less than the stripe size (or perfectly aligned) you will get much better performance. Note that striping is very dependent on the OS and stripe-size. So benchmark your application with different stripe-sizes. See section 5.1.5 Using Your Own Benchmarks. Note that the speed difference for striping is very dependent on the parameters. Depending on how you set the striping parameters and number of disks you may get a difference in orders of magnitude. Note that you have to choose to optimise for random or sequential access.
- For reliability you may want to use RAID 0+1 (striping + mirroring), but in this case you will need 2*N drives to hold N drives of data. This is probably the best option if you have the money for it! You may, however, also have to invest in some volume-management software to handle it efficiently.
- A good option is to have semi-important data (that can be regenerated) on RAID 0 disk while storing really important data (like host information and logs) on a RAID 0+1 or RAID N disk. RAID N can be a problem if you have many writes because of the time to update the parity bits.
- You may also set the parameters for the filesystem that the database uses. One easy change is to mount the filesystem with the noatime option. That makes it skip the updating of the last access time in the inode and by this will avoid some disk seeks.
- On Linux, you can get much more performance (up to 100% under load is not uncommon) by using `hdparm` to configure your disk's interface! The following should be quite good `hdparm` options for MySQL (and probably many other applications):

      hdparm -m 16 -d 1

  Note that the performance/reliability when using the above depends on your hardware, so we strongly suggest that you test your system thoroughly after using `hdparm`! Please consult the `hdparm` man page for more information! If `hdparm` is not used wisely, filesystem corruption may result. Backup everything before experimenting!
- On many operating systems you can mount the disks with the `-o async` option to set the filesystem to be updated asynchronously. If your computer is reasonably stable, this should give you more performance without sacrificing too much reliability. (This flag is on by default on Linux.)
- If you don't need to know when a file was last accessed (which is not really useful on a database server), you can mount your filesystems with the `-o noatime` option.

### 5.6.1 Using Symbolic Links

You can move tables and databases from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space or increase the speed of your system by spreading your tables to different disk.

The recommended way to do this, is to just symlink databases to a different disk and only symlink tables as a last resort.

#### 5.6.1.1 Using Symbolic Links for Databases

The way to symlink a database is to first create a directory on some disk where you have free space and then create a symlink to it from the MySQL database directory.

    shell> mkdir /dr1/databases/test
    shell> ln -s /dr1/databases/test mysqld-datadir

MySQL doesn't support that you link one directory to multiple databases. Replacing a database directory with a symbolic link will work fine as long as you don't make a symbolic link between databases. Suppose you have a database `db1` under the MySQL data directory, and then make a symlink `db2` that points to `db1`:

    shell> cd /path/to/datadir
    shell> ln -s db1 db2

Now, for any table `tbl_a` in `db1`, there also appears to be a table `tbl_a` in `db2`. If one thread updates `db1.tbl_a` and another thread updates `db2.tbl_a`, there will be problems.

If you really need this, you must change the following code in ``mysys/mf_format.c'`:

    if (flag & 32 || (!lstat(to,&stat_buff) && S_ISLNK(stat_buff.st_mode)))

to

    if (1)

On Windows you can use internal symbolic links to directories by compiling MySQL with `-DUSE_SYMDIR`. This allows you to put different databases on different disks. See section 2.6.2.5 Splitting Data Across Different Disks on Windows.

#### 5.6.1.2 Using Symbolic Links for Tables

Before MySQL 4.0 you should not symlink tables, if you are not very careful with them. The problem is that if you run `ALTER TABLE`, `REPAIR TABLE` or `OPTIMIZE TABLE` on a symlinked table, the symlinks will be removed and replaced by the original files. This happens because the above command works by creating a temporary file in the database directory and when the command is complete, replace the original file with the temporary file.

You should not symlink tables on systems that don't have a fully working `realpath()` call. (At least Linux and Solaris support `realpath()`)

In MySQL 4.0 symlinks are fully supported only for `MyISAM` tables. For other table types you will probably get strange problems when doing any of the above mentioned commands.

The handling of symbolic links in MySQL 4.0 works the following way (this is mostly relevant only for `MyISAM` tables).

- In the data directory you will always have the table definition file and the data and index files.
- You can symlink the index file and the data file to different directories independently of the other.
- The symlinking can be done from the operating system (if `mysqld` is not running) or with the `INDEX/DATA DIRECTORY="path-to-dir"` command in `CREATE TABLE`. See section 6.5.3 `CREATE TABLE` Syntax.
- `myisamchk` will not replace a symlink with the data or index file but work directly on the file the symlink points to. Any temporary files will be created in the same directory where the data or index file is located.
- When you drop a table that is using symlinks, both the symlink and the file the symlink points to are dropped. This is a good reason to why you should not run `mysqld` as `root` or allow persons to have write access to the MySQL database directories.
- If you rename a table with `ALTER TABLE RENAME` and you don't move the table to another database, the symlinks in the database directory will be renamed to the new names and the data and index files will be renamed accordingly.
- If you use `ALTER TABLE RENAME` to move a table to another database, the table will be moved to the other database directory and the old symlinks and the files they pointed to will be deleted. (In other words, the new table will not be symlinked.)
- If you are not using symlinks, you should use the `--skip-symlink` option to `mysqld` to ensure that no one can drop or rename a file outside of the `mysqld` data directory.

Things that are not yet supported:

- `ALTER TABLE` ignores all `INDEX/DATA DIRECTORY="path"` options.
- `CREATE TABLE` doesn't report if the table has symbolic links.
- `mysqldump` doesn't include the symbolic link information in the output.
- `BACKUP TABLE` and `RESTORE TABLE` don't respect symbolic links.

# 6 MySQL Language Reference

MySQL has a very complex, but intuitive and easy to learn SQL interface. This chapter describes the various commands, types, and functions you will need to know in order to use MySQL efficiently and effectively. This chapter also serves as a reference to all functionality included in MySQL. In order to use this chapter effectively, you may find it useful to refer to the various indexes.

## 6.1 Language Structure

### 6.1.1 Literals: How to Write Strings and Numbers

This section describes the various ways to write strings and numbers in MySQL. It also covers the various nuances and ``gotchas'' that you may run into when dealing with these basic types in MySQL.

#### 6.1.1.1 Strings

A string is a sequence of characters, surrounded by either single quote (``''`) or double quote (``"'`) characters (only the single quote if you run in ANSI mode). Examples:

    'a string'
    "another string"

Within a string, certain sequences have special meaning. Each of these sequences begins with a backslash (``\'`), known as the escape character. MySQL recognises the following escape sequences:

`\0`
  An ASCII 0 (`NUL`) character.
`\'`
  A single quote (``''`) character.
`\"`
  A double quote (``"'`) character.
`\b`
  A backspace character.
`\n`
  A newline character.
`\r`
  A carriage return character.
`\t`
  A tab character.
`\z`
  ASCII(26) (Control-Z). This character can be encoded to allow you to work around the problem that ASCII(26) stands for END-OF-FILE on Windows. (ASCII(26) will cause problems if you try to use `mysql database < filename`.)
`\\`
  A backslash (``\'`) character.
`\%`
  A ``%'` character. This is used to search for literal instances of ``%'` in contexts where ``%'` would otherwise be interpreted as a wildcard character. See section 6.3.2.1 String Comparison Functions.
`\_`
  A ``_'` character. This is used to search for literal instances of ``_'` in contexts where ``_'` would otherwise be interpreted as a wildcard character. See section 6.3.2.1 String Comparison Functions.

Note that if you use ``\%'` or ``\_'` in some string contexts, these will return the strings ``\%'` and ``\_'` and not ``%'` and ``_'`.

There are several ways to include quotes within a string:

- A ``''` inside a string quoted with ``''` may be written as ``'''`.
- A ``"'` inside a string quoted with ``"'` may be written as ``""'`.
- You can precede the quote character with an escape character (``\'`).
- A ``''` inside a string quoted with ``"'` needs no special treatment and need not be doubled or escaped. In the same way, ``"'` inside a string quoted with ``''` needs no special treatment.

The `SELECT` statements shown here demonstrate how quoting and escaping work:

    mysql> SELECT 'hello', '"hello"', '""hello""', 'hel''lo', '\'hello';
    +-------+---------+-----------+--------+--------+
    | hello | "hello" | ""hello"" | hel'lo | 'hello |
    +-------+---------+-----------+--------+--------+

    mysql> SELECT "hello", "'hello'", "''hello''", "hel""lo", "\"hello";
    +-------+---------+-----------+--------+--------+
    | hello | 'hello' | ''hello'' | hel"lo | "hello |
    +-------+---------+-----------+--------+--------+

    mysql> SELECT "This\nIs\nFour\nlines";
    +--------------------+
    | This
    Is
    Four
    lines |
    +--------------------+

If you want to insert binary data into a string column (such as a `BLOB`), the following characters must be represented by escape sequences:

`NUL`
  ASCII 0. You should represent this by ``\0'` (a backslash and an ASCII ``0'` character).
`\`
  ASCII 92, backslash. Represent this by ``\\'`.
`'`
  ASCII 39, single quote. Represent this by ``\''`.
`"`
  ASCII 34, double quote. Represent this by ``\"'`.

If you write C code, you can use the C API function `mysql_real_escape_string()` to escape characters for the `INSERT` statement. See section 8.4.2 C API Function Overview. In Perl, you can use the `quote` method of the `DBI` package to convert special characters to the proper escape sequences. See section 8.2.2 The `DBI` Interface.

You should use an escape function on any string that might contain any of the special characters listed above!

Alternatively, many MySQL APIs provide some sort of placeholder capability that allows you to insert special markers into a query string, and then bind data values to them when you issue the query. In this case, the API takes case of escaping special characters in the values for you automatically.

#### 6.1.1.2 Numbers

Integers are represented as a sequence of digits. Floats use ``.'` as a decimal separator. Either type of number may be preceded by ``-'` to indicate a negative value.

Examples of valid integers:

    1221
    0
    -32

Examples of valid floating-point numbers:

    294.42
    -32032.6809e+10
    148.00

An integer may be used in a floating-point context; it is interpreted as the equivalent floating-point number.

#### 6.1.1.3 Hexadecimal Values

MySQL supports hexadecimal values. In numeric context these act like an integer (64-bit precision). In string context these act like a binary string where each pair of hex digits is converted to a character:

    mysql> SELECT x'4D7953514C';
             -> MySQL
    mysql> SELECT 0xa+0;
             -> 10
    mysql> SELECT 0x5061756c;
             -> Paul

The `x'hexstring'` syntax (new in 4.0) is based on ANSI SQL and the `0x` syntax is based on ODBC. Hexadecimal strings are often used by ODBC to supply values for `BLOB` columns. You can convert a string or a number to hexadecimal with the `HEX()` function.

#### 6.1.1.4 `NULL` Values

The `NULL` value means ``no data'' and is different from values such as `0` for numeric types or the empty string for string types. See section A.5.3 Problems with `NULL` Values.

`NULL` may be represented by `\N` when using the text file import or export formats (`LOAD DATA INFILE`, `SELECT ... INTO OUTFILE`). See section 6.4.9 `LOAD DATA INFILE` Syntax.

### 6.1.2 Database, Table, Index, Column, and Alias Names

Database, table, index, column, and alias names all follow the same rules in MySQL.

Note that the rules changed starting with MySQL Version 3.23.6 when we introduced quoting of identifiers (database, table, and column names) with ```'`. ``"'` will also work to quote identifiers if you run in ANSI mode. See section 1.7.2 Running MySQL in ANSI Mode.

Identifier | Max length | Allowed characters
Database | 64 | Any character that is allowed in a directory name except ``/'`, ``\'` or ``.'`.
Table | 64 | Any character that is allowed in a file name, except ``/'` or ``.'`.
Column | 64 | All characters.
Alias | 255 | All characters.

Note that in addition to the above, you can't have ASCII(0) or ASCII(255) or the quoting character in an identifier.

Note that if the identifier is a restricted word or contains special characters you must always quote it with a ``` (backtick) when you use it:

    mysql> SELECT * FROM `select` WHERE `select`.id > 100;

See section 6.1.7 Is MySQL Picky About Reserved Words?.

In MySQL versions prior to 3.23.6, the name rules are as follows:

- A name may consist of alphanumeric characters from the current character set and also ``_'` and ``$'`. The default character set is ISO-8859-1 Latin1; this may be changed with the `--default-character-set` option to `mysqld`. See section 4.6.1 The Character Set Used for Data and Sorting.
- A name may start with any character that is legal in a name. In particular, a name may start with a digit (this differs from many other database systems!). However, a name cannot consist only of digits.
- You cannot use the ``.'` character in names because it is used to extend the format by which you can refer to columns (see immediately below).

It is recommended that you do not use names like `1e`, because an expression like `1e+1` is ambiguous. It may be interpreted as the expression `1e + 1` or as the number `1e+1`.

In MySQL you can refer to a column using any of the following forms:

Column reference | Meaning
`col_name` | Column `col_name` from whichever table used in the query contains a column of that name.
`tbl_name.col_name` | Column `col_name` from table `tbl_name` of the current database.
`db_name.tbl_name.col_name` | Column `col_name` from table `tbl_name` of the database `db_name`. This form is available in MySQL Version 3.22 or later.
``column_name`` | A column that is a keyword or contains special characters.

You need not specify a `tbl_name` or `db_name.tbl_name` prefix for a column reference in a statement unless the reference would be ambiguous. For example, suppose tables `t1` and `t2` each contain a column `c`, and you retrieve `c` in a `SELECT` statement that uses both `t1` and `t2`. In this case, `c` is ambiguous because it is not unique among the tables used in the statement, so you must indicate which table you mean by writing `t1.c` or `t2.c`. Similarly, if you are retrieving from a table `t` in database `db1` and from a table `t` in database `db2`, you must refer to columns in those tables as `db1.t.col_name` and `db2.t.col_name`.

The syntax `.tbl_name` means the table `tbl_name` in the current database. This syntax is accepted for ODBC compatibility, because some ODBC programs prefix table names with a ``.'` character.

### 6.1.3 Case Sensitivity in Names

In MySQL, databases and tables correspond to directories and files within those directories. Consequently, the case-sensitivity of the underlying operating system determines the case-sensitivity of database and table names. This means database and table names are case-insensitive in Windows, and case-sensitive in most varieties of Unix (Mac OS X being an exception). See section 1.7.3 MySQL Extensions to ANSI SQL92.

Note: although database and table names are case-insensitive for Windows, you should not refer to a given database or table using different cases within the same query. The following query would not work because it refers to a table both as `my_table` and as `MY_TABLE`:

    mysql> SELECT * FROM my_table WHERE MY_TABLE.col=1;

Column names and column aliases are case-insensitive in all cases.

Aliases on tables are case-sensitive. The following query would not work because it refers to the alias both as `a` and as `A`:

    mysql> SELECT col_name FROM tbl_name AS a
        ->                 WHERE a.col_name = 1 OR A.col_name = 2;

If you have trouble remembering the lettercase for database and table names, adopt a consistent convention, such as always creating databases and tables using lowercase names.

One way to avoid this problem is to start `mysqld` with `-O lower_case_table_names=1`. By default this option is 1 on Windows and 0 on Unix.

If `lower_case_table_names` is 1 MySQL will convert all table names to lowercase on storage and lookup. (From version 4.0.2, this option also applies to database names.) Note that if you change this option, you need to first convert your old table names to lower case before starting `mysqld`.

If you move `MyISAM` files from a Windows to a *nix disk, you may in some cases need to use the ``mysql_fix_extensions'` tool to fix-up the case of the file extensions in each specified database directory (lowercase ``.frm'`, uppercase ``.MYI'` and ``.MYD'`). ``mysql_fix_extensions'` can be found in the ``script'` subdirectory.

### 6.1.4 User Variables

MySQL supports connection-specific user variables with the `@variablename` syntax. A variable name may consist of alphanumeric characters from the current character set and also ``_'`, ``$'`, and ``.'` . The default character set is ISO-8859-1 Latin1; this may be changed with the `--default-character-set` option to `mysqld`. See section 4.6.1 The Character Set Used for Data and Sorting.

Variables don't have to be initialised. They contain `NULL` by default and can store an integer, real, or string value. All variables for a thread are automatically freed when the thread exits.

You can set a variable with the `SET` syntax:

    SET @variable= { integer expression | real expression | string expression }
    [,@variable= ...].

You can also assign a value to a variable in statements other than `SET`. However, in this case the assignment operator is `:=` rather than `=`, because `=` is reserved for comparisons in non-`SET` statements:

    mysql> SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
    +----------------------+------+------+------+
    | @t1:=(@t2:=1)+@t3:=4 | @t1  | @t2  | @t3  |
    +----------------------+------+------+------+
    |                    5 |    5 |    1 |    4 |
    +----------------------+------+------+------+

User variables may be used where expressions are allowed. Note that this does not currently include contexts where a number is explicitly required, such as in the `LIMIT` clause of a `SELECT` statement, or the `IGNORE number LINES` clause of a `LOAD DATA` statement.

Note: in a `SELECT` statement, each expression is evaluated only when it's sent to the client. This means that in the `HAVING`, `GROUP BY`, or `ORDER BY` clause, you can't refer to an expression that involves variables that are set in the `SELECT` part. For example, the following statement will NOT work as expected:

    mysql> SELECT (@aa:=id) AS a, (@aa+3) AS b FROM table_name HAVING b=5;

The reason is that `@aa` will not contain the value of the current row, but the value of `id` for the previous accepted row.

### 6.1.5 System Variables

Starting from MySQL 4.0.3 we provide better access to a lot of system and connection variables. One can change most of them without having to take down the server.

There are two kind of system variables: Thread-specific (or connection-specific) variables that are unique to the current connection and global variables that are used to configure global events. Global variables also are used to set up the initial values of the corresponding thread-specific variables for new connections.

When `mysqld` starts, all global variables are initialised from command line arguments and option files. You can change the value with the `SET GLOBAL` command. When a new thread is created, the thread-specific variables are initialised from the global variables and they will not change even if you issue a new `SET GLOBAL` command.

To set the value for a `GLOBAL` variable, you should use one of the following syntaxes: (Here we use `sort_buffer_size` as an example variable)

    SET GLOBAL sort_buffer_size=value;
    SET @@global.sort_buffer_size=value;

To set the value for a `SESSION` variable, you can use one of the following syntaxes:

    SET SESSION sort_buffer_size=value;
    SET @@session.sort_buffer_size=value;
    SET sort_buffer_size=value;

If you don't specify `GLOBAL` or `SESSION` then `SESSION` is used. See section 5.5.6 `SET` Syntax.

`LOCAL` is a synonym for `SESSION`.

To retrieve the value for a `GLOBAL` variable you can use one of the following commands:

    SELECT @@global.sort_buffer_size;
    SHOW GLOBAL VARIABLES like 'sort_buffer_size';

To retrieve the value for a `SESSION` variable you can use one of the following commands:

    SELECT @@session.sort_buffer_size;
    SHOW SESSION VARIABLES like 'sort_buffer_size';

When you retrieve a variable value with the `@@variable_name` syntax and you don't specify `GLOBAL` or `SESSION` then MySQL will return the thread-specific (`SESSION`) value if it exists. If not, MySQL will return the global value.

The reason for requiring `GLOBAL` for setting `GLOBAL` only variables but not for retrieving them is to ensure that we don't later run into problems if we later would introduce a thread-specific variable with the same name or remove a thread-specific variable. In this case, you could accidentally change the state for the server as a whole, rather than just for your own connection.

The following is a full list of all variables that you change and retrieve and if you can use `GLOBAL` or `SESSION` with them.

Variable name | Value type | Type
autocommit | bool | SESSION
big_tables | bool | SESSION
binlog_cache_size | num | GLOBAL
bulk_insert_buffer_size | num | GLOBAL | SESSION
concurrent_insert | bool | GLOBAL
connect_timeout | num | GLOBAL
convert_character_set | string | SESSION
delay_key_write | OFF | ON | ALL | GLOBAL
delayed_insert_limit | num | GLOBAL
delayed_insert_timeout | num | GLOBAL
delayed_queue_size | num | GLOBAL
error_count | num | LOCAL
flush | bool | GLOBAL
flush_time | num | GLOBAL
foreign_key_checks | bool | SESSION
identity | num | SESSION
insert_id | bool | SESSION
interactive_timeout | num | GLOBAL | SESSION
join_buffer_size | num | GLOBAL | SESSION
key_buffer_size | num | GLOBAL
last_insert_id | bool | SESSION
local_infile | bool | GLOBAL
log_warnings | bool | GLOBAL
long_query_time | num | GLOBAL | SESSION
low_priority_updates | bool | GLOBAL | SESSION
max_allowed_packet | num | GLOBAL | SESSION
max_binlog_cache_size | num | GLOBAL
max_binlog_size | num | GLOBAL
max_connect_errors | num | GLOBAL
max_connections | num | GLOBAL
max_error_count | num | GLOBAL | SESSION
max_delayed_threads | num | GLOBAL
max_heap_table_size | num | GLOBAL | SESSION
max_join_size | num | GLOBAL | SESSION
max_sort_length | num | GLOBAL | SESSION
max_tmp_tables | num | GLOBAL
max_user_connections | num | GLOBAL
max_write_lock_count | num | GLOBAL
myisam_max_extra_sort_file_size | num | GLOBAL | SESSION
myisam_max_sort_file_size | num | GLOBAL | SESSION
myisam_sort_buffer_size | num | GLOBAL | SESSION
net_buffer_length | num | GLOBAL | SESSION
net_read_timeout | num | GLOBAL | SESSION
net_retry_count | num | GLOBAL | SESSION
net_write_timeout | num | GLOBAL | SESSION
query_cache_limit | num | GLOBAL
query_cache_size | num | GLOBAL
query_cache_type | enum | GLOBAL
read_buffer_size | num | GLOBAL | SESSION
read_rnd_buffer_size | num | GLOBAL | SESSION
rpl_recovery_rank | num | GLOBAL
safe_show_database | bool | GLOBAL
server_id | num | GLOBAL
slave_compressed_protocol | bool | GLOBAL
slave_net_timeout | num | GLOBAL
slow_launch_time | num | GLOBAL
sort_buffer_size | num | GLOBAL | SESSION
sql_auto_is_null | bool | SESSION
sql_big_selects | bool | SESSION
sql_big_tables | bool | SESSION
sql_buffer_result | bool | SESSION
sql_log_binlog | bool | SESSION
sql_log_off | bool | SESSION
sql_log_update | bool | SESSION
sql_low_priority_updates | bool | GLOBAL | SESSION
sql_max_join_size | num | GLOBAL | SESSION
sql_quote_show_create | bool | SESSION
sql_safe_updates | bool | SESSION
sql_select_limit | bool | SESSION
sql_slave_skip_counter | num | GLOBAL
sql_warnings | bool | SESSION
table_cache | num | GLOBAL
table_type | enum | GLOBAL | SESSION
thread_cache_size | num | GLOBAL
timestamp | bool | SESSION
tmp_table_size | enum | GLOBAL | SESSION
tx_isolation | enum | GLOBAL | SESSION
version | string | GLOBAL
wait_timeout | num | GLOBAL | SESSION
warning_count | num | LOCAL
unique_checks | bool | SESSION

Variables that are marked with `num` can be given a numerical value. Variables that are marked with `bool` can be set to 0, 1, `ON` or `OFF`. Variables that are of type `enum` should normally be set to one of the available values for the variable, but can also be set to the number that correspond to the enum value. (The first enum value is 0).

Here is a description of some of the variables:

Variable | Description
identity | Alias for last_insert_id (Sybase compatiblity)
sql_low_priority_updates | Alias for low_priority_updates
sql_max_join_size | Alias for max_join_size
delay_key_write_for_all_tables | If this and delay_key_write are set, then all new MyISAM tables that are opened will use delayed key writes.
version | Alias for VERSION() (Sybase (?) compatability)

A description of the other variable definitions can be found in the startup options section, the description of `SHOW VARIABLES` and in the `SET` section. See section 4.1.1 `mysqld` Command-line Options. See section 4.5.6.4 `SHOW VARIABLES`. See section 5.5.6 `SET` Syntax.

### 6.1.6 Comment Syntax

The MySQL server supports the `# to end of line`, `-- to end of line` and `/* in-line or multiple-line */` comment styles:

    mysql> SELECT 1+1;     # This comment continues to the end of line
    mysql> SELECT 1+1;     -- This comment continues to the end of line
    mysql> SELECT 1 /* this is an in-line comment */ + 1;
    mysql> SELECT 1+
    /*
    this is a
    multiple-line comment
    */
    1;

Note that the `--` (double-dash) comment style requires you to have at least one space after the second dash!

Although the server understands the comment syntax just described, there are some limitations on the way that the `mysql` client parses `/* ... */` comments:

- Single-quote and double-quote characters are taken to indicate the beginning of a quoted string, even within a comment. If the quote is not matched by a second quote within the comment, the parser doesn't realise the comment has ended. If you are running `mysql` interactively, you can tell that it has gotten confused like this because the prompt changes from `mysql>` to `'>` or `">`.
- A semicolon is taken to indicate the end of the current SQL statement and anything following it to indicate the beginning of the next statement.

These limitations apply both when you run `mysql` interactively and when you put commands in a file and tell `mysql` to read its input from that file with `mysql < some-file`.

MySQL supports the ``--'` ANSI SQL comment style only if the second dash is followed by a space. See section 1.7.4.7 ``--'` as the Start of a Comment.

### 6.1.7 Is MySQL Picky About Reserved Words?

A common problem stems from trying to create a table with column names that use the names of datatypes or functions built into MySQL, such as `TIMESTAMP` or `GROUP`. You're allowed to do it (for example, `ABS` is an allowed column name), but whitespace is not allowed between a function name and the immediately following ``('` when using functions whose names are also column names.

The following words are explicitly reserved in MySQL. Most of them are forbidden by ANSI SQL92 as column and/or table names (for example, `GROUP`). A few are reserved because MySQL needs them and is (currently) using a `yacc` parser:

Word | Word | Word
`ADD` | `ALL` | `ALTER`
`ANALYZE` | `AND` | `AS`
`ASC` | `AUTO_INCREMENT` | `BDB`
`BERKELEYDB` | `BETWEEN` | `BIGINT`
`BINARY` | `BLOB` | `BOTH`
`BTREE` | `BY` | `CASCADE`
`CASE` | `CHANGE` | `CHAR`
`CHARACTER` | `COLLATE` | `COLUMN`
`COLUMNS` | `CONSTRAINT` | `CREATE`
`CROSS` | `CURRENT_DATE` | `CURRENT_TIME`
`CURRENT_TIMESTAMP` | `DATABASE` | `DATABASES`
`DAY_HOUR` | `DAY_MINUTE` | `DAY_SECOND`
`DEC` | `DECIMAL` | `DEFAULT`
`DELAYED` | `DELETE` | `DESC`
`DESCRIBE` | `DISTINCT` | `DISTINCTROW`
`DOUBLE` | `DROP` | `ELSE`
`ENCLOSED` | `ERRORS` | `ESCAPED`
`EXISTS` | `EXPLAIN` | `FIELDS`
`FLOAT` | `FOR` | `FOREIGN`
`FROM` | `FULLTEXT` | `FUNCTION`
`GEOMETRY` | `GRANT` | `GROUP`
`HASH` | `HAVING` | `HELP`
`HIGH_PRIORITY` | `HOUR_MINUTE` | `HOUR_SECOND`
`IF` | `IGNORE` | `IN`
`INDEX` | `INFILE` | `INNER`
`INNODB` | `INSERT` | `INT`
`INTEGER` | `INTERVAL` | `INTO`
`IS` | `JOIN` | `KEY`
`KEYS` | `KILL` | `LEADING`
`LEFT` | `LIKE` | `LIMIT`
`LINES` | `LOAD` | `LOCK`
`LONG` | `LONGBLOB` | `LONGTEXT`
`LOW_PRIORITY` | `MASTER_SERVER_ID` | `MATCH`
`MEDIUMBLOB` | `MEDIUMINT` | `MEDIUMTEXT`
`MIDDLEINT` | `MINUTE_SECOND` | `MRG_MYISAM`
`NATURAL` | `NOT` | `NULL`
`NUMERIC` | `ON` | `OPTIMIZE`
`OPTION` | `OPTIONALLY` | `OR`
`ORDER` | `OUTER` | `OUTFILE`
`PRECISION` | `PRIMARY` | `PRIVILEGES`
`PROCEDURE` | `PURGE` | `READ`
`REAL` | `REFERENCES` | `REGEXP`
`RENAME` | `REPLACE` | `REQUIRE`
`RESTRICT` | `RETURNS` | `REVOKE`
`RIGHT` | `RLIKE` | `RTREE`
`SELECT` | `SET` | `SHOW`
`SMALLINT` | `SONAME` | `SPATIAL`
`SQL_BIG_RESULT` | `SQL_CALC_FOUND_ROWS` | `SQL_SMALL_RESULT`
`SSL` | `STARTING` | `STRAIGHT_JOIN`
`STRIPED` | `TABLE` | `TABLES`
`TERMINATED` | `THEN` | `TINYBLOB`
`TINYINT` | `TINYTEXT` | `TO`
`TRAILING` | `TYPES` | `UNION`
`UNIQUE` | `UNLOCK` | `UNSIGNED`
`UPDATE` | `USAGE` | `USE`
`USER_RESOURCES` | `USING` | `VALUES`
`VARBINARY` | `VARCHAR` | `VARYING`
`WARNINGS` | `WHEN` | `WHERE`
`WITH` | `WRITE` | `XOR`
`YEAR_MONTH` | `ZEROFILL` |

The following symbols (from the table above) are disallowed by ANSI SQL but allowed by MySQL as column/table names. This is because some of these names are very natural names and a lot of people have already used them.

- `ACTION`
- `BIT`
- `DATE`
- `ENUM`
- `NO`
- `TEXT`
- `TIME`
- `TIMESTAMP`

## 6.2 Column Types

MySQL supports a number of column types, which may be grouped into three categories: numeric types, date and time types, and string (character) types. This section first gives an overview of the types available and summarises the storage requirements for each column type, then provides a more detailed description of the properties of the types in each category. The overview is intentionally brief. The more detailed descriptions should be consulted for additional information about particular column types, such as the allowable formats in which you can specify values.

The column types supported by MySQL are listed below. The following code letters are used in the descriptions:

`M`
  Indicates the maximum display size. The maximum legal display size is 255.
`D`
  Applies to floating-point types and indicates the number of digits following the decimal point. The maximum possible value is 30, but should be no greater than `M`-2.

Square brackets (``['` and ``]'`) indicate parts of type specifiers that are optional.

Note that if you specify `ZEROFILL` for a column, MySQL will automatically add the `UNSIGNED` attribute to the column.

Warning: you should be aware that when you use subtraction between integer values where one is of type `UNSIGNED`, the result will be unsigned! See section 6.3.5 Cast Functions.

`TINYINT[(M)] [UNSIGNED] [ZEROFILL]`
  A very small integer. The signed range is `-128` to `127`. The unsigned range is `0` to `255`.
`BIT`
`BOOL`
  These are synonyms for `TINYINT(1)`.
`SMALLINT[(M)] [UNSIGNED] [ZEROFILL]`
  A small integer. The signed range is `-32768` to `32767`. The unsigned range is `0` to `65535`.
`MEDIUMINT[(M)] [UNSIGNED] [ZEROFILL]`
  A medium-size integer. The signed range is `-8388608` to `8388607`. The unsigned range is `0` to `16777215`.
`INT[(M)] [UNSIGNED] [ZEROFILL]`
  A normal-size integer. The signed range is `-2147483648` to `2147483647`. The unsigned range is `0` to `4294967295`.
`INTEGER[(M)] [UNSIGNED] [ZEROFILL]`
  This is a synonym for `INT`.
`BIGINT[(M)] [UNSIGNED] [ZEROFILL]`
  A large integer. The signed range is `-9223372036854775808` to `9223372036854775807`. The unsigned range is `0` to `18446744073709551615`. Some things you should be aware of with respect to `BIGINT` columns:
  - All arithmetic is done using signed `BIGINT` or `DOUBLE` values, so you shouldn't use unsigned big integers larger than `9223372036854775807` (63 bits) except with bit functions! If you do that, some of the last digits in the result may be wrong because of rounding errors when converting the `BIGINT` to a `DOUBLE`. MySQL 4.0 can handle `BIGINT` in the following cases:
    - Use integers to store big unsigned values in a `BIGINT` column.
    - In `MIN(big_int_column)` and `MAX(big_int_column)`.
    - When using operators (`+`, `-`, `*`, etc.) where both operands are integers.
  - You can always store an exact integer value in a `BIGINT` column by storing it as a string. In this case, MySQL will perform a string-to-number conversion that involves no intermediate double representation.
  - ``-'`, ``+'`, and ``*'` will use `BIGINT` arithmetic when both arguments are integer values! This means that if you multiply two big integers (or results from functions that return integers) you may get unexpected results when the result is larger than `9223372036854775807`.
`FLOAT(precision) [UNSIGNED] [ZEROFILL]`
  A floating-point number. `precision` can be `<=24` for a single-precision floating-point number and between 25 and 53 for a double-precision floating-point number. These types are like the `FLOAT` and `DOUBLE` types described immediately below. `FLOAT(X)` has the same range as the corresponding `FLOAT` and `DOUBLE` types, but the display size and number of decimals are undefined. In MySQL Version 3.23, this is a true floating-point value. In earlier MySQL versions, `FLOAT(precision)` always has 2 decimals. Note that using `FLOAT` may give you some unexpected problems as all calculations in MySQL are done with double precision. See section A.5.6 Solving Problems with No Matching Rows. This syntax is provided for ODBC compatibility.
`FLOAT[(M,D)] [UNSIGNED] [ZEROFILL]`
  A small (single-precision) floating-point number. Allowable values are `-3.402823466E+38` to `-1.175494351E-38`, `0`, and `1.175494351E-38` to `3.402823466E+38`. If `UNSIGNED` is specified, negative values are disallowed. The `M` is the display width and `D` is the number of decimals. `FLOAT` without arguments or `FLOAT(X)` where `X` <= 24 stands for a single-precision floating-point number.
`DOUBLE[(M,D)] [UNSIGNED] [ZEROFILL]`
  A normal-size (double-precision) floating-point number. Allowable values are `-1.7976931348623157E+308` to `-2.2250738585072014E-308`, `0`, and `2.2250738585072014E-308` to `1.7976931348623157E+308`. If `UNSIGNED` is specified, negative values are disallowed. The `M` is the display width and `D` is the number of decimals. `DOUBLE` without arguments or `FLOAT(X)` where 25 <= `X` <= 53 stands for a double-precision floating-point number.
`DOUBLE PRECISION[(M,D)] [UNSIGNED] [ZEROFILL]`
`REAL[(M,D)] [UNSIGNED] [ZEROFILL]`
  These are synonyms for `DOUBLE`.
`DECIMAL[(M[,D])] [UNSIGNED] [ZEROFILL]`
  An unpacked floating-point number. Behaves like a `CHAR` column: ``unpacked'' means the number is stored as a string, using one character for each digit of the value. The decimal point and, for negative numbers, the ``-'` sign, are not counted in `M` (but space for these is reserved). If `D` is 0, values will have no decimal point or fractional part. The maximum range of `DECIMAL` values is the same as for `DOUBLE`, but the actual range for a given `DECIMAL` column may be constrained by the choice of `M` and `D`. If `UNSIGNED` is specified, negative values are disallowed. If `D` is omitted, the default is 0. If `M` is omitted, the default is 10. Prior to MySQL Version 3.23, the `M` argument must include the space needed for the sign and the decimal point.
`DEC[(M[,D])] [UNSIGNED] [ZEROFILL]`
`NUMERIC[(M[,D])] [UNSIGNED] [ZEROFILL]`
  These are synonyms for `DECIMAL`.
`DATE`
  A date. The supported range is `'1000-01-01'` to `'9999-12-31'`. MySQL displays `DATE` values in `'YYYY-MM-DD'` format, but allows you to assign values to `DATE` columns using either strings or numbers. See section 6.2.2.2 The `DATETIME`, `DATE`, and `TIMESTAMP` Types.
`DATETIME`
  A date and time combination. The supported range is `'1000-01-01 00:00:00'` to `'9999-12-31 23:59:59'`. MySQL displays `DATETIME` values in `'YYYY-MM-DD HH:MM:SS'` format, but allows you to assign values to `DATETIME` columns using either strings or numbers. See section 6.2.2.2 The `DATETIME`, `DATE`, and `TIMESTAMP` Types.
`TIMESTAMP[(M)]`
  A timestamp. The range is `'1970-01-01 00:00:00'` to sometime in the year `2037`. MySQL displays `TIMESTAMP` values in `YYYYMMDDHHMMSS`, `YYMMDDHHMMSS`, `YYYYMMDD`, or `YYMMDD` format, depending on whether `M` is `14` (or missing), `12`, `8`, or `6`, but allows you to assign values to `TIMESTAMP` columns using either strings or numbers. A `TIMESTAMP` column is useful for recording the date and time of an `INSERT` or `UPDATE` operation because it is automatically set to the date and time of the most recent operation if you don't give it a value yourself. You can also set it to the current date and time by assigning it a `NULL` value. See section 6.2.2 Date and Time Types. The `M` argument affects only how a `TIMESTAMP` column is displayed; its values always are stored using 4 bytes each. Note that `TIMESTAMP(M)` columns where `M` is 8 or 14 are reported to be numbers while other `TIMESTAMP(M)` columns are reported to be strings. This is just to ensure that one can reliably dump and restore the table with these types! See section 6.2.2.2 The `DATETIME`, `DATE`, and `TIMESTAMP` Types.
`TIME`
  A time. The range is `'-838:59:59'` to `'838:59:59'`. MySQL displays `TIME` values in `'HH:MM:SS'` format, but allows you to assign values to `TIME` columns using either strings or numbers. See section 6.2.2.3 The `TIME` Type.
`YEAR[(2|4)]`
  A year in 2- or 4-digit format (default is 4-digit). The allowable values are `1901` to `2155`, `0000` in the 4-digit year format, and 1970-2069 if you use the 2-digit format (70-69). MySQL displays `YEAR` values in `YYYY` format, but allows you to assign values to `YEAR` columns using either strings or numbers. (The `YEAR` type is unavailable prior to MySQL Version 3.22.) See section 6.2.2.4 The `YEAR` Type.
`[NATIONAL] CHAR(M) [BINARY]`
  A fixed-length string that is always right-padded with spaces to the specified length when stored. The range of `M` is 0 to 255 characters (1 to 255 prior to MySQL Version 3.23). Trailing spaces are removed when the value is retrieved. `CHAR` values are sorted and compared in case-insensitive fashion according to the default character set unless the `BINARY` keyword is given. `NATIONAL CHAR` (or its equivalent short form, `NCHAR`) is the ANSI SQL way to define that a `CHAR` column should use the default CHARACTER set. This is the default in MySQL. `CHAR` is a shorthand for `CHARACTER`. MySQL allows you to create a column of type `CHAR(0)`. This is mainly useful when you have to be compliant with some old applications that depend on the existence of a column but that do not actually use the value. This is also quite nice when you need a column that only can take 2 values: A `CHAR(0)`, that is not defined as `NOT NULL`, will occupy only one bit and can take only 2 values: `NULL` or `""`. See section 6.2.3.1 The `CHAR` and `VARCHAR` Types.
`CHAR`
  This is a synonym for `CHAR(1)`.
`[NATIONAL] VARCHAR(M) [BINARY]`
  A variable-length string. Note: trailing spaces are removed when the value is stored (this differs from the ANSI SQL specification). The range of `M` is 0 to 255 characters (1 to 255 prior to MySQL Version 4.0.2). `VARCHAR` values are sorted and compared in case-insensitive fashion unless the `BINARY` keyword is given. See section 6.5.3.1 Silent Column Specification Changes. `VARCHAR` is a shorthand for `CHARACTER VARYING`. See section 6.2.3.1 The `CHAR` and `VARCHAR` Types.
`TINYBLOB`
`TINYTEXT`
  A `BLOB` or `TEXT` column with a maximum length of 255 (2^8 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. See section 6.2.3.2 The `BLOB` and `TEXT` Types.
`BLOB`
`TEXT`
  A `BLOB` or `TEXT` column with a maximum length of 65535 (2^16 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. See section 6.2.3.2 The `BLOB` and `TEXT` Types.
`MEDIUMBLOB`
`MEDIUMTEXT`
  A `BLOB` or `TEXT` column with a maximum length of 16777215 (2^24 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. See section 6.2.3.2 The `BLOB` and `TEXT` Types.
`LONGBLOB`
`LONGTEXT`
  A `BLOB` or `TEXT` column with a maximum length of 4294967295 (2^32 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. Note that because the server/client protocol and MyISAM tables has currently a limit of 16M per communication packet / table row, you can't yet use this the whole range of this type. See section 6.2.3.2 The `BLOB` and `TEXT` Types.
`ENUM('value1','value2',...)`
  An enumeration. A string object that can have only one value, chosen from the list of values `'value1'`, `'value2'`, `...`, `NULL` or the special `""` error value. An `ENUM` can have a maximum of 65535 distinct values. See section 6.2.3.3 The `ENUM` Type.
`SET('value1','value2',...)`
  A set. A string object that can have zero or more values, each of which must be chosen from the list of values `'value1'`, `'value2'`, `...` A `SET` can have a maximum of 64 members. See section 6.2.3.4 The `SET` Type.

### 6.2.1 Numeric Types

MySQL supports all of the ANSI/ISO SQL92 numeric types. These types include the exact numeric data types (`NUMERIC`, `DECIMAL`, `INTEGER`, and `SMALLINT`), as well as the approximate numeric data types (`FLOAT`, `REAL`, and `DOUBLE PRECISION`). The keyword `INT` is a synonym for `INTEGER`, and the keyword `DEC` is a synonym for `DECIMAL`.

The `NUMERIC` and `DECIMAL` types are implemented as the same type by MySQL, as permitted by the SQL92 standard. They are used for values for which it is important to preserve exact precision, for example with monetary data. When declaring a column of one of these types the precision and scale can be (and usually is) specified; for example:

        salary DECIMAL(5,2)

In this example, `5` (`precision`) represents the number of significant decimal digits that will be stored for values, and `2` (`scale`) represents the number of digits that will be stored following the decimal point. In this case, therefore, the range of values that can be stored in the `salary` column is from `-99.99` to `99.99`. (MySQL can actually store numbers up to `999.99` in this column because it doesn't have to store the sign for positive numbers)

In ANSI/ISO SQL92, the syntax `DECIMAL(p)` is equivalent to `DECIMAL(p,0)`. Similarly, the syntax `DECIMAL` is equivalent to `DECIMAL(p,0)`, where the implementation is allowed to decide the value of `p`. MySQL does not currently support either of these variant forms of the `DECIMAL`/`NUMERIC` data types. This is not generally a serious problem, as the principal benefits of these types derive from the ability to control both precision and scale explicitly.

`DECIMAL` and `NUMERIC` values are stored as strings, rather than as binary floating-point numbers, in order to preserve the decimal precision of those values. One character is used for each digit of the value, the decimal point (if `scale` > 0), and the ``-'` sign (for negative numbers). If `scale` is 0, `DECIMAL` and `NUMERIC` values contain no decimal point or fractional part.

The maximum range of `DECIMAL` and `NUMERIC` values is the same as for `DOUBLE`, but the actual range for a given `DECIMAL` or `NUMERIC` column can be constrained by the `precision` or `scale` for a given column. When such a column is assigned a value with more digits following the decimal point than are allowed by the specified `scale`, the value is rounded to that `scale`. When a `DECIMAL` or `NUMERIC` column is assigned a value whose magnitude exceeds the range implied by the specified (or defaulted) `precision` and `scale`, MySQL stores the value representing the corresponding end point of that range.

As an extension to the ANSI/ISO SQL92 standard, MySQL also supports the integer types `TINYINT`, `MEDIUMINT`, and `BIGINT` as listed in the tables above. Another extension is supported by MySQL for optionally specifying the display width of an integer value in parentheses following the base keyword for the type (for example, `INT(4)`). This optional width specification is used to left-pad the display of values whose width is less than the width specified for the column, but does not constrain the range of values that can be stored in the column, nor the number of digits that will be displayed for values whose width exceeds that specified for the column. When used in conjunction with the optional extension attribute `ZEROFILL`, the default padding of spaces is replaced with zeroes. For example, for a column declared as `INT(5) ZEROFILL`, a value of `4` is retrieved as `00004`. Note that if you store larger values than the display width in an integer column, you may experience problems when MySQL generates temporary tables for some complicated joins, as in these cases MySQL trusts that the data did fit into the original column width.

All integer types can have an optional (non-standard) attribute `UNSIGNED`. Unsigned values can be used when you want to allow only positive numbers in a column and you need a little bigger numeric range for the column.

As of MySQL 4.0.2, floating-point types also can be `UNSIGNED`. As with integer types, this attribute prevents negative values from being stored in the column. Unlike the integer types, the upper range of column values remains the same.

The `FLOAT` type is used to represent approximate numeric data types. The ANSI/ISO SQL92 standard allows an optional specification of the precision (but not the range of the exponent) in bits following the keyword `FLOAT` in parentheses. The MySQL implementation also supports this optional precision specification. When the keyword `FLOAT` is used for a column type without a precision specification, MySQL uses four bytes to store the values. A variant syntax is also supported, with two numbers given in parentheses following the `FLOAT` keyword. With this option, the first number continues to represent the storage requirements for the value in bytes, and the second number specifies the number of digits to be stored and displayed following the decimal point (as with `DECIMAL` and `NUMERIC`). When MySQL is asked to store a number for such a column with more decimal digits following the decimal point than specified for the column, the value is rounded to eliminate the extra digits when the value is stored.

The `REAL` and `DOUBLE PRECISION` types do not accept precision specifications. As an extension to the ANSI/ISO SQL92 standard, MySQL recognises `DOUBLE` as a synonym for the `DOUBLE PRECISION` type. In contrast with the standard's requirement that the precision for `REAL` be smaller than that used for `DOUBLE PRECISION`, MySQL implements both as 8-byte double-precision floating-point values (when not running in ``ANSI mode''). For maximum portability, code requiring storage of approximate numeric data values should use `FLOAT` or `DOUBLE PRECISION` with no specification of precision or number of decimal points.

When asked to store a value in a numeric column that is outside the column type's allowable range, MySQL clips the value to the appropriate endpoint of the range and stores the resulting value instead.

For example, the range of an `INT` column is `-2147483648` to `2147483647`. If you try to insert `-9999999999` into an `INT` column, the value is clipped to the lower endpoint of the range, and `-2147483648` is stored instead. Similarly, if you try to insert `9999999999`, `2147483647` is stored instead.

If the `INT` column is `UNSIGNED`, the size of the column's range is the same but its endpoints shift up to `0` and `4294967295`. If you try to store `-9999999999` and `9999999999`, the values stored in the column become `0` and `4294967296`.

Conversions that occur due to clipping are reported as ``warnings'' for `ALTER TABLE`, `LOAD DATA INFILE`, `UPDATE`, and multi-row `INSERT` statements.

Type | Bytes | From | To
`TINYINT` | 1 | -128 | 127
`SMALLINT` | 2 | -32768 | 32767
`MEDIUMINT` | 3 | -8388608 | 8388607
`INT` | 4 | -2147483648 | 2147483647
`BIGINT` | 8 | -9223372036854775808 | 9223372036854775807

### 6.2.2 Date and Time Types

The date and time types are `DATETIME`, `DATE`, `TIMESTAMP`, `TIME`, and `YEAR`. Each of these has a range of legal values, as well as a ``zero'' value that is used when you specify a really illegal value. Note that MySQL allows you to store certain 'not strictly' legal date values, for example `1999-11-31`. The reason for this is that we think it's the responsibility of the application to handle date checking, not the SQL servers. To make the date checking 'fast', MySQL only checks that the month is in the range of 0-12 and the day is in the range of 0-31. The above ranges are defined this way because MySQL allows you to store, in a `DATE` or `DATETIME` column, dates where the day or month-day is zero. This is extremely useful for applications that need to store a birth-date for which you don't know the exact date. In this case you simply store the date like `1999-00-00` or `1999-01-00`. (You cannot expect to get a correct value from functions like `DATE_SUB()` or `DATE_ADD` for dates like these.)

Here are some general considerations to keep in mind when working with date and time types:

- MySQL retrieves values for a given date or time type in a standard format, but it attempts to interpret a variety of formats for values that you supply (for example, when you specify a value to be assigned to or compared to a date or time type). Nevertheless, only the formats described in the following sections are supported. It is expected that you will supply legal values, and unpredictable results may occur if you use values in other formats.
- Although MySQL tries to interpret values in several formats, it always expects the year part of date values to be leftmost. Dates must be given in year-month-day order (for example, `'98-09-04'`), rather than in the month-day-year or day-month-year orders commonly used elsewhere (for example, `'09-04-98'`, `'04-09-98'`).
- MySQL automatically converts a date or time type value to a number if the value is used in a numeric context, and vice versa.
- When MySQL encounters a value for a date or time type that is out of range or otherwise illegal for the type (see the start of this section), it converts the value to the ``zero'' value for that type. (The exception is that out-of-range `TIME` values are clipped to the appropriate endpoint of the `TIME` range.) The following table shows the format of the ``zero'' value for each type:

  Column type | ``Zero'' value
  `DATETIME` | `'0000-00-00 00:00:00'`
  `DATE` | `'0000-00-00'`
  `TIMESTAMP` | `00000000000000` (length depends on display size)
  `TIME` | `'00:00:00'`
  `YEAR` | `0000`

- The ``zero'' values are special, but you can store or refer to them explicitly using the values shown in the table. You can also do this using the values `'0'` or `0`, which are easier to write.
- ``Zero'' date or time values used through `MyODBC` are converted automatically to `NULL` in `MyODBC` Version 2.50.12 and above, because ODBC can't handle such values.

#### 6.2.2.1 Y2K Issues and Date Types

MySQL itself is Y2K-safe (see section 1.2.5 Year 2000 Compliance), but input values presented to MySQL may not be. Any input containing 2-digit year values is ambiguous, because the century is unknown. Such values must be interpreted into 4-digit form because MySQL stores years internally using four digits.

For `DATETIME`, `DATE`, `TIMESTAMP`, and `YEAR` types, MySQL interprets dates with ambiguous year values using the following rules:

- Year values in the range `00-69` are converted to `2000-2069`.
- Year values in the range `70-99` are converted to `1970-1999`.

Remember that these rules provide only reasonable guesses as to what your data mean. If the heuristics used by MySQL don't produce the correct values, you should provide unambiguous input containing 4-digit year values.

`ORDER BY` will sort 2-digit `YEAR/DATE/DATETIME` types properly.

Note also that some functions like `MIN()` and `MAX()` will convert a `TIMESTAMP/DATE` to a number. This means that a timestamp with a 2-digit year will not work properly with these functions. The fix in this case is to convert the `TIMESTAMP/DATE` to 4-digit year format or use something like `MIN(DATE_ADD(timestamp,INTERVAL 0 DAYS))`.

#### 6.2.2.2 The `DATETIME`, `DATE`, and `TIMESTAMP` Types

The `DATETIME`, `DATE`, and `TIMESTAMP` types are related. This section describes their characteristics, how they are similar, and how they differ.

The `DATETIME` type is used when you need values that contain both date and time information. MySQL retrieves and displays `DATETIME` values in `'YYYY-MM-DD HH:MM:SS'` format. The supported range is `'1000-01-01 00:00:00'` to `'9999-12-31 23:59:59'`. (``Supported'' means that although earlier values might work, there is no guarantee that they will.)

The `DATE` type is used when you need only a date value, without a time part. MySQL retrieves and displays `DATE` values in `'YYYY-MM-DD'` format. The supported range is `'1000-01-01'` to `'9999-12-31'`.

The `TIMESTAMP` column type provides a type that you can use to automatically mark `INSERT` or `UPDATE` operations with the current date and time. If you have multiple `TIMESTAMP` columns, only the first one is updated automatically.

Automatic updating of the first `TIMESTAMP` column occurs under any of the following conditions:

- The column is not specified explicitly in an `INSERT` or `LOAD DATA INFILE` statement.
- The column is not specified explicitly in an `UPDATE` statement and some other column changes value. (Note that an `UPDATE` that sets a column to the value it already has will not cause the `TIMESTAMP` column to be updated, because if you set a column to its current value, MySQL ignores the update for efficiency.)
- You explicitly set the `TIMESTAMP` column to `NULL`.

`TIMESTAMP` columns other than the first may also be set to the current date and time. Just set the column to `NULL` or to `NOW()`.

You can set any `TIMESTAMP` column to a value different from the current date and time by setting it explicitly to the desired value. This is true even for the first `TIMESTAMP` column. You can use this property if, for example, you want a `TIMESTAMP` to be set to the current date and time when you create a row, but not to be changed whenever the row is updated later:

- Let MySQL set the column when the row is created. This will initialise it to the current date and time.
- When you perform subsequent updates to other columns in the row, set the `TIMESTAMP` column explicitly to its current value.

On the other hand, you may find it just as easy to use a `DATETIME` column that you initialise to `NOW()` when the row is created and leave alone for subsequent updates.

`TIMESTAMP` values may range from the beginning of 1970 to sometime in the year 2037, with a resolution of one second. Values are displayed as numbers.

The format in which MySQL retrieves and displays `TIMESTAMP` values depends on the display size, as illustrated by the following table. The `full' `TIMESTAMP` format is 14 digits, but `TIMESTAMP` columns may be created with shorter display sizes:

Column type | Display format
`TIMESTAMP(14)` | `YYYYMMDDHHMMSS`
`TIMESTAMP(12)` | `YYMMDDHHMMSS`
`TIMESTAMP(10)` | `YYMMDDHHMM`
`TIMESTAMP(8)` | `YYYYMMDD`
`TIMESTAMP(6)` | `YYMMDD`
`TIMESTAMP(4)` | `YYMM`
`TIMESTAMP(2)` | `YY`

All `TIMESTAMP` columns have the same storage size, regardless of display size. The most common display sizes are 6, 8, 12, and 14. You can specify an arbitrary display size at table creation time, but values of 0 or greater than 14 are coerced to 14. Odd-valued sizes in the range from 1 to 13 are coerced to the next higher even number.

You can specify `DATETIME`, `DATE`, and `TIMESTAMP` values using any of a common set of formats:

- As a string in either `'YYYY-MM-DD HH:MM:SS'` or `'YY-MM-DD HH:MM:SS'` format. A ``relaxed'' syntax is allowed--any punctuation character may be used as the delimiter between date parts or time parts. For example, `'98-12-31 11:30:45'`, `'98.12.31 11+30+45'`, `'98/12/31 11*30*45'`, and `'98@12@31 11^30^45'` are equivalent.
- As a string in either `'YYYY-MM-DD'` or `'YY-MM-DD'` format. A ``relaxed'' syntax is allowed here, too. For example, `'98-12-31'`, `'98.12.31'`, `'98/12/31'`, and `'98@12@31'` are equivalent.
- As a string with no delimiters in either `'YYYYMMDDHHMMSS'` or `'YYMMDDHHMMSS'` format, provided that the string makes sense as a date. For example, `'19970523091528'` and `'970523091528'` are interpreted as `'1997-05-23 09:15:28'`, but `'971122129015'` is illegal (it has a nonsensical minute part) and becomes `'0000-00-00 00:00:00'`.
- As a string with no delimiters in either `'YYYYMMDD'` or `'YYMMDD'` format, provided that the string makes sense as a date. For example, `'19970523'` and `'970523'` are interpreted as `'1997-05-23'`, but `'971332'` is illegal (it has nonsensical month and day parts) and becomes `'0000-00-00'`.
- As a number in either `YYYYMMDDHHMMSS` or `YYMMDDHHMMSS` format, provided that the number makes sense as a date. For example, `19830905132800` and `830905132800` are interpreted as `'1983-09-05 13:28:00'`.
- As a number in either `YYYYMMDD` or `YYMMDD` format, provided that the number makes sense as a date. For example, `19830905` and `830905` are interpreted as `'1983-09-05'`.
- As the result of a function that returns a value that is acceptable in a `DATETIME`, `DATE`, or `TIMESTAMP` context, such as `NOW()` or `CURRENT_DATE`.

Illegal `DATETIME`, `DATE`, or `TIMESTAMP` values are converted to the ``zero'' value of the appropriate type (`'0000-00-00 00:00:00'`, `'0000-00-00'`, or `00000000000000`).

For values specified as strings that include date part delimiters, it is not necessary to specify two digits for month or day values that are less than `10`. `'1979-6-9'` is the same as `'1979-06-09'`. Similarly, for values specified as strings that include time part delimiters, it is not necessary to specify two digits for hour, minute, or second values that are less than `10`. `'1979-10-30 1:2:3'` is the same as `'1979-10-30 01:02:03'`.

Values specified as numbers should be 6, 8, 12, or 14 digits long. If the number is 8 or 14 digits long, it is assumed to be in `YYYYMMDD` or `YYYYMMDDHHMMSS` format and that the year is given by the first 4 digits. If the number is 6 or 12 digits long, it is assumed to be in `YYMMDD` or `YYMMDDHHMMSS` format and that the year is given by the first 2 digits. Numbers that are not one of these lengths are interpreted as though padded with leading zeros to the closest length.

Values specified as non-delimited strings are interpreted using their length as given. If the string is 8 or 14 characters long, the year is assumed to be given by the first 4 characters. Otherwise, the year is assumed to be given by the first 2 characters. The string is interpreted from left to right to find year, month, day, hour, minute, and second values, for as many parts as are present in the string. This means you should not use strings that have fewer than 6 characters. For example, if you specify `'9903'`, thinking that will represent March, 1999, you will find that MySQL inserts a ``zero'' date into your table. This is because the year and month values are `99` and `03`, but the day part is missing (zero), so the value is not a legal date.

`TIMESTAMP` columns store legal values using the full precision with which the value was specified, regardless of the display size. This has several implications:

- Always specify year, month, and day, even if your column types are `TIMESTAMP(4)` or `TIMESTAMP(2)`. Otherwise, the value will not be a legal date and `0` will be stored.
- If you use `ALTER TABLE` to widen a narrow `TIMESTAMP` column, information will be displayed that previously was ``hidden''.
- Similarly, narrowing a `TIMESTAMP` column does not cause information to be lost, except in the sense that less information is shown when the values are displayed.
- Although `TIMESTAMP` values are stored to full precision, the only function that operates directly on the underlying stored value is `UNIX_TIMESTAMP()`. Other functions operate on the formatted retrieved value. This means you cannot use functions such as `HOUR()` or `SECOND()` unless the relevant part of the `TIMESTAMP` value is included in the formatted value. For example, the `HH` part of a `TIMESTAMP` column is not displayed unless the display size is at least 10, so trying to use `HOUR()` on shorter `TIMESTAMP` values produces a meaningless result.

You can to some extent assign values of one date type to an object of a different date type. However, there may be some alteration of the value or loss of information:

- If you assign a `DATE` value to a `DATETIME` or `TIMESTAMP` object, the time part of the resulting value is set to `'00:00:00'`, because the `DATE` value contains no time information.
- If you assign a `DATETIME` or `TIMESTAMP` value to a `DATE` object, the time part of the resulting value is deleted, because the `DATE` type stores no time information.
- Remember that although `DATETIME`, `DATE`, and `TIMESTAMP` values all can be specified using the same set of formats, the types do not all have the same range of values. For example, `TIMESTAMP` values cannot be earlier than `1970` or later than `2037`. This means that a date such as `'1968-01-01'`, while legal as a `DATETIME` or `DATE` value, is not a valid `TIMESTAMP` value and will be converted to `0` if assigned to such an object.

Be aware of certain pitfalls when specifying date values:

- The relaxed format allowed for values specified as strings can be deceiving. For example, a value such as `'10:11:12'` might look like a time value because of the ``:'` delimiter, but if used in a date context will be interpreted as the year `'2010-11-12'`. The value `'10:45:15'` will be converted to `'0000-00-00'` because `'45'` is not a legal month.
- The MySQL server only performs basic checking on the validity of a date: days `00-31`, months `00-12`, years `1000-9999`. Any date not within this range will revert to `0000-00-00`. Please note that this still allows you to store invalid dates such as `2002-04-31`. It allows web applications to store data from a form without further checking. To ensure a date is valid, perform a check in your application.
- Year values specified as two digits are ambiguous, because the century is unknown. MySQL interprets 2-digit year values using the following rules:
  - Year values in the range `00-69` are converted to `2000-2069`.
  - Year values in the range `70-99` are converted to `1970-1999`.

#### 6.2.2.3 The `TIME` Type

MySQL retrieves and displays `TIME` values in `'HH:MM:SS'` format (or `'HHH:MM:SS'` format for large hours values). `TIME` values may range from `'-838:59:59'` to `'838:59:59'`. The reason the hours part may be so large is that the `TIME` type may be used not only to represent a time of day (which must be less than 24 hours), but also elapsed time or a time interval between two events (which may be much greater than 24 hours, or even negative).

You can specify `TIME` values in a variety of formats:

- As a string in `'D HH:MM:SS.fraction'` format. (Note that MySQL doesn't yet store the fraction for the time column.) One can also use one of the following ``relaxed'' syntax: `HH:MM:SS.fraction`, `HH:MM:SS`, `HH:MM`, `D HH:MM:SS`, `D HH:MM`, `D HH` or `SS`. Here `D` is days between 0-33.
- As a string with no delimiters in `'HHMMSS'` format, provided that it makes sense as a time. For example, `'101112'` is understood as `'10:11:12'`, but `'109712'` is illegal (it has a nonsensical minute part) and becomes `'00:00:00'`.
- As a number in `HHMMSS` format, provided that it makes sense as a time. For example, `101112` is understood as `'10:11:12'`. The following alternative formats are also understood: `SS`, `MMSS`,`HHMMSS`, `HHMMSS.fraction`. Note that MySQL doesn't yet store the fraction part.
- As the result of a function that returns a value that is acceptable in a `TIME` context, such as `CURRENT_TIME`.

For `TIME` values specified as strings that include a time part delimiter, it is not necessary to specify two digits for hours, minutes, or seconds values that are less than `10`. `'8:3:2'` is the same as `'08:03:02'`.

Be careful about assigning ``short'' `TIME` values to a `TIME` column. Without colons, MySQL interprets values using the assumption that the rightmost digits represent seconds. (MySQL interprets `TIME` values as elapsed time rather than as time of day.) For example, you might think of `'1112'` and `1112` as meaning `'11:12:00'` (12 minutes after 11 o'clock), but MySQL interprets them as `'00:11:12'` (11 minutes, 12 seconds). Similarly, `'12'` and `12` are interpreted as `'00:00:12'`. `TIME` values with colons, by contrast, are always treated as time of the day. That is `'11:12'` will mean `'11:12:00'`, not `'00:11:12'`.

Values that lie outside the `TIME` range but are otherwise legal are clipped to the appropriate endpoint of the range. For example, `'-850:00:00'` and `'850:00:00'` are converted to `'-838:59:59'` and `'838:59:59'`.

Illegal `TIME` values are converted to `'00:00:00'`. Note that because `'00:00:00'` is itself a legal `TIME` value, there is no way to tell, from a value of `'00:00:00'` stored in a table, whether the original value was specified as `'00:00:00'` or whether it was illegal.

#### 6.2.2.4 The `YEAR` Type

The `YEAR` type is a 1-byte type used for representing years.

MySQL retrieves and displays `YEAR` values in `YYYY` format. The range is `1901` to `2155`.

You can specify `YEAR` values in a variety of formats:

- As a four-digit string in the range `'1901'` to `'2155'`.
- As a four-digit number in the range `1901` to `2155`.
- As a two-digit string in the range `'00'` to `'99'`. Values in the ranges `'00'` to `'69'` and `'70'` to `'99'` are converted to `YEAR` values in the ranges `2000` to `2069` and `1970` to `1999`.
- As a two-digit number in the range `1` to `99`. Values in the ranges `1` to `69` and `70` to `99` are converted to `YEAR` values in the ranges `2001` to `2069` and `1970` to `1999`. Note that the range for two-digit numbers is slightly different from the range for two-digit strings, because you cannot specify zero directly as a number and have it be interpreted as `2000`. You must specify it as a string `'0'` or `'00'` or it will be interpreted as `0000`.
- As the result of a function that returns a value that is acceptable in a `YEAR` context, such as `NOW()`.

Illegal `YEAR` values are converted to `0000`.

### 6.2.3 String Types

The string types are `CHAR`, `VARCHAR`, `BLOB`, `TEXT`, `ENUM`, and `SET`. This section describes how these types work, their storage requirements, and how to use them in your queries.

Type | Max.size | Bytes
`TINYTEXT` or `TINYBLOB` | 2^8-1 | 255
`TEXT` or `BLOB` | 2^16-1 (64K-1) | 65535
`MEDIUMTEXT` or `MEDIUMBLOB` | 2^24-1 (16M-1) | 16777215
`LONGBLOB` | 2^32-1 (4G-1) | 4294967295

#### 6.2.3.1 The `CHAR` and `VARCHAR` Types

The `CHAR` and `VARCHAR` types are similar, but differ in the way they are stored and retrieved.

The length of a `CHAR` column is fixed to the length that you declare when you create the table. The length can be any value between 1 and 255. (As of MySQL Version 3.23, the length of `CHAR` may be 0 to 255.) When `CHAR` values are stored, they are right-padded with spaces to the specified length. When `CHAR` values are retrieved, trailing spaces are removed.

Values in `VARCHAR` columns are variable-length strings. You can declare a `VARCHAR` column to be any length between 1 and 255, just as for `CHAR` columns. However, in contrast to `CHAR`, `VARCHAR` values are stored using only as many characters as are needed, plus one byte to record the length. Values are not padded; instead, trailing spaces are removed when values are stored. (This space removal differs from the ANSI SQL specification.)

If you assign a value to a `CHAR` or `VARCHAR` column that exceeds the column's maximum length, the value is truncated to fit.

The following table illustrates the differences between the two types of columns by showing the result of storing various string values into `CHAR(4)` and `VARCHAR(4)` columns:

Value | `CHAR(4)` | Storage required | `VARCHAR(4)` | Storage required
`''` | `' '` | 4 bytes | `''` | 1 byte
`'ab'` | `'ab '` | 4 bytes | `'ab'` | 3 bytes
`'abcd'` | `'abcd'` | 4 bytes | `'abcd'` | 5 bytes
`'abcdefgh'` | `'abcd'` | 4 bytes | `'abcd'` | 5 bytes

The values retrieved from the `CHAR(4)` and `VARCHAR(4)` columns will be the same in each case, because trailing spaces are removed from `CHAR` columns upon retrieval.

Values in `CHAR` and `VARCHAR` columns are sorted and compared in case-insensitive fashion, unless the `BINARY` attribute was specified when the table was created. The `BINARY` attribute means that column values are sorted and compared in case-sensitive fashion according to the ASCII order of the machine where the MySQL server is running. `BINARY` doesn't affect how the column is stored or retrieved.

The `BINARY` attribute is sticky. This means that if a column marked `BINARY` is used in an expression, the whole expression is compared as a `BINARY` value.

MySQL may silently change the type of a `CHAR` or `VARCHAR` column at table creation time. See section 6.5.3.1 Silent Column Specification Changes.

#### 6.2.3.2 The `BLOB` and `TEXT` Types

A `BLOB` is a binary large object that can hold a variable amount of data. The four `BLOB` types `TINYBLOB`, `BLOB`, `MEDIUMBLOB`, and `LONGBLOB` differ only in the maximum length of the values they can hold. See section 6.2.6 Column Type Storage Requirements.

The four `TEXT` types `TINYTEXT`, `TEXT`, `MEDIUMTEXT`, and `LONGTEXT` correspond to the four `BLOB` types and have the same maximum lengths and storage requirements. The only difference between `BLOB` and `TEXT` types is that sorting and comparison is performed in case-sensitive fashion for `BLOB` values and case-insensitive fashion for `TEXT` values. In other words, a `TEXT` is a case-insensitive `BLOB`.

If you assign a value to a `BLOB` or `TEXT` column that exceeds the column type's maximum length, the value is truncated to fit.

In most respects, you can regard a `TEXT` column as a `VARCHAR` column that can be as big as you like. Similarly, you can regard a `BLOB` column as a `VARCHAR BINARY` column. The differences are:

- You can have indexes on `BLOB` and `TEXT` columns with MySQL Version 3.23.2 and newer. Older versions of MySQL did not support this.
- There is no trailing-space removal for `BLOB` and `TEXT` columns when values are stored, as there is for `VARCHAR` columns.
- `BLOB` and `TEXT` columns cannot have `DEFAULT` values.

`MyODBC` defines `BLOB` values as `LONGVARBINARY` and `TEXT` values as `LONGVARCHAR`.

Because `BLOB` and `TEXT` values may be extremely long, you may run up against some constraints when using them:

- If you want to use `GROUP BY` or `ORDER BY` on a `BLOB` or `TEXT` column, you must convert the column value into a fixed-length object. The standard way to do this is with the `SUBSTRING` function. For example:

      mysql> SELECT comment FROM tbl_name,SUBSTRING(comment,20) AS substr
          ->                 ORDER BY substr;

  If you don't do this, only the first `max_sort_length` bytes of the column are used when sorting. The default value of `max_sort_length` is 1024; this value can be changed using the `-O` option when starting the `mysqld` server. You can group on an expression involving `BLOB` or `TEXT` values by specifying the column position or by using an alias:

      mysql> SELECT id,SUBSTRING(blob_col,1,100) FROM tbl_name GROUP BY 2;
      mysql> SELECT id,SUBSTRING(blob_col,1,100) AS b FROM tbl_name GROUP BY b;

- The maximum size of a `BLOB` or `TEXT` object is determined by its type, but the largest value you can actually transmit between the client and server is determined by the amount of available memory and the size of the communications buffers. You can change the message buffer size, but you must do so on both the server and client ends. See section 5.5.2 Tuning Server Parameters.

Note that each `BLOB` or `TEXT` value is represented internally by a separately allocated object. This is in contrast to all other column types, for which storage is allocated once per column when the table is opened.

#### 6.2.3.3 The `ENUM` Type

An `ENUM` is a string object whose value normally is chosen from a list of allowed values that are enumerated explicitly in the column specification at table creation time.

The value may also be the empty string (`""`) or `NULL` under certain circumstances:

- If you insert an invalid value into an `ENUM` (that is, a string not present in the list of allowed values), the empty string is inserted instead as a special error value. This string can be distinguished from a 'normal' empty string by the fact that this string has the numerical value 0. More about this later.
- If an `ENUM` is declared `NULL`, `NULL` is also a legal value for the column, and the default value is `NULL`. If an `ENUM` is declared `NOT NULL`, the default value is the first element of the list of allowed values.

Each enumeration value has an index:

- Values from the list of allowable elements in the column specification are numbered beginning with 1.
- The index value of the empty string error value is 0. This means that you can use the following `SELECT` statement to find rows into which invalid `ENUM` values were assigned:

      mysql> SELECT * FROM tbl_name WHERE enum_col=0;

- The index of the `NULL` value is `NULL`.

For example, a column specified as `ENUM("one", "two", "three")` can have any of the values shown here. The index of each value is also shown:

Value | Index
`NULL` | `NULL`
`""` | 0
`"one"` | 1
`"two"` | 2
`"three"` | 3

An enumeration can have a maximum of 65535 elements.

Starting from 3.23.51 trailing spaces are automatically deleted from `ENUM` values when the table is created.

Lettercase is irrelevant when you assign values to an `ENUM` column. However, values retrieved from the column later have lettercase matching the values that were used to specify the allowable values at table creation time.

If you retrieve an `ENUM` in a numeric context, the column value's index is returned. For example, you can retrieve numeric values from an `ENUM` column like this:

    mysql> SELECT enum_col+0 FROM tbl_name;

If you store a number into an `ENUM`, the number is treated as an index, and the value stored is the enumeration member with that index. (However, this will not work with `LOAD DATA`, which treats all input as strings.) It's not advisable to store numbers in an `ENUM` string because it will make things confusing.

`ENUM` values are sorted according to the order in which the enumeration members were listed in the column specification. (In other words, `ENUM` values are sorted according to their index numbers.) For example, `"a"` sorts before `"b"` for `ENUM("a", "b")`, but `"b"` sorts before `"a"` for `ENUM("b", "a")`. The empty string sorts before non-empty strings, and `NULL` values sort before all other enumeration values.

If you want to get all possible values for an `ENUM` column, you should use: `SHOW COLUMNS FROM table_name LIKE enum_column_name` and parse the `ENUM` definition in the second column.

#### 6.2.3.4 The `SET` Type

A `SET` is a string object that can have zero or more values, each of which must be chosen from a list of allowed values specified when the table is created. `SET` column values that consist of multiple set members are specified with members separated by commas (``,'`). A consequence of this is that `SET` member values cannot themselves contain commas.

For example, a column specified as `SET("one", "two") NOT NULL` can have any of these values:

    ""
    "one"
    "two"
    "one,two"

A `SET` can have a maximum of 64 different members.

Starting from 3.23.51 trailing spaces are automatically deleted from `SET` values when the table is created.

MySQL stores `SET` values numerically, with the low-order bit of the stored value corresponding to the first set member. If you retrieve a `SET` value in a numeric context, the value retrieved has bits set corresponding to the set members that make up the column value. For example, you can retrieve numeric values from a `SET` column like this:

    mysql> SELECT set_col+0 FROM tbl_name;

If a number is stored into a `SET` column, the bits that are set in the binary representation of the number determine the set members in the column value. Suppose a column is specified as `SET("a","b","c","d")`. Then the members have the following bit values:

`SET` member | Decimal value | Binary value
`a` | `1` | `0001`
`b` | `2` | `0010`
`c` | `4` | `0100`
`d` | `8` | `1000`

If you assign a value of `9` to this column, that is `1001` in binary, so the first and fourth `SET` value members `"a"` and `"d"` are selected and the resulting value is `"a,d"`.

For a value containing more than one `SET` element, it does not matter what order the elements are listed in when you insert the value. It also does not matter how many times a given element is listed in the value. When the value is retrieved later, each element in the value will appear once, with elements listed according to the order in which they were specified at table creation time. For example, if a column is specified as `SET("a","b","c","d")`, then `"a,d"`, `"d,a"`, and `"d,a,a,d,d"` will all appear as `"a,d"` when retrieved.

If you set a `SET` column to an unsupported value, the value will be ignored.

`SET` values are sorted numerically. `NULL` values sort before non-`NULL` `SET` values.

Normally, you perform a `SELECT` on a `SET` column using the `LIKE` operator or the `FIND_IN_SET()` function:

    mysql> SELECT * FROM tbl_name WHERE set_col LIKE '%value%';
    mysql> SELECT * FROM tbl_name WHERE FIND_IN_SET('value',set_col)>0;

But the following will also work:

    mysql> SELECT * FROM tbl_name WHERE set_col = 'val1,val2';
    mysql> SELECT * FROM tbl_name WHERE set_col & 1;

The first of these statements looks for an exact match. The second looks for values containing the first set member.

If you want to get all possible values for a `SET` column, you should use: `SHOW COLUMNS FROM table_name LIKE set_column_name` and parse the `SET` definition in the second column.

### 6.2.4 Choosing the Right Type for a Column

For the most efficient use of storage, try to use the most precise type in all cases. For example, if an integer column will be used for values in the range between `1` and `99999`, `MEDIUMINT UNSIGNED` is the best type.

Accurate representation of monetary values is a common problem. In MySQL, you should use the `DECIMAL` type. This is stored as a string, so no loss of accuracy should occur. If accuracy is not too important, the `DOUBLE` type may also be good enough.

For high precision, you can always convert to a fixed-point type stored in a `BIGINT`. This allows you to do all calculations with integers and convert results back to floating-point values only when necessary.

### 6.2.5 Using Column Types from Other Database Engines

To make it easier to use code written for SQL implementations from other vendors, MySQL maps column types as shown in the following table. These mappings make it easier to move table definitions from other database engines to MySQL:

Other vendor type | MySQL type
`BINARY(NUM)` | `CHAR(NUM) BINARY`
`CHAR VARYING(NUM)` | `VARCHAR(NUM)`
`FLOAT4` | `FLOAT`
`FLOAT8` | `DOUBLE`
`INT1` | `TINYINT`
`INT2` | `SMALLINT`
`INT3` | `MEDIUMINT`
`INT4` | `INT`
`INT8` | `BIGINT`
`LONG VARBINARY` | `MEDIUMBLOB`
`LONG VARCHAR` | `MEDIUMTEXT`
`MIDDLEINT` | `MEDIUMINT`
`VARBINARY(NUM)` | `VARCHAR(NUM) BINARY`

Column type mapping occurs at table creation time. If you create a table with types used by other vendors and then issue a `DESCRIBE tbl_name` statement, MySQL reports the table structure using the equivalent MySQL types.

### 6.2.6 Column Type Storage Requirements

The storage requirements for each of the column types supported by MySQL are listed by category.

#### 6.2.6.1 Storage requirements for numeric types

Column type | Storage required
`TINYINT` | 1 byte
`SMALLINT` | 2 bytes
`MEDIUMINT` | 3 bytes
`INT` | 4 bytes
`INTEGER` | 4 bytes
`BIGINT` | 8 bytes
`FLOAT(X)` | 4 if X <= 24 or 8 if 25 <= X <= 53
`FLOAT` | 4 bytes
`DOUBLE` | 8 bytes
`DOUBLE PRECISION` | 8 bytes
`REAL` | 8 bytes
`DECIMAL(M,D)` | `M+2` bytes if D > 0, `M+1` bytes if D = 0 (`D`+2, if `M < D`)
`NUMERIC(M,D)` | `M+2` bytes if D > 0, `M+1` bytes if D = 0 (`D`+2, if `M < D`)

#### 6.2.6.2 Storage requirements for date and time types

Column type | Storage required
`DATE` | 3 bytes
`DATETIME` | 8 bytes
`TIMESTAMP` | 4 bytes
`TIME` | 3 bytes
`YEAR` | 1 byte

#### 6.2.6.3 Storage requirements for string types

Column type | Storage required
`CHAR(M)` | `M` bytes, `1 <= M <= 255`
`VARCHAR(M)` | `L`+1 bytes, where `L <= M` and `1 <= M <= 255`
`TINYBLOB`, `TINYTEXT` | `L`+1 bytes, where `L` < 2^8
`BLOB`, `TEXT` | `L`+2 bytes, where `L` < 2^16
`MEDIUMBLOB`, `MEDIUMTEXT` | `L`+3 bytes, where `L` < 2^24
`LONGBLOB`, `LONGTEXT` | `L`+4 bytes, where `L` < 2^32
`ENUM('value1','value2',...)` | 1 or 2 bytes, depending on the number of enumeration values (65535 values maximum)
`SET('value1','value2',...)` | 1, 2, 3, 4 or 8 bytes, depending on the number of set members (64 members maximum)

`VARCHAR` and the `BLOB` and `TEXT` types are variable-length types, for which the storage requirements depend on the actual length of column values (represented by `L` in the preceding table), rather than on the type's maximum possible size. For example, a `VARCHAR(10)` column can hold a string with a maximum length of 10 characters. The actual storage required is the length of the string (`L`), plus 1 byte to record the length of the string. For the string `'abcd'`, `L` is 4 and the storage requirement is 5 bytes.

The `BLOB` and `TEXT` types require 1, 2, 3, or 4 bytes to record the length of the column value, depending on the maximum possible length of the type. See section 6.2.3.2 The `BLOB` and `TEXT` Types.

If a table includes any variable-length column types, the record format will also be variable-length. Note that when a table is created, MySQL may, under certain conditions, change a column from a variable-length type to a fixed-length type, or vice-versa. See section 6.5.3.1 Silent Column Specification Changes.

The size of an `ENUM` object is determined by the number of different enumeration values. One byte is used for enumerations with up to 255 possible values. Two bytes are used for enumerations with up to 65535 values. See section 6.2.3.3 The `ENUM` Type.

The size of a `SET` object is determined by the number of different set members. If the set size is `N`, the object occupies `(N+7)/8` bytes, rounded up to 1, 2, 3, 4, or 8 bytes. A `SET` can have a maximum of 64 members. See section 6.2.3.4 The `SET` Type.

## 6.3 Functions for Use in `SELECT` and `WHERE` Clauses

A `select_expression` or `where_definition` in a SQL statement can consist of any expression using the functions described below.

An expression that contains `NULL` always produces a `NULL` value unless otherwise indicated in the documentation for the operators and functions involved in the expression.

Note: there must be no whitespace between a function name and the parentheses following it. This helps the MySQL parser distinguish between function calls and references to tables or columns that happen to have the same name as a function. Spaces around arguments are permitted, though.

You can force MySQL to accept spaces after the function name by starting `mysqld` with `--ansi` or using the `CLIENT_IGNORE_SPACE` to `mysql_connect()`, but in this case all function names will become reserved words. See section 1.7.2 Running MySQL in ANSI Mode.

For the sake of brevity, examples display the output from the `mysql` program in abbreviated form. So this:

    mysql> SELECT MOD(29,9);
    1 rows in set (0.00 sec)

    +-----------+
    | mod(29,9) |
    +-----------+
    |         2 |
    +-----------+

is displayed like this:

    mysql> SELECT MOD(29,9);
            -> 2

### 6.3.1 Non-Type-Specific Operators and Functions

#### 6.3.1.1 Parentheses

    ( ... )

Use parentheses to force the order of evaluation in an expression. For example:

    mysql> SELECT 1+2*3;
            -> 7
    mysql> SELECT (1+2)*3;
            -> 9

#### 6.3.1.2 Comparison Operators

Comparison operations result in a value of `1` (TRUE), `0` (FALSE), or `NULL`. These functions work for both numbers and strings. Strings are automatically converted to numbers and numbers to strings as needed (as in Perl).

MySQL performs comparisons using the following rules:

- If one or both arguments are `NULL`, the result of the comparison is `NULL`, except for the `<=>` operator.
- If both arguments in a comparison operation are strings, they are compared as strings.
- If both arguments are integers, they are compared as integers.
- Hexadecimal values are treated as binary strings if not compared to a number.
- If one of the arguments is a `TIMESTAMP` or `DATETIME` column and the other argument is a constant, the constant is converted to a timestamp before the comparison is performed. This is done to be more ODBC-friendly.
- In all other cases, the arguments are compared as floating-point (real) numbers.

By default, string comparisons are done in case-independent fashion using the current character set (ISO-8859-1 Latin1 by default, which also works excellently for English).

The following examples illustrate conversion of strings to numbers for comparison operations:

    mysql> SELECT 1 > '6x';
             -> 0
    mysql> SELECT 7 > '6x';
             -> 1
    mysql> SELECT 0 > 'x6';
             -> 0
    mysql> SELECT 0 = 'x6';
             -> 1

`=`
  Equal:

      mysql> SELECT 1 = 0;
              -> 0
      mysql> SELECT '0' = 0;
              -> 1
      mysql> SELECT '0.0' = 0;
              -> 1
      mysql> SELECT '0.01' = 0;
              -> 0
      mysql> SELECT '.01' = 0.01;
              -> 1

`<>`
`!=`
  Not equal:

      mysql> SELECT '.01' <> '0.01';
              -> 1
      mysql> SELECT .01 <> '0.01';
              -> 0
      mysql> SELECT 'zapp' <> 'zappp';
              -> 1

`<=`
  Less than or equal:

      mysql> SELECT 0.1 <= 2;
              -> 1

`<`
  Less than:

      mysql> SELECT 2 < 2;
              -> 0

`>=`
  Greater than or equal:

      mysql> SELECT 2 >= 2;
              -> 1

`>`
  Greater than:

      mysql> SELECT 2 > 2;
              -> 0

`<=>`
  NULL safe equal:

      mysql> SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
              -> 1 1 0

`IS NULL`
`IS NOT NULL`
  Test whether a value is or is not `NULL`:

      mysql> SELECT 1 IS NULL, 0 IS NULL, NULL IS NULL;
              -> 0 0 1
      mysql> SELECT 1 IS NOT NULL, 0 IS NOT NULL, NULL IS NOT NULL;
              -> 1 1 0

  To be able to work good with other programs, MySQL supports the following extra features when using `IS NULL`:
  - You can find the last inserted row with:

        SELECT * FROM tbl_name WHERE auto_col IS NULL

    This can be disabled by setting `SQL_AUTO_IS_NULL=0`. See section 5.5.6 `SET` Syntax.
  - For `NOT NULL` `DATE` and `DATETIME` columns you can find the special date `0000-00-00` by using:

        SELECT * FROM tbl_name WHERE date_column IS NULL

    This is needed to get some ODBC applications to work (as ODBC doesn't support a `0000-00-00` date)
`expr BETWEEN min AND max`
  If `expr` is greater than or equal to `min` and `expr` is less than or equal to `max`, `BETWEEN` returns `1`, otherwise it returns `0`. This is equivalent to the expression `(min <= expr AND expr <= max)` if all the arguments are of the same type. Otherwise type conversion takes place, according to the rules above, but applied to all the three arguments. Note that before 4.0.5 arguments were converted to the type of `expr` instead.

      mysql> SELECT 1 BETWEEN 2 AND 3;
              -> 0
      mysql> SELECT 'b' BETWEEN 'a' AND 'c';
              -> 1
      mysql> SELECT 2 BETWEEN 2 AND '3';
              -> 1
      mysql> SELECT 2 BETWEEN 2 AND 'x-3';
              -> 0

`expr NOT BETWEEN min AND max`
  Same as `NOT (expr BETWEEN min AND max)`.
`expr IN (value,...)`
  Returns `1` if `expr` is any of the values in the `IN` list, else returns `0`. If all values are constants, then all values are evaluated according to the type of `expr` and sorted. The search for the item is then done using a binary search. This means `IN` is very quick if the `IN` value list consists entirely of constants. If `expr` is a case-sensitive string expression, the string comparison is performed in case-sensitive fashion:

      mysql> SELECT 2 IN (0,3,5,'wefwf');
              -> 0
      mysql> SELECT 'wefwf' IN (0,3,5,'wefwf');
              -> 1

`expr NOT IN (value,...)`
  Same as `NOT (expr IN (value,...))`.
`ISNULL(expr)`
  If `expr` is `NULL`, `ISNULL()` returns `1`, otherwise it returns `0`:

      mysql> SELECT ISNULL(1+1);
              -> 0
      mysql> SELECT ISNULL(1/0);
              -> 1

  Note that a comparison of `NULL` values using `=` will always be false!
`COALESCE(list)`
  Returns first non-`NULL` element in list:

      mysql> SELECT COALESCE(NULL,1);
              -> 1
      mysql> SELECT COALESCE(NULL,NULL,NULL);
              -> NULL

`INTERVAL(N,N1,N2,N3,...)`
  Returns `0` if `N` < `N1`, `1` if `N` < `N2` and so on. All arguments are treated as integers. It is required that `N1` < `N2` < `N3` < `...` < `Nn` for this function to work correctly. This is because a binary search is used (very fast):

      mysql> SELECT INTERVAL(23, 1, 15, 17, 30, 44, 200);
              -> 3
      mysql> SELECT INTERVAL(10, 1, 10, 100, 1000);
              -> 2
      mysql> SELECT INTERVAL(22, 23, 30, 44, 200);
              -> 0

If you are comparing case-insensitive strings with any of the standard operators (`=`, `<>`..., but not `LIKE`) trailing whitespace (spaces, tabs and newlines) will be ignored.

    mysql> SELECT "a" ="A \n";
            -> 1

#### 6.3.1.3 Logical Operators

In SQL, all logical operators evaluate to TRUE, FALSE or NULL (UNKNOWN). In MySQL, this is implemented as `1` (TRUE), `0` (FALSE), and `NULL`. Most of this is common between different SQL databases, however some may return any non-zero value for TRUE.

`NOT`
`!`
  Logical NOT. Evaluates to `1` if the operand is `0`, to `0` if the operand is non-zero, and `NOT NULL` returns `NULL`.

      mysql> SELECT NOT 10;
              -> 0
      mysql> SELECT NOT 0;
              -> 1
      mysql> SELECT NOT NULL;
              -> NULL
      mysql> SELECT ! (1+1);
              -> 0
      mysql> SELECT ! 1+1;
              -> 1

  The last example produces `1` because the expression evaluates the same way as `(!1)+1`.
`AND`
`&&`
  Logical AND. Evaluates to `1` if all operands are non-zero and not `NULL`, to `0` if one or more operands are `0`, otherwise `NULL` is returned.

      mysql> SELECT 1 && 1;
              -> 1
      mysql> SELECT 1 && 0;
              -> 0
      mysql> SELECT 1 && NULL;
              -> NULL
      mysql> SELECT 0 && NULL;
              -> 0
      mysql> SELECT NULL && 0;
              -> 0

  Please note that MySQL versions prior to 4.0.5 stop evaluation when a `NULL` is encountered, rather than continuing the process to check for possible `0`s. This means that in these versions, `SELECT (NULL AND 0)` returns `NULL` instead of `0`. In 4.0.5 the code has been re-engineered so that the result will always be as prescribed by ANSI while still using the optimisation wherever possible.
`OR`
`||`
  Logical OR. Evaluates to `1` if any operand is non-zero, to `NULL` if any operand is `NULL`, otherwise `0` is returned.

      mysql> SELECT 1 || 1;
              -> 1
      mysql> SELECT 1 || 0;
              -> 1
      mysql> SELECT 0 || 0;
              -> 0
      mysql> SELECT 0 || NULL;
              -> NULL
      mysql> SELECT 1 || NULL;
              -> 1

`XOR`
  Logical XOR. Returns `NULL` if either operand is `NULL`. For non-`NULL` operands, evaluates to `1` if an odd number of operands is non-zero, otherwise `0` is returned.

      mysql> SELECT 1 XOR 1;
              -> 0
      mysql> SELECT 1 XOR 0;
              -> 1
      mysql> SELECT 1 XOR NULL;
              -> NULL
      mysql> SELECT 1 XOR 1 XOR 1;
              -> 1

  `a XOR b` is mathematically equal to `(a AND (NOT b)) OR ((NOT a) and b)`.

#### 6.3.1.4 Control Flow Functions

`IFNULL(expr1,expr2)`
  If `expr1` is not `NULL`, `IFNULL()` returns `expr1`, else it returns `expr2`. `IFNULL()` returns a numeric or string value, depending on the context in which it is used:

      mysql> SELECT IFNULL(1,0);
              -> 1
      mysql> SELECT IFNULL(NULL,10);
              -> 10
      mysql> SELECT IFNULL(1/0,10);
              -> 10
      mysql> SELECT IFNULL(1/0,'yes');
              -> 'yes'

  In 4.0.6 and above the default result value of `IFNULL(expr1,expr2)` is the more 'general' of the two expressions, in the order `STRING`, `REAL` or `INTEGER`.The difference to earlier MySQL versions are mostly notable when you create a table based on expressions or MySQL has to internally store a value from `IFNULL()` in a temporary table.

      CREATE TABLE foo SELECT IFNULL(1,"test") as test;

  In MySQL 4.0.6 the type for column 'test' is `CHAR(4)` while in earlier versions you would get `BIGINT`.
`NULLIF(expr1,expr2)`
  If `expr1 = expr2` is true, return `NULL` else return `expr1`. This is the same as `CASE WHEN x = y THEN NULL ELSE x END`:

      mysql> SELECT NULLIF(1,1);
              -> NULL
      mysql> SELECT NULLIF(1,2);
              -> 1

  Note that `expr1` is evaluated twice in MySQL if the arguments are equal.
`IF(expr1,expr2,expr3)`
  If `expr1` is TRUE (`expr1 <> 0` and `expr1 <> NULL`) then `IF()` returns `expr2`, else it returns `expr3`. `IF()` returns a numeric or string value, depending on the context in which it is used:

      mysql> SELECT IF(1>2,2,3);
              -> 3
      mysql> SELECT IF(1<2,'yes','no');
              -> 'yes'
      mysql> SELECT IF(STRCMP('test','test1'),'no','yes');
              -> 'no'

  If `expr2` or `expr3` is explicitely `NULL` then the result type of the `IF()` function is the type of the not `NULL` column. (This behaviour is new in MySQL 4.0.3). `expr1` is evaluated as an integer value, which means that if you are testing floating-point or string values, you should do so using a comparison operation:

      mysql> SELECT IF(0.1,1,0);
              -> 0
      mysql> SELECT IF(0.1<>0,1,0);
              -> 1

  In the first case above, `IF(0.1)` returns `0` because `0.1` is converted to an integer value, resulting in a test of `IF(0)`. This may not be what you expect. In the second case, the comparison tests the original floating-point value to see whether it is non-zero. The result of the comparison is used as an integer. The default return type of `IF()` (which may matter when it is stored into a temporary table) is calculated in MySQL Version 3.23 as follows:

  Expression | Return value
  expr2 or expr3 returns string | string
  expr2 or expr3 returns a floating-point value | floating-point
  expr2 or expr3 returns an integer | integer

  If expr2 and expr3 are strings, then the result is case-insensitive if both strings are case-insensitive. (Starting from 3.23.51)
`CASE value WHEN [compare-value] THEN result [WHEN [compare-value] THEN result ...] [ELSE result] END`
`CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END`
  The first version returns the `result` where `value=compare-value`. The second version returns the result for the first condition, which is true. If there was no matching result value, then the result after `ELSE` is returned. If there is no `ELSE` part then `NULL` is returned:

      mysql> SELECT CASE 1 WHEN 1 THEN "one"
                 WHEN 2 THEN "two" ELSE "more" END;
             -> "one"
      mysql> SELECT CASE WHEN 1>0 THEN "true" ELSE "false" END;
             -> "true"
      mysql> SELECT CASE BINARY "B" WHEN "a" THEN 1 WHEN "b" THEN 2 END;
             -> NULL

The type of the return value (`INTEGER`, `DOUBLE` or `STRING`) is the same as the type of the first returned value (the expression after the first `THEN`).

### 6.3.2 String Functions

String-valued functions return `NULL` if the length of the result would be greater than the `max_allowed_packet` server parameter. See section 5.5.2 Tuning Server Parameters.

For functions that operate on string positions, the first position is numbered 1.

`ASCII(str)`
  Returns the ASCII code value of the leftmost character of the string `str`. Returns `0` if `str` is the empty string. Returns `NULL` if `str` is `NULL`:

      mysql> SELECT ASCII('2');
              -> 50
      mysql> SELECT ASCII(2);
              -> 50
      mysql> SELECT ASCII('dx');
              -> 100

  See also the `ORD()` function.
`ORD(str)`
  If the leftmost character of the string `str` is a multi-byte character, returns the code for that character, calculated from the ASCII code values of its constituent characters using this formula: `((first byte ASCII code)*256+(second byte ASCII code))[*256+third byte ASCII code...]`. If the leftmost character is not a multi-byte character, returns the same value that the `ASCII()` function does:

      mysql> SELECT ORD('2');
              -> 50

`CONV(N,from_base,to_base)`
  Converts numbers between different number bases. Returns a string representation of the number `N`, converted from base `from_base` to base `to_base`. Returns `NULL` if any argument is `NULL`. The argument `N` is interpreted as an integer, but may be specified as an integer or a string. The minimum base is `2` and the maximum base is `36`. If `to_base` is a negative number, `N` is regarded as a signed number. Otherwise, `N` is treated as unsigned. `CONV` works with 64-bit precision:

      mysql> SELECT CONV("a",16,2);
              -> '1010'
      mysql> SELECT CONV("6E",18,8);
              -> '172'
      mysql> SELECT CONV(-17,10,-18);
              -> '-H'
      mysql> SELECT CONV(10+"10"+'10'+0xa,10,10);
              -> '40'

`BIN(N)`
  Returns a string representation of the binary value of `N`, where `N` is a longlong (`BIGINT`) number. This is equivalent to `CONV(N,10,2)`. Returns `NULL` if `N` is `NULL`:

      mysql> SELECT BIN(12);
              -> '1100'

`OCT(N)`
  Returns a string representation of the octal value of `N`, where `N` is a longlong number. This is equivalent to `CONV(N,10,8)`. Returns `NULL` if `N` is `NULL`:

      mysql> SELECT OCT(12);
              -> '14'

`HEX(N_or_S)`
  If N_OR_S is a number, returns a string representation of the hexadecimal value of `N`, where `N` is a longlong (`BIGINT`) number. This is equivalent to `CONV(N,10,16)`. If N_OR_S is a string, returns a hexadecimal string of N_OR_S where each character in N_OR_S is converted to 2 hexadecimal digits. This is the invers of the `0xff` strings.

      mysql> SELECT HEX(255);
              -> 'FF'
      mysql> SELECT HEX("abc");
              -> 616263
      mysql> SELECT 0x616263;
              -> "abc"

`CHAR(N,...)`
  `CHAR()` interprets the arguments as integers and returns a string consisting of the characters given by the ASCII code values of those integers. `NULL` values are skipped:

      mysql> SELECT CHAR(77,121,83,81,'76');
              -> 'MySQL'
      mysql> SELECT CHAR(77,77.3,'77.3');
              -> 'MMM'

`CONCAT(str1,str2,...)`
  Returns the string that results from concatenating the arguments. Returns `NULL` if any argument is `NULL`. May have more than 2 arguments. A numeric argument is converted to the equivalent string form:

      mysql> SELECT CONCAT('My', 'S', 'QL');
              -> 'MySQL'
      mysql> SELECT CONCAT('My', NULL, 'QL');
              -> NULL
      mysql> SELECT CONCAT(14.3);
              -> '14.3'

`CONCAT_WS(separator, str1, str2,...)`
  `CONCAT_WS()` stands for CONCAT With Separator and is a special form of `CONCAT()`. The first argument is the separator for the rest of the arguments. The separator can be a string as well as the rest of the arguments. If the separator is `NULL`, the result will be `NULL`. The function will skip any `NULL`s and empty strings, after the separator argument. The separator will be added between the strings to be concatenated:

      mysql> SELECT CONCAT_WS(",","First name","Second name","Last Name");
             -> 'First name,Second name,Last Name'
      mysql> SELECT CONCAT_WS(",","First name",NULL,"Last Name");
             -> 'First name,Last Name'

`LENGTH(str)`
`OCTET_LENGTH(str)`
`CHAR_LENGTH(str)`
`CHARACTER_LENGTH(str)`
  Returns the length of the string `str`:

      mysql> SELECT LENGTH('text');
              -> 4
      mysql> SELECT OCTET_LENGTH('text');
              -> 4

  Note that for `CHAR_LENGTH()` and `CHARACTER_LENGTH()`, multi-byte characters are only counted once.
`BIT_LENGTH(str)`
  Returns the length of the string `str` in bits:

      mysql> SELECT BIT_LENGTH('text');
              -> 32

`LOCATE(substr,str)`
`POSITION(substr IN str)`
  Returns the position of the first occurrence of substring `substr` in string `str`. Returns `0` if `substr` is not in `str`:

      mysql> SELECT LOCATE('bar', 'foobarbar');
              -> 4
      mysql> SELECT LOCATE('xbar', 'foobar');
              -> 0

  This function is multi-byte safe. In MySQL 3.23 this function is case sensitive, while in 4.0 it's only case-sensitive if either argument is a binary string.
`LOCATE(substr,str,pos)`
  Returns the position of the first occurrence of substring `substr` in string `str`, starting at position `pos`. Returns `0` if `substr` is not in `str`:

      mysql> SELECT LOCATE('bar', 'foobarbar',5);
              -> 7

  This function is multi-byte safe. In MySQL 3.23 this function is case sensitive, while in 4.0 it's only case-sensitive if either argument is a binary string.
`INSTR(str,substr)`
  Returns the position of the first occurrence of substring `substr` in string `str`. This is the same as the two-argument form of `LOCATE()`, except that the arguments are swapped:

      mysql> SELECT INSTR('foobarbar', 'bar');
              -> 4
      mysql> SELECT INSTR('xbar', 'foobar');
              -> 0

  This function is multi-byte safe. In MySQL 3.23 this function is case sensitive, while in 4.0 it's only case-sensitive if either argument is a binary string.
`LPAD(str,len,padstr)`
  Returns the string `str`, left-padded with the string `padstr` until `str` is `len` characters long. If `str` is longer than `len'` then it will be shortened to `len` characters.

      mysql> SELECT LPAD('hi',4,'??');
              -> '??hi'

`RPAD(str,len,padstr)`
  Returns the string `str`, right-padded with the string `padstr` until `str` is `len` characters long. If `str` is longer than `len'` then it will be shortened to `len` characters.

      mysql> SELECT RPAD('hi',5,'?');
              -> 'hi???'

`LEFT(str,len)`
  Returns the leftmost `len` characters from the string `str`:

      mysql> SELECT LEFT('foobarbar', 5);
              -> 'fooba'

  This function is multi-byte safe.
`RIGHT(str,len)`
  Returns the rightmost `len` characters from the string `str`:

      mysql> SELECT RIGHT('foobarbar', 4);
              -> 'rbar'

  This function is multi-byte safe.
`SUBSTRING(str,pos,len)`
`SUBSTRING(str FROM pos FOR len)`
`MID(str,pos,len)`
  Returns a substring `len` characters long from string `str`, starting at position `pos`. The variant form that uses `FROM` is ANSI SQL92 syntax:

      mysql> SELECT SUBSTRING('Quadratically',5,6);
              -> 'ratica'

  This function is multi-byte safe.
`SUBSTRING(str,pos)`
`SUBSTRING(str FROM pos)`
  Returns a substring from string `str` starting at position `pos`:

      mysql> SELECT SUBSTRING('Quadratically',5);
              -> 'ratically'
      mysql> SELECT SUBSTRING('foobarbar' FROM 4);
              -> 'barbar'

  This function is multi-byte safe.
`SUBSTRING_INDEX(str,delim,count)`
  Returns the substring from string `str` before `count` occurrences of the delimiter `delim`. If `count` is positive, everything to the left of the final delimiter (counting from the left) is returned. If `count` is negative, everything to the right of the final delimiter (counting from the right) is returned:

      mysql> SELECT SUBSTRING_INDEX('www.mysql.com', '.', 2);
              -> 'www.mysql'
      mysql> SELECT SUBSTRING_INDEX('www.mysql.com', '.', -2);
              -> 'mysql.com'

  This function is multi-byte safe.
`LTRIM(str)`
  Returns the string `str` with leading space characters removed:

      mysql> SELECT LTRIM('  barbar');
              -> 'barbar'

`RTRIM(str)`
  Returns the string `str` with trailing space characters removed:

      mysql> SELECT RTRIM('barbar   ');
              -> 'barbar'

  This function is multi-byte safe.
`TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)`
  Returns the string `str` with all `remstr` prefixes and/or suffixes removed. If none of the specifiers `BOTH`, `LEADING` or `TRAILING` are given, `BOTH` is assumed. If `remstr` is not specified, spaces are removed:

      mysql> SELECT TRIM('  bar   ');
              -> 'bar'
      mysql> SELECT TRIM(LEADING 'x' FROM 'xxxbarxxx');
              -> 'barxxx'
      mysql> SELECT TRIM(BOTH 'x' FROM 'xxxbarxxx');
              -> 'bar'
      mysql> SELECT TRIM(TRAILING 'xyz' FROM 'barxxyz');
              -> 'barx'

  This function is multi-byte safe.
`SOUNDEX(str)`
  Returns a soundex string from `str`. Two strings that sound almost the same should have identical soundex strings. A standard soundex string is 4 characters long, but the `SOUNDEX()` function returns an arbitrarily long string. You can use `SUBSTRING()` on the result to get a standard soundex string. All non-alphanumeric characters are ignored in the given string. All international alpha characters outside the A-Z range are treated as vowels:

      mysql> SELECT SOUNDEX('Hello');
              -> 'H400'
      mysql> SELECT SOUNDEX('Quadratically');
              -> 'Q36324'

`SPACE(N)`
  Returns a string consisting of `N` space characters:

      mysql> SELECT SPACE(6);
              -> '      '

`REPLACE(str,from_str,to_str)`
  Returns the string `str` with all all occurrences of the string `from_str` replaced by the string `to_str`:

      mysql> SELECT REPLACE('www.mysql.com', 'w', 'Ww');
              -> 'WwWwWw.mysql.com'

  This function is multi-byte safe.
`REPEAT(str,count)`
  Returns a string consisting of the string `str` repeated `count` times. If `count <= 0`, returns an empty string. Returns `NULL` if `str` or `count` are `NULL`:

      mysql> SELECT REPEAT('MySQL', 3);
              -> 'MySQLMySQLMySQL'

`REVERSE(str)`
  Returns the string `str` with the order of the characters reversed:

      mysql> SELECT REVERSE('abc');
              -> 'cba'

  This function is multi-byte safe.
`INSERT(str,pos,len,newstr)`
  Returns the string `str`, with the substring beginning at position `pos` and `len` characters long replaced by the string `newstr`:

      mysql> SELECT INSERT('Quadratic', 3, 4, 'What');
              -> 'QuWhattic'

  This function is multi-byte safe.
`ELT(N,str1,str2,str3,...)`
  Returns `str1` if `N` = `1`, `str2` if `N` = `2`, and so on. Returns `NULL` if `N` is less than `1` or greater than the number of arguments. `ELT()` is the complement of `FIELD()`:

      mysql> SELECT ELT(1, 'ej', 'Heja', 'hej', 'foo');
              -> 'ej'
      mysql> SELECT ELT(4, 'ej', 'Heja', 'hej', 'foo');
              -> 'foo'

`FIELD(str,str1,str2,str3,...)`
  Returns the index of `str` in the `str1`, `str2`, `str3`, `...` list. Returns `0` if `str` is not found. `FIELD()` is the complement of `ELT()`:

      mysql> SELECT FIELD('ej', 'Hej', 'ej', 'Heja', 'hej', 'foo');
              -> 2
      mysql> SELECT FIELD('fo', 'Hej', 'ej', 'Heja', 'hej', 'foo');
              -> 0

`FIND_IN_SET(str,strlist)`
  Returns a value `1` to `N` if the string `str` is in the list `strlist` consisting of `N` substrings. A string list is a string composed of substrings separated by ``,'` characters. If the first argument is a constant string and the second is a column of type `SET`, the `FIND_IN_SET()` function is optimised to use bit arithmetic! Returns `0` if `str` is not in `strlist` or if `strlist` is the empty string. Returns `NULL` if either argument is `NULL`. This function will not work properly if the first argument contains a ``,'`:

      mysql> SELECT FIND_IN_SET('b','a,b,c,d');
              -> 2

`MAKE_SET(bits,str1,str2,...)`
  Returns a set (a string containing substrings separated by ``,'` characters) consisting of the strings that have the corresponding bit in `bits` set. `str1` corresponds to bit 0, `str2` to bit 1, etc. `NULL` strings in `str1`, `str2`, `...` are not appended to the result:

      mysql> SELECT MAKE_SET(1,'a','b','c');
              -> 'a'
      mysql> SELECT MAKE_SET(1 | 4,'hello','nice','world');
              -> 'hello,world'
      mysql> SELECT MAKE_SET(0,'a','b','c');
              -> ''

`EXPORT_SET(bits,on,off,[separator,[number_of_bits]])`
  Returns a string where for every bit set in 'bit', you get an 'on' string and for every reset bit you get an 'off' string. Each string is separated with 'separator' (default ',') and only 'number_of_bits' (default 64) of 'bits' is used:

      mysql> SELECT EXPORT_SET(5,'Y','N',',',4)
              -> Y,N,Y,N

`LCASE(str)`
`LOWER(str)`
  Returns the string `str` with all characters changed to lowercase according to the current character set mapping (the default is ISO-8859-1 Latin1):

      mysql> SELECT LCASE('QUADRATICALLY');
              -> 'quadratically'

  This function is multi-byte safe.
`UCASE(str)`
`UPPER(str)`
  Returns the string `str` with all characters changed to uppercase according to the current character set mapping (the default is ISO-8859-1 Latin1):

      mysql> SELECT UCASE('Hej');
              -> 'HEJ'

  This function is multi-byte safe.
`LOAD_FILE(file_name)`
  Reads the file and returns the file contents as a string. The file must be on the server, you must specify the full pathname to the file, and you must have the `FILE` privilege. The file must be readable by all and be smaller than `max_allowed_packet`. If the file doesn't exist or can't be read due to one of the above reasons, the function returns `NULL`:

      mysql> UPDATE tbl_name
                 SET blob_column=LOAD_FILE("/tmp/picture")
                 WHERE id=1;

  If you are not using MySQL Version 3.23, you have to do the reading of the file inside your application and create an `INSERT` statement to update the database with the file information. One way to do this, if you are using the MySQL++ library, can be found at http://www.mysql.com/documentation/mysql++/mysql++-examples.html.
`QUOTE(str)`
  Quotes a string to produce a result that can be used as a properly-escaped data value in a SQL statement. The string is returned surrounded by single quotes and with each instance of single quote (``''`), backslash (``\'`), ASCII NUL, and Control-Z preceded by a backslash. If the argument is `NULL`, the return value is the word ``NULL'' without surrounding single quotes.

      mysql> SELECT QUOTE("Don't");
              -> 'Don\'t!'
      mysql> SELECT QUOTE(NULL);
              -> NULL

MySQL automatically converts numbers to strings as necessary, and vice-versa:

    mysql> SELECT 1+"1";
            -> 2
    mysql> SELECT CONCAT(2,' test');
            -> '2 test'

If you want to convert a number to a string explicitly, pass it as the argument to `CONCAT()`.

If a string function is given a binary string as an argument, the resulting string is also a binary string. A number converted to a string is treated as a binary string. This only affects comparisons.

#### 6.3.2.1 String Comparison Functions

Normally, if any expression in a string comparison is case-sensitive, the comparison is performed in case-sensitive fashion.

`expr LIKE pat [ESCAPE 'escape-char']`
  Pattern matching using SQL simple regular expression comparison. Returns `1` (TRUE) or `0` (FALSE). With `LIKE` you can use the following two wildcard characters in the pattern:

  Char | Description
  `%` | Matches any number of characters, even zero characters
  `_` | Matches exactly one character

      mysql> SELECT 'David!' LIKE 'David_';
              -> 1
      mysql> SELECT 'David!' LIKE '%D%v%';
              -> 1

  To test for literal instances of a wildcard character, precede the character with the escape character. If you don't specify the `ESCAPE` character, ``\'` is assumed:

  String | Description
  `\%` | Matches one `%` character
  `\_` | Matches one `_` character

      mysql> SELECT 'David!' LIKE 'David\_';
              -> 0
      mysql> SELECT 'David_' LIKE 'David\_';
              -> 1

  To specify a different escape character, use the `ESCAPE` clause:

      mysql> SELECT 'David_' LIKE 'David|_' ESCAPE '|';
              -> 1

  The following two statements illustrate that string comparisons are case-insensitive unless one of the operands is a binary string:

      mysql> SELECT 'abc' LIKE 'ABC';
              -> 1
      mysql> SELECT 'abc' LIKE BINARY 'ABC';
              -> 0

  `LIKE` is allowed on numeric expressions! (This is a MySQL extension to the ANSI SQL `LIKE`.)

      mysql> SELECT 10 LIKE '1%';
              -> 1

  Note: Because MySQL uses the C escape syntax in strings (for example, ``\n'`), you must double any ``\'` that you use in your `LIKE` strings. For example, to search for ``\n'`, specify it as ``\\n'`. To search for ``\'`, specify it as ``\\\\'` (the backslashes are stripped once by the parser and another time when the pattern match is done, leaving a single backslash to be matched).
`expr NOT LIKE pat [ESCAPE 'escape-char']`
  Same as `NOT (expr LIKE pat [ESCAPE 'escape-char'])`.
`expr SOUNDS LIKE expr`
  Same as `SOUNDEX(expr)=SOUNDEX(expr)` (available only in version 4.1 or later).
`expr REGEXP pat`
`expr RLIKE pat`
  Performs a pattern match of a string expression `expr` against a pattern `pat`. The pattern can be an extended regular expression. See section G MySQL Regular Expressions. Returns `1` if `expr` matches `pat`, otherwise returns `0`. `RLIKE` is a synonym for `REGEXP`, provided for `mSQL` compatibility. Note: Because MySQL uses the C escape syntax in strings (for example, ``\n'`), you must double any ``\'` that you use in your `REGEXP` strings. As of MySQL Version 3.23.4, `REGEXP` is case-insensitive for normal (not binary) strings:

      mysql> SELECT 'Monty!' REGEXP 'm%y%%';
              -> 0
      mysql> SELECT 'Monty!' REGEXP '.*';
              -> 1
      mysql> SELECT 'new*\n*line' REGEXP 'new\\*.\\*line';
              -> 1
      mysql> SELECT "a" REGEXP "A", "a" REGEXP BINARY "A";
              -> 1  0
      mysql> SELECT "a" REGEXP "^[a-d]";
              -> 1

  `REGEXP` and `RLIKE` use the current character set (ISO-8859-1 Latin1 by default) when deciding the type of a character.
`expr NOT REGEXP pat`
`expr NOT RLIKE pat`
  Same as `NOT (expr REGEXP pat)`.
`STRCMP(expr1,expr2)`
  `STRCMP()` returns `0` if the strings are the same, `-1` if the first argument is smaller than the second according to the current sort order, and `1` otherwise:

      mysql> SELECT STRCMP('text', 'text2');
              -> -1
      mysql> SELECT STRCMP('text2', 'text');
              -> 1
      mysql> SELECT STRCMP('text', 'text');
              -> 0

`MATCH (col1,col2,...) AGAINST (expr)`
`MATCH (col1,col2,...) AGAINST (expr IN BOOLEAN MODE)`
  `MATCH ... AGAINST()` is used for full-text search and returns relevance - similarity measure between the text in columns `(col1,col2,...)` and the query `expr`. Relevance is a positive floating-point number. Zero relevance means no similarity. `MATCH ... AGAINST()` is available in MySQL version 3.23.23 or later. `IN BOOLEAN MODE` extension was added in version 4.0.1. For details and usage examples, see section 6.8 MySQL Full-text Search.

#### 6.3.2.2 Case-Sensitivity

``BINARY``
  The `BINARY` operator casts the string following it to a binary string. This is an easy way to force a column comparison to be case-sensitive even if the column isn't defined as `BINARY` or `BLOB`:

      mysql> SELECT "a" = "A";
              -> 1
      mysql> SELECT BINARY "a" = "A";
              -> 0

  `BINARY string` is a shorthand for `CAST(string AS BINARY)`. See section 6.3.5 Cast Functions. `BINARY` was introduced in MySQL Version 3.23.0. Note that in some context MySQL will not be able to use the index efficiently when you cast an indexed column to `BINARY`.

If you want to compare a blob case-insensitively you can always convert the blob to upper case before doing the comparison:

    SELECT 'A' LIKE UPPER(blob_col) FROM table_name;

We plan to soon introduce casting between different character sets to make string comparison even more flexible.

### 6.3.3 Numeric Functions

#### 6.3.3.1 Arithmetic Operations

The usual arithmetic operators are available. Note that in the case of ``-'`, ``+'`, and ``*'`, the result is calculated with `BIGINT` (64-bit) precision if both arguments are integers! If one of the argument is an unsigned integer, and the other argument is also an integer, the result will be an unsigned integer. See section 6.3.5 Cast Functions.

`+`
  Addition:

      mysql> SELECT 3+5;
              -> 8

`-`
  Subtraction:

      mysql> SELECT 3-5;
              -> -2

`*`
  Multiplication:

      mysql> SELECT 3*5;
              -> 15
      mysql> SELECT 18014398509481984*18014398509481984.0;
              -> 324518553658426726783156020576256.0
      mysql> SELECT 18014398509481984*18014398509481984;
              -> 0

  The result of the last expression is incorrect because the result of the integer multiplication exceeds the 64-bit range of `BIGINT` calculations.
`/`
  Division:

      mysql> SELECT 3/5;
              -> 0.60

  Division by zero produces a `NULL` result:

      mysql> SELECT 102/(1-1);
              -> NULL

  A division will be calculated with `BIGINT` arithmetic only if performed in a context where its result is converted to an integer!

#### 6.3.3.2 Mathematical Functions

All mathematical functions return `NULL` in case of an error.

`-`
  Unary minus. Changes the sign of the argument:

      mysql> SELECT - 2;
              -> -2

  Note that if this operator is used with a `BIGINT`, the return value is a `BIGINT`! This means that you should avoid using `-` on integers that may have the value of `-2^63`!
`ABS(X)`
  Returns the absolute value of `X`:

      mysql> SELECT ABS(2);
              -> 2
      mysql> SELECT ABS(-32);
              -> 32

  This function is safe to use with `BIGINT` values.
`SIGN(X)`
  Returns the sign of the argument as `-1`, `0`, or `1`, depending on whether `X` is negative, zero, or positive:

      mysql> SELECT SIGN(-32);
              -> -1
      mysql> SELECT SIGN(0);
              -> 0
      mysql> SELECT SIGN(234);
              -> 1

`MOD(N,M)`
`%`
  Modulo (like the `%` operator in C). Returns the remainder of `N` divided by `M`:

      mysql> SELECT MOD(234, 10);
              -> 4
      mysql> SELECT 253 % 7;
              -> 1
      mysql> SELECT MOD(29,9);
              -> 2

  This function is safe to use with `BIGINT` values.
`FLOOR(X)`
  Returns the largest integer value not greater than `X`:

      mysql> SELECT FLOOR(1.23);
              -> 1
      mysql> SELECT FLOOR(-1.23);
              -> -2

  Note that the return value is converted to a `BIGINT`!
`CEILING(X)`
  Returns the smallest integer value not less than `X`:

      mysql> SELECT CEILING(1.23);
              -> 2
      mysql> SELECT CEILING(-1.23);
              -> -1

  Note that the return value is converted to a `BIGINT`!
`ROUND(X)`
  Returns the argument `X`, rounded to the nearest integer:

      mysql> SELECT ROUND(-1.23);
              -> -1
      mysql> SELECT ROUND(-1.58);
              -> -2
      mysql> SELECT ROUND(1.58);
              -> 2

  Note that the behaviour of `ROUND()` when the argument is half way between two integers depends on the C library implementation. Some round to the nearest even number, always up, always down, or always toward zero. If you need one kind of rounding, you should use a well-defined function like `TRUNCATE()` or `FLOOR()` instead.
`ROUND(X,D)`
  Returns the argument `X`, rounded to a number with `D` decimals. If `D` is `0`, the result will have no decimal point or fractional part:

      mysql> SELECT ROUND(1.298, 1);
              -> 1.3
      mysql> SELECT ROUND(1.298, 0);
              -> 1

`EXP(X)`
  Returns the value of `e` (the base of natural logarithms) raised to the power of `X`:

      mysql> SELECT EXP(2);
              -> 7.389056
      mysql> SELECT EXP(-2);
              -> 0.135335

`LN(X)`
  Returns the natural logarithm of `X`:

      mysql> SELECT LN(2);
              -> 0.693147
      mysql> SELECT LN(-2);
              -> NULL

  This function was added in MySQL version 4.0.3. It is synonymous with `LOG(X)` in MySQL.
`LOG(X)`
`LOG(B,X)`
  If called with one parameter, this function returns the natural logarithm of `X`:

      mysql> SELECT LOG(2);
              -> 0.693147
      mysql> SELECT LOG(-2);
              -> NULL

  If called with two parameters, this function returns the logarithm of `X` for an arbitary base `B`:

      mysql> SELECT LOG(2,65536);
              -> 16.000000
      mysql> SELECT LOG(1,100);
              -> NULL

  The arbitrary base option was added in MySQL version 4.0.3. `LOG(B,X)` is equivalent to `LOG(X)/LOG(B)`.
`LOG2(X)`
  Returns the base-2 logarithm of `X`:

      mysql> SELECT LOG2(65536);
              -> 16.000000
      mysql> SELECT LOG2(-100);
              -> NULL

  `LOG2()` is useful for finding out how many bits a number would require for storage. This function was added in MySQL version 4.0.3. In earlier versions, you can use `LOG(X)/LOG(2)` instead.
`LOG10(X)`
  Returns the base-10 logarithm of `X`:

      mysql> SELECT LOG10(2);
              -> 0.301030
      mysql> SELECT LOG10(100);
              -> 2.000000
      mysql> SELECT LOG10(-100);
              -> NULL

`POW(X,Y)`
`POWER(X,Y)`
  Returns the value of `X` raised to the power of `Y`:

      mysql> SELECT POW(2,2);
              -> 4.000000
      mysql> SELECT POW(2,-2);
              -> 0.250000

`SQRT(X)`
  Returns the non-negative square root of `X`:

      mysql> SELECT SQRT(4);
              -> 2.000000
      mysql> SELECT SQRT(20);
              -> 4.472136

`PI()`
  Returns the value of PI. The default shown number of decimals is 5, but MySQL internally uses the full double precession for PI.

      mysql> SELECT PI();
              -> 3.141593
      mysql> SELECT PI()+0.000000000000000000;
              -> 3.141592653589793116

`COS(X)`
  Returns the cosine of `X`, where `X` is given in radians:

      mysql> SELECT COS(PI());
              -> -1.000000

`SIN(X)`
  Returns the sine of `X`, where `X` is given in radians:

      mysql> SELECT SIN(PI());
              -> 0.000000

`TAN(X)`
  Returns the tangent of `X`, where `X` is given in radians:

      mysql> SELECT TAN(PI()+1);
              -> 1.557408

`ACOS(X)`
  Returns the arc cosine of `X`, that is, the value whose cosine is `X`. Returns `NULL` if `X` is not in the range `-1` to `1`:

      mysql> SELECT ACOS(1);
              -> 0.000000
      mysql> SELECT ACOS(1.0001);
              -> NULL
      mysql> SELECT ACOS(0);
              -> 1.570796

`ASIN(X)`
  Returns the arc sine of `X`, that is, the value whose sine is `X`. Returns `NULL` if `X` is not in the range `-1` to `1`:

      mysql> SELECT ASIN(0.2);
              -> 0.201358
      mysql> SELECT ASIN('foo');
              -> 0.000000

`ATAN(X)`
  Returns the arc tangent of `X`, that is, the value whose tangent is `X`:

      mysql> SELECT ATAN(2);
              -> 1.107149
      mysql> SELECT ATAN(-2);
              -> -1.107149

`ATAN(Y,X)`
`ATAN2(Y,X)`
  Returns the arc tangent of the two variables `X` and `Y`. It is similar to calculating the arc tangent of `Y / X`, except that the signs of both arguments are used to determine the quadrant of the result:

      mysql> SELECT ATAN(-2,2);
              -> -0.785398
      mysql> SELECT ATAN2(PI(),0);
              -> 1.570796

`COT(X)`
  Returns the cotangent of `X`:

      mysql> SELECT COT(12);
              -> -1.57267341
      mysql> SELECT COT(0);
              -> NULL

`RAND()`
`RAND(N)`
  Returns a random floating-point value in the range `0` to `1.0`. If an integer argument `N` is specified, it is used as the seed value:

      mysql> SELECT RAND();
              -> 0.9233482386203
      mysql> SELECT RAND(20);
              -> 0.15888261251047
      mysql> SELECT RAND(20);
              -> 0.15888261251047
      mysql> SELECT RAND();
              -> 0.63553050033332
      mysql> SELECT RAND();
              -> 0.70100469486881

  You can't use a column with `RAND()` values in an `ORDER BY` clause, because `ORDER BY` would evaluate the column multiple times. In MySQL Version 3.23, you can, however, do: `SELECT * FROM table_name ORDER BY RAND()` This is useful to get a random sample of a set `SELECT * FROM table1,table2 WHERE a=b AND c<d ORDER BY RAND() LIMIT 1000`. Note that a `RAND()` in a `WHERE` clause will be re-evaluated every time the `WHERE` is executed. `RAND()` is not meant to be a perfect random generator, but instead a fast way to generate ad hoc random numbers that will be portable between platforms for the same MySQL version.
`LEAST(X,Y,...)`
  With two or more arguments, returns the smallest (minimum-valued) argument. The arguments are compared using the following rules:
  - If the return value is used in an `INTEGER` context, or all arguments are integer-valued, they are compared as integers.
  - If the return value is used in a `REAL` context, or all arguments are real-valued, they are compared as reals.
  - If any argument is a case-sensitive string, the arguments are compared as case-sensitive strings.
  - In other cases, the arguments are compared as case-insensitive strings:

      mysql> SELECT LEAST(2,0);
              -> 0
      mysql> SELECT LEAST(34.0,3.0,5.0,767.0);
              -> 3.0
      mysql> SELECT LEAST("B","A","C");
              -> "A"

  In MySQL versions prior to Version 3.22.5, you can use `MIN()` instead of `LEAST`.
`GREATEST(X,Y,...)`
  Returns the largest (maximum-valued) argument. The arguments are compared using the same rules as for `LEAST`:

      mysql> SELECT GREATEST(2,0);
              -> 2
      mysql> SELECT GREATEST(34.0,3.0,5.0,767.0);
              -> 767.0
      mysql> SELECT GREATEST("B","A","C");
              -> "C"

  In MySQL versions prior to Version 3.22.5, you can use `MAX()` instead of `GREATEST`.
`DEGREES(X)`
  Returns the argument `X`, converted from radians to degrees:

      mysql> SELECT DEGREES(PI());
              -> 180.000000

`RADIANS(X)`
  Returns the argument `X`, converted from degrees to radians:

      mysql> SELECT RADIANS(90);
              -> 1.570796

`TRUNCATE(X,D)`
  Returns the number `X`, truncated to `D` decimals. If `D` is `0`, the result will have no decimal point or fractional part:

      mysql> SELECT TRUNCATE(1.223,1);
              -> 1.2
      mysql> SELECT TRUNCATE(1.999,1);
              -> 1.9
      mysql> SELECT TRUNCATE(1.999,0);
              -> 1
      mysql> SELECT TRUNCATE(-1.999,1);
              -> -1.9

  Starting from MySQL 3.23.51 all numbers are rounded towards zero. If `D` is negative, then the whole part of the number is zeroed out:

      mysql> SELECT TRUNCATE(122,-2);
             -> 100

  Note that as decimal numbers are normally not stored as exact numbers in computers, but as double values, you may be fooled by the following result:

      mysql> SELECT TRUNCATE(10.28*100,0);
             -> 1027

  The above happens because 10.28 is actually stored as something like 10.2799999999999999.

### 6.3.4 Date and Time Functions

See section 6.2.2 Date and Time Types for a description of the range of values each type has and the valid formats in which date and time values may be specified.

Here is an example that uses date functions. The following query selects all records with a `date_col` value from within the last 30 days:

    mysql> SELECT something FROM tbl_name
               WHERE TO_DAYS(NOW()) - TO_DAYS(date_col) <= 30;

`DAYOFWEEK(date)`
  Returns the weekday index for `date` (`1` = Sunday, `2` = Monday, ... `7` = Saturday). These index values correspond to the ODBC standard.

      mysql> SELECT DAYOFWEEK('1998-02-03');
              -> 3

`WEEKDAY(date)`
  Returns the weekday index for `date` (`0` = Monday, `1` = Tuesday, ... `6` = Sunday):

      mysql> SELECT WEEKDAY('1998-02-03 22:23:00');
              -> 1
      mysql> SELECT WEEKDAY('1997-11-05');
              -> 2

`DAYOFMONTH(date)`
  Returns the day of the month for `date`, in the range `1` to `31`:

      mysql> SELECT DAYOFMONTH('1998-02-03');
              -> 3

`DAYOFYEAR(date)`
  Returns the day of the year for `date`, in the range `1` to `366`:

      mysql> SELECT DAYOFYEAR('1998-02-03');
              -> 34

`MONTH(date)`
  Returns the month for `date`, in the range `1` to `12`:

      mysql> SELECT MONTH('1998-02-03');
              -> 2

`DAYNAME(date)`
  Returns the name of the weekday for `date`:

      mysql> SELECT DAYNAME("1998-02-05");
              -> 'Thursday'

`MONTHNAME(date)`
  Returns the name of the month for `date`:

      mysql> SELECT MONTHNAME("1998-02-05");
              -> 'February'

`QUARTER(date)`
  Returns the quarter of the year for `date`, in the range `1` to `4`:

      mysql> SELECT QUARTER('98-04-01');
              -> 2

`WEEK(date)`
`WEEK(date,first)`
  With a single argument, returns the week for `date`, in the range `0` to `53` (yes, there may be the beginnings of a week 53), for locations where Sunday is the first day of the week. The two-argument form of `WEEK()` allows you to specify whether the week starts on Sunday or Monday and if you the return value should be in range `0-53` or `1-52`. Here is a table for how the second arguments work:

  Value | `Meaning`
  0 | Week starts on Sunday and return value is in range 0-53
  1 | Week starts on Monday and return value is in range 0-53
  2 | Week starts on Sunday and return value is in range 1-53
  3 | Week starts on Monday and return value is in range 1-53 (ISO 8601)

      mysql> SELECT WEEK('1998-02-20');
              -> 7
      mysql> SELECT WEEK('1998-02-20',0);
              -> 7
      mysql> SELECT WEEK('1998-02-20',1);
              -> 8
      mysql> SELECT WEEK('1998-12-31',1);
              -> 53

  Note: in Version 4.0, `WEEK(#,0)` was changed to match the calendar in the USA. Note that if a week is the last week of the previous year, MySQL will return 0 if you don't use 2 or 3 as the optional argument:

      mysql> SELECT YEAR('2000-01-01'), WEEK('2000-01-01',0);
              -> 2000, 0
      mysql> SELECT WEEK('2000-01-01',2);
              -> 52

  One could argue that MySQL should return `52` for the `WEEK()` function as the given date is actually the 52 second week of 1999. We decided to return 0 instead as we want the function to return 'the week number in the given year'. This makes the usage of the `WEEK()` function reliable when combined with other functions that extracts a date part from a date. If you would prefer to know the correct year-week, then you should use the 2 or 3 as the optional argument or use the `YEARWEEK()` function:

      mysql> SELECT YEARWEEK('2000-01-01');
              -> 199952
      mysql> SELECT MID(YEARWEEK('2000-01-01'),5,2);
              -> 52

`YEAR(date)`
  Returns the year for `date`, in the range `1000` to `9999`:

      mysql> SELECT YEAR('98-02-03');
              -> 1998

`YEARWEEK(date)`
`YEARWEEK(date,first)`
  Returns year and week for a date. The second arguments works exactly like the second argument to `WEEK()`. Note that the year may be different from the year in the date argument for the first and the last week of the year:

      mysql> SELECT YEARWEEK('1987-01-01');
              -> 198653

  Note that the week number is different from what the `WEEK()` function would return (`0`) for optional arguments 0 or 1, as `WEEK()` then returns the week in the context of the given year.
`HOUR(time)`
  Returns the hour for `time`, in the range `0` to `23`:

      mysql> SELECT HOUR('10:05:03');
              -> 10

`MINUTE(time)`
  Returns the minute for `time`, in the range `0` to `59`:

      mysql> SELECT MINUTE('98-02-03 10:05:03');
              -> 5

`SECOND(time)`
  Returns the second for `time`, in the range `0` to `59`:

      mysql> SELECT SECOND('10:05:03');
              -> 3

`PERIOD_ADD(P,N)`
  Adds `N` months to period `P` (in the format `YYMM` or `YYYYMM`). Returns a value in the format `YYYYMM`. Note that the period argument `P` is not a date value:

      mysql> SELECT PERIOD_ADD(9801,2);
              -> 199803

`PERIOD_DIFF(P1,P2)`
  Returns the number of months between periods `P1` and `P2`. `P1` and `P2` should be in the format `YYMM` or `YYYYMM`. Note that the period arguments `P1` and `P2` are not date values:

      mysql> SELECT PERIOD_DIFF(9802,199703);
              -> 11

`DATE_ADD(date,INTERVAL expr type)`
`DATE_SUB(date,INTERVAL expr type)`
`ADDDATE(date,INTERVAL expr type)`
`SUBDATE(date,INTERVAL expr type)`
  These functions perform date arithmetic. They are new for MySQL Version 3.22. `ADDDATE()` and `SUBDATE()` are synonyms for `DATE_ADD()` and `DATE_SUB()`. In MySQL Version 3.23, you can use `+` and `-` instead of `DATE_ADD()` and `DATE_SUB()` if the expression on the right side is a date or datetime column. (See example below.) `date` is a `DATETIME` or `DATE` value specifying the starting date. `expr` is an expression specifying the interval value to be added or subtracted from the starting date. `expr` is a string; it may start with a ``-'` for negative intervals. `type` is a keyword indicating how the expression should be interpreted. The related function `EXTRACT(type FROM date)` returns the 'type' interval from the date. The following table shows how the `type` and `expr` arguments are related:

  `type` value | Expected `expr` format
  `SECOND` | `SECONDS`
  `MINUTE` | `MINUTES`
  `HOUR` | `HOURS`
  `DAY` | `DAYS`
  `MONTH` | `MONTHS`
  `YEAR` | `YEARS`
  `MINUTE_SECOND` | `"MINUTES:SECONDS"`
  `HOUR_MINUTE` | `"HOURS:MINUTES"`
  `DAY_HOUR` | `"DAYS HOURS"`
  `YEAR_MONTH` | `"YEARS-MONTHS"`
  `HOUR_SECOND` | `"HOURS:MINUTES:SECONDS"`
  `DAY_MINUTE` | `"DAYS HOURS:MINUTES"`
  `DAY_SECOND` | `"DAYS HOURS:MINUTES:SECONDS"`

  MySQL allows any punctuation delimiter in the `expr` format. Those shown in the table are the suggested delimiters. If the `date` argument is a `DATE` value and your calculations involve only `YEAR`, `MONTH`, and `DAY` parts (that is, no time parts), the result is a `DATE` value. Otherwise, the result is a `DATETIME` value:

      mysql> SELECT "1997-12-31 23:59:59" + INTERVAL 1 SECOND;
              -> 1998-01-01 00:00:00
      mysql> SELECT INTERVAL 1 DAY + "1997-12-31";
              -> 1998-01-01
      mysql> SELECT "1998-01-01" - INTERVAL 1 SECOND;
             -> 1997-12-31 23:59:59
      mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
          ->                 INTERVAL 1 SECOND);
              -> 1998-01-01 00:00:00
      mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
          ->                 INTERVAL 1 DAY);
              -> 1998-01-01 23:59:59
      mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
          ->                 INTERVAL "1:1" MINUTE_SECOND);
              -> 1998-01-01 00:01:00
      mysql> SELECT DATE_SUB("1998-01-01 00:00:00",
          ->                 INTERVAL "1 1:1:1" DAY_SECOND);
              -> 1997-12-30 22:58:59
      mysql> SELECT DATE_ADD("1998-01-01 00:00:00",
          ->                 INTERVAL "-1 10" DAY_HOUR);
              -> 1997-12-30 14:00:00
      mysql> SELECT DATE_SUB("1998-01-02", INTERVAL 31 DAY);
              -> 1997-12-02

  If you specify an interval value that is too short (does not include all the interval parts that would be expected from the `type` keyword), MySQL assumes you have left out the leftmost parts of the interval value. For example, if you specify a `type` of `DAY_SECOND`, the value of `expr` is expected to have days, hours, minutes, and seconds parts. If you specify a value like `"1:10"`, MySQL assumes that the days and hours parts are missing and the value represents minutes and seconds. In other words, `"1:10" DAY_SECOND` is interpreted in such a way that it is equivalent to `"1:10" MINUTE_SECOND`. This is analogous to the way that MySQL interprets `TIME` values as representing elapsed time rather than as time of day. Note that if you add or subtract a date value against something that contains a time part, the date value will be automatically converted to a datetime value:

      mysql> SELECT DATE_ADD("1999-01-01", INTERVAL 1 DAY);
             -> 1999-01-02
      mysql> SELECT DATE_ADD("1999-01-01", INTERVAL 1 HOUR);
             -> 1999-01-01 01:00:00

  If you use really incorrect dates, the result is `NULL`. If you add `MONTH`, `YEAR_MONTH`, or `YEAR` and the resulting date has a day that is larger than the maximum day for the new month, the day is adjusted to the maximum days in the new month:

      mysql> SELECT DATE_ADD('1998-01-30', INTERVAL 1 MONTH);
              -> 1998-02-28

  Note from the preceding example that the word `INTERVAL` and the `type` keyword are not case-sensitive.
`EXTRACT(type FROM date)`
  The `EXTRACT()` function uses the same kinds of interval type specifiers as `DATE_ADD()` or `DATE_SUB()`, but extracts parts from the date rather than performing date arithmetic.

      mysql> SELECT EXTRACT(YEAR FROM "1999-07-02");
             -> 1999
      mysql> SELECT EXTRACT(YEAR_MONTH FROM "1999-07-02 01:02:03");
             -> 199907
      mysql> SELECT EXTRACT(DAY_MINUTE FROM "1999-07-02 01:02:03");
             -> 20102

`TO_DAYS(date)`
  Given a date `date`, returns a daynumber (the number of days since year 0):

      mysql> SELECT TO_DAYS(950501);
              -> 728779
      mysql> SELECT TO_DAYS('1997-10-07');
              -> 729669

  `TO_DAYS()` is not intended for use with values that precede the advent of the Gregorian calendar (1582), because it doesn't take into account the days that were lost when the calendar was changed.
`FROM_DAYS(N)`
  Given a daynumber `N`, returns a `DATE` value:

      mysql> SELECT FROM_DAYS(729669);
              -> '1997-10-07'

  `FROM_DAYS()` is not intended for use with values that precede the advent of the Gregorian calendar (1582), because it doesn't take into account the days that were lost when the calendar was changed.
`DATE_FORMAT(date,format)`
  Formats the `date` value according to the `format` string. The following specifiers may be used in the `format` string:

  Specifier | Description
  `%M` | Month name (`January`..`December`)
  `%W` | Weekday name (`Sunday`..`Saturday`)
  `%D` | Day of the month with English suffix (`0th`, `1st`, `2nd`, `3rd`, etc.)
  `%Y` | Year, numeric, 4 digits
  `%y` | Year, numeric, 2 digits
  `%X` | Year for the week where Sunday is the first day of the week, numeric, 4 digits, used with '%V'
  `%x` | Year for the week, where Monday is the first day of the week, numeric, 4 digits, used with '%v'
  `%a` | Abbreviated weekday name (`Sun`..`Sat`)
  `%d` | Day of the month, numeric (`00`..`31`)
  `%e` | Day of the month, numeric (`0`..`31`)
  `%m` | Month, numeric (`00`..`12`)
  `%c` | Month, numeric (`0`..`12`)
  `%b` | Abbreviated month name (`Jan`..`Dec`)
  `%j` | Day of year (`001`..`366`)
  `%H` | Hour (`00`..`23`)
  `%k` | Hour (`0`..`23`)
  `%h` | Hour (`01`..`12`)
  `%I` | Hour (`01`..`12`)
  `%l` | Hour (`1`..`12`)
  `%i` | Minutes, numeric (`00`..`59`)
  `%r` | Time, 12-hour (`hh:mm:ss [AP]M`)
  `%T` | Time, 24-hour (`hh:mm:ss`)
  `%S` | Seconds (`00`..`59`)
  `%s` | Seconds (`00`..`59`)
  `%p` | `AM` or `PM`
  `%w` | Day of the week (`0`=Sunday..`6`=Saturday)
  `%U` | Week (`00`..`53`), where Sunday is the first day of the week
  `%u` | Week (`00`..`53`), where Monday is the first day of the week
  `%V` | Week (`01`..`53`), where Sunday is the first day of the week. Used with '%X'
  `%v` | Week (`01`..`53`), where Monday is the first day of the week. Used with '%x'
  `%%` | A literal ``%'`.

  All other characters are just copied to the result without interpretation:

      mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00', '%W %M %Y');
              -> 'Saturday October 1997'
      mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00', '%H:%i:%s');
              -> '22:23:00'
      mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00',
                                '%D %y %a %d %m %b %j');
              -> '4th 97 Sat 04 10 Oct 277'
      mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00',
                                '%H %k %I %r %T %S %w');
              -> '22 22 10 10:23:00 PM 22:23:00 00 6'
      mysql> SELECT DATE_FORMAT('1999-01-01', '%X %V');
              -> '1998 52'

  As of MySQL Version 3.23, the ``%'` character is required before format specifier characters. In earlier versions of MySQL, ``%'` was optional. The reason the ranges for the month and day specifiers begin with zero is that MySQL allows incomplete dates such as `'2004-00-00'` to be stored as of MySQL 3.23.
`TIME_FORMAT(time,format)`
  This is used like the `DATE_FORMAT()` function above, but the `format` string may contain only those format specifiers that handle hours, minutes, and seconds. Other specifiers produce a `NULL` value or `0`.
`CURDATE()`
`CURRENT_DATE`
  Returns today's date as a value in `'YYYY-MM-DD'` or `YYYYMMDD` format, depending on whether the function is used in a string or numeric context:

      mysql> SELECT CURDATE();
              -> '1997-12-15'
      mysql> SELECT CURDATE() + 0;
              -> 19971215

`CURTIME()`
`CURRENT_TIME`
  Returns the current time as a value in `'HH:MM:SS'` or `HHMMSS` format, depending on whether the function is used in a string or numeric context:

      mysql> SELECT CURTIME();
              -> '23:50:26'
      mysql> SELECT CURTIME() + 0;
              -> 235026

`NOW()`
`SYSDATE()`
`CURRENT_TIMESTAMP`
  Returns the current date and time as a value in `'YYYY-MM-DD HH:MM:SS'` or `YYYYMMDDHHMMSS` format, depending on whether the function is used in a string or numeric context:

      mysql> SELECT NOW();
              -> '1997-12-15 23:50:26'
      mysql> SELECT NOW() + 0;
              -> 19971215235026

  Note that `NOW()` is only evaluated once per query, namely at the start of query execution. This means that multiple references to `NOW()` within a single query will always give the same time.
`UNIX_TIMESTAMP()`
`UNIX_TIMESTAMP(date)`
  If called with no argument, returns a Unix timestamp (seconds since `'1970-01-01 00:00:00'` GMT) as an unsigned integer. If `UNIX_TIMESTAMP()` is called with a `date` argument, it returns the value of the argument as seconds since `'1970-01-01 00:00:00'` GMT. `date` may be a `DATE` string, a `DATETIME` string, a `TIMESTAMP`, or a number in the format `YYMMDD` or `YYYYMMDD` in local time:

      mysql> SELECT UNIX_TIMESTAMP();
              -> 882226357
      mysql> SELECT UNIX_TIMESTAMP('1997-10-04 22:23:00');
              -> 875996580

  When `UNIX_TIMESTAMP` is used on a `TIMESTAMP` column, the function will return the internal timestamp value directly, with no implicit ``string-to-unix-timestamp'' conversion. If you pass an out-of-range date to `UNIX_TIMESTAMP()` it will return 0, but please note that only basic checking is performed (year 1970-2037, month 01-12, day 01-31). If you want to subtract `UNIX_TIMESTAMP()` columns, you may want to cast the result to signed integers. See section 6.3.5 Cast Functions.
`FROM_UNIXTIME(unix_timestamp)`
  Returns a representation of the `unix_timestamp` argument as a value in `'YYYY-MM-DD HH:MM:SS'` or `YYYYMMDDHHMMSS` format, depending on whether the function is used in a string or numeric context:

      mysql> SELECT FROM_UNIXTIME(875996580);
              -> '1997-10-04 22:23:00'
      mysql> SELECT FROM_UNIXTIME(875996580) + 0;
              -> 19971004222300

`FROM_UNIXTIME(unix_timestamp,format)`
  Returns a string representation of the Unix timestamp, formatted according to the `format` string. `format` may contain the same specifiers as those listed in the entry for the `DATE_FORMAT()` function:

      mysql> SELECT FROM_UNIXTIME(UNIX_TIMESTAMP(),
                                  '%Y %D %M %h:%i:%s %x');
              -> '1997 23rd December 03:43:30 1997'

`SEC_TO_TIME(seconds)`
  Returns the `seconds` argument, converted to hours, minutes, and seconds, as a value in `'HH:MM:SS'` or `HHMMSS` format, depending on whether the function is used in a string or numeric context:

      mysql> SELECT SEC_TO_TIME(2378);
              -> '00:39:38'
      mysql> SELECT SEC_TO_TIME(2378) + 0;
              -> 3938

`TIME_TO_SEC(time)`
  Returns the `time` argument, converted to seconds:

      mysql> SELECT TIME_TO_SEC('22:23:00');
              -> 80580
      mysql> SELECT TIME_TO_SEC('00:39:38');
              -> 2378

### 6.3.5 Cast Functions

The syntax of the `CAST` function is:

    CAST(expression AS type)

    or

    CONVERT(expression,type)

Where type is one of:

- `BINARY`
- `CHAR` (New in 4.0.6)
- `DATE`
- `DATETIME`
- `SIGNED {INTEGER}`
- `TIME`
- `UNSIGNED {INTEGER}`

`CAST()` is ANSI SQL99 syntax and `CONVERT()` is ODBC syntax.

The cast function is mainly useful when you want to create a column with a specific type in a `CREATE ... SELECT`:

    CREATE TABLE new_table SELECT CAST('2000-01-01' AS DATE);

`CAST(string AS BINARY` is the same thing as `BINARY string`. `CAST(expr AS CHAR` threats expression to be a string with the default character set.

To cast a string to a numeric value, you don't normally have to do anything; just use the string value as it would be a number:

    mysql> SELECT 1+'1';
           -> 2

If you use a number in string context the number will automatically be converted to a `BINARY` string.

mysql> SELECT concat("hello you ",2); -> "hello you 2"

MySQL supports arithmetic with both signed and unsigned 64-bit values. If you are using an numerical operations (like `+`) and one of the operands are `unsigned integer`, then the result will be unsigned. You can override this by using the `SIGNED` and `UNSIGNED` cast operators, which will cast the operation to a signed or unsigned 64-bit integer, respectively.

    mysql> SELECT CAST(1-2 AS UNSIGNED)
            -> 18446744073709551615
    mysql> SELECT CAST(CAST(1-2 AS UNSIGNED) AS SIGNED);
            -> -1

Note that if either operation is a floating-point value (In this context `DECIMAL()` is regarded as a floating-point value) the result will be a floating-point value and is not affected by the above rule.

    mysql> SELECT CAST(1 AS UNSIGNED) -2.0
            -> -1.0

If you are using a string in an arithmetic operation, this is converted to a floating-point number.

The `CAST()` and `CONVERT()` functions were added in MySQL 4.0.2.

The handing of unsigned values was changed in MySQL 4.0 to be able to support `BIGINT` values properly. If you have some code that you want to run in both MySQL 4.0 and 3.23 (in which case you probably can't use the CAST function), you can use the following trick to get a signed result when subtracting two unsigned integer columns:

    SELECT (unsigned_column_1+0.0)-(unsigned_column_2+0.0);

The idea is that the columns are converted to floating-point before doing the subtraction.

If you get a problem with `UNSIGNED` columns in your old MySQL application when porting to MySQL 4.0, you can use the `--sql-mode=NO_UNSIGNED_SUBTRACTION` option when starting `mysqld`. Note however that as long as you use this, you will not be able to make efficient use of the `UNSIGNED BIGINT` column type.

### 6.3.6 Other Functions

#### 6.3.6.1 Bit Functions

MySQL uses `BIGINT` (64-bit) arithmetic for bit operations, so these operators have a maximum range of 64 bits.

`|`
  Bitwise OR

      mysql> SELECT 29 | 15;
              -> 31

  The result is an unsigned 64-bit integer.
`&`
  Bitwise AND

      mysql> SELECT 29 & 15;
              -> 13

  The result is an unsigned 64-bit integer.
`^`
  Bitwise XOR

      mysql> SELECT 1 ^ 1;
              -> 0
      mysql> SELECT 1 ^ 0;
              -> 1
      mysql> SELECT 11 ^ 3;
              -> 8

  The result is an unsigned 64-bit integer.
`<<`
  Shifts a longlong (`BIGINT`) number to the left:

      mysql> SELECT 1 << 2;
              -> 4

  The result is an unsigned 64-bit integer.
`>>`
  Shifts a longlong (`BIGINT`) number to the right:

      mysql> SELECT 4 >> 2;
              -> 1

  The result is an unsigned 64-bit integer.
`~`
  Invert all bits:

      mysql> SELECT 5 & ~1;
              -> 4

  The result is an unsigned 64-bit integer.
`BIT_COUNT(N)`
  Returns the number of bits that are set in the argument `N`:

      mysql> SELECT BIT_COUNT(29);
              -> 4

#### 6.3.6.2 Miscellaneous Functions

`DATABASE()`
  Returns the current database name:

      mysql> SELECT DATABASE();
              -> 'test'

  If there is no current database, `DATABASE()` returns the empty string.
`USER()`
`SYSTEM_USER()`
`SESSION_USER()`
  Returns the current MySQL user name:

      mysql> SELECT USER();
              -> 'davida@localhost'

  In MySQL Version 3.22.11 or later, this includes the client hostname as well as the user name. You can extract just the user name part like this (which works whether the value includes a hostname part):

      mysql> SELECT SUBSTRING_INDEX(USER(),"@",1);
              -> 'davida'

`CURRENT_USER()`
  Returns the user name, current session was authentificated as:

      mysql> SELECT USER();
              -> 'davida@localhost'
      mysql> SELECT * FROM mysql.user;
              -> ERROR 1044: Access denied for user: '@localhost' to database 'mysql'
      mysql> SELECT CURRENT_USER();
              -> '@localhost'

`PASSWORD(str)`
  Calculates a password string from the plaintext password `str`. This is the function that is used for encrypting MySQL passwords for storage in the `Password` column of the `user` grant table:

      mysql> SELECT PASSWORD('badpwd');
              -> '7f84554057dd964b'

  `PASSWORD()` encryption is non-reversible. `PASSWORD()` does not perform password encryption in the same way that Unix passwords are encrypted. You should not assume that if your Unix password and your MySQL password are the same, `PASSWORD()` will result in the same encrypted value as is stored in the Unix password file. See `ENCRYPT()`.
`ENCRYPT(str[,salt])`
  Encrypt `str` using the Unix `crypt()` system call. The `salt` argument should be a string with two characters. (As of MySQL Version 3.22.16, `salt` may be longer than two characters.):

      mysql> SELECT ENCRYPT("hello");
              -> 'VxuFAJXVARROc'

  If `crypt()` is not available on your system, `ENCRYPT()` always returns `NULL`. `ENCRYPT()` ignores all but the first 8 characters of `str`, at least on some systems. This will be determined by the behaviour of the underlying `crypt()` system call.
`ENCODE(str,pass_str)`
  Encrypt `str` using `pass_str` as the password. To decrypt the result, use `DECODE()`. The results is a binary string of the same length as `string`. If you want to save it in a column, use a `BLOB` column type.
`DECODE(crypt_str,pass_str)`
  Descrypts the encrypted string `crypt_str` using `pass_str` as the password. `crypt_str` should be a string returned from `ENCODE()`.
`MD5(string)`
  Calculates an MD5 128 bit checksum for the string. The value is returned as a 32 digit hex number that may, for example, be used as a hash key:

      mysql> SELECT MD5("testing");
              -> 'ae2b1fca515949e5d54fb22b8ed95575'

  This is the "RSA Data Security, Inc. MD5 Message-Digest Algorithm".
`SHA1(string)`
`SHA(string)`
  Calculates an SHA1 160 bit checksum for the string, as described in RFC 3174 (Secure Hash Algorithm). The value is returned as a 40 digit hex number, or `NULL` in case the input argument was `NULL`. One of the possible uses for this function is as a hash key. You can also use it as cryptographically safe function for storing passwords.

      mysql> SELECT SHA1("abc");
              -> 'a9993e364706816aba3e25717850c26c9cd0d89d'

  `SHA1()` was added in version 4.0.2, and can be considered a cryptographically more secure equivalent of `MD5()`. `SHA()` is synonym for `SHA1()`.
`AES_ENCRYPT(string,key_string)`
`AES_DECRYPT(string,key_string)`
  These functions allow encryption/decryption of data using the official AES (Advanced Encryption Standard) algorithm, previously known as Rijndael. Encoding with 128 bit key length is used, but you can extend it up to 256 bit by patching the source. We chose 128 bits because it is much faster and it is usually secure enough. The input arguments may be any length. If either argument is `NULL`, the result of this function is also `NULL`. As AES is a block level algorithm, padding is used to encode uneven length strings and so the result string length may be calculated as 16*(trunc(string_length/16)+1). If `AES_DECRYPT()` detects invalid data or incorrect padding, it will return `NULL`. However, it is possible for `AES_DECRYPT()` to return a non-`NULL` value (possibly garbage) if the input data or the key was invalid. You can use the AES functions to store data in an encrypted form by modifying your queries:

      INSERT INTO t VALUES (1,AES_ENCRYPT("text","password"));

  You can get even more security by avoiding transferring the key over the connection for each query, which can be accomplished by storing it in a server side variable at connection time:

      SELECT @password:="my password";
      INSERT INTO t VALUES (1,AES_ENCRYPT("text",@password));

  `AES_ENCRYPT()` and `AES_DECRYPT()` were added in version 4.0.2, and can be considered the most cryptographically secure encryption functions currently available in MySQL.
`DES_ENCRYPT(string_to_encrypt [, (key_number | key_string) ] )`
  Encrypts the string with the given key using the DES algorithm. Note that this function only works if you have configured MySQL with SSL support. See section 4.3.9 Using Secure Connections. The encryption key to use is chosen the following way:

  Argument | Description
  Only one argument | The first key from `des-key-file` is used.
  key number | The given key (0-9) from the `des-key-file` is used.
  string | The given `key_string` will be used to crypt `string_to_encrypt`.

  The return string will be a binary string where the first character will be `CHAR(128 | key_number)`. The 128 is added to make it easier to recognise an encrypted key. If you use a string key, `key_number` will be 127. On error, this function returns `NULL`. The string length for the result will be `new_length= org_length + (8-(org_length % 8))+1`. The `des-key-file` has the following format:

      key_number des_key_string
      key_number des_key_string

  Each `key_number` must be a number in the range from 0 to 9. Lines in the file may be in any order. `des_key_string` is the string that will be used to encrypt the message. Between the number and the key there should be at least one space. The first key is the default key that will be used if you don't specify any key argument to `DES_ENCRYPT()` You can tell MySQL to read new key values from the key file with the `FLUSH DES_KEY_FILE` command. This requires the `Reload_priv` privilege. One benefit of having a set of default keys is that it gives applications a way to check for existence of encrypted column values, without giving the end user the right to decrypt those values.

      mysql> SELECT customer_address FROM customer_table WHERE
             crypted_credit_card = DES_ENCRYPT("credit_card_number");

`DES_DECRYPT(string_to_decrypt [, key_string])`
  Decrypts a string encrypted with `DES_ENCRYPT()`. Note that this function only works if you have configured MySQL with SSL support. See section 4.3.9 Using Secure Connections. If no `key_string` argument is given, `DES_DECRYPT()` examines the first byte of the encrypted string to determine the DES key number that was used to encrypt the original string, then reads the key from the `des-key-file` to decrypt the message. For this to work the user must have the `SUPER` privilege. If you pass this function a `key_string` argument, that string is used as the key for decrypting the message. If the `string_to_decrypt` doesn't look like an encrypted string, MySQL will return the given `string_to_decrypt`. On error, this function returns `NULL`.
`LAST_INSERT_ID([expr])`
  Returns the last automatically generated value that was inserted into an `AUTO_INCREMENT` column. See section 8.4.3.126 `mysql_insert_id()`.

      mysql> SELECT LAST_INSERT_ID();
              -> 195

  The last ID that was generated is maintained in the server on a per-connection basis. It will not be changed by another client. It will not even be changed if you update another `AUTO_INCREMENT` column with a non-magic value (that is, a value that is not `NULL` and not `0`). If you insert many rows at the same time with an insert statement, `LAST_INSERT_ID()` returns the value for the first inserted row. The reason for this is to make it possible to easily reproduce the same `INSERT` statement against some other server. If `expr` is given as an argument to `LAST_INSERT_ID()`, then the value of the argument is returned by the function, and is set as the next value to be returned by `LAST_INSERT_ID()`. This can be used to simulate sequences: First create the table:

      mysql> CREATE TABLE sequence (id INT NOT NULL);
      mysql> INSERT INTO sequence VALUES (0);

  Then the table can be used to generate sequence numbers like this:

      mysql> UPDATE sequence SET id=LAST_INSERT_ID(id+1);

  You can generate sequences without calling `LAST_INSERT_ID()`, but the utility of using the function this way is that the ID value is maintained in the server as the last automatically generated value (multi-user safe). You can retrieve the new ID as you would read any normal `AUTO_INCREMENT` value in MySQL. For example, `LAST_INSERT_ID()` (without an argument) will return the new ID. The C API function `mysql_insert_id()` can also be used to get the value. Note that as `mysql_insert_id()` is only updated after `INSERT` and `UPDATE` statements, so you can't use the C API function to retrieve the value for `LAST_INSERT_ID(expr)` after executing other SQL statements like `SELECT` or `SET`.
`FORMAT(X,D)`
  Formats the number `X` to a format like `'#,###,###.##'`, rounded to `D` decimals. If `D` is `0`, the result will have no decimal point or fractional part:

      mysql> SELECT FORMAT(12332.123456, 4);
              -> '12,332.1235'
      mysql> SELECT FORMAT(12332.1,4);
              -> '12,332.1000'
      mysql> SELECT FORMAT(12332.2,0);
              -> '12,332'

`VERSION()`
  Returns a string indicating the MySQL server version:

      mysql> SELECT VERSION();
              -> '3.23.13-log'

  Note that if your version ends with `-log` this means that logging is enabled.
`CONNECTION_ID()`
  Returns the connection id (`thread_id`) for the connection. Every connection has its own unique id:

      mysql> SELECT CONNECTION_ID();
              -> 1

`GET_LOCK(str,timeout)`
  Tries to obtain a lock with a name given by the string `str`, with a timeout of `timeout` seconds. Returns `1` if the lock was obtained successfully, `0` if the attempt timed out, or `NULL` if an error occurred (such as running out of memory or the thread was killed with `mysqladmin kill`). A lock is released when you execute `RELEASE_LOCK()`, execute a new `GET_LOCK()`, or the thread terminates. This function can be used to implement application locks or to simulate record locks. It blocks requests by other clients for locks with the same name; clients that agree on a given lock string name can use the string to perform cooperative advisory locking:

      mysql> SELECT GET_LOCK("lock1",10);
              -> 1
      mysql> SELECT IS_FREE_LOCK("lock2");
              -> 1
      mysql> SELECT GET_LOCK("lock2",10);
              -> 1
      mysql> SELECT RELEASE_LOCK("lock2");
              -> 1
      mysql> SELECT RELEASE_LOCK("lock1");
              -> NULL

  Note that the second `RELEASE_LOCK()` call returns `NULL` because the lock `"lock1"` was automatically released by the second `GET_LOCK()` call.
`RELEASE_LOCK(str)`
  Releases the lock named by the string `str` that was obtained with `GET_LOCK()`. Returns `1` if the lock was released, `0` if the lock wasn't locked by this thread (in which case the lock is not released), and `NULL` if the named lock didn't exist. The lock will not exist if it was never obtained by a call to `GET_LOCK()` or if it already has been released. The `DO` statement is convinient to use with `RELEASE_LOCK()`. See section 6.4.10 `DO` Syntax.
`IS_FREE_LOCK(str)`
  Checks if the the lock named `str` is free to use (i.e., not locked). Returns `1` if the lock is free (no one is using the lock), `0` if the lock is in use, and `NULL` on errors (like incorrect arguments).
`BENCHMARK(count,expr)`
  The `BENCHMARK()` function executes the expression `expr` repeatedly `count` times. It may be used to time how fast MySQL processes the expression. The result value is always `0`. The intended use is in the `mysql` client, which reports query execution times:

      mysql> SELECT BENCHMARK(1000000,ENCODE("hello","goodbye"));
      +----------------------------------------------+
      | BENCHMARK(1000000,ENCODE("hello","goodbye")) |
      +----------------------------------------------+
      |                                            0 |
      +----------------------------------------------+
      1 row in set (4.74 sec)

  The time reported is elapsed time on the client end, not CPU time on the server end. It may be advisable to execute `BENCHMARK()` several times, and interpret the result with regard to how heavily loaded the server machine is.
`INET_NTOA(expr)`
  Given a numeric network address (4 or 8 byte), returns the dotted-quad representation of the address as a string:

      mysql> SELECT INET_NTOA(3520061480);
             ->  "209.207.224.40"

`INET_ATON(expr)`
  Given the dotted-quad representation of a network address as a string, returns an integer that represents the numeric value of the address. Addresses may be 4 or 8 byte addresses:

      mysql> SELECT INET_ATON("209.207.224.40");
             ->  3520061480

  The generated number is always in network byte order; for example the above number is calculated as `209*256^3 + 207*256^2 + 224*256 +40`.
`MASTER_POS_WAIT(log_name, log_pos)`
  Blocks until the slave reaches the specified position in the master log during replication. If master information is not initialised, returns `NULL`. If the slave is not running, will block and wait until it is started and goes to or past the specified position. If the slave is already past the specified position, returns immediately. The return value is the number of log events it had to wait to get to the specified position, or `NULL` in case of error. Useful for control of master-slave synchronisation, but was originally written to facilitate replication testing.
`FOUND_ROWS()`
  Returns the number of rows that the last `SELECT SQL_CALC_FOUND_ROWS ...` command would have returned, if it had not been restricted with `LIMIT`.

      mysql> SELECT SQL_CALC_FOUND_ROWS * FROM tbl_name
             WHERE id > 100 LIMIT 10;
      mysql> SELECT FOUND_ROWS();

  The second `SELECT` will return a number indicating how many rows the first `SELECT` would have returned had it been written without the `LIMIT` clause. Note that if you are using `SELECT SQL_CALC_FOUND_ROWS ...` MySQL has to calculate all rows in the result set. However, this is faster than if you would not use `LIMIT`, as the result set need not be sent to the client. `SQL_CALC_FOUND_ROWS` is available starting at MySQL version 4.0.0.

### 6.3.7 Functions for Use with `GROUP BY` Clauses

If you use a group function in a statement containing no `GROUP BY` clause, it is equivalent to grouping on all rows.

`COUNT(expr)`
  Returns a count of the number of non-`NULL` values in the rows retrieved by a `SELECT` statement:

      mysql> SELECT student.student_name,COUNT(*)
          ->        FROM student,course
          ->        WHERE student.student_id=course.student_id
          ->        GROUP BY student_name;

  `COUNT(*)` is somewhat different in that it returns a count of the number of rows retrieved, whether they contain `NULL` values. `COUNT(*)` is optimised to return very quickly if the `SELECT` retrieves from one table, no other columns are retrieved, and there is no `WHERE` clause. For example:

      mysql> SELECT COUNT(*) FROM student;

`COUNT(DISTINCT expr,[expr...])`
  Returns a count of the number of different non-`NULL` values:

      mysql> SELECT COUNT(DISTINCT results) FROM student;

  In MySQL you can get the number of distinct expression combinations that don't contain NULL by giving a list of expressions. In ANSI SQL you would have to do a concatenation of all expressions inside `COUNT(DISTINCT ...)`.
`AVG(expr)`
  Returns the average value of `expr`:

      mysql> SELECT student_name, AVG(test_score)
          ->        FROM student
          ->        GROUP BY student_name;

`MIN(expr)`
`MAX(expr)`
  Returns the minimum or maximum value of `expr`. `MIN()` and `MAX()` may take a string argument; in such cases they return the minimum or maximum string value. See section 5.4.3 How MySQL Uses Indexes.

      mysql> SELECT student_name, MIN(test_score), MAX(test_score)
          ->        FROM student
          ->        GROUP BY student_name;

  In `MIN()`, `MAX()` and other aggregate functions, MySQL currently compares `ENUM` and `SET` columns by their string value rather than by the string's relative position in the set. This will be rectified.
`SUM(expr)`
  Returns the sum of `expr`. Note that if the return set has no rows, it returns NULL!
`VARIANCE(expr)`
  Returns the standard variance of `expr`. This is an extension to ANSI SQL (available only in version 4.1 or later).
`STD(expr)`
`STDDEV(expr)`
  Returns the standard deviation of `expr`. This is an extension to ANSI SQL. The `STDDEV()` form of this function is provided for Oracle compatibility.
`BIT_OR(expr)`
  Returns the bitwise `OR` of all bits in `expr`. The calculation is performed with 64-bit (`BIGINT`) precision.
`BIT_AND(expr)`
  Returns the bitwise `AND` of all bits in `expr`. The calculation is performed with 64-bit (`BIGINT`) precision.

MySQL has extended the use of `GROUP BY`. You can use columns or calculations in the `SELECT` expressions that don't appear in the `GROUP BY` part. This stands for any possible value for this group. You can use this to get better performance by avoiding sorting and grouping on unnecessary items. For example, you don't need to group on `customer.name` in the following query:

    mysql> SELECT order.custid,customer.name,MAX(payments)
        ->        FROM order,customer
        ->        WHERE order.custid = customer.custid
        ->        GROUP BY order.custid;

In ANSI SQL, you would have to add `customer.name` to the `GROUP BY` clause. In MySQL, the name is redundant if you don't run in ANSI mode.

Don't use this feature if the columns you omit from the `GROUP BY` part aren't unique in the group! You will get unpredictable results.

In some cases, you can use `MIN()` and `MAX()` to obtain a specific column value even if it isn't unique. The following gives the value of `column` from the row containing the smallest value in the `sort` column:

    SUBSTR(MIN(CONCAT(RPAD(sort,6,' '),column)),7)

See section 3.5.4 The Rows Holding the Group-wise Maximum of a Certain Field.

Note that if you are using MySQL Version 3.22 (or earlier) or if you are trying to follow ANSI SQL, you can't use expressions in `GROUP BY` or `ORDER BY` clauses. You can work around this limitation by using an alias for the expression:

    mysql> SELECT id,FLOOR(value/100) AS val FROM tbl_name
        ->        GROUP BY id,val ORDER BY val;

In MySQL Version 3.23 you can do:

    mysql> SELECT id,FLOOR(value/100) FROM tbl_name ORDER BY RAND();

## 6.4 Data Manipulation: `SELECT`, `INSERT`, `UPDATE`, `DELETE`

### 6.4.1 `SELECT` Syntax

    SELECT [STRAIGHT_JOIN]
           [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
           [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS] [HIGH_PRIORITY]
           [DISTINCT | DISTINCTROW | ALL]
        select_expression,...
        [INTO {OUTFILE | DUMPFILE} 'file_name' export_options]
        [FROM table_references
          [WHERE where_definition]
          [GROUP BY {unsigned_integer | col_name | formula} [ASC | DESC], ...
          [HAVING where_definition]
          [ORDER BY {unsigned_integer | col_name | formula} [ASC | DESC] ,...]
          [LIMIT [offset,] rows | rows OFFSET offset]
          [PROCEDURE procedure_name(argument_list)]
          [FOR UPDATE | LOCK IN SHARE MODE]]

`SELECT` is used to retrieve rows selected from one or more tables. `select_expression` indicates the columns you want to retrieve. `SELECT` may also be used to retrieve rows computed without reference to any table. For example:

    mysql> SELECT 1 + 1;
             -> 2

All keywords used must be given in exactly the order shown above. For example, a `HAVING` clause must come after any `GROUP BY` clause and before any `ORDER BY` clause.

- A `SELECT` expression may be given an alias using `AS`. The alias is used as the expression's column name and can be used with `ORDER BY` or `HAVING` clauses. For example:

      mysql> SELECT CONCAT(last_name,', ',first_name) AS full_name
          FROM mytable ORDER BY full_name;

- It is not allowed to use a column alias in a `WHERE` clause, because the column value may not yet be determined when the `WHERE` clause is executed. See section A.5.4 Problems with `alias`.
- The `FROM table_references` clause indicates the tables from which to retrieve rows. If you name more than one table, you are performing a join. For information on join syntax, see section 6.4.1.1 `JOIN` Syntax. For each table specified, you may optionally specify an alias.

      table_name [[AS] alias] [USE INDEX (key_list)] [IGNORE INDEX (key_list)]

  As of MySQL Version 3.23.12, you can give hints about which index MySQL should use when retrieving information from a table. This is useful if `EXPLAIN` shows that MySQL is using the wrong index from the list of possible indexes. By specifying `USE INDEX (key_list)`, you can tell MySQL to use only one of the possible indexes to find rows in the table. The alternative syntax `IGNORE INDEX (key_list)` can be used to tell MySQL to not use some particular index. `USE/IGNORE KEY` are synonyms for `USE/IGNORE INDEX`.
- You can refer to a table as `tbl_name` (within the current database), or as `dbname.tbl_name` to explicitly specify a database. You can refer to a column as `col_name`, `tbl_name.col_name`, or `db_name.tbl_name.col_name`. You need not specify a `tbl_name` or `db_name.tbl_name` prefix for a column reference in a `SELECT` statement unless the reference would be ambiguous. See section 6.1.2 Database, Table, Index, Column, and Alias Names, for examples of ambiguity that require the more explicit column reference forms.
- A table reference may be aliased using `tbl_name [AS] alias_name`:

      mysql> SELECT t1.name, t2.salary FROM employee AS t1, info AS t2
          ->        WHERE t1.name = t2.name;
      mysql> SELECT t1.name, t2.salary FROM employee t1, info t2
          ->        WHERE t1.name = t2.name;

- Columns selected for output may be referred to in `ORDER BY` and `GROUP BY` clauses using column names, column aliases, or column positions. Column positions begin with 1:

      mysql> SELECT college, region, seed FROM tournament
          ->        ORDER BY region, seed;
      mysql> SELECT college, region AS r, seed AS s FROM tournament
          ->        ORDER BY r, s;
      mysql> SELECT college, region, seed FROM tournament
          ->        ORDER BY 2, 3;

  To sort in reverse order, add the `DESC` (descending) keyword to the name of the column in the `ORDER BY` clause that you are sorting by. The default is ascending order; this may be specified explicitly using the `ASC` keyword.
- You can in the `WHERE` clause use any of the functions that MySQL support. See section 6.3 Functions for Use in `SELECT` and `WHERE` Clauses.
- The `HAVING` clause can refer to any column or alias named in the `select_expression`. It is applied last, just before items are sent to the client, with no optimisation. Don't use `HAVING` for items that should be in the `WHERE` clause. For example, do not write this:

      mysql> SELECT col_name FROM tbl_name HAVING col_name > 0;

  Write this instead:

      mysql> SELECT col_name FROM tbl_name WHERE col_name > 0;

  In MySQL Version 3.22.5 or later, you can also write queries like this:

      mysql> SELECT user,MAX(salary) FROM users
          ->        GROUP BY user HAVING MAX(salary)>10;

  In older MySQL versions, you can write this instead:

      mysql> SELECT user,MAX(salary) AS sum FROM users
          ->        group by user HAVING sum>10;

- The options `DISTINCT`, `DISTINCTROW` and `ALL` specify whether duplicate rows should be returned. The default is (`ALL`), all matching rows are returned. `DISTINCT` and `DISTINCTROW` are synonyms and specify that duplicate rows in the result set should be removed.
- All options beginning with `SQL_`, `STRAIGHT_JOIN`, and `HIGH_PRIORITY` are MySQL extensions to ANSI SQL.
- `HIGH_PRIORITY` will give the `SELECT` higher priority than a statement that updates a table. You should only use this for queries that are very fast and must be done at once. A `SELECT HIGH_PRIORITY` query will run if the table is locked for read even if there is an update statement that is waiting for the table to be free.
- `SQL_BIG_RESULT` can be used with `GROUP BY` or `DISTINCT` to tell the optimiser that the result set will have many rows. In this case, MySQL will directly use disk-based temporary tables if needed. MySQL will also, in this case, prefer sorting to doing a temporary table with a key on the `GROUP BY` elements.
- `SQL_BUFFER_RESULT` will force the result to be put into a temporary table. This will help MySQL free the table locks early and will help in cases where it takes a long time to send the result set to the client.
- `SQL_SMALL_RESULT`, a MySQL-specific option, can be used with `GROUP BY` or `DISTINCT` to tell the optimiser that the result set will be small. In this case, MySQL will use fast temporary tables to store the resulting table instead of using sorting. In MySQL Version 3.23 this shouldn't normally be needed.
- `SQL_CALC_FOUND_ROWS` (version 4.0.0 and up) tells MySQL to calculate how many rows there would be in the result set, disregarding any `LIMIT` clause. The number of rows can then be retrieved with `SELECT FOUND_ROWS()`. See section 6.3.6.2 Miscellaneous Functions. Please note that in versions prior to 4.1.0 this does not work with `LIMIT 0`, which is optimised to return instantly (resulting in a row count of 0). See section 5.2.8 How MySQL Optimises `LIMIT`.
- `SQL_CACHE` tells MySQL to store the query result in the query cache if you are using `QUERY_CACHE_TYPE=2` (`DEMAND`). See section 6.9 MySQL Query Cache.
- `SQL_NO_CACHE` tells MySQL to not allow the query result to be stored in the query cache. See section 6.9 MySQL Query Cache.
- If you use `GROUP BY`, the output rows will be sorted according to the `GROUP BY` as if you would have had an `ORDER BY` over all the fields in the `GROUP BY`. MySQL has extended the `GROUP BY` so that you can also specify `ASC` and `DESC` to `GROUP BY`:

      SELECT a,COUNT(b) FROM test_table GROUP BY a DESC

- MySQL has extended the use of `GROUP BY` to allow you to select fields which are not mentioned in the `GROUP BY` clause. If you are not getting the results you expect from your query, please read the `GROUP BY` description. See section 6.3.7 Functions for Use with `GROUP BY` Clauses.
- `STRAIGHT_JOIN` forces the optimiser to join the tables in the order in which they are listed in the `FROM` clause. You can use this to speed up a query if the optimiser joins the tables in non-optimal order. See section 5.2.1 `EXPLAIN` Syntax (Get Information About a `SELECT`).
- The `LIMIT` clause can be used to constrain the number of rows returned by the `SELECT` statement. `LIMIT` takes one or two numeric arguments. The arguments must be integer constants. If two arguments are given, the first specifies the offset of the first row to return, the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1): To be compatible with PostgreSQL MySQL also supports the syntax: `LIMIT # OFFSET #`.

      mysql> SELECT * FROM table LIMIT 5,10;  # Retrieve rows 6-15

  To retrieve all rows from a certain offset upto the end of the result set, you can use -1 for the second parameter:

      mysql> SELECT * FROM table LIMIT 95,-1; # Retrieve rows 96-last.

  If one argument is given, it indicates the maximum number of rows to return:

      mysql> SELECT * FROM table LIMIT 5;     # Retrieve first 5 rows

  In other words, `LIMIT n` is equivalent to `LIMIT 0,n`.
- The `SELECT ... INTO OUTFILE 'file_name'` form of `SELECT` writes the selected rows to a file. The file is created on the server host and cannot already exist (among other things, this prevents database tables and files such as ``/etc/passwd'` from being destroyed). You must have the `FILE` privilege on the server host to use this form of `SELECT`. `SELECT ... INTO OUTFILE` is mainly intended to let you very quickly dump a table on the server machine. If you want to create the resulting file on some other host than the server host you can't use `SELECT ... INTO OUTFILE`. In this case you should instead use some client program like `mysqldump --tab` or `mysql -e "SELECT ..." > outfile` to generate the file. `SELECT ... INTO OUTFILE` is the complement of `LOAD DATA INFILE`; the syntax for the `export_options` part of the statement consists of the same `FIELDS` and `LINES` clauses that are used with the `LOAD DATA INFILE` statement. See section 6.4.9 `LOAD DATA INFILE` Syntax. In the resulting text file, only the following characters are escaped by the `ESCAPED BY` character:
  - The `ESCAPED BY` character
  - The first character in `FIELDS TERMINATED BY`
  - The first character in `LINES TERMINATED BY`
  Additionally, `ASCII 0` is converted to `ESCAPED BY` followed by 0 (`ASCII 48`). The reason for the above is that you must escape any `FIELDS TERMINATED BY`, `ESCAPED BY`, or `LINES TERMINATED BY` characters to reliably be able to read the file back. `ASCII 0` is escaped to make it easier to view with some pagers. As the resulting file doesn't have to conform to the SQL syntax, nothing else need be escaped. Here follows an example of getting a file in the format used by many old programs.

      SELECT a,b,a+b INTO OUTFILE "/tmp/result.text"
      FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"'
      LINES TERMINATED BY "\n"
      FROM test_table;

- If you use `INTO DUMPFILE` instead of `INTO OUTFILE`, MySQL will only write one row into the file, without any column or line terminations and without any escaping. This is useful if you want to store a blob in a file.
- Note that any file created by `INTO OUTFILE` and `INTO DUMPFILE` is going to be readable for all users! The reason is that the MySQL server can't create a file that is owned by anyone else than the user it's running as (you should never run `mysqld` as root), the file has to be word readable so that you can retrieve the rows.
- If you are using `FOR UPDATE` on a storage engine with page/row locks, the examined rows will be write locked.

#### 6.4.1.1 `JOIN` Syntax

MySQL supports the following `JOIN` syntaxes for use in `SELECT` statements:

    table_reference, table_reference
    table_reference [CROSS] JOIN table_reference
    table_reference INNER JOIN table_reference join_condition
    table_reference STRAIGHT_JOIN table_reference
    table_reference LEFT [OUTER] JOIN table_reference join_condition
    table_reference LEFT [OUTER] JOIN table_reference
    table_reference NATURAL [LEFT [OUTER]] JOIN table_reference
    { OJ table_reference LEFT OUTER JOIN table_reference ON conditional_expr }
    table_reference RIGHT [OUTER] JOIN table_reference join_condition
    table_reference RIGHT [OUTER] JOIN table_reference
    table_reference NATURAL [RIGHT [OUTER]] JOIN table_reference

Where `table_reference` is defined as:

    table_name [[AS] alias] [USE INDEX (key_list)] [IGNORE INDEX (key_list)]

and `join_condition` is defined as:

    ON conditional_expr |
    USING (column_list)

You should generally not have any conditions in the `ON` part that are used to restrict which rows you have in the result set (there are exceptions to this rule). If you want to restrict which rows should be in the result, you have to do this in the `WHERE` clause.

Note that in versions before Version 3.23.17, the `INNER JOIN` didn't take a `join_condition`!

The last `LEFT OUTER JOIN` syntax shown above exists only for compatibility with ODBC:

- A table reference may be aliased using `tbl_name AS alias_name` or `tbl_name alias_name`:

      mysql> SELECT t1.name, t2.salary FROM employee AS t1, info AS t2
          ->        WHERE t1.name = t2.name;

- The `ON` conditional is any conditional of the form that may be used in a `WHERE` clause.
- If there is no matching record for the right table in the `ON` or `USING` part in a `LEFT JOIN`, a row with all columns set to `NULL` is used for the right table. You can use this fact to find records in a table that have no counterpart in another table:

      mysql> SELECT table1.* FROM table1
          ->        LEFT JOIN table2 ON table1.id=table2.id
          ->        WHERE table2.id IS NULL;

  This example finds all rows in `table1` with an `id` value that is not present in `table2` (that is, all rows in `table1` with no corresponding row in `table2`). This assumes that `table2.id` is declared `NOT NULL`, of course. See section 5.2.6 How MySQL Optimises `LEFT JOIN` and `RIGHT JOIN`.
- The `USING` `(column_list)` clause names a list of columns that must exist in both tables. A `USING` clause such as:

      A LEFT JOIN B USING (C1,C2,C3,...)

  is defined to be semantically identical to an `ON` expression like this:

      A.C1=B.C1 AND A.C2=B.C2 AND A.C3=B.C3,...

- The `NATURAL [LEFT] JOIN` of two tables is defined to be semantically equivalent to an `INNER JOIN` or a `LEFT JOIN` with a `USING` clause that names all columns that exist in both tables.
- `INNER JOIN` and `,` (comma) are semantically equivalent. Both do a full join between the tables used. Normally, you specify how the tables should be linked in the WHERE condition.
- `RIGHT JOIN` works analogously as `LEFT JOIN`. To keep code portable across databases, it's recommended to use `LEFT JOIN` instead of `RIGHT JOIN`.
- `STRAIGHT_JOIN` is identical to `JOIN`, except that the left table is always read before the right table. This can be used for those (few) cases where the join optimiser puts the tables in the wrong order.
- As of MySQL Version 3.23.12, you can give hints about which index MySQL should use when retrieving information from a table. This is useful if `EXPLAIN` shows that MySQL is using the wrong index from the list of possible indexes. By specifying `USE INDEX (key_list)`, you can tell MySQL to use only one of the possible indexes to find rows in the table. The alternative syntax `IGNORE INDEX (key_list)` can be used to tell MySQL to not use some particular index. `USE/IGNORE KEY` are synonyms for `USE/IGNORE INDEX`.

Some examples:

    mysql> SELECT * FROM table1,table2 WHERE table1.id=table2.id;
    mysql> SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id;
    mysql> SELECT * FROM table1 LEFT JOIN table2 USING (id);
    mysql> SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id
        ->          LEFT JOIN table3 ON table2.id=table3.id;
    mysql> SELECT * FROM table1 USE INDEX (key1,key2)
        ->          WHERE key1=1 AND key2=2 AND key3=3;
    mysql> SELECT * FROM table1 IGNORE INDEX (key3)
        ->          WHERE key1=1 AND key2=2 AND key3=3;

See section 5.2.6 How MySQL Optimises `LEFT JOIN` and `RIGHT JOIN`.

#### 6.4.1.2 `UNION` Syntax

    SELECT ...
    UNION [ALL]
    SELECT ...
      [UNION
       SELECT ...]

`UNION` is implemented in MySQL 4.0.0.

`UNION` is used to combine the result from many `SELECT` statements into one result set.

The columns listed in the select_expression portion of the `SELECT` should have the same type. The column names used in the first `SELECT` query will be used as the column names for the results returned.

The `SELECT` commands are normal select commands, but with the following restrictions:

- Only the last `SELECT` command can have `INTO OUTFILE`.

If you don't use the keyword `ALL` for the `UNION`, all returned rows will be unique, as if you had done a `DISTINCT` for the total result set. If you specify `ALL`, then you will get all matching rows from all the used `SELECT` statements.

If you want to use an `ORDER BY` for the total `UNION` result, you should use parentheses:

    (SELECT a FROM table_name WHERE a=10 AND B=1 ORDER BY a LIMIT 10)
    UNION
    (SELECT a FROM table_name WHERE a=11 AND B=2 ORDER BY a LIMIT 10)
    ORDER BY a;

### 6.4.2 `HANDLER` Syntax

    HANDLER tbl_name OPEN [ AS alias ]
    HANDLER tbl_name READ index_name { = | >= | <= | < } (value1,value2,...)
        [ WHERE ... ] [LIMIT ... ]
    HANDLER tbl_name READ index_name { FIRST | NEXT | PREV | LAST }
        [ WHERE ... ] [LIMIT ... ]
    HANDLER tbl_name READ { FIRST | NEXT }
        [ WHERE ... ] [LIMIT ... ]
    HANDLER tbl_name CLOSE

The `HANDLER` statement provides direct access to the `MyISAM` table storage engine interface.

The first form of `HANDLER` statement opens a table, making it accessible via subsequent `HANDLER ... READ` statements. This table object is not shared by other threads and will not be closed until the thread calls `HANDLER tbl_name CLOSE` or the thread dies.

The second form fetches one row (or more, specified by `LIMIT` clause) where the index specified complies to the condition and `WHERE` condition is met. If the index consists of several parts (spans over several columns) the values are specified in comma-separated list, providing values only for few first columns is possible.

The third form fetches one row (or more, specified by `LIMIT` clause) from the table in index order, matching `WHERE` condition.

The fourth form (without index specification) fetches one row (or more, specified by `LIMIT` clause) from the table in natural row order (as stored in datafile) matching `WHERE` condition. It is faster than `HANDLER tbl_name READ index_name` when a full table scan is desired.

`HANDLER ... CLOSE` closes a table that was opened with `HANDLER ... OPEN`.

`HANDLER` is a somewhat low-level statement. For example, it does not provide consistency. That is, `HANDLER ... OPEN` does NOT take a snapshot of the table, and does NOT lock the table. This means that after a `HANDLER ... OPEN` is issued, table data can be modified (by this or any other thread) and these modifications may appear only partially in `HANDLER ... NEXT` or `HANDLER ... PREV` scans.

The reasons to use this interface instead of normal SQL are:

- It's faster than `SELECT` because:
  - A designated storage engine is allocated for the thread in `HANDLER open`.
  - There is less parsing involved.
  - No optimiser and no query checking overhead.
  - The used table doesn't have to be locked between two handler requests.
  - The handler interface doesn't have to provide a consistent look of the data (for example dirty-reads are allow), which allows the storage engine to do optimisations that SQL doesn't normally allow.
- It makes it much easier to port applications that uses an ISAM like interface to MySQL.
- It allows one to traverse a database in a manner that is not easy (in some case impossible) to do with SQL. The handler interface is more natural way to look at data when working with applications that provide an interactive user interfaces to the database.

### 6.4.3 `INSERT` Syntax

        INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
            [INTO] tbl_name [(col_name,...)]
            VALUES ((expression | DEFAULT),...),(...),...
            [ ON DUPLICATE KEY UPDATE col_name=expression, ... ]
    or  INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
            [INTO] tbl_name [(col_name,...)]
            SELECT ...
    or  INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
            [INTO] tbl_name
            SET col_name=(expression | DEFAULT), ...
            [ ON DUPLICATE KEY UPDATE col_name=expression, ... ]

`INSERT` inserts new rows into an existing table. The `INSERT ... VALUES` form of the statement inserts rows based on explicitly specified values. The `INSERT ... SELECT` form inserts rows selected from another table or tables. The `INSERT ... VALUES` form with multiple value lists is supported in MySQL Version 3.22.5 or later. The `col_name=expression` syntax is supported in MySQL Version 3.22.10 or later.

`tbl_name` is the table into which rows should be inserted. The column name list or the `SET` clause indicates which columns the statement specifies values for:

- If you specify no column list for `INSERT ... VALUES` or `INSERT ... SELECT`, values for all columns must be provided in the `VALUES()` list or by the `SELECT`. If you don't know the order of the columns in the table, use `DESCRIBE tbl_name` to find out.
- Any column not explicitly given a value is set to its default value. For example, if you specify a column list that doesn't name all the columns in the table, unnamed columns are set to their default values. Default value assignment is described in section 6.5.3 `CREATE TABLE` Syntax. You can also use the keyword `DEFAULT` to set a column to its default value. (New in MySQL 4.0.3.) This makes it easier to write `INSERT` statements that assign values to all but a few columns, because it allows you to avoid writing an incomplete `VALUES()` list (a list that does not include a value for each column in the table). Otherwise, you would have to write out the list of column names corresponding to each value in the `VALUES()` list. MySQL always has a default value for all fields. This is something that is imposed on MySQL to be able to work with both transactional and not transactional tables. Our view is that checking of fields content should be done in the application and not in the database server.
- An `expression` may refer to any column that was set earlier in a value list. For example, you can say this:

      mysql> INSERT INTO tbl_name (col1,col2) VALUES(15,col1*2);

  But not this:

      mysql> INSERT INTO tbl_name (col1,col2) VALUES(col2*2,15);

- If you specify the keyword `LOW_PRIORITY`, execution of the `INSERT` is delayed until no other clients are reading from the table. In this case the client has to wait until the insert statement is completed, which may take a long time if the table is in heavy use. This is in contrast to `INSERT DELAYED`, which lets the client continue at once. See section 6.4.4 `INSERT DELAYED` Syntax. Note that `LOW_PRIORITY` should normally not be used with `MyISAM` tables as this disables concurrent inserts. See section 7.1 `MyISAM` Tables.
- If you specify the keyword `IGNORE` in an `INSERT` with many value rows, any rows that duplicate an existing `PRIMARY` or `UNIQUE` key in the table are ignored and are not inserted. If you do not specify `IGNORE`, the insert is aborted if there is any row that duplicates an existing key value. You can determine with the C API function `mysql_info()` how many rows were inserted into the table.
- If you specify `ON DUPLICATE KEY UPDATE` clause (new in MySQL 4.1.0), and a row is inserted that would cause a duplicate value in `PRIMARY` or `UNIQUE` key, an `UPDATE` of the old row is performed. For example, the command:

      mysql> INSERT INTO table (a,b,c) VALUES (1,2,3)
         --> ON DUPLICATE KEY UPDATE c=c+1;

  in case of column `a` is declared as `UNIQUE` and already holds `1` once, would be identical to the

      mysql> UPDATE table SET c=c+1 WHERE a=1;

  Note: that if column `b` is unique too, the `UPDATE` command would be written as

      mysql> UPDATE table SET c=c+1 WHERE a=1 OR b=2 LIMIT 1;

  and if `a=1 OR b=2` matches several rows, only one row will be updated! In general, one should try to avoid using `ON DUPLICATE KEY` clause on tables with multiple `UNIQUE` keys. When one uses `ON DUPLICATE KEY UPDATE`, the `DELAYED` option is ignored.
- If MySQL was configured using the `DONT_USE_DEFAULT_FIELDS` option, `INSERT` statements generate an error unless you explicitly specify values for all columns that require a non-`NULL` value. See section 2.3.3 Typical `configure` Options.
- You can find the value used for an `AUTO_INCREMENT` column with the `mysql_insert_id` function. See section 8.4.3.126 `mysql_insert_id()`.

If you use `INSERT ... SELECT` or an `INSERT ... VALUES` statement with multiple value lists, you can use the C API function `mysql_info()` to get information about the query. The format of the information string is shown here:

    Records: 100 Duplicates: 0 Warnings: 0

`Duplicates` indicates the number of rows that couldn't be inserted because they would duplicate some existing unique index value. `Warnings` indicates the number of attempts to insert column values that were problematic in some way. Warnings can occur under any of the following conditions:

- Inserting `NULL` into a column that has been declared `NOT NULL`. The column is set to its default value.
- Setting a numeric column to a value that lies outside the column's range. The value is clipped to the appropriate endpoint of the range.
- Setting a numeric column to a value such as `'10.34 a'`. The trailing garbage is stripped and the remaining numeric part is inserted. If the value doesn't make sense as a number at all, the column is set to `0`.
- Inserting a string into a `CHAR`, `VARCHAR`, `TEXT`, or `BLOB` column that exceeds the column's maximum length. The value is truncated to the column's maximum length.
- Inserting a value into a date or time column that is illegal for the column type. The column is set to the appropriate zero value for the type.

#### 6.4.3.1 `INSERT ... SELECT` Syntax

    INSERT [LOW_PRIORITY] [IGNORE] [INTO] tbl_name [(column list)] SELECT ...

With `INSERT ... SELECT` statement you can quickly insert many rows into a table from one or many tables.

    INSERT INTO tblTemp2 (fldID) SELECT tblTemp1.fldOrder_ID FROM tblTemp1 WHERE
    tblTemp1.fldOrder_ID > 100;

The following conditions hold for an `INSERT ... SELECT` statement:

- The target table of the `INSERT` statement cannot appear in the `FROM` clause of the `SELECT` part of the query because it's forbidden in ANSI SQL to `SELECT` from the same table into which you are inserting. (The problem is that the `SELECT` possibly would find records that were inserted earlier during the same run. When using subselect clauses, the situation could easily be very confusing!)
- `AUTO_INCREMENT` columns work as usual.
- You can use the C API function `mysql_info()` to get information about the query. See section 6.4.3 `INSERT` Syntax.
- To ensure that the update log/binary log can be used to re-create the original tables, MySQL will not allow concurrent inserts during `INSERT ... SELECT`.

You can of course also use `REPLACE` instead of `INSERT` to overwrite old rows.

### 6.4.4 `INSERT DELAYED` Syntax

    INSERT DELAYED ...

The `DELAYED` option for the `INSERT` statement is a MySQL-specific option that is very useful if you have clients that can't wait for the `INSERT` to complete. This is a common problem when you use MySQL for logging and you also periodically run `SELECT` and `UPDATE` statements that take a long time to complete. `DELAYED` was introduced in MySQL Version 3.22.15. It is a MySQL extension to ANSI SQL92.

`INSERT DELAYED` only works with `ISAM` and `MyISAM` tables. Note that as `MyISAM` tables supports concurrent `SELECT` and `INSERT`, if there is no free blocks in the middle of the datafile, you very seldom need to use `INSERT DELAYED` with `MyISAM`. See section 7.1 `MyISAM` Tables.

When you use `INSERT DELAYED`, the client will get an OK at once and the row will be inserted when the table is not in use by any other thread.

Another major benefit of using `INSERT DELAYED` is that inserts from many clients are bundled together and written in one block. This is much faster than doing many separate inserts.

Note that currently the queued rows are only stored in memory until they are inserted into the table. This means that if you kill `mysqld` the hard way (`kill -9`) or if `mysqld` dies unexpectedly, any queued rows that weren't written to disk are lost!

The following describes in detail what happens when you use the `DELAYED` option to `INSERT` or `REPLACE`. In this description, the ``thread'' is the thread that received an `INSERT DELAYED` command and ``handler'' is the thread that handles all `INSERT DELAYED` statements for a particular table.

- When a thread executes a `DELAYED` statement for a table, a handler thread is created to process all `DELAYED` statements for the table, if no such handler already exists.
- The thread checks whether the handler has acquired a `DELAYED` lock already; if not, it tells the handler thread to do so. The `DELAYED` lock can be obtained even if other threads have a `READ` or `WRITE` lock on the table. However, the handler will wait for all `ALTER TABLE` locks or `FLUSH TABLES` to ensure that the table structure is up to date.
- The thread executes the `INSERT` statement, but instead of writing the row to the table, it puts a copy of the final row into a queue that is managed by the handler thread. Any syntax errors are noticed by the thread and reported to the client program.
- The client can't report the number of duplicates or the `AUTO_INCREMENT` value for the resulting row; it can't obtain them from the server, because the `INSERT` returns before the insert operation has been completed. If you use the C API, the `mysql_info()` function doesn't return anything meaningful, for the same reason.
- The update log is updated by the handler thread when the row is inserted into the table. In case of multiple-row inserts, the update log is updated when the first row is inserted.
- After every `delayed_insert_limit` rows are written, the handler checks whether any `SELECT` statements are still pending. If so, it allows these to execute before continuing.
- When the handler has no more rows in its queue, the table is unlocked. If no new `INSERT DELAYED` commands are received within `delayed_insert_timeout` seconds, the handler terminates.
- If more than `delayed_queue_size` rows are pending already in a specific handler queue, the thread requesting `INSERT DELAYED` waits until there is room in the queue. This is done to ensure that the `mysqld` server doesn't use all memory for the delayed memory queue.
- The handler thread will show up in the MySQL process list with `delayed_insert` in the `Command` column. It will be killed if you execute a `FLUSH TABLES` command or kill it with `KILL thread_id`. However, it will first store all queued rows into the table before exiting. During this time it will not accept any new `INSERT` commands from another thread. If you execute an `INSERT DELAYED` command after this, a new handler thread will be created. Note that the above means that `INSERT DELAYED` commands have higher priority than normal `INSERT` commands if there is an `INSERT DELAYED` handler already running! Other update commands will have to wait until the `INSERT DELAYED` queue is empty, someone kills the handler thread (with `KILL thread_id`), or someone executes `FLUSH TABLES`.
- The following status variables provide information about `INSERT DELAYED` commands:

  Variable | Meaning
  `Delayed_insert_threads` | Number of handler threads
  `Delayed_writes` | Number of rows written with `INSERT DELAYED`
  `Not_flushed_delayed_rows` | Number of rows waiting to be written

  You can view these variables by issuing a `SHOW STATUS` statement or by executing a `mysqladmin extended-status` command.

Note that `INSERT DELAYED` is slower than a normal INSERT if the table is not in use. There is also the additional overhead for the server to handle a separate thread for each table on which you use `INSERT DELAYED`. This means that you should only use `INSERT DELAYED` when you are really sure you need it!

### 6.4.5 `UPDATE` Syntax

    UPDATE [LOW_PRIORITY] [IGNORE] tbl_name
        SET col_name1=expr1 [, col_name2=expr2, ...]
        [WHERE where_definition]
        [ORDER BY ...]
        [LIMIT #]

`UPDATE` updates columns in existing table rows with new values. The `SET` clause indicates which columns to modify and the values they should be given. The `WHERE` clause, if given, specifies which rows should be updated. Otherwise, all rows are updated. If the `ORDER BY` clause is specified, the rows will be updated in the order that is specified.

If you specify the keyword `LOW_PRIORITY`, execution of the `UPDATE` is delayed until no other clients are reading from the table.

If you specify the keyword `IGNORE`, the update statement will not abort even if we get duplicate key errors during the update. Rows that would cause conflicts will not be updated.

If you access a column from `tbl_name` in an expression, `UPDATE` uses the current value of the column. For example, the following statement sets the `age` column to one more than its current value:

    mysql> UPDATE persondata SET age=age+1;

`UPDATE` assignments are evaluated from left to right. For example, the following statement doubles the `age` column, then increments it:

    mysql> UPDATE persondata SET age=age*2, age=age+1;

If you set a column to the value it currently has, MySQL notices this and doesn't update it.

`UPDATE` returns the number of rows that were actually changed. In MySQL Version 3.22 or later, the C API function `mysql_info()` returns the number of rows that were matched and updated and the number of warnings that occurred during the `UPDATE`.

In MySQL Version 3.23, you can use `LIMIT #` to ensure that only a given number of rows are changed.

Starting with MySQL Version 4.0.4, you can also perform `UPDATE` operations that cover multiple tables:

    UPDATE items,month SET items.price=month.price
    WHERE items.id=month.id;

### 6.4.6 `DELETE` Syntax

    DELETE [LOW_PRIORITY] [QUICK] FROM table_name
           [WHERE where_definition]
           [ORDER BY ...]
           [LIMIT rows]

    or

    DELETE [LOW_PRIORITY] [QUICK] table_name[.*] [,table_name[.*] ...]
           FROM table-references
           [WHERE where_definition]

    or

    DELETE [LOW_PRIORITY] [QUICK]
           FROM table_name[.*], [table_name[.*] ...]
           USING table-references
           [WHERE where_definition]

`DELETE` deletes rows from `table_name` that satisfy the condition given by `where_definition`, and returns the number of records deleted.

If you issue a `DELETE` with no `WHERE` clause, all rows are deleted. If you do this in `AUTOCOMMIT` mode, this works as `TRUNCATE`. See section 6.4.7 `TRUNCATE` Syntax. In MySQL 3.23, `DELETE` without a `WHERE` clause will return zero as the number of affected records.

If you really want to know how many records are deleted when you are deleting all rows, and are willing to suffer a speed penalty, you can use a `DELETE` statement of this form:

    mysql> DELETE FROM table_name WHERE 1>0;

Note that this is much slower than `DELETE FROM table_name` with no `WHERE` clause, because it deletes rows one at a time.

If you specify the keyword `LOW_PRIORITY`, execution of the `DELETE` is delayed until no other clients are reading from the table.

If you specify the word `QUICK` then the storage engine will not merge index leaves during delete, which may speed up certain kind of deletes.

In `MyISAM` tables, deleted records are maintained in a linked list and subsequent `INSERT` operations reuse old record positions. To reclaim unused space and reduce file-sizes, use the `OPTIMIZE TABLE` statement or the `myisamchk` utility to reorganise tables. `OPTIMIZE TABLE` is easier, but `myisamchk` is faster. See section 4.5.1 `OPTIMIZE TABLE` Syntax and section 4.4.6.10 Table Optimisation.

The first multi-table delete format is supported starting from MySQL 4.0.0. The second multi-table delete format is supported starting from MySQL 4.0.2.

The idea is that only matching rows from the tables listed before the `FROM` or before the `USING` clause are deleted. The effect is that you can delete rows from many tables at the same time and also have additional tables that are used for searching.

The `.*` after the table names is there just to be compatible with `Access`:

    DELETE t1,t2 FROM t1,t2,t3 WHERE t1.id=t2.id AND t2.id=t3.id

    or

    DELETE FROM t1,t2 USING t1,t2,t3 WHERE t1.id=t2.id AND t2.id=t3.id

In the above case we delete matching rows just from tables `t1` and `t2`.

`ORDER BY` and using multiple tables in the `DELETE` statement is supported in MySQL 4.0.

If an `ORDER BY` clause is used, the rows will be deleted in that order. This is really only useful in conjunction with `LIMIT`. For example:

    DELETE FROM somelog
    WHERE user = 'jcole'
    ORDER BY timestamp
    LIMIT 1

This will delete the oldest entry (by `timestamp`) where the row matches the `WHERE` clause.

The MySQL-specific `LIMIT rows` option to `DELETE` tells the server the maximum number of rows to be deleted before control is returned to the client. This can be used to ensure that a specific `DELETE` command doesn't take too much time. You can simply repeat the `DELETE` command until the number of affected rows is less than the `LIMIT` value.

### 6.4.7 `TRUNCATE` Syntax

    TRUNCATE TABLE table_name

In 3.23 `TRUNCATE TABLE` is mapped to `COMMIT ; DELETE FROM table_name`. See section 6.4.6 `DELETE` Syntax.

`TRUNCATE TABLE` differs from `DELETE FROM ...` in the following ways:

- Truncate operations drop and re-create the table, which is much faster than deleting rows one by one.
- Not transaction-safe; you will get an error if you have an active transaction or an active table lock.
- Doesn't return the number of deleted rows.
- As long as the table definition file ``table_name.frm'` is valid, the table can be re-created this way, even if the data or index files have become corrupted.

`TRUNCATE` is an Oracle SQL extension.

### 6.4.8 `REPLACE` Syntax

        REPLACE [LOW_PRIORITY | DELAYED]
            [INTO] tbl_name [(col_name,...)]
            VALUES (expression,...),(...),...
    or  REPLACE [LOW_PRIORITY | DELAYED]
            [INTO] tbl_name [(col_name,...)]
            SELECT ...
    or  REPLACE [LOW_PRIORITY | DELAYED]
            [INTO] tbl_name
            SET col_name=expression, col_name=expression,...

`REPLACE` works exactly like `INSERT`, except that if an old record in the table has the same value as a new record on a `UNIQUE` index or `PRIMARY KEY`, the old record is deleted before the new record is inserted. See section 6.4.3 `INSERT` Syntax.

In other words, you can't access the values of the old row from a `REPLACE` statement. In some old MySQL versions it appeared that you could do this, but that was a bug that has been corrected.

To be able to use `REPLACE` you must have `INSERT` and `DELETE` privileges for the table.

When you use a `REPLACE` command, `mysql_affected_rows()` will return 2 if the new row replaced an old row. This is because one row was inserted and then the duplicate was deleted.

This fact makes it easy to determine whether `REPLACE` added or replaced a row: check whether the affected-rows value is 1 (added) or 2 (replaced).

Note that unless you use a `UNIQUE` index or `PRIMARY KEY`, using a `REPLACE` command makes no sense, since it would just do an `INSERT`.

### 6.4.9 `LOAD DATA INFILE` Syntax

    LOAD DATA [LOW_PRIORITY | CONCURRENT] [LOCAL] INFILE 'file_name.txt'
        [REPLACE | IGNORE]
        INTO TABLE tbl_name
        [FIELDS
            [TERMINATED BY '\t']
            [[OPTIONALLY] ENCLOSED BY '']
            [ESCAPED BY '\\' ]
        ]
        [LINES TERMINATED BY '\n']
        [IGNORE number LINES]
        [(col_name,...)]

The `LOAD DATA INFILE` statement reads rows from a text file into a table at a very high speed. If the `LOCAL` keyword is specified, the file is read from the client host. If `LOCAL` is not specified, the file must be located on the server. (`LOCAL` is available in MySQL Version 3.22.6 or later.)

For security reasons, when reading text files located on the server, the files must either reside in the database directory or be readable by all. Also, to use `LOAD DATA INFILE` on server files, you must have the `FILE` privilege on the server host. See section 4.2.7 Privileges Provided by MySQL.

In MySQL 3.23.49 and MySQL 4.0.2 `LOCAL` will only work if you have not started `mysqld` with `--local-infile=0` or if you have not enabled your client to support `LOCAL`. See section 4.2.4 Security issues with LOAD DATA LOCAL.

If you specify the keyword `LOW_PRIORITY`, execution of the `LOAD DATA` statement is delayed until no other clients are reading from the table.

If you specify the keyword `CONCURRENT` with a `MyISAM` table, then other threads can retrieve data from the table while `LOAD DATA` is executing. Using this option will of course affect the performance of `LOAD DATA` a bit even if no other thread is using the table at the same time.

Using `LOCAL` will be a bit slower than letting the server access the files directly, because the contents of the file must travel from the client host to the server host. On the other hand, you do not need the `FILE` privilege to load local files.

If you are using MySQL before Version 3.23.24 you can't read from a FIFO with `LOAD DATA INFILE`. If you need to read from a FIFO (for example the output from gunzip), use `LOAD DATA LOCAL INFILE` instead.

You can also load datafiles by using the `mysqlimport` utility; it operates by sending a `LOAD DATA INFILE` command to the server. The `--local` option causes `mysqlimport` to read datafiles from the client host. You can specify the `--compress` option to get better performance over slow networks if the client and server support the compressed protocol.

When locating files on the server host, the server uses the following rules:

- If an absolute pathname is given, the server uses the pathname as is.
- If a relative pathname with one or more leading components is given, the server searches for the file relative to the server's data directory.
- If a filename with no leading components is given, the server looks for the file in the database directory of the current database.

Note that these rules mean a file given as ``./myfile.txt'` is read from the server's data directory, whereas a file given as ``myfile.txt'` is read from the database directory of the current database. For example, the following `LOAD DATA` statement reads the file ``data.txt'` from the database directory for `db1` because `db1` is the current database, even though the statement explicitly loads the file into a table in the `db2` database:

    mysql> USE db1;
    mysql> LOAD DATA INFILE "data.txt" INTO TABLE db2.my_table;

The `REPLACE` and `IGNORE` keywords control handling of input records that duplicate existing records on unique key values. If you specify `REPLACE`, new rows replace existing rows that have the same unique key value. If you specify `IGNORE`, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored.

If you load data from a local file using the `LOCAL` keyword, the server has no way to stop transmission of the file in the middle of the operation, so the default behaviour is the same as if `IGNORE` is specified.

If you use `LOAD DATA INFILE` on an empty `MyISAM` table, all non-unique indexes are created in a separate batch (like in `REPAIR`). This normally makes `LOAD DATA INFILE` much faster when you have many indexes.

`LOAD DATA INFILE` is the complement of `SELECT ... INTO OUTFILE`. See section 6.4.1 `SELECT` Syntax. To write data from a database to a file, use `SELECT ... INTO OUTFILE`. To read the file back into the database, use `LOAD DATA INFILE`. The syntax of the `FIELDS` and `LINES` clauses is the same for both commands. Both clauses are optional, but `FIELDS` must precede `LINES` if both are specified.

If you specify a `FIELDS` clause, each of its subclauses (`TERMINATED BY`, `[OPTIONALLY] ENCLOSED BY`, and `ESCAPED BY`) is also optional, except that you must specify at least one of them.

If you don't specify a `FIELDS` clause, the defaults are the same as if you had written this:

    FIELDS TERMINATED BY '\t' ENCLOSED BY '' ESCAPED BY '\\'

If you don't specify a `LINES` clause, the default is the same as if you had written this:

    LINES TERMINATED BY '\n'

In other words, the defaults cause `LOAD DATA INFILE` to act as follows when reading input:

- Look for line boundaries at newlines.
- Break lines into fields at tabs.
- Do not expect fields to be enclosed within any quoting characters.
- Interpret occurrences of tab, newline, or ``\'` preceded by ``\'` as literal characters that are part of field values.

Conversely, the defaults cause `SELECT ... INTO OUTFILE` to act as follows when writing output:

- Write tabs between fields.
- Do not enclose fields within any quoting characters.
- Use ``\'` to escape instances of tab, newline or ``\'` that occur within field values.
- Write newlines at the ends of lines.

Note that to write `FIELDS ESCAPED BY '\\'`, you must specify two backslashes for the value to be read as a single backslash.

The `IGNORE number LINES` option can be used to ignore a header of column names at the start of the file:

    mysql> LOAD DATA INFILE "/tmp/file_name" INTO TABLE test IGNORE 1 LINES;

When you use `SELECT ... INTO OUTFILE` in tandem with `LOAD DATA INFILE` to write data from a database into a file and then read the file back into the database later, the field and line handling options for both commands must match. Otherwise, `LOAD DATA INFILE` will not interpret the contents of the file properly. Suppose you use `SELECT ... INTO OUTFILE` to write a file with fields delimited by commas:

    mysql> SELECT * INTO OUTFILE 'data.txt'
        ->          FIELDS TERMINATED BY ','
        ->          FROM ...;

To read the comma-delimited file back in, the correct statement would be:

    mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
        ->           FIELDS TERMINATED BY ',';

If instead you tried to read in the file with the statement shown here, it wouldn't work because it instructs `LOAD DATA INFILE` to look for tabs between fields:

    mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
        ->           FIELDS TERMINATED BY '\t';

The likely result is that each input line would be interpreted as a single field.

`LOAD DATA INFILE` can be used to read files obtained from external sources, too. For example, a file in dBASE format will have fields separated by commas and enclosed in double quotes. If lines in the file are terminated by newlines, the command shown here illustrates the field and line handling options you would use to load the file:

    mysql> LOAD DATA INFILE 'data.txt' INTO TABLE tbl_name
        ->           FIELDS TERMINATED BY ',' ENCLOSED BY '"'
        ->           LINES TERMINATED BY '\n';

Any of the field or line handling options may specify an empty string (`''`). If not empty, the `FIELDS [OPTIONALLY] ENCLOSED BY` and `FIELDS ESCAPED BY` values must be a single character. The `FIELDS TERMINATED BY` and `LINES TERMINATED BY` values may be more than one character. For example, to write lines that are terminated by carriage return-linefeed pairs, or to read a file containing such lines, specify a `LINES TERMINATED BY '\r\n'` clause.

For example, to read a file of jokes, that are separated with a line of `%%`, into a SQL table you can do:

    CREATE TABLE jokes (a INT NOT NULL AUTO_INCREMENT PRIMARY KEY, joke TEXT
    NOT NULL);
    LOAD DATA INFILE "/tmp/jokes.txt" INTO TABLE jokes FIELDS TERMINATED BY ""
    LINES TERMINATED BY "\n%%\n" (joke);

`FIELDS [OPTIONALLY] ENCLOSED BY` controls quoting of fields. For output (`SELECT ... INTO OUTFILE`), if you omit the word `OPTIONALLY`, all fields are enclosed by the `ENCLOSED BY` character. An example of such output (using a comma as the field delimiter) is shown here:

    "1","a string","100.20"
    "2","a string containing a , comma","102.20"
    "3","a string containing a \" quote","102.20"
    "4","a string containing a \", quote and comma","102.20"

If you specify `OPTIONALLY`, the `ENCLOSED BY` character is used only to enclose `CHAR` and `VARCHAR` fields:

    1,"a string",100.20
    2,"a string containing a , comma",102.20
    3,"a string containing a \" quote",102.20
    4,"a string containing a \", quote and comma",102.20

Note that occurrences of the `ENCLOSED BY` character within a field value are escaped by prefixing them with the `ESCAPED BY` character. Also note that if you specify an empty `ESCAPED BY` value, it is possible to generate output that cannot be read properly by `LOAD DATA INFILE`. For example, the output just shown above would appear as shown here if the escape character is empty. Observe that the second field in the fourth line contains a comma following the quote, which (erroneously) appears to terminate the field:

    1,"a string",100.20
    2,"a string containing a , comma",102.20
    3,"a string containing a " quote",102.20
    4,"a string containing a ", quote and comma",102.20

For input, the `ENCLOSED BY` character, if present, is stripped from the ends of field values. (This is true whether `OPTIONALLY` is specified; `OPTIONALLY` has no effect on input interpretation.) Occurrences of the `ENCLOSED BY` character preceded by the `ESCAPED BY` character are interpreted as part of the current field value. In addition, duplicated `ENCLOSED BY` characters occurring within fields are interpreted as single `ENCLOSED BY` characters if the field itself starts with that character. For example, if `ENCLOSED BY '"'` is specified, quotes are handled as shown here:

    "The ""BIG"" boss"  -> The "BIG" boss
    The "BIG" boss      -> The "BIG" boss
    The ""BIG"" boss    -> The ""BIG"" boss

`FIELDS ESCAPED BY` controls how to write or read special characters. If the `FIELDS ESCAPED BY` character is not empty, it is used to prefix the following characters on output:

- The `FIELDS ESCAPED BY` character
- The `FIELDS [OPTIONALLY] ENCLOSED BY` character
- The first character of the `FIELDS TERMINATED BY` and `LINES TERMINATED BY` values
- ASCII `0` (what is actually written following the escape character is ASCII `'0'`, not a zero-valued byte)

If the `FIELDS ESCAPED BY` character is empty, no characters are escaped. It is probably not a good idea to specify an empty escape character, particularly if field values in your data contain any of the characters in the list just given.

For input, if the `FIELDS ESCAPED BY` character is not empty, occurrences of that character are stripped and the following character is taken literally as part of a field value. The exceptions are an escaped ``0'` or ``N'` (for example, `\0` or `\N` if the escape character is ``\'`). These sequences are interpreted as ASCII `0` (a zero-valued byte) and `NULL`. See below for the rules on `NULL` handling.

For more information about ``\'`-escape syntax, see section 6.1.1 Literals: How to Write Strings and Numbers.

In certain cases, field and line handling options interact:

- If `LINES TERMINATED BY` is an empty string and `FIELDS TERMINATED BY` is non-empty, lines are also terminated with `FIELDS TERMINATED BY`.
- If the `FIELDS TERMINATED BY` and `FIELDS ENCLOSED BY` values are both empty (`''`), a fixed-row (non-delimited) format is used. With fixed-row format, no delimiters are used between fields. Instead, column values are written and read using the ``display'' widths of the columns. For example, if a column is declared as `INT(7)`, values for the column are written using 7-character fields. On input, values for the column are obtained by reading 7 characters. Fixed-row format also affects handling of `NULL` values; see below. Note that fixed-size format will not work if you are using a multi-byte character set.

Handling of `NULL` values varies, depending on the `FIELDS` and `LINES` options you use:

- For the default `FIELDS` and `LINES` values, `NULL` is written as `\N` for output and `\N` is read as `NULL` for input (assuming the `ESCAPED BY` character is ``\'`).
- If `FIELDS ENCLOSED BY` is not empty, a field containing the literal word `NULL` as its value is read as a `NULL` value (this differs from the word `NULL` enclosed within `FIELDS ENCLOSED BY` characters, which is read as the string `'NULL'`).
- If `FIELDS ESCAPED BY` is empty, `NULL` is written as the word `NULL`.
- With fixed-row format (which happens when `FIELDS TERMINATED BY` and `FIELDS ENCLOSED BY` are both empty), `NULL` is written as an empty string. Note that this causes both `NULL` values and empty strings in the table to be indistinguishable when written to the file because they are both written as empty strings. If you need to be able to tell the two apart when reading the file back in, you should not use fixed-row format.

Some cases are not supported by `LOAD DATA INFILE`:

- Fixed-size rows (`FIELDS TERMINATED BY` and `FIELDS ENCLOSED BY` both empty) and `BLOB` or `TEXT` columns.
- If you specify one separator that is the same as or a prefix of another, `LOAD DATA INFILE` won't be able to interpret the input properly. For example, the following `FIELDS` clause would cause problems:

      FIELDS TERMINATED BY '"' ENCLOSED BY '"'

- If `FIELDS ESCAPED BY` is empty, a field value that contains an occurrence of `FIELDS ENCLOSED BY` or `LINES TERMINATED BY` followed by the `FIELDS TERMINATED BY` value will cause `LOAD DATA INFILE` to stop reading a field or line too early. This happens because `LOAD DATA INFILE` cannot properly determine where the field or line value ends.

The following example loads all columns of the `persondata` table:

    mysql> LOAD DATA INFILE 'persondata.txt' INTO TABLE persondata;

No field list is specified, so `LOAD DATA INFILE` expects input rows to contain a field for each table column. The default `FIELDS` and `LINES` values are used.

If you wish to load only some of a table's columns, specify a field list:

    mysql> LOAD DATA INFILE 'persondata.txt'
        ->           INTO TABLE persondata (col1,col2,...);

You must also specify a field list if the order of the fields in the input file differs from the order of the columns in the table. Otherwise, MySQL cannot tell how to match up input fields with table columns.

If a row has too few fields, the columns for which no input field is present are set to default values. Default value assignment is described in section 6.5.3 `CREATE TABLE` Syntax.

An empty field value is interpreted differently than if the field value is missing:

- For string types, the column is set to the empty string.
- For numeric types, the column is set to `0`.
- For date and time types, the column is set to the appropriate ``zero'' value for the type. See section 6.2.2 Date and Time Types.

Note that these are the same values that result if you assign an empty string explicitly to a string, numeric, or date or time type explicitly in an `INSERT` or `UPDATE` statement.

`TIMESTAMP` columns are only set to the current date and time if there is a `NULL` value for the column, or (for the first `TIMESTAMP` column only) if the `TIMESTAMP` column is left out from the field list when a field list is specified.

If an input row has too many fields, the extra fields are ignored and the number of warnings is incremented.

`LOAD DATA INFILE` regards all input as strings, so you can't use numeric values for `ENUM` or `SET` columns the way you can with `INSERT` statements. All `ENUM` and `SET` values must be specified as strings!

If you are using the C API, you can get information about the query by calling the API function `mysql_info()` when the `LOAD DATA INFILE` query finishes. The format of the information string is shown here:

    Records: 1  Deleted: 0  Skipped: 0  Warnings: 0

Warnings occur under the same circumstances as when values are inserted via the `INSERT` statement (see section 6.4.3 `INSERT` Syntax), except that `LOAD DATA INFILE` also generates warnings when there are too few or too many fields in the input row. The warnings are not stored anywhere; the number of warnings can only be used as an indication if everything went well. If you get warnings and want to know exactly why you got them, one way to do this is to use `SELECT ... INTO OUTFILE` into another file and compare this to your original input file.

If you need `LOAD DATA` to read from a pipe, you can use the following trick:

    mkfifo /mysql/db/x/x
    chmod 666 /mysql/db/x/x
    cat < /dev/tcp/10.1.1.12/4711 > /nt/mysql/db/x/x
    mysql -e "LOAD DATA INFILE 'x' INTO TABLE x" x

If you are using a version of MySQL older than 3.23.25 you can only do the above with `LOAD DATA LOCAL INFILE`.

For more information about the efficiency of `INSERT` versus `LOAD DATA INFILE` and speeding up `LOAD DATA INFILE`, See section 5.2.9 Speed of `INSERT` Queries.

### 6.4.10 `DO` Syntax

    DO expression, [expression, ...]

Execute the expression but don't return any results. This is a shorthand of `SELECT expression, expression`, but has the advantage that it's slightly faster when you don't care about the result.

This is mainly useful with functions that has side effects, like `RELEASE_LOCK`.

## 6.5 Data Definition: `CREATE`, `DROP`, `ALTER`

### 6.5.1 `CREATE DATABASE` Syntax

    CREATE DATABASE [IF NOT EXISTS] db_name

`CREATE DATABASE` creates a database with the given name. Rules for allowable database names are given in section 6.1.2 Database, Table, Index, Column, and Alias Names. An error occurs if the database already exists and you didn't specify `IF NOT EXISTS`.

Databases in MySQL are implemented as directories containing files that correspond to tables in the database. Because there are no tables in a database when it is initially created, the `CREATE DATABASE` statement only creates a directory under the MySQL data directory.

You can also create databases with `mysqladmin`. See section 4.8 MySQL Client-Side Scripts and Utilities.

### 6.5.2 `DROP DATABASE` Syntax

    DROP DATABASE [IF EXISTS] db_name

`DROP DATABASE` drops all tables in the database and deletes the database. If you do a `DROP DATABASE` on a symbolic linked database, both the link and the original database is deleted. Be VERY careful with this command!

`DROP DATABASE` returns the number of files that were removed from the database directory. Normally, this is three times the number of tables, because normally each table corresponds to a ``.MYD'` file, a ``.MYI'` file, and a ``.frm'` file.

The `DROP DATABASE` command removes from the given database directory all files with the following extensions:

Ext | Ext | Ext | Ext
.BAK | .DAT | .HSH | .ISD
.ISM | .ISM | .MRG | .MYD
.MYI | .db | .frm |

All subdirectories that consists of 2 digits (`RAID` directories) are also removed.

In MySQL Version 3.22 or later, you can use the keywords `IF EXISTS` to prevent an error from occurring if the database doesn't exist.

You can also drop databases with `mysqladmin`. See section 4.8 MySQL Client-Side Scripts and Utilities.

### 6.5.3 `CREATE TABLE` Syntax

    CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name [(create_definition,...)]
    [table_options] [select_statement]

    create_definition:
      col_name type [NOT NULL | NULL] [DEFAULT default_value] [AUTO_INCREMENT]
                [PRIMARY KEY] [reference_definition]
      or    PRIMARY KEY (index_col_name,...)
      or    KEY [index_name] (index_col_name,...)
      or    INDEX [index_name] (index_col_name,...)
      or    UNIQUE [INDEX] [index_name] (index_col_name,...)
      or    FULLTEXT [INDEX] [index_name] (index_col_name,...)
      or    [CONSTRAINT symbol] FOREIGN KEY [index_name] (index_col_name,...)
                [reference_definition]
      or    CHECK (expr)

    type:
            TINYINT[(length)] [UNSIGNED] [ZEROFILL]
      or    SMALLINT[(length)] [UNSIGNED] [ZEROFILL]
      or    MEDIUMINT[(length)] [UNSIGNED] [ZEROFILL]
      or    INT[(length)] [UNSIGNED] [ZEROFILL]
      or    INTEGER[(length)] [UNSIGNED] [ZEROFILL]
      or    BIGINT[(length)] [UNSIGNED] [ZEROFILL]
      or    REAL[(length,decimals)] [UNSIGNED] [ZEROFILL]
      or    DOUBLE[(length,decimals)] [UNSIGNED] [ZEROFILL]
      or    FLOAT[(length,decimals)] [UNSIGNED] [ZEROFILL]
      or    DECIMAL(length,decimals) [UNSIGNED] [ZEROFILL]
      or    NUMERIC(length,decimals) [UNSIGNED] [ZEROFILL]
      or    CHAR(length) [BINARY]
      or    VARCHAR(length) [BINARY]
      or    DATE
      or    TIME
      or    TIMESTAMP
      or    DATETIME
      or    TINYBLOB
      or    BLOB
      or    MEDIUMBLOB
      or    LONGBLOB
      or    TINYTEXT
      or    TEXT
      or    MEDIUMTEXT
      or    LONGTEXT
      or    ENUM(value1,value2,value3,...)
      or    SET(value1,value2,value3,...)

    index_col_name:
            col_name [(length)]

    reference_definition:
            REFERENCES tbl_name [(index_col_name,...)]
                       [MATCH FULL | MATCH PARTIAL]
                       [ON DELETE reference_option]
                       [ON UPDATE reference_option]

    reference_option:
            RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT

    table_options:
    	TYPE = {BDB | HEAP | ISAM | InnoDB | MERGE | MRG_MYISAM | MYISAM }
    or	AUTO_INCREMENT = #
    or	AVG_ROW_LENGTH = #
    or	CHECKSUM = {0 | 1}
    or	COMMENT = "string"
    or	MAX_ROWS = #
    or	MIN_ROWS = #
    or	PACK_KEYS = {0 | 1 | DEFAULT}
    or	PASSWORD = "string"
    or	DELAY_KEY_WRITE = {0 | 1}
    or      ROW_FORMAT= { default | dynamic | fixed | compressed }
    or	RAID_TYPE= {1 | STRIPED | RAID0 } RAID_CHUNKS=#  RAID_CHUNKSIZE=#
    or	UNION = (table_name,[table_name...])
    or	INSERT_METHOD= {NO | FIRST | LAST }
    or      DATA DIRECTORY="absolute path to directory"
    or      INDEX DIRECTORY="absolute path to directory"

    select_statement:
    	[IGNORE | REPLACE] SELECT ...  (Some legal select statement)

`CREATE TABLE` creates a table with the given name in the current database. Rules for allowable table names are given in section 6.1.2 Database, Table, Index, Column, and Alias Names. An error occurs if there is no current database or if the table already exists.

In MySQL Version 3.22 or later, the table name can be specified as `db_name.tbl_name`. This works regardless of whether there is a current database.

From MySQL Version 3.23, you can use the `TEMPORARY` keyword when you create a table. The name is restricted to the current connection, and the temporary table will automatically be deleted when the connection is closed. This means that two different connections can both use the same temporary table name without conflicting with each other or with an existing table of the same name. (The existing table is hidden until the temporary table is deleted.). From MySQL 4.0.2 one must have the `CREATE TEMPORARY TABLES` privilege to be able to create temporary tables.

In MySQL Version 3.23 or later, you can use the keywords `IF NOT EXISTS` so that an error does not occur if the table already exists. Note that there is no verification that the table structures are identical.

Each table `tbl_name` is represented by some files in the database directory. In the case of MyISAM-type tables you will get:

File | Purpose
`tbl_name.frm` | Table definition (form) file
`tbl_name.MYD` | Datafile
`tbl_name.MYI` | Index file

For more information on the properties of the various column types, see section 6.2 Column Types:

- If neither `NULL` nor `NOT NULL` is specified, the column is treated as though `NULL` had been specified.
- An integer column may have the additional attribute `AUTO_INCREMENT`. When you insert a value of `NULL` (recommended) or `0` into an `AUTO_INCREMENT` column, the column is set to `value+1`, where `value` is the largest value for the column currently in the table. `AUTO_INCREMENT` sequences begin with `1`. See section 8.4.3.126 `mysql_insert_id()`. If you delete the row containing the maximum value for an `AUTO_INCREMENT` column, the value will be reused with an `ISAM`, or `BDB` table but not with a `MyISAM` or `InnoDB` table. If you delete all rows in the table with `DELETE FROM table_name` (without a `WHERE`) in `AUTOCOMMIT` mode, the sequence starts over for all table types. Note: there can be only one `AUTO_INCREMENT` column per table, and it must be indexed. MySQL Version 3.23 will also only work properly if the `AUTO_INCREMENT` column only has positive values. Inserting a negative number is regarded as inserting a very large positive number. This is done to avoid precision problems when numbers 'wrap' over from positive to negative and also to ensure that one doesn't accidentally get an `AUTO_INCREMENT` column that contains 0. In MyISAM and BDB tables you can specify `AUTO_INCREMENT` secondary column in a multi-column key. See section 3.5.9 Using `AUTO_INCREMENT`. To make MySQL compatible with some ODBC applications, you can find the last inserted row with the following query:

      SELECT * FROM tbl_name WHERE auto_col IS NULL

- `CREATE TABLE` automatically commits the current InnoDB transaction if the MySQL binlogging is used.
- `NULL` values are handled differently for `TIMESTAMP` columns than for other column types. You cannot store a literal `NULL` in a `TIMESTAMP` column; setting the column to `NULL` sets it to the current date and time. Because `TIMESTAMP` columns behave this way, the `NULL` and `NOT NULL` attributes do not apply in the normal way and are ignored if you specify them. On the other hand, to make it easier for MySQL clients to use `TIMESTAMP` columns, the server reports that such columns may be assigned `NULL` values (which is true), even though `TIMESTAMP` never actually will contain a `NULL` value. You can see this when you use `DESCRIBE tbl_name` to get a description of your table. Note that setting a `TIMESTAMP` column to `0` is not the same as setting it to `NULL`, because `0` is a valid `TIMESTAMP` value.
- A `DEFAULT` value has to be a constant, it can not be a function or an expression. If no `DEFAULT` value is specified for a column, MySQL automatically assigns one. If the column may take `NULL` as a value, the default value is `NULL`. If the column is declared as `NOT NULL`, the default value depends on the column type:
  - For numeric types other than those declared with the `AUTO_INCREMENT` attribute, the default is `0`. For an `AUTO_INCREMENT` column, the default value is the next value in the sequence.
  - For date and time types other than `TIMESTAMP`, the default is the appropriate zero value for the type. For the first `TIMESTAMP` column in a table, the default value is the current date and time. See section 6.2.2 Date and Time Types.
  - For string types other than `ENUM`, the default value is the empty string. For `ENUM`, the default is the first enumeration value (if you haven't explicitly specified another default value with the `DEFAULT` directive).
  Default values must be constants. This means, for example, that you cannot set the default for a date column to be the value of a function such as `NOW()` or `CURRENT_DATE`.
- `KEY` is a synonym for `INDEX`.
- In MySQL, a `UNIQUE` key can have only distinct values. An error occurs if you try to add a new row with a key that matches an existing row.
- A `PRIMARY KEY` is a unique `KEY` with the extra constraint that all key columns must be defined as `NOT NULL`. In MySQL the key is named `PRIMARY`. A table can have only one `PRIMARY KEY`. If you don't have a `PRIMARY KEY` and some applications ask for the `PRIMARY KEY` in your tables, MySQL will return the first `UNIQUE` key, which doesn't have any `NULL` columns, as the `PRIMARY KEY`.
- A `PRIMARY KEY` can be a multiple-column index. However, you cannot create a multiple-column index using the `PRIMARY KEY` key attibute in a column specification. Doing so will mark only that single column as primary. You must use the `PRIMARY KEY(index_col_name, ...)` syntax.
- If the `PRIMARY` or `UNIQUE` key consists of only one column and this is of type integer, you can also refer to it as `_rowid` (new in Version 3.23.11).
- If you don't assign a name to an index, the index will be assigned the same name as the first `index_col_name`, with an optional suffix (`_2`, `_3`, `...`) to make it unique. You can see index names for a table using `SHOW INDEX FROM tbl_name`. See section 4.5.6.1 Retrieving information about Database, Tables, Columns, and Indexes.
- Only the `MyISAM`, `InnoDB`, and `BDB` table types support indexes on columns that can have `NULL` values. In other cases you must declare such columns `NOT NULL` or an error results.
- With `col_name(length)` syntax, you can specify an index that uses only a part of a `CHAR` or `VARCHAR` column. This can make the index file much smaller. See section 5.4.4 Column Indexes.
- Only the `MyISAM` table type supports indexing on `BLOB` and `TEXT` columns. When putting an index on a `BLOB` or `TEXT` column you MUST always specify the length of the index:

      CREATE TABLE test (blob_col BLOB, INDEX(blob_col(10)));

- When you use `ORDER BY` or `GROUP BY` with a `TEXT` or `BLOB` column, only the first `max_sort_length` bytes are used. See section 6.2.3.2 The `BLOB` and `TEXT` Types.
- In MySQL Version 3.23.23 or later, you can also create special `FULLTEXT` indexes. They are used for full-text search. Only the `MyISAM` table type supports `FULLTEXT` indexes. They can be created only from `VARCHAR` and `TEXT` columns. Indexing always happens over the entire column, partial indexing is not supported. See section 6.8 MySQL Full-text Search for details of operation.
- In MySQL Version 3.23.44 or later, `InnoDB` tables support checking of foreign key constraints. See section 7.5 `InnoDB` Tables. Note that the `FOREIGN KEY` syntax in InnoDB is more restricted than the syntax presented above. InnoDB does not allow `index_name` to be specified, and the columns of the referenced table always have to be explicitly named. Of actions, InnoDB supports only `ON DELETE CASCADE` and `ON DELETE SET NULL`. See the InnoDB manual section for the precise syntax. For other table types, MySQL Server does parse the `FOREIGN KEY`, `CHECK`, and `REFERENCES` syntax in `CREATE TABLE` commands, but without further action being taken. See section 1.7.4.5 Foreign Keys.
- Each `NULL` column takes one bit extra, rounded up to the nearest byte.
- The maximum record length in bytes can be calculated as follows:

      row length = 1
                   + (sum of column lengths)
                   + (number of NULL columns + 7)/8
                   + (number of variable-length columns)

- The `table_options` and `SELECT` options are only implemented in MySQL Version 3.23 and above. The different table types are:

  Table type | Description
  BDB or BerkeleyDB | Transaction-safe tables with page locking. See section 7.6 `BDB` or `BerkeleyDB` Tables.
  HEAP | The data for this table is only stored in memory. See section 7.4 `HEAP` Tables.
  ISAM | The original storage engine. See section 7.3 `ISAM` Tables.
  InnoDB | Transaction-safe tables with row locking. See section 7.5 `InnoDB` Tables.
  MERGE | A collection of MyISAM tables used as one table. See section 7.2 `MERGE` Tables.
  MRG_MyISAM | An alias for MERGE tables
  MyISAM | The new binary portable storage engine that is replacing ISAM. See section 7.1 `MyISAM` Tables.

  See section 7 MySQL Table Types. If a table type is specified, and that particular type is not available, MySQL will choose the closest table type to the one that you have specified. For example, if `TYPE=BDB` is specified, and that distribution of MySQL does not support `BDB` tables, the table will be created as `MyISAM` instead. The other table options are used to optimise the behaviour of the table. In most cases, you don't have to specify any of them. The options work for all table types, if not otherwise indicated:

  Option | Description
  `AUTO_INCREMENT` | The next `AUTO_INCREMENT` value you want to set for your table (MyISAM).
  `AVG_ROW_LENGTH` | An approximation of the average row length for your table. You only need to set this for large tables with variable size records.
  `CHECKSUM` | Set this to 1 if you want MySQL to maintain a checksum for all rows (makes the table a little slower to update but makes it easier to find corrupted tables) (MyISAM).
  `COMMENT` | A 60-character comment for your table.
  `MAX_ROWS` | Max number of rows you plan to store in the table.
  `MIN_ROWS` | Minimum number of rows you plan to store in the table.
  `PACK_KEYS` | Set this to 1 if you want to have a smaller index. This usually makes updates slower and reads faster (MyISAM, ISAM). Setting this to 0 will disable all packing of keys. Setting this to `DEFAULT` (MySQL 4.0) will tell the storage engine to only pack long `CHAR`/`VARCHAR` columns.
  `PASSWORD` | Encrypt the ``.frm'` file with a password. This option doesn't do anything in the standard MySQL version.
  `DELAY_KEY_WRITE` | Set this to 1 if want to delay key table updates until the table is closed (MyISAM).
  `ROW_FORMAT` | Defines how the rows should be stored. Currently this option only works with MyISAM tables, which supports the `DYNAMIC` and `FIXED` row formats. See section 7.1.2 `MyISAM` Table Formats.

  When you use a `MyISAM` table, MySQL uses the product of `max_rows * avg_row_length` to decide how big the resulting table will be. If you don't specify any of the above options, the maximum size for a table will be 4G (or 2G if your operating systems only supports 2G tables). The reason for this is just to keep down the pointer sizes to make the index smaller and faster if you don't really need big files. If you don't use `PACK_KEYS`, the default is to only pack strings, not numbers. If you use `PACK_KEYS=1`, numbers will be packed as well. When packing binary number keys, MySQL will use prefix compression. This means that you will only get a big benefit of this if you have many numbers that are the same. Prefix compression means that every key needs one extra byte to indicate how many bytes of the previous key are the same for the next key (note that the pointer to the row is stored in high-byte-first-order directly after the key, to improve compression). This means that if you have many equal keys on two rows in a row, all following 'same' keys will usually only take 2 bytes (including the pointer to the row). Compare this to the ordinary case where the following keys will take storage_size_for_key + pointer_size (usually 4). On the other hand, if all keys are totally different, you will lose 1 byte per key, if the key isn't a key that can have `NULL` values. (In this case the packed key length will be stored in the same byte that is used to mark if a key is `NULL`.)
- If you specify a `SELECT` after the `CREATE` statement, MySQL will create new fields for all elements in the `SELECT`. For example:

      mysql> CREATE TABLE test (a INT NOT NULL AUTO_INCREMENT,
          ->        PRIMARY KEY (a), KEY(b))
          ->        TYPE=MyISAM SELECT b,c FROM test2;

  This will create a `MyISAM` table with three columns, a, b, and c. Notice that the columns from the `SELECT` statement are appended to the right side of the table, not overlapped onto it. Take the following example:

      mysql> SELECT * FROM foo;
      +---+
      | n |
      +---+
      | 1 |
      +---+

      mysql> CREATE TABLE bar (m INT) SELECT n FROM foo;
      Query OK, 1 row affected (0.02 sec)
      Records: 1  Duplicates: 0  Warnings: 0

      mysql> SELECT * FROM bar;
      +------+---+
      | m    | n |
      +------+---+
      | NULL | 1 |
      +------+---+
      1 row in set (0.00 sec)

  For each row in table `foo`, a row is inserted in `bar` with the values from `foo` and default values for the new columns. `CREATE TABLE ... SELECT` will not automatically create any indexes for you. This is done intentionally to make the command as flexible as possible. If you want to have indexes in the created table, you should specify these before the `SELECT` statement:

      mysql> CREATE TABLE bar (UNIQUE (n)) SELECT n FROM foo;

  If any errors occur while copying the data to the table, it will automatically be deleted. To ensure that the update log/binary log can be used to re-create the original tables, MySQL will not allow concurrent inserts during `CREATE TABLE ... SELECT`.
- The `RAID_TYPE` option will help you to break the 2G/4G limit for the MyISAM datafile (not the index file) on operating systems that don't support big files. Note that this option is not recommended for filesystem that supports big files! You can get more speed from the I/O bottleneck by putting `RAID` directories on different physical disks. `RAID_TYPE` will work on any OS, as long as you have configured MySQL with `--with-raid`. For now the only allowed `RAID_TYPE` is `STRIPED` (`1` and `RAID0` are aliases for this). If you specify `RAID_TYPE=STRIPED` for a `MyISAM` table, `MyISAM` will create `RAID_CHUNKS` subdirectories named 00, 01, 02 in the database directory. In each of these directories `MyISAM` will create a `table_name.MYD`. When writing data to the datafile, the `RAID` handler will map the first `RAID_CHUNKSIZE` *1024 bytes to the first file, the next `RAID_CHUNKSIZE` *1024 bytes to the next file and so on.
- `UNION` is used when you want to use a collection of identical tables as one. This only works with `MERGE` tables. See section 7.2 `MERGE` Tables. For the moment you need to have `SELECT`, `UPDATE`, and `DELETE` privileges on the tables you map to a `MERGE` table. All mapped tables must be in the same database as the `MERGE` table.
- If you want to insert data in a `MERGE` table, you have to specify with `INSERT_METHOD` into with table the row should be inserted. See section 7.2 `MERGE` Tables. This option was introduced in MySQL 4.0.0.
- In the created table the `PRIMARY` key will be placed first, followed by all `UNIQUE` keys and then the normal keys. This helps the MySQL optimiser to prioritise which key to use and also more quickly detect duplicated `UNIQUE` keys.
- By using `DATA DIRECTORY="directory"` or `INDEX DIRECTORY="directory"` you can specify where the storage engine should put it's table and index files. Note that the directory should be a full path to the directory (not relative path). This only works for `MyISAM` tables in `MySQL` 4.0, when you are not using the `--skip-symlink` option. See section 5.6.1.2 Using Symbolic Links for Tables.

#### 6.5.3.1 Silent Column Specification Changes

In some cases, MySQL silently changes a column specification from that given in a `CREATE TABLE` statement. (This may also occur with `ALTER TABLE`.):

- `VARCHAR` columns with a length less than four are changed to `CHAR`.
- If any column in a table has a variable length, the entire row is variable-length as a result. Therefore, if a table contains any variable-length columns (`VARCHAR`, `TEXT`, or `BLOB`), all `CHAR` columns longer than three characters are changed to `VARCHAR` columns. This doesn't affect how you use the columns in any way; in MySQL, `VARCHAR` is just a different way to store characters. MySQL performs this conversion because it saves space and makes table operations faster. See section 7 MySQL Table Types.
- `TIMESTAMP` display sizes must be even and in the range from 2 to 14. If you specify a display size of 0 or greater than 14, the size is coerced to 14. Odd-valued sizes in the range from 1 to 13 are coerced to the next higher even number.
- You cannot store a literal `NULL` in a `TIMESTAMP` column; setting it to `NULL` sets it to the current date and time. Because `TIMESTAMP` columns behave this way, the `NULL` and `NOT NULL` attributes do not apply in the normal way and are ignored if you specify them. `DESCRIBE tbl_name` always reports that a `TIMESTAMP` column may be assigned `NULL` values.
- MySQL maps certain column types used by other SQL database vendors to MySQL types. See section 6.2.5 Using Column Types from Other Database Engines.

If you want to see whether MySQL used a column type other than the one you specified, issue a `DESCRIBE tbl_name` statement after creating or altering your table.

Certain other column type changes may occur if you compress a table using `myisampack`. See section 7.1.2.3 Compressed Table Characteristics.

### 6.5.4 `ALTER TABLE` Syntax

    ALTER [IGNORE] TABLE tbl_name alter_spec [, alter_spec ...]

    alter_specification:
            ADD [COLUMN] create_definition [FIRST | AFTER column_name ]
      or    ADD [COLUMN] (create_definition, create_definition,...)
      or    ADD INDEX [index_name] (index_col_name,...)
      or    ADD PRIMARY KEY (index_col_name,...)
      or    ADD UNIQUE [index_name] (index_col_name,...)
      or    ADD FULLTEXT [index_name] (index_col_name,...)
      or	ADD [CONSTRAINT symbol] FOREIGN KEY [index_name] (index_col_name,...)
                [reference_definition]
      or    ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
      or    CHANGE [COLUMN] old_col_name create_definition
                   [FIRST | AFTER column_name]
      or    MODIFY [COLUMN] create_definition [FIRST | AFTER column_name]
      or    DROP [COLUMN] col_name
      or    DROP PRIMARY KEY
      or    DROP INDEX index_name
      or    DISABLE KEYS
      or    ENABLE KEYS
      or    RENAME [TO] new_tbl_name
      or    ORDER BY col
      or    table_options

`ALTER TABLE` allows you to change the structure of an existing table. For example, you can add or delete columns, create or destroy indexes, change the type of existing columns, or rename columns or the table itself. You can also change the comment for the table and type of the table. See section 6.5.3 `CREATE TABLE` Syntax.

If you use `ALTER TABLE` to change a column specification but `DESCRIBE tbl_name` indicates that your column was not changed, it is possible that MySQL ignored your modification for one of the reasons described in section 6.5.3.1 Silent Column Specification Changes. For example, if you try to change a `VARCHAR` column to `CHAR`, MySQL will still use `VARCHAR` if the table contains other variable-length columns.

`ALTER TABLE` works by making a temporary copy of the original table. The alteration is performed on the copy, then the original table is deleted and the new one is renamed. This is done in such a way that all updates are automatically redirected to the new table without any failed updates. While `ALTER TABLE` is executing, the original table is readable by other clients. Updates and writes to the table are stalled until the new table is ready.

Note that if you use any other option to `ALTER TABLE` than `RENAME`, MySQL will always create a temporary table, even if the data wouldn't strictly need to be copied (like when you change the name of a column). We plan to fix this in the future, but as one doesn't normally do `ALTER TABLE` that often this isn't that high on our TODO. For MyISAM tables, you can speed up the index recreation part (which is the slowest part of the recreation process) by setting the `myisam_sort_buffer_size` variable to a high value.

- To use `ALTER TABLE`, you need `ALTER`, `INSERT`, and `CREATE` privileges on the table.
- `IGNORE` is a MySQL extension to ANSI SQL92. It controls how `ALTER TABLE` works if there are duplicates on unique keys in the new table. If `IGNORE` isn't specified, the copy is aborted and rolled back. If `IGNORE` is specified, then for rows with duplicates on a unique key, only the first row is used; the others are deleted.
- You can issue multiple `ADD`, `ALTER`, `DROP`, and `CHANGE` clauses in a single `ALTER TABLE` statement. This is a MySQL extension to ANSI SQL92, which allows only one of each clause per `ALTER TABLE` statement.
- `CHANGE col_name`, `DROP col_name`, and `DROP INDEX` are MySQL extensions to ANSI SQL92.
- `MODIFY` is an Oracle extension to `ALTER TABLE`.
- The optional word `COLUMN` is a pure noise word and can be omitted.
- If you use `ALTER TABLE tbl_name RENAME TO new_name` without any other options, MySQL simply renames the files that correspond to the table `tbl_name`. There is no need to create the temporary table. See section 6.5.5 `RENAME TABLE` Syntax.
- `create_definition` clauses use the same syntax for `ADD` and `CHANGE` as for `CREATE TABLE`. Note that this syntax includes the column name, not just the column type. See section 6.5.3 `CREATE TABLE` Syntax.
- You can rename a column using a `CHANGE old_col_name create_definition` clause. To do so, specify the old and new column names and the type that the column currently has. For example, to rename an `INTEGER` column from `a` to `b`, you can do this:

      mysql> ALTER TABLE t1 CHANGE a b INTEGER;

  If you want to change a column's type but not the name, `CHANGE` syntax still requires two column names even if they are the same. For example:

      mysql> ALTER TABLE t1 CHANGE b b BIGINT NOT NULL;

  However, as of MySQL Version 3.22.16a, you can also use `MODIFY` to change a column's type without renaming it:

      mysql> ALTER TABLE t1 MODIFY b BIGINT NOT NULL;

- If you use `CHANGE` or `MODIFY` to shorten a column for which an index exists on part of the column (for instance, if you have an index on the first 10 characters of a `VARCHAR` column), you cannot make the column shorter than the number of characters that are indexed.
- When you change a column type using `CHANGE` or `MODIFY`, MySQL tries to convert data to the new type as well as possible.
- In MySQL Version 3.22 or later, you can use `FIRST` or `ADD ... AFTER col_name` to add a column at a specific position within a table row. The default is to add the column last. From MySQL Version 4.0.1, you can also use the `FIRST` and `AFTER` keywords in `CHANGE` or `MODIFY`.
- `ALTER COLUMN` specifies a new default value for a column or removes the old default value. If the old default is removed and the column can be `NULL`, the new default is `NULL`. If the column cannot be `NULL`, MySQL assigns a default value, as described in section 6.5.3 `CREATE TABLE` Syntax.
- `DROP INDEX` removes an index. This is a MySQL extension to ANSI SQL92. See section 6.5.8 `DROP INDEX` Syntax.
- If columns are dropped from a table, the columns are also removed from any index of which they are a part. If all columns that make up an index are dropped, the index is dropped as well.
- If a table contains only one column, the column cannot be dropped. If what you intend is to remove the table, use `DROP TABLE` instead.
- `DROP PRIMARY KEY` drops the primary index. If no such index exists, it drops the first `UNIQUE` index in the table. (MySQL marks the first `UNIQUE` key as the `PRIMARY KEY` if no `PRIMARY KEY` was specified explicitly.) If you add a `UNIQUE INDEX` or `PRIMARY KEY` to a table, this is stored before any not `UNIQUE` index so that MySQL can detect duplicate keys as early as possible.
- `ORDER BY` allows you to create the new table with the rows in a specific order. Note that the table will not remain in this order after inserts and deletes. In some cases, it may make sorting easier for MySQL if the table is in order by the column that you wish to order it by later. This option is mainly useful when you know that you are mostly going to query the rows in a certain order; by using this option after big changes to the table, you may be able to get higher performance.
- If you use `ALTER TABLE` on a `MyISAM` table, all non-unique indexes are created in a separate batch (like in `REPAIR`). This should make `ALTER TABLE` much faster when you have many indexes.
- Since MySQL 4.0 the above feature can be activated explicitly. `ALTER TABLE ... DISABLE KEYS` makes MySQL to stop updating non-unique indexes for `MyISAM` table. `ALTER TABLE ... ENABLE KEYS` then should be used to recreate missing indexes. As MySQL does it with special algorithm which is much faster then inserting keys one by one, disabling keys could give a considerable speedup on bulk inserts.
- With the C API function `mysql_info()`, you can find out how many records were copied, and (when `IGNORE` is used) how many records were deleted due to duplication of unique key values.
- The `FOREIGN KEY`, `CHECK`, and `REFERENCES` clauses don't actually do anything, except for InnoDB type tables which support `ADD CONSTRAINT FOREIGN KEY (...) REFERENCES ... (...)`. Note that InnoDB does not allow an `index_name` to be specified. See section 7.5 `InnoDB` Tables. The syntax for other table types is provided only for compatibility, to make it easier to port code from other SQL servers and to run applications that create tables with references. See section 1.7.4 MySQL Differences Compared to ANSI SQL92.

Here is an example that shows some of the uses of `ALTER TABLE`. We begin with a table `t1` that is created as shown here:

    mysql> CREATE TABLE t1 (a INTEGER,b CHAR(10));

To rename the table from `t1` to `t2`:

    mysql> ALTER TABLE t1 RENAME t2;

To change column `a` from `INTEGER` to `TINYINT NOT NULL` (leaving the name the same), and to change column `b` from `CHAR(10)` to `CHAR(20)` as well as renaming it from `b` to `c`:

    mysql> ALTER TABLE t2 MODIFY a TINYINT NOT NULL, CHANGE b c CHAR(20);

To add a new `TIMESTAMP` column named `d`:

    mysql> ALTER TABLE t2 ADD d TIMESTAMP;

To add an index on column `d`, and make column `a` the primary key:

    mysql> ALTER TABLE t2 ADD INDEX (d), ADD PRIMARY KEY (a);

To remove column `c`:

    mysql> ALTER TABLE t2 DROP COLUMN c;

To add a new `AUTO_INCREMENT` integer column named `c`:

    mysql> ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT,
               ADD INDEX (c);

Note that we indexed `c`, because `AUTO_INCREMENT` columns must be indexed, and also that we declare `c` as `NOT NULL`, because indexed columns cannot be `NULL`.

When you add an `AUTO_INCREMENT` column, column values are filled in with sequence numbers for you automatically. You can set the first sequence number by executing `SET INSERT_ID=#` before `ALTER TABLE` or using the `AUTO_INCREMENT = #` table option. See section 5.5.6 `SET` Syntax.

With MyISAM tables, if you don't change the `AUTO_INCREMENT` column, the sequence number will not be affected. If you drop an `AUTO_INCREMENT` column and then add another `AUTO_INCREMENT` column, the numbers will start from 1 again.

See section A.6.1 Problems with `ALTER TABLE`..

### 6.5.5 `RENAME TABLE` Syntax

    RENAME TABLE tbl_name TO new_tbl_name[, tbl_name2 TO new_tbl_name2,...]

The rename is done atomically, which means that no other thread can access any of the tables while the rename is running. This makes it possible to replace a table with an empty one:

    CREATE TABLE new_table (...);
    RENAME TABLE old_table TO backup_table, new_table TO old_table;

The rename is done from left to right, which means that if you want to swap two tables names, you have to:

    RENAME TABLE old_table    TO backup_table,
                 new_table    TO old_table,
                 backup_table TO new_table;

As long as two databases are on the same disk you can also rename from one database to another:

    RENAME TABLE current_db.tbl_name TO other_db.tbl_name;

When you execute `RENAME`, you can't have any locked tables or active transactions. You must also have the `ALTER` and `DROP` privileges on the original table, and the `CREATE` and `INSERT` privileges on the new table.

If MySQL encounters any errors in a multiple-table rename, it will do a reverse rename for all renamed tables to get everything back to the original state.

`RENAME TABLE` was added in MySQL 3.23.23.

### 6.5.6 `DROP TABLE` Syntax

    DROP [TEMPORARY] TABLE [IF EXISTS] tbl_name [, tbl_name,...] [RESTRICT | CASCADE]

`DROP TABLE` removes one or more tables. All table data and the table definition are removed, so be careful with this command!

In MySQL Version 3.22 or later, you can use the keywords `IF EXISTS` to prevent an error from occurring for tables that don't exist.

`RESTRICT` and `CASCADE` are allowed to make porting easier. For the moment they don't do anything.

Note: `DROP TABLE` will automatically commit current active transaction.

`TEMPORARY` is for the moment ignored; In the near future it will be a way to ensure that you don't accidently drop a real table.

### 6.5.7 `CREATE INDEX` Syntax

    CREATE [UNIQUE|FULLTEXT] INDEX index_name
           ON tbl_name (col_name[(length)],... )

The `CREATE INDEX` statement doesn't do anything in MySQL prior to Version 3.22. In Version 3.22 or later, `CREATE INDEX` is mapped to an `ALTER TABLE` statement to create indexes. See section 6.5.4 `ALTER TABLE` Syntax.

Normally, you create all indexes on a table at the time the table itself is created with `CREATE TABLE`. See section 6.5.3 `CREATE TABLE` Syntax. `CREATE INDEX` allows you to add indexes to existing tables.

A column list of the form `(col1,col2,...)` creates a multiple-column index. Index values are formed by concatenating the values of the given columns.

For `CHAR` and `VARCHAR` columns, indexes can be created that use only part of a column, using `col_name(length)` syntax. (On `BLOB` and `TEXT` columns the length is required.) The statement shown here creates an index using the first 10 characters of the `name` column:

    mysql> CREATE INDEX part_of_name ON customer (name(10));

Because most names usually differ in the first 10 characters, this index should not be much slower than an index created from the entire `name` column. Also, using partial columns for indexes can make the index file much smaller, which could save a lot of disk space and might also speed up `INSERT` operations!

Note that you can only add an index on a column that can have `NULL` values or on a `BLOB`/`TEXT` column if you are using MySQL Version 3.23.2 or newer and are using the `MyISAM` table type.

For more information about how MySQL uses indexes, see section 5.4.3 How MySQL Uses Indexes.

`FULLTEXT` indexes can index only `VARCHAR` and `TEXT` columns, and only in `MyISAM` tables. `FULLTEXT` indexes are available in MySQL Version 3.23.23 and later. section 6.8 MySQL Full-text Search.

### 6.5.8 `DROP INDEX` Syntax

    DROP INDEX index_name ON tbl_name

`DROP INDEX` drops the index named `index_name` from the table `tbl_name`. `DROP INDEX` doesn't do anything in MySQL prior to Version 3.22. In Version 3.22 or later, `DROP INDEX` is mapped to an `ALTER TABLE` statement to drop the index. See section 6.5.4 `ALTER TABLE` Syntax.

## 6.6 Basic MySQL User Utility Commands

### 6.6.1 `USE` Syntax

    USE db_name

The `USE db_name` statement tells MySQL to use the `db_name` database as the default database for subsequent queries. The database remains current until the end of the session or until another `USE` statement is issued:

    mysql> USE db1;
    mysql> SELECT COUNT(*) FROM mytable;      # selects from db1.mytable
    mysql> USE db2;
    mysql> SELECT COUNT(*) FROM mytable;      # selects from db2.mytable

Making a particular database current by means of the `USE` statement does not preclude you from accessing tables in other databases. The following example accesses the `author` table from the `db1` database and the `editor` table from the `db2` database:

    mysql> USE db1;
    mysql> SELECT author_name,editor_name FROM author,db2.editor
        ->        WHERE author.editor_id = db2.editor.editor_id;

The `USE` statement is provided for Sybase compatibility.

### 6.6.2 `DESCRIBE` Syntax (Get Information About Columns)

    {DESCRIBE | DESC} tbl_name [col_name | wild]

`DESCRIBE` is a shortcut for `SHOW COLUMNS FROM`. See section 4.5.6.1 Retrieving information about Database, Tables, Columns, and Indexes.

`DESCRIBE` provides information about a table's columns. `col_name` may be a column name or a string containing the SQL ``%'` and ``_'` wildcard characters.

If the column types are different from what you expect them to be based on a `CREATE TABLE` statement, note that MySQL sometimes changes column types. See section 6.5.3.1 Silent Column Specification Changes.

This statement is provided for Oracle compatibility.

The `SHOW` statement provides similar information. See section 4.5.6 `SHOW` Syntax.

## 6.7 MySQL Transactional and Locking Commands

### 6.7.1 `BEGIN/COMMIT/ROLLBACK` Syntax

By default, MySQL runs in `autocommit` mode. This means that as soon as you execute an update, MySQL will store the update on disk.

If you are using transactions safe tables (like `InnoDB`, `BDB`, you can put MySQL into non-`autocommit` mode with the following command:

    SET AUTOCOMMIT=0

After this you must use `COMMIT` to store your changes to disk or `ROLLBACK` if you want to ignore the changes you have made since the beginning of your transaction.

If you want to switch from `AUTOCOMMIT` mode for one series of statements, you can use the `BEGIN` or `BEGIN WORK` statement:

    BEGIN;
    SELECT @A:=SUM(salary) FROM table1 WHERE type=1;
    UPDATE table2 SET summmary=@A WHERE type=1;
    COMMIT;

Note that if you are using non-transaction-safe tables, the changes will be stored at once, independent of the status of the `autocommit` mode.

If you do a `ROLLBACK` when you have updated a non-transactional table you will get an error (`ER_WARNING_NOT_COMPLETE_ROLLBACK`) as a warning. All transactional safe tables will be restored but any non-transactional table will not change.

If you are using `BEGIN` or `SET AUTOCOMMIT=0`, you should use the MySQL binary log for backups instead of the older update log. Transactions are stored in the binary log in one chunk, upon `COMMIT`, to ensure that transactions which are rolled back are not stored. See section 4.9.4 The Binary Update Log.

The following commands automatically end a transaction (as if you had done a `COMMIT` before executing the command):

Command | Command | Command
`ALTER TABLE` | `BEGIN` | `CREATE INDEX`
`DROP DATABASE` | `DROP TABLE` | `RENAME TABLE`
`TRUNCATE` |  |

You can change the isolation level for transactions with `SET TRANSACTION ISOLATION LEVEL ...`. See section 6.7.3 `SET TRANSACTION` Syntax.

### 6.7.2 `LOCK TABLES/UNLOCK TABLES` Syntax

    LOCK TABLES tbl_name [AS alias] {READ [LOCAL] | [LOW_PRIORITY] WRITE}
                [, tbl_name [AS alias] {READ [LOCAL] | [LOW_PRIORITY] WRITE} ...]
    ...
    UNLOCK TABLES

`LOCK TABLES` locks tables for the current thread. `UNLOCK TABLES` releases any locks held by the current thread. All tables that are locked by the current thread are automatically unlocked when the thread issues another `LOCK TABLES`, or when the connection to the server is closed.

To use `LOCK TABLES` in MySQL 4.0.2 you need the global `LOCK TABLES` privilege and a `SELECT` privilege on the involved tables. In MySQL 3.23 you need to have `SELECT`, `insert`, `DELETE` and `UPDATE` privileges for the tables.

The main reasons to use `LOCK TABLES` are for emulating transactions or getting more speed when updating tables. This is explained in more detail later.

If a thread obtains a `READ` lock on a table, that thread (and all other threads) can only read from the table. If a thread obtains a `WRITE` lock on a table, then only the thread holding the lock can read from or write to the table. Other threads are blocked.

The difference between `READ LOCAL` and `READ` is that `READ LOCAL` allows non-conflicting `INSERT` statements to execute while the lock is held. This can't however be used if you are going to manipulate the database files outside MySQL while you hold the lock.

When you use `LOCK TABLES`, you must lock all tables that you are going to use and you must use the same alias that you are going to use in your queries! If you are using a table multiple times in a query (with aliases), you must get a lock for each alias!

`WRITE` locks normally have higher priority than `READ` locks, to ensure that updates are processed as soon as possible. This means that if one thread obtains a `READ` lock and then another thread requests a `WRITE` lock, subsequent `READ` lock requests will wait until the `WRITE` thread has gotten the lock and released it. You can use `LOW_PRIORITY WRITE` locks to allow other threads to obtain `READ` locks while the thread is waiting for the `WRITE` lock. You should only use `LOW_PRIORITY WRITE` locks if you are sure that there will eventually be a time when no threads will have a `READ` lock.

`LOCK TABLES` works as follows:

1. Sort all tables to be locked in a internally defined order (from the user standpoint the order is undefined).
2. If a table is locked with a read and a write lock, put the write lock before the read lock.
3. Lock one table at a time until the thread gets all locks.

This policy ensures that table locking is deadlock free. There is however other things one needs to be aware of with this schema:

If you are using a `LOW_PRIORITY WRITE` lock for a table, this means only that MySQL will wait for this particlar lock until there is no threads that wants a `READ` lock. When the thread has got the `WRITE` lock and is waiting to get the lock for the next table in the lock table list, all other threads will wait for the `WRITE` lock to be released. If this becomes a serious problem with your application, you should consider converting some of your tables to transactions safe tables.

You can safely kill a thread that is waiting for a table lock with `KILL`. See section 4.5.5 `KILL` Syntax.

Note that you should not lock any tables that you are using with `INSERT DELAYED`. This is because that in this case the `INSERT` is done by a separate thread.

Normally, you don't have to lock tables, as all single `UPDATE` statements are atomic; no other thread can interfere with any other currently executing SQL statement. There are a few cases when you would like to lock tables anyway:

- If you are going to run many operations on a bunch of tables, it's much faster to lock the tables you are going to use. The downside is, of course, that no other thread can update a `READ`-locked table and no other thread can read a `WRITE`-locked table. The reason some things are faster under `LOCK TABLES` is that MySQL will not flush the key cache for the locked tables until `UNLOCK TABLES` is called (normally the key cache is flushed after each SQL statement). This speeds up inserting/updateing/deletes on `MyISAM` tables.
- If you are using a storage engine in MySQL that doesn't support transactions, you must use `LOCK TABLES` if you want to ensure that no other thread comes between a `SELECT` and an `UPDATE`. The example shown here requires `LOCK TABLES` in order to execute safely:

      mysql> LOCK TABLES trans READ, customer WRITE;
      mysql> SELECT SUM(value) FROM trans WHERE customer_id=some_id;
      mysql> UPDATE customer SET total_value=sum_from_previous_statement
          ->        WHERE customer_id=some_id;
      mysql> UNLOCK TABLES;

  Without `LOCK TABLES`, there is a chance that another thread might insert a new row in the `trans` table between execution of the `SELECT` and `UPDATE` statements.

By using incremental updates (`UPDATE customer SET value=value+new_value`) or the `LAST_INSERT_ID()` function, you can avoid using `LOCK TABLES` in many cases.

You can also solve some cases by using the user-level lock functions `GET_LOCK()` and `RELEASE_LOCK()`. These locks are saved in a hash table in the server and implemented with `pthread_mutex_lock()` and `pthread_mutex_unlock()` for high speed. See section 6.3.6.2 Miscellaneous Functions.

See section 5.3.1 How MySQL Locks Tables, for more information on locking policy.

You can lock all tables in all databases with read locks with the `FLUSH TABLES WITH READ LOCK` command. See section 4.5.3 `FLUSH` Syntax. This is very convenient way to get backups if you have a filesystem, like Veritas, that can take snapshots in time.

NOTE: `LOCK TABLES` is not transaction-safe and will automatically commit any active transactions before attempting to lock the tables.

### 6.7.3 `SET TRANSACTION` Syntax

    SET [GLOBAL | SESSION] TRANSACTION ISOLATION LEVEL
    { READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE }

Sets the transaction isolation level for the global, whole session or the next transaction.

The default behaviour is to set the isolation level for the next (not started) transaction. If you use the `GLOBAL` keyword, the statement sets the default transaction level globally for all new connections created from that point on. You will need the `SUPER` privilege to do do this. Using the `SESSION` keyword sets the default transaction level for all future transactions performed on the current connection.

You can set the default global isolation level for `mysqld` with `--transaction-isolation=...`. See section 4.1.1 `mysqld` Command-line Options.

## 6.8 MySQL Full-text Search

As of Version 3.23.23, MySQL has support for full-text indexing and searching. Full-text indexes in MySQL are an index of type `FULLTEXT`. `FULLTEXT` indexes are used with `MyISAM` tables and can be created from `CHAR`, `VARCHAR`, or `TEXT` columns at `CREATE TABLE` time or added later with `ALTER TABLE` or `CREATE INDEX`. For large datasets, it will be much faster to load your data into a table that has no `FULLTEXT` index, then create the index with `ALTER TABLE` (or `CREATE INDEX`). Loading data into a table that already has a `FULLTEXT` index will be slower.

Full-text searching is performed with the `MATCH()` function.

    mysql> CREATE TABLE articles (
        ->   id INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY,
        ->   title VARCHAR(200),
        ->   body TEXT,
        ->   FULLTEXT (title,body)
        -> );
    Query OK, 0 rows affected (0.00 sec)

    mysql> INSERT INTO articles VALUES
        -> (NULL,'MySQL Tutorial', 'DBMS stands for DataBase ...'),
        -> (NULL,'How To Use MySQL Efficiently', 'After you went through a ...'),
        -> (NULL,'Optimising MySQL','In this tutorial we will show ...'),
        -> (NULL,'1001 MySQL Tricks','1. Never run mysqld as root. 2. ...'),
        -> (NULL,'MySQL vs. YourSQL', 'In the following database comparison ...'),
        -> (NULL,'MySQL Security', 'When configured properly, MySQL ...');
    Query OK, 6 rows affected (0.00 sec)
    Records: 6  Duplicates: 0  Warnings: 0

    mysql> SELECT * FROM articles
        ->          WHERE MATCH (title,body) AGAINST ('database');
    +----+-------------------+------------------------------------------+
    | id | title             | body                                     |
    +----+-------------------+------------------------------------------+
    |  5 | MySQL vs. YourSQL | In the following database comparison ... |
    |  1 | MySQL Tutorial    | DBMS stands for DataBase ...             |
    +----+-------------------+------------------------------------------+
    2 rows in set (0.00 sec)

The `MATCH()` function performs a natural language search for a string against a text collection (a set of of one or more columns included in a `FULLTEXT` index). The search string is given as the argument to `AGAINST()`. The search is performed in case-insensitive fashion. For every row in the table, `MATCH()` returns a relevance value, that is, a similarity measure between the search string and the text in that row in the columns named in the `MATCH()` list.

When `MATCH()` is used in a `WHERE` clause (see example above) the rows returned are automatically sorted with highest relevance first. Relevance values are non-negative floating-point numbers. Zero relevance means no similarity. Relevance is computed based on the number of words in the row, the number of unique words in that row, the total number of words in the collection, and the number of documents (rows) that contain a particular word.

It is also possible to perform a boolean mode search. This is explained later in the section.

The preceding example is a basic illustration showing how to use the `MATCH()` function. Rows are returned in order of decreasing relevance.

The next example shows how to retrieve the relevance values explicitly. As neither `WHERE` nor `ORDER BY` clauses are present, returned rows are not ordered.

    mysql> SELECT id,MATCH (title,body) AGAINST ('Tutorial') FROM articles;
    +----+-----------------------------------------+
    | id | MATCH (title,body) AGAINST ('Tutorial') |
    +----+-----------------------------------------+
    |  1 |                        0.64840710366884 |
    |  2 |                                       0 |
    |  3 |                        0.66266459031789 |
    |  4 |                                       0 |
    |  5 |                                       0 |
    |  6 |                                       0 |
    +----+-----------------------------------------+
    6 rows in set (0.00 sec)

The following example is more complex. The query returns the relevance and still sorts the rows in order of decreasing relevance. To achieve this result, you should specify `MATCH()` twice. This will cause no additional overhead, because the MySQL optimiser will notice that the two `MATCH()` calls are identical and invoke the full-text search code only once.

    mysql> SELECT id, body, MATCH (title,body) AGAINST
        -> ('Security implications of running MySQL as root') AS score
        -> FROM articles WHERE MATCH (title,body) AGAINST
        -> ('Security implications of running MySQL as root');
    +----+-------------------------------------+-----------------+
    | id | body                                | score           |
    +----+-------------------------------------+-----------------+
    |  4 | 1. Never run mysqld as root. 2. ... | 1.5055546709332 |
    |  6 | When configured properly, MySQL ... |   1.31140957288 |
    +----+-------------------------------------+-----------------+
    2 rows in set (0.00 sec)

MySQL uses a very simple parser to split text into words. A ``word'' is any sequence of characters consisting of letters, digits, ``''`, and ``_'`. Any ``word'' that is present in the stopword list or is just too short (3 characters or less) is ignored.

Every correct word in the collection and in the query is weighted according to its significance in the query or collection. This way, a word that is present in many documents will have lower weight (and may even have a zero weight), because it has lower semantic value in this particular collection. Otherwise, if the word is rare, it will receive a higher weight. The weights of the words are then combined to compute the relevance of the row.

Such a technique works best with large collections (in fact, it was carefully tuned this way). For very small tables, word distribution does not reflect adequately their semantic value, and this model may sometimes produce bizarre results.

    mysql> SELECT * FROM articles WHERE MATCH (title,body) AGAINST ('MySQL');
    Empty set (0.00 sec)

The search for the word `MySQL` produces no results in the above example, because that word is present in more than half the rows. As such, it is effectively treated as a stopword (that is, a word with zero semantic value). This is the most desirable behaviour -- a natural language query should not return every second row from a 1GB table.

A word that matches half of rows in a table is less likely to locate relevant documents. In fact, it will most likely find plenty of irrelevant documents. We all know this happens far too often when we are trying to find something on the Internet with a search engine. It is with this reasoning that such rows have been assigned a low semantic value in this particular dataset.

As of Version 4.0.1, MySQL can also perform boolean full-text searches using the `IN BOOLEAN MODE` modifier.

    mysql> SELECT * FROM articles WHERE MATCH (title,body)
        ->     AGAINST ('+MySQL -YourSQL' IN BOOLEAN MODE);
    +----+------------------------------+-------------------------------------+
    | id | title                        | body                                |
    +----+------------------------------+-------------------------------------+
    |  1 | MySQL Tutorial               | DBMS stands for DataBase ...        |
    |  2 | How To Use MySQL Efficiently | After you went through a ...        |
    |  3 | Optimising MySQL             | In this tutorial we will show ...   |
    |  4 | 1001 MySQL Tricks            | 1. Never run mysqld as root. 2. ... |
    |  6 | MySQL Security               | When configured properly, MySQL ... |
    +----+------------------------------+-------------------------------------+

This query retrieved all the rows that contain the word `MySQL` (note: the 50% threshold is not used), but that do not contain the word `YourSQL`. Note that a boolean mode search does not auto-magically sort rows in order of decreasing relevance. You can see this from result of the preceding query, where the row with the highest relevance (the one that contains `MySQL` twice) is listed last, not first. A boolean full-text search can also work even without a `FULLTEXT` index, although it would be slow.

The boolean full-text search capability supports the following operators:

`+`
  A leading plus sign indicates that this word must be present in every row returned.
`-`
  A leading minus sign indicates that this word must not be present in any row returned.
``
  By default (when neither plus nor minus is specified) the word is optional, but the rows that contain it will be rated higher. This mimicks the behaviour of `MATCH() ... AGAINST()` without the `IN BOOLEAN MODE` modifier.
`< >`
  These two operators are used to change a word's contribution to the relevance value that is assigned to a row. The `<` operator decreases the contribution and the `>` operator increases it. See the example below.
`( )`
  Parentheses are used to group words into subexpressions.
`~`
  A leading tilde acts as a negation operator, causing the word's contribution to the row relevance to be negative. It's useful for marking noise words. A row that contains such a word will be rated lower than others, but will not be excluded altogether, as it would be with the `-` operator.
`*`
  An asterisk is the truncation operator. Unlike the other operators, it should be appended to the word, not prepended.
`"`
  The phrase, that is enclosed in double quotes `"`, matches only rows that contain this phrase literally, as it was typed.

And here are some examples:

`apple banana`
  find rows that contain at least one of these words.
`+apple +juice`
  ... both words.
`+apple macintosh`
  ... word ``apple'', but rank it higher if it also contain ``macintosh''.
`+apple -macintosh`
  ... word ``apple'' but not ``macintosh''.
`+apple +(>pie <strudel)`
  ... ``apple'' and ``pie'', or ``apple'' and ``strudel'' (in any order), but rank ``apple pie'' higher than ``apple strudel''.
`apple*`
  ... ``apple'', ``apples'', ``applesauce'', and ``applet''.
`"some words"`
  ... ``some words of wisdom'', but not ``some noise words''.

### 6.8.1 Full-text Restrictions

- All parameters to the `MATCH()` function must be columns from the same table that is part of the same `FULLTEXT` index, unless the `MATCH()` is `IN BOOLEAN MODE`.
- The `MATCH()` column list must exactly match the column list in some `FULLTEXT` index definition for the table, unless this `MATCH()` is `IN BOOLEAN MODE`.
- The argument to `AGAINST()` must be a constant string.

### 6.8.2 Fine-tuning MySQL Full-text Search

Unfortunately, full-text search has few user-tunable parameters yet, although adding some is very high on the TODO. If you have a MySQL source distribution (see section 2.3 Installing a MySQL Source Distribution), you can exert more control over full-text searching behaviour.

Note that full-text search was carefully tuned for the best searching effectiveness. Modifying the default behaviour will, in most cases, only make the search results worse. Do not alter the MySQL sources unless you know what you are doing!

- The minimum length of words to be indexed is defined by the MySQL variable `ft_min_word_len`. See section 4.5.6.4 `SHOW VARIABLES`. Change it to the value you prefer, and rebuild your `FULLTEXT` indexes. (This variable is only available from MySQL version 4.0.)
- The stopword list is defined in ``myisam/ft_static.c'` Modify it to your taste, recompile MySQL, and rebuild your `FULLTEXT` indexes.
- The 50% threshold is determined by the particular weighting scheme chosen. To disable it, change the following line in ``myisam/ftdefs.h'`:

      #define GWS_IN_USE GWS_PROB

  To:

      #define GWS_IN_USE GWS_FREQ

  Then recompile MySQL. There is no need to rebuild the indexes in this case. Note: by doing this you severely decrease MySQL's ability to provide adequate relevance values for the `MATCH()` function. If you really need to search for such common words, it would be better to search using `IN BOOLEAN MODE` instead, which does not observe the 50% threshold.
- Sometimes the search engine maintainer would like to change the operators used for boolean full-text searches. These are defined by the `ft_boolean_syntax` variable. See section 4.5.6.4 `SHOW VARIABLES`. Still, this variable is read-only, its value is set in ``myisam/ft_static.c'`.

For those changes that require you to rebuild your `FULLTEXT` indexes, the easiest way to do so for a MyISAM table is to use the following statement, which rebuilds the index file:

    mysql> REPAIR TABLE tbl_name QUICK;

### 6.8.3 Full-text Search TODO

- Make all operations with `FULLTEXT` index faster.
- Proximity operators
- Support for "always-index words". They could be any strings the user wants to treat as words, examples are "C++", "AS/400", "TCP/IP", etc.
- Support for full-text search in `MERGE` tables.
- Support for multi-byte charsets.
- Make stopword list to depend of the language of the data.
- Stemming (dependent of the language of the data, of course).
- Generic user-suppliable UDF preparser.
- Make the model more flexible (by adding some adjustable parameters to `FULLTEXT` in `CREATE/ALTER TABLE`).

## 6.9 MySQL Query Cache

From version 4.0.1, `MySQL server` features a `Query Cache`. When in use, the query cache stores the text of a `SELECT` query together with the corresponding result that was sent to the client. If an identical query is later received, the server will retrieve the results from the query cache rather than parsing and executing the same query again.

NOTE: The query cache does not return stale data. When data is modified, any relevant entries in the query cache are flushed.

The query cache is extremely useful in an environment where (some) tables don't change very often and you have a lot of identical queries. This is a typical situation for many web servers that use a lot of dynamic content.

Below is some performance data for the query cache. (These results were generated by running the MySQL benchmark suite on a Linux Alpha 2 x 500 MHz with 2GB RAM and a 64MB query cache):

- If all of the queries you're performing are simple (such as selecting a row from a table with one row); but still differ so that the queries can not be cached, the overhead for having the query cache active is 13%. This could be regarded as the worst case scenario. However, in real life, queries are much more complicated than our simple example so the overhead is normally significantly lower.
- Searches after one row in a one row table is 238% faster. This can be regarded as close to the minimum speedup to be expected for a query that is cached.
- If you want to disable the query cache code set `query_cache_size=0`. By disabling the query cache code there is no noticeable overhead. (query cache can be excluded from code with help of configure option `--without-query-cache`)

### 6.9.1 How The Query Cache Operates

Queries are compared before parsing, thus

    SELECT * FROM TABLE

and

    Select * from table

are regarded as different queries for query cache, so queries need to be exactly the same (byte for byte) to be seen as identical. In addition, a query may be seen as different if for instance one client is using a new communication protocol format or another character set than another client.

Queries that uses different databases, uses different protocol versions or the uses different default character sets are considered different queries and cached separately.

The cache does work for `SELECT CALC_ROWS ...` and `SELECT FOUND_ROWS() ...` type queries because the number of found rows is also stored in the cache.

If a table changes (`INSERT`, `UPDATE`, `DELETE`, `TRUNCATE`, `ALTER` or `DROP TABLE|DATABASE`), then all cached queries that used this table (possibly through a `MRG_MyISAM` table!) become invalid and are removed from the cache.

Transactional `InnoDB` tables that have been changed will be invalidated when a `COMMIT` is performed.

A query cannot be cached if it contains one of the functions:

Function | Function | Function
`User-Defined Functions` | `CONNECTION_ID` | `FOUND_ROWS`
`GET_LOCK` | `RELEASE_LOCK` | `LOAD_FILE`
`MASTER_POS_WAIT` | `NOW` | `SYSDATE`
`CURRENT_TIMESTAMP` | `CURDATE` | `CURRENT_DATE`
`CURTIME` | `CURRENT_TIME` | `DATABASE`
`ENCRYPT` (with one parameter) | `LAST_INSERT_ID` | `RAND`
`UNIX_TIMESTAMP` (without parameters) | `USER` | `BENCHMARK`

Nor can a query be cached if it contains user variables, references the mysql system database, is of the form `SELECT ... IN SHARE MODE`, or of the form `SELECT * FROM AUTOINCREMENT_FIELD IS NULL` (to retrieve last insert id - ODBC work around).

However, `FOUND ROWS()` will return the correct value, even if the preceding query was fetched from the cache.

In case a query does not use any tables, or uses temporary tables, or if the user has a column privilege for any of the involved tables, that query will not be cached.

Before a query is fetched from the query cache, MySQL will check that the user has SELECT privilege to all the involved databases and tables. If this is not the case, the cached result will not be used.

### 6.9.2 Query Cache Configuration

The query cache adds a few `MySQL` system variables for `mysqld` which may be set in a configuration file, on the command-line when starting `mysqld`.

- `query_cache_limit` Don't cache results that are bigger than this. (Default 1M).
- `query_cache_size` The amount of memory (specified in bytes) allocated to store results from old queries. If this is 0, the query cache is disabled (default).
- `query_cache_type` This may be set (only numeric) to

  Option | Description
  0 | (OFF, don't cache or retrieve results)
  1 | (ON, cache all results except `SELECT SQL_NO_CACHE ...` queries)
  2 | (DEMAND, cache only `SELECT SQL_CACHE ...` queries)

Inside a thread (connection), the behaviour of the query cache can be changed from the default. The syntax is as follows:

`QUERY_CACHE_TYPE = OFF | ON | DEMAND` `QUERY_CACHE_TYPE = 0 | 1 | 2`

Option | Description
0 or OFF | Don't cache or retrieve results.
1 or ON | Cache all results except `SELECT SQL_NO_CACHE ...` queries.
2 or DEMAND | Cache only `SELECT SQL_CACHE ...` queries.

### 6.9.3 Query Cache Options in `SELECT`

There are two possible query cache related parameters that may be specified in a `SELECT` query:

Option | Description
`SQL_CACHE` | If `QUERY_CACHE_TYPE` is `DEMAND`, allow the query to be cached. If `QUERY_CACHE_TYPE` is `ON`, this is the default. If `QUERY_CACHE_TYPE` is `OFF`, do nothing.
`SQL_NO_CACHE` | Make this query non-cachable, don't allow this query to be stored in the cache.

### 6.9.4 Query Cache Status and Maintenance

With the `FLUSH QUERY CACHE` command you can defragment the query cache to better utilise its memory. This command will not remove any queries from the cache. `FLUSH TABLES` also flushes the query cache.

The `RESET QUERY CACHE` command removes all query results from the query cache.

You can monitor query cache performance in `SHOW STATUS`:

Variable | Description
`Qcache_queries_in_cache` | Number of queries registered in the cache.
`Qcache_inserts` | Number of queries added to the cache.
`Qcache_hits` | Number of cache hits.
`Qcache_lowmem_prunes` | Number of queries that were deleted from cache because of low memory.
`Qcache_not_cached` | Number of non-cached queries (not cachable, or due to `QUERY_CACHE_TYPE`).
`Qcache_free_memory` | Amount of free memory for query cache.
`Qcache_total_blocks` | Total number of blocks in query cache.
`Qcache_free_blocks` | Number of free memory blocks in query cache.

Total number of queries = `Qcache_inserts` + `Qcache_hits` + `Qcache_not_cached`.

The query cache uses variable length blocks, so `Qcache_total_blocks` and `Qcache_free_blocks` may indicate query cache memory fragmentation. After `FLUSH QUERY CACHE` only a single (big) free block remains.

Note: Every query needs a minimum of two blocks (one for the query text and one or more for the query results). Also, every table that is used by a query needs one block, but if two or more queries use same table only one block needs to be allocated.

You can use the `Qcache_lowmem_prunes` status variable to tune the query cache size. It counts the number of queries that have been removed from the cache to free up memory for caching new queries. The query cache uses a `least recently used` (`LRU`) strategy to decide which queries to remove from the cache.

# 7 MySQL Table Types

As of MySQL Version 3.23.6, you can choose between three basic table formats (`ISAM`, `HEAP` and `MyISAM`. Newer MySQL may support additional table type (`InnoDB`, or `BDB`), depending on how you compile it.

When you create a new table, you can tell MySQL which table type it should use for the table. MySQL will always create a ``.frm'` file to hold the table and column definitions. Depending on the table type, the index and data will be stored in other files.

Note that to use `InnoDB` tables you have to use at least the `innodb_data_file_path` startup option. See section 7.5.2 InnoDB Startup Options.

The default table type in MySQL is `MyISAM`. If you are trying to use a table type that is not compiled-in or activated, MySQL will instead create a table of type `MyISAM`. This is a very useful feature when you want to copy tables between different SQL servers that supports different table types (like copying tables to a slave that is optimised for speed by not having transactional tables). This automatic table changing can however also be very confusing for new MySQL users. We plan to fix this by introducing warnings in the new new client-server protocol in version 4.1 and giving a warning when a table type is automatically changed.

You can convert tables between different types with the `ALTER TABLE` statement. See section 6.5.4 `ALTER TABLE` Syntax.

Note that MySQL supports two different kinds of tables: transaction-safe tables (`InnoDB` and `BDB`) and not transaction-safe tables (`HEAP`, `ISAM`, `MERGE`, and `MyISAM`).

Advantages of transaction-safe tables (TST):

- Safer. Even if MySQL crashes or you get hardware problems, you can get your data back, either by automatic recovery or from a backup + the transaction log.
- You can combine many statements and accept these all in one go with the `COMMIT` command.
- You can execute `ROLLBACK` to ignore your changes (if you are not running in auto-commit mode).
- If an update fails, all your changes will be restored. (With NTST tables all changes that have taken place are permanent)
- Can provide better concurrency if the table gets many updates concurrently with reads.

Advantages of not transaction-safe tables (NTST):

- Much faster as there is no transaction overhead.
- Will use less disk space as there is no overhead of transactions.
- Will use less memory to do updates.

You can combine TST and NTST tables in the same statements to get the best of both worlds.

## 7.1 `MyISAM` Tables

`MyISAM` is the default table type in MySQL Version 3.23. It's based on the `ISAM` code and has a lot of useful extensions.

The index is stored in a file with the ``.MYI'` (MYIndex) extension, and the data is stored in a file with the ``.MYD'` (MYData) extension. You can check/repair `MyISAM` tables with the `myisamchk` utility. See section 4.4.6.7 Using `myisamchk` for Crash Recovery. You can compress `MyISAM` tables with `myisampack` to take up much less space. See section 4.7.4 `myisampack`, The MySQL Compressed Read-only Table Generator.

The following is new in `MyISAM`:

- There is a flag in the `MyISAM` file that indicates whether the table was closed correctly. If `mysqld` is started with `--myisam-recover`, `MyISAM` tables will automatically be checked and/or repaired on open if the table wasn't closed properly.
- You can `INSERT` new rows in a table that doesn't have free blocks in the middle of the datafile, at the same time other threads are reading from the table (concurrent insert). An free block can come from an update of a dynamic length row with much data to a row with less data or when deleting rows. When all free blocks are used up, all future inserts will be concurrent again.
- Support for big files (63-bit) on filesystems/operating systems that support big files.
- All data is stored with the low byte first. This makes the data machine and OS independent. The only requirement is that the machine uses two's-complement signed integers (as every machine for the last 20 years has) and IEEE floating-point format (also totally dominant among mainstream machines). The only area of machines that may not support binary compatibility are embedded systems (because they sometimes have peculiar processors). There is no big speed penalty in storing data low byte first; the bytes in a table row is normally unaligned and it doesn't take that much more power to read an unaligned byte in order than in reverse order. The actual fetch-column-value code is also not time critical compared to other code.
- All number keys are stored with high byte first to give better index compression.
- Internal handling of one `AUTO_INCREMENT` column. `MyISAM` will automatically update this on `INSERT/UPDATE`. The `AUTO_INCREMENT` value can be reset with `myisamchk`. This will make `AUTO_INCREMENT` columns faster (at least 10%) and old numbers will not be reused as with the old `ISAM`. Note that when an `AUTO_INCREMENT` is defined on the end of a multi-part-key the old behaviour is still present.
- When inserted in sorted order (as when you are using an `AUTO_INCREMENT` column) the key tree will be split so that the high node only contains one key. This will improve the space utilisation in the key tree.
- `BLOB` and `TEXT` columns can be indexed.
- `NULL` values are allowed in indexed columns. This takes 0-1 bytes/key.
- Maximum key length is 500 bytes by default (can be changed by recompiling). In cases of keys longer than 250 bytes, a bigger key block size than the default of 1024 bytes is used for this key.
- Maximum number of keys/table is 32 as default. This can be enlarged to 64 without having to recompile `myisamchk`.
- `myisamchk` will mark tables as checked if one runs it with `--update-state`. `myisamchk --fast` will only check those tables that don't have this mark.
- `myisamchk -a` stores statistics for key parts (and not only for whole keys as in `ISAM`).
- Dynamic size rows will now be much less fragmented when mixing deletes with updates and inserts. This is done by automatically combining adjacent deleted blocks and by extending blocks if the next block is deleted.
- `myisampack` can pack `BLOB` and `VARCHAR` columns.
- You can use put the datafile and index file on different directories to get more speed (with the `DATA/INDEX DIRECTORY="path"` option to `CREATE TABLE`). See section 6.5.3 `CREATE TABLE` Syntax.

`MyISAM` also supports the following things, which MySQL will be able to use in the near future:

- Support for a true `VARCHAR` type; a `VARCHAR` column starts with a length stored in 2 bytes.
- Tables with `VARCHAR` may have fixed or dynamic record length.
- `VARCHAR` and `CHAR` may be up to 64K. All key segments have their own language definition. This will enable MySQL to have different language definitions per column.
- A hashed computed index can be used for `UNIQUE`. This will allow you to have `UNIQUE` on any combination of columns in a table. (You can't search on a `UNIQUE` computed index, however.)

Note that index files are usually much smaller with `MyISAM` than with `ISAM`. This means that `MyISAM` will normally use less system resources than `ISAM`, but will need more CPU time when inserting data into a compressed index.

The following options to `mysqld` can be used to change the behaviour of `MyISAM` tables. See section 4.5.6.4 `SHOW VARIABLES`.

Option | Description
`--myisam-recover=#` | Automatic recovery of crashed tables.
`-O myisam_sort_buffer_size=#` | Buffer used when recovering tables.
`--delay-key-write=ALL` | Don't flush key buffers between writes for any MyISAM table
`-O myisam_max_extra_sort_file_size=#` | Used to help MySQL to decide when to use the slow but safe key cache index create method. Note that this parameter is given in megabytes before 4.0.3 and in bytes starting from this version.
`-O myisam_max_sort_file_size=#` | Don't use the fast sort index method to created index if the temporary file would get bigger than this. Note that this parameter is given in megabytes before 4.0.3 and in bytes starting from this version.
`-O bulk_insert_buffer_size=#` | Size of tree cache used in bulk insert optimisation. Note that this is a limit per thread!

The automatic recovery is activated if you start `mysqld` with `--myisam-recover=#`. See section 4.1.1 `mysqld` Command-line Options. On open, the table is checked if it's marked as crashed or if the open count variable for the table is not 0 and you are running with `--skip-external-locking`. If either of the above is true the following happens.

- The table is checked for errors.
- If we found an error, try to do a fast repair (with sorting and without re-creating the datafile) of the table.
- If the repair fails because of an error in the datafile (for example a duplicate key error), we try again, but this time we re-create the datafile.
- If the repair fails, retry once more with the old repair option method (write row by row without sorting) which should be able to repair any type of error with little disk requirements..

If the recover wouldn't be able to recover all rows from a previous completed statement and you didn't specify `FORCE` as an option to `myisam-recover`, then the automatic repair will abort with an error message in the error file:

    Error: Couldn't repair table: test.g00pages

If you in this case had used the `FORCE` option you would instead have got a warning in the error file:

    Warning: Found 344 of 354 rows when repairing ./test/g00pages

Note that if you run automatic recover with the `BACKUP` option, you should have a `cron` script that automatically moves file with names like ``tablename-datetime.BAK'` from the database directories to a backup media.

See section 4.1.1 `mysqld` Command-line Options.

### 7.1.1 Space Needed for Keys

MySQL can support different index types, but the normal type is ISAM or MyISAM. These use a B-tree index, and you can roughly calculate the size for the index file as `(key_length+4)/0.67`, summed over all keys. (This is for the worst case when all keys are inserted in sorted order and we don't have any compressed keys.)

String indexes are space compressed. If the first index part is a string, it will also be prefix compressed. Space compression makes the index file smaller than the above figures if the string column has a lot of trailing space or is a `VARCHAR` column that is not always used to the full length. Prefix compression is used on keys that start with a string. Prefix compression helps if there are many strings with an identical prefix.

In `MyISAM` tables, you can also prefix compress numbers by specifying `PACK_KEYS=1` when you create the table. This helps when you have many integer keys that have an identical prefix when the numbers are stored high-byte first.

### 7.1.2 `MyISAM` Table Formats

`MyISAM` supports 3 different table types. Two of them are chosen automatically depending on the type of columns you are using. The third, compressed tables, can only be created with the `myisampack` tool.

When you `CREATE` or `ALTER` a table you can for tables that doesn't have `BLOB`s force the table format to `DYNAMIC` or `FIXED` with the `ROW_FORMAT=#` table option. In the future you will be able to compress/decompress tables by specifying `ROW_FORMAT=compressed | default` to `ALTER TABLE`. See section 6.5.3 `CREATE TABLE` Syntax.

#### 7.1.2.1 Static (Fixed-length) Table Characteristics

This is the default format. It's used when the table contains no `VARCHAR`, `BLOB`, or `TEXT` columns.

This format is the simplest and most secure format. It is also the fastest of the on-disk formats. The speed comes from the easy way data can be found on disk. When looking up something with an index and static format it is very simple. Just multiply the row number by the row length.

Also, when scanning a table it is very easy to read a constant number of records with each disk read.

The security is evidenced if your computer crashes when writing to a fixed-size MyISAM file, in which case `myisamchk` can easily figure out where each row starts and ends. So it can usually reclaim all records except the partially written one. Note that in MySQL all indexes can always be reconstructed:

- All `CHAR`, `NUMERIC`, and `DECIMAL` columns are space-padded to the column width.
- Very quick.
- Easy to cache.
- Easy to reconstruct after a crash, because records are located in fixed positions.
- Doesn't have to be reorganised (with `myisamchk`) unless a huge number of records are deleted and you want to return free disk space to the operating system.
- Usually requires more disk space than dynamic tables.

#### 7.1.2.2 Dynamic Table Characteristics

This format is used if the table contains any `VARCHAR`, `BLOB`, or `TEXT` columns or if the table was created with `ROW_FORMAT=dynamic`.

This format is a little more complex because each row has to have a header that says how long it is. One record can also end up at more than one location when it is made longer at an update.

You can use `OPTIMIZE table` or `myisamchk` to defragment a table. If you have static data that you access/change a lot in the same table as some `VARCHAR` or `BLOB` columns, it might be a good idea to move the dynamic columns to other tables just to avoid fragmentation:

- All string columns are dynamic (except those with a length less than 4).
- Each record is preceded by a bitmap indicating which columns are empty (`''`) for string columns, or zero for numeric columns. (This isn't the same as columns containing `NULL` values.) If a string column has a length of zero after removal of trailing spaces, or a numeric column has a value of zero, it is marked in the bit map and not saved to disk. Non-empty strings are saved as a length byte plus the string contents.
- Usually takes much less disk space than fixed-length tables.
- Each record uses only as much space as is required. If a record becomes larger, it is split into as many pieces as are required. This results in record fragmentation.
- If you update a row with information that extends the row length, the row will be fragmented. In this case, you may have to run `myisamchk -r` from time to time to get better performance. Use `myisamchk -ei tbl_name` for some statistics.
- Not as easy to reconstruct after a crash, because a record may be fragmented into many pieces and a link (fragment) may be missing.
- The expected row length for dynamic sized records is:

      3
      + (number of columns + 7) / 8
      + (number of char columns)
      + packed size of numeric columns
      + length of strings
      + (number of NULL columns + 7) / 8

  There is a penalty of 6 bytes for each link. A dynamic record is linked whenever an update causes an enlargement of the record. Each new link will be at least 20 bytes, so the next enlargement will probably go in the same link. If not, there will be another link. You may check how many links there are with `myisamchk -ed`. All links may be removed with `myisamchk -r`.

#### 7.1.2.3 Compressed Table Characteristics

This is a read-only type that is generated with the optional `myisampack` tool (`pack_isam` for `ISAM` tables):

- All MySQL distributions, even those that existed before MySQL went GPL, can read tables that were compressed with `myisampack`.
- Compressed tables take very little disk space. This minimises disk usage, which is very nice when using slow disks (like CD-ROMs).
- Each record is compressed separately (very little access overhead). The header for a record is fixed (1-3 bytes) depending on the biggest record in the table. Each column is compressed differently. Some of the compression types are:
  - There is usually a different Huffman table for each column.
  - Suffix space compression.
  - Prefix space compression.
  - Numbers with value `0` are stored using 1 bit.
  - If values in an integer column have a small range, the column is stored using the smallest possible type. For example, a `BIGINT` column (8 bytes) may be stored as a `TINYINT` column (1 byte) if all values are in the range `0` to `255`.
  - If a column has only a small set of possible values, the column type is converted to `ENUM`.
  - A column may use a combination of the above compressions.
- Can handle fixed- or dynamic-length records.
- Can be uncompressed with `myisamchk`.

### 7.1.3 `MyISAM` Table Problems

The file format that MySQL uses to store data has been extensively tested, but there are always circumstances that may cause database tables to become corrupted.

#### 7.1.3.1 Corrupted `MyISAM` Tables

Even if the MyISAM table format is very reliable (all changes to a table is written before the SQL statements returns) , you can still get corrupted tables if some of the following things happens:

- The `mysqld` process being killed in the middle of a write.
- Unexpected shutdown of the computer (for example, if the computer is turned off).
- A hardware error.
- You are using an external program (like myisamchk) on a live table.
- A software bug in the MySQL or MyISAM code.

Typial typical symptoms for a corrupt table is:

- You get the error `Incorrect key file for table: '...'. Try to repair it` while selecting data from the table.
- Queries doesn't find rows in the table or returns incomplete data.

You can check if a table is ok with the command `CHECK TABLE`. See section 4.4.4 `CHECK TABLE` Syntax.

You can repair a corrupted table with `REPAIR TABLE`. See section 4.4.5 `REPAIR TABLE` Syntax. You can also repair a table, when `mysqld` is not running with the `myisamchk` command. `myisamchk syntax`.

If your tables get corrupted a lot you should try to find the reason for this! See section A.4.1 What To Do If MySQL Keeps Crashing.

In this case the most important thing to know is if the table got corrupted if the `mysqld` died (one can easily verify this by checking if there is a recent row `restarted mysqld` in the mysqld error file). If this isn't the case, then you should try to make a test case of this. See section E.1.6 Making a Test Case When You Experience Table Corruption.

#### 7.1.3.2 Clients is using or hasn't closed the table properly

Each `MyISAM` ``.MYI'` file has in the header a counter that can be used to check if a table has been closed properly.

If you get the following warning from `CHECK TABLE` or `myisamchk`:

    # clients is using or hasn't closed the table properly

this means that this counter has come out of sync. This doesn't mean that the table is corrupted, but means that you should at least do a check on the table to verify that it's okay.

The counter works as follows:

- The first time a table is updated in MySQL, a counter in the header of the index files is incremented.
- The counter is not changed during further updates.
- When the last instance of a table is closed (because of a `FLUSH` or because there isn't room in the table cache) the counter is decremented if the table has been updated at any point.
- When you repair the table or check the table and it was okay, the counter is reset to 0.
- To avoid problems with interaction with other processes that may do a check on the table, the counter is not decremented on close if it was 0.

In other words, the only ways this can go out of sync are:

- The `MyISAM` tables are copied without a `LOCK` and `FLUSH TABLES`.
- MySQL has crashed between an update and the final close. (Note that the table may still be okay, as MySQL always issues writes for everything between each statement.)
- Someone has done a `myisamchk --recover` or `myisamchk --update-state`on a table that was in use by `mysqld`.
- Many `mysqld` servers are using the table and one has done a `REPAIR` or `CHECK` of the table while it was in use by another server. In this setup the `CHECK` is safe to do (even if you will get the warning from other servers), but `REPAIR` should be avoided as it currently replaces the datafile with a new one, which is not signaled to the other servers.

## 7.2 `MERGE` Tables

`MERGE` tables are new in MySQL Version 3.23.25. The code is still in gamma, but should be resonable stable.

A `MERGE` table (also known as a `MRG_MyISAM` table) is a collection of identical `MyISAM` tables that can be used as one. You can only `SELECT`, `DELETE`, and `UPDATE` from the collection of tables. If you `DROP` the `MERGE` table, you are only dropping the `MERGE` specification.

Note that `DELETE FROM merge_table` used without a `WHERE` will only clear the mapping for the table, not delete everything in the mapped tables. (We plan to fix this in 4.1).

With identical tables we mean that all tables are created with identical column and key information. You can't merge tables in which the columns are packed differently, doesn't have exactly the same columns, or have the keys in different order. However, some of the tables can be compressed with `myisampack`. See section 4.7.4 `myisampack`, The MySQL Compressed Read-only Table Generator.

When you create a `MERGE` table, you will get a ``.frm'` table definition file and a ``.MRG'` table list file. The ``.MRG'` just contains a list of the index files (``.MYI'` files) that should be used as one. All used tables must be in the same database as the `MERGE` table itself.

For the moment, you need to have `SELECT`, `UPDATE`, and `DELETE` privileges on the tables you map to a `MERGE` table.

`MERGE` tables can help you solve the following problems:

- Easily manage a set of log tables. For example, you can put data from different months into separate files, compress some of them with `myisampack`, and then create a `MERGE` to use these as one.
- Give you more speed. You can split a big read-only table based on some criteria and then put the different table part on different disks. A `MERGE` table on this could be much faster than using the big table. (You can, of course, also use a RAID to get the same kind of benefits.)
- Do more efficient searches. If you know exactly what you are looking after, you can search in just one of the split tables for some queries and use a `MERGE` table for others. You can even have many different `MERGE` tables active, with possible overlapping files.
- More efficient repairs. It's easier to repair the individual files that are mapped to a `MERGE` file than trying to repair a really big file.
- Instant mapping of many files as one. A `MERGE` table uses the index of the individual tables. It doesn't need to maintain an index of its one. This makes `MERGE` table collections VERY fast to make or remap. Note that you must specify the key definitions when you create a `MERGE` table!.
- If you have a set of tables that you join to a big table on demand or batch, you should instead create a `MERGE` table on them on demand. This is much faster and will save a lot of disk space.
- Go around the file-size limit for the operating system.
- You can create an alias/synonym for a table by just using `MERGE` over one table. There shouldn't be any really notable performance impacts of doing this (only a couple of indirect calls and `memcpy()` calls for each read).

The disadvantages with `MERGE` tables are:

- You can only use identical `MyISAM` tables for a `MERGE` table.
- `REPLACE` doesn't work.
- `MERGE` tables uses more file descriptors. If you are using a `MERGE` table that maps over 10 tables and 10 users are using this, you are using 10*10 + 10 file descriptors. (10 datafiles for 10 users and 10 shared index files.)
- Key reads are slower. When you do a read on a key, the `MERGE` storage engine will need to issue a read on all underlying tables to check which one most closely matches the given key. If you then do a "read-next" then the `MERGE` storage engine will need to search the read buffers to find the next key. Only when one key buffer is used up, the storage engine will need to read the next key block. This makes `MERGE` keys much slower on `eq_ref` searches, but not much slower on `ref` searches. See section 5.2.1 `EXPLAIN` Syntax (Get Information About a `SELECT`).
- You can't do `DROP TABLE`, `ALTER TABLE`, `DELETE FROM table_name` without a `WHERE` clause, `REPAIR TABLE`, `TRUNCATE TABLE`, `OPTIMIZE TABLE`, or `ANALYZE TABLE` on any of the table that is mapped by a `MERGE` table that is "open". If you do this, the `MERGE` table may still refer to the original table and you will get unexpected results. The easiest way to get around this deficiency is to issue the `FLUSH TABLES` command, ensuring no `MERGE` tables remain "open".

When you create a `MERGE` table you have to specify with `UNION(list-of-tables)` which tables you want to use as one. Optionally you can specify with `INSERT_METHOD` if you want insert for the `MERGE` table to happen in the first or last table in the `UNION` list. If you don't specify `INSERT_METHOD` or specify `NO`, then all `INSERT` commands on the `MERGE` table will return an error.

The following example shows you how to use `MERGE` tables:

    CREATE TABLE t1 (a INT AUTO_INCREMENT PRIMARY KEY, message CHAR(20));
    CREATE TABLE t2 (a INT AUTO_INCREMENT PRIMARY KEY, message CHAR(20));
    INSERT INTO t1 (message) VALUES ("Testing"),("table"),("t1");
    INSERT INTO t2 (message) VALUES ("Testing"),("table"),("t2");
    CREATE TABLE total (a INT AUTO_INCREMENT PRIMARY KEY, message CHAR(20))
                 TYPE=MERGE UNION=(t1,t2) INSERT_METHOD=LAST;

Note that you can also manipulate the ``.MRG'` file directly from the outside of the MySQL server:

    shell> cd /mysql-data-directory/current-database
    shell> ls -1 t1.MYI t2.MYI > total.MRG
    shell> mysqladmin flush-tables

Now you can do things like:

    mysql> SELECT * FROM total;
    +---+---------+
    | a | message |
    +---+---------+
    | 1 | Testing |
    | 2 | table   |
    | 3 | t1      |
    | 1 | Testing |
    | 2 | table   |
    | 3 | t2      |
    +---+---------+

Note that the `a` column, though declared as `PRIMARY KEY`, is not really unique, as `MERGE` table cannot enforce uniqueness over a set of underlying `MyISAM` tables.

To remap a `MERGE` table you can do one of the following:

- `DROP` the table and re-create it
- Use `ALTER TABLE table_name UNION(...)`
- Change the ``.MRG'` file and issue a `FLUSH TABLE` on the `MERGE` table and all underlying tables to force the storage engine to read the new definition file.

### 7.2.1 `MERGE` Table Problems

The following are the known problems with `MERGE` tables:

- A `MERGE` table cannot maintain `UNIQUE` constraints over the whole table. When you do `INSERT`, the data goes into the first or last table (according to `INSERT_METHOD=xxx`) and this `MyISAM` table ensures that the data are unique, but it knows nothing about others `MyISAM` tables.
- `DELETE FROM merge_table` used without a `WHERE` will only clear the mapping for the table, not delete everything in the mapped tables.
- `RENAME TABLE` on a table used in an active `MERGE` table may corrupt the table. This will be fixed in MySQL 4.0.x.
- Creation of a table of type `MERGE` doesn't check if the underlying tables are of compatible types. If you use `MERGE` tables in this fashion, you are very likely to run into strange problems.
- If you use `ALTER TABLE` to first add an `UNIQUE` index to a table used in a `MERGE` table and then use `ALTER TABLE` to add a normal index on the `MERGE` table, the key order will be different for the tables if there was an old non-unique key in the table. This is because `ALTER TABLE` puts `UNIQUE` keys before normal keys to be able to detect duplicate keys as early as possible.
- The range optimiser can't yet use `MERGE` table efficiently and may sometimes produce non-optimal joins. This will be fixed in MySQL 4.0.x.
- `DROP TABLE` on a table that is in use by a `MERGE` table will not work on Windows because the `MERGE` storage engine does the table mapping hidden from the upper layer of MySQL. Because Windows doesn't allow you to drop files that are open, you first must flush all `MERGE` tables (with `FLUSH TABLES`) or drop the `MERGE` table before dropping the table. We will fix this at the same time we introduce `VIEW`s.

## 7.3 `ISAM` Tables

You can also use the deprecated `ISAM` table type. This will disappear rather soon (probably in MySQL 5.0) because `MyISAM` is a better implementation of the same thing. `ISAM` uses a `B-tree` index. The index is stored in a file with the ``.ISM'` extension, and the data is stored in a file with the ``.ISD'` extension. You can check/repair `ISAM` tables with the `isamchk` utility. See section 4.4.6.7 Using `myisamchk` for Crash Recovery.

`ISAM` has the following features/properties:

- Compressed and fixed-length keys
- Fixed and dynamic record length
- 16 keys with 16 key parts/key
- Max key length 256 (default)
- Data is stored in machine format; this is fast, but is machine/OS dependent.

Most of the things true for `MyISAM` tables are also true for `ISAM` tables. See section 7.1 `MyISAM` Tables. The major differences compared to `MyISAM` tables are:

- `ISAM` tables are not binary portable across OS/Platforms.
- Can't handle tables > 4G.
- Only support prefix compression on strings.
- Smaller key limits.
- Dynamic tables get more fragmented.
- Tables are compressed with `pack_isam` rather than with `myisampack`.

If you want to convert an `ISAM` table to a `MyISAM` table so that you can use utilities such as `mysqlcheck`, use an `ALTER TABLE` statement:

    mysql> ALTER TABLE tbl_name TYPE = MYISAM;

The embedded MySQL versions doesn't support `ISAM` tables.

## 7.4 `HEAP` Tables

`HEAP` tables use a hashed index and are stored in memory. This makes them very fast, but if MySQL crashes you will lose all data stored in them. `HEAP` is very useful for temporary tables!

The MySQL internal `HEAP` tables use 100% dynamic hashing without overflow areas. There is no extra space needed for free lists. `HEAP` tables also don't have problems with delete + inserts, which normally is common with hashed tables:

    mysql> CREATE TABLE test TYPE=HEAP SELECT ip,SUM(downloads) AS down
        ->                   FROM log_table GROUP BY ip;
    mysql> SELECT COUNT(ip),AVG(down) FROM test;
    mysql> DROP TABLE test;

Here are some things you should consider when you use `HEAP` tables:

- You should always use specify `MAX_ROWS` in the `CREATE` statement to ensure that you accidentally do not use all memory.
- Indexes will only be used with `=` and `<=>` (but are VERY fast).
- `HEAP` tables can only use whole keys to search for a row; compare this to `MyISAM` tables where any prefix of the key can be used to find rows.
- `HEAP` tables use a fixed record length format.
- `HEAP` doesn't support `BLOB`/`TEXT` columns.
- `HEAP` doesn't support `AUTO_INCREMENT` columns.
- Prior to MySQL 4.0.2, `HEAP` doesn't support an index on a `NULL` column.
- You can have non-unique keys in a `HEAP` table (this isn't common for hashed tables).
- `HEAP` tables are shared between all clients (just like any other table).
- You can't search for the next entry in order (that is, to use the index to do an `ORDER BY`).
- Data for `HEAP` tables are allocated in small blocks. The tables are 100% dynamic (on inserting). No overflow areas and no extra key space are needed. Deleted rows are put in a linked list and are reused when you insert new data into the table.
- You need enough extra memory for all `HEAP` tables that you want to use at the same time.
- To free memory, you should execute `DELETE FROM heap_table`, `TRUNCATE heap_table` or `DROP TABLE heap_table`.
- MySQL cannot find out approximately how many rows there are between two values (this is used by the range optimiser to decide which index to use). This may affect some queries if you change a `MyISAM` table to a `HEAP` table.
- To ensure that you accidentally don't do anything foolish, you can't create `HEAP` tables bigger than `max_heap_table_size`.

The memory needed for one row in a `HEAP` table is:

    SUM_OVER_ALL_KEYS(max_length_of_key + sizeof(char*) * 2)
    + ALIGN(length_of_row+1, sizeof(char*))

`sizeof(char*)` is 4 on 32-bit machines and 8 on 64-bit machines.

## 7.5 `InnoDB` Tables

### 7.5.1 InnoDB Tables Overview

InnoDB provides MySQL with a transaction-safe (`ACID` compliant) storage engine with commit, rollback, and crash recovery capabilities. InnoDB does locking on row level and also provides an Oracle-style consistent non-locking read in `SELECT`s. These features increase multiuser concurrency and performance. There is no need for lock escalation in InnoDB, because row level locks in InnoDB fit in very small space. InnoDB tables support `FOREIGN KEY` constraints as the first table type in MySQL.

InnoDB has been designed for maximum performance when processing large data volumes. Its CPU efficiency is probably not matched by any other disk-based relational database engine.

Technically, InnoDB is a complete database backend placed under MySQL. InnoDB has its own buffer pool for caching data and indexes in main memory. InnoDB stores its tables and indexes in a tablespace, which may consist of several files. This is different from, for example, MyISAM tables where each table is stored as a separate file. InnoDB tables can be of any size also on those operating systems where file-size is limited to 2 GB.

You can find the latest information about InnoDB at http://www.innodb.com/. The most up-to-date version of the InnoDB manual is always placed there, and you can also order commercial licenses and support for InnoDB.

InnoDB is currently (October 2001) used in production at several large database sites requiring high performance. The famous Internet news site Slashdot.org runs on InnoDB. Mytrix, Inc. stores over 1 TB of data in InnoDB, and another site handles an average load of 800 inserts/updates per second in InnoDB.

InnoDB tables are included in the MySQL source distribution starting from 3.23.34a and are activated in the MySQL -Max binary. For Windows the -Max binaries are contained in the standard distribution.

If you have downloaded a binary version of MySQL that includes support for InnoDB, simply follow the instructions of the MySQL manual for installing a binary version of MySQL. If you already have MySQL-3.23 installed, then the simplest way to install MySQL -Max is to replace the server executable ``mysqld'` with the corresponding executable in the -Max distribution. MySQL and MySQL -Max differ only in the server executable. See section 2.2.7 Installing a MySQL Binary Distribution. See section 4.7.5 `mysqld-max`, An Extended `mysqld` Server.

To compile MySQL with InnoDB support, download MySQL-3.23.34a or newer version from http://www.mysql.com/ and configure MySQL with the `--with-innodb` option. See the MySQL manual about installing a MySQL source distribution. See section 2.3 Installing a MySQL Source Distribution.

    cd /path/to/source/of/mysql-3.23.37
    ./configure --with-innodb

To use InnoDB you have to specify InnoDB startup options in your ``my.cnf'` or ``my.ini'` file. The minimal way to modify it is to add to the `[mysqld]` section the line

    innodb_data_file_path=ibdata:30M

but to get good performance it is best that you specify options as recommended. See section 7.5.2 InnoDB Startup Options.

InnoDB is distributed under the GNU GPL License Version 2 (of June 1991). In the source distribution of MySQL, InnoDB appears as a subdirectory.

### 7.5.2 InnoDB Startup Options

To use InnoDB tables in MySQL-Max-3.23 you MUST specify configuration parameters in the `[mysqld]` section of the configuration file ``my.cnf'`, or on Windows optionally in ``my.ini'`.

At the minimum, in 3.23 you must specify `innodb_data_file_path` where you specify the names and the sizes of datafiles. If you do not mention `innodb_data_home_dir` in ``my.cnf'` the default is to create these files to the `datadir` of MySQL. If you specify `innodb_data_home_dir` as an empty string, then you can give absolute paths to your data files in `innodb_data_file_path`. In MySQL-4.0 you do not need to specify even `innodb_data_file_path`: the default for it is to create an auto-extending 10 MB file ``ibdata1'` to the `datadir` of MySQL. (In MySQL-4.0.0 and 4.0.1 the datafile is 64 MB and not auto-extending.)

If you don't want to use `InnoDB` tables, you can add the `skip-innodb` option to your MySQL option file.

But to get good performance you MUST explicitly set the InnoDB parameters listed in the following examples.

Starting from versions 3.23.50 and 4.0.2 InnoDB allows the last datafile on the `innodb_data_file_path` line to be specified as auto-extending. The syntax for `innodb_data_file_path` is then the following:

    pathtodatafile:sizespecification;pathtodatafile:sizespecification;...
    ...  ;pathtodatafile:sizespecification[:autoextend[:max:sizespecification]]

If you specify the last datafile with the autoextend option, InnoDB will extend the last datafile if it runs out of free space in the tablespace. The increment is 8 MB at a time. An example:

    innodb_data_home_dir =
    innodb_data_file_path = /ibdata/ibdata1:100M:autoextend

instructs InnoDB to create just a single datafile whose initial size is 100 MB and which is extended in 8 MB blocks when space runs out. If the disk becomes full you may want to add another data file to another disk, for example. Then you have to look the size of ``ibdata1'`, round the size downward to the closest multiple of 1024 * 1024 bytes (= 1 MB), and specify the rounded size of ``ibdata1'` explicitly in `innodb_data_file_path`. After that you can add another datafile:

    innodb_data_home_dir =
    innodb_data_file_path = /ibdata/ibdata1:988M;/disk2/ibdata2:50M:autoextend

Be cautious on filesystems where the maximum file-size is 2 GB! InnoDB is not aware of the OS maximum file-size. On those filesystems you might want to specify the max size for the datafile:

    innodb_data_home_dir =
    innodb_data_file_path = /ibdata/ibdata1:100M:autoextend:max:2000M

A simple ``my.cnf'` example. Suppose you have a computer with 128 MB RAM and one hard disk. Below is an example of possible configuration parameters in ``my.cnf'` or ``my.ini'` for InnoDB. We assume you are running MySQL-Max-3.23.50 or later, or MySQL-4.0.2 or later. This example suits most users, both on Unix and Windows, who do not want to distribute InnoDB datafiles and log files on several disks. This creates an auto-extending data file ``ibdata1'` and two InnoDB log files ``ib_logfile0'` and ``ib_logfile1'` to the `datadir` of MySQL (typically ``/mysql/data'`). Also the small archived InnoDB log file ``ib_arch_log_0000000000'` ends up in the `datadir`.

    [mysqld]
    # You can write your other MySQL server options here
    # ...
    #                                  Data file(s) must be able to
    #                                  hold your data and indexes.
    #                                  Make sure you have enough
    #                                  free disk space.
    innodb_data_file_path = ibdata1:10M:autoextend
    #                                  Set buffer pool size to
    #                                  50 - 80 % of your computer's
    #                                  memory
    set-variable = innodb_buffer_pool_size=70M
    set-variable = innodb_additional_mem_pool_size=10M
    #                                  Set the log file size to about
    #                                  25 % of the buffer pool size
    set-variable = innodb_log_file_size=20M
    set-variable = innodb_log_buffer_size=8M
    #                                  Set ..flush_log_at_trx_commit
    #                                  to 0 if you can afford losing
    #                                  some last transactions
    innodb_flush_log_at_trx_commit=1

Check that the MySQL server has the rights to create files in `datadir`.

Note that datafiles must be < 2G in some file systems! The combined size of the log files must be < 4G. The combined size of datafiles must be >= 10 MB.

When you for the first time create an InnoDB database, it is best that you start the MySQL server from the command prompt. Then InnoDB will print the information about the database creation to the screen, and you see what is happening. See below next section what the printout should look like. For example, in Windows you can start ``mysqld-max.exe'` with:

    your-path-to-mysqld>mysqld-max --console

Where to put ``my.cnf'` or ``my.ini'` in Windows? The rules for Windows are the following:

- Only one of ``my.cnf'` or ``my.ini'` should be created.
- The ``my.cnf'` file should be placed in the root directory of the drive ``C:'`.
- The ``my.ini'` file should be placed in the WINDIR directory, e.g, ``C:\WINDOWS'` or ``C:\WINNT'`. You can use the `SET` command of MS-DOS to print the value of WINDIR.
- If your PC uses a boot loader where the ``C:'` drive is not the boot drive, then your only option is to use the ``my.ini'` file.

Where to specify options in Unix? On Unix ``mysqld'` reads options from the following files, if they exist, in the following order:

- ``/etc/my.cnf'` Global options.
- ``COMPILATION_DATADIR/my.cnf'` Server-specific options.
- ``defaults-extra-file'` The file specified with `--defaults-extra-file=...`.
- ``~/.my.cnf'` User-specific options.

``COMPILATION_DATADIR'` is the MySQL data directory which was specified as a `./configure` option when ``mysqld'` was compiled (typically ``/usr/local/mysql/data'` for a binary installation or ``/usr/local/var'` for a source installation).

If you are not sure from where ``mysqld'` reads its ``my.cnf'` or ``my.ini'`, you can give the path as the first command-line option to the server: `mysqld --defaults-file=your_path_to_my_cnf`.

InnoDB forms the directory path to a datafile by textually catenating `innodb_data_home_dir` to a datafile name or path in `innodb_data_file_path`, adding a possible slash or backslash in between if needed. If the keyword `innodb_data_home_dir` is not mentioned in ``my.cnf'` at all, the default for it is the 'dot' directory ``./'` which means the `datadir` of MySQL.

An advanced ``my.cnf'` example. Suppose you have a Linux computer with 2 GB RAM and three 60 GB hard disks (at directory paths ``/'`, ``/dr2'` and ``/dr3'`). Below is an example of possible configuration parameters in ``my.cnf'` for InnoDB.

Note that InnoDB does not create directories: you have to create them yourself. Use the Unix or MS-DOS `mkdir` command to create the data and log group home directories.

    [mysqld]
    # You can write your other MySQL server options here
    # ...
    innodb_data_home_dir =
    #                                  Data files must be able to
    #                                  hold your data and indexes
    innodb_data_file_path = /ibdata/ibdata1:2000M;/dr2/ibdata/ibdata2:2000M:autoextend
    #                                  Set buffer pool size to
    #                                  50 - 80 % of your computer's
    #                                  memory, but make sure on Linux
    #                                  x86 total memory usage is
    #                                  < 2 GB
    set-variable = innodb_buffer_pool_size=1G
    set-variable = innodb_additional_mem_pool_size=20M
    innodb_log_group_home_dir = /dr3/iblogs
    #                                  .._log_arch_dir must be the same
    #                                  as .._log_group_home_dir
    innodb_log_arch_dir = /dr3/iblogs
    set-variable = innodb_log_files_in_group=3
    #                                  Set the log file size to about
    #                                  15 % of the buffer pool size
    set-variable = innodb_log_file_size=150M
    set-variable = innodb_log_buffer_size=8M
    #                                  Set ..flush_log_at_trx_commit to
    #                                  0 if you can afford losing
    #                                  some last transactions
    innodb_flush_log_at_trx_commit=1
    set-variable = innodb_lock_wait_timeout=50
    #innodb_flush_method=fdatasync
    #set-variable = innodb_thread_concurrency=5

Note that we have placed the two datafiles on different disks. InnoDB will fill the tablespace formed by the datafiles from bottom up. In some cases it will improve the performance of the database if all data is not placed on the same physical disk. Putting log files on a different disk from data is very often beneficial for performance. You can also use raw disk partitions (raw devices) as datafiles. In some Unixes they speed up I/O. See the manual section on InnoDB file space management about how to specify them in ``my.cnf'`.

Warning: on Linux x86 you must be careful you do not set memory usage too high. glibc will allow the process heap to grow over thread stacks, which will crash your server. It is a risk if the value of

    innodb_buffer_pool_size + key_buffer +
    max_connections * (sort_buffer + read_buffer_size) + max_connections * 2 MB

is close to 2 GB or exceeds 2 GB. Each thread will use a stack (often 2 MB, but in MySQL AB binaries only 256 kB) and in the worst case also `sort_buffer + read_buffer_size` additional memory.

How to tune other ``mysqld'` server parameters? Typical values which suit most users are:

    skip-locking
    set-variable = max_connections=200
    set-variable = read_buffer_size=1M
    set-variable = sort_buffer=1M
    #                                  Set key_buffer to 5 - 50%
    #                                  of your RAM depending on how
    #                                  much you use MyISAM tables, but
    #                                  keep key_buffer + InnoDB
    #                                  buffer pool size < 80% of
    #                                  your RAM
    set-variable = key_buffer=...

Note that some parameters are given using the numeric ``my.cnf'` parameter format: `set-variable = innodb... = 123`, others (string and boolean parameters) with another format: `innodb_... = ...` .

The meanings of the configuration parameters are the following:

Option | Description
`innodb_data_home_dir` | The common part of the directory path for all InnoDB datafiles. If you do not mentioned this option in ``my.cnf'` the default is the `datadir` of MySQL. You can specify this also as an empty string, in which case you can use absolute file paths in `innodb_data_file_path`.
`innodb_data_file_path` | Paths to individual datafiles and their sizes. The full directory path to each datafile is acquired by concatenating innodb_data_home_dir to the paths specified here. The file-sizes are specified in megabytes, hence the 'M' after the size specification above. InnoDB also understands the abbreviation 'G', 1G meaning 1024M. Starting from 3.23.44 you can set the file-size bigger than 4 GB on those operating systems which support big files. On some operating systems files must be < 2 GB. The sum of the sizes of the files must be at least 10 MB.
`innodb_mirrored_log_groups` | Number of identical copies of log groups we keep for the database. Currently this should be set to 1.
`innodb_log_group_home_dir` | Directory path to InnoDB log files.
`innodb_log_files_in_group` | Number of log files in the log group. InnoDB writes to the files in a circular fashion. Value 3 is recommended here.
`innodb_log_file_size` | Size of each log file in a log group in megabytes. Sensible values range from 1M to 1/nth of the size of the buffer pool specified below, where n is the number of log files in the group. The bigger the value, the less checkpoint flush activity is needed in the buffer pool, saving disk I/O. But bigger log files also mean that recovery will be slower in case of a crash. The combined size of log files must be < 4 GB on 32-bit computers.
`innodb_log_buffer_size` | The size of the buffer which InnoDB uses to write log to the log files on disk. Sensible values range from 1M to 8M. A big log buffer allows large transactions to run without a need to write the log to disk until the transaction commit. Thus, if you have big transactions, making the log buffer big will save disk I/O.
`innodb_flush_log_at_trx_commit` | Normally this is set to 1, meaning that at a transaction commit the log is flushed to disk, and the modifications made by the transaction become permanent, and survive a database crash. If you are willing to compromise this safety, and you are running small transactions, you may set this to 0 to reduce disk I/O to the logs.
`innodb_log_arch_dir` | The directory where fully written log files would be archived if we used log archiving. The value of this parameter should currently be set the same as `innodb_log_group_home_dir`.
`innodb_log_archive` | This value should currently be set to 0. As recovery from a backup is done by MySQL using its own log files, there is currently no need to archive InnoDB log files.
`innodb_buffer_pool_size` | The size of the memory buffer InnoDB uses to cache data and indexes of its tables. The bigger you set this the less disk I/O is needed to access data in tables. On a dedicated database server you may set this parameter up to 80% of the machine physical memory size. Do not set it too large, though, because competition of the physical memory may cause paging in the operating system.
`innodb_additional_mem_pool_size` | Size of a memory pool InnoDB uses to store data dictionary information and other internal data structures. A sensible value for this might be 2M, but the more tables you have in your application the more you will need to allocate here. If InnoDB runs out of memory in this pool, it will start to allocate memory from the operating system, and write warning messages to the MySQL error log.
`innodb_file_io_threads` | Number of file I/O threads in InnoDB. Normally, this should be 4, but on Windows disk I/O may benefit from a larger number.
`innodb_lock_wait_timeout` | Timeout in seconds an InnoDB transaction may wait for a lock before being rolled back. InnoDB automatically detects transaction deadlocks in its own lock table and rolls back the transaction. If you use `LOCK TABLES` command, or other transaction-safe storage engines than InnoDB in the same transaction, then a deadlock may arise which InnoDB cannot notice. In cases like this the timeout is useful to resolve the situation.
`innodb_flush_method` | (Available from 3.23.40 up.) The default value for this is `fdatasync`. Another option is `O_DSYNC`.

### 7.5.3 Creating InnoDB Tablespace

Suppose you have installed MySQL and have edited ``my.cnf'` so that it contains the necessary InnoDB configuration parameters. Before starting MySQL you should check that the directories you have specified for InnoDB datafiles and log files exist and that you have access rights to those directories. InnoDB cannot create directories, only files. Check also you have enough disk space for the data and log files.

When you now start MySQL, InnoDB will start creating your datafiles and log files. InnoDB will print something like the following:

    ~/mysqlm/sql > mysqld
    InnoDB: The first specified datafile /home/heikki/data/ibdata1
    did not exist:
    InnoDB: a new database to be created!
    InnoDB: Setting file /home/heikki/data/ibdata1 size to 134217728
    InnoDB: Database physically writes the file full: wait...
    InnoDB: datafile /home/heikki/data/ibdata2 did not exist:
    new to be created
    InnoDB: Setting file /home/heikki/data/ibdata2 size to 262144000
    InnoDB: Database physically writes the file full: wait...
    InnoDB: Log file /home/heikki/data/logs/ib_logfile0 did not exist:
    new to be created
    InnoDB: Setting log file /home/heikki/data/logs/ib_logfile0 size to 5242880
    InnoDB: Log file /home/heikki/data/logs/ib_logfile1 did not exist:
    new to be created
    InnoDB: Setting log file /home/heikki/data/logs/ib_logfile1 size to 5242880
    InnoDB: Log file /home/heikki/data/logs/ib_logfile2 did not exist:
    new to be created
    InnoDB: Setting log file /home/heikki/data/logs/ib_logfile2 size to 5242880
    InnoDB: Started
    mysqld: ready for connections

A new InnoDB database has now been created. You can connect to the MySQL server with the usual MySQL client programs like `mysql`. When you shut down the MySQL server with ``mysqladmin shutdown'`, InnoDB output will be like the following:

    010321 18:33:34  mysqld: Normal shutdown
    010321 18:33:34  mysqld: Shutdown Complete
    InnoDB: Starting shutdown...
    InnoDB: Shutdown completed

You can now look at the datafiles and logs directories and you will see the files created. The log directory will also contain a small file named ``ib_arch_log_0000000000'`. That file resulted from the database creation, after which InnoDB switched off log archiving. When MySQL is again started, the output will be like the following:

    ~/mysqlm/sql > mysqld
    InnoDB: Started
    mysqld: ready for connections

#### 7.5.3.1 If Something Goes Wrong in Database Creation

If InnoDB prints an operating system error in a file operation, usually the problem is one of the following:

- You did not create InnoDB data or log directories.
- ``mysqld'` does not have the rights to create files in those directories.
- ``mysqld'` does not read the right ``my.cnf'` or ``my.ini'` file, and consequently does not see the options you specified.
- The disk is full or a disk quota is exceeded.
- You have created a subdirectory whose name is equal to a datafile you specified.
- There is a syntax error in `innodb_data_home_dir` or `innodb_data_file_path`.

If something goes wrong in an InnoDB database creation, you should delete all files created by InnoDB. This means all datafiles, all log files, the small archived log file, and in the case you already did create some InnoDB tables, delete also the corresponding ``.frm'` files for these tables from the MySQL database directories. Then you can try the InnoDB database creation again.

### 7.5.4 Creating InnoDB Tables

Suppose you have started the MySQL client with the command `mysql test`. To create a table in the InnoDB format you must specify `TYPE = InnoDB` in the table creation SQL command:

    CREATE TABLE CUSTOMER (A INT, B CHAR (20), INDEX (A)) TYPE = InnoDB;

This SQL command will create a table and an index on column `A` into the InnoDB tablespace consisting of the datafiles you specified in ``my.cnf'`. In addition MySQL will create a file ``CUSTOMER.frm'` to the MySQL database directory ``test'`. Internally, InnoDB will add to its own data dictionary an entry for table `'test/CUSTOMER'`. Thus you can create a table of the same name `CUSTOMER` in another database of MySQL, and the table names will not collide inside InnoDB.

You can query the amount of free space in the InnoDB tablespace by issuing the table status command of MySQL for any table you have created with `TYPE = InnoDB`. Then the amount of free space in the tablespace appears in the table comment section in the output of `SHOW`. An example:

    SHOW TABLE STATUS FROM test LIKE 'CUSTOMER'

Note that the statistics `SHOW` gives about InnoDB tables are only approximate: they are used in SQL optimisation. Table and index reserved sizes in bytes are accurate, though.

#### 7.5.4.1 Converting MyISAM Tables to InnoDB

InnoDB does not have a special optimisation for separate index creation. Therefore it does not pay to export and import the table and create indexes afterwards. The fastest way to alter a table to InnoDB is to do the inserts directly to an InnoDB table, that is, use `ALTER TABLE ... TYPE=INNODB`, or create an empty InnoDB table with identical definitions and insert the rows with `INSERT INTO ... SELECT * FROM ...`.

To get better control over the insertion process, it may be good to insert big tables in pieces:

    INSERT INTO newtable SELECT * FROM oldtable
       WHERE yourkey > something AND yourkey <= somethingelse;

After all data has been inserted you can rename the tables.

During the conversion of big tables you should set the InnoDB buffer pool size big to reduce disk I/O. Not bigger than 80% of the physical memory, though. You should set InnoDB log files big, and also the log buffer large.

Make sure you do not run out of tablespace: InnoDB tables take a lot more space than MyISAM tables. If an `ALTER TABLE` runs out of space, it will start a rollback, and that can take hours if it is disk-bound. In inserts InnoDB uses the insert buffer to merge secondary index records to indexes in batches. That saves a lot of disk I/O. In rollback no such mechanism is used, and the rollback can take 30 times longer than the insertion.

In the case of a runaway rollback, if you do not have valuable data in your database, it is better that you kill the database process and delete all InnoDB data and log files and all InnoDB table ``.frm'` files, and start your job again, rather than wait for millions of disk I/Os to complete.

#### 7.5.4.2 Foreign Key Constraints

Starting from version 3.23.43b InnoDB features foreign key constraints. InnoDB is the first MySQL table type which allows you to define foreign key constraints to guard the integrity of your data.

The syntax of a foreign key constraint definition in InnoDB:

    [CONSTRAINT symbol] FOREIGN KEY (index_col_name, ...)
                      REFERENCES table_name (index_col_name, ...)
                      [ON DELETE CASCADE | ON DELETE SET NULL]

Both tables have to be InnoDB type and there must be an index where the foreign key and the referenced key are listed as the first columns. InnoDB does not auto-create indexes on foreign keys or referenced keys: you have to create them explicitly.

Corresponding columns in the foreign key and the referenced key must have similar internal data types inside InnoDB so that they can be compared without a type conversion. The size and the signedness of integer types has to be the same. The length of string types need not be the same.

Starting from version 3.23.50 you can also associate the `ON DELETE CASCADE` or `ON DELETE SET NULL` clause with the foreign key constraint.

If `ON DELETE CASCADE` is specified, and a row in the parent table is deleted, then InnoDB automatically deletes also all those rows in the child table whose foreign key values are equal to the referenced key value in the parent row. If `ON DELETE SET NULL` is specified, the child rows are automatically updated so that the columns in the foreign key are set to the SQL NULL value.

Starting from version 3.23.50, InnoDB does not check foreign key constraints on those foreign key or referenced key values which contain a NULL column.

Starting from version 3.23.50 the InnoDB parser allows you to use backquotes (`) around table and column names in the `FOREIGN KEY ... REFERENCES ...` clause but the InnoDB parser is not yet aware of the option `lower_case_table_names` you can specify in ``my.cnf'`.

An example:

    CREATE TABLE parent(id INT NOT NULL, PRIMARY KEY (id)) TYPE=INNODB;
    CREATE TABLE child(id INT, parent_id INT, INDEX par_ind (parent_id),
                 FOREIGN KEY (parent_id) REFERENCES parent(id)
                 ON DELETE SET NULL
    ) TYPE=INNODB;

If MySQL gives the error number 1005 from a `CREATE TABLE` statement, and the error message string refers to errno 150, then the table creation failed because a foreign key constraint was not correctly formed. Similarly, if an `ALTER TABLE` fails and it refers to errno 150, that means a foreign key definition would be incorrectly formed for the altered table.

Starting from version 3.23.50 InnoDB allows you to add a new foreign key constraint to a table through

    ALTER TABLE yourtablename
    ADD [CONSTRAINT symbol] FOREIGN KEY (...) REFERENCES anothertablename(...)

Remember to create the required indexes first, though. In InnoDB versions < 3.23.50 `ALTER TABLE` or `CREATE INDEX` should not be used in connection with tables which have foreign key constraints or which are referenced in foreign key constraints: Any `ALTER TABLE` removes all foreign key constrainst defined for the table. You should not use `ALTER TABLE` to the referenced table either, but use `DROP TABLE` and `CREATE TABLE` to modify the schema. When MySQL does an `ALTER TABLE` it may internally use `RENAME TABLE`, and that will confuse the foreign key costraints which refer to the table. A `CREATE INDEX` statement is in MySQL processed as an `ALTER TABLE`, and these restrictions apply also to it.

When doing foreign key checks InnoDB sets shared row level locks on child or parent records it has to look at. InnoDB checks foreign key constraints immediately: the check is not deferred to transaction commit.

InnoDB allows you to drop any table even though that would break the foreign key constraints which reference the table. When you drop a table the constraints which were defined in its create statement are also dropped.

If you re-create a table which was dropped, it has to have a definition which conforms to the foreign key constraints referencing it. It must have the right column names and types, and it must have indexes on the referenced keys, as stated above. If these are not satisfied, MySQL returns error number 1005 and refers to errno 150 in the error message string.

Starting from version 3.23.50 InnoDB returns the foreign key definitions of a table when you call

    SHOW CREATE TABLE yourtablename

Then also ``mysqldump'` produces correct definitions of tables to the dump file, and does not forget about the foreign keys.

You can also list the foreign key constraints for a table `T` with

    SHOW TABLE STATUS FROM yourdatabasename LIKE 'T'

The foreign key constraints are listed in the table comment of the output.

### 7.5.5 Adding and Removing InnoDB Data and Log Files

From version 3.23.50 and 4.0.2 you can specify the last InnoDB datafile to `autoextend`. Alternatively, you can increase to your tablespace by specifying an additional datafile. To do this you have to shut down the MySQL server, edit the ``my.cnf'` file adding a new datafile to `innodb_data_file_path`, and then start the MySQL server again.

Currently you cannot remove a datafile from InnoDB. To decrease the size of your database you have to use ``mysqldump'` to dump all your tables, create a new database, and import your tables to the new database.

If you want to change the number or the size of your InnoDB log files, you have to shut down MySQL and make sure that it shuts down without errors. Then copy the old log files into a safe place just in case something went wrong in the shutdown and you will need them to recover the database. Delete then the old log files from the log file directory, edit ``my.cnf'`, and start MySQL again. InnoDB will tell you at the startup that it is creating new log files.

### 7.5.6 Backing up and Recovering an InnoDB Database

The key to safe database management is taking regular backups.

InnoDB Hot Backup is an online backup tool you can use to backup your InnoDB database while it is running. InnoDB Hot Backup does not require you to shut down your database and it does not set any locks or disturb your normal database processing. InnoDB Hot Backup is a non-free additional tool which is not included in the standard MySQL distribution. See the InnoDB Hot Backup homepage http://www.innodb.com/hotbackup.html for detailed information and screenshots.

If you are able to shut down your MySQL server, then to take a 'binary' backup of your database you have to do the following:

- Shut down your MySQL database and make sure it shuts down without errors.
- Copy all your datafiles into a safe place.
- Copy all your InnoDB log files to a safe place.
- Copy your ``my.cnf'` configuration file(s) to a safe place.
- Copy all the ``.frm'` files for your InnoDB tables into a safe place.

In addition to taking the binary backups described above, you should also regularly take dumps of your tables with ``mysqldump'`. The reason to this is that a binary file may be corrupted without you noticing it. Dumped tables are stored into text files which are human-readable and much simpler than database binary files. Seeing table corruption from dumped files is easier, and since their format is simpler, the chance for serious data corruption in them is smaller.

A good idea is to take the dumps at the same time you take a binary backup of your database. You have to shut out all clients from your database to get a consistent snapshot of all your tables into your dumps. Then you can take the binary backup, and you will then have a consistent snapshot of your database in two formats.

To be able to recover your InnoDB database to the present from the binary backup described above, you have to run your MySQL database with the general logging and log archiving of MySQL switched on. Here by the general logging we mean the logging mechanism of the MySQL server which is independent of InnoDB logs.

To recover from a crash of your MySQL server process, the only thing you have to do is to restart it. InnoDB will automatically check the logs and perform a roll-forward of the database to the present. InnoDB will automatically roll back uncommitted transactions which were present at the time of the crash. During recovery, InnoDB will print out something like the following:

    ~/mysqlm/sql > mysqld
    InnoDB: Database was not shut down normally.
    InnoDB: Starting recovery from log files...
    InnoDB: Starting log scan based on checkpoint at
    InnoDB: log sequence number 0 13674004
    InnoDB: Doing recovery: scanned up to log sequence number 0 13739520
    InnoDB: Doing recovery: scanned up to log sequence number 0 13805056
    InnoDB: Doing recovery: scanned up to log sequence number 0 13870592
    InnoDB: Doing recovery: scanned up to log sequence number 0 13936128
    ...
    InnoDB: Doing recovery: scanned up to log sequence number 0 20555264
    InnoDB: Doing recovery: scanned up to log sequence number 0 20620800
    InnoDB: Doing recovery: scanned up to log sequence number 0 20664692
    InnoDB: 1 uncommitted transaction(s) which must be rolled back
    InnoDB: Starting rollback of uncommitted transactions
    InnoDB: Rolling back trx no 16745
    InnoDB: Rolling back of trx no 16745 completed
    InnoDB: Rollback of uncommitted transactions completed
    InnoDB: Starting an apply batch of log records to the database...
    InnoDB: Apply batch completed
    InnoDB: Started
    mysqld: ready for connections

If your database gets corrupted or your disk fails, you have to do the recovery from a backup. In the case of corruption, you should first find a backup which is not corrupted. From a backup do the recovery from the general log files of MySQL according to instructions in the MySQL manual.

#### 7.5.6.1 Checkpoints

InnoDB implements a checkpoint mechanism called a fuzzy checkpoint. InnoDB will flush modified database pages from the buffer pool in small batches, there is no need to flush the buffer pool in one single batch, which would in practice stop processing of user SQL statements for a while.

In crash recovery InnoDB looks for a checkpoint label written to the log files. It knows that all modifications to the database before the label are already present on the disk image of the database. Then InnoDB scans the log files forward from the place of the checkpoint applying the logged modifications to the database.

InnoDB writes to the log files in a circular fashion. All committed modifications which make the database pages in the buffer pool different from the images on disk must be available in the log files in case InnoDB has to do a recovery. This means that when InnoDB starts to reuse a log file in the circular fashion, it has to make sure that the database page images on disk already contain the modifications logged in the log file InnoDB is going to reuse. In other words, InnoDB has to make a checkpoint and often this involves flushing of modified database pages to disk.

The above explains why making your log files very big may save disk I/O in checkpointing. It can make sense to set the total size of the log files as big as the buffer pool or even bigger. The drawback in big log files is that crash recovery can last longer because there will be more log to apply to the database.

### 7.5.7 Moving an InnoDB Database to Another Machine

InnoDB data and log files are binary-compatible on all platforms if the floating-point number format on the machines is the same. You can move an InnoDB database simply by copying all the relevant files, which we already listed in the previous section on backing up a database. If the floating-point formats on the machines are different but you have not used `FLOAT` or `DOUBLE` data types in your tables then the procedure is the same: just copy the relevant files. If the formats are different and your tables contain floating-point data, you have to use ``mysqldump'` and ``mysqlimport'` to move those tables.

A performance tip is to switch off auto-commit mode when you import data into your database, assuming your tablespace has enough space for the big rollback segment the big import transaction will generate. Do the commit only after importing a whole table or a segment of a table.

### 7.5.8 InnoDB Transaction Model

In the InnoDB transaction model the goal has been to combine the best properties of a multi-versioning database to traditional two-phase locking. InnoDB does locking on row level and runs queries by default as non-locking consistent reads, in the style of Oracle. The lock table in InnoDB is stored so space-efficiently that lock escalation is not needed: typically several users are allowed to lock every row in the database, or any random subset of the rows, without InnoDB running out of memory.

In InnoDB all user activity happens inside transactions. If the autocommit mode is used in MySQL, then each SQL statement will form a single transaction. MySQL always starts a new connection with the autocommit mode switched on.

If the autocommit mode is switched off with `SET AUTOCOMMIT = 0`, then we can think that a user always has a transaction open. If he issues the SQL `COMMIT` or `ROLLBACK` statement, it ends the current transaction, and a new starts. Both statements will release all InnoDB locks that were set during the current transaction. A `COMMIT` means that the changes made in the current transaction are made permanent and become visible to other users. A `ROLLBACK`, on the other hand, cancels all modifications made by the current transaction.

If the connection has `AUTOCOMMIT = 1`, then the user can still perform a multi-statement transaction by starting it with `BEGIN` and ending it with `COMMIT` or `ROLLBACK`.

In terms of the SQL-1992 transaction isolation levels, the InnoDB default is `REPEATABLE READ`. Starting from version 4.0.5, InnoDB offers all 4 different transaction isolation levels described by the SQL-1992 standard. You can set the default isolation level for all connections in the `[mysqld]` section of ``my.cnf'`:

    transaction-isolation = {READ-UNCOMMITTED | READ-COMMITTED
                             | REPEATABLE-READ | SERIALIZABLE}

A user can change the isolation level of a single session or all new incoming connections with the

    SET [SESSION | GLOBAL] TRANSACTION ISOLATION LEVEL
                           {READ UNCOMMITTED | READ COMMITTED
                            | REPEATABLE READ | SERIALIZABLE}

SQL statement. Note that there are no hyphens in level names in the SQL syntax. If you specify the keyword `GLOBAL` in the above statement, it will determine the initial isolation level of new incoming connections, but will not change the isolation level of old connections. Any user is free to change the isolation level of his session, even in the middle of a transaction. In versions < 3.23.50 `SET TRANSACTION` had no effect on InnoDB tables. In versions < 4.0.5 only `REPEATABLE READ` and `SERIALIZABLE` were available.

You can query the global and session transaction isolation levels with:

    SELECT @@global.tx_isolation;
    SELECT @@tx_isolation;

In row level locking InnoDB uses so-called next-key locking. That means that besides index records, InnoDB can also lock the 'gap' before an index record to block insertions by other users immediately before the index record. A next-key lock means a lock which locks an index record and the gap before it. A gap lock means a lock which only locks a gap before some index record.

A detailed description of each isolation level in InnoDB:

- `READ UNCOMMITTED` This is also called 'dirty read': non-locking `SELECT`s are performed so that we do not look at a possible earlier version of a record; thus they are not 'consistent' reads under this isolation level; otherwise this level works like `READ COMMITTED`.
- `READ COMMITTED` Somewhat Oracle-like isolation level. All `SELECT ... FOR UPDATE` and `SELECT ... LOCK IN SHARE MODE` statements only lock the index records, NOT the gaps before them, and thus allow free inserting of new records next to locked records. `UPDATE` and `DELETE` which use a unique index with a unique search condition, only lock the index record found, not the gap before it. But still in range type `UPDATE` and `DELETE` InnoDB must set next-key or gap locks and block insertions by other users to the gaps covered by the range. This is necessary since 'phantom rows' have to be blocked for MySQL replication and recovery to work. Consistent reads behave like in Oracle: each consistent read, even within the same transaction, sets and reads its own fresh snapshot.
- `REPEATABLE READ` This is the default isolation level of InnoDB. `SELECT ... FOR UPDATE`, `SELECT ... LOCK IN SHARE MODE`, `UPDATE`, and `DELETE` which use a unique index with a unique search condition, only lock the index record found, not the gap before it. Otherwise these operations employ next-key locking, locking the index range scanned with next-key or gap locks, and block new insertions by other users. In consistent reads there is an important difference from the previous isolation level: in this level all consistent reads within the same transaction read the same snapshot established by the first read. This convention means that if you issue several plain `SELECT`s within the same transaction, these `SELECT`s are consistent also with respect to each other.
- `SERIALIZABLE` This level is like the previous one, but all plain `SELECT`s are implicitly converted to `SELECT ... LOCK IN SHARE MODE`.

#### 7.5.8.1 Consistent Read

A consistent read means that InnoDB uses its multi-versioning to present to a query a snapshot of the database at a point in time. The query will see the changes made by exactly those transactions that committed before that point of time, and no changes made by later or uncommitted transactions. The exception to this rule is that the query will see the changes made by the transaction itself which issues the query.

If you are running with the default `REPEATABLE READ` isolation level, then all consistent reads within the same transaction read the snapshot established by the first such read in that transaction. You can get a fresher snapshot for your queries by committing the current transaction and after that issuing new queries.

Consistent read is the default mode in which InnoDB processes `SELECT` statements in `READ COMMITTED` and `REPEATABLE READ` isolation levels. A consistent read does not set any locks on the tables it accesses, and therefore other users are free to modify those tables at the same time a consistent read is being performed on the table.

#### 7.5.8.2 Locking Reads

A consistent read is not convenient in some circumstances. Suppose you want to add a new row into your table `CHILD`, and make sure that the child already has a parent in table `PARENT`.

Suppose you use a consistent read to read the table `PARENT` and indeed see the parent of the child in the table. Can you now safely add the child row to table `CHILD`? No, because it may happen that meanwhile some other user has deleted the parent row from the table `PARENT`, and you are not aware of that.

The solution is to perform the `SELECT` in a locking mode, `LOCK IN SHARE MODE`.

    SELECT * FROM PARENT WHERE NAME = 'Jones' LOCK IN SHARE MODE;

Performing a read in share mode means that we read the latest available data, and set a shared mode lock on the rows we read. If the latest data belongs to a yet uncommitted transaction of another user, we will wait until that transaction commits. A shared mode lock prevents others from updating or deleting the row we have read. After we see that the above query returns the parent `'Jones'`, we can safely add his child to table `CHILD`, and commit our transaction. This example shows how to implement referential integrity in your application code.

Let us look at another example: we have an integer counter field in a table `CHILD_CODES` which we use to assign a unique identifier to each child we add to table `CHILD`. Obviously, using a consistent read or a shared mode read to read the present value of the counter is not a good idea, since then two users of the database may see the same value for the counter, and we will get a duplicate key error when we add the two children with the same identifier to the table.

In this case there are two good ways to implement the reading and incrementing of the counter: (1) update the counter first by incrementing it by 1 and only after that read it, or (2) read the counter first with a lock mode `FOR UPDATE`, and increment after that:

    SELECT COUNTER_FIELD FROM CHILD_CODES FOR UPDATE;
    UPDATE CHILD_CODES SET COUNTER_FIELD = COUNTER_FIELD + 1;

A `SELECT ... FOR UPDATE` will read the latest available data setting exclusive locks on each row it reads. Thus it sets the same locks a searched SQL `UPDATE` would set on the rows.

#### 7.5.8.3 Next-key Locking: Avoiding the Phantom Problem

In row level locking InnoDB uses an algorithm called next-key locking. InnoDB does the row level locking so that when it searches or scans an index of a table, it sets shared or exclusive locks on the index records in encounters. Thus the row level locks are more precisely called index record locks.

The locks InnoDB sets on index records also affect the 'gap' before that index record. If a user has a shared or exclusive lock on record R in an index, then another user cannot insert a new index record immediately before R in the index order. This locking of gaps is done to prevent the so-called phantom problem. Suppose I want to read and lock all children with identifier bigger than 100 from table `CHILD`, and update some field in the selected rows.

    SELECT * FROM CHILD WHERE ID > 100 FOR UPDATE;

Suppose there is an index on table `CHILD` on column `ID`. Our query will scan that index starting from the first record where `ID` is bigger than 100. Now, if the locks set on the index records would not lock out inserts made in the gaps, a new child might meanwhile be inserted to the table. If now I in my transaction execute

    SELECT * FROM CHILD WHERE ID > 100 FOR UPDATE;

again, I will see a new child in the result set the query returns. This is against the isolation principle of transactions: a transaction should be able to run so that the data it has read does not change during the transaction. If we regard a set of rows as a data item, then the new 'phantom' child would break this isolation principle.

When InnoDB scans an index it can also lock the gap after the last record in the index. Just that happens in the previous example: the locks set by InnoDB will prevent any insert to the table where `ID` would be bigger than 100.

You can use next-key locking to implement a uniqueness check in your application: if you read your data in share mode and do not see a duplicate for a row you are going to insert, then you can safely insert your row and know that the next-key lock set on the successor of your row during the read will prevent anyone meanwhile inserting a duplicate for your row. Thus the next-key locking allows you to 'lock' the non-existence of something in your table.

#### 7.5.8.4 Locks Set by Different SQL Statements in InnoDB

- `SELECT ... FROM ...` : this is a consistent read, reading a snapshot of the database and setting no locks.
- `SELECT ... FROM ... LOCK IN SHARE MODE` : sets shared next-key locks on all index records the read encounters.
- `SELECT ... FROM ... FOR UPDATE` : sets exclusive next-key locks on all index records the read encounters.
- `INSERT INTO ... VALUES (...)` : sets an exclusive lock on the inserted row; note that this lock is not a next-key lock and does not prevent other users from inserting to the gap before the inserted row. If a duplicate key error occurs, sets a shared lock on the duplicate index record.
- `INSERT INTO T SELECT ... FROM S WHERE ...` sets an exclusive (non-next-key) lock on each row inserted into `T`. Does the search on `S` as a consistent read, but sets shared next-key locks on `S` if the MySQL logging is on. InnoDB has to set locks in the latter case because in roll-forward recovery from a backup every SQL statement has to be executed in exactly the same way as it was done originally.
- `CREATE TABLE ... SELECT ...` performs the `SELECT` as a consistent read or with shared locks, like in the previous item.
- `REPLACE` is done like an insert if there is no collision on a unique key. Otherwise, an exclusive next-key lock is placed on the row which has to be updated.
- `UPDATE ... SET ... WHERE ...` : sets an exclusive next-key lock on every record the search encounters.
- `DELETE FROM ... WHERE ...` : sets an exclusive next-key lock on every record the search encounters.
- If a `FOREIGN KEY` constraint is defined on a table, any insert, update, or delete which requires checking of the constraint condition sets shared record level locks on the records it looks at to check the constraint. Also in the case where the constraint fails, InnoDB sets these locks.
- `LOCK TABLES ...` : sets table locks. In the implementation the MySQL layer of code sets these locks. The automatic deadlock detection of InnoDB cannot detect deadlocks where such table locks are involved: see the following section. Also, since MySQL does know about row level locks, it is possible that you get a table lock on a table where another user currently has row level locks. But that does not put transaction integerity into danger. See section 7.5.13 Restrictions on InnoDB Tables.

#### 7.5.8.5 Deadlock Detection and Rollback

InnoDB automatically detects a deadlock of transactions and rolls back a transaction or transactions to prevent the deadlock. Starting from version 4.0.5, InnoDB will try to pick small transactions to roll back. The size of a transaction is determined by the number of rows it has inserted, updated, or deleted. Previous to 4.0.5, InnoDB always rolled back the transaction whose lock request was the last one to build a deadlock, that is, a cycle in the waits-for graph of transactions.

InnoDB cannot detect deadlocks where a lock set by a MySQL `LOCK TABLES` statement is involved, or if a lock set in another storage engine than InnoDB is involved. You have to resolve these situations using `innodb_lock_wait_timeout` set in ``my.cnf'`.

When InnoDB performs a complete rollback of a transaction, all the locks of the transaction are released. However, if just a single SQL statement is rolled back as a result of an error, some of the locks set by the SQL statement may be preserved. This is because InnoDB stores row locks in a format where it cannot afterwards know which was set by which SQL statement.

#### 7.5.8.6 An Example of How the Consistent Read Works in InnoDB

Suppose you are running on the default `REPEATABLE READ` isolation level. When you issue a consistent read, that is, an ordinary `SELECT` statement, InnoDB will give your transaction a timepoint according to which your query sees the database. Thus, if transaction B deletes a row and commits after your timepoint was assigned, then you will not see the row deleted. Similarly with inserts and updates.

You can advance your timepoint by committing your transaction and then doing another `SELECT`.

This is called multi-versioned concurrency control.

                      User A                 User B

                  SET AUTOCOMMIT=0;      SET AUTOCOMMIT=0;
    time
    |             SELECT * FROM t;
    |             empty set
    |                                    INSERT INTO t VALUES (1, 2);
    |
    v             SELECT * FROM t;
                  empty set
                                         COMMIT;

                  SELECT * FROM t;
                  empty set;

                  COMMIT;

                  SELECT * FROM t;
                  ---------------------
                  |    1    |    2    |
                  ---------------------

Thus user A sees the row inserted by B only when B has committed the insert, and A has committed his own transaction so that the timepoint is advanced past the commit of B.

If you want to see the ``freshest'' state of the database, you should use a locking read:

    SELECT * FROM t LOCK IN SHARE MODE;

#### 7.5.8.7 How to cope with deadlocks?

Deadlocks are a classic problem in transactional databases, but they are not dangerous, unless they are so frequent that you cannot run certain transactions at all. Normally you have to write your applications so that they are always prepared to re-issue a transaction if it gets rolled back because of a deadlock.

InnoDB uses automatic row level locking. You can get deadlocks even in the case of transactions which just insert or delete a single row. That is because these operations are not really 'atomic': they automatically set locks on the (possibly several) index records of the row inserted/deleted.

You can cope with deadlocks and reduce the number of them with the following tricks:

- Use `SHOW INNODB STATUS` in MySQL versions >= 3.23.52 and >= 4.0.3 to determine the cause of the latest deadlock. That can help you to tune your application to avoid deadlocks.
- Always be prepared to re-issue a transaction if it fails in a deadlock. Deadlocks are not dangerous. Just try again.
- Commit your transactions often. Small transactions are less prone to collide.
- If you are using locking reads `SELECT ... FOR UPDATE` or `... LOCK IN SHARE MODE`, try using a lower isolation level `READ COMMITTED`.
- Access your tables and rows in a fixed order. Then transactions will form nice queues, and do not deadlock.
- Add good indexes to your tables. Then your queries need to scan less index records and consequently set less locks. Use `EXPLAIN SELECT` to determine that MySQL picks appropriate indexes for your queries.
- Use less locking: if you can afford a `SELECT` to return data from an old snapshot, do not add the clause `FOR UPDATE` or `LOCK IN SHARE MODE` to it. Using `READ COMMITTED` isolation level is good here, because each consistent read within the same transaction reads from its own fresh snapshot.
- If nothing helps, serialize your transactions with table level locks: `LOCK TABLES t1 WRITE, t2 READ, ... ; [do something with tables t1 and t2 here]; UNLOCK TABLES`. Table level locks make you transactions to queue nicely, and deadlocks are avoided. Note that `LOCK TABLES` implicitly starts a transaction, just like the command `BEGIN`, and `UNLOCK TABLES` implicitly ends the transaction in a `COMMIT`.
- Another solution to serialize transactions is to create an auxiliary 'semaphore' table where there is just a single row. Each transaction updates that row before accessing other tables. In that way all transactions happen in a serial fashion. Note that then also the InnoDB instant deadlock detection algorithm works, because the serializing lock is a row level lock. In MySQL table level locks we have to resort to the timeout method to resolve a deadlock.

#### 7.5.8.8 Performance Tuning Tips

1. If the Unix ``top'` or the Windows ``Task Manager'` shows that the CPU usage percentage with your workload is less than 70%, your workload is probably disk-bound. Maybe you are making too many transaction commits, or the buffer pool is too small. Making the buffer pool bigger can help, but do not set it bigger than 80% of physical memory.

2. Wrap several modifications into one transaction. InnoDB must flush the log to disk at each transaction commit, if that transaction made modifications to the database. Since the rotation speed of a disk is typically at most 167 revolutions/second, that constrains the number of commits to the same 167/second if the disk does not fool the operating system.

3. If you can afford the loss of some latest committed transactions, you can set the ``my.cnf'` parameter `innodb_flush_log_at_trx_commit` to zero. InnoDB tries to flush the log anyway once in a second, though the flush is not guaranteed.

4. Make your log files big, even as big as the buffer pool. When InnoDB has written the log files full, it has to write the modified contents of the buffer pool to disk in a checkpoint. Small log files will cause many unnecessary disk writes. The drawback in big log files is that recovery time will be longer.

5. Also the log buffer should be quite big, say 8 MB.

6. (Relevant from 3.23.39 up.) In some versions of Linux and Unix, flushing files to disk with the Unix `fdatasync` and other similar methods is surprisingly slow. The default method InnoDB uses is the `fdatasync` function. If you are not satisfied with the database write performance, you may try setting `innodb_flush_method` in ``my.cnf'` to `O_DSYNC`, though O_DSYNC seems to be slower on most systems.

7. In importing data to InnoDB, make sure that MySQL does not have `autocommit=1` on. Then every insert requires a log flush to disk. Put before your plain SQL import file line

    SET AUTOCOMMIT=0;

and after it

    COMMIT;

If you use the ``mysqldump'` option `--opt`, you will get dump files which are fast to import also to an InnoDB table, even without wrapping them to the above `SET AUTOCOMMIT=0; ... COMMIT;` wrappers.

8. Beware of big rollbacks of mass inserts: InnoDB uses the insert buffer to save disk I/O in inserts, but in a corresponding rollback no such mechanism is used. A disk-bound rollback can take 30 times the time of the corresponding insert. Killing the database process will not help because the rollback will start again at the database startup. The only way to get rid of a runaway rollback is to increase the buffer pool so that the rollback becomes CPU-bound and runs fast, or delete the whole InnoDB database.

9. Beware also of other big disk-bound operations. Use `DROP TABLE` or `TRUNCATE` (from MySQL-4.0 up) to empty a table, not `DELETE FROM yourtable`.

10. Use the multi-line `INSERT` to reduce communication overhead between the client and the server if you need to insert many rows:

    INSERT INTO yourtable VALUES (1, 2), (5, 5);

This tip is of course valid for inserts into any table type, not just InnoDB.

#### 7.5.8.9 The InnoDB Monitor

Starting from version 3.23.41 InnoDB includes the InnoDB Monitor which prints information on the InnoDB internal state. When switched on, InnoDB Monitor will make the MySQL server ``mysqld'` to print data (note: the MySQL client will not print anything) to the standard output about once every 15 seconds. This data is useful in performance tuning. On Windows you must start `mysqld-max` from a MS-DOS prompt with the `--standalone --console` options to direct the output to the MS-DOS prompt window.

There is a separate `innodb_lock_monitor` which prints the same information as `innodb_monitor` plus information on locks set by each transaction.

The printed information includes data on:

- lock waits of a transactions,
- semaphore waits of threads,
- pending file I/O requests,
- buffer pool statistics, and
- purge and insert buffer merge activity of the main thread of InnoDB.

You can start InnoDB Monitor through the following SQL command:

    CREATE TABLE innodb_monitor(a int) type = innodb;

and stop it by

    DROP TABLE innodb_monitor;

The `CREATE TABLE` syntax is just a way to pass a command to the InnoDB engine through the MySQL SQL parser: the created table is not relevant at all for InnoDB Monitor. If you shut down the database when the monitor is running, and you want to start the monitor again, you have to drop the table before you can issue a new `CREATE TABLE` to start the monitor. This syntax may change in a future release.

A sample output of the InnoDB Monitor:

    ================================
    010809 18:45:06 INNODB MONITOR OUTPUT
    ================================
    --------------------------
    LOCKS HELD BY TRANSACTIONS
    --------------------------
    LOCK INFO:
    Number of locks in the record hash table 1294
    LOCKS FOR TRANSACTION ID 0 579342744
    TABLE LOCK table test/mytable trx id 0 582333343 lock_mode IX

    RECORD LOCKS space id 0 page no 12758 n bits 104 table test/mytable index
    PRIMARY trx id 0 582333343 lock_mode X
    Record lock, heap no 2 PHYSICAL RECORD: n_fields 74; 1-byte offs FALSE;
    info bits 0
     0: len 4; hex 0001a801; asc ;; 1: len 6; hex 000022b5b39f; asc ";;
     2: len 7; hex 000002001e03ec; asc ;; 3: len 4; hex 00000001;
    ...
    -----------------------------------------------
    CURRENT SEMAPHORES RESERVED AND SEMAPHORE WAITS
    -----------------------------------------------
    SYNC INFO:
    Sorry, cannot give mutex list info in non-debug version!
    Sorry, cannot give rw-lock list info in non-debug version!
    -----------------------------------------------------
    SYNC ARRAY INFO: reservation count 6041054, signal count 2913432
    4a239430 waited for by thread 49627477 op. S-LOCK file NOT KNOWN line 0
    Mut ex 0 sp 5530989 r 62038708 sys 2155035;
    rws 0 8257574 8025336; rwx 0 1121090 1848344
    -----------------------------------------------------
    CURRENT PENDING FILE I/O'S
    --------------------------
    Pending normal aio reads:
    Reserved slot, messages 40157658 4a4a40b8
    Reserved slot, messages 40157658 4a477e28
    ...
    Reserved slot, messages 40157658 4a4424a8
    Reserved slot, messages 40157658 4a39ea38
    Total of 36 reserved aio slots
    Pending aio writes:
    Total of 0 reserved aio slots
    Pending insert buffer aio reads:
    Total of 0 reserved aio slots
    Pending log writes or reads:
    Reserved slot, messages 40158c98 40157f98
    Total of 1 reserved aio slots
    Pending synchronous reads or writes:
    Total of 0 reserved aio slots
    -----------
    BUFFER POOL
    -----------
    LRU list length 8034
    Free list length 0
    Flush list length 999
    Buffer pool size in pages 8192
    Pending reads 39
    Pending writes: LRU 0, flush list 0, single page 0
    Pages read 31383918, created 51310, written 2985115
    ----------------------------
    END OF INNODB MONITOR OUTPUT
    ============================
    010809 18:45:22 InnoDB starts purge
    010809 18:45:22 InnoDB purged 0 pages

Some notes on the output:

- If the section LOCKS HELD BY TRANSACTIONS reports lock waits, then your application may have lock contention. The output can also help to trace reasons for transaction deadlocks.
- Section SYNC INFO will report reserved semaphores if you compile InnoDB with `UNIV_SYNC_DEBUG` defined in ``univ.i'`.
- Section SYNC ARRAY INFO reports threads waiting for a semaphore and statistics on how many times threads have needed a spin or a wait on a mutex or a rw-lock semaphore. A big number of threads waiting for semaphores may be a result of disk I/O, or contention problems inside InnoDB. Contention can be due to heavy parallelism of queries, or problems in operating system thread scheduling.
- Section CURRENT PENDING FILE I/O'S lists pending file I/O requests. A large number of these indicates that the workload is disk I/O-bound.
- Section BUFFER POOL gives you statistics on pages read and written. You can calculate from these numbers how many datafile I/Os your queries are currently doing.

### 7.5.9 Implementation of Multi-versioning

Since InnoDB is a multi-versioned database, it must keep information of old versions of rows in the tablespace. This information is stored in a data structure we call a rollback segment after an analogous data structure in Oracle.

InnoDB internally adds two fields to each row stored in the database. A 6-byte field tells the transaction identifier for the last transaction which inserted or updated the row. Also a deletion is internally treated as an update where a special bit in the row is set to mark it as deleted. Each row also contains a 7-byte field called the roll pointer. The roll pointer points to an undo log record written to the rollback segment. If the row was updated, then the undo log record contains the information necessary to rebuild the content of the row before it was updated.

InnoDB uses the information in the rollback segment to perform the undo operations needed in a transaction rollback. It also uses the information to build earlier versions of a row for a consistent read.

Undo logs in the rollback segment are divided into insert and update undo logs. Insert undo logs are only needed in transaction rollback and can be discarded as soon as the transaction commits. Update undo logs are used also in consistent reads, and they can be discarded only after there is no transaction present for which InnoDB has assigned a snapshot that in a consistent read could need the information in the update undo log to build an earlier version of a database row.

You must remember to commit your transactions regularly, also those transactions which only issue consistent reads. Otherwise InnoDB cannot discard data from the update undo logs, and the rollback segment may grow too big, filling up your tablespace.

The physical size of an undo log record in the rollback segment is typically smaller than the corresponding inserted or updated row. You can use this information to calculate the space need for your rollback segment.

In our multi-versioning scheme a row is not physically removed from the database immediately when you delete it with an SQL statement. Only when InnoDB can discard the update undo log record written for the deletion, it can also physically remove the corresponding row and its index records from the database. This removal operation is called a purge, and it is quite fast, usually taking the same order of time as the SQL statement which did the deletion.

### 7.5.10 Table and Index Structures

MySQL stores its data dictionary information of tables in ``.frm'` files in database directories. But every InnoDB type table also has its own entry in InnoDB internal data dictionaries inside the tablespace. When MySQL drops a table or a database, it has to delete both a ``.frm'` file or files, and the corresponding entries inside the InnoDB data dictionary. This is the reason why you cannot move InnoDB tables between databases simply by moving the ``.frm'` files, and why `DROP DATABASE` did not work for InnoDB type tables in MySQL versions <= 3.23.43.

Every InnoDB table has a special index called the clustered index where the data of the rows is stored. If you define a `PRIMARY KEY` on your table, then the index of the primary key will be the clustered index.

If you do not define a primary key for your table, InnoDB will internally generate a clustered index where the rows are ordered by the row id InnoDB assigns to the rows in such a table. The row id is a 6-byte field which monotonically increases as new rows are inserted. Thus the rows ordered by the row id will be physically in the insertion order.

Accessing a row through the clustered index is fast, because the row data will be on the same page where the index search leads us. In many databases the data is traditionally stored on a different page from the index record. If a table is large, the clustered index architecture often saves a disk I/O when compared to the traditional solution.

The records in non-clustered indexes (we also call them secondary indexes), in InnoDB contain the primary key value for the row. InnoDB uses this primary key value to search for the row from the clustered index. Note that if the primary key is long, the secondary indexes will use more space.

#### 7.5.10.1 Physical Structure of an Index

All indexes in InnoDB are B-trees where the index records are stored in the leaf pages of the tree. The default size of an index page is 16 kB. When new records are inserted, InnoDB tries to leave 1 / 16 of the page free for future insertions and updates of the index records.

If index records are inserted in a sequential (ascending or descending) order, the resulting index pages will be about 15/16 full. If records are inserted in a random order, then the pages will be 1/2 - 15/16 full. If the fillfactor of an index page drops below 1/2, InnoDB will try to contract the index tree to free the page.

#### 7.5.10.2 Insert Buffering

It is a common situation in a database application that the primary key is a unique identifier and new rows are inserted in the ascending order of the primary key. Thus the insertions to the clustered index do not require random reads from a disk.

On the other hand, secondary indexes are usually non-unique and insertions happen in a relatively random order into secondary indexes. This would cause a lot of random disk I/Os without a special mechanism used in InnoDB.

If an index record should be inserted to a non-unique secondary index, InnoDB checks if the secondary index page is already in the buffer pool. If that is the case, InnoDB will do the insertion directly to the index page. But, if the index page is not found from the buffer pool, InnoDB inserts the record to a special insert buffer structure. The insert buffer is kept so small that it entirely fits in the buffer pool, and insertions can be made to it very fast.

The insert buffer is periodically merged to the secondary index trees in the database. Often we can merge several insertions on the same page in of the index tree, and hence save disk I/Os. It has been measured that the insert buffer can speed up insertions to a table up to 15 times.

#### 7.5.10.3 Adaptive Hash Indexes

If a database fits almost entirely in main memory, then the fastest way to perform queries on it is to use hash indexes. InnoDB has an automatic mechanism which monitors index searches made to the indexes defined for a table, and if InnoDB notices that queries could benefit from building of a hash index, such an index is automatically built.

But note that the hash index is always built based on an existing B-tree index on the table. InnoDB can build a hash index on a prefix of any length of the key defined for the B-tree, depending on what search pattern InnoDB observes on the B-tree index. A hash index can be partial: it is not required that the whole B-tree index is cached in the buffer pool. InnoDB will build hash indexes on demand to those pages of the index which are often accessed.

In a sense, through the adaptive hash index mechanism InnoDB adapts itself to ample main memory, coming closer to the architecture of main memory databases.

#### 7.5.10.4 Physical Record Structure

- Each index record in InnoDB contains a header of 6 bytes. The header is used to link consecutive records together, and also in the row level locking.
- Records in the clustered index contain fields for all user-defined columns. In addition, there is a 6-byte field for the transaction id and a 7-byte field for the roll pointer.
- If the user has not defined a primary key for a table, then each clustered index record contains also a 6-byte row id field.
- Each secondary index record contains also all the fields defined for the clustered index key.
- A record contains also a pointer to each field of the record. If the total length of the fields in a record is < 128 bytes, then the pointer is 1 byte, else 2 bytes.

#### 7.5.10.5 How an Auto-increment Column Works in InnoDB

After a database startup, when a user first does an insert to a table `T` where an auto-increment column has been defined, and the user does not provide an explicit value for the column, then InnoDB executes `SELECT MAX(auto-inc-column) FROM T`, and assigns that value incremented by one to the column and the auto-increment counter of the table. We say that the auto-increment counter for table `T` has been initialised.

InnoDB follows the same procedure in initializing the auto-increment counter for a freshly created table.

Note that if the user specifies in an insert the value 0 to the auto-increment column, then InnoDB treats the row like the value would not have been specified.

After the auto-increment counter has been initialised, if a user inserts a row where he explicitly specifies the column value, and the value is bigger than the current counter value, then the counter is set to the specified column value. If the user does not explicitly specify a value, then InnoDB increments the counter by one and assigns its new value to the column.

The auto-increment mechanism, when assigning values from the counter, bypasses locking and transaction handling. Therefore you may also get gaps in the number sequence if you roll back transactions which have got numbers from the counter.

The behaviour of auto-increment is not defined if a user gives a negative value to the column or if the value becomes bigger than the maximum integer that can be stored in the specified integer type.

### 7.5.11 File Space Management and Disk I/O

#### 7.5.11.1 Disk I/O

In disk I/O InnoDB uses asynchronous I/O. On Windows NT it uses the native asynchronous I/O provided by the operating system. On Unix, InnoDB uses simulated asynchronous I/O built into InnoDB: InnoDB creates a number of I/O threads to take care of I/O operations, such as read-ahead. In a future version we will add support for simulated aio on Windows NT and native aio on those versions of Unix which have one.

On Windows NT InnoDB uses non-buffered I/O. That means that the disk pages InnoDB reads or writes are not buffered in the operating system file cache. This saves some memory bandwidth.

Starting from 3.23.41 InnoDB uses a novel file flush technique called doublewrite. It adds safety to crash recovery after an operating system crash or a power outage, and improves performance on most Unix flavors by reducing the need for fsync operations.

Doublewrite means that InnoDB before writing pages to a datafile first writes them to a contiguous tablespace area called the doublewrite buffer. Only after the write and the flush to the doublewrite buffer has completed, InnoDB writes the pages to their proper positions in the datafile. If the operating system crashes in the middle of a page write, InnoDB will in recovery find a good copy of the page from the doublewrite buffer.

Starting from 3.23.41 you can also use a raw disk partition as a datafile, though this has not been tested yet. When you create a new datafile you have to put the keyword `newraw` immediately after the data file-size in `innodb_data_file_path`. The partition must be >= than you specify as the size. Note that 1M in InnoDB is 1024 x 1024 bytes, while in disk specifications 1 MB usually means 1000 000 bytes.

    innodb_data_file_path=hdd1:5Gnewraw;hdd2:2Gnewraw

When you start the database again you must change the keyword to `raw`. Otherwise, InnoDB will write over your partition!

    innodb_data_file_path=hdd1:5Graw;hdd2:2Graw

By using a raw disk you can on some Unixes perform unbuffered I/O.

There are two read-ahead heuristics in InnoDB: sequential read-ahead and random read-ahead. In sequential read-ahead InnoDB notices that the access pattern to a segment in the tablespace is sequential. Then InnoDB will post in advance a batch of reads of database pages to the I/O system. In random read-ahead InnoDB notices that some area in a tablespace seems to be in the process of being fully read into the buffer pool. Then InnoDB posts the remaining reads to the I/O system.

#### 7.5.11.2 File Space Management

The datafiles you define in the configuration file form the tablespace of InnoDB. The files are simply catenated to form the tablespace, there is no striping in use. Currently you cannot directly instruct where the space is allocated for your tables, except by using the following fact: from a newly created tablespace InnoDB will allocate space starting from the low end.

The tablespace consists of database pages whose default size is 16 kB. The pages are grouped into extents of 64 consecutive pages. The 'files' inside a tablespace are called segments in InnoDB. The name of the rollback segment is somewhat misleading because it actually contains many segments in the tablespace.

For each index in InnoDB we allocate two segments: one is for non-leaf nodes of the B-tree, the other is for the leaf nodes. The idea here is to achieve better sequentiality for the leaf nodes, which contain the data.

When a segment grows inside the tablespace, InnoDB allocates the first 32 pages to it individually. After that InnoDB starts to allocate whole extents to the segment. InnoDB can add to a large segment up to 4 extents at a time to ensure good sequentiality of data.

Some pages in the tablespace contain bitmaps of other pages, and therefore a few extents in an InnoDB tablespace cannot be allocated to segments as a whole, but only as individual pages.

When you issue a query `SHOW TABLE STATUS FROM ... LIKE ...` to ask for available free space in the tablespace, InnoDB will report the extents which are definitely free in the tablespace. InnoDB always reserves some extents for clean-up and other internal purposes; these reserved extents are not included in the free space.

When you delete data from a table, InnoDB will contract the corresponding B-tree indexes. It depends on the pattern of deletes if that frees individual pages or extents to the tablespace, so that the freed space is available for other users. Dropping a table or deleting all rows from it is guaranteed to release the space to other users, but remember that deleted rows can be physically removed only in a purge operation after they are no longer needed in transaction rollback or consistent read.

#### 7.5.11.3 Defragmenting a Table

If there are random insertions or deletions in the indexes of a table, the indexes may become fragmented. By fragmentation we mean that the physical ordering of the index pages on the disk is not close to the alphabetical ordering of the records on the pages, or that there are many unused pages in the 64-page blocks which were allocated to the index.

It can speed up index scans if you periodically use `mysqldump` to dump the table to a text file, drop the table, and reload it from the dump. Another way to do the defragmenting is to `ALTER` the table type to `MyISAM` and back to `InnoDB` again. Note that a `MyISAM` table must fit in a single file on your operating system.

If the insertions to and index are always ascending and records are deleted only from the end, then the file space management algorithm of InnoDB guarantees that fragmentation in the index will not occur.

### 7.5.12 Error Handling

The error handling in InnoDB is not always the same as specified in the ANSI SQL standards. According to the ANSI standard, any error during an SQL statement should cause the rollback of that statement. InnoDB sometimes rolls back only part of the statement, or the whole transaction. The following list specifies the error handling of InnoDB.

- If you run out of file space in the tablespace, you will get the MySQL `'Table is full'` error and InnoDB rolls back the SQL statement.
- A transaction deadlock or a timeout in a lock wait make InnoDB to roll back the whole transaction.
- A duplicate key error only rolls back the insert of that particular row, even in a statement like `INSERT INTO ... SELECT ...`. This will probably change so that the SQL statement will be rolled back if you have not specified the `IGNORE` option in your statement.
- A 'row too long' error rolls back the SQL statement.
- Other errors are mostly detected by the MySQL layer of code, and they roll back the corresponding SQL statement.

### 7.5.13 Restrictions on InnoDB Tables

- Warning: do NOT convert MySQL system tables from MyISAM TO InnoDB tables! This is not supported; if you do this MySQL will not restart until you restore the old system tables from a backup or re-generate them with the `mysql_install_db` script.
- `SHOW TABLE STATUS` does not give accurate statistics on InnoDB tables, except for the physical size reserved by the table. The row count is only a rough estimate used in SQL optimisation.
- If you try to create a unique index on a prefix of a column you will get an error:

      CREATE TABLE T (A CHAR(20), B INT, UNIQUE (A(5))) TYPE = InnoDB;

  If you create a non-unique index on a prefix of a column, InnoDB will create an index over the whole column.
- `INSERT DELAYED` is not supported for InnoDB tables.
- The MySQL `LOCK TABLES` operation does not know of InnoDB row level locks set in already completed SQL statements: this means that you can get a table lock on a table even if there still exist transactions of other users which have row level locks on the same table. Thus your operations on the table may have to wait if they collide with these locks of other users. Also a deadlock is possible. However, this does not endanger transaction integrity, because the row level locks set by InnoDB will always take care of the integrity. Also, a table lock prevents other transactions from acquiring more row level locks (in a conflicting lock mode) on the table.
- You cannot have a key on a `BLOB` or `TEXT` column.
- A table cannot contain more than 1000 columns.
- `DELETE FROM TABLE` does not regenerate the table but instead deletes all rows, one by one, which is not that fast. In future versions of MySQL you can use `TRUNCATE` which is fast.
- The default database page size in InnoDB is 16 kB. By recompiling the code one can set it from 8 kB to 64 kB. The maximun row length is slightly less than half of a database page in versions <= 3.23.40 of InnoDB. Starting from source release 3.23.41 BLOB and TEXT columns are allowed to be < 4 GB, the total row length must also be < 4 GB. InnoDB does not store fields whose size is <= 128 bytes on separate pages. After InnoDB has modified the row by storing long fields on separate pages, the remaining length of the row must be less than half a database page. The maximun key length is 7000 bytes.
- On some operating systems datafiles must be < 2 GB. The combined size of log files must be < 4 GB.
- The maximum tablespace size is 4 billion database pages. This is also the maximum size for a table. The minimum tablespace size is 10 MB.

### 7.5.14 InnoDB Change History

#### 7.5.14.1 MySQL/InnoDB-4.0.5, November 18, 2002

- InnoDB now supports also transaction isolation levels READ COMMITTED and READ UNCOMMITTED. READ COMMITTED more closely emulates Oracle and makes porting of applications from Oracle to MySQL easier.
- Deadlock resolution is now selective: we try to pick as victims transactions with less modified or inserted rows.
- FOREIGN KEY definitions are now aware of the lower_case_table_names setting in my.cnf.
- SHOW CREATE TABLE does not output the database name to a FOREIGN KEY definition if the referred table is in the same database as the table.
- InnoDB does a consistency check to most index pages before writing them to a data file.
- If you set innodb_force_recovery > 0, InnoDB tries to jump over corrupt index records and pages when doing SELECT * FROM table. This helps in dumping.
- InnoDB now again uses asynchronous unbuffered i/o in Windows 2000 and XP; only unbuffered simulated async i/o in NT, 95/98/ME.
- Fixed a bug: the InnoDB range estimator greatly exaggerated the size of a short index range if the paths to the endpoints of the range in the index tree happened to branch already in the root. This could cause unnecessary table scans in SQL queries. The fix will also be backported to 3.23.54.
- Fixed a bug present in 3.23.52, 4.0.3, 4.0.4: InnoDB startup could take very long or even crash on some Win 95/98/ME computers.
- Fixed a bug: the AUTO-INC lock was held to the end of the transaction if it was granted after a lock wait. This could cause unnecessary deadlocks.
- Fixed a bug: if SHOW INNODB STATUS, innodb_monitor, or innodb_lock_monitor had to print several hundred transactions in one report, and the output became truncated, InnoDB would hang, printing to the error log many waits for a mutex created at srv0srv.c, line 1621.
- Fixed a bug: SHOW INNODB STATUS on Unix always reported average file read size as 0 bytes.
- Fixed a potential bug in 4.0.4: InnoDB now does ORDER BY ... DESC like MyISAM.
- Fixed a bug: DROP TABLE could cause crash or a hang if there was a rollback concurrently running on the table. The fix will only be backported to 3.23 if this appears a real problem for users.
- Fixed a bug: ORDER BY could fail if you had not created a primary key to a table, but had defined several indexes of which at least one was a UNIQUE index with all its columns declared as NOT NULL.
- Fixed a bug: a lock wait timeout in connection with ON DELETE CASCADE could cause corruption in indexes.
- Fixed a bug: if a SELECT was done with a unique key from a primary index, and the search matched to a delete-marked record, InnoDB could return the NEXT record.
- Outstanding bugs: in 4.0.4 two bugs were introduced to AUTO_INCREMENT. REPLACE can cause the counter to be left 1 too low. A deadlock or a lock wait timeout can cause the same problem. These will be fixed in 4.0.6.

#### 7.5.14.2 MySQL/InnoDB-3.23.53, October 9, 2002

- We again use unbuffered disk i/o to data files in Windows. Win XP and Win 2000 read performance seems to be very poor with normal i/o.
- Tuned range estimator so that index range scans are preferred over full index scans.
- Allow dropping and creating a table even if innodb_force_recovery is set. One can use this to drop a table which would cause a crash in rollback or purge, or if a failed table import causes a runaway rollback in recovery.
- Fixed a bug present in 3.23.52, 4.0.3, 4.0.4: InnoDB startup could take very long or even crash on some Win 95/98/ME computers.
- Fixed a bug: fast shutdown (which is the default) sometimes was slowed down by purge and insert buffer merge.
- Fixed a bug: doing a big SELECT from a table where no rows were visible in a consistent read could cause a very long (> 600 seconds) semaphore wait in btr0cur.c line 310.
- Fixed a bug: the AUTO-INC lock was held to the end of the transaction if it was granted after a lock wait. This could cause unnecessary deadlocks.
- Fixed a bug: if you created a temporary table inside LOCK TABLES, and used that temporary table, that caused an assertion failure in ha_innobase.cc.
- Fixed a bug: if SHOW INNODB STATUS, innodb_monitor, or innodb_lock_monitor had to print several hundred transactions in one report, and the output became truncated, InnoDB would hang, printing to the error log many waits for a mutex created at srv0srv.c, line 1621.
- Fixed a bug: SHOW INNODB STATUS on Unix always reported average file read size as 0 bytes.

#### 7.5.14.3 MySQL/InnoDB-4.0.4, October 2, 2002

- We again use unbuffered disk i/o in Windows. Win XP and Win 2000 read performance seems to be very poor with normal i/o.
- Increased the max key length of InnoDB tables from 500 to 1024 bytes.
- Increased the table comment field in SHOW TABLE STATUS so that up to 16000 characters of foreign key definitions can be printed there.
- The auto-increment counter is no longer incremented if an insert of a row immediately fails in an error.
- Allow dropping and creating a table even if innodb_force_recovery is set. One can use this to drop a table which would cause a crash in rollback or purge, or if a failed table import causes a runaway rollback in recovery.
- Fixed a bug: Using ORDER BY primarykey DESC in 4.0.3 causes an assertion failure in btr0pcur.c, line 203.
- Fixed a bug: fast shutdown (which is the default) sometimes was slowed down by purge and insert buffer merge.
- Fixed a bug: doing a big SELECT from a table where no rows were visible in a consistent read could cause a very long (> 600 seconds) semaphore wait in btr0cur.c line 310.
- Fixed a bug: if the MySQL query cache was used, it did not get invalidated by a modification done by ON DELETE CASCADE or ...SET NULL.
- Fixed a bug: if you created a temporary table inside LOCK TABLES, and used that temporary table, that caused an assertion failure in ha_innodb.cc.
- Fixed a bug: if you set innodb_flush_log_at_trx_commit to 1, SHOW VARIABLES would show its value as 16 million.

#### 7.5.14.4 MySQL/InnoDB-4.0.3, August 28, 2002

- Removed unnecessary deadlocks when inserts have to wait for a locking read, update, or delete to release its next-key lock.
- The MySQL `HANDLER` SQL commands now work also for `InnoDB` type tables. `InnoDB` does the `HANDLER` reads always as consistent reads. `HANDLER` is a direct access path to read individual indexes of tables. In some cases `HANDLER` can be used as a substitute of server-side cursors.
- Fixed a bug in 4.0.2: even a simple insert could crash the AIX version.
- Fixed a bug: if you used in a table name characters whose code is > 127, in DROP TABLE InnoDB could assert on line 155 of pars0sym.c.
- Compilation from source now provides a working version both on HP-UX-11 and HP-UX-10.20. The source of 4.0.2 worked only on 11, and the source of 3.23.52 only on 10.20.
- Fixed a bug: if compiled on 64-bit Solaris, InnoDB produced a bus error at startup.

#### 7.5.14.5 MySQL/InnoDB-3.23.52, August 16, 2002

- The feature set of 3.23 will be frozen from this version on. New features will go the the 4.0 branch, and only bug fixes will be made to the 3.23 branch.
- Many CPU-bound join queries now run faster. On Windows also many other CPU-bound queries run faster.
- A new SQL command SHOW INNODB STATUS returns the output of the InnoDB Monitor to the client. The InnoDB Monitor now prints detailed info on the latest detected deadlock.
- InnoDB made the SQL query optimizer to avoid too much index-only range scans and choose full table scans instead. This is now fixed.
- "BEGIN" and "COMMIT" are now added in the binlog around transactions. The MySQL replication now respects transaction borders: a user will no longer see half transactions in replication slaves.
- A replication slave now prints in crash recovery the last master binlog position it was able to recover to.
- A new setting innodb_flush_log_at_trx_commit=2 makes InnoDB to write the log to the operating system file cache at each commit. This is almost as fast as the setting innodb_flush_log_at_trx_commit=0, and the setting 2 also has the nice feature that in a crash where the operating system does not crash, no committed transaction is lost. If the operating system crashes or there is a power outage, then the setting 2 is no safer than the setting 0.
- Added checksum fields to log blocks.
- SET FOREIGN_KEY_CHECKS=0 helps in importing tables in an arbitrary order which does not respect the foreign key rules.
- SET UNIQUE_CHECKS=0 speeds up table imports into InnoDB if you have UNIQUE constraints on secondary indexes.
- SHOW TABLE STATUS now lists also possible ON DELETE CASCADE or ON DELETE SET NULL in the comment field of the table.
- When CHECK TABLE is run on any InnoDB type table, it now checks also the adaptive hash index for all tables.
- If you defined ON DELETE CASCADE or SET NULL and updated the referenced key in the parent row, InnoDB deleted or updated the child row. This is now changed to conform to SQL-92: you get the error 'Cannot delete parent row'.
- Improved the auto-increment algorithm: now the first insert or SHOW TABLE STATUS initializes the auto-inc counter for the table. This removes almost all surprising deadlocks caused by SHOW TABLE STATUS.
- Aligned some buffers used in reading and writing to data files. This allows using unbuffered raw devices as data files in Linux.
- Fixed a bug: If you updated the primary key of a table so that only the case of characters changed, that could cause assertion failures, mostly in page0page.ic line 515.
- Fixed a bug: If you delete or update a row referenced in a foreign key constraint and the foreign key check has to wait for a lock, then the check may report an erroneous result. This affects also the ON DELETE... operation.
- Fixed a bug: A deadlock or a lock wait timeout error in InnoDB causes InnoDB to roll back the whole transaction, but MySQL could still write the earlier SQL statements to the binlog, even though InnoDB rolled them back. This could, for example, cause replicated databases to get out-of-sync.
- Fixed a bug: If the database happened to crash in the middle of a commit, then the recovery might leak tablespace pages.
- Fixed a bug: If you specified a non-latin1 character set in my.cnf, then, in contrary to what is stated in the manual, in a foreign key constraint a string type column had to have the same length specification in the referencing table and the referenced table.
- Fixed a bug: DROP TABLE or DROP DATABASE could fail if there simultaneously was a CREATE TABLE running.
- Fixed a bug: If you configured the buffer pool bigger than 2 GB in a 32-bit computer, InnoDB would assert in buf0buf.ic line 214.
- Fixed a bug: on 64-bit computers updating rows which contained the SQL NULL in some column could cause the undo log and the ordinary log to become corrupt.
- Fixed a bug: innodb_log_monitor caused a hang if it suppressed lock prints for a page.
- Fixed a bug: in the HP-UX-10.20 version mutexes would leak and cause race conditions and crashes in any part of InnoDB code.
- Fixed a bug: if you ran in the AUTOCOMMIT mode, executed a SELECT, and immediately after that a RENAME TABLE, then RENAME would fail and MySQL would complain about error 1192.
- Fixed a bug: if compiled on 64-bit Solaris, InnoDB produced a bus error at startup.

#### 7.5.14.6 MySQL/InnoDB-4.0.2, July 10, 2002

- InnoDB is essentially the same as InnoDB-3.23.51.
- If no innodb_data_file_path is specified, InnoDB at the database creation now creates a 10 MB auto-extending data file ibdata1 to the datadir of MySQL. In 4.0.1 the file was 64 MB and not auto-extending.

#### 7.5.14.7 MySQL/InnoDB-3.23.51, June 12, 2002

- Fixed a bug: a join could result in a seg fault in copying of a BLOB or TEXT column if some of the BLOB or TEXT columns in the table contained SQL NULL values.
- Fixed a bug: if you added self-referential foreign key constraints with ON DELETE CASCADE to tables and a row deletion caused InnoDB to attempt the deletion of the same row twice because of a cascading delete, then you got an assertion failure.
- Fixed a bug: if you use MySQL 'user level locks' and close a connection, then InnoDB may assert in ha_innobase.cc, line 302.

#### 7.5.14.8 MySQL/InnoDB-3.23.50, April 23, 2002

- InnoDB now supports an auto-extending last data file. You do not need to preallocate the whole data file at the database startup.
- Made several changes to facilitate the use of the InnoDB Hot Backup tool. It is a separate non-free tool you can use to take online backups of your database without shutting down the server or setting any locks.
- If you want to run the InnoDB Hot Backup tool on an auto-extending data file you have to upgrade it to version ibbackup-0.35.
- The log scan phase in crash recovery will now run much faster.
- Starting from this server version, the hot backup tool truncates unused ends in the backup InnoDB data files.
- To allow the hot backup tool to work, on Windows we no longer use unbuffered i/o or native async i/o; instead we use the same simulated async i/o as on Unix.
- You can now define the ON DELETE CASCADE or ON DELETE SET NULL clause on foreign keys.
- FOREIGN KEY constraints now survive ALTER TABLE and CREATE INDEX.
- We suppress the FOREIGN KEY check if any of the column values in the foreign key or referenced key to be checked is the SQL NULL. This is compatible with Oracle, for example.
- SHOW CREATE TABLE now lists also foreign key constraints. Also mysqldump no longer forgets about foreign keys in table definitions.
- You can now add a new foreign key constraint with ALTER TABLE ... ADD CONSTRAINT FOREIGN KEY (...) REFERENCES ... (...).
- FOREIGN KEY definitions now allow backquotes around table and column names.
- MySQL command SET TRANSACTION ISOLATION LEVEL ... has now the following effect on InnoDB tables: if a transaction is defined as SERIALIZABLE then InnoDB conceptually adds LOCK IN SHARE MODE to all consistent reads. If a transaction is defined to have any other isolation level, then InnoDB obeys its default locking strategy which is REPEATABLE READ.
- SHOW TABLE STATUS no longer sets an x-lock at the end of an auto-increment index if the auto-increment counter has already been initialized. This removes in almost all cases the surprising deadlocks caused by SHOW TABLE STATUS.
- Fixed a bug: in a CREATE TABLE statement the string 'foreign' followed by a non-space character confused the FOREIGN KEY parser and caused table creation to fail with errno 150.

#### 7.5.14.9 MySQL/InnoDB-3.23.49, February 17, 2002

- Fixed a bug: if you called DROP DATABASE for a database on which there simultaneously were running queries, the MySQL server could crash or hang. Crashes fixed, but a full fix has to wait some changes in the MySQL layer of code.
- Fixed a bug: on Windows one had to put the database name in lower case for DROP DATABASE to work. Fixed in 3.23.49: case no longer matters on Windows. On Unix the database name remains case-sensitive.
- Fixed a bug: if one defined a non-latin1 character set as the default character set, then definition of foreign key constraints could fail in an assertion failure in dict0crea.c, reporting an internal error 17.

#### 7.5.14.10 MySQL/InnoDB-3.23.48, February 9, 2002

- Tuned the SQL optimizer to favor more often index searches over table scans.
- Fixed a performance problem when several large SELECT queries are run concurrently on a multiprocessor Linux computer. Large CPU-bound SELECT queries will now also generally run faster on all platforms.
- If MySQL binlogging is used, InnoDB now prints after crash recovery the latest MySQL binlog file name and the position in that file (= byte offset) InnoDB was able to recover to. This is useful, for example, when resynchronizing a master and a slave database in replication.
- Added better error messages to help in installation problems.
- One can now recover also MySQL temporary tables which have become orphaned inside the InnoDB tablespace.
- InnoDB now prevents a FOREIGN KEY declaration where the signedness is not the same in the referencing and referenced integer columns.
- Fixed a bug: calling SHOW CREATE TABLE or SHOW TABLE STATUS could cause memory corruption and make mysqld to crash. Especially at risk was mysqldump, because it calls frequently SHOW CREATE TABLE.
- Fixed a bug: if on Unix you did an ALTER TABLE to an InnoDB table and simultaneously did queries to it, mysqld could crash with an assertion failure in row0row.c, line 474.
- Fixed a bug: if inserts to several tables containing an auto-inc column were wrapped inside one LOCK TABLES, InnoDB asserted in lock0lock.c.
- In 3.23.47 we allowed several NULLS in a UNIQUE secondary index. But CHECK TABLE was not relaxed: it reports the table as corrupt. CHECK TABLE no longer complains in this situation.
- Fixed a bug: on Sparc and other high-endian processors SHOW VARIABLES showed innodb_flush_log_at_trx_commit and other boolean-valued startup parameters always OFF even if they were switched on.
- Fixed a bug: if you ran mysqld-max-nt as a service on Windows NT/2000, the service shutdown did not always wait long enough for the InnoDB shutdown to finish.

#### 7.5.14.11 MySQL/InnoDB-3.23.47, December 28, 2001

- Recovery happens now faster, especially in a lightly loaded system, because background checkpointing has been made more frequent.
- InnoDB allows now several similar key values in a UNIQUE secondary index if those values contain SQL NULLs. Thus the convention is now the same as in MyISAM tables.
- InnoDB gives a better row count estimate for a table which contains BLOBs.
- In a FOREIGN KEY constraint InnoDB is now case-insensitive to column names, and in Windows also to table names.
- InnoDB allows a FOREIGN KEY column of CHAR type to refer to a column of VARCHAR type, and vice versa. MySQL silently changes the type of some columns between CHAR and VARCHAR, and these silent changes do not hinder FOREIGN KEY declaration any more.
- Recovery has been made more resilient to corruption of log files.
- Unnecessary statistics calculation has been removed from queries which generate a temporary table. Some ORDER BY and DISTINCT queries will now run much faster.
- MySQL now knows that the table scan of an InnoDB table is done through the primary key. This will save a sort in some ORDER BY queries.
- The maximum key length of InnoDB tables is again restricted to 500 bytes. The MySQL interpreter is not able to handle longer keys.
- The default value of innodb_lock_wait_timeout was changed from infinite to 50 seconds, the default value of innodb_file_io_threads from 9 to 4.

#### 7.5.14.12 MySQL/InnoDB-4.0.1, December 23, 2001

- InnoDB is the same as in 3.23.47.
- In 4.0.0 the MySQL interpreter did not know the syntax LOCK IN SHARE MODE. This has been fixed.
- In 4.0.0 multi-table delete did not work for transactional tables. This has been fixed.

#### 7.5.14.13 MySQL/InnoDB-3.23.46, November 30, 2001

- This is the same as 3.23.45.

#### 7.5.14.14 MySQL/InnoDB-3.23.45, November 23, 2001

- This is a bugfix release.
- In versions 3.23.42-.44 when creating a table on Windows you have to use lower case letters in the database name to be able to access the table. Fixed in 3.23.45.
- InnoDB now flushes stdout and stderr every 10 seconds: if these are redirected to files, the file contents can be better viewed with an editor.
- Fixed an assertion failure in .44, in trx0trx.c, line 178 when you drop a table which has the .frm file but does not exist inside InnoDB.
- Fixed a bug in the insert buffer. The insert buffer tree could get into an inconsistent state, causing a crash, and also crashing the recovery. This bug could appear especially in large table imports or alterations.
- Fixed a bug in recovery: InnoDB could go into an infinite loop constantly printing a warning message that it cannot find free blocks from the buffer pool.
- Fixed a bug: when you created a temporary table of the InnoDB type, and then used ALTER TABLE to it, the MySQL server could crash.
- Prevented creation of MySQL system tables 'mysql.user', 'mysql.host', or 'mysql.db', in the InnoDB type.
- Fixed a bug which can cause an assertion failure in 3.23.44 in srv0srv.c, line 1728.

#### 7.5.14.15 MySQL/InnoDB-3.23.44, November 2, 2001

- You can define foreign key constraints on InnoDB tables. An example: FOREIGN KEY (col1) REFERENCES table2(col2).
- You can create > 4 GB data files in those file systems that allow it.
- Improved InnoDB monitors, including a new innodb_table_monitor which allows you to print the contents of the InnoDB internal data dictionary.
- DROP DATABASE will now work also for InnoDB tables.
- Accent characters in the default character set latin1 will be ordered according to the MySQL ordering.<br> NOTE: if you are using latin1 and have inserted characters whose code is > 127 to an indexed CHAR column, you should run CHECK TABLE on your table when you upgrade to 3.23.43, and drop and reimport the table if CHECK TABLE reports an error!
- InnoDB will calculate better table cardinality estimates.
- Change in deadlock resolution: in .43 a deadlock rolls back only the SQL statement, in .44 it will roll back the whole transaction.
- Deadlock, lock wait timeout, and foreign key constraint violations (no parent row, child rows exist) now return native MySQL error codes 1213, 1205, 1216, 1217, respectively.
- A new my.cnf parameter innodb_thread_concurrency helps in performance tuning in high concurrency environments.
- A new my.cnf option innodb_force_recovery will help you in dumping tables from a corrupted database.
- A new my.cnf option innodb_fast_shutdown will speed up shutdown. Normally InnoDB does a full purge and an insert buffer merge at shutdown.
- Raised maximum key length to 7000 bytes from a previous limit of 500 bytes.
- Fixed a bug in replication of auto-inc columns with multiline inserts.
- Fixed a bug when the case of letters changes in an update of an indexed secondary column.
- Fixed a hang when there are > 24 data files.
- Fixed a crash when MAX(col) is selected from an empty table, and col is a not the first column in a multi-column index.
- Fixed a bug in purge which could cause crashes.

#### 7.5.14.16 MySQL/InnoDB-3.23.43, October 4, 2001

- This is essentially the same as InnoDB-3.23.42.

#### 7.5.14.17 MySQL/InnoDB-3.23.42, September 9, 2001

- Fixed a bug which corrupted the table if the primary key of a > 8000-byte row was updated.
- There are now 3 types of InnoDB Monitors: innodb_monitor, innodb_lock_monitor, and innodb_tablespace_monitor. innodb_monitor now prints also buffer pool hit rate and the total number of rows inserted, updated, deleted, read.
- Fixed a bug in RENAME TABLE.
- Fixed a bug in replication with an auto-increment column.

#### 7.5.14.18 MySQL/InnoDB-3.23.41, August 13, 2001

- Support for < 4 GB rows. The previous limit was 8000 bytes.
- Use the doublewrite file flush method.
- Raw disk partitions supported as data files.
- InnoDB Monitor.
- Several hang bugs fixed and an ORDER BY bug ('Sort aborted') fixed.

#### 7.5.14.19 MySQL/InnoDB-3.23.40, July 16, 2001

- Only a few rare bugs fixed.

#### 7.5.14.20 MySQL/InnoDB-3.23.39, June 13, 2001

- CHECK TABLE now works for InnoDB tables.
- A new my.cnf parameter innodb_unix_file_flush_method introduced. It can be used to tune disk write performance.
- An auto-increment column now gets new values past the transaction mechanism. This saves CPU time and eliminates transaction deadlocks in new value assignment.
- Several bug fixes, most notably the rollback bug in 3.23.38.

#### 7.5.14.21 MySQL/InnoDB-3.23.38, May 12, 2001

- The new syntax SELECT ... LOCK IN SHARE MODE is introduced.
- InnoDB now calls fsync after every disk write and calculates a checksum for every database page it writes or reads, which will reveal disk defects.
- Several bug fixes.

### 7.5.15 InnoDB Contact Information

Contact information of Innobase Oy, producer of the InnoDB engine. Web site: http://www.innodb.com/. E-mail: Heikki.Tuuri@innodb.com

    phone: 358-9-6969 3250 (office) 358-40-5617367 (mobile)
    Innobase Oy Inc.
    World Trade Center Helsinki
    Aleksanterinkatu 17
    P.O.Box 800
    00101 Helsinki
    Finland

## 7.6 `BDB` or `BerkeleyDB` Tables

### 7.6.1 Overview of `BDB` Tables

Support for `BDB` tables is included in the MySQL source distribution starting from Version 3.23.34 and is activated in the MySQL-Max binary.

BerkeleyDB, available at http://www.sleepycat.com/ has provided MySQL with a transactional storage engine. By using BerkeleyDB tables, your tables may have a greater chance of surviving crashes, and also provides `COMMIT` and `ROLLBACK` on transactions. The MySQL source distribution comes with a `BDB` distribution that has a couple of small patches to make it work more smoothly with MySQL. You can't use a non-patched `BDB` version with MySQL.

We at MySQL AB are working in close cooperation with Sleepycat to keep the quality of the MySQL/BDB interface high.

When it comes to supporting `BDB` tables, we are committed to help our users to locate the problem and help creating a reproducable test case for any problems involving `BDB` tables. Any such test case will be forwarded to Sleepycat who in turn will help us find and fix the problem. As this is a two stage operation, any problems with `BDB` tables may take a little longer for us to fix than for other storage engines. However, as the BerkeleyDB code itself has been used by many other applications than MySQL, we don't envision any big problems with this. See section 1.4.1 Support Offered by MySQL AB.

### 7.6.2 Installing `BDB`

If you have downloaded a binary version of MySQL that includes support for BerkeleyDB, simply follow the instructions for installing a binary version of MySQL. See section 2.2.7 Installing a MySQL Binary Distribution. See section 4.7.5 `mysqld-max`, An Extended `mysqld` Server.

To compile MySQL with Berkeley DB support, download MySQL Version 3.23.34 or newer and configure `MySQL` with the `--with-berkeley-db` option. See section 2.3 Installing a MySQL Source Distribution.

    cd /path/to/source/of/mysql-3.23.34
    ./configure --with-berkeley-db

Please refer to the manual provided with the `BDB` distribution for more updated information.

Even though Berkeley DB is in itself very tested and reliable, the MySQL interface is still considered beta quality. We are actively improving and optimising it to get it stable very soon.

### 7.6.3 `BDB` startup options

If you are running with `AUTOCOMMIT=0` then your changes in `BDB` tables will not be updated until you execute `COMMIT`. Instead of commit you can execute `ROLLBACK` to forget your changes. See section 6.7.1 `BEGIN/COMMIT/ROLLBACK` Syntax.

If you are running with `AUTOCOMMIT=1` (the default), your changes will be committed immediately. You can start an extended transaction with the `BEGIN WORK` SQL command, after which your changes will not be committed until you execute `COMMIT` (or decide to `ROLLBACK` the changes).

The following options to `mysqld` can be used to change the behaviour of `BDB` tables:

Option | Description
`--bdb-home=directory` | Base directory for `BDB` tables. This should be the same directory you use for `--datadir`.
`--bdb-lock-detect=#` | Berkeley lock detect. One of (`DEFAULT`, `OLDEST`, `RANDOM`, or `YOUNGEST`).
`--bdb-logdir=directory` | Berkeley DB log file directory.
`--bdb-no-sync` | Don't synchronously flush logs.
`--bdb-no-recover` | Don't start Berkeley DB in recover mode.
`--bdb-shared-data` | Start Berkeley DB in multi-process mode (Don't use `DB_PRIVATE` when initialising Berkeley DB)
`--bdb-tmpdir=directory` | Berkeley DB temporary file directory.
`--skip-bdb` | Disable usage of `BDB` tables.
`-O bdb_max_lock=1000` | Set the maximum number of locks possible. See section 4.5.6.4 `SHOW VARIABLES`.

If you use `--skip-bdb`, MySQL will not initialise the Berkeley DB library and this will save a lot of memory. Of course, you cannot use `BDB` tables if you are using this option. If you try to create a `BDB` table, MySQL will instead create a `MyISAM` table.

Normally you should start `mysqld` without `--bdb-no-recover` if you intend to use `BDB` tables. This may, however, give you problems when you try to start `mysqld` if the `BDB` log files are corrupted. See section 2.4.2 Problems Starting the MySQL Server.

With `bdb_max_lock` you can specify the maximum number of locks (10000 by default) you can have active on a `BDB` table. You should increase this if you get errors of type `bdb: Lock table is out of available locks` or `Got error 12 from ...` when you have do long transactions or when `mysqld` has to examine a lot of rows to calculate the query.

You may also want to change `binlog_cache_size` and `max_binlog_cache_size` if you are using big multi-line transactions. See section 6.7.1 `BEGIN/COMMIT/ROLLBACK` Syntax.

### 7.6.4 Characteristics of `BDB` tables:

- To be able to rollback transactions, the `BDB` storage engine maintains log files. For maximum performance you should place these on another disk than your databases by using the `--bdb-logdir` option.
- MySQL performs a checkpoint each time a new `BDB` log file is started, and removes any log files that are not needed for current transactions. One can also run `FLUSH LOGS` at any time to checkpoint the Berkeley DB tables. For disaster recovery, one should use table backups plus MySQL's binary log. See section 4.4.1 Database Backups. Warning: If you delete old log files that are in use, `BDB` will not be able to do recovery at all and you may lose data if something goes wrong.
- MySQL requires a `PRIMARY KEY` in each `BDB` table to be able to refer to previously read rows. If you don't create one, MySQL will create an maintain a hidden `PRIMARY KEY` for you. The hidden key has a length of 5 bytes and is incremented for each insert attempt.
- If all columns you access in a `BDB` table are part of the same index or part of the primary key, then MySQL can execute the query without having to access the actual row. In a `MyISAM` table the above holds only if the columns are part of the same index.
- The `PRIMARY KEY` will be faster than any other key, as the `PRIMARY KEY` is stored together with the row data. As the other keys are stored as the key data + the `PRIMARY KEY`, it's important to keep the `PRIMARY KEY` as short as possible to save disk and get better speed.
- `LOCK TABLES` works on `BDB` tables as with other tables. If you don't use `LOCK TABLE`, MySQL will issue an internal multiple-write lock on the table to ensure that the table will be properly locked if another thread issues a table lock.
- Internal locking in `BDB` tables is done on page level.
- `SELECT COUNT(*) FROM table_name` is slow as `BDB` tables doesn't maintain a count of the number of rows in the table.
- Scanning is slower than with `MyISAM` tables as one has data in `BDB` tables stored in B-trees and not in a separate datafile.
- The application must always be prepared to handle cases where any change of a `BDB` table may make an automatic rollback and any read may fail with a deadlock error.
- Keys are not compressed to previous keys as with `ISAM` or `MyISAM` tables. In other words, the key information will take a little more space in `BDB` tables compared to `MyISAM` tables which don't use `PACK_KEYS=0`.
- There are often holes in the `BDB` table to allow you to insert new rows in the middle of the key tree. This makes `BDB` tables somewhat larger than `MyISAM` tables.
- The optimiser needs to know an approximation of the number of rows in the table. MySQL solves this by counting inserts and maintaining this in a separate segment in each `BDB` table. If you don't issue a lot of `DELETE` or `ROLLBACK` statements, this number should be accurate enough for the MySQL optimiser, but as MySQL only stores the number on close, it may be incorrect if MySQL dies unexpectedly. It should not be fatal even if this number is not 100% correct. One can update the number of rows by executing `ANALYZE TABLE` or `OPTIMIZE TABLE`. See section 4.5.2 `ANALYZE TABLE` Syntax . See section 4.5.1 `OPTIMIZE TABLE` Syntax.
- If you get full disk with a `BDB` table, you will get an error (probably error 28) and the transaction should roll back. This is in contrast with `MyISAM` and `ISAM` tables where `mysqld` will wait for enough free disk before continuing.

### 7.6.5 Things we need to fix for `BDB` in the near future:

- It's very slow to open many `BDB` tables at the same time. If you are going to use `BDB` tables, you should not have a very big table cache (like >256) and you should use `--no-auto-rehash` with the `mysql` client. We plan to partly fix this in 4.0.
- `SHOW TABLE STATUS` doesn't yet provide that much information for `BDB` tables.
- Optimise performance.
- Change to not use page locks at all when we are scanning tables.

### 7.6.6 Operating systems supported by `BDB`

If you after having built MySQL with support for `BDB` tables get the following error in the log file when you start `mysqld`:

    bdb: architecture lacks fast mutexes: applications cannot be threaded
    Can't init dtabases

This means that `BDB` tables are not supported for your architecture. In this case you have to rebuild MySQL without `BDB` table support.

Note: The following list is not complete; we will update it as we receive more information about this.

Currently we know that the `BDB` storage engine works with the following operating systems:

- Linux 2.x intel
- Solaris SPARC
- Caldera (SCO) OpenServer
- Caldera (SCO) UnixWare 7.0.1

It doesn't work with the following operating systems:

- Linux 2.x Alpha
- Max OS X

### 7.6.7 Restrictions on `BDB` Tables

Here follows the restrictions you have when using `BDB` tables:

- `BDB` tables store in the ``.db'` file the path to the file as it was created. (This was done to be able to detect locks in a multi-user environment that supports symlinks). The effect of this is that `BDB` tables are not movable between directories!
- When taking backups of `BDB` tables, you have to either use `mysqldump` or take a backup of all `table_name.db` files and the `BDB` log files. The `BDB` log files are the files in the base data directory named `log.XXXXXXXXXX` (ten digits); The `BDB` storage engine stores unfinished transactions in the log files and requires these to be present when `mysqld` starts.

### 7.6.8 Errors That May Occur When Using `BDB` Tables

- If you get the following error in the `hostname.err log` when starting `mysqld`:

      bdb:  Ignoring log file: .../log.XXXXXXXXXX: unsupported log version #

  it means that the new `BDB` version doesn't support the old log file format. In this case you have to delete all `BDB` logs from your database directory (the files with names that have the format `log.XXXXXXXXXX`) and restart `mysqld`. We would also recommend you to do a `mysqldump --opt` of your old `BDB` tables, delete the old tables, and restore the dump.
- If you are not running in auto-commit mode and delete a table you are using by another thread you may get the following error messages in the MySQL error file:

      001119 23:43:56  bdb:  Missing log fileid entry
      001119 23:43:56  bdb:  txn_abort: Log undo failed for LSN:
                             1 3644744: Invalid

  This is not fatal but we don't recommend that you delete tables if you are not in auto-commit mode, until this problem is fixed (the fix is not trivial).

# 8 MySQL APIs

This chapter describes the APIs available for MySQL, where to get them, and how to use them. The C API is the most extensively covered, as it was developed by the MySQL team, and is the basis for most of the other APIs.

## 8.1 MySQL PHP API

PHP is a server-side, HTML-embedded scripting language that may be used to create dynamic web pages. It contains support for accessing several databases, including MySQL. PHP may be run as a separate program or compiled as a module for use with the Apache web server.

The distribution and documentation are available at the PHP web site (http://www.php.net/).

### 8.1.1 Common Problems with MySQL and PHP

- Error: "Maximum Execution Time Exceeded" This is a PHP limit; go into the ``php3.ini'` file and set the maximum execution time up from 30 seconds to something higher, as needed. It is also not a bad idea to double the ram allowed per script to 16MB instead of 8 MB.
- Error: "Fatal error: Call to unsupported or undefined function mysql_connect() in .." This means that your PHP version isn't compiled with MySQL support. You can either compile a dynamic MySQL module and load it into PHP or recompile PHP with built-in MySQL support. This is described in detail in the PHP manual.
- Error: "undefined reference to `uncompress'" This means that the client library is compiled with support for a compressed client/server protocol. The fix is to add `-lz` last when linking with `-lmysqlclient`.

## 8.2 MySQL Perl API

This section documents the Perl `DBI` interface. The former interface was called `mysqlperl`. `DBI`/`DBD` now is the recommended Perl interface, so `mysqlperl` is obsolete and is not documented here.

### 8.2.1 `DBI` with `DBD::mysql`

`DBI` is a generic interface for many databases. That means that you can write a script that works with many different database engines without change. You need a DataBase Driver (DBD) defined for each database type. For MySQL, this driver is called `DBD::mysql`.

For more information on the Perl5 DBI, please visit the `DBI` web page and read the documentation:

    http://dbi.perl.org/

For more information on Object Oriented Programming (OOP) as defined in Perl5, see the Perl OOP page:

    http://language.perl.com/info/documentation.html

Note that if you want to use transactions with Perl, you need to have `Msql-Mysql-modules` version 1.2216 or newer.

Installation instructions for MySQL Perl support are given in section 2.7 Perl Installation Comments.

### 8.2.2 The `DBI` Interface

Portable DBI Methods

Method | Description
`connect` | Establishes a connection to a database server.
`disconnect` | Disconnects from the database server.
`prepare` | Prepares a SQL statement for execution.
`execute` | Executes prepared statements.
`do` | Prepares and executes a SQL statement.
`quote` | Quotes string or `BLOB` values to be inserted.
`fetchrow_array` | Fetches the next row as an array of fields.
`fetchrow_arrayref` | Fetches next row as a reference array of fields.
`fetchrow_hashref` | Fetches next row as a reference to a hashtable.
`fetchall_arrayref` | Fetches all data as an array of arrays.
`finish` | Finishes a statement and lets the system free resources.
`rows` | Returns the number of rows affected.
`data_sources` | Returns an array of databases available on localhost.
`ChopBlanks` | Controls whether `fetchrow_*` methods trim spaces.
`NUM_OF_PARAMS` | The number of placeholders in the prepared statement.
`NULLABLE` | Which columns can be `NULL`.
`trace` | Perform tracing for debugging.

MySQL-specific Methods

Method | Description
`insertid` | The latest `AUTO_INCREMENT` value.
`is_blob` | Which columns are `BLOB` values.
`is_key` | Which columns are keys.
`is_num` | Which columns are numeric.
`is_pri_key` | Which columns are primary keys.
`is_not_null` | Which columns CANNOT be `NULL`. See `NULLABLE`.
`length` | Maximum possible column sizes.
`max_length` | Maximum column sizes actually present in result.
`NAME` | Column names.
`NUM_OF_FIELDS` | Number of fields returned.
`table` | Table names in returned set.
`type` | All column types.

The Perl methods are described in more detail in the following sections. Variables used for method return values have these meanings:

`$dbh`
  Database handle
`$sth`
  Statement handle
`$rc`
  Return code (often a status)
`$rv`
  Return value (often a row count)

Portable DBI Methods

`connect($data_source, $username, $password)`
  Use the `connect` method to make a database connection to the data source. The `$data_source` value should begin with `DBI:driver_name:`. Example uses of `connect` with the `DBD::mysql` driver:

      $dbh = DBI->connect("DBI:mysql:$database", $user, $password);
      $dbh = DBI->connect("DBI:mysql:$database:$hostname",
                          $user, $password);
      $dbh = DBI->connect("DBI:mysql:$database:$hostname:$port",
                          $user, $password);

  If the user name and/or password are undefined, `DBI` uses the values of the `DBI_USER` and `DBI_PASS` environment variables, respectively. If you don't specify a hostname, it defaults to `'localhost'`. If you don't specify a port number, it defaults to the default MySQL port (3306). As of `Msql-Mysql-modules` Version 1.2009, the `$data_source` value allows certain modifiers:
  `mysql_read_default_file=file_name`
    Read ``filename'` as an option file. For information on option files, see section 4.1.2 ``my.cnf'` Option Files.
  `mysql_read_default_group=group_name`
    The default group when reading an option file is normally the `[client]` group. By specifying the `mysql_read_default_group` option, the default group becomes the `[group_name]` group.
  `mysql_compression=1`
    Use compressed communication between the client and server (MySQL Version 3.22.3 or later).
  `mysql_socket=/path/to/socket`
    Specify the pathname of the Unix socket that is used to connect to the server (MySQL Version 3.21.15 or later).
  Multiple modifiers may be given; each must be preceded by a semicolon. For example, if you want to avoid hardcoding the user name and password into a `DBI` script, you can take them from the user's ``~/.my.cnf'` option file instead by writing your `connect` call like this:

      $dbh = DBI->connect("DBI:mysql:$database"
                      . ";mysql_read_default_file=$ENV{HOME}/.my.cnf",
                      $user, $password);

  This call will read options defined for the `[client]` group in the option file. If you wanted to do the same thing but use options specified for the `[perl]` group as well, you could use this:

      $dbh = DBI->connect("DBI:mysql:$database"
                      . ";mysql_read_default_file=$ENV{HOME}/.my.cnf"
                      . ";mysql_read_default_group=perl",
                      $user, $password);

`disconnect`
  The `disconnect` method disconnects the database handle from the database. This is typically called right before you exit from the program. Example:

      $rc = $dbh->disconnect;

`prepare($statement)`
  Prepares a SQL statement for execution by the database engine and returns a statement handle `($sth)`, which you can use to invoke the `execute` method. Typically you handle `SELECT` statements (and `SELECT`-like statements such as `SHOW`, `DESCRIBE`, and `EXPLAIN`) by means of `prepare` and `execute`. Example:

      $sth = $dbh->prepare($statement)
          or die "Can't prepare $statement: $dbh->errstr\n";

`execute`
  The `execute` method executes a prepared statement. For non-`SELECT` statements, `execute` returns the number of rows affected. If no rows are affected, `execute` returns `"0E0"`, which Perl treats as zero but regards as true. If an error occurs, `execute` returns `undef`. For `SELECT` statements, `execute` only starts the SQL query in the database; you need to use one of the `fetch_*` methods described here to retrieve the data. Example:

      $rv = $sth->execute
                or die "can't execute the query: $sth->errstr;

`do($statement)`
  The `do` method prepares and executes a SQL statement and returns the number of rows affected. If no rows are affected, `do` returns `"0E0"`, which Perl treats as zero but regards as true. This method is generally used for non-`SELECT` statements that cannot be prepared in advance (due to driver limitations) or that do not need to be executed more than once (inserts, deletes, etc.). Example:

      $rv = $dbh->do($statement)
              or die "Can't execute $statement: $dbh- >errstr\n";

  Generally the 'do' statement is much faster (and is preferable) than prepare/execute for statements that don't contain parameters.
`quote($string)`
  The `quote` method is used to "escape" any special characters contained in the string and to add the required outer quotation marks. Example:

      $sql = $dbh->quote($string)

`fetchrow_array`
  This method fetches the next row of data and returns it as an array of field values. Example:

      while(@row = $sth->fetchrow_array) {
              print qw($row[0]\t$row[1]\t$row[2]\n);
      }

`fetchrow_arrayref`
  This method fetches the next row of data and returns it as a reference to an array of field values. Example:

      while($row_ref = $sth->fetchrow_arrayref) {
              print qw($row_ref->[0]\t$row_ref->[1]\t$row_ref->[2]\n);
      }

`fetchrow_hashref`
  This method fetches a row of data and returns a reference to a hash table containing field name/value pairs. This method is not nearly as efficient as using array references as demonstrated above. Example:

      while($hash_ref = $sth->fetchrow_hashref) {
              print qw($hash_ref->{firstname}\t$hash_ref->{lastname}\t\
                      $hash_ref- > title}\n);
      }

`fetchall_arrayref`
  This method is used to get all the data (rows) to be returned from the SQL statement. It returns a reference to an array of references to arrays for each row. You access or print the data by using a nested loop. Example:

      my $table = $sth->fetchall_arrayref
                      or die "$sth->errstr\n";
      my($i, $j);
      for $i ( 0 .. $#{$table} ) {
              for $j ( 0 .. $#{$table->[$i]} ) {
                      print "$table->[$i][$j]\t";
              }
              print "\n";
      }

`finish`
  Indicates that no more data will be fetched from this statement handle. You call this method to free up the statement handle and any system resources associated with it. Example:

      $rc = $sth->finish;

`rows`
  Returns the number of rows changed (updated, deleted, etc.) by the last command. This is usually used after a non-`SELECT` `execute` statement. Example:

      $rv = $sth->rows;

`NULLABLE`
  Returns a reference to an array of values that indicate whether columns may contain `NULL` values. The possible values for each array element are 0 or the empty string if the column cannot be `NULL`, 1 if it can, and 2 if the column's `NULL` status is unknown. Example:

      $null_possible = $sth->{NULLABLE};

`NUM_OF_FIELDS`
  This attribute indicates the number of fields returned by a `SELECT` or `SHOW FIELDS` statement. You may use this for checking whether a statement returned a result: A zero value indicates a non-`SELECT` statement like `INSERT`, `DELETE`, or `UPDATE`. Example:

      $nr_of_fields = $sth->{NUM_OF_FIELDS};

`data_sources($driver_name)`
  This method returns an array containing names of databases available to the MySQL server on the host `'localhost'`. Example:

      @dbs = DBI->data_sources("mysql");

`ChopBlanks`
  This attribute determines whether the `fetchrow_*` methods will chop leading and trailing blanks from the returned values. Example:

      $sth->{'ChopBlanks'} =1;

`trace($trace_level)`
`trace($trace_level, $trace_filename)`
  The `trace` method enables or disables tracing. When invoked as a `DBI` class method, it affects tracing for all handles. When invoked as a database or statement handle method, it affects tracing for the given handle (and any future children of the handle). Setting `$trace_level` to 2 provides detailed trace information. Setting `$trace_level` to 0 disables tracing. Trace output goes to the standard error output by default. If `$trace_filename` is specified, the file is opened in append mode and output for all traced handles is written to that file. Example:

      DBI->trace(2);                # trace everything
      DBI->trace(2,"/tmp/dbi.out"); # trace everything to
                                    # /tmp/dbi.out
      $dth->trace(2);               # trace this database handle
      $sth->trace(2);               # trace this statement handle

  You can also enable `DBI` tracing by setting the `DBI_TRACE` environment variable. Setting it to a numeric value is equivalent to calling `DBI->(value)`. Setting it to a pathname is equivalent to calling `DBI->(2,value)`.

MySQL-specific Methods

The methods shown here are MySQL-specific and not part of the `DBI` standard. Several of them are now deprecated: `is_blob`, `is_key`, `is_num`, `is_pri_key`, `is_not_null`, `length`, `max_length`, and `table`. Where `DBI`-standard alternatives exist, they are noted here:

`insertid`
  If you use the `AUTO_INCREMENT` feature of MySQL, the new auto-incremented values will be stored here. Example:

      $new_id = $sth->{insertid};

  As an alternative, you can use `$dbh->{'mysql_insertid'}`.
`is_blob`
  Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column is a `BLOB`. Example:

      $keys = $sth->{is_blob};

`is_key`
  Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column is a key. Example:

      $keys = $sth->{is_key};

`is_num`
  Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column contains numeric values. Example:

      $nums = $sth->{is_num};

`is_pri_key`
  Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column is a primary key. Example:

      $pri_keys = $sth->{is_pri_key};

`is_not_null`
  Returns a reference to an array of boolean values; for each element of the array, a value of FALSE indicates that this column may contain `NULL` values. Example:

      $not_nulls = $sth->{is_not_null};

  `is_not_null` is deprecated; it is preferable to use the `NULLABLE` attribute (described above), because that is a DBI standard.
`length`
`max_length`
  Each of these methods returns a reference to an array of column sizes. The `length` array indicates the maximum possible sizes that each column may be (as declared in the table description). The `max_length` array indicates the maximum sizes actually present in the result table. Example:

      $lengths = $sth->{length};
      $max_lengths = $sth->{max_length};

`NAME`
  Returns a reference to an array of column names. Example:

      $names = $sth->{NAME};

`table`
  Returns a reference to an array of table names. Example:

      $tables = $sth->{table};

`type`
  Returns a reference to an array of column types. Example:

      $types = $sth->{type};

### 8.2.3 More `DBI`/`DBD` Information

You can use the `perldoc` command to get more information about `DBI`.

    perldoc DBI
    perldoc DBI::FAQ
    perldoc DBD::mysql

You can also use the `pod2man`, `pod2html`, etc., tools to translate to other formats.

You can find the latest `DBI` information at the `DBI` web page: http://dbi.perl.org/.

## 8.3 MySQL ODBC Support

MySQL provides support for ODBC by means of the `MyODBC` program. This chapter will teach you how to install `MyODBC`, and how to use it. Here, you will also find a list of common programs that are known to work with `MyODBC`.

### 8.3.1 How To Install MyODBC

`MyODBC` 2.50 is a 32-bit ODBC 2.50 specification level 0 (with level 1 and level 2 features) driver for connecting an ODBC-aware application to MySQL. `MyODBC` works on Windows 9x/Me/NT/2000/XP and most Unix platforms. `MyODBC` 3.51 is an enhanced version with ODBC 3.5x specification level 1 (complete core API + level 2 features).

`MyODBC` is `Open Source`, and you can find the newest version at http://www.mysql.com/downloads/api-myodbc.html. Please note that the 2.50.x versions are `LGPL` licensed, whereas the 3.51.x versions are `GPL` licensed.

If you have problem with `MyODBC` and your program also works with OLEDB, you should try the OLEDB driver.

Normally you only need to install `MyODBC` on Windows machines. You only need `MyODBC` for Unix if you have a program like ColdFusion that is running on the Unix machine and uses ODBC to connect to the databases.

If you want to install `MyODBC` on a Unix box, you will also need an `ODBC` manager. `MyODBC` is known to work with most of the Unix ODBC managers. See section 1.6.1 MySQL Portals.

To install `MyODBC` on Windows, you should download the appropriate `MyODBC` ``.zip'` file, unpack it with `WinZIP` or some similar program, and execute the ``SETUP.EXE'` file.

On Windows/NT/XP you may get the following error when trying to install `MyODBC`:

    An error occurred while copying C:\WINDOWS\SYSTEM\MFC30.DLL. Restart
    Windows and try installing again (before running any applications which
    use ODBC)

The problem in this case is that some other program is using ODBC and because of how Windows is designed, you may not in this case be able to install a new ODBC drivers with Microsoft's ODBC setup program. In most cases you can continue by just pressing `Ignore` to copy the rest of the MyODBC files and the final installation should still work. If this doesn't work, the solution is to reboot your computer in ``safe mode`` (Choose this by pressing F8 just before your machine starts Windows during rebooting), install `MyODBC`, and reboot to normal mode.

- To make a connection to a Unix box from a Windows box, with an ODBC application (one that doesn't support MySQL natively), you must first install `MyODBC` on the Windows machine.
- The user and Windows machine must have the access privileges to the MySQL server on the Unix machine. This is set up with the `GRANT` command. See section 4.3.1 `GRANT` and `REVOKE` Syntax.
- You must create an ODBC DSN entry as follows:
  - Open the Control Panel on the Windows machine.
  - Double-click the ODBC Data Sources 32-bit icon.
  - Click the tab User DSN.
  - Click the button Add.
  - Select MySQL in the screen Create New Data Source and click the Finish button.
  - The MySQL Driver default configuration screen is shown. See section 8.3.2 How to Fill in the Various Fields in the ODBC Administrator Program.
- Now start your application and select the ODBC driver with the DSN you specified in the ODBC administrator.

Notice that there are other configuration options on the screen of MySQL (trace, don't prompt on connect, etc) that you can try if you run into problems.

### 8.3.2 How to Fill in the Various Fields in the ODBC Administrator Program

There are three possibilities for specifying the server name on Windows95:

- Use the IP address of the server.
- Add a file ``\windows\lmhosts'` with the following information:

      ip hostname

  For example:

      194.216.84.21 my_hostname

- Configure the PC to use DNS.

Example of how to fill in the `ODBC setup`:

    Windows DSN name:   test
    Description:        This is my test database
    MySql Database:     test
    Server:             194.216.84.21
    User:               monty
    Password:           my_password
    Port:

The value for the `Windows DSN name` field is any name that is unique in your Windows ODBC setup.

You don't have to specify values for the `Server`, `User`, `Password`, or `Port` fields in the ODBC setup screen. However, if you do, the values will be used as the defaults later when you attempt to make a connection. You have the option of changing the values at that time.

If the port number is not given, the default port (3306) is used.

If you specify the option `Read options from C:\my.cnf`, the groups `client` and `odbc` will be read from the ``C:\my.cnf'` file. You can use all options that are usable by `mysql_options()`. See section 8.4.3.159 `mysql_options()`.

### 8.3.3 Connect parameters for MyODBC

One can specify the following parameters for `MyODBC` on the `[Servername]` section of an ``ODBC.INI'` file or through the `InConnectionString` argument in the `SQLDriverConnect()` call.

Parameter | Default value | Comment
user | ODBC (on Windows) | The username used to connect to MySQL.
server | localhost | The hostname of the MySQL server.
database |  | The default database.
option | 0 | A integer by which you can specify how `MyODBC` should work. See below.
port | 3306 | The TCP/IP port to use if `server` is not `localhost`.
stmt |  | A statement that will be executed when connecting to `MySQL`.
password |  | The password for the `server` `user` combination.
socket |  | The socket or Windows pipe to connect to.

The option argument is used to tell `MyODBC` that the client isn't 100% ODBC compliant. On Windows, one normally sets the option flag by toggling the different options on the connection screen but one can also set this in the opton argument. The following options are listed in the same order as they appear in the `MyODBC` connect screen:

Bit | Description
1 | The client can't handle that `MyODBC` returns the real width of a column.
2 | The client can't handle that MySQL returns the true value of affected rows. If this flag is set then MySQL returns 'found rows' instead. One must have MySQL 3.21.14 or newer to get this to work.
4 | Make a debug log in c:\myodbc.log. This is the same as putting `MYSQL_DEBUG=d:t:O,c::\myodbc.log` in ``AUTOEXEC.BAT'`
8 | Don't set any packet limit for results and parameters.
16 | Don't prompt for questions even if driver would like to prompt
32 | Simulate a ODBC 1.0 driver in some context.
64 | Ignore use of database name in 'database.table.column'.
128 | Force use of ODBC manager cursors (experimental).
256 | Disable the use of extended fetch (experimental).
512 | Pad CHAR fields to full column length.
1024 | SQLDescribeCol() will return fully qualifed column names
2048 | Use the compressed server/client protocol
4096 | Tell server to ignore space after function name and before `'('` (needed by PowerBuilder). This will make all function names keywords!
8192 | Connect with named pipes to a `mysqld` server running on NT.
16384 | Change LONGLONG columns to INT columns (some applications can't handle LONGLONG).
32768 | Return 'user' as Table_qualifier and Table_owner from SQLTables (experimental)
65536 | Read parameters from the `client` and `odbc` groups from ``my.cnf'`
131072 | Add some extra safety checks (should not bee needed but...)

If you want to have many options, you should add the above flags! For example setting option to 12 (4+8) gives you debugging without package limits!

The default ``MYODBC.DLL'` is compiled for optimal performance. If you want to to debug `MyODBC` (for example to enable tracing), you should instead use ``MYODBCD.DLL'`. To install this file, copy ``MYODBCD.DLL'` over the installed ``MYODBC.DLL'` file.

### 8.3.4 How to Report Problems with MyODBC

`MyODBC` has been tested with Access, Admndemo.exe, C++-Builder, Borland Builder 4, Centura Team Developer (formerly Gupta SQL/Windows), ColdFusion (on Solaris and NT with svc pack 5), Crystal Reports, DataJunction, Delphi, ERwin, Excel, iHTML, FileMaker Pro, FoxPro, Notes 4.5/4.6, SBSS, Perl DBD-ODBC, Paradox, Powerbuilder, Powerdesigner 32 bit, VC++, and Visual Basic.

If you know of any other applications that work with `MyODBC`, please send mail to myodbc@lists.mysql.com about this!

With some programs you may get an error like: `Another user has modifies the record that you have modified`. In most cases this can be solved by doing one of the following things:

- Add a primary key for the table if there isn't one already.
- Add a timestamp column if there isn't one already.
- Only use double float fields. Some programs may fail when they compare single floats.

If the above doesn't help, you should do a `MyODBC` trace file and try to figure out why things go wrong.

### 8.3.5 Programs Known to Work with MyODBC

Most programs should work with `MyODBC`, but for each of those listed here, we have tested it ourselves or received confirmation from some user that it works:

Program
  Comment
Access
  To make Access work:
  - If you are using Access 2000, you should get and install the newest (version 2.6 or above) Microsoft MDAC (`Microsoft Data Access Components`) from http://www.microsoft.com/data/. This will fix the following bug in Access: when you export data to MySQL, the table and column names aren't specified. Another way to around this bug is to upgrade to MyODBC Version 2.50.33 and MySQL Version 3.23.x, which together provide a workaround for this bug! You should also get and apply the Microsoft Jet 4.0 Service Pack 5 (SP5) which can be found here http://support.microsoft.com/support/kb/articles/Q 239/1/14.ASP. This will fix some cases where columns are marked as `#deleted#` in Access. Note that if you are using MySQL Version 3.22, you must to apply the MDAC patch and use MyODBC 2.50.32 or 2.50.34 and above to go around this problem.
  - For all Access versions, you should enable the MyODBC option flag `Return matching rows`. For Access 2.0, you should additionally enable `Simulate ODBC 1.0`.
  - You should have a timestamp in all tables you want to be able to update. For maximum portability `TIMESTAMP(14)` or simple `TIMESTAMP` is recommended instead of other `TIMESTAMP(X)` variations.
  - You should have a primary key in the table. If not, new or updated rows may show up as `#DELETED#`.
  - Only use `DOUBLE` float fields. Access fails when comparing with single floats. The symptom usually is that new or updated rows may show up as `#DELETED#` or that you can't find or update rows.
  - If you are linking a table through MyODBC, which has `BIGINT` as one of the column, then the results will be displayed as `#DELETED`. The work around solution is:
    - Have one more dummy column with `TIMESTAMP` as the data type, preferably `TIMESTAMP(14)`.
    - Check the `'Change BIGINT columns to INT'` in connection options dialog in ODBC DSN Administrator
    - Delete the table link from access and re-create it.
    It still displays the previous records as `#DELETED#`, but newly added/updated records will be displayed properly.
  - If you still get the error `Another user has changed your data` after adding a `TIMESTAMP` column, the following trick may help you: Don't use `table` data sheet view. Create instead a form with the fields you want, and use that `form` data sheet view. You should set the `DefaultValue` property for the `TIMESTAMP` column to `NOW()`. It may be a good idea to hide the `TIMESTAMP` column from view so your users are not confused.
  - In some cases, Access may generate illegal SQL queries that MySQL can't understand. You can fix this by selecting `"Query|SQLSpecific|Pass-Through"` from the Access menu.
  - Access on NT will report `BLOB` columns as `OLE OBJECTS`. If you want to have `MEMO` columns instead, you should change the column to `TEXT` with `ALTER TABLE`.
  - Access can't always handle `DATE` columns properly. If you have a problem with these, change the columns to `DATETIME`.
  - If you have in Access a column defined as `BYTE`, Access will try to export this as `TINYINT` instead of `TINYINT UNSIGNED`. This will give you problems if you have values > 127 in the column!
ADO
  When you are coding with the ADO API and `MyODBC` you need to put attention in some default properties that aren't supported by the MySQL server. For example, using the `CursorLocation Property` as `adUseServer` will return for the `RecordCount Property` a result of -1. To have the right value, you need to set this property to `adUseClient`, like is showing in the VB code here:

      Dim myconn As New ADODB.Connection
      Dim myrs As New Recordset
      Dim mySQL As String
      Dim myrows As Long

      myconn.Open "DSN=MyODBCsample"
      mySQL = "SELECT * from user"
      myrs.Source = mySQL
      Set myrs.ActiveConnection = myconn
      myrs.CursorLocation = adUseClient
      myrs.Open
      myrows = myrs.RecordCount

      myrs.Close
      myconn.Close

  Another workaround is to use a `SELECT COUNT(*)` statement for a similar query to get the correct row count.
Active server pages (ASP)
  You should use the option flag `Return matching rows`.
BDE applications
  To get these to work, you should set the option flags `Don't optimize column widths` and `Return matching rows`.
Borland Builder 4
  When you start a query you can use the property `Active` or use the method `Open`. Note that `Active` will start by automatically issuing a `SELECT * FROM ...` query that may not be a good thing if your tables are big!
ColdFusion (On Unix)
  The following information is taken from the ColdFusion documentation: Use the following information to configure ColdFusion Server for Linux to use the unixODBC driver with `MyODBC` for MySQL data sources. Allaire has verified that `MyODBC` Version 2.50.26 works with MySQL Version 3.22.27 and ColdFusion for Linux. (Any newer version should also work.) You can download `MyODBC` at http://www.mysql.com/downloads/api-myodbc.html ColdFusion Version 4.5.1 allows you to us the ColdFusion Administrator to add the MySQL data source. However, the driver is not included with ColdFusion Version 4.5.1. Before the MySQL driver will appear in the ODBC datasources drop-down list, you must build and copy the `MyODBC` driver to ``/opt/coldfusion/lib/libmyodbc.so'`. The Contrib directory contains the program ``mydsn-xxx.zip'` which allows you to build and remove the DSN registry file for the MyODBC driver on Coldfusion applications.
DataJunction
  You have to change it to output `VARCHAR` rather than `ENUM`, as it exports the latter in a manner that causes MySQL grief.
Excel
  Works. A few tips:
  - If you have problems with dates, try to select them as strings using the `CONCAT()` function. For example:

        select CONCAT(rise_time), CONCAT(set_time)
            from sunrise_sunset;

    Values retrieved as strings this way should be correctly recognised as time values by Excel97. The purpose of `CONCAT()` in this example is to fool ODBC into thinking the column is of ``string type''. Without the `CONCAT()`, ODBC knows the column is of time type, and Excel does not understand that. Note that this is a bug in Excel, because it automatically converts a string to a time. This would be great if the source was a text file, but is plain stupid when the source is an ODBC connection that reports exact types for each column.
Word
  To retrieve data from MySQL to Word/Excel documents, you need to use the `MyODBC` driver and the Add-in Microsoft Query help. For example, create a db with a table containing 2 columns of text:
  - Insert rows using the `mysql` client command-line tool.
  - Create a DSN file using the ODBC manager, for example, ``my'` for the db above.
  - Open the Word application.
  - Create a blank new documentation.
  - Using the tool bar called Database, press the button insert database.
  - Press the button Get Data.
  - At the right hand of the screen Get Data, press the button Ms Query.
  - In the Ms Query create a New Data Source using the DSN file my.
  - Select the new query.
  - Select the columns that you want.
  - Make a filter if you want.
  - Make a Sort if you want.
  - Select Return Data to Microsoft Word.
  - Click Finish.
  - Click Insert data and select the records.
  - Click OK and you see the rows in your Word document.
odbcadmin
  Test program for ODBC.
Delphi
  You must use BDE Version 3.2 or newer. Set the `Don't optimize column width` option field when connecting to MySQL. Also, here is some potentially useful Delphi code that sets up both an ODBC entry and a BDE entry for `MyODBC` (the BDE entry requires a BDE Alias Editor that is free at a Delphi Super Page near you. (Thanks to Bryan Brunton bryan@flesherfab.com for this):

      fReg:= TRegistry.Create;
        fReg.OpenKey('\Software\ODBC\ODBC.INI\DocumentsFab', True);
        fReg.WriteString('Database', 'Documents');
        fReg.WriteString('Description', ' ');
        fReg.WriteString('Driver', 'C:\WINNT\System32\myodbc.dll');
        fReg.WriteString('Flag', '1');
        fReg.WriteString('Password', '');
        fReg.WriteString('Port', ' ');
        fReg.WriteString('Server', 'xmark');
        fReg.WriteString('User', 'winuser');
        fReg.OpenKey('\Software\ODBC\ODBC.INI\ODBC Data Sources', True);
        fReg.WriteString('DocumentsFab', 'MySQL');
        fReg.CloseKey;
        fReg.Free;

        Memo1.Lines.Add('DATABASE NAME=');
        Memo1.Lines.Add('USER NAME=');
        Memo1.Lines.Add('ODBC DSN=DocumentsFab');
        Memo1.Lines.Add('OPEN MODE=READ/WRITE');
        Memo1.Lines.Add('BATCH COUNT=200');
        Memo1.Lines.Add('LANGDRIVER=');
        Memo1.Lines.Add('MAX ROWS=-1');
        Memo1.Lines.Add('SCHEMA CACHE DIR=');
        Memo1.Lines.Add('SCHEMA CACHE SIZE=8');
        Memo1.Lines.Add('SCHEMA CACHE TIME=-1');
        Memo1.Lines.Add('SQLPASSTHRU MODE=SHARED AUTOCOMMIT');
        Memo1.Lines.Add('SQLQRYMODE=');
        Memo1.Lines.Add('ENABLE SCHEMA CACHE=FALSE');
        Memo1.Lines.Add('ENABLE BCD=FALSE');
        Memo1.Lines.Add('ROWSET SIZE=20');
        Memo1.Lines.Add('BLOBS TO CACHE=64');
        Memo1.Lines.Add('BLOB SIZE=32');

        AliasEditor.Add('DocumentsFab','MySQL',Memo1.Lines);

C++ Builder
  Tested with BDE Version 3.0. The only known problem is that when the table schema changes, query fields are not updated. BDE, however, does not seem to recognise primary keys, only the index PRIMARY, though this has not been a problem.
Vision
  You should use the option flag `Return matching rows`.
Visual Basic
  To be able to update a table, you must define a primary key for the table. Visual Basic with ADO can't handle big integers. This means that some queries like `SHOW PROCESSLIST` will not work properly. The fix is to set the option `OPTION=16384` in the ODBC connect string or to set the `Change BIGINT columns to INT` option in the MyODBC connect screen. You may also want to set the `Return matching rows` option.
VisualInterDev
  If you get the error `[Microsoft][ODBC Driver Manager] Driver does not support this parameter` the reason may be that you have a `BIGINT` in your result. Try setting the `Change BIGINT columns to INT` option in the MyODBC connect screen.
Visual Objects
  You should use the option flag `Don't optimize column widths`.

### 8.3.6 How to Get the Value of an `AUTO_INCREMENT` Column in ODBC

A common problem is how to get the value of an automatically generated ID from an `INSERT`. With ODBC, you can do something like this (assuming that `auto` is an `AUTO_INCREMENT` field):

    INSERT INTO foo (auto,text) VALUES(NULL,'text');
    SELECT LAST_INSERT_ID();

Or, if you are just going to insert the ID into another table, you can do this:

    INSERT INTO foo (auto,text) VALUES(NULL,'text');
    INSERT INTO foo2 (id,text) VALUES(LAST_INSERT_ID(),'text');

See section 8.4.6.3 How Can I Get the Unique ID for the Last Inserted Row?.

For the benefit of some ODBC applications (at least Delphi and Access), the following query can be used to find a newly inserted row:

    SELECT * FROM tbl_name WHERE auto IS NULL;

### 8.3.7 Reporting Problems with MyODBC

If you encounter difficulties with `MyODBC`, you should start by making a log file from the ODBC manager (the log you get when requesting logs from ODBCADMIN) and a `MyODBC` log.

To get a `MyODBC` log, you need to do the following:

1. Ensure that you are using ``myodbcd.dll'` and not ``myodbc.dll'`. The easiest way to do this is to get ``myodbcd.dll'` from the MyODBC distribution and copy it over the ``myodbc.dll'`, which is probably in your ``C:\windows\system32'` or ``C:\winnt\system32'` directory. Note that you probably want to restore the old myodbc.dll file when you have finished testing, as this is a lot faster than ``myodbcd.dll'`.
2. Tag the `Trace MyODBC' option flag in the `MyODBC` connect/configure screen. The log will be written to file ``C:\myodbc.log'`. If the trace option is not remembered when you are going back to the above screen, it means that you are not using the `myodbcd.dll` driver (see the item above).
3. Start your application and try to get it to fail.

Check the `MyODBC trace file`, to find out what could be wrong. You should be able to find out the issued queries by searching after the string `>mysql_real_query` in the ``myodbc.log'` file.

You should also try duplicating the queries in the `mysql` monitor or `admndemo` to find out if the error is MyODBC or MySQL.

If you find out something is wrong, please only send the relevant rows (max 40 rows) to myodbc@lists.mysql.com. Please never send the whole MyODBC or ODBC log file!

If you are unable to find out what's wrong, the last option is to make an archive (tar or zip) that contains a MyODBC trace file, the ODBC log file, and a README file that explains the problem. You can send this to ftp://support.mysql.com/pub/mysql/secret/. Only we at MySQL AB will have access to the files you upload, and we will be very discrete with the data!

If you can create a program that also shows this problem, please upload this too!

If the program works with some other SQL server, you should make an ODBC log file where you do exactly the same thing in the other SQL server.

Remember that the more information you can supply to us, the more likely it is that we can fix the problem!

## 8.4 MySQL C API

The C API code is distributed with MySQL. It is included in the `mysqlclient` library and allows C programs to access a database.

Many of the clients in the MySQL source distribution are written in C. If you are looking for examples that demonstrate how to use the C API, take a look at these clients. You can find these in the `clients` directory in the MySQL source distribution.

Most of the other client APIs (all except Java) use the `mysqlclient` library to communicate with the MySQL server. This means that, for example, you can take advantage of many of the same environment variables that are used by other client programs, because they are referenced from the library. See section 4.8 MySQL Client-Side Scripts and Utilities, for a list of these variables.

The client has a maximum communication buffer size. The size of the buffer that is allocated initially (16K bytes) is automatically increased up to the maximum size (the maximum is 16M). Because buffer sizes are increased only as demand warrants, simply increasing the default maximum limit does not in itself cause more resources to be used. This size check is mostly a check for erroneous queries and communication packets.

The communication buffer must be large enough to contain a single SQL statement (for client-to-server traffic) and one row of returned data (for server-to-client traffic). Each thread's communication buffer is dynamically enlarged to handle any query or row up to the maximum limit. For example, if you have `BLOB` values that contain up to 16M of data, you must have a communication buffer limit of at least 16M (in both server and client). The client's default maximum is 16M, but the default maximum in the server is 1M. You can increase this by changing the value of the `max_allowed_packet` parameter when the server is started. See section 5.5.2 Tuning Server Parameters.

The MySQL server shrinks each communication buffer to `net_buffer_length` bytes after each query. For clients, the size of the buffer associated with a connection is not decreased until the connection is closed, at which time client memory is reclaimed.

For programming with threads, see section 8.4.8 How to Make a Threaded Client. For creating a stand-alone application which includes the "server" and "client" in the same program (and does not communicate with an external MySQL server), see section 8.4.9 libmysqld, the Embedded MySQL Server Library.

### 8.4.1 C API Datatypes

`MYSQL`
  This structure represents a handle to one database connection. It is used for almost all MySQL functions.
`MYSQL_RES`
  This structure represents the result of a query that returns rows (`SELECT`, `SHOW`, `DESCRIBE`, `EXPLAIN`). The information returned from a query is called the result set in the remainder of this section.
`MYSQL_ROW`
  This is a type-safe representation of one row of data. It is currently implemented as an array of counted byte strings. (You cannot treat these as null-terminated strings if field values may contain binary data, because such values may contain null bytes internally.) Rows are obtained by calling `mysql_fetch_row()`.
`MYSQL_FIELD`
  This structure contains information about a field, such as the field's name, type, and size. Its members are described in more detail here. You may obtain the `MYSQL_FIELD` structures for each field by calling `mysql_fetch_field()` repeatedly. Field values are not part of this structure; they are contained in a `MYSQL_ROW` structure.
`MYSQL_FIELD_OFFSET`
  This is a type-safe representation of an offset into a MySQL field list. (Used by `mysql_field_seek()`.) Offsets are field numbers within a row, beginning at zero.
`my_ulonglong`
  The type used for the number of rows and for `mysql_affected_rows()`, `mysql_num_rows()`, and `mysql_insert_id()`. This type provides a range of `0` to `1.84e19`. On some systems, attempting to print a value of type `my_ulonglong` will not work. To print such a value, convert it to `unsigned long` and use a `%lu` print format. Example:

      printf (Number of rows: %lu\n", (unsigned long) mysql_num_rows(result));

The `MYSQL_FIELD` structure contains the members listed here:

`char * name`
  The name of the field, as a null-terminated string.
`char * table`
  The name of the table containing this field, if it isn't a calculated field. For calculated fields, the `table` value is an empty string.
`char * def`
  The default value of this field, as a null-terminated string. This is set only if you use `mysql_list_fields()`.
`enum enum_field_types type`
  The type of the field. The `type` value may be one of the following:

  Type value | Type description
  `FIELD_TYPE_TINY` | `TINYINT` field
  `FIELD_TYPE_SHORT` | `SMALLINT` field
  `FIELD_TYPE_LONG` | `INTEGER` field
  `FIELD_TYPE_INT24` | `MEDIUMINT` field
  `FIELD_TYPE_LONGLONG` | `BIGINT` field
  `FIELD_TYPE_DECIMAL` | `DECIMAL` or `NUMERIC` field
  `FIELD_TYPE_FLOAT` | `FLOAT` field
  `FIELD_TYPE_DOUBLE` | `DOUBLE` or `REAL` field
  `FIELD_TYPE_TIMESTAMP` | `TIMESTAMP` field
  `FIELD_TYPE_DATE` | `DATE` field
  `FIELD_TYPE_TIME` | `TIME` field
  `FIELD_TYPE_DATETIME` | `DATETIME` field
  `FIELD_TYPE_YEAR` | `YEAR` field
  `FIELD_TYPE_STRING` | String (`CHAR` or `VARCHAR`) field
  `FIELD_TYPE_BLOB` | `BLOB` or `TEXT` field (use `max_length` to determine the maximum length)
  `FIELD_TYPE_SET` | `SET` field
  `FIELD_TYPE_ENUM` | `ENUM` field
  `FIELD_TYPE_NULL` | `NULL`-type field
  `FIELD_TYPE_CHAR` | Deprecated; use `FIELD_TYPE_TINY` instead

  You can use the `IS_NUM()` macro to test whether a field has a numeric type. Pass the `type` value to `IS_NUM()` and it will evaluate to TRUE if the field is numeric:

      if (IS_NUM(field->type))
          printf("Field is numeric\n");

`unsigned int length`
  The width of the field, as specified in the table definition.
`unsigned int max_length`
  The maximum width of the field for the result set (the length of the longest field value for the rows actually in the result set). If you use `mysql_store_result()` or `mysql_list_fields()`, this contains the maximum length for the field. If you use `mysql_use_result()`, the value of this variable is zero.
`unsigned int flags`
  Different bit-flags for the field. The `flags` value may have zero or more of the following bits set:

  Flag value | Flag description
  `NOT_NULL_FLAG` | Field can't be `NULL`
  `PRI_KEY_FLAG` | Field is part of a primary key
  `UNIQUE_KEY_FLAG` | Field is part of a unique key
  `MULTIPLE_KEY_FLAG` | Field is part of a non-unique key
  `UNSIGNED_FLAG` | Field has the `UNSIGNED` attribute
  `ZEROFILL_FLAG` | Field has the `ZEROFILL` attribute
  `BINARY_FLAG` | Field has the `BINARY` attribute
  `AUTO_INCREMENT_FLAG` | Field has the `AUTO_INCREMENT` attribute
  `ENUM_FLAG` | Field is an `ENUM` (deprecated)
  `SET_FLAG` | Field is a `SET` (deprecated)
  `BLOB_FLAG` | Field is a `BLOB` or `TEXT` (deprecated)
  `TIMESTAMP_FLAG` | Field is a `TIMESTAMP` (deprecated)

  Use of the `BLOB_FLAG`, `ENUM_FLAG`, `SET_FLAG`, and `TIMESTAMP_FLAG` flags is deprecated because they indicate the type of a field rather than an attribute of its type. It is preferable to test `field->type` against `FIELD_TYPE_BLOB`, `FIELD_TYPE_ENUM`, `FIELD_TYPE_SET`, or `FIELD_TYPE_TIMESTAMP` instead. The following example illustrates a typical use of the `flags` value:

      if (field->flags & NOT_NULL_FLAG)
          printf("Field can't be null\n");

  You may use the following convenience macros to determine the boolean status of the `flags` value:

  Flag status | Description
  `IS_NOT_NULL(flags)` | True if this field is defined as `NOT NULL`
  `IS_PRI_KEY(flags)` | True if this field is a primary key
  `IS_BLOB(flags)` | True if this field is a `BLOB` or `TEXT` (deprecated; test `field->type` instead)

`unsigned int decimals`
  The number of decimals for numeric fields.

### 8.4.2 C API Function Overview

The functions available in the C API are listed here and are described in greater detail in a later section. See section 8.4.3 C API Function Descriptions.

Function | Description
mysql_affected_rows() | Returns the number of rows changed/deleted/inserted by the last `UPDATE`, `DELETE`, or `INSERT` query.
mysql_change_user() | Changes user and database on an open connection.
mysql_character_set_name() | Returns the name of the default character set for the connection.
mysql_close() | Closes a server connection.
mysql_connect() | Connects to a MySQL server. This function is deprecated; use `mysql_real_connect()` instead.
mysql_create_db() | Creates a database. This function is deprecated; use the SQL command `CREATE DATABASE` instead.
mysql_data_seek() | Seeks to an arbitrary row in a query result set.
mysql_debug() | Does a `DBUG_PUSH` with the given string.
mysql_drop_db() | Drops a database. This function is deprecated; use the SQL command `DROP DATABASE` instead.
mysql_dump_debug_info() | Makes the server write debug information to the log.
mysql_eof() | Determines whether the last row of a result set has been read. This function is deprecated; `mysql_errno()` or `mysql_error()` may be used instead.
mysql_errno() | Returns the error number for the most recently invoked MySQL function.
mysql_error() | Returns the error message for the most recently invoked MySQL function.
mysql_escape_string() | Escapes special characters in a string for use in a SQL statement.
mysql_fetch_field() | Returns the type of the next table field.
mysql_fetch_field_direct() | Returns the type of a table field, given a field number.
mysql_fetch_fields() | Returns an array of all field structures.
mysql_fetch_lengths() | Returns the lengths of all columns in the current row.
mysql_fetch_row() | Fetches the next row from the result set.
mysql_field_seek() | Puts the column cursor on a specified column.
mysql_field_count() | Returns the number of result columns for the most recent query.
mysql_field_tell() | Returns the position of the field cursor used for the last `mysql_fetch_field()`.
mysql_free_result() | Frees memory used by a result set.
mysql_get_client_info() | Returns client version information.
mysql_get_host_info() | Returns a string describing the connection.
mysql_get_proto_info() | Returns the protocol version used by the connection.
mysql_get_server_info() | Returns the server version number.
mysql_info() | Returns information about the most recently executed query.
mysql_init() | Gets or initialises a `MYSQL` structure.
mysql_insert_id() | Returns the ID generated for an `AUTO_INCREMENT` column by the previous query.
mysql_kill() | Kills a given thread.
mysql_list_dbs() | Returns database names matching a simple regular expression.
mysql_list_fields() | Returns field names matching a simple regular expression.
mysql_list_processes() | Returns a list of the current server threads.
mysql_list_tables() | Returns table names matching a simple regular expression.
mysql_num_fields() | Returns the number of columns in a result set.
mysql_num_rows() | Returns the number of rows in a result set.
mysql_options() | Sets connect options for `mysql_connect()`.
mysql_ping() | Checks whether the connection to the server is working, reconnecting as necessary.
mysql_query() | Executes a SQL query specified as a null-terminated string.
mysql_real_connect() | Connects to a MySQL server.
mysql_real_escape_string() | Escapes special characters in a string for use in a SQL statement, taking into account the current charset of the connection.
mysql_real_query() | Executes a SQL query specified as a counted string.
mysql_reload() | Tells the server to reload the grant tables.
mysql_row_seek() | Seeks to a row in a result set, using value returned from `mysql_row_tell()`.
mysql_row_tell() | Returns the row cursor position.
mysql_select_db() | Selects a database.
mysql_shutdown() | Shuts down the database server.
mysql_stat() | Returns the server status as a string.
mysql_store_result() | Retrieves a complete result set to the client.
mysql_thread_id() | Returns the current thread ID.
mysql_thread_safe() | Returns 1 if the clients are compiled as thread-safe.
mysql_use_result() | Initiates a row-by-row result set retrieval.

To connect to the server, call `mysql_init()` to initialise a connection handler, then call `mysql_real_connect()` with that handler (along with other information such as the hostname, user name, and password). Upon connection, `mysql_real_connect()` sets the `reconnect` flag (part of the MYSQL structure) to a value of `1`. This flag indicates, in the event that a query cannot be performed because of a lost connection, to try reconnecting to the server before giving up. When you are done with the connection, call `mysql_close()` to terminate it.

While a connection is active, the client may send SQL queries to the server using `mysql_query()` or `mysql_real_query()`. The difference between the two is that `mysql_query()` expects the query to be specified as a null-terminated string whereas `mysql_real_query()` expects a counted string. If the string contains binary data (which may include null bytes), you must use `mysql_real_query()`.

For each non-`SELECT` query (for example, `INSERT`, `UPDATE`, `DELETE`), you can find out how many rows were changed (affected) by calling `mysql_affected_rows()`.

For `SELECT` queries, you retrieve the selected rows as a result set. (Note that some statements are `SELECT`-like in that they return rows. These include `SHOW`, `DESCRIBE`, and `EXPLAIN`. They should be treated the same way as `SELECT` statements.)

There are two ways for a client to process result sets. One way is to retrieve the entire result set all at once by calling `mysql_store_result()`. This function acquires from the server all the rows returned by the query and stores them in the client. The second way is for the client to initiate a row-by-row result set retrieval by calling `mysql_use_result()`. This function initialises the retrieval, but does not actually get any rows from the server.

In both cases, you access rows by calling `mysql_fetch_row()`. With `mysql_store_result()`, `mysql_fetch_row()` accesses rows that have already been fetched from the server. With `mysql_use_result()`, `mysql_fetch_row()` actually retrieves the row from the server. Information about the size of the data in each row is available by calling `mysql_fetch_lengths()`.

After you are done with a result set, call `mysql_free_result()` to free the memory used for it.

The two retrieval mechanisms are complementary. Client programs should choose the approach that is most appropriate for their requirements. In practice, clients tend to use `mysql_store_result()` more commonly.

An advantage of `mysql_store_result()` is that because the rows have all been fetched to the client, you not only can access rows sequentially, you can move back and forth in the result set using `mysql_data_seek()` or `mysql_row_seek()` to change the current row position within the result set. You can also find out how many rows there are by calling `mysql_num_rows()`. On the other hand, the memory requirements for `mysql_store_result()` may be very high for large result sets and you are more likely to encounter out-of-memory conditions.

An advantage of `mysql_use_result()` is that the client requires less memory for the result set because it maintains only one row at a time (and because there is less allocation overhead, `mysql_use_result()` can be faster). Disadvantages are that you must process each row quickly to avoid tying up the server, you don't have random access to rows within the result set (you can only access rows sequentially), and you don't know how many rows are in the result set until you have retrieved them all. Furthermore, you must retrieve all the rows even if you determine in mid-retrieval that you've found the information you were looking for.

The API makes it possible for clients to respond appropriately to queries (retrieving rows only as necessary) without knowing whether or not the query is a `SELECT`. You can do this by calling `mysql_store_result()` after each `mysql_query()` (or `mysql_real_query()`). If the result set call succeeds, the query was a `SELECT` and you can read the rows. If the result set call fails, call `mysql_field_count()` to determine whether a result was actually to be expected. If `mysql_field_count()` returns zero, the query returned no data (indicating that it was an `INSERT`, `UPDATE`, `DELETE`, etc.), and was not expected to return rows. If `mysql_field_count()` is non-zero, the query should have returned rows, but didn't. This indicates that the query was a `SELECT` that failed. See the description for `mysql_field_count()` for an example of how this can be done.

Both `mysql_store_result()` and `mysql_use_result()` allow you to obtain information about the fields that make up the result set (the number of fields, their names and types, etc.). You can access field information sequentially within the row by calling `mysql_fetch_field()` repeatedly, or by field number within the row by calling `mysql_fetch_field_direct()`. The current field cursor position may be changed by calling `mysql_field_seek()`. Setting the field cursor affects subsequent calls to `mysql_fetch_field()`. You can also get information for fields all at once by calling `mysql_fetch_fields()`.

For detecting and reporting errors, MySQL provides access to error information by means of the `mysql_errno()` and `mysql_error()` functions. These return the error code or error message for the most recently invoked function that can succeed or fail, allowing you to determine when an error occurred and what it was.

### 8.4.3 C API Function Descriptions

In the descriptions here, a parameter or return value of `NULL` means `NULL` in the sense of the C programming language, not a MySQL `NULL` value.

Functions that return a value generally return a pointer or an integer. Unless specified otherwise, functions returning a pointer return a non-`NULL` value to indicate success or a `NULL` value to indicate an error, and functions returning an integer return zero to indicate success or non-zero to indicate an error. Note that ``non-zero'' means just that. Unless the function description says otherwise, do not test against a value other than zero:

    if (result)                   /* correct */
        ... error ...

    if (result < 0)               /* incorrect */
        ... error ...

    if (result == -1)             /* incorrect */
        ... error ...

When a function returns an error, the Errors subsection of the function description lists the possible types of errors. You can find out which of these occurred by calling `mysql_errno()`. A string representation of the error may be obtained by calling `mysql_error()`.

#### 8.4.3.1 `mysql_affected_rows()`

`my_ulonglong mysql_affected_rows(MYSQL *mysql)`

#### 8.4.3.2 Description

Returns the number of rows changed by the last `UPDATE`, deleted by the last `DELETE` or inserted by the last `INSERT` statement. May be called immediately after `mysql_query()` for `UPDATE`, `DELETE`, or `INSERT` statements. For `SELECT` statements, `mysql_affected_rows()` works like `mysql_num_rows()`.

#### 8.4.3.3 Return Values

An integer greater than zero indicates the number of rows affected or retrieved. Zero indicates that no records where updated for an `UPDATE` statement, no rows matched the `WHERE` clause in the query or that no query has yet been executed. -1 indicates that the query returned an error or that, for a `SELECT` query, `mysql_affected_rows()` was called prior to calling `mysql_store_result()`.

#### 8.4.3.4 Errors

None.

#### 8.4.3.5 Example

    mysql_query(&mysql,"UPDATE products SET cost=cost*1.25 WHERE group=10");
    printf("%ld products updated",(long) mysql_affected_rows(&mysql));

If one specifies the flag `CLIENT_FOUND_ROWS` when connecting to `mysqld`, `mysql_affected_rows()` will return the number of rows matched by the `WHERE` statement for `UPDATE` statements.

Note that when one uses a `REPLACE` command, `mysql_affected_rows()` will return 2 if the new row replaced and old row. This is because in this case one row was inserted and then the duplicate was deleted.

#### 8.4.3.6 `mysql_change_user()`

`my_bool mysql_change_user(MYSQL *mysql, const char *user, const char *password, const char *db)`

#### 8.4.3.7 Description

Changes the user and causes the database specified by `db` to become the default (current) database on the connection specified by `mysql`. In subsequent queries, this database is the default for table references that do not include an explicit database specifier.

This function was introduced in MySQL Version 3.23.3.

`mysql_change_user()` fails unless the connected user can be authenticated or if he doesn't have permission to use the database. In this case the user and database are not changed

The `db` parameter may be set to `NULL` if you don't want to have a default database.

Starting from MySQL 4.0.6 this command will always `ROLLBACK` any active transactions, close all temporary tables, unlock all locked tables and reset the state as if one had done a new connect. This will happen even if the user didn't change.

#### 8.4.3.8 Return Values

Zero for success. Non-zero if an error occurred.

#### 8.4.3.9 Errors

The same that you can get from `mysql_real_connect()`.

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.
`ER_UNKNOWN_COM_ERROR`
  The MySQL server doesn't implement this command (probably an old server)
`ER_ACCESS_DENIED_ERROR`
  The user or password was wrong.
`ER_BAD_DB_ERROR`
  The database didn't exist.
`ER_DBACCESS_DENIED_ERROR`
  The user did not have access rights to the database.
`ER_WRONG_DB_NAME`
  The database name was too long.

#### 8.4.3.10 Example

    if (mysql_change_user(&mysql, "user", "password", "new_database"))
    {
       fprintf(stderr, "Failed to change user.  Error: %s\n",
               mysql_error(&mysql));
    }

#### 8.4.3.11 `mysql_character_set_name()`

`const char *mysql_character_set_name(MYSQL *mysql)`

#### 8.4.3.12 Description

Returns the default character set for the current connection.

#### 8.4.3.13 Return Values

The default character set

#### 8.4.3.14 Errors

None.

#### 8.4.3.15 `mysql_close()`

`void mysql_close(MYSQL *mysql)`

#### 8.4.3.16 Description

Closes a previously opened connection. `mysql_close()` also deallocates the connection handle pointed to by `mysql` if the handle was allocated automatically by `mysql_init()` or `mysql_connect()`.

#### 8.4.3.17 Return Values

None.

#### 8.4.3.18 Errors

None.

#### 8.4.3.19 `mysql_connect()`

`MYSQL *mysql_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd)`

#### 8.4.3.20 Description

This function is deprecated. It is preferable to use `mysql_real_connect()` instead.

`mysql_connect()` attempts to establish a connection to a MySQL database engine running on `host`. `mysql_connect()` must complete successfully before you can execute any of the other API functions, with the exception of `mysql_get_client_info()`.

The meanings of the parameters are the same as for the corresponding parameters for `mysql_real_connect()` with the difference that the connection parameter may be `NULL`. In this case the C API allocates memory for the connection structure automatically and frees it when you call `mysql_close()`. The disadvantage of this approach is that you can't retrieve an error message if the connection fails. (To get error information from `mysql_errno()` or `mysql_error()`, you must provide a valid `MYSQL` pointer.)

#### 8.4.3.21 Return Values

Same as for `mysql_real_connect()`.

#### 8.4.3.22 Errors

Same as for `mysql_real_connect()`.

#### 8.4.3.23 `mysql_create_db()`

`int mysql_create_db(MYSQL *mysql, const char *db)`

#### 8.4.3.24 Description

Creates the database named by the `db` parameter.

This function is deprecated. It is preferable to use `mysql_query()` to issue a SQL `CREATE DATABASE` statement instead.

#### 8.4.3.25 Return Values

Zero if the database was created successfully. Non-zero if an error occurred.

#### 8.4.3.26 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.27 Example

    if(mysql_create_db(&mysql, "my_database"))
    {
       fprintf(stderr, "Failed to create new database.  Error: %s\n",
               mysql_error(&mysql));
    }

#### 8.4.3.28 `mysql_data_seek()`

`void mysql_data_seek(MYSQL_RES *result, my_ulonglong offset)`

#### 8.4.3.29 Description

Seeks to an arbitrary row in a query result set. This requires that the result set structure contains the entire result of the query, so `mysql_data_seek()` may be used in conjunction only with `mysql_store_result()`, not with `mysql_use_result()`.

The offset should be a value in the range from 0 to `mysql_num_rows(result)-1`.

#### 8.4.3.30 Return Values

None.

#### 8.4.3.31 Errors

None.

#### 8.4.3.32 `mysql_debug()`

`void mysql_debug(const char *debug)`

#### 8.4.3.33 Description

Does a `DBUG_PUSH` with the given string. `mysql_debug()` uses the Fred Fish debug library. To use this function, you must compile the client library to support debugging. See section E.1 Debugging a MySQL server. See section E.2 Debugging a MySQL client.

#### 8.4.3.34 Return Values

None.

#### 8.4.3.35 Errors

None.

#### 8.4.3.36 Example

The call shown here causes the client library to generate a trace file in ``/tmp/client.trace'` on the client machine:

    mysql_debug("d:t:O,/tmp/client.trace");

#### 8.4.3.37 `mysql_drop_db()`

`int mysql_drop_db(MYSQL *mysql, const char *db)`

#### 8.4.3.38 Description

Drops the database named by the `db` parameter.

This function is deprecated. It is preferable to use `mysql_query()` to issue a SQL `DROP DATABASE` statement instead.

#### 8.4.3.39 Return Values

Zero if the database was dropped successfully. Non-zero if an error occurred.

#### 8.4.3.40 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.41 Example

    if(mysql_drop_db(&mysql, "my_database"))
      fprintf(stderr, "Failed to drop the database: Error: %s\n",
              mysql_error(&mysql));

#### 8.4.3.42 `mysql_dump_debug_info()`

`int mysql_dump_debug_info(MYSQL *mysql)`

#### 8.4.3.43 Description

Instructs the server to write some debug information to the log. For this to work, the connected user must have the `SUPER` privilege.

#### 8.4.3.44 Return Values

Zero if the command was successful. Non-zero if an error occurred.

#### 8.4.3.45 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.46 `mysql_eof()`

`my_bool mysql_eof(MYSQL_RES *result)`

#### 8.4.3.47 Description

This function is deprecated. `mysql_errno()` or `mysql_error()` may be used instead.

`mysql_eof()` determines whether the last row of a result set has been read.

If you acquire a result set from a successful call to `mysql_store_result()`, the client receives the entire set in one operation. In this case, a `NULL` return from `mysql_fetch_row()` always means the end of the result set has been reached and it is unnecessary to call `mysql_eof()`. When used with `mysql_store_result()`, `mysql_eof()` will always return true.

On the other hand, if you use `mysql_use_result()` to initiate a result set retrieval, the rows of the set are obtained from the server one by one as you call `mysql_fetch_row()` repeatedly. Because an error may occur on the connection during this process, a `NULL` return value from `mysql_fetch_row()` does not necessarily mean the end of the result set was reached normally. In this case, you can use `mysql_eof()` to determine what happened. `mysql_eof()` returns a non-zero value if the end of the result set was reached and zero if an error occurred.

Historically, `mysql_eof()` predates the standard MySQL error functions `mysql_errno()` and `mysql_error()`. Because those error functions provide the same information, their use is preferred over `mysql_eof()`, which is now deprecated. (In fact, they provide more information, because `mysql_eof()` returns only a boolean value whereas the error functions indicate a reason for the error when one occurs.)

#### 8.4.3.48 Return Values

Zero if no error occurred. Non-zero if the end of the result set has been reached.

#### 8.4.3.49 Errors

None.

#### 8.4.3.50 Example

The following example shows how you might use `mysql_eof()`:

    mysql_query(&mysql,"SELECT * FROM some_table");
    result = mysql_use_result(&mysql);
    while((row = mysql_fetch_row(result)))
    {
        // do something with data
    }
    if(!mysql_eof(result))  // mysql_fetch_row() failed due to an error
    {
        fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
    }

However, you can achieve the same effect with the standard MySQL error functions:

    mysql_query(&mysql,"SELECT * FROM some_table");
    result = mysql_use_result(&mysql);
    while((row = mysql_fetch_row(result)))
    {
        // do something with data
    }
    if(mysql_errno(&mysql))  // mysql_fetch_row() failed due to an error
    {
        fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
    }

#### 8.4.3.51 `mysql_errno()`

`unsigned int mysql_errno(MYSQL *mysql)`

#### 8.4.3.52 Description

For the connection specified by `mysql`, `mysql_errno()` returns the error code for the most recently invoked API function that can succeed or fail. A return value of zero means that no error occurred. Client error message numbers are listed in the MySQL ``errmsg.h'` header file. Server error message numbers are listed in ``mysqld_error.h'`. In the MySQL source distribution you can find a complete list of error messages and error numbers in the file ``Docs/mysqld_error.txt'`.

#### 8.4.3.53 Return Values

An error code value. Zero if no error occurred.

#### 8.4.3.54 Errors

None.

#### 8.4.3.55 `mysql_error()`

`char *mysql_error(MYSQL *mysql)`

#### 8.4.3.56 Description

For the connection specified by `mysql`, `mysql_error()` returns the error message for the most recently invoked API function that can succeed or fail. An empty string (`""`) is returned if no error occurred. This means the following two tests are equivalent:

    if(mysql_errno(&mysql))
    {
        // an error occurred
    }

    if(mysql_error(&mysql)[0] != '\0')
    {
        // an error occurred
    }

The language of the client error messages may be changed by recompiling the MySQL client library. Currently you can choose error messages in several different languages. See section 4.6.2 Non-English Error Messages.

#### 8.4.3.57 Return Values

A character string that describes the error. An empty string if no error occurred.

#### 8.4.3.58 Errors

None.

#### 8.4.3.59 `mysql_escape_string()`

You should use `mysql_real_escape_string()` instead!

This function is identical to `mysql_real_escape_string()` except that `mysql_real_escape_string()` takes a connection handler as its first argument and escapes the string according to the current character set. `mysql_escape_string()` does not take a connection argument and does not respect the current charset setting.

#### 8.4.3.60 `mysql_fetch_field()`

`MYSQL_FIELD *mysql_fetch_field(MYSQL_RES *result)`

#### 8.4.3.61 Description

Returns the definition of one column of a result set as a `MYSQL_FIELD` structure. Call this function repeatedly to retrieve information about all columns in the result set. `mysql_fetch_field()` returns `NULL` when no more fields are left.

`mysql_fetch_field()` is reset to return information about the first field each time you execute a new `SELECT` query. The field returned by `mysql_fetch_field()` is also affected by calls to `mysql_field_seek()`.

If you've called `mysql_query()` to perform a `SELECT` on a table but have not called `mysql_store_result()`, MySQL returns the default blob length (8K bytes) if you call `mysql_fetch_field()` to ask for the length of a `BLOB` field. (The 8K size is chosen because MySQL doesn't know the maximum length for the `BLOB`. This should be made configurable sometime.) Once you've retrieved the result set, `field->max_length` contains the length of the largest value for this column in the specific query.

#### 8.4.3.62 Return Values

The `MYSQL_FIELD` structure for the current column. `NULL` if no columns are left.

#### 8.4.3.63 Errors

None.

#### 8.4.3.64 Example

    MYSQL_FIELD *field;

    while((field = mysql_fetch_field(result)))
    {
        printf("field name %s\n", field->name);
    }

#### 8.4.3.65 `mysql_fetch_fields()`

`MYSQL_FIELD *mysql_fetch_fields(MYSQL_RES *result)`

#### 8.4.3.66 Description

Returns an array of all `MYSQL_FIELD` structures for a result set. Each structure provides the field definition for one column of the result set.

#### 8.4.3.67 Return Values

An array of `MYSQL_FIELD` structures for all columns of a result set.

#### 8.4.3.68 Errors

None.

#### 8.4.3.69 Example

    unsigned int num_fields;
    unsigned int i;
    MYSQL_FIELD *fields;

    num_fields = mysql_num_fields(result);
    fields = mysql_fetch_fields(result);
    for(i = 0; i < num_fields; i++)
    {
       printf("Field %u is %s\n", i, fields[i].name);
    }

#### 8.4.3.70 `mysql_fetch_field_direct()`

`MYSQL_FIELD *mysql_fetch_field_direct(MYSQL_RES *result, unsigned int fieldnr)`

#### 8.4.3.71 Description

Given a field number `fieldnr` for a column within a result set, returns that column's field definition as a `MYSQL_FIELD` structure. You may use this function to retrieve the definition for an arbitrary column. The value of `fieldnr` should be in the range from 0 to `mysql_num_fields(result)-1`.

#### 8.4.3.72 Return Values

The `MYSQL_FIELD` structure for the specified column.

#### 8.4.3.73 Errors

None.

#### 8.4.3.74 Example

    unsigned int num_fields;
    unsigned int i;
    MYSQL_FIELD *field;

    num_fields = mysql_num_fields(result);
    for(i = 0; i < num_fields; i++)
    {
        field = mysql_fetch_field_direct(result, i);
        printf("Field %u is %s\n", i, field->name);
    }

#### 8.4.3.75 `mysql_fetch_lengths()`

`unsigned long *mysql_fetch_lengths(MYSQL_RES *result)`

#### 8.4.3.76 Description

Returns the lengths of the columns of the current row within a result set. If you plan to copy field values, this length information is also useful for optimisation, because you can avoid calling `strlen()`. In addition, if the result set contains binary data, you must use this function to determine the size of the data, because `strlen()` returns incorrect results for any field containing null characters.

The length for empty columns and for columns containing `NULL` values is zero. To see how to distinguish these two cases, see the description for `mysql_fetch_row()`.

#### 8.4.3.77 Return Values

An array of unsigned long integers representing the size of each column (not including any terminating null characters). `NULL` if an error occurred.

#### 8.4.3.78 Errors

`mysql_fetch_lengths()` is valid only for the current row of the result set. It returns `NULL` if you call it before calling `mysql_fetch_row()` or after retrieving all rows in the result.

#### 8.4.3.79 Example

    MYSQL_ROW row;
    unsigned long *lengths;
    unsigned int num_fields;
    unsigned int i;

    row = mysql_fetch_row(result);
    if (row)
    {
        num_fields = mysql_num_fields(result);
        lengths = mysql_fetch_lengths(result);
        for(i = 0; i < num_fields; i++)
        {
             printf("Column %u is %lu bytes in length.\n", i, lengths[i]);
        }
    }

#### 8.4.3.80 `mysql_fetch_row()`

`MYSQL_ROW mysql_fetch_row(MYSQL_RES *result)`

#### 8.4.3.81 Description

Retrieves the next row of a result set. When used after `mysql_store_result()`, `mysql_fetch_row()` returns `NULL` when there are no more rows to retrieve. When used after `mysql_use_result()`, `mysql_fetch_row()` returns `NULL` when there are no more rows to retrieve or if an error occurred.

The number of values in the row is given by `mysql_num_fields(result)`. If `row` holds the return value from a call to `mysql_fetch_row()`, pointers to the values are accessed as `row[0]` to `row[mysql_num_fields(result)-1]`. `NULL` values in the row are indicated by `NULL` pointers.

The lengths of the field values in the row may be obtained by calling `mysql_fetch_lengths()`. Empty fields and fields containing `NULL` both have length 0; you can distinguish these by checking the pointer for the field value. If the pointer is `NULL`, the field is `NULL`; otherwise, the field is empty.

#### 8.4.3.82 Return Values

A `MYSQL_ROW` structure for the next row. `NULL` if there are no more rows to retrieve or if an error occurred.

#### 8.4.3.83 Errors

`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.84 Example

    MYSQL_ROW row;
    unsigned int num_fields;
    unsigned int i;

    num_fields = mysql_num_fields(result);
    while ((row = mysql_fetch_row(result)))
    {
       unsigned long *lengths;
       lengths = mysql_fetch_lengths(result);
       for(i = 0; i < num_fields; i++)
       {
           printf("[%.*s] ", (int) lengths[i], row[i] ? row[i] : "NULL");
       }
       printf("\n");
    }

#### 8.4.3.85 `mysql_field_count()`

`unsigned int mysql_field_count(MYSQL *mysql)`

If you are using a version of MySQL earlier than Version 3.22.24, you should use `unsigned int mysql_num_fields(MYSQL *mysql)` instead.

#### 8.4.3.86 Description

Returns the number of columns for the most recent query on the connection.

The normal use of this function is when `mysql_store_result()` returned `NULL` (and thus you have no result set pointer). In this case, you can call `mysql_field_count()` to determine whether `mysql_store_result()` should have produced a non-empty result. This allows the client program to take proper action without knowing whether the query was a `SELECT` (or `SELECT`-like) statement. The example shown here illustrates how this may be done.

See section 8.4.6.1 Why Is It that After `mysql_query()` Returns Success, `mysql_store_result()` Sometimes Returns `NULL`?.

#### 8.4.3.87 Return Values

An unsigned integer representing the number of fields in a result set.

#### 8.4.3.88 Errors

None.

#### 8.4.3.89 Example

    MYSQL_RES *result;
    unsigned int num_fields;
    unsigned int num_rows;

    if (mysql_query(&mysql,query_string))
    {
        // error
    }
    else // query succeeded, process any data returned by it
    {
        result = mysql_store_result(&mysql);
        if (result)  // there are rows
        {
            num_fields = mysql_num_fields(result);
            // retrieve rows, then call mysql_free_result(result)
        }
        else  // mysql_store_result() returned nothing; should it have?
        {
            if(mysql_field_count(&mysql) == 0)
            {
                // query does not return data
                // (it was not a SELECT)
                num_rows = mysql_affected_rows(&mysql);
            }
            else // mysql_store_result() should have returned data
            {
                fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
            }
        }
    }

An alternative is to replace the `mysql_field_count(&mysql)` call with `mysql_errno(&mysql)`. In this case, you are checking directly for an error from `mysql_store_result()` rather than inferring from the value of `mysql_field_count()` whether the statement was a `SELECT`.

#### 8.4.3.90 `mysql_field_seek()`

`MYSQL_FIELD_OFFSET mysql_field_seek(MYSQL_RES *result, MYSQL_FIELD_OFFSET offset)`

#### 8.4.3.91 Description

Sets the field cursor to the given offset. The next call to `mysql_fetch_field()` will retrieve the field definition of the column associated with that offset.

To seek to the beginning of a row, pass an `offset` value of zero.

#### 8.4.3.92 Return Values

The previous value of the field cursor.

#### 8.4.3.93 Errors

None.

#### 8.4.3.94 `mysql_field_tell()`

`MYSQL_FIELD_OFFSET mysql_field_tell(MYSQL_RES *result)`

#### 8.4.3.95 Description

Returns the position of the field cursor used for the last `mysql_fetch_field()`. This value can be used as an argument to `mysql_field_seek()`.

#### 8.4.3.96 Return Values

The current offset of the field cursor.

#### 8.4.3.97 Errors

None.

#### 8.4.3.98 `mysql_free_result()`

`void mysql_free_result(MYSQL_RES *result)`

#### 8.4.3.99 Description

Frees the memory allocated for a result set by `mysql_store_result()`, `mysql_use_result()`, `mysql_list_dbs()`, etc. When you are done with a result set, you must free the memory it uses by calling `mysql_free_result()`.

#### 8.4.3.100 Return Values

None.

#### 8.4.3.101 Errors

None.

#### 8.4.3.102 `mysql_get_client_info()`

`char *mysql_get_client_info(void)`

#### 8.4.3.103 Description

Returns a string that represents the client library version.

#### 8.4.3.104 Return Values

A character string that represents the MySQL client library version.

#### 8.4.3.105 Errors

None.

#### 8.4.3.106 `mysql_get_host_info()`

`char *mysql_get_host_info(MYSQL *mysql)`

#### 8.4.3.107 Description

Returns a string describing the type of connection in use, including the server host name.

#### 8.4.3.108 Return Values

A character string representing the server host name and the connection type.

#### 8.4.3.109 Errors

None.

#### 8.4.3.110 `mysql_get_proto_info()`

`unsigned int mysql_get_proto_info(MYSQL *mysql)`

#### 8.4.3.111 Description

Returns the protocol version used by current connection.

#### 8.4.3.112 Return Values

An unsigned integer representing the protocol version used by the current connection.

#### 8.4.3.113 Errors

None.

#### 8.4.3.114 `mysql_get_server_info()`

`char *mysql_get_server_info(MYSQL *mysql)`

#### 8.4.3.115 Description

Returns a string that represents the server version number.

#### 8.4.3.116 Return Values

A character string that represents the server version number.

#### 8.4.3.117 Errors

None.

#### 8.4.3.118 `mysql_info()`

`char *mysql_info(MYSQL *mysql)`

#### 8.4.3.119 Description

Retrieves a string providing information about the most recently executed query, but only for the statements listed here. For other statements, `mysql_info()` returns `NULL`. The format of the string varies depending on the type of query, as described here. The numbers are illustrative only; the string will contain values appropriate for the query.

`INSERT INTO ... SELECT ...`
  String format: `Records: 100 Duplicates: 0 Warnings: 0`
`INSERT INTO ... VALUES (...),(...),(...)...`
  String format: `Records: 3 Duplicates: 0 Warnings: 0`
`LOAD DATA INFILE ...`
  String format: `Records: 1 Deleted: 0 Skipped: 0 Warnings: 0`
`ALTER TABLE`
  String format: `Records: 3 Duplicates: 0 Warnings: 0`
`UPDATE`
  String format: `Rows matched: 40 Changed: 40 Warnings: 0`

Note that `mysql_info()` returns a non-`NULL` value for the `INSERT ... VALUES` statement only if multiple value lists are specified in the statement.

#### 8.4.3.120 Return Values

A character string representing additional information about the most recently executed query. `NULL` if no information is available for the query.

#### 8.4.3.121 Errors

None.

#### 8.4.3.122 `mysql_init()`

`MYSQL *mysql_init(MYSQL *mysql)`

#### 8.4.3.123 Description

Allocates or initialises a `MYSQL` object suitable for `mysql_real_connect()`. If `mysql` is a `NULL` pointer, the function allocates, initialises, and returns a new object. Otherwise, the object is initialised and the address of the object is returned. If `mysql_init()` allocates a new object, it will be freed when `mysql_close()` is called to close the connection.

#### 8.4.3.124 Return Values

An initialised `MYSQL*` handle. `NULL` if there was insufficient memory to allocate a new object.

#### 8.4.3.125 Errors

In case of insufficient memory, `NULL` is returned.

#### 8.4.3.126 `mysql_insert_id()`

`my_ulonglong mysql_insert_id(MYSQL *mysql)`

#### 8.4.3.127 Description

Returns the ID generated for an `AUTO_INCREMENT` column by the previous query. Use this function after you have performed an `INSERT` query into a table that contains an `AUTO_INCREMENT` field.

Note that `mysql_insert_id()` returns `0` if the previous query does not generate an `AUTO_INCREMENT` value. If you need to save the value for later, be sure to call `mysql_insert_id()` immediately after the query that generates the value.

`mysql_insert_id()` is updated after `INSERT` and `UPDATE` statements that generate an `AUTO_INCREMENT` value or that set a column value to `LAST_INSERT_ID(expr)`. See section 6.3.6.2 Miscellaneous Functions.

Also note that the value of the SQL `LAST_INSERT_ID()` function always contains the most recently generated `AUTO_INCREMENT` value, and is not reset between queries because the value of that function is maintained in the server.

#### 8.4.3.128 Return Values

The value of the `AUTO_INCREMENT` field that was updated by the previous query. Returns zero if there was no previous query on the connection or if the query did not update an `AUTO_INCREMENT` value.

#### 8.4.3.129 Errors

None.

#### 8.4.3.130 `mysql_kill()`

`int mysql_kill(MYSQL *mysql, unsigned long pid)`

#### 8.4.3.131 Description

Asks the server to kill the thread specified by `pid`.

#### 8.4.3.132 Return Values

Zero for success. Non-zero if an error occurred.

#### 8.4.3.133 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.134 `mysql_list_dbs()`

`MYSQL_RES *mysql_list_dbs(MYSQL *mysql, const char *wild)`

#### 8.4.3.135 Description

Returns a result set consisting of database names on the server that match the simple regular expression specified by the `wild` parameter. `wild` may contain the wildcard characters ``%'` or ``_'`, or may be a `NULL` pointer to match all databases. Calling `mysql_list_dbs()` is similar to executing the query `SHOW databases [LIKE wild]`.

You must free the result set with `mysql_free_result()`.

#### 8.4.3.136 Return Values

A `MYSQL_RES` result set for success. `NULL` if an error occurred.

#### 8.4.3.137 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_OUT_OF_MEMORY`
  Out of memory.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.138 `mysql_list_fields()`

`MYSQL_RES *mysql_list_fields(MYSQL *mysql, const char *table, const char *wild)`

#### 8.4.3.139 Description

Returns a result set consisting of field names in the given table that match the simple regular expression specified by the `wild` parameter. `wild` may contain the wildcard characters ``%'` or ``_'`, or may be a `NULL` pointer to match all fields. Calling `mysql_list_fields()` is similar to executing the query `SHOW COLUMNS FROM tbl_name [LIKE wild]`.

Note that it's recommended that you use `SHOW COLUMNS FROM tbl_name` instead of `mysql_list_fields()`.

You must free the result set with `mysql_free_result()`.

#### 8.4.3.140 Return Values

A `MYSQL_RES` result set for success. `NULL` if an error occurred.

#### 8.4.3.141 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.142 `mysql_list_processes()`

`MYSQL_RES *mysql_list_processes(MYSQL *mysql)`

#### 8.4.3.143 Description

Returns a result set describing the current server threads. This is the same kind of information as that reported by `mysqladmin processlist` or a `SHOW PROCESSLIST` query.

You must free the result set with `mysql_free_result()`.

#### 8.4.3.144 Return Values

A `MYSQL_RES` result set for success. `NULL` if an error occurred.

#### 8.4.3.145 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.146 `mysql_list_tables()`

`MYSQL_RES *mysql_list_tables(MYSQL *mysql, const char *wild)`

#### 8.4.3.147 Description

Returns a result set consisting of table names in the current database that match the simple regular expression specified by the `wild` parameter. `wild` may contain the wildcard characters ``%'` or ``_'`, or may be a `NULL` pointer to match all tables. Calling `mysql_list_tables()` is similar to executing the query `SHOW tables [LIKE wild]`.

You must free the result set with `mysql_free_result()`.

#### 8.4.3.148 Return Values

A `MYSQL_RES` result set for success. `NULL` if an error occurred.

#### 8.4.3.149 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.150 `mysql_num_fields()`

`unsigned int mysql_num_fields(MYSQL_RES *result)`

or

`unsigned int mysql_num_fields(MYSQL *mysql)`

The second form doesn't work on MySQL Version 3.22.24 or newer. To pass a `MYSQL*` argument, you must use `unsigned int mysql_field_count(MYSQL *mysql)` instead.

#### 8.4.3.151 Description

Returns the number of columns in a result set.

Note that you can get the number of columns either from a pointer to a result set or to a connection handle. You would use the connection handle if `mysql_store_result()` or `mysql_use_result()` returned `NULL` (and thus you have no result set pointer). In this case, you can call `mysql_field_count()` to determine whether `mysql_store_result()` should have produced a non-empty result. This allows the client program to take proper action without knowing whether or not the query was a `SELECT` (or `SELECT`-like) statement. The example shown here illustrates how this may be done.

See section 8.4.6.1 Why Is It that After `mysql_query()` Returns Success, `mysql_store_result()` Sometimes Returns `NULL`?.

#### 8.4.3.152 Return Values

An unsigned integer representing the number of fields in a result set.

#### 8.4.3.153 Errors

None.

#### 8.4.3.154 Example

    MYSQL_RES *result;
    unsigned int num_fields;
    unsigned int num_rows;

    if (mysql_query(&mysql,query_string))
    {
        // error
    }
    else // query succeeded, process any data returned by it
    {
        result = mysql_store_result(&mysql);
        if (result)  // there are rows
        {
            num_fields = mysql_num_fields(result);
            // retrieve rows, then call mysql_free_result(result)
        }
        else  // mysql_store_result() returned nothing; should it have?
        {
            if (mysql_errno(&mysql))
    	{
               fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
    	}
            else if (mysql_field_count(&mysql) == 0)
            {
                // query does not return data
                // (it was not a SELECT)
                num_rows = mysql_affected_rows(&mysql);
            }
        }
    }

An alternative (if you know that your query should have returned a result set) is to replace the `mysql_errno(&mysql)` call with a check if `mysql_field_count(&mysql)` is = 0. This will only happen if something went wrong.

#### 8.4.3.155 `mysql_num_rows()`

`my_ulonglong mysql_num_rows(MYSQL_RES *result)`

#### 8.4.3.156 Description

Returns the number of rows in the result set.

The use of `mysql_num_rows()` depends on whether you use `mysql_store_result()` or `mysql_use_result()` to return the result set. If you use `mysql_store_result()`, `mysql_num_rows()` may be called immediately. If you use `mysql_use_result()`, `mysql_num_rows()` will not return the correct value until all the rows in the result set have been retrieved.

#### 8.4.3.157 Return Values

The number of rows in the result set.

#### 8.4.3.158 Errors

None.

#### 8.4.3.159 `mysql_options()`

`int mysql_options(MYSQL *mysql, enum mysql_option option, const char *arg)`

#### 8.4.3.160 Description

Can be used to set extra connect options and affect behaviour for a connection. This function may be called multiple times to set several options.

`mysql_options()` should be called after `mysql_init()` and before `mysql_connect()` or `mysql_real_connect()`.

The `option` argument is the option that you want to set; the `arg` argument is the value for the option. If the option is an integer, then `arg` should point to the value of the integer.

Possible options values:

Option | Argument type | Function
`MYSQL_OPT_CONNECT_TIMEOUT` | `unsigned int *` | Connect timeout in seconds.
`MYSQL_OPT_COMPRESS` | Not used | Use the compressed client/server protocol.
`MYSQL_OPT_LOCAL_INFILE` | optional pointer to uint | If no pointer is given or if pointer points to an `unsigned int != 0` the command `LOAD LOCAL INFILE` is enabled.
`MYSQL_OPT_NAMED_PIPE` | Not used | Use named pipes to connect to a MySQL server on NT.
`MYSQL_INIT_COMMAND` | `char *` | Command to execute when connecting to the MySQL server. Will automatically be re-executed when reconnecting.
`MYSQL_READ_DEFAULT_FILE` | `char *` | Read options from the named option file instead of from ``my.cnf'`.
`MYSQL_READ_DEFAULT_GROUP` | `char *` | Read options from the named group from ``my.cnf'` or the file specified with `MYSQL_READ_DEFAULT_FILE`.

Note that the group `client` is always read if you use `MYSQL_READ_DEFAULT_FILE` or `MYSQL_READ_DEFAULT_GROUP`.

The specified group in the option file may contain the following options:

Option | Description
`connect-timeout` | Connect timeout in seconds. On Linux this timeout is also used for waiting for the first answer from the server.
`compress` | Use the compressed client/server protocol.
`database` | Connect to this database if no database was specified in the connect command.
`debug` | Debug options.
`disable-local-infile` | Disable use of `LOAD DATA LOCAL`.
`host` | Default host name.
`init-command` | Command to execute when connecting to MySQL server. Will automatically be re-executed when reconnecting.
`interactive-timeout` | Same as specifying `CLIENT_INTERACTIVE` to `mysql_real_connect()`. See section 8.4.3.171 `mysql_real_connect()`.
`local-infile[=(0|1)]` | If no argument or argument != 0 then enable use of `LOAD DATA LOCAL`.
`password` | Default password.
`pipe` | Use named pipes to connect to a MySQL server on NT.
`port` | Default port number.
`return-found-rows` | Tell `mysql_info()` to return found rows instead of updated rows when using `UPDATE`.
`socket` | Default socket number.
`user` | Default user.

Note that `timeout` has been replaced by `connect-timeout`, but `timeout` will still work for a while.

For more information about option files, see section 4.1.2 ``my.cnf'` Option Files.

#### 8.4.3.161 Return Values

Zero for success. Non-zero if you used an unknown option.

#### 8.4.3.162 Example

    MYSQL mysql;

    mysql_init(&mysql);
    mysql_options(&mysql,MYSQL_OPT_COMPRESS,0);
    mysql_options(&mysql,MYSQL_READ_DEFAULT_GROUP,"odbc");
    if (!mysql_real_connect(&mysql,"host","user","passwd","database",0,NULL,0))
    {
        fprintf(stderr, "Failed to connect to database: Error: %s\n",
              mysql_error(&mysql));
    }

The above requests the client to use the compressed client/server protocol and read the additional options from the `odbc` section in the ``my.cnf'` file.

#### 8.4.3.163 `mysql_ping()`

`int mysql_ping(MYSQL *mysql)`

#### 8.4.3.164 Description

Checks whether the connection to the server is working. If it has gone down, an automatic reconnection is attempted.

This function can be used by clients that remain idle for a long while, to check whether the server has closed the connection and reconnect if necessary.

#### 8.4.3.165 Return Values

Zero if the server is alive. Non-zero if an error occurred.

#### 8.4.3.166 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.167 `mysql_query()`

`int mysql_query(MYSQL *mysql, const char *query)`

#### 8.4.3.168 Description

Executes the SQL query pointed to by the null-terminated string `query`. The query must consist of a single SQL statement. You should not add a terminating semicolon (``;'`) or `\g` to the statement.

`mysql_query()` cannot be used for queries that contain binary data; you should use `mysql_real_query()` instead. (Binary data may contain the ``\0'` character, which `mysql_query()` interprets as the end of the query string.)

If you want to know if the query should return a result set or not, you can use `mysql_field_count()` to check for this. See section 8.4.3.85 `mysql_field_count()`.

#### 8.4.3.169 Return Values

Zero if the query was successful. Non-zero if an error occurred.

#### 8.4.3.170 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.171 `mysql_real_connect()`

`MYSQL *mysql_real_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd, const char *db, unsigned int port, const char *unix_socket, unsigned int client_flag)`

#### 8.4.3.172 Description

`mysql_real_connect()` attempts to establish a connection to a MySQL database engine running on `host`. `mysql_real_connect()` must complete successfully before you can execute any of the other API functions, with the exception of `mysql_get_client_info()`.

The parameters are specified as follows:

- The first parameter should be the address of an existing `MYSQL` structure. Before calling `mysql_real_connect()` you must call `mysql_init()` to initialise the `MYSQL` structure. You can change a lot of connect options with the `mysql_options()` call. See section 8.4.3.159 `mysql_options()`.
- The value of `host` may be either a hostname or an IP address. If `host` is `NULL` or the string `"localhost"`, a connection to the local host is assumed. If the OS supports sockets (Unix) or named pipes (Windows), they are used instead of TCP/IP to connect to the server.
- The `user` parameter contains the user's MySQL login ID. If `user` is `NULL`, the current user is assumed. Under Unix, this is the current login name. Under Windows ODBC, the current user name must be specified explicitly. See section 8.3.2 How to Fill in the Various Fields in the ODBC Administrator Program.
- The `passwd` parameter contains the password for `user`. If `passwd` is `NULL`, only entries in the `user` table for the user that have a blank (empty) password field will be checked for a match. This allows the database administrator to set up the MySQL privilege system in such a way that users get different privileges depending on whether or not they have specified a password. Note: Do not attempt to encrypt the password before calling `mysql_real_connect()`; password encryption is handled automatically by the client API.
- `db` is the database name. If `db` is not `NULL`, the connection will set the default database to this value.
- If `port` is not 0, the value will be used as the port number for the TCP/IP connection. Note that the `host` parameter determines the type of the connection.
- If `unix_socket` is not `NULL`, the string specifies the socket or named pipe that should be used. Note that the `host` parameter determines the type of the connection.
- The value of `client_flag` is usually 0, but can be set to a combination of the following flags in very special circumstances:

  Flag name | Flag description
  `CLIENT_COMPRESS` | Use compression protocol.
  `CLIENT_FOUND_ROWS` | Return the number of found (matched) rows, not the number of affected rows.
  `CLIENT_IGNORE_SPACE` | Allow spaces after function names. Makes all functions names reserved words.
  `CLIENT_INTERACTIVE` | Allow `interactive_timeout` seconds (instead of `wait_timeout` seconds) of inactivity before closing the connection.
  `CLIENT_NO_SCHEMA` | Don't allow the `db_name.tbl_name.col_name` syntax. This is for ODBC. It causes the parser to generate an error if you use that syntax, which is useful for trapping bugs in some ODBC programs.
  `CLIENT_ODBC` | The client is an ODBC client. This changes `mysqld` to be more ODBC-friendly.
  `CLIENT_SSL` | Use SSL (encrypted protocol).

#### 8.4.3.173 Return Values

A `MYSQL*` connection handle if the connection was successful, `NULL` if the connection was unsuccessful. For a successful connection, the return value is the same as the value of the first parameter.

#### 8.4.3.174 Errors

`CR_CONN_HOST_ERROR`
  Failed to connect to the MySQL server.
`CR_CONNECTION_ERROR`
  Failed to connect to the local MySQL server.
`CR_IPSOCK_ERROR`
  Failed to create an IP socket.
`CR_OUT_OF_MEMORY`
  Out of memory.
`CR_SOCKET_CREATE_ERROR`
  Failed to create a Unix socket.
`CR_UNKNOWN_HOST`
  Failed to find the IP address for the hostname.
`CR_VERSION_ERROR`
  A protocol mismatch resulted from attempting to connect to a server with a client library that uses a different protocol version. This can happen if you use a very old client library to connect to a new server that wasn't started with the `--old-protocol` option.
`CR_NAMEDPIPEOPEN_ERROR`
  Failed to create a named pipe on Windows.
`CR_NAMEDPIPEWAIT_ERROR`
  Failed to wait for a named pipe on Windows.
`CR_NAMEDPIPESETSTATE_ERROR`
  Failed to get a pipe handler on Windows.
`CR_SERVER_LOST`
  If `connect_timeout` > 0 and it took longer then `connect_timeout` seconds to connect to the server or if the server died while executing the `init-command`.

#### 8.4.3.175 Example

    MYSQL mysql;

    mysql_init(&mysql);
    mysql_options(&mysql,MYSQL_READ_DEFAULT_GROUP,"your_prog_name");
    if (!mysql_real_connect(&mysql,"host","user","passwd","database",0,NULL,0))
    {
        fprintf(stderr, "Failed to connect to database: Error: %s\n",
              mysql_error(&mysql));
    }

By using `mysql_options()` the MySQL library will read the `[client]` and `[your_prog_name]` sections in the ``my.cnf'` file which will ensure that your program will work, even if someone has set up MySQL in some non-standard way.

Note that upon connection, `mysql_real_connect()` sets the `reconnect` flag (part of the `MYSQL` structure) to a value of `1`. This flag indicates, in the event that a query cannot be performed because of a lost connection, to try reconnecting to the server before giving up.

#### 8.4.3.176 `mysql_real_escape_string()`

`unsigned long mysql_real_escape_string(MYSQL *mysql, char *to, const char *from, unsigned long length)`

#### 8.4.3.177 Description

This function is used to create a legal SQL string that you can use in a SQL statement. See section 6.1.1.1 Strings.

The string in `from` is encoded to an escaped SQL string, taking into account the current character set of the connection. The result is placed in `to` and a terminating null byte is appended. Characters encoded are `NUL` (ASCII 0), ``\n'`, ``\r'`, ``\'`, ``''`, ``"'`, and Control-Z (see section 6.1.1 Literals: How to Write Strings and Numbers). (Strictly speaking, MySQL requires only that backslash and the quote character used to quote the string in the query be escaped. This function quotes the other characters to make them easier to read in log files.)

The string pointed to by `from` must be `length` bytes long. You must allocate the `to` buffer to be at least `length*2+1` bytes long. (In the worst case, each character may need to be encoded as using two bytes, and you need room for the terminating null byte.) When `mysql_real_escape_string()` returns, the contents of `to` will be a null-terminated string. The return value is the length of the encoded string, not including the terminating null character.

#### 8.4.3.178 Example

    char query[1000],*end;

    end = strmov(query,"INSERT INTO test_table values(");
    *end++ = '\'';
    end += mysql_real_escape_string(&mysql, end,"What's this",11);
    *end++ = '\'';
    *end++ = ',';
    *end++ = '\'';
    end += mysql_real_escape_string(&mysql, end,"binary data: \0\r\n",16);
    *end++ = '\'';
    *end++ = ')';

    if (mysql_real_query(&mysql,query,(unsigned int) (end - query)))
    {
       fprintf(stderr, "Failed to insert row, Error: %s\n",
               mysql_error(&mysql));
    }

The `strmov()` function used in the example is included in the `mysqlclient` library and works like `strcpy()` but returns a pointer to the terminating null of the first parameter.

#### 8.4.3.179 Return Values

The length of the value placed into `to`, not including the terminating null character.

#### 8.4.3.180 Errors

None.

#### 8.4.3.181 `mysql_real_query()`

`int mysql_real_query(MYSQL *mysql, const char *query, unsigned long length)`

#### 8.4.3.182 Description

Executes the SQL query pointed to by `query`, which should be a string `length` bytes long. The query must consist of a single SQL statement. You should not add a terminating semicolon (``;'`) or `\g` to the statement.

You must use `mysql_real_query()` rather than `mysql_query()` for queries that contain binary data, because binary data may contain the ``\0'` character. In addition, `mysql_real_query()` is faster than `mysql_query()` because it does not call `strlen()` on the query string.

If you want to know if the query should return a result set or not, you can use `mysql_field_count()` to check for this. See section 8.4.3.85 `mysql_field_count()`.

#### 8.4.3.183 Return Values

Zero if the query was successful. Non-zero if an error occurred.

#### 8.4.3.184 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.185 `mysql_reload()`

`int mysql_reload(MYSQL *mysql)`

#### 8.4.3.186 Description

Asks the MySQL server to reload the grant tables. The connected user must have the `RELOAD` privilege.

This function is deprecated. It is preferable to use `mysql_query()` to issue a SQL `FLUSH PRIVILEGES` statement instead.

#### 8.4.3.187 Return Values

Zero for success. Non-zero if an error occurred.

#### 8.4.3.188 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.189 `mysql_row_seek()`

`MYSQL_ROW_OFFSET mysql_row_seek(MYSQL_RES *result, MYSQL_ROW_OFFSET offset)`

#### 8.4.3.190 Description

Sets the row cursor to an arbitrary row in a query result set. This requires that the result set structure contains the entire result of the query, so `mysql_row_seek()` may be used in conjunction only with `mysql_store_result()`, not with `mysql_use_result()`.

The offset should be a value returned from a call to `mysql_row_tell()` or to `mysql_row_seek()`. This value is not simply a row number; if you want to seek to a row within a result set using a row number, use `mysql_data_seek()` instead.

#### 8.4.3.191 Return Values

The previous value of the row cursor. This value may be passed to a subsequent call to `mysql_row_seek()`.

#### 8.4.3.192 Errors

None.

#### 8.4.3.193 `mysql_row_tell()`

`MYSQL_ROW_OFFSET mysql_row_tell(MYSQL_RES *result)`

#### 8.4.3.194 Description

Returns the current position of the row cursor for the last `mysql_fetch_row()`. This value can be used as an argument to `mysql_row_seek()`.

You should use `mysql_row_tell()` only after `mysql_store_result()`, not after `mysql_use_result()`.

#### 8.4.3.195 Return Values

The current offset of the row cursor.

#### 8.4.3.196 Errors

None.

#### 8.4.3.197 `mysql_select_db()`

`int mysql_select_db(MYSQL *mysql, const char *db)`

#### 8.4.3.198 Description

Causes the database specified by `db` to become the default (current) database on the connection specified by `mysql`. In subsequent queries, this database is the default for table references that do not include an explicit database specifier.

`mysql_select_db()` fails unless the connected user can be authenticated as having permission to use the database.

#### 8.4.3.199 Return Values

Zero for success. Non-zero if an error occurred.

#### 8.4.3.200 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.201 `mysql_shutdown()`

`int mysql_shutdown(MYSQL *mysql)`

#### 8.4.3.202 Description

Asks the database server to shut down. The connected user must have `SHUTDOWN` privileges.

#### 8.4.3.203 Return Values

Zero for success. Non-zero if an error occurred.

#### 8.4.3.204 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.205 `mysql_stat()`

`char *mysql_stat(MYSQL *mysql)`

#### 8.4.3.206 Description

Returns a character string containing information similar to that provided by the `mysqladmin status` command. This includes uptime in seconds and the number of running threads, questions, reloads, and open tables.

#### 8.4.3.207 Return Values

A character string describing the server status. `NULL` if an error occurred.

#### 8.4.3.208 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.209 `mysql_store_result()`

`MYSQL_RES *mysql_store_result(MYSQL *mysql)`

#### 8.4.3.210 Description

You must call `mysql_store_result()` or `mysql_use_result()` for every query that successfully retrieves data (`SELECT`, `SHOW`, `DESCRIBE`, `EXPLAIN`).

You don't have to call `mysql_store_result()` or `mysql_use_result()` for other queries, but it will not do any harm or cause any notable performance if you call `mysql_store_result()` in all cases. You can detect if the query didn't have a result set by checking if `mysql_store_result()` returns 0 (more about this later one).

If you want to know if the query should return a result set or not, you can use `mysql_field_count()` to check for this. See section 8.4.3.85 `mysql_field_count()`.

`mysql_store_result()` reads the entire result of a query to the client, allocates a `MYSQL_RES` structure, and places the result into this structure.

`mysql_store_result()` returns a null pointer if the query didn't return a result set (if the query was, for example, an `INSERT` statement).

`mysql_store_result()` also returns a null pointer if reading of the result set failed. You can check if you got an error by checking if `mysql_error()` doesn't return a null pointer, if `mysql_errno()` returns <> 0, or if `mysql_field_count()` returns <> 0.

An empty result set is returned if there are no rows returned. (An empty result set differs from a null pointer as a return value.)

Once you have called `mysql_store_result()` and got a result back that isn't a null pointer, you may call `mysql_num_rows()` to find out how many rows are in the result set.

You can call `mysql_fetch_row()` to fetch rows from the result set, or `mysql_row_seek()` and `mysql_row_tell()` to obtain or set the current row position within the result set.

You must call `mysql_free_result()` once you are done with the result set.

See section 8.4.6.1 Why Is It that After `mysql_query()` Returns Success, `mysql_store_result()` Sometimes Returns `NULL`?.

#### 8.4.3.211 Return Values

A `MYSQL_RES` result structure with the results. `NULL` if an error occurred.

#### 8.4.3.212 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_OUT_OF_MEMORY`
  Out of memory.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

#### 8.4.3.213 `mysql_thread_id()`

`unsigned long mysql_thread_id(MYSQL *mysql)`

#### 8.4.3.214 Description

Returns the thread ID of the current connection. This value can be used as an argument to `mysql_kill()` to kill the thread.

If the connection is lost and you reconnect with `mysql_ping()`, the thread ID will change. This means you should not get the thread ID and store it for later. You should get it when you need it.

#### 8.4.3.215 Return Values

The thread ID of the current connection.

#### 8.4.3.216 Errors

None.

#### 8.4.3.217 `mysql_use_result()`

`MYSQL_RES *mysql_use_result(MYSQL *mysql)`

#### 8.4.3.218 Description

You must call `mysql_store_result()` or `mysql_use_result()` for every query that successfully retrieves data (`SELECT`, `SHOW`, `DESCRIBE`, `EXPLAIN`).

`mysql_use_result()` initiates a result set retrieval but does not actually read the result set into the client like `mysql_store_result()` does. Instead, each row must be retrieved individually by making calls to `mysql_fetch_row()`. This reads the result of a query directly from the server without storing it in a temporary table or local buffer, which is somewhat faster and uses much less memory than `mysql_store_result()`. The client will only allocate memory for the current row and a communication buffer that may grow up to `max_allowed_packet` bytes.

On the other hand, you shouldn't use `mysql_use_result()` if you are doing a lot of processing for each row on the client side, or if the output is sent to a screen on which the user may type a `^S` (stop scroll). This will tie up the server and prevent other threads from updating any tables from which the data is being fetched.

When using `mysql_use_result()`, you must execute `mysql_fetch_row()` until a `NULL` value is returned, otherwise, the unfetched rows will be returned as part of the result set for your next query. The C API will give the error `Commands out of sync; you can't run this command now` if you forget to do this!

You may not use `mysql_data_seek()`, `mysql_row_seek()`, `mysql_row_tell()`, `mysql_num_rows()`, or `mysql_affected_rows()` with a result returned from `mysql_use_result()`, nor may you issue other queries until the `mysql_use_result()` has finished. (However, after you have fetched all the rows, `mysql_num_rows()` will accurately return the number of rows fetched.)

You must call `mysql_free_result()` once you are done with the result set.

#### 8.4.3.219 Return Values

A `MYSQL_RES` result structure. `NULL` if an error occurred.

#### 8.4.3.220 Errors

`CR_COMMANDS_OUT_OF_SYNC`
  Commands were executed in an improper order.
`CR_OUT_OF_MEMORY`
  Out of memory.
`CR_SERVER_GONE_ERROR`
  The MySQL server has gone away.
`CR_SERVER_LOST`
  The connection to the server was lost during the query.
`CR_UNKNOWN_ERROR`
  An unknown error occurred.

### 8.4.4 C Threaded Function Descriptions

You need to use the following functions when you want to create a threaded client. See section 8.4.8 How to Make a Threaded Client.

#### 8.4.4.1 `my_init()`

`void my_init(void)`

#### 8.4.4.2 Description

This function needs to be called once in the program before calling any MySQL function. This initialises some global variables that MySQL needs. If you are using a thread-safe client library, this will also call `mysql_thread_init()` for this thread.

This is automatically called by `mysql_init()`, `mysql_server_init()` and `mysql_connect()`.

#### 8.4.4.3 Return Values

None.

#### 8.4.4.4 `mysql_thread_init()`

`my_bool mysql_thread_init(void)`

#### 8.4.4.5 Description

This function needs to be called for each created thread to initialise thread-specific variables.

This is automatically called by `my_init()` and `mysql_connect()`.

#### 8.4.4.6 Return Values

None.

#### 8.4.4.7 `mysql_thread_end()`

`void mysql_thread_end(void)`

#### 8.4.4.8 Description

This function needs to be called before calling `pthread_exit()` to free memory allocated by `mysql_thread_init()`.

Note that this function is not invoked automatically by the client library. It must be called explicitly to avoid a memory leak.

#### 8.4.4.9 Return Values

None.

#### 8.4.4.10 `mysql_thread_safe()`

`unsigned int mysql_thread_safe(void)`

#### 8.4.4.11 Description

This function indicates whether the client is compiled as thread-safe.

#### 8.4.4.12 Return Values

1 is the client is thread-safe, 0 otherwise.

### 8.4.5 C Embedded Server Function Descriptions

You must use the following functions if you want to allow your application to be linked against the embedded MySQL server library. See section 8.4.9 libmysqld, the Embedded MySQL Server Library.

If the program is linked with `-lmysqlclient` instead of `-lmysqld`, these functions do nothing. This makes it possible to choose between using the embedded MySQL server and a stand-alone server without modifying any code.

#### 8.4.5.1 `mysql_server_init()`

`int mysql_server_init(int argc, char **argv, char **groups)`

#### 8.4.5.2 Description

This function must be called once in the program using the embedded server before calling any other MySQL function. It starts up the server and initialises any subsystems (`mysys`, InnoDB, etc.) that the server uses. If this function is not called, the program will crash. If you are using the DBUG package that comes with MySQL, you should call this after you have called `MY_INIT()`.

The `argc` and `argv` arguments are analogous to the arguments to `main()`. The first element of `argv` is ignored (it typically contains the program name). For convenience, `argc` may be `0` (zero) if there are no command-line arguments for the server. `mysql_server_init()` makes a copy of the arguments so it's safe to destroy `argv` or `groups` after the call.

The `NULL`-terminated list of strings in `groups` selects which groups in the option files will be active. See section 4.1.2 ``my.cnf'` Option Files. For convenience, `groups` may be `NULL`, in which case the `[server]` and `[emedded]` groups will be active.

#### 8.4.5.3 Example

    #include <mysql.h>
    #include <stdlib.h>

    static char *server_args[] = {
      "this_program",       /* this string is not used */
      "--datadir=.",
      "--key_buffer_size=32M"
    };
    static char *server_groups[] = {
      "embedded",
      "server",
      "this_program_SERVER",
      (char *)NULL
    };

    int main(void) {
      mysql_server_init(sizeof(server_args) / sizeof(char *),
                        server_args, server_groups);

      /* Use any MySQL API functions here */

      mysql_server_end();

      return EXIT_SUCCESS;
    }

#### 8.4.5.4 Return Values

0 if okay, 1 if an error occurred.

#### 8.4.5.5 `mysql_server_end()`

`void mysql_server_end(void)`

#### 8.4.5.6 Description

This function must be called once in the program after all other MySQL functions. It shuts down the embedded server.

#### 8.4.5.7 Return Values

None.

### 8.4.6 Common questions and problems when using the C API

#### 8.4.6.1 Why Is It that After `mysql_query()` Returns Success, `mysql_store_result()` Sometimes Returns `NULL`?

It is possible for `mysql_store_result()` to return `NULL` following a successful call to `mysql_query()`. When this happens, it means one of the following conditions occurred:

- There was a `malloc()` failure (for example, if the result set was too large).
- The data couldn't be read (an error occurred on the connection).
- The query returned no data (for example, it was an `INSERT`, `UPDATE`, or `DELETE`).

You can always check whether the statement should have produced a non-empty result by calling `mysql_field_count()`. If `mysql_field_count()` returns zero, the result is empty and the last query was a statement that does not return values (for example, an `INSERT` or a `DELETE`). If `mysql_field_count()` returns a non-zero value, the statement should have produced a non-empty result. See the description of the `mysql_field_count()` function for an example.

You can test for an error by calling `mysql_error()` or `mysql_errno()`.

#### 8.4.6.2 What Results Can I Get From a Query?

In addition to the result set returned by a query, you can also get the following information:

- `mysql_affected_rows()` returns the number of rows affected by the last query when doing an `INSERT`, `UPDATE`, or `DELETE`. An exception is that if `DELETE` is used without a `WHERE` clause, the table is re-created empty, which is much faster! In this case, `mysql_affected_rows()` returns zero for the number of records affected.
- `mysql_num_rows()` returns the number of rows in a result set. With `mysql_store_result()`, `mysql_num_rows()` may be called as soon as `mysql_store_result()` returns. With `mysql_use_result()`, `mysql_num_rows()` may be called only after you have fetched all the rows with `mysql_fetch_row()`.
- `mysql_insert_id()` returns the ID generated by the last query that inserted a row into a table with an `AUTO_INCREMENT` index. See section 8.4.3.126 `mysql_insert_id()`.
- Some queries (`LOAD DATA INFILE ...`, `INSERT INTO ... SELECT ...`, `UPDATE`) return additional information. The result is returned by `mysql_info()`. See the description for `mysql_info()` for the format of the string that it returns. `mysql_info()` returns a `NULL` pointer if there is no additional information.

#### 8.4.6.3 How Can I Get the Unique ID for the Last Inserted Row?

If you insert a record in a table containing a column that has the `AUTO_INCREMENT` attribute, you can get the most recently generated ID by calling the `mysql_insert_id()` function.

You can also retrieve the ID by using the `LAST_INSERT_ID()` function in a query string that you pass to `mysql_query()`.

You can check if an `AUTO_INCREMENT` index is used by executing the following code. This also checks if the query was an `INSERT` with an `AUTO_INCREMENT` index:

    if (mysql_error(&mysql)[0] == 0 &&
        mysql_num_fields(result) == 0 &&
        mysql_insert_id(&mysql) != 0)
    {
        used_id = mysql_insert_id(&mysql);
    }

The most recently generated ID is maintained in the server on a per-connection basis. It will not be changed by another client. It will not even be changed if you update another `AUTO_INCREMENT` column with a non-magic value (that is, a value that is not `NULL` and not `0`).

If you want to use the ID that was generated for one table and insert it into a second table, you can use SQL statements like this:

    INSERT INTO foo (auto,text)
        VALUES(NULL,'text');              # generate ID by inserting NULL
    INSERT INTO foo2 (id,text)
        VALUES(LAST_INSERT_ID(),'text');  # use ID in second table

#### 8.4.6.4 Problems Linking with the C API

When linking with the C API, the following errors may occur on some systems:

    gcc -g -o client test.o -L/usr/local/lib/mysql -lmysqlclient -lsocket -lnsl

    Undefined        first referenced
     symbol          in file
    floor            /usr/local/lib/mysql/libmysqlclient.a(password.o)
    ld: fatal: Symbol referencing errors. No output written to client

If this happens on your system, you must include the math library by adding `-lm` to the end of the compile/link line.

### 8.4.7 Building Client Programs

If you compile MySQL clients that you've written yourself or that you obtain from a third-party, they must be linked using the `-lmysqlclient -lz` option on the link command. You may also need to specify a `-L` option to tell the linker where to find the library. For example, if the library is installed in ``/usr/local/mysql/lib'`, use `-L/usr/local/mysql/lib -lmysqlclient -lz` on the link command.

For clients that use MySQL header files, you may need to specify a `-I` option when you compile them (for example, `-I/usr/local/mysql/include`), so the compiler can find the header files.

### 8.4.8 How to Make a Threaded Client

The client library is almost thread-safe. The biggest problem is that the subroutines in ``net.c'` that read from sockets are not interrupt safe. This was done with the thought that you might want to have your own alarm that can break a long read to a server. If you install interrupt handlers for the `SIGPIPE` interrupt, the socket handling should be thread-safe.

In the older binaries we distribute on our web site (http://www.mysql.com/), the client libraries are not normally compiled with the thread-safe option (the Windows binaries are by default compiled to be thread-safe). Newer binary distributions should have both a normal and a thread-safe client library.

To get a threaded client where you can interrupt the client from other threads and set timeouts when talking with the MySQL server, you should use the `-lmysys`, `-lmystrings`, and `-ldbug` libraries and the `net_serv.o` code that the server uses.

If you don't need interrupts or timeouts, you can just compile a thread-safe client library `(mysqlclient_r)` and use this. See section 8.4 MySQL C API. In this case you don't have to worry about the `net_serv.o` object file or the other MySQL libraries.

When using a threaded client and you want to use timeouts and interrupts, you can make great use of the routines in the ``thr_alarm.c'` file. If you are using routines from the `mysys` library, the only thing you must remember is to call `my_init()` first! See section 8.4.4 C Threaded Function Descriptions.

All functions except `mysql_real_connect()` are by default thread-safe. The following notes describe how to compile a thread-safe client library and use it in a thread-safe manner. (The notes below for `mysql_real_connect()` actually apply to `mysql_connect()` as well, but because `mysql_connect()` is deprecated, you should be using `mysql_real_connect()` anyway.)

To make `mysql_real_connect()` thread-safe, you must recompile the client library with this command:

    shell> ./configure --enable-thread-safe-client

This will create a thread-safe client library `libmysqlclient_r`. (Assuming your OS has a thread-safe `gethostbyname_r()` function.) This library is thread-safe per connection. You can let two threads share the same connection with the following caveats:

- Two threads can't send a query to the MySQL server at the same time on the same connection. In particular, you have to ensure that between a `mysql_query()` and `mysql_store_result()` no other thread is using the same connection.
- Many threads can access different result sets that are retrieved with `mysql_store_result()`.
- If you use `mysql_use_result`, you have to ensure that no other thread is using the same connection until the result set is closed. However, it really is best for threaded clients that share the same connection to use `mysql_store_result()`.
- If you want to use multiple threads on the same connection, you must have a mutex lock around your `mysql_query()` and `mysql_store_result()` call combination. Once `mysql_store_result()` is ready, the lock can be released and other threads may query the same connection.
- If you program with POSIX threads, you can use `pthread_mutex_lock()` and `pthread_mutex_unlock()` to establish and release a mutex lock.

You need to know the following if you have a thread that is calling MySQL functions which did not create the connection to the MySQL database:

When you call `mysql_init()` or `mysql_connect()`, MySQL will create a thread-specific variable for the thread that is used by the debug library (among other things).

If you call a MySQL function, before the thread has called `mysql_init()` or `mysql_connect()`, the thread will not have the necessary thread-specific variables in place and you are likely to end up with a core dump sooner or later.

The get things to work smoothly you have to do the following:

1. Call `my_init()` at the start of your program if it calls any other MySQL function before calling `mysql_real_connect()`.
2. Call `mysql_thread_init()` in the thread handler before calling any MySQL function.
3. In the thread, call `mysql_thread_end()` before calling `pthread_exit()`. This will free the memory used by MySQL thread-specific variables.

You may get some errors because of undefined symbols when linking your client with `libmysqlclient_r`. In most cases this is because you haven't included the thread libraries on the link/compile line.

### 8.4.9 libmysqld, the Embedded MySQL Server Library

#### 8.4.9.1 Overview of the Embedded MySQL Server Library

The embedded MySQL server library makes it possible to run a full-featured MySQL server inside the client application. The main benefits are increased speed and more simple management for embedded applications.

The API is identical for the embedded MySQL version and the client/server version. To change an old threaded application to use the embedded library, you normally only have to add calls to the following functions:

Function | When to call
`mysql_server_init()` | Should be called before any other MySQL function is called, preferably early in the `main()` function.
`mysql_server_end()` | Should be called before your program exits.
`mysql_thread_init()` | Should be called in each thread you create that will access MySQL.
`mysql_thread_end()` | Should be called before calling `pthread_exit()`

Then you must link your code with ``libmysqld.a'` instead of ``libmysqlclient.a'`.

The above `mysql_server_xxx` functions are also included in ``libmysqlclient.a'` to allow you to change between the embedded and the client/server version by just linking your application with the right library. See section 8.4.5.1 `mysql_server_init()`.

#### 8.4.9.2 Compiling Programs with `libmysqld`

To get a `libmysqld` library you should configure MySQL with the `--with-embedded-server` option.

When you link your program with `libmysqld`, you must also include the system-specific `pthread` libraries and some libraries that the MySQL server uses. You can get the full list of libraries by executing `mysql_config --libmysqld-libs`.

The correct flags for compiling and linking a threaded program must be used, even if you do not directly call any thread functions in your code.

#### 8.4.9.3 Restrictions when using the Embedded MySQL Server

The embedded server has the following limitations:

- No support for ISAM tables. (This is mainly done to make the library smaller)
- No UDF functions.
- No stack trace on core dump.
- No internal RAID support. (This is not normally needed as most OS has nowadays support for big files).
- You can set this up as a server or a master (no replication).
- You can't connect to the embedded server from an outside process with sockets or TCP/IP.

Some of these limitations can be changed by editing the ``mysql_embed.h'` include file and recompiling MySQL.

#### 8.4.9.4 Using Option Files with the Embedded Server

The following is the recommended way to use option files to make it easy to switch between a client/server application and one where MySQL is embedded. See section 4.1.2 ``my.cnf'` Option Files.

- Put common options in the `[server]` section. These will be read by both MySQL versions.
- Put client/server-specific options in the `[mysqld]` section.
- Put embedded MySQL-specific options in the `[embedded]` section.
- Put application-specific options in a `[ApplicationName_SERVER]` section.

#### 8.4.9.5 Things left to do in Embedded Server (TODO)

- We are going to provide options to leave out some parts of MySQL to make the library smaller.
- There is still a lot of speed optimisation to do.
- Errors are written to stderr. We will add an option to specify a filename for these.
- We have to change InnoDB to not be so verbose when using in the embedded version.

#### 8.4.9.6 A Simple Embedded Server Example

This example program and makefile should work without any changes on a Linux or FreeBSD system. For other operating systems, minor changes will be needed. This example is designed to give enough details to understand the problem, without the clutter that is a necessary part of a real application.

To try out the example, create an ``test_libmysqld'` directory at the same level as the mysql-4.0 source directory. Save the ``test_libmysqld.c'` source and the ``GNUmakefile'` in the directory, and run GNU ``make'` from inside the ``test_libmysqld'` directory.

``test_libmysqld.c'`

    /*
     * A simple example client, using the embedded MySQL server library
     */

    #include <mysql.h>
    #include <stdarg.h>
    #include <stdio.h>
    #include <stdlib.h>

    MYSQL *db_connect(const char *dbname);
    void db_disconnect(MYSQL *db);
    void db_do_query(MYSQL *db, const char *query);

    const char *server_groups[] = {
      "test_libmysqld_SERVER", "embedded", "server", NULL
    };

    int
    main(int argc, char **argv)
    {
      MYSQL *one, *two;

      /* mysql_server_init() must be called before any other mysql
       * functions.
       *
       * You can use mysql_server_init(0, NULL, NULL), and it will
       * initialise the server using groups = {
       *   "server", "embedded", NULL
       *  }.
       *
       * In your $HOME/.my.cnf file, you probably want to put:

    [test_libmysqld_SERVER]
    language = /path/to/source/of/mysql/sql/share/english

       * You could, of course, modify argc and argv before passing
       * them to this function.  Or you could create new ones in any
       * way you like.  But all of the arguments in argv (except for
       * argv[0], which is the program name) should be valid options
       * for the MySQL server.
       *
       * If you link this client against the normal mysqlclient
       * library, this function is just a stub that does nothing.
       */
      mysql_server_init(argc, argv, (char **)server_groups);

      one = db_connect("test");
      two = db_connect(NULL);

      db_do_query(one, "SHOW TABLE STATUS");
      db_do_query(two, "SHOW DATABASES");

      mysql_close(two);
      mysql_close(one);

      /* This must be called after all other mysql functions */
      mysql_server_end();

      exit(EXIT_SUCCESS);
    }

    static void
    die(MYSQL *db, char *fmt, ...)
    {
      va_list ap;
      va_start(ap, fmt);
      vfprintf(stderr, fmt, ap);
      va_end(ap);
      (void)putc('\n', stderr);
      if (db)
        db_disconnect(db);
      exit(EXIT_FAILURE);
    }

    MYSQL *
    db_connect(const char *dbname)
    {
      MYSQL *db = mysql_init(NULL);
      if (!db)
        die(db, "mysql_init failed: no memory");
      /*
       * Notice that the client and server use separate group names.
       * This is critical, because the server will not accept the
       * client's options, and vice versa.
       */
      mysql_options(db, MYSQL_READ_DEFAULT_GROUP, "test_libmysqld_CLIENT");
      if (!mysql_real_connect(db, NULL, NULL, NULL, dbname, 0, NULL, 0))
        die(db, "mysql_real_connect failed: %s", mysql_error(db));

      return db;
    }

    void
    db_disconnect(MYSQL *db)
    {
      mysql_close(db);
    }

    void
    db_do_query(MYSQL *db, const char *query)
    {
      if (mysql_query(db, query) != 0)
        goto err;

      if (mysql_field_count(db) > 0)
      {
        MYSQL_RES   *res;
        MYSQL_ROW    row, end_row;
        int num_fields;

        if (!(res = mysql_store_result(db)))
          goto err;
        num_fields = mysql_num_fields(res);
        while ((row = mysql_fetch_row(res)))
        {
          (void)fputs(">> ", stdout);
          for (end_row = row + num_fields; row < end_row; ++row)
            (void)printf("%s\t", row ? (char*)*row : "NULL");
          (void)fputc('\n', stdout);
        }
        (void)fputc('\n', stdout);
      }
      else
        (void)printf("Affected rows: %lld\n", mysql_affected_rows(db));

      return;

    err:
      die(db, "db_do_query failed: %s [%s]", mysql_error(db), query);
    }

``GNUmakefile'`

    # This assumes the MySQL software is installed in /usr/local/mysql
    inc      := /usr/local/mysql/include/mysql
    lib      := /usr/local/mysql/lib

    # If you have not installed the MySQL software yet, try this instead
    #inc      := $(HOME)/mysql-4.0/include
    #lib      := $(HOME)/mysql-4.0/libmysqld

    CC       := gcc
    CPPFLAGS := -I$(inc) -D_THREAD_SAFE -D_REENTRANT
    CFLAGS   := -g -W -Wall
    LDFLAGS  := -static
    # You can change -lmysqld to -lmysqlclient to use the
    # client/server library
    LDLIBS    = -L$(lib) -lmysqld -lz -lm -lcrypt

    ifneq (,$(shell grep FreeBSD /COPYRIGHT 2>/dev/null))
    # FreeBSD
    LDFLAGS += -pthread
    else
    # Assume Linux
    LDLIBS += -lpthread
    endif

    # This works for simple one-file test programs
    sources := $(wildcard *.c)
    objects := $(patsubst %c,%o,$(sources))
    targets := $(basename $(sources))

    all: $(targets)

    clean:
    	rm -f $(targets) $(objects) *.core

#### 8.4.9.7 Licensing the Embedded Server

The MySQL source code is covered by the GNU GPL license (see section H GNU General Public License). One result of this is that any program which includes, by linking with `libmysqld`, the MySQL source code must be released as free software (under a license compatible with the GPL).

We encourage everyone to promote free software by releasing code under the GPL or a compatible license. For those who are not able to do this, another option is to purchase a commercial licence for the MySQL code from MySQL AB. For details, please see section 1.4.3 MySQL Licenses.

## 8.5 MySQL C++ APIs

Two APIs are available in the MySQL Contrib directory (http://www.mysql.com/Downloads/Contrib/).

### 8.5.1 Borland C++

You can compile the MySQL Windows source with Borland C++ 5.02. (The Windows source includes only projects for Microsoft VC++, for Borland C++ you have to do the project files yourself.)

One known problem with Borland C++ is that it uses a different structure alignment than VC++. This means that you will run into problems if you try to use the default `libmysql.dll` libraries (that was compiled with VC++) with Borland C++. You can do one of the following to avoid this problem.

- You can use the static MySQL libraries for Borland C++ that you can find on http://www.mysql.com/downloads/os-win32.html.
- Only call `mysql_init()` with `NULL` as an argument, not a pre-allocated MYSQL struct.

## 8.6 MySQL Java Connectivity (JDBC)

There are 2 supported JDBC drivers for MySQL (the Connector/J driver and the Resin JDBC driver). You can find a copy of the Connector/J driver at http://www.mysql.com/products/connector-j/ and the Resin driver at http://www.caucho.com/projects/jdbc-mysql/index.xtp For documentation consult any JDBC documentation and the driver's own documentation for MySQL-specific features.

## 8.7 MySQL Python APIs

The MySQL Contrib directory (http://www.mysql.com/Downloads/Contrib/) contains a Python interface written by Joseph Skinner.

## 8.8 MySQL Tcl APIs

http://www.binevolve.com/~tdarugar/tcl-sql/ (Tcl at binevolve). The Contrib directory (http://www.mysql.com/Downloads/Contrib/) contains a Tcl interface that is based on msqltcl 1.50.

## 8.9 MySQL Eiffel wrapper

The MySQL Contrib directory (http://www.mysql.com/Downloads/Contrib/) contains an Eiffel wrapper written by Michael Ravits.

# 9 Extending MySQL

## 9.1 MySQL Internals

This chapter describes a lot of things that you need to know when working on the MySQL code. If you plan to contribute to MySQL development, want to have access to the bleeding-edge in-between versions code, or just want to keep track of development, follow the instructions in section 2.3.4 Installing from the Development Source Tree. If you are interested in MySQL internals, you should also subscribe to our `internals` mailing list. This list is relatively low traffic. For details on how to subscribe, please see section 1.6.2.1 The MySQL Mailing Lists. All developers at MySQL AB are on the `internals` list and we help other people who are working on the MySQL code. Feel free to use this list both to ask questions about the code and to send patches that you would like to contribute to the MySQL project!

### 9.1.1 MySQL Threads

The MySQL server creates the following threads:

- The TCP/IP connection thread handles all connection requests and creates a new dedicated thread to handle the authentication and SQL query processing for each connection.
- On Windows NT there is a named pipe handler thread that does the same work as the TCP/IP connection thread on named pipe connect requests.
- The signal thread handles all signals. This thread also normally handles alarms and calls `process_alarm()` to force timeouts on connections that have been idle too long.
- If `mysqld` is compiled with `-DUSE_ALARM_THREAD`, a dedicated thread that handles alarms is created. This is only used on some systems where there are problems with `sigwait()` or if one wants to use the `thr_alarm()` code in ones application without a dedicated signal handling thread.
- If one uses the `--flush_time=#` option, a dedicated thread is created to flush all tables at the given interval.
- Every connection has its own thread.
- Every different table on which one uses `INSERT DELAYED` gets its own thread.
- If you use `--master-host`, a slave replication thread will be started to read and apply updates from the master.

`mysqladmin processlist` only shows the connection, `INSERT DELAYED`, and replication threads.

### 9.1.2 MySQL Test Suite

Until recently, our main full-coverage test suite was based on proprietary customer data and for that reason has not been publicly available. The only publicly available part of our testing process consisted of the `crash-me` test, a Perl DBI/DBD benchmark found in the `sql-bench` directory, and miscellaneous tests located in `tests` directory. The lack of a standardised publicly available test suite has made it difficult for our users, as well developers, to do regression tests on the MySQL code. To address this problem, we have created a new test system that is included in the source and binary distributions starting in Version 3.23.29.

The current set of test cases doesn't test everything in MySQL, but it should catch most obvious bugs in the SQL processing code, OS/library issues, and is quite thorough in testing replication. Our eventual goal is to have the tests cover 100% of the code. We welcome contributions to our test suite. You may especially want to contribute tests that examine the functionality critical to your system, as this will ensure that all future MySQL releases will work well with your applications.

#### 9.1.2.1 Running the MySQL Test Suite

The test system consist of a test language interpreter (`mysqltest`), a shell script to run all tests(`mysql-test-run`), the actual test cases written in a special test language, and their expected results. To run the test suite on your system after a build, type `make test` or `mysql-test/mysql-test-run` from the source root. If you have installed a binary distribution, `cd` to the install root (eg. `/usr/local/mysql`), and do `scripts/mysql-test-run`. All tests should succeed. If not, you should try to find out why and report the problem if this is a bug in MySQL. See section 9.1.2.3 Reporting Bugs in the MySQL Test Suite.

If you have a copy of `mysqld` running on the machine where you want to run the test suite you do not have to stop it, as long as it is not using ports `9306` and `9307`. If one of those ports is taken, you should edit `mysql-test-run` and change the values of the master and/or slave port to one that is available.

You can run one individual test case with `mysql-test/mysql-test-run test_name`.

If one test fails, you should test running `mysql-test-run` with the `--force` option to check if any other tests fails.

#### 9.1.2.2 Extending the MySQL Test Suite

You can use the `mysqltest` language to write your own test cases. Unfortunately, we have not yet written full documentation for it - we plan to do this shortly. You can, however, look at our current test cases and use them as an example. The following points should help you get started:

- The tests are located in `mysql-test/t/*.test`
- A test case consists of `;` terminated statements and is similar to the input of `mysql` command-line client. A statement by default is a query to be sent to MySQL server, unless it is recognised as internal command (eg. `sleep`).
- All queries that produce results@-e.g., `SELECT`, `SHOW`, `EXPLAIN`, etc., must be preceded with `@/path/to/result/file`. The file must contain the expected results. An easy way to generate the result file is to run `mysqltest -r < t/test-case-name.test` from `mysql-test` directory, and then edit the generated result files, if needed, to adjust them to the expected output. In that case, be very careful about not adding or deleting any invisible characters - make sure to only change the text and/or delete lines. If you have to insert a line, make sure the fields are separated with a hard tab, and there is a hard tab at the end. You may want to use `od -c` to make sure your text editor has not messed anything up during edit. We, of course, hope that you will never have to edit the output of `mysqltest -r` as you only have to do it when you find a bug.
- To be consistent with our setup, you should put your result files in `mysql-test/r` directory and name them `test_name.result`. If the test produces more than one result, you should use `test_name.a.result`, `test_name.b.result`, etc.
- If a statement returns an error, you should on the line before the statement specify with the `--error error-number`. The error number can be a list of possible error numbers separated with `','`.
- If you are writing a replication test case, you should on the first line of the test file, put `source include/master-slave.inc;`. To switch between master and slave, use `connection master;` and `connection slave;`. If you need to do something on an alternate connection, you can do `connection master1;` for the master, and `connection slave1;` for the slave.
- If you need to do something in a loop, you can use something like this:

      let $1=1000;
      while ($1)
      {
       # do your queries here
       dec $1;
      }

- To sleep between queries, use the `sleep` command. It supports fractions of a second, so you can do `sleep 1.3;`, for example, to sleep 1.3 seconds.
- To run the slave with additional options for your test case, put them in the command-line format in `mysql-test/t/test_name-slave.opt`. For the master, put them in `mysql-test/t/test_name-master.opt`.
- If you have a question about the test suite, or have a test case to contribute, e-mail to internals@lists.mysql.com. As the list does not accept attachments, you should ftp all the relevant files to: ftp://support.mysql.com/pub/mysql/Incoming/

#### 9.1.2.3 Reporting Bugs in the MySQL Test Suite

If your MySQL version doesn't pass the test suite you should do the following:

- Don't send a bug report before you have found out as much as possible of what when wrong! When you do it, please use the `mysqlbug` script so that we can get information about your system and `MySQL` version. See section 1.6.2.3 How to Report Bugs or Problems.
- Make sure to include the output of `mysql-test-run`, as well as contents of all `.reject` files in `mysql-test/r` directory.
- If a test in the test suite fails, check if the test fails also when run by its own:

      cd mysql-test
      mysql-test-run --local test-name

  If this fails, then you should configure MySQL with `--with-debug` and run `mysql-test-run` with the `--debug` option. If this also fails send the trace file ``var/tmp/master.trace'` to ftp://support.mysql.com/pub/mysql/secret so that we can examine it. Please remember to also include a full description of your system, the version of the mysqld binary and how you compiled it.
- Try also to run `mysql-test-run` with the `--force` option to see if there is any other test that fails.
- If you have compiled MySQL yourself, check our manual for how to compile MySQL on your platform or, preferable, use one of the binaries we have compiled for you at http://www.mysql.com/downloads/. All our standard binaries should pass the test suite !
- If you get an error, like `Result length mismatch` or `Result content mismatch` it means that the output of the test didn't match exactly the expected output. This could be a bug in MySQL or that your mysqld version produces slight different results under some circumstances. Failed test results are put in a file with the same base name as the result file with the `.reject` extension. If your test case is failing, you should do a diff on the two files. If you cannot see how they are different, examine both with `od -c` and also check their lengths.
- If a test fails totally, you should check the logs file in the `mysql-test/var/log` directory for hints of what went wrong.
- If you have compiled MySQL with debugging you can try to debug this by running `mysql-test-run` with the `--gdb` and/or `--debug` options. See section E.1.2 Creating Trace Files. If you have not compiled MySQL for debugging you should probably do that. Just specify the `--with-debug` options to `configure`! See section 2.3 Installing a MySQL Source Distribution.

## 9.2 Adding New Functions to MySQL

There are two ways to add new functions to MySQL:

- You can add the function through the user-definable function (UDF) interface. User-definable functions are added and removed dynamically using the `CREATE FUNCTION` and `DROP FUNCTION` statements. See section 9.2.1 `CREATE FUNCTION/DROP FUNCTION` Syntax.
- You can add the function as a native (built in) MySQL function. Native functions are compiled into the `mysqld` server and become available on a permanent basis.

Each method has advantages and disadvantages:

- If you write a user-definable function, you must install the object file in addition to the server itself. If you compile your function into the server, you don't need to do that.
- You can add UDFs to a binary MySQL distribution. Native functions require you to modify a source distribution.
- If you upgrade your MySQL distribution, you can continue to use your previously installed UDFs. For native functions, you must repeat your modifications each time you upgrade.

Whichever method you use to add new functions, they may be used just like native functions such as `ABS()` or `SOUNDEX()`.

### 9.2.1 `CREATE FUNCTION/DROP FUNCTION` Syntax

    CREATE [AGGREGATE] FUNCTION function_name RETURNS {STRING|REAL|INTEGER}
           SONAME shared_library_name

    DROP FUNCTION function_name

A user-definable function (UDF) is a way to extend MySQL with a new function that works like native (built in) MySQL functions such as `ABS()` and `CONCAT()`.

`AGGREGATE` is a new option for MySQL Version 3.23. An `AGGREGATE` function works exactly like a native MySQL `GROUP` function like `SUM` or `COUNT()`.

`CREATE FUNCTION` saves the function's name, type, and shared library name in the `mysql.func` system table. You must have the `INSERT` and `DELETE` privileges for the `mysql` database to create and drop functions.

All active functions are reloaded each time the server starts, unless you start `mysqld` with the `--skip-grant-tables` option. In this case, UDF initialisation is skipped and UDFs are unavailable. (An active function is one that has been loaded with `CREATE FUNCTION` and not removed with `DROP FUNCTION`.)

For instructions on writing user-definable functions, see section 9.2 Adding New Functions to MySQL. For the UDF mechanism to work, functions must be written in C or C++, your operating system must support dynamic loading and you must have compiled `mysqld` dynamically (not statically).

Note that to make `AGGREGATE` work, you must have a `mysql.func` table that contains the column `type`. If this is not the case, you should run the script `mysql_fix_privilege_tables` to get this fixed.

### 9.2.2 Adding a New User-definable Function

For the UDF mechanism to work, functions must be written in C or C++ and your operating system must support dynamic loading. The MySQL source distribution includes a file ``sql/udf_example.cc'` that defines 5 new functions. Consult this file to see how UDF calling conventions work.

For `mysqld` to be able to use UDF functions, you should configure MySQL with `--with-mysqld-ldflags=-rdynamic` The reason is that to on many platforms (including Linux) you can load a dynamic library (with `dlopen()`) from a static linked program, which you would get if you are using `--with-mysqld-ldflags=-all-static` If you want to use an UDF that needs to access symbols from `mysqld` (like the `methaphone` example in ``sql/udf_example.cc'` that uses `default_charset_info`), you must link the program with `-rdynamic` (see `man dlopen`).

For each function that you want to use in SQL statements, you should define corresponding C (or C++) functions. In the discussion below, the name ``xxx'' is used for an example function name. To distinquish between SQL and C/C++ usage, `XXX()` (uppercase) indicates a SQL function call, and `xxx()` (lowercase) indicates a C/C++ function call.

The C/C++ functions that you write to implement the interface for `XXX()` are:

`xxx()` (required)
  The main function. This is where the function result is computed. The correspondence between the SQL type and return type of your C/C++ function is shown here:

  SQL type | C/C++ type
  `STRING` | `char *`
  `INTEGER` | `long long`
  `REAL` | `double`

`xxx_init()` (optional)
  The initialisation function for `xxx()`. It can be used to:
  - Check the number of arguments to `XXX()`.
  - Check that the arguments are of a required type or, alternatively, tell MySQL to coerce arguments to the types you want when the main function is called.
  - Allocate any memory required by the main function.
  - Specify the maximum length of the result.
  - Specify (for `REAL` functions) the maximum number of decimals.
  - Specify whether the result can be `NULL`.
`xxx_deinit()` (optional)
  The deinitialisation function for `xxx()`. It should deallocate any memory allocated by the initialisation function.

When a SQL statement invokes `XXX()`, MySQL calls the initialisation function `xxx_init()` to let it perform any required setup, such as argument checking or memory allocation. If `xxx_init()` returns an error, the SQL statement is aborted with an error message and the main and deinitialisation functions are not called. Otherwise, the main function `xxx()` is called once for each row. After all rows have been processed, the deinitialisation function `xxx_deinit()` is called so it can perform any required cleanup.

For aggregate functions (like `SUM()`), you must also provide the following functions:

`xxx_reset()` (required)
  Reset sum and insert the argument as the initial value for a new group.
`xxx_add()` (required)
  Add the argument to the old sum.

When using aggregate UDF functions MySQL works the following way:

1. Call `xxx_init()` to let the aggregate function allocate the memory it will need to store results.
2. Sort the table according to the `GROUP BY` expression.
3. For the first row in a new group, call the `xxx_reset()` function.
4. For each new row that belongs in the same group, call the `xxx_add()` function.
5. When the group changes or after the last row has been processed, call `xxx()` to get the result for the aggregate.
6. Repeat 3-5 until all rows has been processed
7. Call `xxx_deinit()` to let the UDF free any memory it has allocated.

All functions must be thread-safe (not just the main function, but the initialisation and deinitialisation functions as well). This means that you are not allowed to allocate any global or static variables that change! If you need memory, you should allocate it in `xxx_init()` and free it in `xxx_deinit()`.

#### 9.2.2.1 UDF Calling Sequences for simple functions

The main function should be declared as shown here. Note that the return type and parameters differ, depending on whether you will declare the SQL function `XXX()` to return `STRING`, `INTEGER`, or `REAL` in the `CREATE FUNCTION` statement:

For `STRING` functions:

    char *xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *result, unsigned long *length,
              char *is_null, char *error);

For `INTEGER` functions:

    long long xxx(UDF_INIT *initid, UDF_ARGS *args,
                  char *is_null, char *error);

For `REAL` functions:

    double xxx(UDF_INIT *initid, UDF_ARGS *args,
                  char *is_null, char *error);

The initialisation and deinitialisation functions are declared like this:

    my_bool xxx_init(UDF_INIT *initid, UDF_ARGS *args, char *message);

    void xxx_deinit(UDF_INIT *initid);

The `initid` parameter is passed to all three functions. It points to a `UDF_INIT` structure that is used to communicate information between functions. The `UDF_INIT` structure members are listed below. The initialisation function should fill in any members that it wishes to change. (To use the default for a member, leave it unchanged.):

`my_bool maybe_null`
  `xxx_init()` should set `maybe_null` to `1` if `xxx()` can return `NULL`. The default value is `1` if any of the arguments are declared `maybe_null`.
`unsigned int decimals`
  Number of decimals. The default value is the maximum number of decimals in the arguments passed to the main function. (For example, if the function is passed `1.34`, `1.345`, and `1.3`, the default would be 3, because `1.345` has 3 decimals.
`unsigned int max_length`
  The maximum length of the string result. The default value differs depending on the result type of the function. For string functions, the default is the length of the longest argument. For integer functions, the default is 21 digits. For real functions, the default is 13 plus the number of decimals indicated by `initid->decimals`. (For numeric functions, the length includes any sign or decimal point characters.) If you want to return a blob, you can set this to 65K or 16M; this memory is not allocated but used to decide which column type to use if there is a need to temporary store the data.
`char *ptr`
  A pointer that the function can use for its own purposes. For example, functions can use `initid->ptr` to communicate allocated memory between functions. In `xxx_init()`, allocate the memory and assign it to this pointer:

      initid->ptr = allocated_memory;

  In `xxx()` and `xxx_deinit()`, refer to `initid->ptr` to use or deallocate the memory.

#### 9.2.2.2 UDF Calling Sequences for aggregate functions

Here follows a description of the different functions you need to define when you want to create an aggregate UDF function.

    char *xxx_reset(UDF_INIT *initid, UDF_ARGS *args,
                    char *is_null, char *error);

This function is called when MySQL finds the first row in a new group. In the function you should reset any internal summary variables and then set the given argument as the first argument in the group.

In many cases this is implemented internally by reseting all variables and then calling `xxx_add()`.

    char *xxx_add(UDF_INIT *initid, UDF_ARGS *args,
                  char *is_null, char *error);

This function is called for all rows that belongs to the same group, except for the first row. In this you should add the value in UDF_ARGS to your internal summary variable.

The `xxx()` function should be declared identical as when you define a simple UDF function. See section 9.2.2.1 UDF Calling Sequences for simple functions.

This function is called when all rows in the group has been processed. You should normally never access the `args` variable here but return your value based on your internal summary variables.

All argument processing in `xxx_reset()` and `xxx_add()` should be done identically as for normal UDF functions. See section 9.2.2.3 Argument Processing.

The return value handling in `xxx()` should be done identically as for a normal UDF. See section 9.2.2.4 Return Values and Error Handling.

The pointer argument to `is_null` and `error` is the same for all calls to `xxx_reset()`, `xxx_add()` and `xxx()`. You can use this to remember that you got an error or if the `xxx()` function should return `NULL`. Note that you should not store a string into `*error`! This is just a 1 byte flag!

`is_null` is reset for each group (before calling `xxx_reset()`. `error` is never reset.

If `isnull` or `error` are set after `xxx()` then MySQL will return `NULL` as the result for the group function.

#### 9.2.2.3 Argument Processing

The `args` parameter points to a `UDF_ARGS` structure that has the members listed here:

`unsigned int arg_count`
  The number of arguments. Check this value in the initialisation function if you want your function to be called with a particular number of arguments. For example:

      if (args->arg_count != 2)
      {
          strcpy(message,"XXX() requires two arguments");
          return 1;
      }

`enum Item_result *arg_type`
  The types for each argument. The possible type values are `STRING_RESULT`, `INT_RESULT`, and `REAL_RESULT`. To make sure that arguments are of a given type and return an error if they are not, check the `arg_type` array in the initialisation function. For example:

      if (args->arg_type[0] != STRING_RESULT ||
          args->arg_type[1] != INT_RESULT)
      {
          strcpy(message,"XXX() requires a string and an integer");
          return 1;
      }

  As an alternative to requiring your function's arguments to be of particular types, you can use the initialisation function to set the `arg_type` elements to the types you want. This causes MySQL to coerce arguments to those types for each call to `xxx()`. For example, to specify coercion of the first two arguments to string and integer, do this in `xxx_init()`:

      args->arg_type[0] = STRING_RESULT;
      args->arg_type[1] = INT_RESULT;

`char **args`
  `args->args` communicates information to the initialisation function about the general nature of the arguments your function was called with. For a constant argument `i`, `args->args[i]` points to the argument value. (See below for instructions on how to access the value properly.) For a non-constant argument, `args->args[i]` is `0`. A constant argument is an expression that uses only constants, such as `3` or `4*7-2` or `SIN(3.14)`. A non-constant argument is an expression that refers to values that may change from row to row, such as column names or functions that are called with non-constant arguments. For each invocation of the main function, `args->args` contains the actual arguments that are passed for the row currently being processed. Functions can refer to an argument `i` as follows:
  - An argument of type `STRING_RESULT` is given as a string pointer plus a length, to allow handling of binary data or data of arbitrary length. The string contents are available as `args->args[i]` and the string length is `args->lengths[i]`. You should not assume that strings are null-terminated.
  - For an argument of type `INT_RESULT`, you must cast `args->args[i]` to a `long long` value:

        long long int_val;
        int_val = *((long long*) args->args[i]);

  - For an argument of type `REAL_RESULT`, you must cast `args->args[i]` to a `double` value:

        double    real_val;
        real_val = *((double*) args->args[i]);

`unsigned long *lengths`
  For the initialisation function, the `lengths` array indicates the maximum string length for each argument. You should not change these. For each invocation of the main function, `lengths` contains the actual lengths of any string arguments that are passed for the row currently being processed. For arguments of types `INT_RESULT` or `REAL_RESULT`, `lengths` still contains the maximum length of the argument (as for the initialisation function).

#### 9.2.2.4 Return Values and Error Handling

The initialisation function should return `0` if no error occurred and `1` otherwise. If an error occurs, `xxx_init()` should store a null-terminated error message in the `message` parameter. The message will be returned to the client. The message buffer is `MYSQL_ERRMSG_SIZE` characters long, but you should try to keep the message to less than 80 characters so that it fits the width of a standard terminal screen.

The return value of the main function `xxx()` is the function value, for `long long` and `double` functions. A string functions should return a pointer to the result and store the length of the string in the `length` arguments.

Set these to the contents and length of the return value. For example:

    memcpy(result, "result string", 13);
    *length = 13;

The `result` buffer that is passed to the calc function is 255 byte big. If your result fits in this, you don't have to worry about memory allocation for results.

If your string function needs to return a string longer than 255 bytes, you must allocate the space for it with `malloc()` in your `xxx_init()` function or your `xxx()` function and free it in your `xxx_deinit()` function. You can store the allocated memory in the `ptr` slot in the `UDF_INIT` structure for reuse by future `xxx()` calls. See section 9.2.2.1 UDF Calling Sequences for simple functions.

To indicate a return value of `NULL` in the main function, set `is_null` to `1`:

    *is_null = 1;

To indicate an error return in the main function, set the `error` parameter to `1`:

    *error = 1;

If `xxx()` sets `*error` to `1` for any row, the function value is `NULL` for the current row and for any subsequent rows processed by the statement in which `XXX()` was invoked. (`xxx()` will not even be called for subsequent rows.) Note: in MySQL versions prior to 3.22.10, you should set both `*error` and `*is_null`:

    *error = 1;
    *is_null = 1;

#### 9.2.2.5 Compiling and Installing User-definable Functions

Files implementing UDFs must be compiled and installed on the host where the server runs. This process is described below for the example UDF file ``udf_example.cc'` that is included in the MySQL source distribution. This file contains the following functions:

- `metaphon()` returns a metaphon string of the string argument. This is something like a soundex string, but it's more tuned for English.
- `myfunc_double()` returns the sum of the ASCII values of the characters in its arguments, divided by the sum of the length of its arguments.
- `myfunc_int()` returns the sum of the length of its arguments.
- `sequence([const int])` returns an sequence starting from the given number or 1 if no number has been given.
- `lookup()` returns the IP number for a hostname.
- `reverse_lookup()` returns the hostname for an IP number. The function may be called with a string `"xxx.xxx.xxx.xxx"` or four numbers.

A dynamically loadable file should be compiled as a sharable object file, using a command something like this:

    shell> gcc -shared -o udf_example.so myfunc.cc

You can easily find out the correct compiler options for your system by running this command in the ``sql'` directory of your MySQL source tree:

    shell> make udf_example.o

You should run a compile command similar to the one that `make` displays, except that you should remove the `-c` option near the end of the line and add `-o udf_example.so` to the end of the line. (On some systems, you may need to leave the `-c` on the command.)

Once you compile a shared object containing UDFs, you must install it and tell MySQL about it. Compiling a shared object from ``udf_example.cc'` produces a file named something like ``udf_example.so'` (the exact name may vary from platform to platform). Copy this file to some directory searched by `ld`, such as ``/usr/lib'`. On many systems, you can set the `LD_LIBRARY` or `LD_LIBRARY_PATH` environment variable to point at the directory where you have your UDF function files. The `dlopen` manual page tells you which variable you should use on your system. You should set this in `mysql.server` or `safe_mysqld` startup scripts and restart `mysqld`.

After the library is installed, notify `mysqld` about the new functions with these commands:

    mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "udf_example.so";
    mysql> CREATE FUNCTION myfunc_double RETURNS REAL SONAME "udf_example.so";
    mysql> CREATE FUNCTION myfunc_int RETURNS INTEGER SONAME "udf_example.so";
    mysql> CREATE FUNCTION lookup RETURNS STRING SONAME "udf_example.so";
    mysql> CREATE FUNCTION reverse_lookup
        ->        RETURNS STRING SONAME "udf_example.so";
    mysql> CREATE AGGREGATE FUNCTION avgcost
        ->        RETURNS REAL SONAME "udf_example.so";

Functions can be deleted using `DROP FUNCTION`:

    mysql> DROP FUNCTION metaphon;
    mysql> DROP FUNCTION myfunc_double;
    mysql> DROP FUNCTION myfunc_int;
    mysql> DROP FUNCTION lookup;
    mysql> DROP FUNCTION reverse_lookup;
    mysql> DROP FUNCTION avgcost;

The `CREATE FUNCTION` and `DROP FUNCTION` statements update the system table `func` in the `mysql` database. The function's name, type and shared library name are saved in the table. You must have the `INSERT` and `DELETE` privileges for the `mysql` database to create and drop functions.

You should not use `CREATE FUNCTION` to add a function that has already been created. If you need to reinstall a function, you should remove it with `DROP FUNCTION` and then reinstall it with `CREATE FUNCTION`. You would need to do this, for example, if you recompile a new version of your function, so that `mysqld` gets the new version. Otherwise, the server will continue to use the old version.

Active functions are reloaded each time the server starts, unless you start `mysqld` with the `--skip-grant-tables` option. In this case, UDF initialisation is skipped and UDFs are unavailable. (An active function is one that has been loaded with `CREATE FUNCTION` and not removed with `DROP FUNCTION`.)

### 9.2.3 Adding a New Native Function

The procedure for adding a new native function is described here. Note that you cannot add native functions to a binary distribution because the procedure involves modifying MySQL source code. You must compile MySQL yourself from a source distribution. Also note that if you migrate to another version of MySQL (for example, when a new version is released), you will need to repeat the procedure with the new version.

To add a new native MySQL function, follow these steps:

1. Add one line to ``lex.h'` that defines the function name in the `sql_functions[]` array.
2. If the function prototype is simple (just takes zero, one, two or three arguments), you should in lex.h specify SYM(FUNC_ARG#) (where # is the number of arguments) as the second argument in the `sql_functions[]` array and add a function that creates a function object in ``item_create.cc'`. Take a look at `"ABS"` and `create_funcs_abs()` for an example of this. If the function prototype is complicated (for example takes a variable number of arguments), you should add two lines to ``sql_yacc.yy'`. One indicates the preprocessor symbol that `yacc` should define (this should be added at the beginning of the file). Then define the function parameters and add an ``item'' with these parameters to the `simple_expr` parsing rule. For an example, check all occurrences of `ATAN` in ``sql_yacc.yy'` to see how this is done.
3. In ``item_func.h'`, declare a class inheriting from `Item_num_func` or `Item_str_func`, depending on whether your function returns a number or a string.
4. In ``item_func.cc'`, add one of the following declarations, depending on whether you are defining a numeric or string function:

       double   Item_func_newname::val()
       longlong Item_func_newname::val_int()
       String  *Item_func_newname::Str(String *str)

   If you inherit your object from any of the standard items (like `Item_num_func` you probably only have to define one of the above functions and let the parent object take care of the other functions. For example, the `Item_str_func` class defines a `val()` function that executes `atof()` on the value returned by `::str()`.
5. You should probably also define the following object function:

       void Item_func_newname::fix_length_and_dec()

   This function should at least calculate `max_length` based on the given arguments. `max_length` is the maximum number of characters the function may return. This function should also set `maybe_null = 0` if the main function can't return a `NULL` value. The function can check if any of the function arguments can return `NULL` by checking the arguments `maybe_null` variable. You can take a look at `Item_func_mod::fix_length_and_dec` for a typical example of how to do this.

All functions must be thread-safe (in other words, don't use any global or static variables in the functions without protecting them with mutexes).

If you want to return `NULL`, from `::val()`, `::val_int()` or `::str()` you should set `null_value` to 1 and return 0.

For `::str()` object functions, there are some additional considerations to be aware of:

- The `String *str` argument provides a string buffer that may be used to hold the result. (For more information about the `String` type, take a look at the ``sql_string.h'` file.)
- The `::str()` function should return the string that holds the result or `(char*) 0` if the result is `NULL`.
- All current string functions try to avoid allocating any memory unless absolutely necessary!

## 9.3 Adding New Procedures to MySQL

In MySQL, you can define a procedure in C++ that can access and modify the data in a query before it is sent to the client. The modification can be done on row-by-row or `GROUP BY` level.

We have created an example procedure in MySQL Version 3.23 to show you what can be done.

Additionally we recommend you to take a look at `mylua`. With this you can use the LUA language to load a procedure at runtime into `mysqld`.

### 9.3.1 Procedure Analyse

`analyse([max elements,[max memory]])`

This procedure is defined in the ``sql/sql_analyse.cc'`. This examines the result from your query and returns an analysis of the results:

- `max elements` (default 256) is the maximum number of distinct values `analyse` will notice per column. This is used by `analyse` to check if the optimal column type should be of type `ENUM`.
- `max memory` (default 8192) is the maximum memory `analyse` should allocate per column while trying to find all distinct values.

    SELECT ... FROM ... WHERE ... PROCEDURE ANALYSE([max elements,[max memory]])

### 9.3.2 Writing a Procedure

For the moment, the only documentation for this is the source.

You can find all information about procedures by examining the following files:

- ``sql/sql_analyse.cc'`
- ``sql/procedure.h'`
- ``sql/procedure.cc'`
- ``sql/sql_select.cc'`

# A Problems and Common Errors

This chapter lists some common problems and error messages that users have run into. You will learn how to figure out what the problem is, and what to do to solve it. You will also find proper solutions to some common problems.

## A.1 How to Determine What Is Causing Problems

When you run into problems, the first thing you should do is to find out which program / piece of equipment is causing problems:

- If you have one of the following symptoms, then it is probably a hardware (like memory, motherboard, CPU, or hard disk) or kernel problem:
  - The keyboard doesn't work. This can normally be checked by pressing Caps Lock. If the Caps Lock light doesn't change you have to replace your keyboard. (Before doing this, you should try to reboot your computer and check all cables to the keyboard.)
  - The mouse pointer doesn't move.
  - The machine doesn't answer to a remote machine's pings.
  - Different, unrelated programs don't behave correctly.
  - If your system rebooted unexpectedly (a faulty user level program should never be able to take down your system).
  In this case you should start by checking all your cables and run some diagnostic tool to check your hardware! You should also check if there are any patches, updates, or service packs for your operating system that could likely solve your problems. Check also that all your libraries (like glibc) are up to date. It's always good to use a machine with ECC memory to discover memory problems early!
- If your keyboard is locked up, you may be able to fix this by logging into your machine from another machine and execute `kbd_mode -a` on it.
- Please examine your system log file (/var/log/messages or similar) for reasons for your problems. If you think the problem is in MySQL then you should also examine MySQL's log files. See section 4.9.3 The Update Log.
- If you don't think you have hardware problems, you should try to find out which program is causing problems. Try using `top`, `ps`, `taskmanager`, or some similar program, to check which program is taking all CPU or is locking the machine.
- Check with `top`, `df`, or a similar program if you are out of memory, disk space, open files, or some other critical resource.
- If the problem is some runaway process, you can always try to kill it. If it doesn't want to die, there is probably a bug in the operating system.

If after you have examined all other possibilities and you have concluded that it's the MySQL server or a MySQL client that is causing the problem, it's time to do a bug report for our mailing list or our support team. In the bug report, try to give a very detailed description of how the system is behaving and what you think is happening. You should also state why you think it's MySQL that is causing the problems. Take into consideration all the situations in this chapter. State any problems exactly how they appear when you examine your system. Use the 'cut and paste' method for any output and/or error messages from programs and/or log files!

Try to describe in detail which program is not working and all symptoms you see! We have in the past received many bug reports that just state "the system doesn't work". This doesn't provide us with any information about what could be the problem.

If a program fails, it's always useful to know:

- Has the program in question made a segmentation fault (core dumped)?
- Is the program taking up the whole CPU? Check with `top`. Let the program run for a while, it may be evaluating something heavy.
- If it's the `mysqld` server that is causing problems, can you do `mysqladmin -u root ping` or `mysqladmin -u root processlist`?
- What does a client program say (try with `mysql`, for example) when you try to connect to the MySQL server? Does the client jam? Do you get any output from the program?

When sending a bug report, you should of follow the outlines described in this manual. See section 1.6.2.2 Asking Questions or Reporting Bugs.

## A.2 Common Errors When Using MySQL

This section lists some errors that users frequently get. You will find descriptions of the errors, and how to solve the problem here.

### A.2.1 `Access denied` Error

See section 4.2.6 How the Privilege System Works, and especially. See section 4.2.11 Causes of `Access denied` Errors.

### A.2.2 `MySQL server has gone away` Error

This section also covers the related `Lost connection to server during query` error.

The most common reason for the `MySQL server has gone away` error is that the server timed out and closed the connection. By default, the server closes the connection after 8 hours if nothing has happened. You can change the time limit by setting the `wait_timeout` variable when you start `mysqld`.

Another common reason to receive the `MySQL server has gone away` error is because you have issued a ``close'' on your MySQL connection and then tried to run a query on the closed connection.

If you have a script, you just have to issue the query again for the client to do an automatic reconnection.

You normally can get the following error codes in this case (which one you get is OS-dependent):

Error code | Description
`CR_SERVER_GONE_ERROR` | The client couldn't send a question to the server.
`CR_SERVER_LOST` | The client didn't get an error when writing to the server, but it didn't get a full answer (or any answer) to the question.

You will also get this error if someone has kills the running thread with `kill #threadid#`.

You can check that the MySQL hasn't died by executing `mysqladmin version` and examining the uptime. If the problem is that mysqld crashed you should concentrate one finding the reason for the crash. You should in this case start by checking if issuing the query again will kill MySQL again. See section A.4.1 What To Do If MySQL Keeps Crashing.

You can also get these errors if you send a query to the server that is incorrect or too large. If `mysqld` gets a packet that is too large or out of order, it assumes that something has gone wrong with the client and closes the connection. If you need big queries (for example, if you are working with big `BLOB` columns), you can increase the query limit by starting `mysqld` with the `-O max_allowed_packet=#` option (default 1M). The extra memory is allocated on demand, so `mysqld` will use more memory only when you issue a big query or when `mysqld` must return a big result row!

If you want to make a bug report regarding this problem, be sure that you include the following information:

- Include information if MySQL died or not. (You can find this in the `hostname.err file`. See section A.4.1 What To Do If MySQL Keeps Crashing.
- If a specific query kills `mysqld` and the involved tables where checked with `CHECK TABLE` before you did the query, can you do a test case for this? See section E.1.6 Making a Test Case When You Experience Table Corruption.
- What is the value of the `wait_timeout` variable in the MySQL server ? `mysqladmin variables` gives you the value of this
- Have you tried to run `mysqld` with `--log` and check if the issued query appears in the log ?

See section 1.6.2.2 Asking Questions or Reporting Bugs.

### A.2.3 `Can't connect to [local] MySQL server` Error

A MySQL client on Unix can connect to the `mysqld` server in two different ways: Unix sockets, which connect through a file in the file system (default ``/tmp/mysqld.sock'`) or TCP/IP, which connects through a port number. Unix sockets are faster than TCP/IP but can only be used when connecting to a server on the same computer. Unix sockets are used if you don't specify a hostname or if you specify the special hostname `localhost`.

On Windows, if the `mysqld` server is running on 9x/Me, you can connect only via TCP/IP. If the server is running on NT/2000/XP and mysqld is started with `--enable-named-pipe`, you can also connect with named pipes. The name of the named pipe is MySQL. If you don't give a hostname when connecting to `mysqld`, a MySQL client will first try to connect to the named pipe, and if this doesn't work it will connect to the TCP/IP port. You can force the use of named pipes on Windows by using `.` as the hostname.

The error (2002) `Can't connect to ...` normally means that there isn't a MySQL server running on the system or that you are using a wrong socket file or TCP/IP port when trying to connect to the `mysqld` server.

Start by checking (using `ps` or the task manager on Windows) that there is a process running named `mysqld` on your server! If there isn't any `mysqld` process, you should start one. See section 2.4.2 Problems Starting the MySQL Server.

If a `mysqld` process is running, you can check the server by trying these different connections (the port number and socket pathname might be different in your setup, of course):

    shell> mysqladmin version
    shell> mysqladmin variables
    shell> mysqladmin -h `hostname` version variables
    shell> mysqladmin -h `hostname` --port=3306 version
    shell> mysqladmin -h 'ip for your host' version
    shell> mysqladmin --socket=/tmp/mysql.sock version

Note the use of backquotes rather than forward quotes with the `hostname` command; these cause the output of `hostname` (that is, the current hostname) to be substituted into the `mysqladmin` command.

Here are some reasons the `Can't connect to local MySQL server` error might occur:

- `mysqld` is not running.
- You are running on a system that uses MIT-pthreads. If you are running on a system that doesn't have native threads, `mysqld` uses the MIT-pthreads package. See section 2.2.2 Operating Systems Supported by MySQL. However, not all MIT-pthreads versions support Unix sockets. On a system without sockets support you must always specify the hostname explicitly when connecting to the server. Try using this command to check the connection to the server:

      shell> mysqladmin -h `hostname` version

- Someone has removed the Unix socket that `mysqld` uses (default ``/tmp/mysqld.sock'`). You might have a `cron` job that removes the MySQL socket (for example, a job that removes old files from the ``/tmp'` directory). You can always run `mysqladmin version` and check that the socket `mysqladmin` is trying to use really exists. The fix in this case is to change the `cron` job to not remove ``mysqld.sock'` or to place the socket somewhere else. See section A.4.5 How to Protect or Change the MySQL Socket File ``/tmp/mysql.sock'`.
- You have started the `mysqld` server with the `--socket=/path/to/socket` option. If you change the socket pathname for the server, you must also notify the MySQL clients about the new path. You can do this by providing the socket path as an argument to the client. See section A.4.5 How to Protect or Change the MySQL Socket File ``/tmp/mysql.sock'`.
- You are using Linux and one thread has died (core dumped). In this case you must kill the other `mysqld` threads (for example, with the `mysql_zap` script before you can start a new MySQL server. See section A.4.1 What To Do If MySQL Keeps Crashing.
- You may not have read and write privilege to either the directory that holds the socket file or privilege to the socket file itself. In this case you have to either change the privilege for the directory / file or restart `mysqld` so that it uses a directory that you can access.

If you get the error message `Can't connect to MySQL server on some_hostname`, you can try the following things to find out what the problem is :

- Check if the server is up by doing `telnet your-host-name tcp-ip-port-number` and press Enter a couple of times. If there is a MySQL server running on this port you should get a responses that includes the version number of the running MySQL server. If you get an error like `telnet: Unable to connect to remote host: Connection refused`, then there is no server running on the given port.
- Try connecting to the `mysqld` daemon on the local machine and check the TCP/IP port that `mysqld` it's configured to use (variable `port`) with `mysqladmin variables`.
- Check that your `mysqld` server is not started with the `--skip-networking` option.

### A.2.4 `Host '...' is blocked` Error

If you get an error like this:

    Host 'hostname' is blocked because of many connection errors.
    Unblock with 'mysqladmin flush-hosts'

this means that `mysqld` has gotten a lot (`max_connect_errors`) of connect requests from the host `'hostname'` that have been interrupted in the middle. After `max_connect_errors` failed requests, `mysqld` assumes that something is wrong (like an attack from a cracker), and blocks the site from further connections until someone executes the command `mysqladmin flush-hosts`.

By default, `mysqld` blocks a host after 10 connection errors. You can easily adjust this by starting the server like this:

    shell> safe_mysqld -O max_connect_errors=10000 &

Note that if you get this error message for a given host, you should first check that there isn't anything wrong with TCP/IP connections from that host. If your TCP/IP connections aren't working, it won't do you any good to increase the value of the `max_connect_errors` variable!

### A.2.5 `Too many connections` Error

If you get the error `Too many connections` when you try to connect to MySQL, this means that there is already `max_connections` clients connected to the `mysqld` server.

If you need more connections than the default (100), then you should restart `mysqld` with a bigger value for the `max_connections` variable.

Note that `mysqld` actually allows (`max_connections`+1) clients to connect. The last connection is reserved for a user with the `SUPER` privilege. By not giving this privilege to normal users (they shouldn't need this), an administrator with this privilege can log in and use `SHOW PROCESSLIST` to find out what could be wrong. See section 4.5.6.6 `SHOW PROCESSLIST`.

The maximum number of connects MySQL is depending on how good the thread library is on a given platform. Linux or Solaris should be able to support 500-1000 simultaneous connections, depending on how much RAM you have and what your clients are doing.

### A.2.6 `Some non-transactional changed tables couldn't be rolled back` Error

If you get the error/warning: `Warning: Some non-transactional changed tables couldn't be rolled back` when trying to do a `ROLLBACK`, this means that some of the tables you used in the transaction didn't support transactions. These non-transactional tables will not be affected by the `ROLLBACK` statement.

The most typical case when this happens is when you have tried to create a table of a type that is not supported by your `mysqld` binary. If `mysqld` doesn't support a table type (or if the table type is disabled by a startup option) , it will instead create the table type with the table type that is most resembles to the one you requested, probably `MyISAM`.

You can check the table type for a table by doing:

`SHOW TABLE STATUS LIKE 'table_name'`. See section 4.5.6.2 `SHOW TABLE STATUS`.

You can check the extensions your `mysqld` binary supports by doing:

`show variables like 'have_%'`. See section 4.5.6.4 `SHOW VARIABLES`.

### A.2.7 `Out of memory` Error

If you issue a query and get something like the following error:

    mysql: Out of memory at line 42, 'malloc.c'
    mysql: needed 8136 byte (8k), memory in use: 12481367 bytes (12189k)
    ERROR 2008: MySQL client ran out of memory

note that the error refers to the MySQL client `mysql`. The reason for this error is simply that the client does not have enough memory to store the whole result.

To remedy the problem, first check that your query is correct. Is it reasonable that it should return so many rows? If so, you can use `mysql --quick`, which uses `mysql_use_result()` to retrieve the result set. This places less of a load on the client (but more on the server).

### A.2.8 `Packet too large` Error

When a MySQL client or the `mysqld` server gets a packet bigger than `max_allowed_packet` bytes, it issues a `Packet too large` error and closes the connection.

In MySQL 3.23 the biggest possible packet is 16M (due to limits in the client/server protocol). In MySQL 4.0.1 and up, this is only limited by the amount on memory you have on your server (up to a theoretical maximum of 2G).

A communication packet is a single SQL statement sent to the MySQL server or a single row that is sent to the client.

When a MySQL client or the `mysqld` server gets a packet bigger than `max_allowed_packet` bytes, it issues a `Packet too large` error and closes the connection. With some clients, you may also get `Lost connection to MySQL server during query` error if the communication packet is too big.

Note that both the client and the server has it's own `max_allowed_packet` variable. If you want to handle big packets, you have to increase this variable both in the client and in the server.

It's safe to increase this variable as memory is only allocated when needed; this variable is more a precaution to catch wrong packets between the client/server and also to ensure that you don't accidentally use big packets so that you run out of memory.

If you are using the `mysql` client, you may specify a bigger buffer by starting the client with `mysql --set-variable=max_allowed_packet=8M`. Other clients have different methods to set this variable. Please note that `--set-variable` is deprecated since MySQL 4.0, just use `--max-allowed-packet=8M` instead. You can use the option file to set `max_allowed_packet` to a larger size in `mysqld`. For example, if you are expecting to store the full length of a `MEDIUMBLOB` into a table, you'll need to start the server with the `set-variable=max_allowed_packet=16M` option.

You can also get strange problems with large packets if you are using big blobs, but you haven't given `mysqld` access to enough memory to handle the query. If you suspect this is the case, try adding `ulimit -d 256000` to the beginning of the `safe_mysqld` script and restart `mysqld`.

### A.2.9 Communication Errors / Aborted Connection

Starting with `MySQL 3.23.40` you only get the `Aborted connection` error of you start `mysqld` with `--warnings`.

If you find errors like the following in your error log.

    010301 14:38:23  Aborted connection 854 to db: 'users' user: 'josh'

See section 4.9.1 The Error Log.

This means that something of the following has happened:

- The client program did not call `mysql_close()` before exit.
- The client had been sleeping more than `wait_timeout` or `interactive_timeout` without doing any requests. See section 4.5.6.4 `SHOW VARIABLES`. See section 4.5.6.4 `SHOW VARIABLES`.
- The client program ended abruptly in the middle of the transfer.

When the above happens, the server variable `Aborted_clients` is incremented.

The server variable `Aborted_connects` is incremented when:

- When a connection packet doesn't contain the right information.
- When the user didn't have privileges to connect to a database.
- When a user uses a wrong password.
- When it takes more than `connect_timeout` seconds to get a connect package. See section 4.5.6.4 `SHOW VARIABLES`.

Note that the above could indicate that someone is trying to break into your database!

Other reasons for problems with Aborted clients / Aborted connections.

- Usage of duplex Ethernet protocol, both half and full with Linux. Many Linux Ethernet drivers have this bug. You should test for this bug by transferring a huge file via ftp between these two machines. If a transfer goes in burst-pause-burst-pause ... mode then you are experiencing a Linux duplex syndrome. The only solution to this problem is switching of both half and full duplexing on hubs and switches.
- Some problem with the thread library that causes interrupts on reads.
- Badly configured TCP/IP.
- Faulty Ethernets or hubs or switches, cables ... This can be diagnosed properly only by replacing hardware.
- `max_allowed_packet` is too small or queries require more memory than you have alloacated for `mysqld`. See section A.2.8 `Packet too large` Error.

### A.2.10 `The table is full` Error

There is a couple of different cases when you can get this error:

- You are using an older MySQL version (before 3.23.0) when an in-memory temporary table becomes larger than `tmp_table_size` bytes. To avoid this problem, you can use the `-O tmp_table_size=#` option to make `mysqld` increase the temporary table size or use the SQL option `BIG_TABLES` before you issue the problematic query. See section 5.5.6 `SET` Syntax. You can also start `mysqld` with the `--big-tables` option. This is exactly the same as using `BIG_TABLES` for all queries. In MySQL Version 3.23, in-memory temporary tables will automatically be converted to a disk-based `MyISAM` table after the table size gets bigger than `tmp_table_size`.
- You are using `InnoDB` tables and run out of room in the `InnoDB` tablespace. In this case the solution is to extend the `InnoDB` tablespace.
- You are using `ISAM` or `MyISAM` tables on an OS that only supports files of 2G in size and you have hit this limit for the data or index file.
- You are using `MyISAM` tables and the needed data or index size is bigger than what MySQL has allocated pointers for. (If you don't specify `MAX_ROWS` to `CREATE TABLE` MySQL will only allocate pointers to hold 4G of data). You can check the maximum data/index sizes by doing

      SHOW TABLE STATUS FROM database LIKE 'table_name';

  or using `myisamchk -dv database/table_name`. If this is the problem, you can fix it by doing something like:

      ALTER TABLE table_name MAX_ROWS=1000000000 AVG_ROW_LENGTH=nnn;

  You only have to specify `AVG_ROW_LENGTH` for tables with BLOB/TEXT fields as in this case MySQL can't optimise the space required based only on the number of rows.

### A.2.11 `Can't create/write to file` Error

If you get an error for some queries of type:

    Can't create/write to file '\\sqla3fe_0.ism'.

this means that MySQL can't create a temporary file for the result set in the given temporary directory. (The above error is a typical error message on Windows, and the Unix error message is similar.) The fix is to start `mysqld` with `--tmpdir=path` or to add to your option file:

    [mysqld]
    tmpdir=C:/temp

assuming that the ``c:\\temp'` directory exists. See section 4.1.2 ``my.cnf'` Option Files.

Check also the error code that you get with `perror`. One reason may also be a disk full error;

    shell> perror 28
    Error code  28:  No space left on device

### A.2.12 `Commands out of sync` Error in Client

If you get `Commands out of sync; you can't run this command now` in your client code, you are calling client functions in the wrong order!

This can happen, for example, if you are using `mysql_use_result()` and try to execute a new query before you have called `mysql_free_result()`. It can also happen if you try to execute two queries that return data without a `mysql_use_result()` or `mysql_store_result()` in between.

### A.2.13 `Ignoring user` Error

If you get the following error:

`Found wrong password for user: 'some_user@some_host'; ignoring user`

this means that when `mysqld` was started or when it reloaded the permissions tables, it found an entry in the `user` table with an invalid password. As a result, the entry is simply ignored by the permission system.

Possible causes of and fixes for this problem:

- You may be running a new version of `mysqld` with an old `user` table. You can check this by executing `mysqlshow mysql user` to see if the password field is shorter than 16 characters. If so, you can correct this condition by running the `scripts/add_long_password` script.
- The user has an old password (8 characters long) and you didn't start `mysqld` with the `--old-protocol` option. Update the user in the `user` table with a new password or restart `mysqld` with `--old-protocol`.
- You have specified a password in the `user` table without using the `PASSWORD()` function. Use `mysql` to update the user in the `user` table with a new password. Make sure to use the `PASSWORD()` function:

      mysql> UPDATE user SET password=PASSWORD('your password')
          ->             WHERE user='XXX';

### A.2.14 `Table 'xxx' doesn't exist` Error

If you get the error `Table 'xxx' doesn't exist` or `Can't find file: 'xxx' (errno: 2)`, this means that no table exists in the current database with the name `xxx`.

Note that as MySQL uses directories and files to store databases and tables, the database and table names are case-sensitive! (On Windows the databases and tables names are not case-sensitive, but all references to a given table within a query must use the same case!)

You can check which tables you have in the current database with `SHOW TABLES`. See section 4.5.6 `SHOW` Syntax.

### A.2.15 `Can't initialize character set xxx` error

If you get an error like:

    MySQL Connection Failed: Can't initialize character set xxx

This means one of the following things:

- The character set is a multi-byte character set and you have no support for the character set in the client. In this case you need to recompile the client with `--with-charset=xxx` or with `--with-extra-charsets=xxx`. See section 2.3.3 Typical `configure` Options. All standard MySQL binaries are compiled with `--with-extra-character-sets=complex` which will enable support for all multi-byte character sets. See section 4.6.1 The Character Set Used for Data and Sorting.
- The character set is a simple character set which is not compiled into `mysqld` and the character set definition files are not in the place where the client expects to find them. In this case you need to:
  - Recompile the client with support for the character set. See section 2.3.3 Typical `configure` Options.
  - Specify to the client where the character set definition files are. For many clients you can do this with the `--character-sets-dir=path-to-charset-dir` option.
  - Copy the character definition files to the path where the client expects them to be.

### A.2.16 File Not Found

If you get `ERROR '...' not found (errno: 23)`, `Can't open file: ... (errno: 24)`, or any other error with `errno 23` or `errno 24` from MySQL, it means that you haven't allocated enough file descriptors for MySQL. You can use the `perror` utility to get a description of what the error number means:

    shell> perror 23
    File table overflow
    shell> perror 24
    Too many open files
    shell> perror 11
    Resource temporarily unavailable

The problem here is that `mysqld` is trying to keep open too many files simultaneously. You can either tell `mysqld` not to open so many files at once or increase the number of file descriptors available to `mysqld`.

To tell `mysqld` to keep open fewer files at a time, you can make the table cache smaller by using the `-O table_cache=32` option to `safe_mysqld` (the default value is 64). Reducing the value of `max_connections` will also reduce the number of open files (the default value is 90).

To change the number of file descriptors available to `mysqld`, you can use the option `--open-files-limit=#` to `safe_mysqld` or `-O open-files-limit=#` to `mysqld`. See section 4.5.6.4 `SHOW VARIABLES`. The easiest way to do that is to add the option to your option file. See section 4.1.2 ``my.cnf'` Option Files. If you have an old `mysqld` version that doesn't support this, you can edit the `safe_mysqld` script. There is a commented-out line `ulimit -n 256` in the script. You can remove the `'#'` character to uncomment this line, and change the number 256 to affect the number of file descriptors available to `mysqld`.

`ulimit` (and `open-files-limit`) can increase the number of file descriptors, but only up to the limit imposed by the operating system. There is also a 'hard' limit that can only be overrided if you start `safe_mysqld` or `mysqld` as root (just remember that you need to also use the `--user=...` option in this case). If you need to increase the OS limit on the number of file descriptors available to each process, consult the documentation for your operating system.

Note that if you run the `tcsh` shell, `ulimit` will not work! `tcsh` will also report incorrect values when you ask for the current limits! In this case you should start `safe_mysqld` with `sh`!

## A.3 Installation Related Issues

### A.3.1 Problems When Linking with the MySQL Client Library

If you are linking your program and you get errors for unreferenced symbols that start with `mysql_`, like the following:

    /tmp/ccFKsdPa.o: In function `main':
    /tmp/ccFKsdPa.o(.text+0xb): undefined reference to `mysql_init'
    /tmp/ccFKsdPa.o(.text+0x31): undefined reference to `mysql_real_connect'
    /tmp/ccFKsdPa.o(.text+0x57): undefined reference to `mysql_real_connect'
    /tmp/ccFKsdPa.o(.text+0x69): undefined reference to `mysql_error'
    /tmp/ccFKsdPa.o(.text+0x9a): undefined reference to `mysql_close'

you should be able to solve this by adding `-Lpath-to-the-mysql-library -lmysqlclient` last on your link line.

If you get `undefined reference` errors for the `uncompress` or `compress` function, add `-lz` last on your link line and try again!

If you get `undefined reference` errors for functions that should exist on your system, like `connect`, check the man page for the function in question, for which libraries you should add to the link line!

If you get `undefined reference` errors for functions that don't exist on your system, like the following:

    mf_format.o(.text+0x201): undefined reference to `__lxstat'

it usually means that your library is compiled on a system that is not 100% compatible with yours. In this case you should download the latest MySQL source distribution and compile this yourself. See section 2.3 Installing a MySQL Source Distribution.

If you are trying to run a program and you then get errors for unreferenced symbols that start with `mysql_` or that the `mysqlclient` library can't be found, this means that your system can't find the share ``libmysqlclient.so'` library.

The fix for this is to tell your system to search after shared libraries where the library is located by one of the following methods:

- Add the path to the directory where you have ``libmysqlclient.so'` the `LD_LIBRARY_PATH` environment variable.
- Add the path to the directory where you have ``libmysqlclient.so'` the `LD_LIBRARY` environment variable.
- Copy ``libmysqlclient.so'` to some place that is searched by your system, like ``/lib'`, and update the shared library information by executing `ldconfig`.

Another way to solve this problem is to link your program statically, with `-static`, or by removing the dynamic MySQL libraries before linking your code. In the second case you should be sure that no other programs are using the dynamic libraries!

### A.3.2 How to Run MySQL As a Normal User

The MySQL server `mysqld` can be started and run by any user. In order to change `mysqld` to run as a Unix user `user_name`, you must do the following:

1. Stop the server if it's running (use `mysqladmin shutdown`).
2. Change the database directories and files so that `user_name` has privileges to read and write files in them (you may need to do this as the Unix `root` user):

       shell> chown -R user_name /path/to/mysql/datadir

   If directories or files within the MySQL data directory are symlinks, you'll also need to follow those links and change the directories and files they point to. `chown -R` may not follow symlinks for you.
3. Start the server as user `user_name`, or, if you are using MySQL Version 3.22 or later, start `mysqld` as the Unix `root` user and use the `--user=user_name` option. `mysqld` will switch to run as the Unix user `user_name` before accepting any connections.
4. To start the server as the given user name automatically at system startup time, add a `user` line that specifies the user name to the `[mysqld]` group of the ``/etc/my.cnf'` option file or the ``my.cnf'` option file in the server's data directory. For example:

       [mysqld]
       user=user_name

At this point, your `mysqld` process should be running fine and dandy as the Unix user `user_name`. One thing hasn't changed, though: the contents of the permissions tables. By default (right after running the permissions table install script `mysql_install_db`), the MySQL user `root` is the only user with permission to access the `mysql` database or to create or drop databases. Unless you have changed those permissions, they still hold. This shouldn't stop you from accessing MySQL as the MySQL `root` user when you're logged in as a Unix user other than `root`; just specify the `-u root` option to the client program.

Note that accessing MySQL as `root`, by supplying `-u root` on the command-line, has nothing to do with MySQL running as the Unix `root` user, or, indeed, as another Unix user. The access permissions and user names of MySQL are completely separate from Unix user names. The only connection with Unix user names is that if you don't provide a `-u` option when you invoke a client program, the client will try to connect using your Unix login name as your MySQL user name.

If your Unix box itself isn't secured, you should probably at least put a password on the MySQL `root` users in the access tables. Otherwise, any user with an account on that machine can run `mysql -u root db_name` and do whatever he likes.

### A.3.3 Problems with File Permissions

If you have problems with file permissions, for example, if `mysql` issues the following error message when you create a table:

    ERROR: Can't find file: 'path/with/filename.frm' (Errcode: 13)

then the environment variable `UMASK` might be set incorrectly when `mysqld` starts up. The default umask value is `0660`. You can change this behaviour by starting `safe_mysqld` as follows:

    shell> UMASK=384  # = 600 in octal
    shell> export UMASK
    shell> /path/to/safe_mysqld &

By default MySQL will create database and `RAID` directories with permission type 0700. You can modify this behaviour by setting the `UMASK_DIR` variable. If you set this, new directories are created with the combined `UMASK` and `UMASK_DIR`. For example, if you want to give group access to all new directories, you can do:

    shell> UMASK_DIR=504  # = 770 in octal
    shell> export UMASK_DIR
    shell> /path/to/safe_mysqld &

In MySQL Version 3.23.25 and above, MySQL assumes that the value for `UMASK` and `UMASK_DIR` is in octal if it starts with a zero.

See section F Environment Variables.

## A.4 Administration Related Issues

### A.4.1 What To Do If MySQL Keeps Crashing

All MySQL versions are tested on many platforms before they are released. This doesn't mean that there aren't any bugs in MySQL, but it means if there are bugs, they are very few and can be hard to find. If you have a problem, it will always help if you try to find out exactly what crashes your system, as you will have a much better chance of getting this fixed quickly.

First, you should try to find out whether the problem is that the `mysqld` daemon dies or whether your problem has to do with your client. You can check how long your `mysqld` server has been up by executing `mysqladmin version`. If `mysqld` has died, you may find the reason for this in the file ``mysql-data-directory/`hostname`.err'`. See section 4.9.1 The Error Log.

Many crashes of MySQL are caused by corrupted index / data files. MySQL will update the data on disk, with the `write()` system call, after every SQL statement and before the client is notified about the result. (This is not true if you are running with `delay_key_write`, in which case only the data is written.) This means that the data is safe even if `mysqld` crashes, as the OS will ensure that the not flushed data is written to disk. You can force MySQL to sync everything to disk after every SQL command by starting `mysqld` with `--flush`.

The above means that normally you shouldn't get corrupted tables unless:

- Someone/something killed `mysqld` or the machine in the middle of an update.
- You have found a bug in `mysqld` that caused it to die in the middle of an update.
- Someone is manipulating the data/index files outside of mysqld without locking the table properly.
- If you are running many `mysqld` servers on the same data on a system that doesn't support good filesystem locks (normally handled by the `lockd` daemon ) or if you are running multiple servers with `--skip-external-locking`
- You have a crashed index/datafile that contains very wrong data that got `mysqld` confused.
- You have found a bug in the data storage code. This isn't that likely, but it's at least possible. In this case you can try to change the file type to another storage engine by using `ALTER TABLE` on a repaired copy of the table!

Because it is very difficult to know why something is crashing, first try to check whether things that work for others crash for you. Please try the following things:

Take down the `mysqld` daemon with `mysqladmin shutdown`, run `myisamchk --silent --force */*.MYI` on all tables, and restart the `mysqld` daemon. This will ensure that you are running from a clean state. See section 4 Database Administration.
- Use `mysqld --log` and try to determine from the information in the log whether some specific query kills the server. About 95% of all bugs are related to a particular query! Normally this is one of the last queries in the log file just before MySQL restarted. See section 4.9.2 The General Query Log. If you can repeatadly kill MySQL with one of the queries, even when you have checked all tables just before doing the query, then you have been able to locate the bug and should do a bug report for this! See section 1.6.2.3 How to Report Bugs or Problems.
- Try to make a test case that we can use to reproduce the problem. See section E.1.6 Making a Test Case When You Experience Table Corruption.
- Try running the included mysql-test test and the MySQL benchmarks. See section 9.1.2 MySQL Test Suite. They should test MySQL rather well. You can also add code that to the benchmarks to simulates your application! The benchmarks can be found in the ``bench'` directory in the source distribution or, for a binary distribution, in the ``sql-bench'` directory under your MySQL installation directory.
- Try `fork_test.pl` and `fork2_test.pl`.
- If you configure MySQL for debugging, it will be much easier to gather information about possible errors if something goes wrong. Reconfigure MySQL with the `--with-debug` option or `--with-debug=full` to `configure` and then recompile. See section E.1 Debugging a MySQL server.
- Configuring MySQL for debugging causes a safe memory allocator to be included that can find some errors. It also provides a lot of output about what is happening.
- Have you applied the latest patches for your operating system?
- Use the `--skip-external-locking` option to `mysqld`. On some systems, the `lockd` lock manager does not work properly; the `--skip-external-locking` option tells `mysqld` not to use external locking. (This means that you cannot run 2 `mysqld` servers on the same data and that you must be careful if you use `myisamchk`, but it may be instructive to try the option as a test.)
- Have you tried `mysqladmin -u root processlist` when `mysqld` appears to be running but not responding? Sometimes `mysqld` is not comatose even though you might think so. The problem may be that all connections are in use, or there may be some internal lock problem. `mysqladmin processlist` will usually be able to make a connection even in these cases, and can provide useful information about the current number of connections and their status.
- Run the command `mysqladmin -i 5 status` or `mysqladmin -i 5 -r status` or in a separate window to produce statistics while you run your other queries.
- Try the following:
  1. Start `mysqld` from `gdb` (or in another debugger). See section E.1.3 Debugging mysqld under gdb.
  2. Run your test scripts.
  3. Print the backtrace and the local variables at the 3 lowest levels. In gdb you can do this with the following commands when `mysqld` has crashed inside gdb:

         backtrace
         info local
         up
         info local
         up
         info local

     With gdb you can also examine which threads exist with `info threads` and switch to a specific thread with `thread #`, where `#` is the thread id.
- Try to simulate your application with a Perl script to force MySQL to crash or misbehave.
- Send a normal bug report. See section 1.6.2.3 How to Report Bugs or Problems. Be even more detailed than usual. Because MySQL works for many people, it may be that the crash results from something that exists only on your computer (for example, an error that is related to your particular system libraries).
- If you have a problem with tables with dynamic-length rows and you are not using `BLOB/TEXT` columns (but only `VARCHAR` columns), you can try to change all `VARCHAR` to `CHAR` with `ALTER TABLE`. This will force MySQL to use fixed-size rows. Fixed-size rows take a little extra space, but are much more tolerant to corruption! The current dynamic row code has been in use at MySQL AB for at least 3 years without any problems, but by nature dynamic-length rows are more prone to errors, so it may be a good idea to try the above to see if it helps!

### A.4.2 How to Reset a Forgotten Root Password

If you never set a `root` password for MySQL, then the server will not require a password at all for connecting as `root`. It is recommended to always set a password for each user. See section 4.2.2 How to Make MySQL Secure Against Crackers.

If you have set a `root` password, but forgot what it was, you can set a new password with the following procedure:

1. Take down the `mysqld` server by sending a `kill` (not `kill -9`) to the `mysqld` server. The pid is stored in a ``.pid'` file, which is normally in the MySQL database directory:

       shell> kill `cat /mysql-data-directory/hostname.pid`

   You must be either the Unix `root` user or the same user `mysqld` runs as to do this.
2. Restart `mysqld` with the `--skip-grant-tables` option.
3. Set a new password with the `mysqladmin password` command:

       shell> mysqladmin -u root password 'mynewpassword'

4. Now you can either stop `mysqld` and restart it normally, or just load the privilege tables with:

       shell> mysqladmin -h hostname flush-privileges

5. After this, you should be able to connect using the new password.

Alternatively, you can set the new password using the `mysql` client:

1. Take down and restart `mysqld` with the `--skip-grant-tables` option as described above.
2. Connect to the `mysqld` server with:

       shell> mysql -u root mysql

3. Issue the following commands in the `mysql` client:

       mysql> UPDATE user SET Password=PASSWORD('mynewpassword')
           ->             WHERE User='root';
       mysql> FLUSH PRIVILEGES;

4. After this, you should be able to connect using the new password.
5. You can now stop `mysqld` and restart it normally.

### A.4.3 How MySQL Handles a Full Disk

When a disk-full condition occurs, MySQL does the following:

- It checks once every minute to see whether there is enough space to write the current row. If there is enough space, it continues as if nothing had happened.
- Every 6 minutes it writes an entry to the log file warning about the disk full condition.

To alleviate the problem, you can take the following actions:

- To continue, you only have to free enough disk space to insert all records.
- To abort the thread, you must send a `mysqladmin kill` to the thread. The thread will be aborted the next time it checks the disk (in 1 minute).
- Note that other threads may be waiting for the table that caused the disk full condition. If you have several ``locked'' threads, killing the one thread that is waiting on the disk-full condition will allow the other threads to continue.

Exceptions to the above behaveour is when you use `REPAIR` or `OPTIMIZE` or when the indexes are created in a batch after an `LOAD DATA INFILE` or after an `ALTER TABLE` statement.

All of the above commands may use big temporary files that left to themself would cause big problems for the rest of the system. If MySQL gets disk full while doing any of the above operations, it will remove the big temporary files and mark the table as crashed (except for `ALTER TABLE`, in which the old table will be left unchanged).

### A.4.4 Where MySQL Stores Temporary Files

MySQL uses the value of the `TMPDIR` environment variable as the pathname of the directory in which to store temporary files. If you don't have `TMPDIR` set, MySQL uses the system default, which is normally ``/tmp'` or ``/usr/tmp'`. If the filesystem containing your temporary file directory is too small, you should edit `safe_mysqld` to set `TMPDIR` to point to a directory in a filesystem where you have enough space! You can also set the temporary directory using the `--tmpdir` option to `mysqld`.

MySQL creates all temporary files as hidden files. This ensures that the temporary files will be removed if `mysqld` is terminated. The disadvantage of using hidden files is that you will not see a big temporary file that fills up the filesystem in which the temporary file directory is located.

When sorting (`ORDER BY` or `GROUP BY`), MySQL normally uses one or two temporary files. The maximum disk-space needed is:

    (length of what is sorted + sizeof(database pointer))
    * number of matched rows
    * 2

`sizeof(database pointer)` is usually 4, but may grow in the future for really big tables.

For some `SELECT` queries, MySQL also creates temporary SQL tables. These are not hidden and have names of the form ``SQL_*'`.

`ALTER TABLE` creates a temporary table in the same directory as the original table.

### A.4.5 How to Protect or Change the MySQL Socket File ``/tmp/mysql.sock'`

If you have problems with the fact that anyone can delete the MySQL communication socket ``/tmp/mysql.sock'`, you can, on most versions of Unix, protect your ``/tmp'` filesystem by setting the `sticky` bit on it. Log in as `root` and do the following:

    shell> chmod +t /tmp

This will protect your ``/tmp'` filesystem so that files can be deleted only by their owners or the superuser (`root`).

You can check if the `sticky` bit is set by executing `ls -ld /tmp`. If the last permission bit is `t`, the bit is set.

You can change the place where MySQL uses / puts the socket file the following ways:

- Specify the path in a global or local option file. For example, put in `/etc/my.cnf`:

      [client]
      socket=path-for-socket-file

      [mysqld]
      socket=path-for-socket-file

  See section 4.1.2 ``my.cnf'` Option Files.
- Specifying this on the command-line to `safe_mysqld` and most clients with the `--socket=path-for-socket-file` option.
- Specify the path to the socket in the `MYSQL_UNIX_PORT` environment variable.
- Defining the path with the `configure` option `--with-unix-socket-path=path-for-socket-file`. See section 2.3.3 Typical `configure` Options.

You can test that the socket works with this command:

    shell> mysqladmin --socket=/path/to/socket version

### A.4.6 Time Zone Problems

If you have a problem with `SELECT NOW()` returning values in GMT and not your local time, you have to set the `TZ` environment variable to your current time zone. This should be done for the environment in which the server runs, for example, in `safe_mysqld` or `mysql.server`. See section F Environment Variables.

## A.5 Query Related Issues

### A.5.1 Case-Sensitivity in Searches

By default, MySQL searches are case-insensitive (although there are some character sets that are never case-insensitive, such as `czech`). That means that if you search with `col_name LIKE 'a%'`, you will get all column values that start with `A` or `a`. If you want to make this search case-sensitive, use something like `INSTR(col_name, "A")=1` to check a prefix. Or use `STRCMP(col_name, "A") = 0` if the column value must be exactly `"A"`.

Simple comparison operations (`>=, >, = , < , <=`, sorting and grouping) are based on each character's ``sort value''. Characters with the same sort value (like E, e and é) are treated as the same character!

In older MySQL versions `LIKE` comparisons where done on the uppercase value of each character (E == e but E <> é). In newer MySQL versions `LIKE` works just like the other comparison operators.

If you want a column always to be treated in case-sensitive fashion, declare it as `BINARY`. See section 6.5.3 `CREATE TABLE` Syntax.

If you are using Chinese data in the so-called big5 encoding, you want to make all character columns `BINARY`. This works because the sorting order of big5 encoding characters is based on the order of ASCII codes.

### A.5.2 Problems Using `DATE` Columns

The format of a `DATE` value is `'YYYY-MM-DD'`. According to ANSI SQL, no other format is allowed. You should use this format in `UPDATE` expressions and in the WHERE clause of `SELECT` statements. For example:

    mysql> SELECT * FROM tbl_name WHERE date >= '1997-05-05';

As a convenience, MySQL automatically converts a date to a number if the date is used in a numeric context (and vice versa). It is also smart enough to allow a ``relaxed'' string form when updating and in a `WHERE` clause that compares a date to a `TIMESTAMP`, `DATE`, or a `DATETIME` column. (Relaxed form means that any punctuation character may be used as the separator between parts. For example, `'1998-08-15'` and `'1998#08#15'` are equivalent.) MySQL can also convert a string containing no separators (such as `'19980815'`), provided it makes sense as a date.

The special date `'0000-00-00'` can be stored and retrieved as `'0000-00-00'.` When using a `'0000-00-00'` date through `MyODBC`, it will automatically be converted to `NULL` in `MyODBC` Version 2.50.12 and above, because ODBC can't handle this kind of date.

Because MySQL performs the conversions described above, the following statements work:

    mysql> INSERT INTO tbl_name (idate) VALUES (19970505);
    mysql> INSERT INTO tbl_name (idate) VALUES ('19970505');
    mysql> INSERT INTO tbl_name (idate) VALUES ('97-05-05');
    mysql> INSERT INTO tbl_name (idate) VALUES ('1997.05.05');
    mysql> INSERT INTO tbl_name (idate) VALUES ('1997 05 05');
    mysql> INSERT INTO tbl_name (idate) VALUES ('0000-00-00');

    mysql> SELECT idate FROM tbl_name WHERE idate >= '1997-05-05';
    mysql> SELECT idate FROM tbl_name WHERE idate >= 19970505;
    mysql> SELECT MOD(idate,100) FROM tbl_name WHERE idate >= 19970505;
    mysql> SELECT idate FROM tbl_name WHERE idate >= '19970505';

However, the following will not work:

    mysql> SELECT idate FROM tbl_name WHERE STRCMP(idate,'19970505')=0;

`STRCMP()` is a string function, so it converts `idate` to a string and performs a string comparison. It does not convert `'19970505'` to a date and perform a date comparison.

Note that MySQL does very limited checking whether the date is correct. If you store an incorrect date, such as `'1998-2-31'`, the wrong date will be stored.

Because MySQL packs dates for storage, it can't store any given date as it would not fit onto the result buffer. The rules for accepting a date are:

- If MySQL can store and retrieve a given date, the wrong date is accepted for `DATE` and `DATETIME` columns.
- All days values between 0-31 are accepted for any date. This makes it very convenient for web applications where you ask year, month and day in 3 different fields.
- The day or month field may be zero. This is convenient if you want to store a birthdate in a `DATE` column and you only know part of the date.

If the date cannot be converted to any reasonable value, a `0` is stored in the `DATE` field, which will be retrieved as `0000-00-00`. This is both a speed and convenience issue as we believe that the database's responsiblity is to retrieve the same date you stored (even if the data was not logically correct in all cases). We think it is up to the application to check the dates, and not the server.

### A.5.3 Problems with `NULL` Values

The concept of the `NULL` value is a common source of confusion for newcomers to SQL, who often think that `NULL` is the same thing as an empty string `""`. This is not the case! For example, the following statements are completely different:

    mysql> INSERT INTO my_table (phone) VALUES (NULL);
    mysql> INSERT INTO my_table (phone) VALUES ("");

Both statements insert a value into the `phone` column, but the first inserts a `NULL` value and the second inserts an empty string. The meaning of the first can be regarded as ``phone number is not known'' and the meaning of the second can be regarded as ``she has no phone''.

In SQL, the `NULL` value is always false in comparison to any other value, even `NULL`. An expression that contains `NULL` always produces a `NULL` value unless otherwise indicated in the documentation for the operators and functions involved in the expression. All columns in the following example return `NULL`:

    mysql> SELECT NULL,1+NULL,CONCAT('Invisible',NULL);

If you want to search for column values that are `NULL`, you cannot use the `=NULL` test. The following statement returns no rows, because `expr = NULL` is FALSE, for any expression:

    mysql> SELECT * FROM my_table WHERE phone = NULL;

To look for `NULL` values, you must use the `IS NULL` test. The following shows how to find the `NULL` phone number and the empty phone number:

    mysql> SELECT * FROM my_table WHERE phone IS NULL;
    mysql> SELECT * FROM my_table WHERE phone = "";

Note that you can only add an index on a column that can have `NULL` values if you are using MySQL Version 3.23.2 or newer and are using the `MyISAM` or `InnoDB` table type. In earlier versions and with other table types, you must declare such columns `NOT NULL`. This also means you cannot then insert `NULL` into an indexed column.

When reading data with `LOAD DATA INFILE`, empty columns are updated with `''`. If you want a `NULL` value in a column, you should use `\N` in the text file. The literal word `'NULL'` may also be used under some circumstances. See section 6.4.9 `LOAD DATA INFILE` Syntax.

When using `ORDER BY`, `NULL` values are presented first. If you sort in descending order using `DESC`, `NULL` values are presented last. When using `GROUP BY`, all `NULL` values are regarded as equal.

To help with `NULL` handling, you can use the `IS NULL` and `IS NOT NULL` operators and the `IFNULL()` function.

For some column types, `NULL` values are handled specially. If you insert `NULL` into the first `TIMESTAMP` column of a table, the current date and time is inserted. If you insert `NULL` into an `AUTO_INCREMENT` column, the next number in the sequence is inserted.

### A.5.4 Problems with `alias`

You can use an alias to refer to a column in the `GROUP BY`, `ORDER BY`, or in the `HAVING` part. Aliases can also be used to give columns better names:

    SELECT SQRT(a*b) as rt FROM table_name GROUP BY rt HAVING rt > 0;
    SELECT id,COUNT(*) AS cnt FROM table_name GROUP BY id HAVING cnt > 0;
    SELECT id AS "Customer identity" FROM table_name;

Note that ANSI SQL doesn't allow you to refer to an alias in a `WHERE` clause. This is because when the `WHERE` code is executed the column value may not yet be determined. For example, the following query is illegal:

    SELECT id,COUNT(*) AS cnt FROM table_name WHERE cnt > 0 GROUP BY id;

The `WHERE` statement is executed to determine which rows should be included in the `GROUP BY` part while `HAVING` is used to decide which rows from the result set should be used.

### A.5.5 Deleting Rows from Related Tables

As MySQL doesn't yet support subselects, nor the use of more than one table in the `DELETE` statement (prior to Version 4.0), you should use the following approach to delete rows from 2 related tables:

1. `SELECT` the rows based on some `WHERE` condition in the main table.
2. `DELETE` the rows in the main table based on the same condition.
3. `DELETE FROM related_table WHERE related_column IN (selected_rows)`.

If the total number of characters in the query with `related_column` is more than 1,048,576 (the default value of `max_allowed_packet`, you should split it into smaller parts and execute multiple `DELETE` statements. You will probably get the fastest `DELETE` by only deleting 100-1000 `related_column` id's per query if the `related_column` is an index. If the `related_column` isn't an index, the speed is independent of the number of arguments in the `IN` clause.

### A.5.6 Solving Problems with No Matching Rows

If you have a complicated query that has many tables and that doesn't return any rows, you should use the following procedure to find out what is wrong with your query:

1. Test the query with `EXPLAIN` and check if you can find something that is obviously wrong. See section 5.2.1 `EXPLAIN` Syntax (Get Information About a `SELECT`).
2. Select only those fields that are used in the `WHERE` clause.
3. Remove one table at a time from the query until it returns some rows. If the tables are big, it's a good idea to use `LIMIT 10` with the query.
4. Do a `SELECT` for the column that should have matched a row against the table that was last removed from the query.
5. If you are comparing `FLOAT` or `DOUBLE` columns with numbers that have decimals, you can't use `'='`. This problem is common in most computer languages because floating-point values are not exact values. In most cases, changing the `FLOAT` to a `DOUBLE` will fix this. See section A.5.7 Problems with Floating-Point Comparison.
6. If you still can't figure out what's wrong, create a minimal test that can be run with `mysql test < query.sql` that shows your problems. You can create a test file with `mysqldump --quick database tables > query.sql`. Open the file in an editor, remove some insert lines (if there are too many of these), and add your select statement at the end of the file. Test that you still have your problem by doing:

       shell> mysqladmin create test2
       shell> mysql test2 < query.sql

   Post the test file using `mysqlbug` to mysql@lists.mysql.com.

### A.5.7 Problems with Floating-Point Comparison

floating-point numbers cause confusion sometimes, because these numbers are not stored as exact values inside computer architecture. What one can see on the screen usually is not the exact value of the number.

Field types `FLOAT`, `DOUBLE` and `DECIMAL` are such.

    CREATE TABLE t1 (i INT, d1 DECIMAL(9,2), d2 DECIMAL(9,2));
    INSERT INTO t1 VALUES (1, 101.40, 21.40), (1, -80.00, 0.00),
    (2, 0.00, 0.00), (2, -13.20, 0.00), (2, 59.60, 46.40),
    (2, 30.40, 30.40), (3, 37.00, 7.40), (3, -29.60, 0.00),
    (4, 60.00, 15.40), (4, -10.60, 0.00), (4, -34.00, 0.00),
    (5, 33.00, 0.00), (5, -25.80, 0.00), (5, 0.00, 7.20),
    (6, 0.00, 0.00), (6, -51.40, 0.00);

    mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b
        -> FROM t1 GROUP BY i HAVING a <> b;
    +------+--------+-------+
    | i    | a      | b     |
    +------+--------+-------+
    |    1 |  21.40 | 21.40 |
    |    2 |  76.80 | 76.80 |
    |    3 |   7.40 |  7.40 |
    |    4 |  15.40 | 15.40 |
    |    5 |   7.20 |  7.20 |
    |    6 | -51.40 |  0.00 |
    +------+--------+-------+

The result is correct. Although the first five records look like they shouldn't pass the comparison test, they may do so because the difference between the numbers show up around tenth decimal, or so depending on computer architecture.

The problem cannot be solved by using ROUND() (or similar function), because the result is still a floating-point number. Example:

    mysql> SELECT i, ROUND(SUM(d1), 2) AS a, ROUND(SUM(d2), 2) AS b
        -> FROM t1 GROUP BY i HAVING a <> b;
    +------+--------+-------+
    | i    | a      | b     |
    +------+--------+-------+
    |    1 |  21.40 | 21.40 |
    |    2 |  76.80 | 76.80 |
    |    3 |   7.40 |  7.40 |
    |    4 |  15.40 | 15.40 |
    |    5 |   7.20 |  7.20 |
    |    6 | -51.40 |  0.00 |
    +------+--------+-------+

This is what the numbers in row 'a' look like:

    mysql> SELECT i, ROUND(SUM(d1), 2)*1.0000000000000000 AS a,
        -> ROUND(SUM(d2), 2) AS b FROM t1 GROUP BY i HAVING a <> b;
    +------+----------------------+-------+
    | i    | a                    | b     |
    +------+----------------------+-------+
    |    1 |  21.3999999999999986 | 21.40 |
    |    2 |  76.7999999999999972 | 76.80 |
    |    3 |   7.4000000000000004 |  7.40 |
    |    4 |  15.4000000000000004 | 15.40 |
    |    5 |   7.2000000000000002 |  7.20 |
    |    6 | -51.3999999999999986 |  0.00 |
    +------+----------------------+-------+

Depending on the computer architecture you may or may not see similar results. Each CPU may evaluate floating-point numbers differently. For example in some machines you may get 'right' results by multiplaying both arguments with 1, an example follows.

WARNING: NEVER TRUST THIS METHOD IN YOUR APPLICATION, THIS IS AN EXAMPLE OF A WRONG METHOD!!!

    mysql> SELECT i, ROUND(SUM(d1), 2)*1 AS a, ROUND(SUM(d2), 2)*1 AS b
        -> FROM t1 GROUP BY i HAVING a <> b;
    +------+--------+------+
    | i    | a      | b    |
    +------+--------+------+
    |    6 | -51.40 | 0.00 |
    +------+--------+------+

The reason why the above example seems to be working is that on the particular machine where the test was done, the CPU floating-point arithmetics happens to round the numbers to same, but there is no rule that any CPU should do so, so it cannot be trusted.

The correct way to do floating-point number comparison is to first decide on what is the wanted tolerance between the numbers and then do the comparsion against the tolerance number. For example, if we agree on that floating-point numbers should be regarded the same, if they are same with precision of one of ten thousand (0.0001), the comparsion should be done like this:

    mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b FROM t1
        -> GROUP BY i HAVING ABS(a - b) > 0.0001;
    +------+--------+------+
    | i    | a      | b    |
    +------+--------+------+
    |    6 | -51.40 | 0.00 |
    +------+--------+------+
    1 row in set (0.00 sec)

And vice versa, if we wanted to get rows where the numbers are the same, the test would be:

    mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b FROM t1
        -> GROUP BY i HAVING ABS(a - b) < 0.0001;
    +------+-------+-------+
    | i    | a     | b     |
    +------+-------+-------+
    |    1 | 21.40 | 21.40 |
    |    2 | 76.80 | 76.80 |
    |    3 |  7.40 |  7.40 |
    |    4 | 15.40 | 15.40 |
    |    5 |  7.20 |  7.20 |
    +------+-------+-------+

## A.6 Table Definition Related Issues

### A.6.1 Problems with `ALTER TABLE`.

`ALTER TABLE` changes a table to the current character set. If you during `ALTER TABLE` get a duplicate key error, then the cause is either that the new character sets maps to keys to the same value or that the table is corrupted, in which case you should run `REPAIR TABLE` on the table.

If `ALTER TABLE` dies with an error like this:

    Error on rename of './database/name.frm' to './database/B-a.frm' (Errcode: 17)

the problem may be that MySQL has crashed in a previous `ALTER TABLE` and there is an old table named ``A-something'` or ``B-something'` lying around. In this case, go to the MySQL data directory and delete all files that have names starting with `A-` or `B-`. (You may want to move them elsewhere instead of deleting them.)

`ALTER TABLE` works the following way:

- Create a new table named ``A-xxx'` with the requested changes.
- All rows from the old table are copied to ``A-xxx'`.
- The old table is renamed ``B-xxx'`.
- ``A-xxx'` is renamed to your old table name.
- ``B-xxx'` is deleted.

If something goes wrong with the renaming operation, MySQL tries to undo the changes. If something goes seriously wrong (this shouldn't happen, of course), MySQL may leave the old table as ``B-xxx'`, but a simple rename on the system level should get your data back.

### A.6.2 How To Change the Order of Columns in a Table

The whole point of SQL is to abstract the application from the data storage format. You should always specify the order in which you wish to retrieve your data. For example:

    SELECT col_name1, col_name2, col_name3 FROM tbl_name;

will return columns in the order `col_name1`, `col_name2`, `col_name3`, whereas:

    SELECT col_name1, col_name3, col_name2 FROM tbl_name;

will return columns in the order `col_name1`, `col_name3`, `col_name2`.

You should never, in an application, use `SELECT *` and retrieve the columns based on their position, because the order in which columns are returned cannot be guaranteed over time. A simple change to your database may cause your application to fail rather dramatically.

If you want to change the order of columns anyway, you can do it as follows:

1. Create a new table with the columns in the right order.
2. Execute `INSERT INTO new_table SELECT fields-in-new_table-order FROM old_table`.
3. Drop or rename `old_table`.
4. `ALTER TABLE new_table RENAME old_table`.

### A.6.3 TEMPORARY TABLE problems

The following are a list of the limitations with `TEMPORARY TABLES`.

- A temporary table can only be of type `HEAP`, `ISAM`, `MyISAM` or `InnoDB`.
- You can't use temporary tables more than once in the same query. For example, the following doesn't work.

      mysql> SELECT * FROM temporary_table, temporary_table AS t2;

  We plan to fix the above in 4.0.
- You can't use `RENAME` on a `TEMPORARY` table. Note that `ALTER TABLE org_name RENAME new_name` works! We plan to fix the above in 4.0.

# B Contributed Programs

Many users of MySQL have contributed very useful support tools and add-ons.

A list of what is available at http://www.mysql.com/Downloads/Contrib/ (or any mirror) is shown here.

Please visit our Software Portal at http://www.mysql.com/portal/software/. The community facilities there also allow for your input!

If you want to build MySQL support for the Perl `DBI`/`DBD` interface, you should fetch the `Data-Dumper`, `DBI`, and `Msql-Mysql-modules` files and install them. See section 2.7 Perl Installation Comments.

Note: The programs listed here can be freely downloaded and used. They are copyrighted by their respective owners. Please see individual product documentation for more details on licensing and terms. MySQL AB assumes no liability for the correctness of the information in this chapter or for the proper operation of the programs listed herein.

## B.1 APIs

- Perl Modules
  - http://www.mysql.com/Downloads/Contrib/Data-Dumper-2.101.tar.gz Perl `Data-Dumper` module. Useful with `DBI`/`DBD` support for older Perl installations.
  - http://www.mysql.com/Downloads/Contrib/DBI-1.18.tar.gz Perl `DBI` module.
  - http://www.mysql.com/Downloads/Contrib/KAMXbase1.2.tar.gz Convert between ``.dbf'` files and MySQL tables. Perl module written by Pratap Pereira pereira@ee.eng.ohio-state.edu, extended by Kevin A. McGrail kmcgrail@digital1.peregrinehw.com. This converter can handle MEMO fields.
  - http://www.mysql.com/Downloads/Contrib/Msql-Mysql-modules-1.2218.tar.gz Perl `DBD` module to access mSQL and MySQL databases.
  - http://www.mysql.com/Downloads/Contrib/Data-ShowTable-3.3.tar.gz Perl `Data-ShowTable` module. Useful with `DBI`/`DBD` support.
  - http://www.mysql.com/Downloads/Contrib/HandySQL-1.1.tar.gz HandySQL is a MySQL access module. It offers a C interface embedded in Perl and is approximately 20% faster than regular DBI.
- OLEDB
  - http://www.mysql.com/Downloads/Win32/MyOLEDB3.exe MyOLEDB 3.0 installation package from SWSoft.
  - http://www.mysql.com/Downloads/Win32/mysql-oledb-3.0.0.zip Source for MyOLEDB 3.0.
  - http://www.mysql.com/Downloads/Win32/MySamples.zip Examples and documentation for MyOLEDB.
  - http://www.mysql.com/Downloads/Win32/MyOLEDB.chm Help files for MyOLEDB.
  - http://www.mysql.com/Downloads/Win32/libmyodbc.zip Static MyODBC library used for build MyOLEDB. Based on MyODBC code.
- C++
  - http://www.mysql.com/Downloads/Contrib/mysql-c++-0.02.tar.gz MySQL C++ wrapper library. By Roland Haenel, rh@ginster.net.
  - http://www.mysql.com/Downloads/Contrib/MyDAO.tar.gz MySQL C++ API. By Satish spitfire@pn3.vsnl.net.in. Inspired by Roland Haenel's C++ API and Ed Carp's MyC library.
  - http://www.mysql.com/download_mysql++.html MySQL C++ API (more than just a wrapper library). Originally by kevina@clark.net. Nowadays maintained by Sinisa at MySQL AB.
  - http://nelsonjr.homepage.com/NJrAPI/ A C++ database independent library that supports MySQL.
- Delphi
  - http://www.mysql.com/Downloads/Contrib/DelphiMySQL2.zip Delphi interface to `libmysql.dll`, by bsilva@umesd.k12.or.us.
  - http://www.mysql.com/Downloads/Contrib/Udmysql.pas A wrapper for libmysql.dll for usage in Delphi. By Reiner Sombrowsky.
  - http://www.fichtner.net/delphi/mysql.delphi.phtml A Delphi Interface to MySQL, with source code. By Matthias Fichtner.
  - http://www.productivity.org/projects/tmysql/ `TmySQL`, a library to use MySQL with Delphi.
  - https://sourceforge.net/projects/zeoslib/ Zeos Library is a set of delphi native datasets and database components for MySql, PostgreSql, Interbase, MS SQL, Oracle, DB/2. Also it includes development tools such as Database Explorer and Database Designer.
  - http://www.mysql.com/Downloads/Contrib/Win32/SBMySQL50Share.exe Delphi 5 Shareware MySQL Dataset Components.
- http://www.mysql.com/Downloads/Contrib/mysql-ruby-2.2.0.tar.gz MySQL Ruby module. By TOMITA Masahiro tommy@tmtm.org Ruby is an Object-Oriented Interpreter Language (http://www.netlab.co.jp/ruby/).
- http://www.mysql.com/Downloads/Contrib/JdmMysqlDriver-0.1.0.tar.gz A VisualWorks 3.0 Smalltalk driver for MySQL. By joshmiller@earthlink.net.
- http://www.mysql.com/Downloads/Contrib/Db.py Python module with caching. By gandalf@rosmail.com.
- http://www.mysql.com/Downloads/Contrib/MySQLmodule-1.4.tar.gz Python interface for MySQL. By Joseph Skinner joe@earthlight.co.nz. Modified by Joerg Senekowitsch senekow@ibm.net.
- http://www.mysql.com/Downloads/Contrib/MySQL-python-0.3.0.tar.gz MySQLdb Python is an DB-API v2.0-compliant interface to MySQL. Transactions are supported if the server and tables support them. It is thread-safe, and contains a compatibility module for older code written for the no-longer-maintained MySQLmodule interface.
- http://www.mysql.com/Downloads/Contrib/mysql_mex_12.tar.gz An interface program for the Matlab program by MathWorks. The interface is done by Kimmo Uutela and John Fisher (not by Mathworks). Check http://boojum.hut.fi/~kuutela/mysqlmex.html for more information.
- http://www.mysql.com/Downloads/Contrib/mysqltcl-1.53.tar.gz Tcl interface for MySQL. Based on ``msqltcl-1.50.tar.gz'`. For version 2.0 and more info, see http://www.xdobry.de/mysqltcl/.
- http://www.mysql.com/Downloads/Contrib/MyC-0.1.tar.gz A Visual Basic-like API, by Ed Carp.
- http://www.mysql.com/Downloads/Contrib/Vdb-dflts-2.1.tar.gz This is a new version of a set of library utilities intended to provide a generic interface to SQL database engines such that your application becomes a 3-tiered application. The advantage is that you can easily switch between and move to other database engines by implementing one file for the new backend without making any changes to your applications. By damian@cablenet.net.
- http://www.mysql.com/Downloads/Contrib/DbFramework-1.10.tar.gz DbFramework is a collection of classes for manipulating MySQL databases. The classes are loosely based on the CDIF Data Model Subject Area. By Paul Sharpe paul@miraclefish.com.
- http://www.mysql.com/Downloads/Contrib/pike-mysql-1.4.tar.gz MySQL module for pike. For use with the Roxen web server.
- http://www.mysql.com/Downloads/Contrib/squile.tar.gz Module for `guile` that allows `guile` to interact with SQL databases. By Hal Roberts.
- http://www.mysql.com/Downloads/Contrib/stk-mysql.tar.gz Interface for Stk. Stk is the Tk widgets with Scheme underneath instead of Tcl. By Terry Jones.
- http://www.mysql.com/Downloads/Contrib/eiffel-wrapper-1.0.tar.gz Eiffel wrapper by Michael Ravits.
- http://www.mysql.com/Downloads/Contrib/SQLmy0.06.tgz FlagShip Replaceable Database Driver (RDD) for MySQL. By Alejandro Fernandez Herrero. The Flagship RDD homepage is at http://www.fship.com/rdds.html.
- http://www.mysql.com/Downloads/Contrib/mydsn-1.0.zip Binary and source for `mydsn.dll`. mydsn should be used to build and remove the DSN registry file for the MyODBC driver in Coldfusion applications. By Miguel Angel Solórzano.
- http://www.mysql.com/Downloads/Contrib/MySQL-ADA95_API.zip An ADA95 interface to the MySQL API. By Francois Fabien.
- http://www.mysql.com/Downloads/Contrib/MyTool-DLL_for_VB_and_MySQL.zip A DLL with MySQL C API for Visual Basic. By Ken Menzel kenm@icarz.com.
- http://www.mysql.com/Downloads/Contrib/MYSQLX.EXE MySQL ActiveX Object for directly accessing your MySQL servers from IIS/ASP, VB, VC++ skipping the slower ODBC methods. Fully updatable, multi-threaded with full support for all MySQL fieldtypes (version 2001.1.1). By SciBit http://www.scibit.com/.
- http://www.fastflow.it/mylua/ MyLUA home page; how to use the LUA language to write MySQL `PROCEDURE` that can be loaded runtime.
  - http://www.mysql.com/Downloads/Contrib/lua-4.0.tar.gz LUA 4.0
  - http://www.mysql.com/Downloads/Contrib/mylua-3.23.32.1.tar.gz Patch for MySQL 3.23.32 to use LUA 4.0. By Cristian Giussani.
- http://www.mysql.com/Downloads/Contrib/patched_myodbc.zip Patch (for Omniform 4.0 support) to the MyODBC driver. By Thomas Thaele tthaele@papenmeier.de

## B.2 Converters

- http://www.mysql.com/Downloads/Contrib/mssql2mysql.txt Converter from MS-SQL to MySQL. By Michael Kofler. The mssql2mysql home page is at http://www.kofler.cc/mysql/mssql2mysql.html.
- http://www.mysql.com/Downloads/Contrib/dbf2mysql-1.14.tar.gz Convert between ``.dbf'` files and MySQL tables. By Maarten Boekhold (boekhold@cindy.et.tudelft.nl), William Volkman, and Michael Widenius. This converter includes rudimentary read-only support for MEMO fields.
- http://www.mysql.com/Downloads/Contrib/dbf2mysql-1.13.tgz Convert between ``.dbf'` files and MySQL tables. By Maarten Boekhold, boekhold@cindy.et.tudelft.nl, and Michael Widenius. This converter can't handle MEMO fields.
- http://www.mysql.com/Downloads/Contrib/dbf2mysql.zip Convert between FoxPro ``.dbf'` files and MySQL tables on Windows. By Alexander Eltsyn, ae@nica.ru or ae@usa.net.
- http://www.mysql.com/Downloads/Contrib/dbf2sql.zip Short and simple prg that can help you transport your data from foxpro table into MySQL table. By Danko Josic.
- http://www.mysql.com/Downloads/Contrib/dump2h-1.20.gz Convert from `mysqldump` output to a C header file. By Harry Brueckner, brueckner@mail.respublica.de.
- http://www.mysql.com/Downloads/Contrib/exportsql.txt A script that is similar to `access_to_mysql.txt`, except that this one is fully configurable, has better type conversion (including detection of `TIMESTAMP` fields), provides warnings and suggestions while converting, quotes all special characters in text and binary data, and so on. It will also convert to `mSQL` v1 and v2, and is free of charge for anyone. See http://www.cynergi.net/exportsql/ for the latest version. By Pedro Freire, support@cynergi.net. Note: Doesn't work with Access2!
- http://www.mysql.com/Downloads/Contrib/access_to_mysql.txt Paste this function into an Access module of a database that has the tables you want to export. See also `exportsql`. By Brian Andrews. Note: Doesn't work with Access2!
- http://www.mysql.com/Downloads/Contrib/importsql.txt A script that does the exact reverse of `exportsql.txt`. That is, it imports data from MySQL into an Access database via ODBC. This is very handy when combined with exportsql, because it lets you use Access for all DB design and administration, and synchronise with your actual MySQL server either way. Free of charge. See http://www.netdive.com/freebies/importsql/ for any updates. Created by Laurent Bossavit of NetDIVE. Note: doesn't work with Access2!
- http://www.mysql.com/Downloads/Contrib/mdb2sql.bas Converter from Access97 to MySQL by Moshe Gurvich.
- http://www.mysql.com/Downloads/Contrib/msql2mysqlWrapper-1.0.tgz A C wrapper from `mSQL` to MySQL. By alfred@sb.net
- http://www.mysql.com/Downloads/Contrib/sqlconv.pl A simple script that can be used to copy fields from one MySQL table to another in bulk. Basically, you can run `mysqldump` and pipe it to the `sqlconv.pl` script. The script will parse through the `mysqldump` output and will rearrange the fields so they can be inserted into a new table. An example is when you want to create a new table for a different site you are working on, but the table is just a bit different (that is - fields in different order, etc.). By Steve Shreeve.
- http://www.mysql.com/Downloads/Contrib/oracledump Perl program to convert Oracle databases to MySQL. Has same output format as mysqldump. By Johan Andersson.
- http://www.mysql.com/Downloads/Contrib/excel2mysql Perl program to import Excel spreadsheets into a MySQL database. By Stephen Hurd shurd@sk.sympatico.ca
- http://www.mysql.com/Downloads/Contrib/T2S_100.ZIP. Windows program to convert text files to MySQL databases. By Asaf Azulay.

## B.3 Utilities

- http://worldcommunity.com/opensource/utilities/mysql_backup.html MySQL Backup is a backup script for MySQL. By Peter F. Brown.
- http://www.mysql.com/Downloads/Contrib/mysql_watchdog.pl Monitor the MySQL daemon for possible lockups. By Yermo Lamers, yml@yml.com.
- http://www.mysql.com/Downloads/Contrib/mysql_structure_dumper.tar.gz
- http://www.mysql.com/Downloads/Contrib/mysql_structure_dumper.tgz Prints the structure of every table in a database. By Thomas Wana.
- http://www.mysql.com/Downloads/Contrib/mysqlsync. A Perl script to keep remote copies of a MySQL database in sync with a central master copy. By Mark Jeftovic. markjr@easydns.com.
- http://www.mysql.com/Downloads/Contrib/MySQLTutor-0.2.tar.gz. MySQLTutor. A MySQL tutorial for beginners.
- http://www.mysql.com/Downloads/Contrib/MySQLDB.zip
- http://www.mysql.com/Downloads/Contrib/MySQLDB-readme.html. A COM library for MySQL by Alok Singh.
- http://www.mysql.com/Downloads/Contrib/mysql_replicate.pl Perl program that handles replication. By elble@icculus.nsg.nwu.edu
- http://www.mysql.com/Downloads/Contrib/DBIx-TextIndex-0.02.tar.gz Perl script that uses reverse indexing to handle text searching. By Daniel Koch.
- http://www.mysql.com/Downloads/Contrib/dbcheck Perl script that takes a backup of tables before running isamchk on them. By Elizabeth.
- http://www.mysql.com/Downloads/Contrib/mybackup.
- http://www.mswanson.com/mybackup (mybackup home page) Wrapper for mysqldump to backup all databases. By Marc Swanson.
- http://www.mysql.com/Downloads/Contrib/mdu.pl.gz Prints the storage usage of a MySQL database.

# C Credits

This appendix lists the developers, contributors, and supporters that have helped to make MySQL what it is today.

## C.1 Developers at MySQL AB

These are the developers that are or have been employed by `MySQL AB` to work on the `MySQL` database software, roughly in the order they started to work with us. Following each developer is a small list of the tasks that the developer is responsible for, or the accomplishments they have made. All developers are involved in support.

Michael (Monty) Widenius
  - Lead developer and main author of the MySQL server (`mysqld`).
  - New functions for the string library.
  - Most of the `mysys` library.
  - The `ISAM` and `MyISAM` libraries (B-tree index file handlers with index compression and different record formats).
  - The `HEAP` library. A memory table system with our superior full dynamic hashing. In use since 1981 and published around 1984.
  - The `replace` program (take a look at it, it's COOL!).
  - `MyODBC`, the ODBC driver for Windows95.
  - Fixing bugs in MIT-pthreads to get it to work for MySQL Server. And also Unireg, a curses-based application tool with many utilities.
  - Porting of `mSQL` tools like `msqlperl`, `DBD`/`DBI`, and `DB2mysql`.
  - Most of `crash-me` and the foundation for the MySQL benchmarks.
David Axmark
  - Initial main writer of the Reference Manual, including enhancements to `texi2html`.
  - Automatic web site updating from the manual.
  - Initial Autoconf, Automake, and Libtool support.
  - Licensing.
  - Parts of all the text files. (Nowadays only the ``README'` is left. The rest ended up in the manual.)
  - Lots of testing of new features.
  - Our in-house Free Software legal expert.
  - Mailing list maintainer (who never has the time to do it right...).
  - Our original portability code (more than 10 years old now). Nowadays only some parts of `mysys` are left.
  - Someone for Monty to call in the middle of the night when he just got that new feature to work.
  - Chief "Open Sourcerer" (MySQL community relations).
Jani Tolonen
  - `mysqlimport`
  - A lot of extensions to the command-line clients.
  - `PROCEDURE ANALYSE()`
Sinisa Milivojevic
  - Compression (with `zlib`) in the client/server protocol.
  - Perfect hashing for the lexical analyser phase.
  - Multi-row `INSERT`
  - `mysqldump` -e option
  - `LOAD DATA LOCAL INFILE`
  - `SQL_CALC_FOUND_ROWS` `SELECT` option
  - `--max-user-connections=...` option
  - `net_read` and `net_write_timeout`
  - `GRANT`/`REVOKE` and `SHOW GRANTS FOR`
  - New client-server protocol for 4.0
  - `UNION` in 4.0
  - Multi-table `DELETE`/`UPDATE`
  - Derived tables in 4.1
  - User resources management
  - Initial developer of the `MySQL++` C++ API and the `MySQLGUI` client.
Tonu Samuel (past developer)
  - VIO interface (the foundation for the encrypted client/server protocol).
  - MySQL Filesystem (a way to use MySQL databases as files and directories).
  - The `CASE` expression.
  - The `MD5()` and `COALESCE()` functions.
  - `RAID` support for `MyISAM` tables.
Sasha Pachev
  - Initial implementation of replication (upto version 4.0).
  - `SHOW CREATE TABLE`.
  - `mysql-bench`
Matt Wagner
  - MySQL test suite.
  - Webmaster (until 2002).
  - Coordinator of development.
Miguel Solorzano
  - Win32 development and release builds.
  - Windows NT server code.
  - WinMySQLAdmin
Timothy Smith (past developer)
  - Dynamic character sets support.
  - configure, RPMs and other parts of the build system.
  - Initial developer of `libmysqld`, the embedded server.
Sergei Golubchik
  - Full-text search.
  - Added keys to the `MERGE` library.
Jeremy Cole
  - Proofreading and editing this fine manual.
  - `ALTER TABLE ... ORDER BY ...`.
  - `UPDATE ... ORDER BY ...`.
  - `DELETE ... ORDER BY ...`.
Indrek Siitan
  - Designing/programming of our web interface.
  - Author of our newsletter management system.
Jorge del Conde
  - `MySQLCC` (`MySQL Control Center`)
  - Win32 development
  - Initial implementation of the website portals.
Venu Anuganti
  - MyODBC 3.51
  - New client/server protocol for 4.1
Arjen Lentz
  - Maintainer of the MySQL Reference Manual.
  - Preparing the O'Reilly printed edition of the manual.
Alexander (Bar) Barkov, Alexey (Holyfoot) Botchkov, and Ramil Kallimullin
  - Spatial data (GIS) and R-Trees implementation for 4.1
  - Unicode and character sets for 4.1
Alexander (Sanja) Belkin
  - Query cache in 4.0
  - Implementation of subqueries (4.1).
Aleksey (Walrus) Kishkin and Alexey (Ranger) Stroganov
  - Benchmarks design and analysis.
  - Maintenance of the MySQL test suite.
Zak Greant
  - Open Source advocate, MySQL community relations.
Carsten Pedersen
  - The MySQL Certification program.
Lenz Grimmer
  - Production (build and release) engineering.
Peter Zaitsev
  - `SHA1()`, `AES_ENCRYPT()` and `AES_DECRYPT()` functions.
  - Debugging, cleaning up various features.
Alexander (Salle) Keremidarski
  - Documentation of existing MySQL code/algorithms.
  - Debugging.
Per-Erik Martin
  - Lead developer for stored procedures and triggers.
Jim Winstead
  - Lead web developer.
Mark Matthews
  - Connector/J driver (Java).

## C.2 Contributors to MySQL

While `MySQL AB` owns all copyrights in the `MySQL server` and the `MySQL manual`, we wish to recognise those who have made contributions of one kind or another to the `MySQL distribution`. Contributors are listed here, in somewhat random order:

Paul DuBois
  Ongoing help with making this manual correct and understandable. That includes rewriting Monty's and David's attempts at English into English as other people know it.
Gianmassimo Vigazzola qwerg@mbox.vol.it or qwerg@tin.it
  The initial port to Win32/NT.
Kim Aldale
  Helped to rewrite Monty's and David's early attempts at English into English.
Per Eric Olsson
  For more or less constructive criticism and real testing of the dynamic record format.
Irena Pancirov irena@mail.yacc.it
  Win32 port with Borland compiler. `mysqlshutdown.exe` and `mysqlwatch.exe`
David J. Hughes
  For the effort to make a shareware SQL database. At TcX, the predecessor of MySQL AB, we started with `mSQL`, but found that it couldn't satisfy our purposes so instead we wrote a SQL interface to our application builder Unireg. `mysqladmin` and `mysql` client are programs that were largely influenced by their `mSQL` counterparts. We have put a lot of effort into making the MySQL syntax a superset of `mSQL`. Many of the API's ideas are borrowed from `mSQL` to make it easy to port free `mSQL` programs to the MySQL API. The MySQL software doesn't contain any code from `mSQL`. Two files in the distribution (``client/insert_test.c'` and ``client/select_test.c'`) are based on the corresponding (non-copyrighted) files in the `mSQL` distribution, but are modified as examples showing the changes necessary to convert code from `mSQL` to MySQL Server. (`mSQL` is copyrighted David J. Hughes.)
Fred Fish
  For his excellent C debugging and trace library. Monty has made a number of smaller improvements to the library (speed and additional options).
Richard A. O'Keefe
  For his public domain string library.
Henry Spencer
  For his regex library, used in `WHERE column REGEXP regexp`.
Free Software Foundation
  From whom we got an excellent compiler (`gcc`), the `libc` library (from which we have borrowed ``strto.c'` to get some code working in Linux), and the `readline` library (for the `mysql` client).
Free Software Foundation & The XEmacs development team
  For a really great editor/environment used by almost everybody at MySQL AB/TcX/detron.
Patrick Lynch
  For helping us acquire http://www.mysql.com/.
Fred Lindberg
  For setting up qmail to handle the MySQL mailing list and for the incredible help we got in managing the MySQL mailing lists.
Igor Romanenko igor@frog.kiev.ua
  `mysqldump` (previously `msqldump`, but ported and enhanced by Monty).
Yuri Dario
  For keeping up and extending the MySQL OS/2 port.
Tim Bunce, Alligator Descartes
  For the `DBD` (Perl) interface.
Tim Bunce
  Author of `mysqlhotcopy`.
Andreas Koenig a.koenig@mind.de
  For the Perl interface for MySQL Server.
Eugene Chan eugene@acenet.com.sg
  For porting PHP for MySQL Server.
Michael J. Miller Jr. mke@terrapin.turbolift.com
  For the first MySQL manual. And a lot of spelling/language fixes for the FAQ (that turned into the MySQL manual a long time ago).
Yan Cailin
  First translator of the MySQL Reference Manual into simplified chinese in early 2000 on which the Big5 and HK coded (http://mysql.hitstar.com/) versions were based. Personal home page at linuxdb.yeah.net.
Giovanni Maruzzelli maruzz@matrice.it
  For porting iODBC (Unix ODBC).
Chris Provenzano
  Portable user level pthreads. From the copyright: This product includes software developed by Chris Provenzano, the University of California, Berkeley, and contributors. We are currently using version 1_60_beta6 patched by Monty (see ``mit-pthreads/Changes-mysql'`).
Xavier Leroy Xavier.Leroy@inria.fr
  The author of LinuxThreads (used by the MySQL Server on Linux).
Zarko Mocnik zarko.mocnik@dem.si
  Sorting for Slovenian language and the ``cset.tar.gz'` module that makes it easier to add other character sets.
"TAMITO" tommy@valley.ne.jp
  The `_MB` character set macros and the ujis and sjis character sets.
Joshua Chamas joshua@chamas.com
  Base for concurrent insert, extended date syntax, debugging on NT, and answering on the MySQL mailing list.
Yves Carlier Yves.Carlier@rug.ac.be
  `mysqlaccess`, a program to show the access rights for a user.
Rhys Jones rhys@wales.com (And GWE Technologies Limited)
  For JDBC, a module to extract data from a MySQL Database with a Java client.
Dr Xiaokun Kelvin ZHU X.Zhu@brad.ac.uk
  Further development of the JDBC driver and other MySQL-related Java tools.
James Cooper pixel@organic.com
  For setting up a searchable mailing list archive at his site.
Rick Mehalick Rick_Mehalick@i-o.com
  For `xmysql`, a graphical X client for MySQL Server.
Doug Sisk sisk@wix.com
  For providing RPM packages of MySQL for RedHat Linux.
Diemand Alexander V. axeld@vial.ethz.ch
  For providing RPM packages of MySQL for RedHat Linux-Alpha.
Antoni Pamies Olive toni@readysoft.es
  For providing RPM versions of a lot of MySQL clients for Intel and SPARC.
Jay Bloodworth jay@pathways.sde.state.sc.us
  For providing RPM versions for MySQL Version 3.21.
Jochen Wiedmann wiedmann@neckar-alb.de
  For maintaining the Perl `DBD::mysql` module.
Therrien Gilbert gilbert@ican.net, Jean-Marc Pouyot jmp@scalaire.fr
  French error messages.
Petr Snajdr, snajdr@pvt.net
  Czech error messages.
Jaroslaw Lewandowski jotel@itnet.com.pl
  Polish error messages.
Miguel Angel Fernandez Roiz
  Spanish error messages.
Roy-Magne Mo rmo@www.hivolda.no
  Norwegian error messages and testing of Version 3.21.#.
Timur I. Bakeyev root@timur.tatarstan.ru
  Russian error messages.
brenno@dewinter.com & Filippo Grassilli phil@hyppo.com
  Italian error messages.
Dirk Munzinger dirk@trinity.saar.de
  German error messages.
Billik Stefan billik@sun.uniag.sk
  Slovak error messages.
Stefan Saroiu tzoompy@cs.washington.edu
  Romanian error messages.
Peter Feher
  Hungarian error messages.
Roberto M. Serqueira
  Portugise error messages.
Carsten H. Pedersen
  Danish error messages.
Arjen G. Lentz
  Dutch error messages, completing earlier partial translation (also work on consistency and spelling).
David Sacerdote davids@secnet.com
  Ideas for secure checking of DNS hostnames.
Wei-Jou Chen jou@nematic.ieo.nctu.edu.tw
  Some support for Chinese(BIG5) characters.
Wei He hewei@mail.ied.ac.cn
  A lot of functionality for the Chinese(GBK) character set.
Zeev Suraski bourbon@netvision.net.il
  `FROM_UNIXTIME()` time formatting, `ENCRYPT()` functions, and `bison` advisor. Active mailing list member.
Luuk de Boer luuk@wxs.nl
  Ported (and extended) the benchmark suite to `DBI`/`DBD`. Have been of great help with `crash-me` and running benchmarks. Some new date functions. The `mysql_setpermissions` script.
Jay Flaherty fty@mediapulse.com
  Big parts of the Perl `DBI`/`DBD` section in the manual.
Paul Southworth pauls@etext.org, Ray Loyzaga yar@cs.su.oz.au
  Proof-reading of the Reference Manual.
Alexis Mikhailov root@medinf.chuvashia.su
  User-definable functions (UDFs); `CREATE FUNCTION` and `DROP FUNCTION`.
Andreas F. Bobak bobak@relog.ch
  The `AGGREGATE` extension to UDF functions.
Ross Wakelin R.Wakelin@march.co.uk
  Help to set up InstallShield for MySQL-Win32.
Jethro Wright III jetman@li.net
  The ``libmysql.dll'` library.
James Pereria jpereira@iafrica.com
  Mysqlmanager, a Win32 GUI tool for administrating MySQL Server.
Curt Sampson cjs@portal.ca
  Porting of MIT-pthreads to NetBSD/Alpha and NetBSD 1.3/i386.
Antony T. Curtis antony.curtis@olcs.net
  Porting of the MySQL Database software to OS/2.
Martin Ramsch m.ramsch@computer.org
  Examples in the MySQL Tutorial.
Steve Harvey
  For making `mysqlaccess` more secure.
Konark IA-64 Centre of Persistent Systems Private Limited
  http://www.pspl.co.in/konark/. Help with the Win64 port of the MySQL server.
Albert Chin-A-Young.
  Configure updates for Tru64, large file support and better TCP wrappers support.
John Birrell
  Emulation of `pthread_mutex()` for OS/2.
Benjamin Pflugmann
  Extended `MERGE` tables to handle `INSERTS`. Active member on the MySQL mailing lists.
Guilhem Bichot
  Fixed handling of exponents for `DECIMAL`. Author of `mysql_tableinfo`.
Jocelyn Fournier
  For excellent spotting and reporting innumerable bugs (especially in the 4.1 subselect code)

Other contributors, bugfinders, and testers: James H. Thompson, Maurizio Menghini, Wojciech Tryc, Luca Berra, Zarko Mocnik, Wim Bonis, Elmar Haneke, jehamby@lightside, psmith@BayNetworks.com, duane@connect.com.au, Ted Deppner ted@psyber.com, Mike Simons, Jaakko Hyvatti.

And lots of bug report/patches from the folks on the mailing list.

A big tribute goes to those that help us answer questions on the `mysql@lists.mysql.com` mailing list:

Daniel Koch dkoch@amcity.com
  Irix setup.
Luuk de Boer luuk@wxs.nl
  Benchmark questions.
Tim Sailer tps@users.buoy.com
  `DBD-mysql` questions.
Boyd Lynn Gerber gerberb@zenez.com
  SCO-related questions.
Richard Mehalick RM186061@shellus.com
  `xmysql`-related questions and basic installation questions.
Zeev Suraski bourbon@netvision.net.il
  Apache module configuration questions (log & auth), PHP-related questions, SQL syntax-related questions and other general questions.
Francesc Guasch frankie@citel.upc.es
  General questions.
Jonathan J Smith jsmith@wtp.net
  Questions pertaining to OS-specifics with Linux, SQL syntax, and other things that might need some work.
David Sklar sklar@student.net
  Using MySQL from PHP and Perl.
Alistair MacDonald A.MacDonald@uel.ac.uk
  Not yet specified, but is flexible and can handle Linux and maybe HP-UX. Will try to get user to use `mysqlbug`.
John Lyon jlyon@imag.net
  Questions about installing MySQL on Linux systems, using either ``.rpm'` files or compiling from source.
Lorvid Ltd. lorvid@WOLFENET.com
  Simple billing/license/support/copyright issues.
Patrick Sherrill patrick@coconet.com
  ODBC and VisualC++ interface questions.
Randy Harmon rjharmon@uptimecomputers.com
  `DBD`, Linux, some SQL syntax questions.

## C.3 Supporters to MySQL

While `MySQL AB` owns all copyrights in the `MySQL server` and the `MySQL manual`, we wish to recognise the following companies, which helped us finance the development of the `MySQL server`, such as by paying us for developing a new feature or giving us hardware for development of the `MySQL server`.

VA Linux / Andover.net
  Funded replication.
NuSphere
  Editing of the MySQL manual.
Stork Design studio
  The MySQL web site in use between 1998-2000.
Intel
  Contributed to development on Windows and Linux platforms.
Compaq
  Contributed to Development on Linux/Alpha.
SWSoft
  Development on the embedded `mysqld` version.
FutureQuest
  `--skip-show-database`

# D MySQL Change History

This appendix lists the changes from version to version in the MySQL source code.

We are now working actively on MySQL 4.x and will only provide critical bug fixes for MySQL 3.23. We update this section as we add new features, so that everybody can follow the development.

Our TODO section contains what further plans we have for 4.x. See section 1.8 MySQL and The Future (The TODO).

Note that we tend to update the manual at the same time we make changes to MySQL. If you find a version listed here that you can't find on the MySQL download page (http://www.mysql.com/downloads/), this means that the version has not yet been released!

## D.1 Changes in release 4.1.x (Alpha)

Version 4.1 of the MySQL server includes many enhancements and new features:

- `INSERT ... ON DUPLICATE KEY UPDATE` syntax.
- Subqueries: `SELECT * from t1 where t1.a=(SELECT t2.b FROM t2)`.
- Derived tables: `SELECT a from t1, (select * from t2) WHERE t1.a=t2.a`
- Character sets to be defined per column, table and database.
- Unicode (UTF8) support.
- `BTREE` index on `HEAP` tables.
- Support for GIS (Geometrical data).
- `SHOW WARNINGS`; Shows warnings for the last command.
- Faster binary protocol used with prepared statements.

For a full list of changes, please refer to the changelog sections for each individual 4.1.x release.

### D.1.1 Changes in release 4.1.0

- Added a support for `UNIONs` in derived tables
- Timestamp is now returned as string of type `'YYYY-MM-DD HH:MM:DD'`.
- Faster binary protocol used with prepared statements.
- In `CREATE TABLE foo (a int not null primary key)` the `PRIMARY` word is now optional.
- In `CREATE TABLE` the attribute `SERIAL` is now an alias for `BIGINT NOT NULL AUTO_INCREMENT UNIQUE`.
- `SELECT ... FROM DUAL` is an alias for `SELECT ...`. (To be compatible with some other databases).
- If once creates a too long `CHAR`/`VARCHAR` it's now automatically changed to `TEXT` or `BLOB`; One will get a warning in this case.
- One can specify the different `BLOB`/`TEXT` types with the syntax `BLOB(length)` and `TEXT(length)`. MySQL will automatically change it to one of the internal `BLOB`/`TEXT` types.
- `CHAR BYTE` is an alias for `CHAR BINARY`.
- `VARCHARACTER` is an alias for `VARCHAR`.
- New operators `integer MOD integer` and `integer DIV integer`.
- `SERIAL DEFAULT VALUE` is an alias for `AUTO_INCREMENT`.
- `TRUE` and `FALSE` are added as alias for 0 and 1.
- Aliases are now forced in derived tables, as per SQL-99
- Fixed `SELECT .. LIMIT 0` to return proper row count for `SQL_CALC_FOUND_ROWS`.
- One can specify many temporary directories to be used in a round-robin fashion with: `--tmpdir=dirname1:dirname2:dirname3`.
- Subqueries: `SELECT * from t1 where t1.a=(SELECT t2.b FROM t2)`.
- Derived tables: `SELECT a from t1, (select * from t2) WHERE t1.a=t2.a`
- Character sets to be defined per column, table and database.
- Unicode (UTF8) support.
- `BTREE` index on `HEAP` tables.
- Faster embedded server.
- One can add a comment per column in `CREATE TABLE`.
- `SHOW FULL COLUMNS FROM table_name` shows column comments.
- `ALTER DATABASE`.
- Support for GIS (Geometrical data).
- `SHOW WARNINGS`; Shows warnings from the last command.
- One can specify a column type for a colum in `CREATE TABLE ... SELECT` by defining the column in the `CREATE` part.

      CREATE TABLE foo (a tinyint not null) SELECT b+1 AS 'a' FROM bar;

- `expr SOUNDS LIKE expr` same as `SOUNDEX(expr)=SOUNDEX(expr)`.
- `VARIANCE(expr)` returns the standard variance of `expr`

## D.2 Changes in release 4.0.x (Gamma)

Version 4.0 of the MySQL server includes many enhancements and new features:

- The `InnoDB` table type is now included in the standard binaries, adding transactions, row-level locking, and foreign keys. See section 7.5 `InnoDB` Tables.
- A query cache, offering vastly increased performance for many applications. By caching complete result sets, later identical queries can return instantly. See section 6.9 MySQL Query Cache.
- Improved full-text indexing with boolean mode, truncation, and phrase searching. See section 6.8 MySQL Full-text Search.
- Enhanced `MERGE` tables, now supporting `INSERT`s and `AUTO_INCREMENT`. See section 7.2 `MERGE` Tables.
- `UNION` syntax in `SELECT`. See section 6.4.1.2 `UNION` Syntax.
- Multi-table `DELETE` statements. See section 6.4.6 `DELETE` Syntax.
- `libmysqld`, the embedded server library. See section 8.4.9 libmysqld, the Embedded MySQL Server Library.
- Additional `GRANT` privilege options for even tighter control and security. See section 4.3.1 `GRANT` and `REVOKE` Syntax.
- Management of user resources in the `GRANT` system, particularly useful for ISPs and other hosting providers. See section 4.3.6 Limiting user resources.
- Dynamic server variables, allowing configuration changes without having to take down the server. See section 5.5.6 `SET` Syntax.
- Improved replication code and features. See section 4.10 Replication in MySQL.
- Numerous new functions and options.
- Changes to existing code for enhanced performance and reliability.

For a full list of changes, please refer to the changelog sections for each individual 4.0.x release.

### D.2.1 Changes in release 4.0.7 (Gamma)

Functionality added or changed:

Bugs fixed:

### D.2.2 Changes in release 4.0.6 (Gamma)

Functionality added or changed:

- Added syntax support for `CHARACTER SET xxx` and `CHARSET=xxx` table options (to be able to read table dumps from 4.1).
- Fixed replication bug that caused the slave to loose its position in some cases when the replication log was rotated.
- Fixed that a slave will restart from the start of a transaction if it's killed in the middle of one.
- Moved the manual pages from ``man'` to ``man/man1'` in the binary distributions.
- The default type returned by `IFNULL(A,B)` is now set to be the more 'general' of the types of `A` and `B`. (The order is `STRING`, `REAL` or `INTEGER`).
- Moved the ``mysql.server'` startup script in the RPM packages from ``/etc/rc.d/init.d/mysql'` to ``/etc/init.d/mysql'` (which almost all current Linux distributions support for LSB compliance).
- Added `Qcache_lowmem_prunes` status variable (number of queries that were deleted from cache because of low memory).
- Fixed `mysqlcheck` so it can deal with table names containing dashes.
- Bulk insert optimisation (see section 4.5.6.4 `SHOW VARIABLES`) is no longer used when inserting small (less than 100) number of rows.
- Optimisation added for queries like `SELECT ... FROM merge_table WHERE indexed_column=constant_expr`.
- Added functions `LOCALTIME` and `LOCALTIMESTAMP` as synonyms for `NOW()`.
- `CEIL` is now an alias for `CEILING`.
- The `CURRENT_USER()` function can be used to get a `user@host` value as it was matched in the `GRANT` system. See section 6.3.6.2 Miscellaneous Functions.
- Fixed `CHECK` constraints to be compatible with ANSI SQL. This made `CHECK` a reserved word. (Checking of `CHECK` constraints is still not implemented).
- Added `CAST(... as CHAR)`.
- Added PostgreSQL compatible `LIMIT` syntax: `SELECT ... LIMIT # OFFSET #`
- `mysql_change_user()` will now reset the connection to the state of a fresh connect (Ie, `ROLLBACK` any active transaction, close all temporary tables, reset all user variables etc..)

Bugs fixed:

- Make `--lower-case-table-names` default on MacOSX as the file system is case sensitive.
- Transactions in `AUTOCOMMIT=0` mode didn't rotate binary log.
- A fix for the bug in a `SELECT` with joined tables with `ORDER BY` and `LIMIT` clause when filesort had to be used. In that case `LIMIT` was applied to filesort of one of the tables, although it could not be. This fix solved problems with `LEFT JOIN` too.
- `mysql_server_init()` now makes a copy of all arguments. This fixes a problem when using the embedded server in C# program.
- Fixed buffer overrun in `libmysqlclient` library that allowed a malicious `MySQL` server to crash the client application.
- Fixed security-related bug in `mysql_change_user()` handling. All users are strongly recommended to upgrade to version 4.0.6.
- Fixed bug that prevented `--chroot` command-line option of `mysqld` from working.
- Fixed bug in phrase operator `"..."` in boolean full-text search.
- Fixed bug that caused `OPTIMIZE TABLE` to corrupt the table under some rare circumstances.
- Part rewrite of multi-table-update to optimise it, make it safer and more bug free.
- `LOCK TABLES` now works together with multi-table-update and multi-table-delete.
- `--replicate-do=xxx` didn't work for `UPDATE` commands. (Bug introduced in 4.0.0)
- Fixed shutdown problem on Mac OS X.

### D.2.3 Changes in release 4.0.5 (13 Nov 2002)

Functionality added or changed:

- Changed handling of last argument in `WEEK()` so that one can get week number according to the ISO 8601 specification. (Old code should still work).
- Fixed that `INSERT DELAYED` threads doesn't hang on `Waiting for INSERT` when one sends a `SIGHUP` to mysqld.
- Change that `AND` works according to SQL ANSI99 when it comes to `NULL` handling. In practice, this only affects queries where you do something like `WHERE ... NOT (NULL AND 0)`.
- `mysqld` will now resolve `basedir` to its full path (with `realpath()`). This enables one to use relative symlinks to the MySQL installation directory. This will however cause `show variables` to report different directories on systems where there is a symbolic link in the path.
- Fixed that MySQL will not use index scan on index disabled with `IGNORE INDEX` or `USE INDEX`. to be ignored.
- Added `--use-frm` option to `mysqlcheck`. When used with `REPAIR`, it gets the table structure from the `.frm` file, so the table can be repaired even if the `.MYI` header is corrupted.
- Fixed bug in `MAX()` optimisation when used with `JOIN` and `ON` expressions.
- Added support for reading of MySQL 4.1 table definition files.
- `BETWEEN` behaviour changed (see section 6.3.1.2 Comparison Operators). Now `datetime_col BETWEEN timestamp AND timestamp` should work as expected.
- One can create `TEMPORARY` `MERGE` tables now.
- `DELETE FROM myisam_table` now shrinks not only the ``.MYD'` file but also the ``.MYI'` file.
- When one uses the `--open-files-limit=#` option to `mysqld_safe` it's now passed on to `mysqld`.
- Changed output from `EXPLAIN` from `'where used'` to `'Using where'` to make it more in line with other output.
- Removed variable `safe_show_database` as it was not used anymore.
- Updated source tree to be built using `automake 1.5` and `libtool 1.4`.
- Fixed an inadvertently changed option (`--ignore-space`) back to the original `--ignore-spaces` in `mysqlclient`. (Both syntaxes will work).
- Don't require `UPDATE` privilege when using `REPLACE`.
- Added support for `DROP TEMPORARY TABLE ...`, to be used to make replication safer.
- When transactions are enabled, all commands that update temporary tables inside a `BEGIN/COMMIT` are now stored in the binary log on `COMMIT` and not stored if one does `ROLLBACK`. This fixes some problems with non-transactional temporary tables used inside transactions.
- Allow braces in joins in all positions. Formerly, things like `SELECT * FROM (t2 LEFT JOIN t3 USING (a)), t1` worked, but not `SELECT * FROM t1, (t2 LEFT JOIN t3 USING (a))`. Note that braces are simply removed, they do not change the way the join is executed.
- InnoDB now supports also isolation levels `READ UNCOMMITTED` and `READ COMMITTED`. For a detailed InnoDB changelog, see section 7.5.14 InnoDB Change History in this manual.

Bugs fixed:

- Fixed bug in `MAX()` optimisation when used with `JOIN` and `ON` expressions.
- Fixed that `INSERT DELAY` threads don't hang on `Waiting for INSERT` when one sends a `SIGHUP` to `mysqld`.
- Fixed that MySQL will not use an index scan on an index that has been disabled with `IGNORE INDEX` or `USE INDEX`.
- Corrected test for `root` user in `mysqld_safe`.
- Fixed error message issued when storage engine cannot do `CHECK` or `REPAIR`.
- Fixed rare core dump problem in complicated `GROUP BY` queries that didn't return any result.
- Fixed `mysqlshow` to work properly with wildcarded database names and with database names that contain underscores.
- Portability fixes to get MySQL to compile cleanly with Sun Forte 5.0.
- Fixed `MyISAM` crash when using dynamic-row tables with huge numbers of packed fields.
- Fixed query cache behaviour with `BDB` transactions.
- Fixed possible floating point exception in `MATCH` relevance calculations.
- Fixed bug in full-text search `IN BOOLEAN MODE` that made `MATCH` to return incorrect relevance value in some complex joins.
- Fixed a bug that limited `MyISAM` key length to a value slightly less that 500. It is exactly 500 now.
- Fixed that `GROUP BY` on columns that may have a `NULL` value doesn't always use disk based temporary tables.
- The filename argument for the `--des-key-file` argument to `mysqld` is interpreted relative to the data directory if given as a relative pathname.
- Removed a condition that temp table with index on column that can be `NULL` has to be `MyISAM`. This was okay for 3.23, but not needed in 4.*. This resulted in slowdown in many queries since 4.0.2.
- Small code improvement in multi-table updates.
- Fixed a newly introduced bug that caused `ORDER BY ... LIMIT #` to not return all rows.
- Fixed a bug in multi-table deletes when outer join is used on an empty table, which gets first to be deleted.
- Fixed a bug in multi-table updates when a single table is updated.
- Fixed bug that caused `REPAIR TABLE` and `myisamchk` to corrupt `FULLTEXT` indexes.
- Fixed bug with caching the `mysql` grant table database. Now queries in this database are not cached in the query cache.
- Small fix in `mysqld_safe` for some shells.
- Give error if a `MyISAM` `MERGE` table has more than 2 ^ 32 rows and MySQL was not compiled with with `-DBIG_TABLES`.
- Fixed some `ORDER BY ... DESC` problems with `InnoDB` tables.

### D.2.4 Changes in release 4.0.4 (29 Sep 2002)

- Fixed bug where `GRANT`/`REVOKE` failed if hostname was given in non-matching case.
- Don't give warning in `LOAD DATA INFILE` when setting a `timestamp` to a string value of `'0'`.
- Fixed bug in `myisamchk -R` mode.
- Fixed bug that caused `mysqld` to crash on `REVOKE`.
- Fixed bug in `ORDER BY` when there is a constant in the `SELECT` statement.
- One didn't get an error message if `mysqld` couldn't open the privilege tables.
- `SET PASSWORD FOR ...` closed the connection in case of errors (bug from 4.0.3).
- Increased max possible `max_allowed_packet` in `mysqld` to 1GB.
- Fixed bug when doing a multi-line `INSERT` on a table with an `AUTO_INCREMENT` key which was not in the first part of the key.
- Changed `LOAD DATA INFILE` to not recreate index if the table had rows from before.
- Fixed overrun bug when calling `AES_DECRYPT()` with incorrect arguments.
- `--skip-ssl` can now be used to disable SSL in the MySQL clients, even if one is using other SSL options in an option file or previously on the command line.
- Fixed bug in `MATCH ... AGAINST( ... IN BOOLEAN MODE)` used with `ORDER BY`.
- Added `LOCK TABLES` and `CREATE TEMPORARY TABLES` privilege on the database level. One must run the `mysql_fix_privilege_tables` script on old installations to activate these.
- In `SHOW TABLE ... STATUS`, compressed tables sometimes showed up as `dynamic`.
- `SELECT @@[global|session].var_name` didn't report `global | session` in the result column name.
- Fixed problem in replication that `FLUSH LOGS` in a circular replication setup created an infinite number of binary log files. Now a `rotate-binary-log` command in the binary log will not cause slaves to rotate logs.
- Removed `STOP EVENT` from binary log when doing `FLUSH LOGS`.
- Disable the use of `SHOW NEW MASTER FOR SLAVE` as this needs to be completely changed in 4.1.
- Fixed a bug with constant expression (e.g. field of a one-row table, or field from a table, referenced by a `UNIQUE` key) appeared in `ORDER BY` part of `SELECT DISTINCT`.
- `--log-binary=a.b.c` now properly strips off `.b.c`.
- `FLUSH LOGS` removed numerical extension for all future update logs.
- `GRANT ... REQUIRE` didn't store the SSL information in the `mysql.user` table if SSL was not enabled in the server.
- `GRANT ... REQUIRE NONE` can now be used to remove SSL information.
- `AND` is now optional between `REQUIRE` options.
- `REQUIRE` option was not properly saved, which could cause strange output in `SHOW GRANTS`.
- Fixed that `mysqld --help` reports correct values for `--datadir` and `--bind-address`.
- Fixed that one can drop UDFs that didn't exist when `mysqld` was started.
- Fixed core dump problem with `SHOW VARIABLES` on some 64 bit systems (like Solaris sparc).
- Fixed a bug in my_getopt; `--set-variable` syntax didn't work for those options that didn't have a valid variable in my_option struct. This affected at least `default-table-type` option.
- Fixed a bug from 4.0.2 that caused `REPAIR TABLE` and `myisamchk --recover` to fail on tables with duplicates in a unique key.
- Fixed a bug from 4.0.3 in calculating the default field type for some functions. This affected queries of type `CREATE TABLE table_name SELECT expression(),...`
- Fixed bug in queries of type `SELECT * FROM table-list GROUP BY ...` and `SELECT DISTINCT * FROM ...`.
- Fixed bug with the `--slow-log` when logging an administrator command (like `FLUSH TABLES`).
- Fixed a bug that `OPTIMIZE` of locked and modified table, reported table corruption.
- Fixed a bug in my_getopt in handling of special prefixes (`--skip-`, `--enable-`). `--skip-external-locking` didn't work and the bug may have affected other similar options.
- Fixed bug in checking for output file name of the `tee` option.
- Added some more optimisation to use index for `SELECT ... FROM many_tables .. ORDER BY key limit #`
- Fixed problem in `SHOW OPEN TABLES` when a user didn't have access permissions to one of the opened tables.

### D.2.5 Changes in release 4.0.3 (26 Aug 2002: Beta)

- Fixed problem with `configure ... --localstatedir=...`.
- Cleaned up `mysql.server` script.
- Fixed a bug in `mysqladmin shutdown` when pid file was modified while `mysqladmin` was still waiting for the previous one to disappear. This could happen during a very quick restart and caused `mysqladmin` to hang until `shutdown_timeout` seconds had passed.
- Don't increment warnings when setting `AUTO_INCREMENT` columns to `NULL` in `LOAD DATA INFILE`.
- Fixed all boolean type variables/options to work with the old syntax, e.g. all of these work: --lower-case-table-names, --lower-case-table-names=1, -O lower-case-table-names=1, --set-variable=lower-case-table-names=1
- Fixed shutdown problem (SIGTERM signal handling) on Solaris. (Bug from 4.0.2).
- `SHOW MASTER STATUS` now returns an empty set if binary log is not enabled.
- `SHOW SLAVE STATUS` now returns an empty set if slave is not initialised.
- Don't update MyISAM index file on update if not strictly necessary.
- Fixed bug in `SELECT DISTINCT ... FROM many_tables ORDER BY not-used-column`.
- Fixed a bug with `BIGINT`s and quoted strings.
- Added `QUOTE()` function that performs SQL quoting to produce values that can be used as data values in queries.
- Changed variable `DELAY_KEY_WRITE` to an enum to allow one set `DELAY_KEY_WRITE` for all tables without taking down the server.
- Changed behaviour of `IF(condition,column,NULL)` so that it returns the value of the column type.
- Made `safe_mysqld` a symlink to `mysqld_safe` in binary distribution.
- Fixed security bug when having an empty database name in the `user.db` table.
- Fixed some problems with `CREATE TABLE ... SELECT function()`.
- `mysqld` now has the option `--temp-pool` enabled by default as this gives better performance with some operating systems.
- Fixed hang in `CHANGE MASTER TO` if the slave thread died very quickly.
- Big cleanup in replication code (less logging, better error messages, etc..)
- If the `--code-file` option is specified, the server calls `setrlimit()` to set the maximum allowed core file size to unlimited, so core files can be generated.
- Fixed bug in query cache after temporary table creation.
- Added `--count=N` (`-c`) option to `mysqladmin`, to make the program do only `N` iterations. To be used with `--sleep` (`-i`). Useful in scripts.
- Fixed bug in multi-table `UPDATE`: when updating a table, `do_select()` became confused about reading records from a cache.
- Fixed bug in multi-table `UPDATE` when several fields were referenced from a single table
- Fixed bug in truncating nonexisting table.
- Fixed bug in `REVOKE` that caused user resources to be randomly set.
- Fixed bug in `GRANT` for the new `CREATE TEMPORARY TABLE` privilege.
- Fixed bug in multi-table `DELETE` when tables are re-ordered in the table initialisation method and ref_lengths are of different sizes.
- Fixed two bugs in `SELECT DISTINCT` with large tables.
- Fixed bug in query cache initialisation with very small query cache size.
- Allow `DEFAULT` with `INSERT` statement.
- The startup parameters `myisam_max_sort_file_size` and `myisam_max_extra_sort_file_size` are now given in bytes, not megabytes.
- External system locking of `MyISAM`/`ISAM` files is now turned off by default. One can turn this on with `--external-locking`. (For most users this is never needed).
- Fixed core dump bug with `INSERT ... SET db_name.table_name.colname=''`.
- Fixed client hangup bug when using some SQL commands with wrong syntax.
- Fixed a timing bug in `DROP DATABASE`
- New `SET [GLOBAL | SESSION]` syntax to change thread-specific and global server variables at runtime.
- Added variable `slave_compressed_protocol`.
- Renamed variable `query_cache_startup_type` to `query_cache_type`, `myisam_bulk_insert_tree_size` to `bulk_insert_buffer_size`, `record_buffer` to `read_buffer_size` and `record_rnd_buffer` to `record_rnd_buffer_size`.
- Renamed some SQL variables, but old names will still work until 5.0. See section 2.5.2 Upgrading From Version 3.23 to Version 4.0.
- Renamed `--skip-locking` to `--skip-external-locking`.
- Removed unused variable `query_buffer_size`.
- Fixed a bug that made the pager option in the `mysql` client non-functional.
- Added full `AUTO_INCREMENT` support to `MERGE` tables.
- Extended `LOG()` function to accept an optional arbitrary base parameter. See section 6.3.3.2 Mathematical Functions.
- Added `LOG2()` function (useful for finding out how many bits a number would require for storage).
- Added `LN()` natural logarithm function for compatibility with other databases. It is synonymous with `LOG(X)`.

### D.2.6 Changes in release 4.0.2 (01 Jul 2002)

- Cleaned up `NULL` handling for default values in `DESCRIBE table_name`.
- Fixed `truncate()` to round up negative values to the nearest integer.
- Changed `--chroot=path` option to execute `chroot()` immediately after all options have been parsed.
- Don't allow database names that contain ``\'`.
- `lower_case_table_names` now also affects database names.
- Added `XOR` operator (logical and bitwise `XOR`) with `^` as a synonym for bitwise `XOR`.
- Added function `IS_FREE_LOCK("lock_name")`. Based on code contributed by Hartmut Holzgraefe hartmut@six.de.
- Removed `mysql_ssl_clear()` from C API, as it was not needed.
- `DECIMAL` and `NUMERIC` types can now read exponential numbers.
- Added `SHA1()` function to calculate 160 bit hash value as described in RFC 3174 (Secure Hash Algorithm). This function can be considered a cryptographically more secure equivalent of `MD5()`. See section 6.3.6.2 Miscellaneous Functions.
- Added `AES_ENCRYPT()` and `AES_DECRYPT()` functions to perform encryption according to AES standard (Rijndael). See section 6.3.6.2 Miscellaneous Functions.
- Added `--single-transaction` option to `mysqldump`, allowing a consistent dump of `InnoDB` tables. See section 4.8.5 `mysqldump`, Dumping Table Structure and Data.
- Fixed bug in `innodb_log_group_home_dir` in `SHOW VARIABLES`.
- Fixed a bug in optimiser with merge tables when non-unique values are used in summing up (causing crashes).
- Fixed a bug in optimiser when a range specified makes index grouping impossible (causing crashes).
- Fixed a rare bug when `FULLTEXT` index is present and no tables are used.
- Added privileges `CREATE TEMPORARY TABLES`, `EXECUTE`, `LOCK TABLES`, `REPLICATION CLIENT`, `REPLICATION SLAVE`, `SHOW DATABASES` and `SUPER`. To use these, you must have run the `mysql_fix_privilege_tables` script after upgrading.
- Fixed query cache align data bug.
- Fixed mutex bug in replication when reading from master fails.
- Added missing mutex in `TRUNCATE TABLE`; This fixes some core dump/hangup problems when using `TRUNCATE TABLE`.
- Fixed bug in multi-table `DELETE` when optimiser uses only indices.
- Fixed that `ALTER TABLE table_name RENAME new_table_name` is as fast as `RENAME TABLE`.
- Fixed bug in `GROUP BY` with two or more fields, where at least one field can contain `NULL` values.
- Use `Turbo Boyer-Moore` algorithm to speed up `LIKE "%keyword%"` searches.
- Fixed bug in `DROP DATABASE` with symlink.
- Fixed crash in `REPAIR ... USE_FRM`.
- Fixed bug in `EXPLAIN` with `LIMIT offset != 0`.
- Fixed bug in phrase operator `"..."` in boolean full-text search.
- Fixed bug that caused duplicated rows when using truncation operator `*` in boolean full-text search.
- Fixed bug in truncation operator of boolean full-text search (wrong results when there are only `+word*`s in the query).
- Fixed bug in boolean full-text search that caused a crash when an identical `MATCH` expression that did not use an index appeared twice.
- Query cache is now automatically disabled in `mysqldump`.
- Fixed problem on Windows 98 that made sending of results very slow.
- Boolean full-text search weighting scheme changed to something more reasonable.
- Fixed bug in boolean full-text search that caused MySQL to ignore queries of `ft_min_word_len` characters.
- Boolean full-text search now supports ``phrase searches''.
- New configure option `--without-query-cache`.
- Memory allocation strategy for ``root memory'' changed. Block size now grows with number of allocated blocks.
- `INET_NTOA()` now returns `NULL` if you give it an argument that is too large (greater than the value corresponding to `255.255.255.255`).
- Fix `SQL_CALC_FOUND_ROWS` to work with `UNION`s. It will work only if the first `SELECT` has this option and if there is global `LIMIT` for the entire statement. For the moment, this requires using parentheses for individual `SELECT` queries within the statement.
- Fixed bug in `SQL_CALC_FOUND_ROWS` and `LIMIT`.
- Don't give an error for `CREATE TABLE ...(... VARCHAR(0))`.
- Fixed `SIGINT` and `SIGQUIT` problems in ``mysql.cc'` on Linux with some `glibc` versions.
- Fixed bug in ``convert.cc'`, which is caused by having an incorrect `net_store_length()` linked in the `CONVERT::store()` method.
- `DOUBLE` and `FLOAT` columns now honor the `UNSIGNED` flag on storage.
- `InnoDB` now retains foreign key constraints through `ALTER TABLE` and `CREATE/DROP INDEX`.
- `InnoDB` now allows foreign key constraints to be added through the `ALTER TABLE` syntax.
- `InnoDB` tables can now be set to automatically grow in size (autoextend).
- Added `--ignore-lines=n` option to `mysqlimport`. This has the same effect as the `IGNORE n LINES` clause for `LOAD DATA`.
- Fixed bug in `UNION` with last offset being transposed to total result set.
- `REPAIR ... USE_FRM` added.
- Fixed that `DEFAULT_SELECT_LIMIT` is always imposed on `UNION` result set.
- Fixed that some `SELECT` options can appear only in the first `SELECT`.
- Fixed bug with `LIMIT` with `UNION`, where last select is in the braces.
- Fixed that full-text works fine with `UNION` operations.
- Fixed bug with indexless boolean full-text search.
- Fixed bug that sometimes appeared when full-text search was used with ``const'' tables.
- Fixed incorrect error value when doing a `SELECT` with an empty `HEAP` table.
- Use `ORDER BY column DESC` now sorts `NULL` values first. (In other words, `NULL` values sort first in all cases, whether or not `DESC` is specified.)
- Fixed bug in `WHERE key_name='constant' ORDER BY key_name DESC`.
- Fixed bug in `SELECT DISTINCT ... ORDER BY DESC` optimisation.
- Fixed bug in `... HAVING 'GROUP_FUNCTION'(xxx) IS [NOT] NULL`.
- Fixed bug in truncation operator for boolean full-text search.
- Allow value of `--user=#` option for `mysqld` to be specified as a numeric user ID.
- Fixed a bug where `SQL_CALC_ROWS` returned an incorrect value when used with one table and `ORDER BY` and with `InnoDB` tables.
- Fixed that `SELECT 0 LIMIT 0` doesn't hang thread.
- Fixed some problems with `USE/IGNORE INDEX` when using many keys with the same start column.
- Don't use table scan with `BerkeleyDB` and `InnoDB` tables when we can use an index that covers the whole row.
- Optimised `InnoDB` sort-buffer handling to take less memory.
- Fixed bug in multi-table `DELETE` and `InnoDB` tables.
- Fixed problem with `TRUNCATE` and `InnoDB` tables that produced the error `Can't execute the given command because you have active locked tables or an active transaction`.
- Added `NO_UNSIGNED_SUBTRACTION` to the set of flags that may be specified with the `--sql-mode` option for `mysqld`. It disables unsigned arithmetic rules when it comes to subtraction. (This will make MySQL 4.0 behave more closely to 3.23 with `UNSIGNED` columns).
- The result returned for all bit functions (`|`, `<<`, ...) is now of type `unsigned integer`.
- Added detection of `nan` values in `MyISAM` to make it possible to repair tables with `nan` in float or double columns.
- Fixed new bug in `myisamchk` where it didn't correctly update number of ``parts'' in the `MyISAM` index file.
- Changed to use `autoconf` 2.52 (from `autoconf` 2.13).
- Fixed optimisation problem where the MySQL Server was in ``preparing'' state for a long time when selecting from an empty table which had contained a lot of rows.
- Fixed bug in complicated join with `const` tables. This fix also improves performance a bit when referring to another table from a `const` table.
- First pre-version of multi-table `UPDATE` statement.
- Fixed bug in multi-table `DELETE`.
- Fixed bug in `SELECT CONCAT(argument_list) ... GROUP BY 1`.
- `INSERT ... SELECT` did a full rollback in case of an error. Fixed so that we only roll back the last statement in the current transaction.
- Fixed bug with empty expression for boolean full-text search.
- Fixed core dump bug in updating full-text key from/to `NULL`.
- ODBC compatibility: Added `BIT_LENGTH()` function.
- Fixed core dump bug in `GROUP BY BINARY column`.
- Added support for `NULL` keys in `HEAP` tables.
- Use index for `ORDER BY` in queries of type: `SELECT * FROM t WHERE key_part1=1 ORDER BY key_part1 DESC,key_part2 DESC`
- Fixed bug in `FLUSH QUERY CACHE`.
- Added `CAST()` and `CONVERT()` functions. The `CAST` and `CONVERT` functions are nearly identical and mainly useful when you want to create a column with a specific type in a `CREATE ... SELECT` statement. For more information, read section 6.3.5 Cast Functions.
- `CREATE ... SELECT` on `DATE` and `TIME` functions now create columns of the expected type.
- Changed order in which keys are created in tables.
- Added new columns `Null` and `Index_type` to `SHOW INDEX` output.
- Added `--no-beep` and `--prompt` options to `mysql` command-line client.
- New feature: management of user resources.

      GRANT ... WITH MAX_QUERIES_PER_HOUR N1
                     MAX_UPDATES_PER_HOUR N2
                     MAX_CONNECTIONS_PER_HOUR N3;

  See section 4.3.6 Limiting user resources.
- Added `mysql_secure_installation` to the ``scripts/'` directory.

### D.2.7 Changes in release 4.0.1 (23 Dec 2001)

- Fixed bug when `HANDLER` was used with some unsupported table type.
- `mysqldump` now puts `ALTER TABLE tbl_name DISABLE KEYS` and `ALTER TABLE tbl_name ENABLE KEYS` in the sql dump.
- Added `mysql_fix_extensions` script.
- Fixed stack overrun problem with `LOAD DATA FROM MASTER` on OSF/1.
- Fixed shutdown problem on HP-UX.
- Added `DES_ENCRYPT()` and `DES_DECRYPT()` functions.
- Added `FLUSH DES_KEY_FILE` statement.
- Added `--des-key-file` option to `mysqld`.
- `HEX(string)` now returns the characters in `string` converted to hexadecimal.
- Fixed problem with `GRANT` when using `lower_case_table_names=1`.
- Changed `SELECT ... IN SHARE MODE` to `SELECT ... LOCK IN SHARE MODE` (as in MySQL 3.23).
- A new query cache to cache results from identical `SELECT` queries.
- Fixed core dump bug on 64-bit machines when it got an incorrect communication packet.
- `MATCH ... AGAINST(... IN BOOLEAN MODE)` can now work without `FULLTEXT` index.
- Fixed slave to replicate from 3.23 master.
- Miscellaneous replication fixes/cleanup.
- Got shutdown to work on Mac OS X.
- Added ``myisam/ft_dump'` utility for low-level inspection of `FULLTEXT` indexes.
- Fixed bug in `DELETE ... WHERE ... MATCH ...`.
- Added support for `MATCH ... AGAINST(... IN BOOLEAN MODE)`. Note: you must rebuild your tables with `ALTER TABLE tablename TYPE=MyISAM` to be able to use boolean full-text search.
- `LOCATE()` and `INSTR()` are now case-sensitive if either argument is a binary string.
- Changed `RAND()` initialisation so that `RAND(N)` and `RAND(N+1)` are more distinct.
- Fixed core dump bug in `UPDATE ... ORDER BY`.
- Changed `INSERT INTO ... SELECT` to stop on errors by default.
- Ignore `DATA DIRECTORY` and `INDEX DIRECTORY` directives on Windows.
- Added boolean full-text search code. It should be considered early alpha.
- Extended `MODIFY` and `CHANGE` in `ALTER TABLE` to accept the `FIRST` and `AFTER` keywords.
- Indexes are now used with `ORDER BY` on a whole `InnoDB` table.

### D.2.8 Changes in release 4.0.0 (Oct 2001: Alpha)

- Added `--xml` option to `mysql` for producing XML output.
- Added full-text variables `ft_min_word_len`, `ft_max_word_len`, and `ft_max_word_len_for_sort`.
- Added documentation for `libmysqld`, the embedded MySQL server library. Also added example programs (a `mysql` client and `mysqltest` test program) which use `libmysqld`.
- Removed all Gemini hooks from MySQL server.
- Removed `my_thread_init()` and `my_thread_end()` from ``mysql_com.h'`, and added `mysql_thread_init()` and `mysql_thread_end()` to ``mysql.h'`.
- Support for communication packets > 16M. In 4.0.1 we will extend `MyISAM` to be able to handle these.
- Secure connections (with SSL).
- Unsigned `BIGINT` constants now work. `MIN()` and `MAX()` now handle signed and unsigned `BIGINT` numbers correctly.
- New character set `latin1_de` which provides correct German sorting.
- `STRCMP()` now uses the current character set when doing comparisons, which means that the default comparison behaviour now is case-insensitive.
- `TRUNCATE TABLE` and `DELETE FROM tbl_name` are now separate functions. One bonus is that `DELETE FROM tbl_name` now returns the number of deleted rows, rather than zero.
- `DROP DATABASE` now executes a `DROP TABLE` on all tables in the database, which fixes a problem with `InnoDB` tables.
- Added support for `UNION`.
- Added support for multi-table `DELETE` operations.
- A new `HANDLER` interface to `MyISAM` tables.
- Added support for `INSERT` on `MERGE` tables. Patch from Benjamin Pflugmann.
- Changed `WEEK(#,0)` to match the calendar in the USA.
- `COUNT(DISTINCT)` is about 30% faster.
- Speed up all internal list handling.
- Speed up `IS NULL`, `ISNULL()` and some other internal primitives.
- Full-text index creation now is much faster.
- Tree-like cache to speed up bulk inserts and `myisam_bulk_insert_tree_size` variable.
- Searching on packed (`CHAR`/`VARCHAR`) keys is now much faster.
- Optimised queries of type: `SELECT DISTINCT * from tbl_name ORDER by key_part1 LIMIT #`.
- `SHOW CREATE TABLE` now shows all table attributes.
- `ORDER BY ... DESC` can now use keys.
- `LOAD DATA FROM MASTER` ``auto-magically'' sets up a slave.
- Renamed `safe_mysqld` to `mysqld_safe` to make this name more in line with other MySQL scripts/commands.
- Added support for symbolic links to `MyISAM` tables. Symlink handling is now enabled by default for Windows.
- Added `SQL_CALC_FOUND_ROWS` and `FOUND_ROWS()`. This makes it possible to know how many rows a query would have returned without a `LIMIT` clause.
- Changed output format of `SHOW OPEN TABLES`.
- Allow `SELECT expression LIMIT ...`.
- Added `IDENTITY` as a synonym for `AUTO_INCREMENT` (like Sybase).
- Added `ORDER BY` syntax to `UPDATE` and `DELETE`.
- `SHOW INDEXES` is now a synonym for `SHOW INDEX`.
- Added `ALTER TABLE tbl_name DISABLE KEYS` and `ALTER TABLE tbl_name ENABLE KEYS` commands.
- Allow use of `IN` as a synonym for `FROM` in `SHOW` commands.
- Implemented ``repair by sort'' for `FULLTEXT` indexes. `REPAIR TABLE`, `ALTER TABLE`, and `OPTIMIZE TABLE` for tables with `FULLTEXT` indexes are now up to 100 times faster.
- Allow ANSI SQL syntax `X'hexadecimal-number'`.
- Cleaned up global lock handling for `FLUSH TABLES WITH READ LOCK`.
- Fixed problem with `DATETIME = constant` in `WHERE` optimisation.
- Added `--master-data` and `--no-autocommit` options to `mysqldump`. (Thanks to Brian Aker for this.)
- Added script `mysql_explain_log.sh` to distribution. (Thanks to mobile.de).

## D.3 Changes in release 3.23.x (Stable)

The 3.23 release has several major features that are not present in previous versions. We have added three new table types:

`MyISAM`
  A new ISAM library which is tuned for SQL and supports large files.
`InnoDB`
  A transaction-safe storage engine that supports row level locking, and many Oracle-like features.
`BerkeleyDB` or `BDB`
  Uses the Berkeley DB library from Sleepycat Software to implement transaction-safe tables.

Note that only `MyISAM` is available in the standard binary distribution.

The 3.23 release also includes support for database replication between a master and many slaves, full-text indexing, and much more.

All new features are being developed in the 4.x version. Only bug fixes and minor enhancements to existing features will be added to 3.23.

The replication code and BerkeleyDB code is still not as tested and as the rest of the code, so we will probably need to do a couple of future releases of 3.23 with small fixes for this part of the code. As long as you don't use these features, you should be quite safe with MySQL 3.23!

Note that the above doesn't mean that replication or Berkeley DB don't work. We have done a lot of testing of all code, including replication and `BDB` without finding any problems. It only means that not as many users use this code as the rest of the code and because of this we are not yet 100% confident in this code.

### D.3.1 Changes in release 3.23.55

- Make `--lower-case-table-names` default on MacOSX as the file system is case sensitive.
- Fixed a bug in ``scripts/mysqld_safe.sh'` in `NOHUP_NICENESS` testing.
- Transactions in `AUTOCOMMIT=0` mode didn't rotate binary log.
- Fixed a bug in `scripts/make_binary_distribution` that resulted in a remaining `@HOSTNAME@` variable instead of replacing it with the correct path to the `hostname` binary.
- Fixed a very unlikely bug that could cause `SHOW PROCESSLIST` to core dump in pthread_mutex_unlock() if a new thread was connecting.
- Forbid `SLAVE STOP` if the thread executing the query has locked tables. This removes a possible deadlock situation.

### D.3.2 Changes in release 3.23.54 (5 Dec 2002)

- Fixed a bug, that allowed to crash `mysqld` with a specially crafted packet.
- Fixed a rare crash (double `free`'d pointer) when altering a temporary table.
- Fixed buffer overrun in `libmysqlclient` library that allowed malicious `MySQL` server to crash the client application.
- Fixed security-related bug in `mysql_change_user()` handling. All users are strongly recommended to upgrade to the version 3.23.54.
- Fixed bug that prevented `--chroot` command-line option of `mysqld` from working.
- Fixed bug that made `OPTIMIZE TABLE` to corrupt the table under some rare circumstances.
- Fixed `mysqlcheck` so it can deal with table names containing dashes.
- Fixed shutdown problem on Mac OS X.
- Fixed bug with comparing an indexed `NULL` field with `<=> NULL`.
- Fixed bug that caused `IGNORE INDEX` and `USE INDEX` sometimes to be ignored.
- Fixed rare core dump problem in complicated `GROUP BY` queries that didn't return any result.
- Fixed a bug where `MATCH ... AGAINST () >=0` was treated as if it was `>`.
- One can create `TEMPORARY` `MERGE` tables now.
- Fixed that `--core-file` works on Linux (at least on kernel 2.4.18).
- Fixed a problem with `BDB` and `ALTER TABLE`.
- Fixed reference to freed memory when doing complicated `GROUP BY ... ORDER BY` queries. Symptom was that `mysqld` died in function `send_fields`.
- Allocate heap rows in smaller blocks to get better memory usage.
- Fixed memory allocation bug when storing `BLOB` values in internal temporary tables used for some (unlikely) `GROUP BY` queries.
- Fixed a bug in key optimizing handling where the expression `WHERE column_name = key_column_name` was calculated as true for `NULL` values.
- Fixed core dump bug when doing `LEFT JOIN ... WHERE key_column=NULL`.
- Fixed `MyISAM` crash when using dynamic-row tables with huge numbers of packed fields.
- Updated source tree to be built using `automake 1.5` and `libtool 1.4`.

### D.3.3 Changes in release 3.23.53 (09 Oct 2002)

- Fixed crash when `SHOW INNODB STATUS` was used and `skip-innodb` was defined.
- Fixed possible memory corruption bug in binary log file handling when slave rotated the logs (only affected 3.23, not 4.0).
- Fixed problem in `LOCK TABLES` on windows when one connects to a database that contains upper case letters.
- Fixed that `--skip-show-databases` doesn't reset the `--port` option.
- Small fix in `safe_mysqld` for some shells.
- Fixed that `FLUSH STATUS` doesn't reset `Delayed_insert_threads`.
- Fixed core dump bug when using the `BINARY` cast on a `NULL` value.
- Fixed race condition when someone did a `GRANT` at the same time a new user logged in or did a `USE DATABASE`.
- Fixed bug in `ALTER TABLE` and `RENAME TABLE` when running with `-O lower_case_table_names=1` (typically on windows) when giving the table name in uppercase.
- Fixed that `-O lower_case_table_names=1` also converts database names to lower case.
- Fixed unlikely core dump with `SELECT ... ORDER BY ... LIMIT`.
- Changed `AND/OR` to report that they can return NULL. This fixes a bug in `GROUP BY` on `AND/OR` expressions that return `NULL`.
- Fixed a bug that `OPTIMIZE` of locked and modified MyISAM table, reported table corruption.
- Fixed a `BDB`-related `ALTER TABLE` bug with dropping a column and shutting down immediately thereafter.
- Fixed problem with `configure ... --localstatedir=...`.
- Fixed problem with `UNSIGNED BIGINT` on AIX (again).
- Fixed bug in pthread_mutex_trylock() on HPUX 11.0.
- Multithreaded stress tests for InnoDB.

### D.3.4 Changes in release 3.23.52 (14 Aug 2002)

- Wrap `BEGIN`/`COMMIT` around transaction in the binary log. This makes replication honour transactions.
- Fixed security bug when having an empty database name in the `user.db` table.
- Changed initialisation of `RND()` to make it less predicatable.
- Fixed problem with `GROUP BY` on result with expression that created a `BLOB` field.
- Fixed problem with `GROUP BY` on columns that have `NULL` values. To solve this we now create an `MyISAM` temporary table when doing a `GROUP BY` on a possible `NULL` item. From MySQL 4.0.5 we can use in memory `HEAP` tables for this case.
- Fixed problem with privilege tables when downgrading from 4.0.2 to 3.23.
- Fixed thread bug in `SLAVE START`, `SLAVE STOP` and automatic repair of MyISAM tables that could cause table cache to be corrupted.
- Fixed possible thread related key-cache-corruption problem with `OPTIMIZE TABLE` and `REPAIR TABLE`.
- Added name of 'administrator command' logs.
- Fixed bug with creating an auto-increment value on second part of a `UNIQUE()` key where first part could contain NULL values.
- Don't write slave-timeout reconnects to the error log.
- Fixed bug with slave net read timeouting
- Fixed a core-dump bug with `MERGE` tables and `MAX()` function.
- Fixed bug in `ALTER TABLE` with `BDB` tables.
- Fixed bug when logging `LOAD DATA INFILE` to binary log with no active database.
- Fixed a bug in range optimiser (causing crashes).
- Fixed possible problem in replication when doing `DROP DATABASE` on a database with `InnoDB` tables.
- Fixed that `mysql_info()` returns 0 for 'Duplicates' when using `INSERT DELAYED IGNORE`.
- Added `-DHAVE_BROKEN_REALPATH` to the Mac OS X (darwin) compile options in ``configure.in'` to fix a failure under high load.

### D.3.5 Changes in release 3.23.51 (31 May 2002)

- Fix bug with closing tags missing slash for `mysqldump` XML output.
- Remove end space from `ENUM` values. (This fixed a problem with `SHOW CREATE TABLE`.)
- Fixed bug in `CONCAT_WS()` that cut the result.
- Changed name of server variables `Com_show_master_stat` to `Com_show_master_status` and `Com_show_slave_stat` to `Com_show_slave_status`.
- Changed handling of `gethostbyname()` to make the client library thread-safe even if `gethostbyname_r` doesn't exist.
- Fixed core-dump problem when giving a wrong password string to `GRANT`.
- Fixed bug in `DROP DATABASE` with symlinked directory.
- Fixed optimisation problem with `DATETIME` and value outside `DATETIME` range.
- Removed Sleepycat's `BDB` doc files from the source tree, as they're not needed (MySQL covers `BDB` in its own documentation).
- Fixed MIT-pthreads to compile with `glibc` 2.2 (needed for `make dist`).
- Fixed the `FLOAT(X+1,X)` is not converted to `FLOAT(X+2,X)`. (This also affected `DECIMAL`, `DOUBLE` and `REAL` types)
- Fixed the result from `IF()` is case in-sensitive if the second and third arguments are case sensitive.
- Fixed core dump problem on OSF/1 in `gethostbyname_r`.
- Fixed that underflowed decimal fields are not zero filled.
- If we get an overflow when inserting `'+11111'` for `DECIMAL(5,0) UNSIGNED` columns, we will just drop the sign.
- Fixed optimisation bug with `ISNULL(expression_which_cannot_be_null)` and `ISNULL(constant_expression)`.
- Fixed host lookup bug in the `glibc` library that we used with the 3.23.50 Linux-x86 binaries.

### D.3.6 Changes in release 3.23.50 (21 Apr 2002)

- Fixed buffer overflow problem if someone specified a too long datadir parameter to mysqld
- Add missing `<row>` tags for `mysqldump` XML output.
- Fixed problem with `crash-me` and `gcc` 3.0.4.
- Fixed that `@@unknown_variable` doesn't hang server.
- Added `@@VERSION` as a synonym for `VERSION()`.
- `SHOW VARIABLES LIKE 'xxx'` is now case-insensitive.
- Fixed timeout for `GET_LOCK()` on HP-UX with DCE threads.
- Fixed memory allocation bug in the glibc library used to build Linux binaries, which caused mysqld to die in 'free()'.
- Fixed `SIGINT` and `SIGQUIT` problems in `mysql`.
- Fixed bug in character table converts when used with big ( > 64K) strings.
- `InnoDB` now retains foreign key constraints through `ALTER TABLE` and `CREATE/DROP INDEX`.
- `InnoDB` now allows foreign key constraints to be added through the `ALTER TABLE` syntax.
- `InnoDB` tables can now be set to automatically grow in size (autoextend).
- Our Linux RPMS and binaries are now compiled with `gcc` 3.0.4, which should make them a bit faster.
- Fixed some buffer overflow problems when reading startup parameters.
- Because of problems on shutdown we have now disabled named pipes on windows by default. One can enable named pipes by starting mysqld with `--enable-named-pipe`.
- Fixed bug when using `WHERE key_column = 'J' or key_column='j'`.
- Fixed core-dump bug when using `--log-bin` with `LOAD DATA INFILE` without an active database.
- Fixed bug in `RENAME TABLE` when used with `lower_case_table_names=1` (default on Windows).
- Fixed unlikely core-dump bug when using `DROP TABLE` on a table that was in use by a thread that also used queries on only temporary tables.
- Fixed problem with `SHOW CREATE TABLE` and `PRIMARY KEY` when using 32 indexes.
- Fixed that one can use `SET PASSWORD` for the anonymous user.
- Fixed core dump bug when reading client groups from option files using `mysql_options()`.
- Memory leak (16 bytes per every corrupted table) closed.
- Fixed binary builds to use `--enable-local-infile`.
- Update source to work with new version of `bison`.
- Updated shell scripts to now agree with new POSIX standard.
- Fixed bug where `DATE_FORMAT()` returned empty string when used with `GROUP BY`.

### D.3.7 Changes in release 3.23.49

- Don't give warning for a statement that is only a comment; this is needed for `mysqldump --disable-keys` to work.
- Fixed unlikely caching bug when doing a join without keys. In this case the last used field for a table always returned `NULL`.
- Added options to make `LOAD DATA LOCAL INFILE` more secure.
- MySQL binary release 3.23.48 for Linux contained a new `glibc` library, which has serious problems under high load and RedHat 7.2. The 3.23.49 binary release doesn't have this problem.
- Fixed shutdown problem on NT.

### D.3.8 Changes in release 3.23.48 (07 Feb 2002)

- Added `--xml` option to `mysqldump` for producing XML output.
- Changed to use `autoconf` 2.52 (from `autoconf` 2.13)
- Fixed bug in complicated join with `const` tables.
- Added internal safety checks for `InnoDB`.
- Some `InnoDB` variables were always shown in `SHOW VARIABLES` as `OFF` on high-byte-first systems (like SPARC).
- Fixed problem with one thread using an `InnoDB` table and another thread doing an `ALTER TABLE` on the same table. Before that, `mysqld` could crash with an assertion failure in ``row0row.c'`, line 474.
- Tuned the `InnoDB` SQL optimiser to favor index searches more often over table scans.
- Fixed a performance problem with `InnoDB` tables when several large `SELECT` queries are run concurrently on a multiprocessor Linux computer. Large CPU-bound `SELECT` queries will now also generally run faster on all platforms.
- If MySQL binlogging is used, `InnoDB` now prints after crash recovery the latest MySQL binlog name and the offset `InnoDB` was able to recover to. This is useful, for example, when resynchronising a master and a slave database in replication.
- Added better error messages to help in installation problems of `InnoDB` tables.
- It is now possible to recover MySQL temporary tables that have become orphaned inside the `InnoDB` tablespace.
- `InnoDB` now prevents a `FOREIGN KEY` declaration where the signedness is not the same in the referencing and referenced integer columns.
- Calling `SHOW CREATE TABLE` or `SHOW TABLE STATUS` could cause memory corruption and make `mysqld` crash. Especially at risk was `mysqldump`, because it frequently calls `SHOW CREATE TABLE`.
- If inserts to several tables containing an `AUTO_INCREMENT` column were wrapped inside one `LOCK TABLES`, `InnoDB` asserted in ``lock0lock.c'`.
- In 3.23.47 we allowed several `NULL` values in a `UNIQUE` secondary index for an `InnoDB` table. But `CHECK TABLE` was not relaxed: it reports the table as corrupt. `CHECK TABLE` no longer complains in this situation.
- `SHOW GRANTS` now shows `REFERENCES` instead of `REFERENCE`.

### D.3.9 Changes in release 3.23.47 (27 Dec 2001)

- Fixed bug when using the following construct: `SELECT ... WHERE key=@var_name OR key=@var_name2`
- Restrict `InnoDB` keys to 500 bytes.
- `InnoDB` now supports `NULL` in keys.
- Fixed shutdown problem on HP-UX. (Introduced in 3.23.46)
- Fixed core dump bug in replication when using `SELECT RELEASE_LOCK()`.
- Added new command: `DO expression,[expression]`
- Added `slave-skip-errors` option.
- Added statistics variables for all MySQL commands. (`SHOW STATUS` is now much longer.)
- Fixed default values for `InnoDB` tables.
- Fixed that `GROUP BY expr DESC` works.
- Fixed bug when using `t1 LEFT JOIN t2 ON t2.key=constant`.
- `mysql_config` now also works with binary (relocated) distributions.

### D.3.10 Changes in release 3.23.46 (29 Nov 2001)

- Fixed problem with aliased temporary table replication.
- `InnoDB` and `BDB` tables will now use index when doing an `ORDER BY` on the whole table.
- Fixed bug where one got an empty set instead of a DEADLOCK error when using `BDB` tables.
- One can now kill `ANALYZE`, `REPAIR`, and `OPTIMIZE TABLE` when the thread is waiting to get a lock on the table.
- Fixed race condition in `ANALYZE TABLE`.
- Fixed bug when joining with caching (unlikely to happen).
- Fixed race condition when using the binary log and `INSERT DELAYED` which could cause the binary log to have rows that were not yet written to `MyISAM` tables.
- Changed caching of binary log to make replication slightly faster.
- Fixed bug in replication on Mac OS X.

### D.3.11 Changes in release 3.23.45 (22 Nov 2001)

- `(UPDATE|DELETE) ...WHERE MATCH` bugfix.
- shutdown should now work on Darwin (Mac OS X).
- Fixed core dump when repairing corrupted packed `MyISAM` files.
- `--core-file` now works on Solaris.
- Fix a bug which could cause `InnoDB` to complain if it cannot find free blocks from the buffer cache during recovery.
- Fixed bug in `InnoDB` insert buffer B-tree handling that could cause crashes.
- Fixed bug in `InnoDB` lock timeout handling.
- Fixed core dump bug in `ALTER TABLE` on a `TEMPORARY` `InnoDB` table.
- Fixed bug in `OPTIMIZE TABLE` that reset index cardinality if it was up to date.
- Fixed problem with `t1 LEFT_JOIN t2 ... WHERE t2.date_column IS NULL` when date_column was declared as `NOT NULL`.
- Fixed bug with `BDB` tables and keys on `BLOB` columns.
- Fixed bug in `MERGE` tables on OS with 32-bit file pointers.
- Fixed bug in `TIME_TO_SEC()` when using negative values.

### D.3.12 Changes in release 3.23.44 (31 Oct 2001)

- Fixed `Rows_examined` count in slow query log.
- Fixed bug when using a reference to an `AVG()` column in `HAVING`.
- Fixed that date functions that require correct dates, like `DAYOFYEAR(column)`, will return `NULL` for `0000-00-00` dates.
- Fixed bug in const-propagation when comparing columns of different types. (`SELECT * FROM date_col="2001-01-01" and date_col=time_col`)
- Fixed bug that caused error message `Can't write, because of unique constraint` with some `GROUP BY` queries.
- Fixed problem with `sjis` character strings used within quoted table names.
- Fixed core dump when using `CREATE ... FULLTEXT` keys with other storage engines than `MyISAM`.
- Don't use `signal()` on Windows because this appears to not be 100% reliable.
- Fixed bug when doing `WHERE col_name=NULL` on an indexed column that had `NULL` values.
- Fixed bug when doing `LEFT JOIN ... ON (col_name = constant) WHERE col_name = constant`.
- When using replications, aborted queries that contained `%` could cause a core dump.
- `TCP_NODELAY` was not used on some systems. (Speed problem.)
- Applied portability fixes for OS/2. (Patch by Yuri Dario.)

The following changes are for `InnoDB` tables:

- Add missing `InnoDB` variables to `SHOW VARIABLES`.
- Foreign keys checking is now done for `InnoDB` tables.
- `DROP DATABASE` now works also for `InnoDB` tables.
- `InnoDB` now supports datafiles and raw disk partitions bigger than 4 GB on those operating systems that have big files.
- `InnoDB` calculates better table cardinality estimates for the MySQL optimiser.
- Accent characters in the default character set `latin1` are ordered according to the MySQL ordering. Note: if you are using `latin1` and have inserted characters whose code is greater than 127 into an indexed `CHAR` column, you should run `CHECK TABLE` on your table when you upgrade to 3.23.44, and drop and reimport the table if `CHECK TABLE` reports an error!
- A new ``my.cnf'` parameter, `innodb_thread_concurrency`, helps in performance tuning in heavily concurrent environments.
- A new ``my.cnf'` parameter, `innodb_fast_shutdown`, speeds up server shutdown.
- A new ``my.cnf'` parameter, `innodb_force_recovery`, helps to save your data in case the disk image of the database becomes corrupt.
- `innodb_monitor` has been improved and a new `innodb_table_monitor` added.
- Increased maximum key length from 500 to 7000 bytes.
- Fixed a bug in replication of `AUTO_INCREMENT` columns with multiple-line inserts.
- Fixed a bug when the case of letters changes in an update of an indexed secondary column.
- Fixed a hang when there are > 24 datafiles.
- Fixed a crash when `MAX(col)` is selected from an empty table, and `col` is not the first column in a multi-column index.
- Fixed a bug in purge which could cause crashes.

### D.3.13 Changes in release 3.23.43 (04 Oct 2001)

- Fixed a bug in `INSERT DELAYED` and `FLUSH TABLES` introduced in 3.23.42.
- Fixed unlikely bug, which returned non-matching rows, in `SELECT` with many tables and multi-column indexes and 'range' type.
- Fixed an unlikely core dump bug when doing `EXPLAIN SELECT` when using many tables and `ORDER BY`.
- Fixed bug in `LOAD DATA FROM MASTER` when using table with `CHECKSUM=1`.
- Added unique error message when one gets a DEADLOCK during a transaction with `BDB` tables.
- Fixed problem with `BDB` tables and `UNIQUE` columns defined as `NULL`.
- Fixed problem with `myisampack` when using pre-space filled `CHAR` columns.
- Applied patch from Yuri Dario for OS/2.
- Fixed bug in `--safe-user-create`.

### D.3.14 Changes in release 3.23.42 (08 Sep 2001)

- Fixed problem when using `LOCK TABLES` and `BDB` tables.
- Fixed problem with `REPAIR TABLE` on `MyISAM` tables with row lengths in the range from 65517 to 65520 bytes.
- Fixed rare hang when doing `mysqladmin shutdown` when there was a lot of activity in other threads.
- Fixed problem with `INSERT DELAYED` where delay thread could be hanging on `upgrading locks` with no apparent reason.
- Fixed problem with `myisampack` and `BLOB`.
- Fixed problem when one edited ``.MRG'` tables by hand. (Patch from Benjamin Pflugmann).
- Enforce that all tables in a `MERGE` table come from the same database.
- Fixed bug with `LOAD DATA INFILE` and transactional tables.
- Fix bug when using `INSERT DELAYED` with wrong column definition.
- Fixed core dump during `REPAIR` of some particularly broken tables.
- Fixed bug in `InnoDB` and `AUTO_INCREMENT` columns.
- Fixed bug in `InnoDB` and `RENAME TABLE` columns.
- Fixed critical bug in `InnoDB` and `BLOB` columns. If you have used `BLOB` columns larger than 8000 bytes in an `InnoDB` table, it is necessary to dump the table with `mysqldump`, drop it and restore it from the dump.
- Applied large patch for OS/2 from Yuri Dario.
- Fixed problem with `InnoDB` when one could get the error `Can't execute the given command...` even when no transaction was active.
- Applied some minor fixes that concern Gemini.
- Use real arithmetic operations even in integer context if not all arguments are integers. (Fixes uncommon bug in some integer contexts).
- Don't force everything to lowercase on Windows. (To fix problem with Windows and `ALTER TABLE`). Now `--lower_case_names` also works on Unix.
- Fixed that automatic rollback is done when thread end doesn't lock other threads.

### D.3.15 Changes in release 3.23.41 (11 Aug 2001)

- Added `--sql-mode=option[,option[,option]]` option to `mysqld`. See section 4.1.1 `mysqld` Command-line Options.
- Fixed possible problem with `shutdown` on Solaris where the ``.pid'` file wasn't deleted.
- `InnoDB` now supports < 4 GB rows. The former limit was 8000 bytes.
- The `doublewrite` file flush method is used in `InnoDB`. It reduces the need for Unix `fsync()` calls to a fraction and improves performance on most Unix flavors.
- You can now use the `InnoDB` Monitor to print a lot of `InnoDB` state information, including locks, to the standard output. This is useful in performance tuning.
- Several bugs which could cause hangs in `InnoDB` have been fixed.
- Split `record_buffer` to `record_buffer` and `record_rnd_buffer`. To make things compatible to previous MySQL versions, if `record_rnd_buffer` is not set, then it takes the value of `record_buffer`.
- Fixed optimising bug in `ORDER BY` where some `ORDER BY` parts where wrongly removed.
- Fixed overflow bug with `ALTER TABLE` and `MERGE` tables.
- Added prototypes for `my_thread_init()` and `my_thread_end()` to ``mysql_com.h'`
- Added `--safe-user-create` option to `mysqld`.
- Fixed bug in `SELECT DISTINCT ... HAVING` that caused error message `Can't find record in #...`

### D.3.16 Changes in release 3.23.40

- Fixed problem with `--low-priority-updates` and `INSERT` statements.
- Fixed bug in slave thread when under some rare circumstances it could get 22 bytes ahead on the offset in the master.
- Added `slave_net_timeout` for replication.
- Fixed problem with `UPDATE` and `BDB` tables.
- Fixed hard bug in `BDB` tables when using key parts.
- Fixed problem when using `GRANT FILE ON database.* ...`; previously we added the `DROP` privilege for the database.
- Fixed `DELETE FROM tbl_name ... LIMIT 0` and `UPDATE FROM tbl_name ... LIMIT 0`, which acted as though the `LIMIT` clause was not present (they deleted or updated all selected rows).
- `CHECK TABLE` now checks if an `AUTO_INCREMENT` column contains the value 0.
- Sending a `SIGHUP` to `mysqld` will now only flush the logs, not reset the replication.
- Fixed parser to allow floats of type `1.0e1` (no sign after `e`).
- Option `--force` to `myisamchk` now also updates states.
- Added option `--warnings` to `mysqld`. Now `mysqld` prints the error `Aborted connection` only if this option is used.
- Fixed problem with `SHOW CREATE TABLE` when you didn't have a `PRIMARY KEY`.
- Properly fixed the rename of `innodb_unix_file_flush_method` variable to `innodb_flush_method`.
- Fixed bug when converting `BIGINT UNSIGNED` to `DOUBLE`. This caused a problem when doing comparisons with `BIGINT` values outside of the signed range.
- Fixed bug in `BDB` tables when querying empty tables.
- Fixed a bug when using `COUNT(DISTINCT)` with `LEFT JOIN` and there weren't any matching rows.
- Removed all documentation referring to the `GEMINI` table type. `GEMINI` is not released under an `Open Source` license.

### D.3.17 Changes in release 3.23.39 (12 Jun 2001)

- The `AUTO_INCREMENT` sequence wasn't reset when dropping and adding an `AUTO_INCREMENT` column.
- `CREATE ... SELECT` now creates non-unique indexes delayed.
- Fixed problem where `LOCK TABLES tbl_name READ` followed by `FLUSH TABLES` put an exclusive lock on the table.
- `REAL @variable` values were represented with only 2 digits when converted to strings.
- Fixed problem that client ``hung'' when `LOAD TABLE FROM MASTER` failed.
- `myisamchk --fast --force` will no longer repair tables that only had the open count wrong.
- Added functions to handle symbolic links to make life easier in 4.0.
- We are now using the `-lcma` thread library on HP-UX 10.20 so that MySQL will be more stable on HP-UX.
- Fixed problem with `IF()` and number of decimals in the result.
- Fixed date-part extraction functions to work with dates where day and/or month is 0.
- Extended argument length in option files from 256 to 512 chars.
- Fixed problem with shutdown when `INSERT DELAYED` was waiting for a `LOCK TABLE`.
- Fixed core dump bug in `InnoDB` when tablespace was full.
- Fixed problem with `MERGE` tables and big tables (> 4G) when using `ORDER BY`.

### D.3.18 Changes in release 3.23.38 (09 May 2001)

- Fixed a bug when `SELECT` from `MERGE` table sometimes results in incorrectly ordered rows.
- Fixed a bug in `REPLACE()` when using the `ujis` character set.
- Applied Sleepycat `BDB` patches 3.2.9.1 and 3.2.9.2.
- Added `--skip-stack-trace` option to `mysqld`.
- `CREATE TEMPORARY` now works with `InnoDB` tables.
- `InnoDB` now promotes sub keys to whole keys.
- Added option `CONCURRENT` to `LOAD DATA`.
- Better error message when slave `max_allowed_packet` is too low to read a very long log event from the master.
- Fixed bug when too many rows where removed when using `SELECT DISTINCT ... HAVING`.
- `SHOW CREATE TABLE` now returns `TEMPORARY` for temporary tables.
- Added `Rows_examined` to slow query log.
- Fixed problems with function returning empty string when used together with a group function and a `WHERE` that didn't match any rows.
- New program `mysqlcheck`.
- Added database name to output for administrative commands like `CHECK`, `REPAIR`, `OPTIMIZE`.
- Lots of portability fixes for `InnoDB`.
- Changed optimiser so that queries like `SELECT * FROM tbl_name,tbl_name2 ... ORDER BY key_part1 LIMIT #` will use index on `key_part1` instead of `filesort`.
- Fixed bug when doing `LOCK TABLE to_table WRITE,...; INSERT INTO to_table... SELECT ...` when `to_table` was empty.
- Fixed bug with `LOCK TABLE` and `BDB` tables.

### D.3.19 Changes in release 3.23.37 (17 Apr 2001)

- Fixed a bug when using `MATCH()` in `HAVING` clause.
- Fixed a bug when using `HEAP` tables with `LIKE`.
- Added `--mysql-version` option to `safe_mysqld`
- Changed `INNOBASE` to `InnoDB` (because the `INNOBASE` name was already used). All `configure` options and `mysqld` start options now use `innodb` instead of `innobase`. This means that before upgrading to this version, you have to change any configuration files where you have used `innobase` options!
- Fixed bug when using indexes on `CHAR(255) NULL` columns.
- Slave thread will now be started even if `master-host` is not set, as long as `server-id` is set and valid ``master.info'` is present.
- Partial updates (terminated with kill) are now logged with a special error code to the binary log. Slave will refuse to execute them if the error code indicates the update was terminated abnormally, and will have to be recovered with `SET SQL_SLAVE_SKIP_COUNTER=1; SLAVE START` after a manual sanity check/correction of data integrity.
- Fixed bug that erroneously logged a drop of internal temporary table on thread termination to the binary log -- this bug affected replication.
- Fixed a bug in `REGEXP` on 64-bit machines.
- `UPDATE` and `DELETE` with `WHERE unique_key_part IS NULL` didn't update/delete all rows.
- Disabled `INSERT DELAYED` for tables that support transactions.
- Fixed bug when using date functions on `TEXT`/`BLOB` column with wrong date format.
- UDFs now also work on Windows. (Patch by Ralph Mason.)
- Fixed bug in `ALTER TABLE` and `LOAD DATA INFILE` that disabled key-sorting. These commands should now be faster in most cases.
- Fixed performance bug where reopened tables (tables that had been waiting for `FLUSH` or `REPAIR`) would not use indexes for the next query.
- Fixed problem with `ALTER TABLE` to `InnoDB` tables on FreeBSD.
- Added `mysqld` variables `myisam_max_sort_file_size` and `myisam_max_extra_sort_file_size`.
- Initialise signals early to avoid problem with signals in `InnoDB`.
- Applied patch for the `tis620` character set to make comparisons case-independent and to fix a bug in `LIKE` for this character set. Note: All tables that uses the `tis620` character set must be fixed with `myisamchk -r` or `REPAIR TABLE` !
- Added `--skip-safemalloc` option to `mysqld`.

### D.3.20 Changes in release 3.23.36 (27 Mar 2001)

- Fixed a bug that allowed use of database names containing a ``.'` character. This fixes a serious security issue when `mysqld` is run as root.
- Fixed bug when thread creation failed (could happen when doing a lot of connections in a short time).
- Fixed some problems with `FLUSH TABLES` and `TEMPORARY` tables. (Problem with freeing the key cache and error `Can't reopen table...`.)
- Fixed a problem in `InnoDB` with other character sets than `latin1` and another problem when using many columns.
- Fixed bug that caused a core dump when using a very complex query involving `DISTINCT` and summary functions.
- Added `SET TRANSACTION ISOLATION LEVEL ...`
- Added `SELECT ... FOR UPDATE`.
- Fixed bug where the number of affected rows was not returned when MySQL was compiled without transaction support.
- Fixed a bug in `UPDATE` where keys weren't always used to find the rows to be updated.
- Fixed a bug in `CONCAT_WS()` where it returned incorrect results.
- Changed `CREATE ... SELECT` and `INSERT ... SELECT` to not allow concurrent inserts as this could make the binary log hard to repeat. (Concurrent inserts are enabled if you are not using the binary or update log.)
- Changed some macros to be able to use fast mutex with `glibc` 2.2.

### D.3.21 Changes in release 3.23.35 (15 Mar 2001)

- Fixed newly introduced bug in `ORDER BY`.
- Fixed wrong define `CLIENT_TRANSACTIONS`.
- Fixed bug in `SHOW VARIABLES` when using `INNOBASE` tables.
- Setting and using user variables in `SELECT DISTINCT` didn't work.
- Tuned `SHOW ANALYZE` for small tables.
- Fixed handling of arguments in the benchmark script `run-all-tests`.

### D.3.22 Changes in release 3.23.34a

- Added extra files to the distribution to allow `INNOBASE` support to be compiled.

### D.3.23 Changes in release 3.23.34 (10 Mar 2001)

- Added the `INNOBASE` storage engine and the `BDB` storage engine to the MySQL source distribution.
- Updated the documentation about `GEMINI` tables.
- Fixed a bug in `INSERT DELAYED` that caused threads to hang when inserting `NULL` into an `AUTO_INCREMENT` column.
- Fixed a bug in `CHECK TABLE` / `REPAIR TABLE` that could cause a thread to hang.
- `REPLACE` will not replace a row that conflicts with an `AUTO_INCREMENT` generated key.
- `mysqld` now only sets `CLIENT_TRANSACTIONS` in `mysql->server_capabilities` if the server supports a transaction-safe storage engine.
- Fixed `LOAD DATA INFILE` to allow numeric values to be read into `ENUM` and `SET` columns.
- Improved error diagnostic for slave thread exit.
- Fixed bug in `ALTER TABLE ... ORDER BY`.
- Added `max_user_connections` variable to `mysqld`.
- Limit query length for replication by `max_allowed_packet`, not the arbitrary limit of 4 MB.
- Allow space around `=` in argument to `--set-variable`.
- Fixed problem in automatic repair that could leave some threads in state `Waiting for table`.
- `SHOW CREATE TABLE` now displays the `UNION()` for `MERGE` tables.
- `ALTER TABLE` now remembers the old `UNION()` definition.
- Fixed bug when replicating timestamps.
- Fixed bug in bidirectional replication.
- Fixed bug in the `BDB` storage engine that occurred when using an index on multi-part key where a key part may be `NULL`.
- Fixed `MAX()` optimisation on sub-key for `BDB` tables.
- Fixed problem where garbage results were returned when using `BDB` tables and `BLOB` or `TEXT` fields when joining many tables.
- Fixed a problem with `BDB` tables and `TEXT` columns.
- Fixed bug when using a `BLOB` key where a const row wasn't found.
- Fixed that `mysqlbinlog` writes the timestamp value for each query. This ensures that one gets same values for date functions like `NOW()` when using `mysqlbinlog` to pipe the queries to another server.
- Allow `--skip-gemini`, `--skip-bdb`, and `--skip-innodb` options to be specified when invoking `mysqld`, even if these storage engines are not compiled in to `mysqld`.
- One can now do `GROUP BY ... DESC`.
- Fixed a deadlock in the `SET` code, when one ran `SET @foo=bar`, where `bar` is a column reference, an error was not properly generated.

### D.3.24 Changes in release 3.23.33 (09 Feb 2001)

- Fixed DNS lookups not to use the same mutex as the hostname cache. This will enable known hosts to be quickly resolved even if a DNS lookup takes a long time.
- Added `--character-sets-dir` option to `myisampack`.
- Removed warnings when running `REPAIR TABLE ... EXTENDED`.
- Fixed a bug that caused a core dump when using `GROUP BY` on an alias, where the alias was the same as an existing column name.
- Added `SEQUENCE()` as an example UDF function.
- Changed `mysql_install_db` to use `BINARY` for `CHAR` columns in the privilege tables.
- Changed `TRUNCATE tbl_name` to `TRUNCATE TABLE tbl_name` to use the same syntax as Oracle. Until 4.0 we will also allow `TRUNCATE tbl_name` to not crash old code.
- Fixed ``no found rows'' bug in `MyISAM` tables when a `BLOB` was first part of a multi-part key.
- Fixed bug where `CASE` didn't work with `GROUP BY`.
- Added `--sort-recover` option to `myisamchk`.
- `myisamchk -S` and `OPTIMIZE TABLE` now work on Windows.
- Fixed bug when using `DISTINCT` on results from functions that referred to a group function, like:

      SELECT a, DISTINCT SEC_TO_TIME(SUM(a))
      FROM tbl_name GROUP BY a, b;

- Fixed buffer overrun in `libmysqlclient` library. Fixed bug in handling `STOP` event after `ROTATE` event in replication.
- Fixed another buffer overrun in `DROP DATABASE`.
- Added `Table_locks_immediate` and `Table_locks_waited` status variables.
- Fixed bug in replication that broke slave server start with existing ``master.info'`. This fixes a bug introduced in 3.23.32.
- Added `SET SQL_SLAVE_SKIP_COUNTER=n` command to recover from replication glitches without a full database copy.
- Added `max_binlog_size` variable; the binary log will be rotated automatically when the size crosses the limit.
- Added `Last_error`, `Last_errno`, and `Slave_skip_counter` variables to `SHOW SLAVE STATUS`.
- Fixed bug in `MASTER_POS_WAIT()` function.
- Execute core dump handler on `SIGILL`, and `SIGBUS` in addition to `SIGSEGV`.
- On x86 Linux, print the current query and thread (connection) id, if available, in the core dump handler.
- Fixed several timing bugs in the test suite.
- Extended `mysqltest` to take care of the timing issues in the test suite.
- `ALTER TABLE` can now be used to change the definition for a `MERGE` table.
- Fixed creation of `MERGE` tables on Windows.
- Portability fixes for OpenBSD and OS/2.
- Added `--temp-pool` option to `mysqld`. Using this option will cause most temporary files created to use a small set of names, rather than a unique name for each new file. This is to work around a problem in the Linux kernel dealing with creating a bunch of new files with different names. With the old behaviour, Linux seems to "leak" memory, as it's being allocated to the directory entry cache instead of the disk cache.

### D.3.25 Changes in release 3.23.32 (22 Jan 2001: Stable)

- Changed code to get around compiler bug in Compaq C++ on OSF/1, that broke `BACKUP`, `RESTORE`, `CHECK`, `REPAIR`, and `ANALYZE TABLE`.
- Added option `FULL` to `SHOW COLUMNS`. Now we show the privilege list for the columns only if this option is given.
- Fixed bug in `SHOW LOGS` when there weren't any `BDB` logs.
- Fixed a timing problem in replication that could delay sending an update to the client until a new update was done.
- Don't convert field names when using `mysql_list_fields()`. This is to keep this code compatible with `SHOW FIELDS`.
- `MERGE` tables didn't work on Windows.
- Fixed problem with `SET PASSWORD=...` on Windows.
- Added missing ``my_config.h'` to RPM distribution.
- `TRIM("foo" from "foo")` didn't return an empty string.
- Added `--with-version-suffix` option to `configure`.
- Fixed core dump when client aborted connection without `mysql_close()`.
- Fixed a bug in `RESTORE TABLE` when trying to restore from a non-existent directory.
- Fixed a bug which caused a core dump on the slave when replicating `SET PASSWORD`.
- Added `MASTER_POS_WAIT()`.

### D.3.26 Changes in release 3.23.31 (17 Jan 2001)

- The test suite now tests all reachable `BDB` interface code. During testing we found and fixed many errors in the interface code.
- Using `HAVING` on an empty table could produce one result row when it shouldn't.
- Fixed the MySQL RPM not to depend on Perl5 anymore.
- Fixed some problems with `HEAP` tables on Windows.
- `SHOW TABLE STATUS` didn't show correct average row length for tables larger than 4G.
- `CHECK TABLE ... EXTENDED` didn't check row links for fixed size tables.
- Added option `MEDIUM` to `CHECK TABLE`.
- Fixed problem when using `DECIMAL()` keys on negative numbers.
- `HOUR()` (and some other `TIME` functions) on a `CHAR` column always returned `NULL`.
- Fixed security bug in something (please upgrade if you are using an earlier MySQL 3.23 version).
- Fixed buffer overflow bug when writing a certain error message.
- Added usage of `setrlimit()` on Linux to get `-O --open-files-limit=#` to work on Linux.
- Added `bdb_version` variable to `mysqld`.
- Fixed bug when using expression of type:

      SELECT ... FROM t1 LEFT JOIN t2 ON (t1.a=t2.a) WHERE t1.a=t2.a

  In this case the test in the `WHERE` clause was wrongly optimised away.
- Fixed bug in `MyISAM` when deleting keys with possible `NULL` values, but the first key-column was not a prefix-compressed text column.
- Fixed `mysql.server` to read the `[mysql.server]` option file group rather than the `[mysql_server]` group.
- Fixed `safe_mysqld` and `mysql.server` to also read the `server` option section.
- Added `Threads_created` status variable to `mysqld`.

### D.3.27 Changes in release 3.23.30 (04 Jan 2001)

- Added `SHOW OPEN TABLES` command.
- Fixed that `myisamdump` works against old `mysqld` servers.
- Fixed `myisamchk -k#` so that it works again.
- Fixed a problem with replication when the binary log file went over 2G on 32-bit systems.
- `LOCK TABLES` will now automatically start a new transaction.
- Changed `BDB` tables to not use internal subtransactions and reuse open files to get more speed.
- Added `--mysqld=#` option to `safe_mysqld`.
- Allow hex constants in the `--fields-*-by` and `--lines-terminated-by` options to `mysqldump` and `mysqlimport`. By Paul DuBois.
- Added `--safe-show-database` option to `mysqld`.
- Added `have_bdb`, `have_gemini`, `have_innobase`, `have_raid` and `have_openssl` to `SHOW VARIABLES` to make it easy to test for supported extensions.
- Added `--open-files-limit` option to `mysqld`.
- Changed `--open-files` option to `--open-files-limit` in `safe_mysqld`.
- Fixed a bug where some rows were not found with `HEAP` tables that had many keys.
- Fixed that `--bdb-no-sync` works.
- Changed `--bdb-recover` to `--bdb-no-recover` as recover should be on by default.
- Changed the default number of `BDB` locks to 10000.
- Fixed a bug from 3.23.29 when allocating the shared structure needed for `BDB` tables.
- Changed `mysqld_multi.sh` to use configure variables. Patch by Christopher McCrory.
- Added fixing of include files for Solaris 2.8.
- Fixed bug with `--skip-networking` on Debian Linux.
- Fixed problem that some temporary files where reported as having the name `UNOPENED` in error messages.
- Fixed bug when running two simultaneous `SHOW LOGS` queries.

### D.3.28 Changes in release 3.23.29 (16 Dec 2000)

- Configure updates for Tru64, large file support, and better TCP wrapper support. By Albert Chin-A-Young.
- Fixed bug in `<=>` operator.
- Fixed bug in `REPLACE` with `BDB` tables.
- `LPAD()` and `RPAD()` will shorten the result string if it's longer than the length argument.
- Added `SHOW LOGS` command.
- Remove unused `BDB` logs on shutdown.
- When creating a table, put `PRIMARY` keys first, followed by `UNIQUE` keys.
- Fixed a bug in `UPDATE` involving multi-part keys where one specified all key parts both in the update and the `WHERE` part. In this case MySQL could try to update a record that didn't match the whole `WHERE` part.
- Changed drop table to first drop the tables and then the ``.frm'` file.
- Fixed a bug in the hostname cache which caused `mysqld` to report the hostname as `''` in some error messages.
- Fixed a bug with `HEAP` type tables; the variable `max_heap_table_size` wasn't used. Now either `MAX_ROWS` or `max_heap_table_size` can be used to limit the size of a `HEAP` type table.
- Changed the default server-id to 1 for masters and 2 for slaves to make it easier to use the binary log.
- Renamed `bdb_lock_max` variable to `bdb_max_lock`.
- Added support for `AUTO_INCREMENT` on sub-fields for `BDB` tables.
- Added `ANALYZE` of `BDB` tables.
- In `BDB` tables, we now store the number of rows; this helps to optimise queries when we need an approximation of the number of rows.
- If we get an error in a multi-row statement, we now only roll back the last statement, not the entire transaction.
- If you do a `ROLLBACK` when you have updated a non-transactional table you will get an error as a warning.
- Added `--bdb-shared-data` option to `mysqld`.
- Added `Slave_open_temp_tables` status variable to `mysqld`
- Added `binlog_cache_size` and `max_binlog_cache_size` variables to `mysqld`.
- `DROP TABLE`, `RENAME TABLE`, `CREATE INDEX` and `DROP INDEX` are now transaction endpoints.
- If you do a `DROP DATABASE` on a symbolically linked database, both the link and the original database is deleted.
- Fixed `DROP DATABASE` to work on OS/2.
- Fixed bug when doing a `SELECT DISTINCT ... table1 LEFT JOIN table2 ...` when `table2` was empty.
- Added `--abort-slave-event-count` and `--disconnect-slave-event-count` options to `mysqld` for debugging and testing of replication.
- Fixed replication of temporary tables. Handles everything except slave server restart.
- `SHOW KEYS` now shows whether key is `FULLTEXT`.
- New script `mysqld_multi`. See section 4.7.3 `mysqld_multi`, A Program for Managing Multiple MySQL Servers.
- Added new script, `mysql-multi.server.sh`. Thanks to Tim Bunce Tim.Bunce@ig.co.uk for modifying `mysql.server` to easily handle hosts running many `mysqld` processes.
- `safe_mysqld`, `mysql.server`, and `mysql_install_db` have been modified to use `mysql_print_defaults` instead of various hacks to read the ``my.cnf'` files. In addition, the handling of various paths has been made more consistent with how `mysqld` handles them by default.
- Automatically remove Berkeley DB transaction logs that no longer are in use.
- Fixed bug with several `FULLTEXT` indexes in one table.
- Added a warning if number of rows changes on `REPAIR`/`OPTIMIZE`.
- Applied patches for OS/2 by `Yuri Dario`.
- `FLUSH TABLES tbl_name` didn't always flush the index tree to disk properly.
- `--bootstrap` is now run in a separate thread. This fixes a problem that caused `mysql_install_db` to core dump on some Linux machines.
- Changed `mi_create()` to use less stack space.
- Fixed bug with optimiser trying to over-optimise `MATCH()` when used with `UNIQUE` key.
- Changed `crash-me` and the MySQL benchmarks to also work with FrontBase.
- Allow `RESTRICT` and `CASCADE` after `DROP TABLE` to make porting easier.
- Reset status variable which could cause problem if one used `--slow-log`.
- Added `connect_timeout` variable to `mysql` and `mysqladmin`.
- Added `connect-timeout` as an alias for `timeout` for option files read by `mysql_options()`.

### D.3.29 Changes in release 3.23.28 (22 Nov 2000: Gamma)

- Added new options `--pager[=...]`, `--no-pager`, `--tee=...` and `--no-tee` to the `mysql` client. The new corresponding interactive commands are `pager`, `nopager`, `tee` and `notee`. See section 4.8.2 `mysql`, The Command-line Tool, `mysql --help` and the interactive help for more information.
- Fixed crash when automatic repair of `MyISAM` table failed.
- Fixed a major performance bug in the table locking code when one constantly had a lot of `SELECT`, `UPDATE` and `INSERT` statements running. The symptom was that the `UPDATE` and `INSERT` queries were locked for a long time while new `SELECT` statements were executed before the updates.
- When reading `options_files` with `mysql_options()` the `return-found-rows` option was ignored.
- One can now specify `interactive-timeout` in the option file that is read by `mysql_options()`. This makes it possible to force programs that run for a long time (like `mysqlhotcopy`) to use the `interactive_timeout` time instead of the `wait_timeout` time.
- Added to the slow query log the time and the user name for each logged query. If you are using `--log-long-format` then also queries that do not use an index are logged, even if the query takes less than `long_query_time` seconds.
- Fixed a problem in `LEFT JOIN` which caused all columns in a reference table to be `NULL`.
- Fixed a problem when using `NATURAL JOIN` without keys.
- Fixed a bug when using a multi-part keys where the first part was of type `TEXT` or `BLOB`.
- `DROP` of temporary tables wasn't stored in the update/binary log.
- Fixed a bug where `SELECT DISTINCT * ... LIMIT #` only returned one row.
- Fixed a bug in the assembler code in `strstr()` for SPARC and cleaned up the ``global.h'` header file to avoid a problem with bad aliasing with the compiler submitted with RedHat 7.0. (Reported by Trond Eivind Glomsrød)
- The `--skip-networking` option now works properly on NT.
- Fixed a long outstanding bug in the `ISAM` tables when a row with a length of more than 65K was shortened by a single byte.
- Fixed a bug in `MyISAM` when running multiple updating processes on the same table.
- Allow one to use `FLUSH TABLE tbl_name`.
- Added `--replicate-ignore-table`, `--replicate-do-table`, `--replicate-wild-ignore-table`, and `--replicate-wild-do-table` options to `mysqld`.
- Changed all log files to use our own `IO_CACHE` mechanism instead of `FILE` to avoid OS problems when there are many files open.
- Added `--open-files` and `--timezone` options to `safe_mysqld`.
- Fixed a fatal bug in `CREATE TEMPORARY TABLE ... SELECT ...`.
- Fixed a problem with `CREATE TABLE ... SELECT NULL`.
- Added variables `large_file_support`,`net_read_timeout`, `net_write_timeout` and `query_buffer_size` to `SHOW VARIABLES`.
- Added status variables `created_tmp_files` and `sort_merge_passes` to `SHOW STATUS`.
- Fixed a bug where we didn't allow an index name after the `FOREIGN KEY` definition.
- Added `TRUNCATE table_name` as a synonym for `DELETE FROM table_name`.
- Fixed a bug in a `BDB` key compare function when comparing part keys.
- Added `bdb_lock_max` variable to `mysqld`.
- Added more tests to the benchmark suite.
- Fixed an overflow bug in the client code when using overly long database names.
- `mysql_connect()` now aborts on Linux if the server doesn't answer in `timeout` seconds.
- `SLAVE START` did not work if you started with `--skip-slave-start` and had not explicitly run `CHANGE MASTER TO`.
- Fixed the output of `SHOW MASTER STATUS` to be consistent with `SHOW SLAVE STATUS`. (It now has no directory in the log name.)
- Added `PURGE MASTER LOGS TO`.
- Added `SHOW MASTER LOGS`.
- Added `--safemalloc-mem-limit` option to `mysqld` to simulate memory shortage when compiled with the `--with-debug=full` option.
- Fixed several core dumps in out-of-memory conditions.
- `SHOW SLAVE STATUS` was using an uninitialised mutex if the slave had not been started yet.
- Fixed bug in `ELT()` and `MAKE_SET()` when the query used a temporary table.
- `CHANGE MASTER TO` without specifying `MASTER_LOG_POS` would set it to 0 instead of 4 and hit the magic number in the master binlog.
- `ALTER TABLE ... ORDER BY ...` syntax added. This will create the new table with the rows in a specific order.

### D.3.30 Changes in release 3.23.27 (24 Oct 2000)

- Fixed a bug where the automatic repair of `MyISAM` tables sometimes failed when the datafile was corrupt.
- Fixed a bug in `SHOW CREATE` when using `AUTO_INCREMENT` columns.
- Changed `BDB` tables to use new compare function in Berkeley DB 3.2.3.
- You can now use Unix sockets with MIT-pthreads.
- Added the `latin5` (turkish) character set.
- Small portability fixes.

### D.3.31 Changes in release 3.23.26 (18 Oct 2000)

- Renamed `FLUSH MASTER` and `FLUSH SLAVE` to `RESET MASTER` and `RESET SLAVE`.
- Fixed `<>` to work properly with `NULL`.
- Fixed a problem with `SUBSTRING_INDEX()` and `REPLACE()`. (Patch by Alexander Igonitchev)
- Fix `CREATE TEMPORARY TABLE IF NOT EXISTS` not to produce an error if the table exists.
- If you don't create a `PRIMARY KEY` in a `BDB` table, a hidden `PRIMARY KEY` will be created.
- Added read-only-key optimisation to `BDB` tables.
- `LEFT JOIN` in some cases preferred a full table scan when there was no `WHERE` clause.
- When using `--log-slow-queries`, don't count the time waiting for a lock.
- Fixed bug in lock code on Windows which could cause the key cache to report that the key file was crashed even if it was okay.
- Automatic repair of `MyISAM` tables if you start `mysqld` with `--myisam-recover`.
- Removed the `TYPE=` keyword from `CHECK` and `REPAIR`. Allow `CHECK` options to be combined. (You can still use `TYPE=`, but this usage is deprecated.)
- Fixed mutex bug in the binary replication log -- long update queries could be read only in part by the slave if it did it at the wrong time, which was not fatal, but resulted in a performance-degrading reconnect and a scary message in the error log.
- Changed the format of the binary log -- added magic number, server version, binlog version. Added server id and query error code for each query event.
- Replication thread from the slave now will kill all the stale threads from the same server.
- Long replication user names were not being handled properly.
- Added `--replicate-rewrite-db` option to `mysqld`.
- Added `--skip-slave-start` option to `mysqld`.
- Updates that generated an error code (such as `INSERT INTO foo(some_key) values (1),(1)`) erroneously terminated the slave thread.
- Added optimisation of queries where `DISTINCT` is only used on columns from some of the tables.
- Allow floating-point numbers where there is no sign after the exponent (like `1e1`).
- `SHOW GRANTS` didn't always show all column grants.
- Added `--default-extra-file=#` option to all MySQL clients.
- Columns referenced in `INSERT` statements now are initialised properly.
- `UPDATE` didn't always work when used with a range on a timestamp that was part of the key that was used to find rows.
- Fixed a bug in `FULLTEXT` index when inserting a `NULL` column.
- Changed to use `mkstemp()` instead of `tempnam()`. Based on a patch from John Jones.

### D.3.32 Changes in release 3.23.25 (29 Sep 2000)

- Fixed that `databasename` works as second argument to `mysqlhotcopy`.
- The values for the `UMASK` and `UMASK_DIR` environment variables now can be specified in octal by beginning the value with a zero.
- Added `RIGHT JOIN`. This makes `RIGHT` a reserved word.
- Added `@@IDENTITY` as a synonym for `LAST_INSERT_ID()`. (This is for MSSQL compatibility.)
- Fixed a bug in `myisamchk` and `REPAIR` when using `FULLTEXT` index.
- `LOAD DATA INFILE` now works with FIFOs. (Patch by Toni L. Harbaugh-Blackford.)
- `FLUSH LOGS` broke replication if you specified a log name with an explicit extension as the value of the `log-bin` option.
- Fixed a bug in `MyISAM` with packed multi-part keys.
- Fixed crash when using `CHECK TABLE` on Windows.
- Fixed a bug where `FULLTEXT` index always used the `koi8_ukr` character set.
- Fixed privilege checking for `CHECK TABLE`.
- The `MyISAM` repair/reindex code didn't use the `--tmpdir` option for its temporary files.
- Added `BACKUP TABLE` and `RESTORE TABLE`.
- Fixed core dump on `CHANGE MASTER TO` when the slave did not have the master to start with.
- Fixed incorrect `Time` in the processlist for `Connect` of the slave thread.
- The slave now logs when it connects to the master.
- Fixed a core dump bug when doing `FLUSH MASTER` if you didn't specify a filename argument to `--log-bin`.
- Added missing ``ha_berkeley.x'` files to the MySQL Windows distribution.
- Fixed some mutex bugs in the log code that could cause thread blocks if new log files couldn't be created.
- Added lock time and number of selected processed rows to slow query log.
- Added `--memlock` option to `mysqld` to lock `mysqld` in memory on systems with the `mlockall()` call (like in Solaris).
- `HEAP` tables didn't use keys properly. (Bug from 3.23.23.)
- Added better support for `MERGE` tables (keys, mapping, creation, documentation...). See section 7.2 `MERGE` Tables.
- Fixed bug in `mysqldump` from 3.23 which caused some `CHAR` columns not to be quoted.
- Merged `analyze`, `check`, `optimize` and repair code.
- `OPTIMIZE TABLE` is now mapped to `REPAIR` with statistics and sorting of the index tree. This means that for the moment it only works on `MyISAM` tables.
- Added a pre-alloced block to root_malloc to get fewer mallocs.
- Added a lot of new statistics variables.
- Fixed `ORDER BY` bug with `BDB` tables.
- Removed warning that `mysqld` couldn't remove the ``.pid'` file under Windows.
- Changed `--log-isam` to log `MyISAM` tables instead of isam tables.
- Fixed `CHECK TABLE` to work on Windows.
- Added file mutexes to make `pwrite()` safe on Windows.

### D.3.33 Changes in release 3.23.24 (08 Sep 2000)

- Added `created_tmp_disk_tables` variable to `mysqld`.
- To make it possible to reliably dump and restore tables with `TIMESTAMP(X)` columns, MySQL now reports columns with `X` other than 14 or 8 to be strings.
- Changed sort order for `latin1` as it was before MySQL Version 3.23.23. Any table that was created or modified with 3.23.22 must be repaired if it has `CHAR` columns that may contain characters with ASCII values greater than 128!
- Fixed small memory leak introduced from 3.23.22 when creating a temporary table.
- Fixed problem with `BDB` tables and reading on a unique (not primary) key.
- Restored the `win1251` character set (it's now only marked deprecated).

### D.3.34 Changes in release 3.23.23 (01 Sep 2000)

- Changed sort order for 'German'; all tables created with 'German' sortorder must be repaired with `REPAIR TABLE` or `myisamchk` before use!
- Added `--core-file` option to `mysqld` to get a core file on Linux if `mysqld` dies on the `SIGSEGV` signal.
- MySQL client `mysql` now starts with option `--no-named-commands` (`-g`) by default. This option can be disabled with `--enable-named-commands` (`-G`). This may cause incompatibility problems in some cases, for example, in SQL scripts that use named commands without a semicolon, etc. ! Long format commands still work from the first line.
- Fixed a problem when using many pending `DROP TABLE` statements at the same time.
- Optimiser didn't use keys properly when using `LEFT JOIN` on an empty table.
- Added shorter help text when invoking `mysqld` with incorrect options.
- Fixed non-fatal `free()` bug in `mysqlimport`.
- Fixed bug in `MyISAM` index handling of `DECIMAL`/`NUMERIC` keys.
- Fixed a bug in concurrent insert in `MyISAM` tables. In some contexts, usage of `MIN(key_part)` or `MAX(key_part)` returned an empty set.
- Updated `mysqlhotcopy` to use the new `FLUSH TABLES table_list` syntax. Only tables which are being backed up are flushed now.
- Changed behaviour of `--enable-thread-safe-client` so that both non-threaded (`-lmysqlclient`) and threaded (`-lmysqlclient_r`) libraries are built. Users who linked against a threaded `-lmysqlclient` will need to link against `-lmysqlclient_r` now.
- Added atomic `RENAME TABLE` command.
- Don't count `NULL` values in `COUNT(DISTINCT ...)`.
- Changed `ALTER TABLE`, `LOAD DATA INFILE` on empty tables and `INSERT ... SELECT ...` on empty tables to create non-unique indexes in a separate batch with sorting. This will make the above calls much faster when you have many indexes.
- `ALTER TABLE` now logs the first used insert_id correctly.
- Fixed crash when adding a default value to a `BLOB` column.
- Fixed a bug with `DATE_ADD/DATE_SUB` where it returned a datetime instead of a date.
- Fixed a problem with the thread cache which made some threads show up as `***DEAD***` in `SHOW PROCESSLIST`.
- Fixed a lock in our thr_rwlock code, which could make selects that run at the same time as concurrent inserts crash. This only affects systems that don't have the `pthread_rwlock_rdlock` code.
- When deleting rows with a non-unique key in a `HEAP` table, all rows weren't always deleted.
- Fixed bug in range optimiser for `HEAP` tables for searches on a part index.
- Fixed `SELECT` on part keys to work with `BDB` tables.
- Fixed `INSERT INTO bdb_table ... SELECT` to work with `BDB` tables.
- `CHECK TABLE` now updates key statistics for the table.
- `ANALYZE TABLE` will now only update tables that have been changed since the last `ANALYZE`. Note that this is a new feature and tables will not be marked to be analysed until they are updated in any way with 3.23.23 or newer. For older tables, you have to do `CHECK TABLE` to update the key distribution.
- Fixed some minor privilege problems with `CHECK`, `ANALYZE`, `REPAIR` and `SHOW CREATE` commands.
- Added `CHANGE MASTER TO` statement.
- Added `FAST`, `QUICK` `EXTENDED` check types to `CHECK TABLES`.
- Changed `myisamchk` so that `--fast` and `--check-only-changed` are also honored with `--sort-index` and `--analyze`.
- Fixed fatal bug in `LOAD TABLE FROM MASTER` that did not lock the table during index re-build.
- `LOAD DATA INFILE` broke replication if the database was excluded from replication.
- More variables in `SHOW SLAVE STATUS` and `SHOW MASTER STATUS`.
- `SLAVE STOP` now will not return until the slave thread actually exits.
- Full-text search via the `MATCH()` function and `FULLTEXT` index type (for `MyISAM` files). This makes `FULLTEXT` a reserved word.

### D.3.35 Changes in release 3.23.22 (31 Jul 2000)

- Fixed that `lex_hash.h` is created properly for each MySQL distribution.
- Fixed that `MASTER` and `COLLECTION` are not reserved words.
- The log generated by `--slow-query-log` didn't contain the whole queries.
- Fixed that open transactions in `BDB` tables are rolled back if the connection is closed unexpectedly.
- Added workaround for a bug in `gcc` 2.96 (intel) and `gcc` 2.9 (IA64) in `gen_lex_hash.c`.
- Fixed memory leak in the client library when using `host=` in the ``my.cnf'` file.
- Optimised functions that manipulate the hours/minutes/seconds.
- Fixed bug when comparing the result of `DATE_ADD()`/`DATE_SUB()` against a number.
- Changed the meaning of `-F, --fast` for `myisamchk`. Added `-C, --check-only-changed` option to `myisamchk`.
- Added `ANALYZE tbl_name` to update key statistics for tables.
- Changed binary items `0x...` to be regarded as integers by default.
- Fix for SCO and `SHOW PROCESSLIST`.
- Added `auto-rehash` on reconnect for the `mysql` client.
- Fixed a newly introduced bug in `MyISAM`, where the index file couldn't get bigger than 64M.
- Added `SHOW MASTER STATUS` and `SHOW SLAVE STATUS`.

### D.3.36 Changes in release 3.23.21

- Added `mysql_character_set_name()` function to the MySQL C API.
- Made the update log ASCII 0 safe.
- Added the `mysql_config` script.
- Fixed problem when using `<` or `>` with a char column that was only partly indexed.
- One would get a core dump if the log file was not readable by the MySQL user.
- Changed `mysqladmin` to use `CREATE DATABASE` and `DROP DATABASE` statements instead of the old deprecated API calls.
- Fixed `chown` warning in `safe_mysqld`.
- Fixed a bug in `ORDER BY` that was introduced in 3.23.19.
- Only optimise the `DELETE FROM tbl_name` to do a drop+create of the table if we are in `AUTOCOMMIT` mode (needed for `BDB` tables).
- Added extra checks to avoid index corruption when the `ISAM`/`MyISAM` index files get full during an `INSERT`/`UPDATE`.
- `myisamchk` didn't correctly update row checksum when used with `-ro` (this only gave a warning in subsequent runs).
- Fixed bug in `REPAIR TABLE` so that it works with tables without indexes.
- Fixed buffer overrun in `DROP DATABASE`.
- `LOAD TABLE FROM MASTER` is sufficiently bug-free to announce it as a feature.
- `MATCH` and `AGAINST` are now reserved words.

### D.3.37 Changes in release 3.23.20

- Fixed bug in 3.23.19; `DELETE FROM tbl_name` removed the ``.frm'` file.
- Added `SHOW CREATE TABLE`.

### D.3.38 Changes in release 3.23.19

- Changed copyright for all files to GPL for the server code and utilities and to LGPL for the client libraries.
- Fixed bug where all rows matching weren't updated on a `MyISAM` table when doing update based on key on a table with many keys and some key changed values.
- The Linux MySQL RPM's and binaries are now statically linked with a linuxthread version that has faster mutex handling when used with MySQL.
- `ORDER BY` can now use `REF` keys to find subsets of the rows that need to be sorted.
- Changed name of `print_defaults` program to `my_print_defaults` to avoid name confusion.
- Fixed `NULLIF()` to work according to ANSI SQL99.
- Added `net_read_timeout` and `net_write_timeout` as startup parameters to `mysqld`.
- Fixed bug that destroyed index when doing `myisamchk --sort-records` on a table with prefix compressed index.
- Added `pack_isam` and `myisampack` to the standard MySQL distribution.
- Added the syntax `BEGIN WORK` (the same as `BEGIN`).
- Fixed core dump bug when using `ORDER BY` on a `CONV()` expression.
- Added `LOAD TABLE FROM MASTER`.
- Added `FLUSH MASTER` and `FLUSH SLAVE`.
- Fixed big/little endian problem in the replication.

### D.3.39 Changes in release 3.23.18

- Fixed a problem from 3.23.17 when choosing character set on the client side.
- Added `FLUSH TABLES WITH READ LOCK` to make a global lock suitable for making a copy of MySQL datafiles.
- `CREATE TABLE ... SELECT ... PROCEDURE` now works.
- Internal temporary tables will now use compressed index when using `GROUP BY` on `VARCHAR/CHAR` columns.
- Fixed a problem when locking the same table with both a `READ` and a `WRITE` lock.
- Fixed problem with `myisamchk` and `RAID` tables.

### D.3.40 Changes in release 3.23.17

- Fixed a bug in `FIND_IN_SET()` when the first argument was `NULL`.
- Added table locks to Berkeley DB.
- Fixed a bug with `LEFT JOIN` and `ORDER BY` where the first table had only one matching row.
- Added 4 sample ``my.cnf'` example files in the ``support-files'` directory.
- Fixed `duplicated key` problem when doing big `GROUP BY` operations. (This bug was probably introduced in 3.23.15.)
- Changed syntax for `INNER JOIN` to match ANSI SQL.
- Added `NATURAL JOIN` syntax.
- A lot of fixes in the `BDB` interface.
- Added handling of `--no-defaults` and `--defaults-file` to `safe_mysqld.sh` and `mysql_install_db.sh`.
- Fixed bug in reading compressed tables with many threads.
- Fixed that `USE INDEX` works with `PRIMARY` keys.
- Added `BEGIN` statement to start a transaction in `AUTOCOMMIT` mode.
- Added support for symbolic links for Windows.
- Changed protocol to let client know if the server is in `AUTOCOMMIT` mode and if there is a pending transaction. If there is a pending transaction, the client library will give an error before reconnecting to the server to let the client know that the server did a rollback. The protocol is still backward-compatible with old clients.
- `KILL` now works on a thread that is locked on a 'write' to a dead client.
- Fixed memory leak in the replication slave thread.
- Added new `log-slave-updates` option to `mysqld`, to allow daisy-chaining the slaves.
- Fixed compile error on FreeBSD and other systems where `pthread_t` is not the same as `int`.
- Fixed master shutdown aborting the slave thread.
- Fixed a race condition in `INSERT DELAYED` code when doing `ALTER TABLE`.
- Added deadlock detection sanity checks to `INSERT DELAYED`.

### D.3.41 Changes in release 3.23.16

- Added `SLAVE START` and `SLAVE STOP` statements.
- Added `TYPE=QUICK` option to `CHECK` and to `REPAIR`.
- Fixed bug in `REPAIR TABLE` when the table was in use by other threads.
- Added a thread cache to make it possible to debug MySQL with `gdb` when one does a lot of reconnects. This will also improve systems where you can't use persistent connections.
- Lots of fixes in the Berkeley DB interface.
- `UPDATE IGNORE` will not abort if an update results in a `DUPLICATE_KEY` error.
- Put `CREATE TEMPORARY TABLE` commands in the update log.
- Fixed bug in handling of masked IP numbers in the privilege tables.
- Fixed bug with `delay_key_write` tables and `CHECK TABLE`.
- Added `replicate-do-db` and `replicate-ignore-db` options to `mysqld`, to restrict which databases get replicated.
- Added `SQL_LOG_BIN` option.

### D.3.42 Changes in release 3.23.15 (May 2000: Beta)

- To start `mysqld` as `root`, you must now use the `--user=root` option.
- Added interface to Berkeley DB. (This is not yet functional; play with it at your own risk!)
- Replication between master and slaves.
- Fixed bug that other threads could steal a lock when a thread had a lock on a table and did a `FLUSH TABLES` command.
- Added the `slow_launch_time` variable and the `Slow_launch_threads` status variable to `mysqld`. These can be examined with `mysqladmin variables` and `mysqladmin extended-status`.
- Added functions `INET_NTOA()` and `INET_ATON()`.
- The default type of `IF()` now depends on the second and third arguments and not only on the second argument.
- Fixed case when `myisamchk` could go into a loop when trying to repair a crashed table.
- Don't write `INSERT DELAYED` to update log if `SQL_LOG_UPDATE=0`.
- Fixed problem with `REPLACE` on `HEAP` tables.
- Added possible character sets and time zone to `SHOW VARIABLES` output.
- Fixed bug in locking code that could result in locking problems with concurrent inserts under high load.
- Fixed a problem with `DELETE` of many rows on a table with compressed keys where MySQL scanned the index to find the rows.
- Fixed problem with `CHECK` on table with deleted keyblocks.
- Fixed a bug in reconnect (at the client side) where it didn't free memory properly in some contexts.
- Fixed problems in update log when using `LAST_INSERT_ID()` to update a table with an `AUTO_INCREMENT` key.
- Added `NULLIF()` function.
- Fixed bug when using `LOAD DATA INFILE` on a table with `BLOB/TEXT` columns.
- Optimised `MyISAM` to be faster when inserting keys in sorted order.
- `EXPLAIN SELECT ...` now also prints out whether MySQL needs to create a temporary table or use file sorting when resolving the `SELECT`.
- Added optimisation to skip `ORDER BY` parts where the part is a constant expression in the `WHERE` part. Indexes can now be used even if the `ORDER BY` doesn't match the index exactly, as long as all the unused index parts and all the extra `ORDER BY` columns are constants in the `WHERE` clause. See section 5.4.3 How MySQL Uses Indexes.
- `UPDATE` and `DELETE` on a whole unique key in the `WHERE` part are now faster than before.
- Changed `RAID_CHUNKSIZE` to be in 1024-byte increments.
- Fixed core dump in `LOAD_FILE(NULL)`.

### D.3.43 Changes in release 3.23.14

- Added `mysql_real_escape_string()` function to the MySQL C API.
- Fixed a bug in `CONCAT()` where one of the arguments was a function that returned a modified argument.
- Fixed a critical bug in `myisamchk`, where it updated the header in the index file when one only checked the table. This confused the `mysqld` daemon if it updated the same table at the same time. Now the status in the index file is only updated if one uses `--update-state`. With older `myisamchk` versions you should use `--read-only` when only checking tables, if there is the slightest chance that the `mysqld` server is working on the table at the same time!
- Fixed that `DROP TABLE` is logged in the update log.
- Fixed problem when searching on `DECIMAL()` key field where the column data contained leading zeros.
- Fix bug in `myisamchk` when the `AUTO_INCREMENT` column isn't the first key.
- Allow `DATETIME` in ISO8601 format: 2000-03-12T12:00:00
- Dynamic character sets. A `mysqld` binary can now handle many different character sets (you can choose which when starting `mysqld`).
- Added command `REPAIR TABLE`.
- Added `mysql_thread_safe()` function to the MySQL C API.
- Added the `UMASK_DIR` environment variable.
- Added `CONNECTION_ID()` function to return the client connection thread ID.
- When using `=` on `BLOB` or `VARCHAR BINARY` keys, where only a part of the column was indexed, the whole column of the result row wasn't compared.
- Fix for `sjis` character set and `ORDER BY`.
- When running in ANSI mode, don't allow columns to be used that aren't in the `GROUP BY` part.

### D.3.44 Changes in release 3.23.13

- Fixed problem when doing locks on the same table more than 2 times in the same `LOCK TABLE` command; this fixed the problem one got when running the test-ATIS test with `--fast` or `--check-only-changed`.
- Added `SQL_BUFFER_RESULT` option to `SELECT`.
- Removed end space from double/float numbers in results from temporary tables.
- Added `CHECK TABLE` command.
- Added changes for `MyISAM` in 3.23.12 that didn't get into the source distribution because of CVS problems.
- Fixed bug so that `mysqladmin shutdown` will wait for the local server to close down.
- Fixed a possible endless loop when calculating timestamp.
- Added `print_defaults` program to the ``.rpm'` files. Removed `mysqlbug` from the client ``.rpm'` file.

### D.3.45 Changes in release 3.23.12 (07 Mar 2000)

- Fixed bug in `MyISAM` involving `REPLACE ... SELECT ...` which could give a corrupted table.
- Fixed bug in `myisamchk` where it incorrectly reset the `AUTO_INCREMENT` value.
- LOTS of patches for Linux Alpha. MySQL now appears to be relatively stable on Alpha.
- Changed `DISTINCT` on `HEAP` temporary tables to use hashed keys to quickly find duplicated rows. This mostly concerns queries of type `SELECT DISTINCT ... GROUP BY ...`. This fixes a problem where not all duplicates were removed in queries of the above type. In addition, the new code is MUCH faster.
- Added patches to make MySQL compile on Mac OS X.
- Added `IF NOT EXISTS` clause to `CREATE DATABASE`.
- Added `--all-databases` and `--databases` options to `mysqldump` to allow dumping of many databases at the same time.
- Fixed bug in compressed `DECIMAL()` index in `MyISAM` tables.
- Fixed bug when storing 0 into a timestamp.
- When doing `mysqladmin shutdown` on a local connection, `mysqladmin` now waits until the PID file is gone before terminating.
- Fixed core dump with some `COUNT(DISTINCT ...)` queries.
- Fixed that `myisamchk` works properly with RAID tables.
- Fixed problem with `LEFT JOIN` and `key_field IS NULL`.
- Fixed bug in `net_clear()` which could give the error `Aborted connection` in the MySQL clients.
- Added options `USE INDEX (key_list)` and `IGNORE INDEX (key_list)` as parameters in `SELECT`.
- `DELETE` and `RENAME` should now work on `RAID` tables.

### D.3.46 Changes in release 3.23.11

- Allow the `ALTER TABLE tbl_name ADD (field_list)` syntax.
- Fixed problem with optimiser that could sometimes use incorrect keys.
- Fixed that `GRANT/REVOKE ALL PRIVILEGES` doesn't affect `GRANT OPTION`.
- Removed extra ``)'` from the output of `SHOW GRANTS`.
- Fixed problem when storing numbers in timestamps.
- Fix problem with timezones that have half hour offsets.
- Allow the syntax `UNIQUE INDEX` in `CREATE` statements.
- `mysqlhotcopy` - fast online hot-backup utility for local MySQL databases. By Tim Bunce.
- New more secure `mysqlaccess`. Thanks to Steve Harvey for this.
- Added `--i-am-a-dummy` and `--safe-updates` options to `mysql`.
- Added `select_limit` and `max_join_size` variables to `mysql`.
- Added `SQL_MAX_JOIN_SIZE` and `SQL_SAFE_UPDATES` options.
- Added `READ LOCAL` lock that doesn't lock the table for concurrent inserts. (This is used by `mysqldump`.)
- Changed that `LOCK TABLES ... READ` doesn't anymore allow concurrent inserts.
- Added `--skip-delay-key-write` option to `mysqld`.
- Fixed security problem in the protocol regarding password checking.
- `_rowid` can now be used as an alias for an integer type unique indexed column.
- Added back blocking of `SIGPIPE` when compiling with `--thread-safe-clients` to make things safe for old clients.

### D.3.47 Changes in release 3.23.10

- Fixed bug in 3.23.9 where memory wasn't properly freed when using `LOCK TABLES`.

### D.3.48 Changes in release 3.23.9

- Fixed problem that affected queries that did arithmetic on group functions.
- Fixed problem with timestamps and `INSERT DELAYED`.
- Fixed that `date_col BETWEEN const_date AND const_date` works.
- Fixed problem when only changing a 0 to `NULL` in a table with `BLOB/TEXT` columns.
- Fixed bug in range optimiser when using many key parts and or on the middle key parts: `WHERE K1=1 and K3=2 and (K2=2 and K4=4 or K2=3 and K4=5)`
- Added `source` command to `mysql` to allow reading of batch files inside the `mysql` client. Original patch by Matthew Vanecek.
- Fixed critical problem with the `WITH GRANT OPTION` option.
- Don't give an unnecessary `GRANT` error when using tables from many databases in the same query.
- Added VIO wrapper (needed for SSL support; by Andrei Errapart and Tõnu Samuel).
- Fixed optimiser problem on `SELECT` when using many overlapping indexes. MySQL should now be able to choose keys even better when there are many keys to choose from.
- Changed optimiser to prefer a range key instead of a ref key when the range key can uses more columns than the ref key (which only can use columns with `=`). For example, the following type of queries should now be faster: `SELECT * from key_part_1=const and key_part_2 > const2`
- Fixed bug that a change of all `VARCHAR` columns to `CHAR` columns didn't change row type from dynamic to fixed.
- Disabled floating-point exceptions for FreeBSD to fix core dump when doing `SELECT FLOOR(POW(2,63))`.
- Renamed `mysqld` startup option from `--delay-key-write` to `--delay-key-write-for-all-tables`.
- Added `read-next-on-key` to `HEAP` tables. This should fix all problems with `HEAP` tables when using non-`UNIQUE` keys.
- Added option to print default arguments to all clients.
- Added `--log-slow-queries` option to `mysqld` to log all queries that take a long time to a separate log file with a time indicating how long the query took.
- Fixed core dump when doing `WHERE key_col=RAND(...)`.
- Fixed optimisation bug in `SELECT ... LEFT JOIN ... key_col IS NULL`, when `key_col` could contain `NULL` values.
- Fixed problem with 8-bit characters as separators in `LOAD DATA INFILE`.

### D.3.49 Changes in release 3.23.8 (02 Jan 2000)

- Fixed problem when handling indexfiles larger than 8G.
- Added latest patches to MIT-pthreads for NetBSD.
- Fixed problem with timezones that are < GMT - 11.
- Fixed a bug when deleting packed keys in `NISAM`.
- Fixed problem with `ISAM` when doing some `ORDER BY ... DESC` queries.
- Fixed bug when doing a join on a text key which didn't cover the whole key.
- Option `--delay-key-write` didn't enable delayed key writing.
- Fixed update of `TEXT` column which involved only case changes.
- Fixed that `INSERT DELAYED` doesn't update timestamps that are given.
- Added function `YEARWEEK()` and options `x`, `X`, `v` and `V` to `DATE_FORMAT()`.
- Fixed problem with `MAX(indexed_column)` and `HEAP` tables.
- Fixed problem with `BLOB NULL` keys and `LIKE "prefix%"`.
- Fixed problem with `MyISAM` and fixed-length rows < 5 bytes.
- Fixed problem that could cause MySQL to touch freed memory when doing very complicated `GROUP BY` queries.
- Fixed core dump if you got a crashed table where an `ENUM` field value was too big.

### D.3.50 Changes in release 3.23.7 (10 Dec 1999)

- Fixed workaround under Linux to avoid problems with `pthread_mutex_timedwait`, which is used with `INSERT DELAYED`. See section 2.6.1 Linux Notes (All Linux Versions).
- Fixed that one will get a 'disk full' error message if one gets disk full when doing sorting (instead of waiting until we got more disk space).
- Fixed a bug in `MyISAM` with keys > 250 characters.
- In `MyISAM` one can now do an `INSERT` at the same time as other threads are reading from the table.
- Added `max_write_lock_count` variable to `mysqld` to force a `READ` lock after a certain number of `WRITE` locks.
- Inverted flag `delay_key_write` on `show variables`.
- Renamed `concurrency` variable to `thread_concurrency`.
- The following functions are now multi-byte-safe: `LOCATE(substr,str)`, `POSITION(substr IN str)`, `LOCATE(substr,str,pos)`, `INSTR(str,substr)`, `LEFT(str,len)`, `RIGHT(str,len)`, `SUBSTRING(str,pos,len)`, `SUBSTRING(str FROM pos FOR len)`, `MID(str,pos,len)`, `SUBSTRING(str,pos)`, `SUBSTRING(str FROM pos)`, `SUBSTRING_INDEX(str,delim,count)`, `RTRIM(str)`, `TRIM([[BOTH | TRAILING] [remstr] FROM] str)`, `REPLACE(str,from_str,to_str)`, `REVERSE(str)`, `INSERT(str,pos,len,newstr)`, `LCASE(str)`, `LOWER(str)`, `UCASE(str)` and `UPPER(str)`; patch by Wei He.
- Fix core dump when releasing a lock from a non-existent table.
- Remove locks on tables before starting to remove duplicates.
- Added option `FULL` to `SHOW PROCESSLIST`.
- Added option `--verbose` to `mysqladmin`.
- Fixed problem when automatically converting `HEAP` to `MyISAM`.
- Fixed bug in `HEAP` tables when doing insert + delete + insert + scan the table.
- Fixed bugs on Alpha with `REPLACE()` and `LOAD DATA INFILE`.
- Added `interactive_timeout` variable to `mysqld`.
- Changed the argument to `mysql_data_seek()` from `ulong` to `ulonglong`.

### D.3.51 Changes in release 3.23.6

- Added `-O lower_case_table_names={0|1}` option to `mysqld` to allow users to force table names to lowercase.
- Added `SELECT ... INTO DUMPFILE`.
- Added `--ansi` option to `mysqld` to make some functions ANSI SQL compatible.
- Temporary table names now start with `#sql`.
- Added quoting of identifiers with ``` (`"` in `--ansi` mode).
- Changed to use `snprintf()` when printing floats to avoid some buffer overflows on FreeBSD.
- Made `FLOOR()` overflow safe on FreeBSD.
- Added `--quote-names` option to `mysqldump`.
- Fixed bug that one could make a part of a `PRIMARY KEY NOT NULL`.
- Fixed `encrypt()` to be thread-safe and not reuse buffer.
- Added `mysql_odbc_escape_string()` function to support big5 characters in MyODBC.
- Rewrote the storage engine to use classes. This introduces a lot of new code, but will make table handling faster and better.
- Added patch by Sasha for user-defined variables.
- Changed that `FLOAT` and `DOUBLE` (without any length modifiers) no longer are fixed decimal point numbers.
- Changed the meaning of `FLOAT(X)`: Now this is the same as `FLOAT` if `X` <= 24 and a `DOUBLE` if 24 < `X` <= 53.
- `DECIMAL(X)` is now an alias for `DECIMAL(X,0)` and `DECIMAL` is now an alias for `DECIMAL(10,0)`. The same goes for `NUMERIC`.
- Added option `ROW_FORMAT={default | dynamic | fixed | compressed}` to `CREATE_TABLE`.
- `DELETE FROM table_name` didn't work on temporary tables.
- Changed function `CHAR_LENGTH()` to be multi-byte character safe.
- Added function `ORD(string)`.

### D.3.52 Changes in release 3.23.5 (20 Oct 1999)

- Fixed some Y2K problems in the new date handling in 3.23.
- Fixed problem with `SELECT DISTINCT ... ORDER BY RAND()`.
- Added patches by Sergei A. Golubchik for text searching on the `MyISAM` level.
- Fixed cache overflow problem when using full joins without keys.
- Fixed some configure issues.
- Some small changes to make parsing faster.
- Adding a column after the last field with `ALTER TABLE` didn't work.
- Fixed problem when using an `AUTO_INCREMENT` column in two keys
- With `MyISAM`, you now can have an `AUTO_INCREMENT` column as a key sub part: `CREATE TABLE foo (a INT NOT NULL AUTO_INCREMENT, b CHAR(5), PRIMARY KEY (b,a))`
- Fixed bug in `MyISAM` with packed char keys that could be `NULL`.
- `AS` on field name with `CREATE TABLE table_name SELECT ...` didn't work.
- Allow use of `NATIONAL` and `NCHAR` when defining character columns. This is the same as not using `BINARY`.
- Don't allow `NULL` columns in a `PRIMARY KEY` (only in `UNIQUE` keys).
- Clear `LAST_INSERT_ID()` if one uses this in ODBC: `WHERE auto_increment_column IS NULL`. This seems to fix some problems with Access.
- `SET SQL_AUTO_IS_NULL=0|1` now turns on/off the handling of searching after the last inserted row with `WHERE auto_increment_column IS NULL`.
- Added new variable `concurrency` to `mysqld` for Solaris.
- Added `--relative` option to `mysqladmin` to make `extended-status` more useful to monitor changes.
- Fixed bug when using `COUNT(DISTINCT ...)` on an empty table.
- Added support for the Chinese character set GBK.
- Fixed problem with `LOAD DATA INFILE` and `BLOB` columns.
- Added bit operator `~` (negation).
- Fixed problem with `UDF` functions.

### D.3.53 Changes in release 3.23.4 (28 Sep 1999)

- Inserting a `DATETIME` into a `TIME` column no longer will try to store 'days' in it.
- Fixed problem with storage of float/double on little endian machines. (This affected `SUM()`.)
- Added connect timeout on TCP/IP connections.
- Fixed problem with `LIKE "%"` on an index that may have `NULL` values.
- `REVOKE ALL PRIVILEGES` didn't revoke all privileges.
- Allow creation of temporary tables with same name as the original table.
- When granting a user a `GRANT` option for a database, he couldn't grant privileges to other users.
- New command: `SHOW GRANTS FOR user` (by Sinisa).
- New `date_add` syntax: `date/datetime + INTERVAL # interval_type`. By Joshua Chamas.
- Fixed privilege check for `LOAD DATA REPLACE`.
- Automatic fixing of broken include files on Solaris 2.7
- Some configure issues to fix problems with big filesystem detection.
- `REGEXP` is now case-insensitive if you use non-binary strings.

### D.3.54 Changes in release 3.23.3

- Added patches for MIT-pthreads on NetBSD.
- Fixed range bug in `MyISAM`.
- `ASC` is now the default again for `ORDER BY`.
- Added `LIMIT` to `UPDATE`.
- Added `mysql_change_user()` function to the MySQL C API.
- Added character set to `SHOW VARIABLES`.
- Added support of `--[whitespace]` comments.
- Allow `INSERT into tbl_name VALUES ()`, that is, you may now specify an empty value list to insert a row in which each column is set to its default value.
- Changed `SUBSTRING(text FROM pos)` to conform to ANSI SQL. (Before this construct returned the rightmost `pos` characters.)
- `SUM()` with `GROUP BY` returned 0 on some systems.
- Changed output for `SHOW TABLE STATUS`.
- Added `DELAY_KEY_WRITE` option to `CREATE TABLE`.
- Allow `AUTO_INCREMENT` on any key part.
- Fixed problem with `YEAR(NOW())` and `YEAR(CURDATE())`.
- Added `CASE` construct.
- New function `COALESCE()`.

### D.3.55 Changes in release 3.23.2 (09 Aug 1999)

- Fixed range optimiser bug: `SELECT * FROM table_name WHERE key_part1 >= const AND (key_part2 = const OR key_part2 = const)`. The bug was that some rows could be duplicated in the result.
- Running `myisamchk` without `-a` updated the index distribution incorrectly.
- `SET SQL_LOW_PRIORITY_UPDATES=1` was causing a parse error.
- You can now update index columns that are used in the `WHERE` clause. `UPDATE tbl_name SET KEY=KEY+1 WHERE KEY > 100`
- Date handling should now be a bit faster.
- Added handling of fuzzy dates (dates where day or month is 0), such as `'1999-01-00'`.
- Fixed optimisation of `SELECT ... WHERE key_part1=const1 AND key_part_2=const2 AND key_part1=const4 AND key_part2=const4`; indextype should be `range` instead of `ref`.
- Fixed `egcs` 1.1.2 optimiser bug (when using `BLOB`s) on Linux Alpha.
- Fixed problem with `LOCK TABLES` combined with `DELETE FROM table`.
- `MyISAM` tables now allow keys on `NULL` and `BLOB/TEXT` columns.
- The following join is now much faster: `SELECT ... FROM t1 LEFT JOIN t2 ON ... WHERE t2.not_null_column IS NULL`.
- `ORDER BY` and `GROUP BY` can be done on functions.
- Changed handling of 'const_item' to allow handling of `ORDER BY RAND()`.
- Indexes are now used for `WHERE key_column = function`.
- Indexes are now used for `WHERE key_column = col_name` even if the columns are not identically packed.
- Indexes are now used for `WHERE col_name IS NULL`.
- Changed heap tables to be stored in low_byte_first order (to make it easy to convert to `MyISAM` tables)
- Automatic change of `HEAP` temporary tables to `MyISAM` tables in case of 'table is full' errors.
- Added `--init-file=file_name` option to `mysqld`.
- Added `COUNT(DISTINCT value, [value, ...])`.
- `CREATE TEMPORARY TABLE` now creates a temporary table, in its own namespace, that is automatically deleted if connection is dropped.
- New reserved words (required for `CASE`): `CASE, THEN, WHEN, ELSE and END`.
- New functions `EXPORT_SET()` and `MD5()`.
- Support for the GB2312 Chinese character set.

### D.3.56 Changes in release 3.23.1

- Fixed some compilation problems.

### D.3.57 Changes in release 3.23.0 (05 Aug 1999: Alpha)

- A new storage engine library (`MyISAM`) with a lot of new features. See section 7.1 `MyISAM` Tables.
- You can create in-memory `HEAP` tables which are extremely fast for lookups.
- Support for big files (63-bit) on OSes that support big files.
- New function `LOAD_FILE(filename)` to get the contents of a file as a string value.
- New operator `<=>` which will act as `=` but will return TRUE if both arguments are `NULL`. This is useful for comparing changes between tables.
- Added the ODBC 3.0 `EXTRACT(interval FROM datetime)` function.
- Columns defined as `FLOAT(X)` are not rounded on storage and may be in scientific notation (1.0 E+10) when retrieved.
- `REPLACE` is now faster than before.
- Changed `LIKE` character comparison to behave as `=`; This means that `'e' LIKE ''e'` is now true. (If the line doesn't display correctly, the latter 'e' is a French 'e' with a dot above.)
- `SHOW TABLE STATUS` returns a lot of information about the tables.
- Added `LIKE` to the `SHOW STATUS` command.
- Added `Privileges` column to `SHOW COLUMNS`.
- Added `Packed` and `Comment` columns to `SHOW INDEX`.
- Added comments to tables (with `CREATE TABLE ... COMMENT "xxx"`).
- Added `UNIQUE`, as in `CREATE TABLE table_name (col int not null UNIQUE)`
- New create syntax: `CREATE TABLE table_name SELECT ...`
- New create syntax: `CREATE TABLE IF NOT EXISTS ...`
- Allow creation of `CHAR(0)` columns.
- `DATE_FORMAT()` now requires ``%'` before any format character.
- `DELAYED` is now a reserved word (sorry about that :( ).
- An example procedure is added: `analyse`, file: ``sql_analyse.c'`. This will describe the data in your query. Try the following:

      SELECT ... FROM ...
      WHERE ... PROCEDURE ANALYSE([max elements,[max memory]])

  This procedure is extremely useful when you want to check the data in your table!
- `BINARY` cast to force a string to be compared in case-sensitive fashion.
- Added `--skip-show-database` option to `mysqld`.
- Check whether a row has changed in an `UPDATE` now also works with `BLOB`/`TEXT` columns.
- Added the `INNER` join syntax. NOTE: This made `INNER` a reserved word!
- Added support for netmasks to the hostname in the MySQL grant tables. You can specify a netmask using the `IP/NETMASK` syntax.
- If you compare a `NOT NULL DATE/DATETIME` column with `IS NULL`, this is changed to a compare against `0` to satisfy some ODBC applications. (By shreeve@uci.edu.)
- `NULL IN (...)` now returns `NULL` instead of `0`. This will ensure that `null_column NOT IN (...)` doesn't match `NULL` values.
- Fix storage of floating-point values in `TIME` columns.
- Changed parsing of `TIME` strings to be more strict. Now the fractional second part is detected (and currently skipped). The following formats are supported:
  - [[DAYS] [H]H:]MM:]SS[.fraction]
  - [[[[[H]H]H]H]MM]SS[.fraction]
- Detect (and ignore) fractional second part from `DATETIME`.
- Added the `LOW_PRIORITY` attribute to `LOAD DATA INFILE`.
- The default index name now uses the same case as the column name on which the index name is based.
- Changed default number of connections to 100.
- Use bigger buffers when using `LOAD DATA INFILE`.
- `DECIMAL(x,y)` now works according to ANSI SQL.
- Added aggregate UDF functions. Thanks to Andreas F. Bobak (bobak@relog.ch) for this!
- `LAST_INSERT_ID()` is now updated for `INSERT INTO ... SELECT`.
- Some small changes to the join table optimiser to make some joins faster.
- `SELECT DISTINCT` is much faster; it uses the new `UNIQUE` functionality in `MyISAM`. One difference compared to MySQL Version 3.22 is that the output of `DISTINCT` is not sorted anymore.
- All C client API macros are now functions to make shared libraries more reliable. Because of this, you can no longer call `mysql_num_fields()` on a `MYSQL` object, you must use `mysql_field_count()` instead.
- Added use of `LIBWRAP`; patch by Henning P. Schmiedehausen.
- Don't allow `AUTO_INCREMENT` for other than numerical columns.
- Using `AUTO_INCREMENT` will now automatically make the column `NOT NULL`.
- Show `NULL` as the default value for `AUTO_INCREMENT` columns.
- Added `SQL_BIG_RESULT`; `SQL_SMALL_RESULT` is now default.
- Added a shared library RPM. This enhancement was contributed by David Fox (dsfox@cogsci.ucsd.edu).
- Added `--enable-large-files` and `--disable-large-files` switches to `configure`. See ``configure.in'` for some systems where this is automatically turned off because of broken implementations.
- Upgraded `readline` to 4.0.
- New `CREATE TABLE` options: `PACK_KEYS` and `CHECKSUM`.
- Added `--default-table-type` option to `mysqld`.

## D.4 Changes in release 3.22.x (Older; still supported)

The 3.22 version has faster and safer connect code than version 3.21, as well as a lot of new nice enhancements. As there aren't really any major changes, upgrading from 3.21 to 3.22 should be very easy and painless. See section 2.5.4 Upgrading from Version 3.21 to Version 3.22.

### D.4.1 Changes in release 3.22.35

- Fixed problem with `STD()`.
- Merged changes from the newest `ISAM` library from 3.23.
- Fixed problem with `INSERT DELAYED`.
- Fixed a bug core dump when using a `LEFT JOIN`/`STRAIGHT_JOIN` on a table with only one row.

### D.4.2 Changes in release 3.22.34

- Fixed problem with `GROUP BY` on `TINYBLOB` columns; this caused bugzilla to not show rows in some queries.
- Had to do total recompile of the Windows binary version as VC++ didn't compile all relevant files for 3.22.33 :(

### D.4.3 Changes in release 3.22.33

- Fixed problems in Windows when locking tables with `LOCK TABLE`.
- Quicker kill of `SELECT DISTINCT` queries.

### D.4.4 Changes in release 3.22.32 (14 Feb 2000)

- Fixed problem when storing numbers in timestamps.
- Fix problem with timezones that have half hour offsets.
- Added `mysqlhotcopy`, a fast online hot-backup utility for local MySQL databases. By Tim Bunce.
- New more secure `mysqlaccess`. Thanks to Steve Harvey for this.
- Fixed security problem in the protocol regarding password checking.
- Fixed problem that affected queries that did arithmetic on `GROUP` functions.
- Fixed a bug in the `ISAM` code when deleting rows on tables with packed indexes.

### D.4.5 Changes in release 3.22.31

- A few small fixes for the Windows version.

### D.4.6 Changes in release 3.22.30

- Fixed optimiser problem on `SELECT` when using many overlapping indexes.
- Disabled floating-point exceptions for FreeBSD to fix core dump when doing `SELECT FLOOR(POW(2,63))`.
- Added print of default arguments options to all clients.
- Fixed critical problem with the `WITH GRANT OPTION` option.
- Fixed non-critical Y2K problem when writing short date to log files.

### D.4.7 Changes in release 3.22.29 (02 Jan 2000)

- Upgraded the configure and include files to match the latest 3.23 version. This should increase portability and make it easier to build shared libraries.
- Added latest patches to MIT-pthreads for NetBSD.
- Fixed problem with timezones that are < GMT -11.
- Fixed a bug when deleting packed keys in NISAM.
- Fixed problem that could cause MySQL to touch freed memory when doing very complicated `GROUP BY` queries.
- Fixed core dump if you got a crashed table where an `ENUM` field value was too big.
- Added `mysqlshutdown.exe` and `mysqlwatch.exe` to the Windows distribution.
- Fixed problem when doing `ORDER BY` on a reference key.
- Fixed that `INSERT DELAYED` doesn't update timestamps that are given.

### D.4.8 Changes in release 3.22.28 (20 Oct 1999)

- Fixed problem with `LEFT JOIN` and `COUNT()` on a column which was declared `NULL` + and it had a `DEFAULT` value.
- Fixed core dump problem when using `CONCAT()` in a `WHERE` clause.
- Fixed problem with `AVG()` and `STD()` with `NULL` values.

### D.4.9 Changes in release 3.22.27

- Fixed prototype in ``my_ctype.h'` when using other character sets.
- Some configure issues to fix problems with big filesystem detection.
- Fixed problem when sorting on big `BLOB` columns.
- `ROUND()` will now work on Windows.

### D.4.10 Changes in release 3.22.26 (16 Sep 1999)

- Fixed core dump with empty `BLOB/TEXT` column argument to `REVERSE()`.
- Extended `/*! */` with version numbers.
- Changed `SUBSTRING(text FROM pos)` to conform to ANSI SQL. (Before this construct returned the rightmost 'pos' characters.)
- Fixed problem with `LOCK TABLES` combined with `DELETE FROM table`
- Fixed problem that `INSERT ... SELECT` didn't use `BIG_TABLES`.
- `SET SQL_LOW_PRIORITY_UPDATES=#` didn't work.
- Password wasn't updated correctly if privileges didn't change on: `GRANT ... IDENTIFIED BY`
- Fixed range optimiser bug in `SELECT * FROM table_name WHERE key_part1 >= const AND (key_part2 = const OR key_part2 = const)`.
- Fixed bug in compression key handling in `ISAM`.

### D.4.11 Changes in release 3.22.25

- Fixed some small problems with the installation.

### D.4.12 Changes in release 3.22.24 (05 Jul 1999)

- `DATA` is not a reserved word anymore.
- Fixed optimiser bug with tables with only one row.
- Fixed bug when using `LOCK TABLES table_name READ; FLUSH TABLES;`
- Applied some patches for HP-UX.
- `isamchk` should now work on Windows.
- Changed ``configure'` to not use big file handling on Linux as this crashes some RedHat 6.0 systems

### D.4.13 Changes in release 3.22.23 (08 Jun 1999)

- Upgraded to use Autoconf 2.13, Automake 1.4 and `libtool` 1.3.2.
- Better support for SCO in `configure`.
- Added option `--defaults-file=###` to option file handling to force use of only one specific option file.
- Extended `CREATE` syntax to ignore MySQL Version 3.23 keywords.
- Fixed deadlock problem when using `INSERT DELAYED` on a table locked with `LOCK TABLES`.
- Fixed deadlock problem when using `DROP TABLE` on a table that was locked by another thread.
- Add logging of `GRANT/REVOKE` commands in the update log.
- Fixed `isamchk` to detect a new error condition.
- Fixed bug in `NATURAL LEFT JOIN`.

### D.4.14 Changes in release 3.22.22 (30 Apr 1999)

- Fixed problem in the C API when you called `mysql_close()` directly after `mysql_init()`.
- Better client error message when you can't open socket.
- Fixed `delayed_insert_thread` counting when you couldn't create a new delayed_insert thread.
- Fixed bug in `CONCAT()` with many arguments.
- Added patches for DEC 3.2 and SCO.
- Fixed path-bug when installing MySQL as a service on NT.
- The MySQL-Windows version is now compiled with VC++ 6.0 instead of with VC++ 5.0.
- New installation setup for MySQL-Windows.

### D.4.15 Changes in release 3.22.21

- Fixed problem with `DELETE FROM TABLE` when table was locked by another thread.
- Fixed bug in `LEFT JOIN` involving empty tables.
- Changed the `mysql.db` column from `CHAR(32)` to `CHAR(60)`.
- `MODIFY` and `DELAYED` are not reserved words anymore.
- Fixed a bug when storing days in a `TIME` column.
- Fixed a problem with `Host '...' is not allowed to connect to this MySQL server` after one had inserted a new MySQL user with a `GRANT` command.
- Changed to use `TCP_NODELAY` also on Linux (should give faster TCP/IP connections).

### D.4.16 Changes in release 3.22.20 (18 Mar 1999)

- Fixed `STD()` for big tables when result should be 0.
- The update log didn't have newlines on some operating systems.
- `INSERT DELAYED` had some garbage at end in the update log.

### D.4.17 Changes in release 3.22.19 (Mar 1999: Stable)

- Fixed bug in `mysql_install_db` (from 3.22.17).
- Changed default key cache size to 8M.
- Fixed problem with queries that needed temporary tables with `BLOB` columns.

### D.4.18 Changes in release 3.22.18

- Fixes a fatal problem in 3.22.17 on Linux; after `shutdown` not all threads died properly.
- Added option `-O flush_time=#` to `mysqld`. This is mostly useful on Windows and tells how often MySQL should close all unused tables and flush all updated tables to disk.
- Fixed problem that a `VARCHAR` column compared with `CHAR` column didn't use keys efficiently.

### D.4.19 Changes in release 3.22.17

- Fixed a core dump problem when using `--log-update` and connecting without a default database.
- Fixed some `configure` and portability problems.
- Using `LEFT JOIN` on tables that had circular dependencies caused `mysqld` to hang forever.

### D.4.20 Changes in release 3.22.16 (Feb 1999: Gamma)

- `mysqladmin processlist` could kill the server if a new user logged in.
- `DELETE FROM tbl_name WHERE key_column=col_name` didn't find any matching rows. Fixed.
- `DATE_ADD(column, ...)` didn't work.
- `INSERT DELAYED` could deadlock with status 'upgrading lock'
- Extended `ENCRYPT()` to take longer salt strings than 2 characters.
- `longlong2str` is now much faster than before. For `Intel x86` platforms, this function is written in optimised assembler.
- Added the `MODIFY` keyword to `ALTER TABLE`.

### D.4.21 Changes in release 3.22.15

- `GRANT` used with `IDENTIFIED BY` didn't take effect until privileges were flushed.
- Name change of some variables in `SHOW STATUS`.
- Fixed problem with `ORDER BY` with 'only index' optimisation when there were multiple key definitions for a used column.
- `DATE` and `DATETIME` columns are now up to 5 times faster than before.
- `INSERT DELAYED` can be used to let the client do other things while the server inserts rows into a table.
- `LEFT JOIN USING (col1,col2)` didn't work if one used it with tables from 2 different databases.
- `LOAD DATA LOCAL INFILE` didn't work in the Unix version because of a missing file.
- Fixed problems with `VARCHAR`/`BLOB` on very short rows (< 4 bytes); error 127 could occur when deleting rows.
- Updating `BLOB/TEXT` through formulas didn't work for short (< 256 char) strings.
- When you did a `GRANT` on a new host, `mysqld` could die on the first connect from this host.
- Fixed bug when one used `ORDER BY` on column name that was the same name as an alias.
- Added `BENCHMARK(loop_count,expression)` function to time expressions.

### D.4.22 Changes in release 3.22.14

- Allow empty arguments to `mysqld` to make it easier to start from shell scripts.
- Setting a `TIMESTAMP` column to `NULL` didn't record the timestamp value in the update log.
- Fixed lock handler bug when one did `INSERT INTO TABLE ... SELECT ... GROUP BY`.
- Added a patch for `localtime_r()` on Windows so that it will not crash anymore if your date is > 2039, but instead will return a time of all zero.
- Names for user-defined functions are no longer case-sensitive.
- Added escape of `^Z` (ASCII 26) to `\Z` as `^Z` doesn't work with pipes on Windows.
- `mysql_fix_privileges` adds a new column to the `mysql.func` to support aggregate UDF functions in future MySQL releases.

### D.4.23 Changes in release 3.22.13

- Saving `NOW()`, `CURDATE()` or `CURTIME()` directly in a column didn't work.
- `SELECT COUNT(*) ... LEFT JOIN ...` didn't work with no `WHERE` part.
- Updated ``config.guess'` to allow MySQL to configure on UnixWare 7.0.x.
- Changed the implementation of `pthread_cond()` on the Windows version. `get_lock()` now correctly times out on Windows!

### D.4.24 Changes in release 3.22.12

- Fixed problem when using `DATE_ADD()` and `DATE_SUB()` in a `WHERE` clause.
- You can now set the password for a user with the `GRANT ... TO user IDENTIFIED BY 'password'` syntax.
- Fixed bug in `GRANT` checking with `SELECT` on many tables.
- Added missing file `mysql_fix_privilege_tables` to the RPM distribution. This is not run by default because it relies on the client package.
- Added option `SQL_SMALL_RESULT` to `SELECT` to force use of fast temporary tables when you know that the result set will be small.
- Allow use of negative real numbers without a decimal point.
- Day number is now adjusted to maximum days in month if the resulting month after `DATE_ADD`/`DATE_SUB()` doesn't have enough days.
- Fix that `GRANT` compares columns in case-insensitive fashion.
- Fixed a bug in ``sql_list.h'` that made `ALTER TABLE` dump core in some contexts.
- The hostname in `user@hostname` can now include ``.'` and ``-'` without quotes in the context of the `GRANT`, `REVOKE` and `SET PASSWORD FOR ...` statements.
- Fix for `isamchk` for tables which need big temporary files.

### D.4.25 Changes in release 3.22.11

- Important: You must run the `mysql_fix_privilege_tables` script when you upgrade to this version! This is needed because of the new `GRANT` system. If you don't do this, you will get `Access denied` when you try to use `ALTER TABLE`, `CREATE INDEX`, or `DROP INDEX`.
- `GRANT` to allow/deny users table and column access.
- Changed `USER()` to return a value in `user@host` format. Formerly it returned only `user`.
- Changed the syntax for how to set `PASSWORD` for another user.
- New command `FLUSH STATUS` that resets most status variables to zero.
- New status variables: `aborted_threads`, `aborted_connects`.
- New option variable: `connection_timeout`.
- Added support for Thai sorting (by Pruet Boonma pruet@ds90.intanon.nectec.or.th).
- Slovak and japanese error messages.
- Configuration and portability fixes.
- Added option `SET SQL_WARNINGS=1` to get a warning count also for simple inserts.
- MySQL now uses `SIGTERM` instead of `SIGQUIT` with shutdown to work better on FreeBSD.
- Added option `\G` (print vertically) to `mysql`.
- `SELECT HIGH_PRIORITY ...` killed `mysqld`.
- `IS NULL` on a `AUTO_INCREMENT` column in a `LEFT JOIN` didn't work as expected.
- New function `MAKE_SET()`.

### D.4.26 Changes in release 3.22.10

- `mysql_install_db` no longer starts the MySQL server! You should start `mysqld` with `safe_mysqld` after installing it! The MySQL RPM will, however, start the server as before.
- Added `--bootstrap` option to `mysqld` and recoded `mysql_install_db` to use it. This will make it easier to install MySQL with RPMs.
- Changed `+`, `-` (sign and minus), `*`, `/`, `%`, `ABS()` and `MOD()` to be `BIGINT` aware (64-bit safe).
- Fixed a bug in `ALTER TABLE` that caused `mysqld` to crash.
- MySQL now always reports the conflicting key values when a duplicate key entry occurs. (Before this was only reported for `INSERT`.)
- New syntax: `INSERT INTO tbl_name SET col_name=value, col_name=value, ...`
- Most errors in the ``.err'` log are now prefixed with a time stamp.
- Added option `MYSQL_INIT_COMMAND` to `mysql_options()` to make a query on connect or reconnect.
- Added option `MYSQL_READ_DEFAULT_FILE` and `MYSQL_READ_DEFAULT_GROUP` to `mysql_options()` to read the following parameters from the MySQL option files: `port`, `socket`, `compress`, `password`, `pipe`, `timeout`, `user`, `init-command`, `host` and `database`.
- Added `maybe_null` to the UDF structure.
- Added option `IGNORE` to `INSERT` statements with many rows.
- Fixed some problems with sorting of the `koi8` character sets; users of `koi8` must run `isamchk -rq` on each table that has an index on a `CHAR` or `VARCHAR` column.
- New script `mysql_setpermission`, by Luuk de Boer. It allows easy creation of new users with permissions for specific databases.
- Allow use of hexadecimal strings (0x...) when specifying a constant string (like in the column separators with `LOAD DATA INFILE`).
- Ported to OS/2 (thanks to Antony T. Curtis antony.curtis@olcs.net).
- Added more variables to `SHOW STATUS` and changed format of output to be like `SHOW VARIABLES`.
- Added `extended-status` command to `mysqladmin` which will show the new status variables.

### D.4.27 Changes in release 3.22.9

- `SET SQL_LOG_UPDATE=0` caused a lockup of the server.
- New SQL command: `FLUSH [ TABLES | HOSTS | LOGS | PRIVILEGES ] [, ...]`
- New SQL command: `KILL` `thread_id`.
- Added casts and changed include files to make MySQL easier to compile on AIX and DEC OSF/1 4.x
- Fixed conversion problem when using `ALTER TABLE` from a `INT` to a short `CHAR()` column.
- Added `SELECT HIGH_PRIORITY`; this will get a lock for the `SELECT` even if there is a thread waiting for another `SELECT` to get a `WRITE LOCK`.
- Moved `wild_compare()` to string class to be able to use `LIKE` on `BLOB`/`TEXT` columns with `\0`.
- Added `ESCAPE` option to `LIKE`.
- Added a lot more output to `mysqladmin debug`.
- You can now start `mysqld` on Windows with the `--flush` option. This will flush all tables to disk after each update. This makes things much safer on the Windows platforms but also much slower.

### D.4.28 Changes in release 3.22.8

- Czech character sets should now work much better. You must also install http://www.mysql.com/Downloads/Patches/czech-3.22.8-patch. This patch should also be installed if you are using a character set which uses `my_strcoll()`! The patch should always be safe to install (for any system), but as this patch changes `ISAM` internals it's not yet in the default distribution.
- `DATE_ADD()` and `DATE_SUB()` didn't work with group functions.
- `mysql` will now also try to reconnect on `USE DATABASE` commands.
- Fix problem with `ORDER BY` and `LEFT JOIN` and `const` tables.
- Fixed problem with `ORDER BY` if the first `ORDER BY` column was a key and the rest of the `ORDER BY` columns wasn't part of the key.
- Fixed a big problem with `OPTIMIZE TABLE`.
- MySQL clients on NT will now by default first try to connect with named pipes and after this with TCP/IP.
- Fixed a problem with `DROP TABLE` and `mysqladmin shutdown` on Windows (a fatal bug from 3.22.6).
- Fixed problems with `TIME columns` and negative strings.
- Added an extra thread signal loop on shutdown to avoid some error messages from the client.
- MySQL now uses the next available number as extension for the update log file.
- Added patches for UNIXWARE 7.

### D.4.29 Changes in release 3.22.7 (Sep 1998: Beta)

- Added `LIMIT` clause for the `DELETE` statement.
- You can now use the `/*! ... */` syntax to hide MySQL-specific keywords when you write portable code. MySQL will parse the code inside the comments as if the surrounding `/*!` and `*/` comment characters didn't exist.
- `OPTIMIZE TABLE tbl_name` can now be used to reclaim disk space after many deletes. Currently, this uses `ALTER TABLE` to regenerate the table, but in the future it will use an integrated `isamchk` for more speed.
- Upgraded `libtool` to get the configure more portable.
- Fixed slow `UPDATE` and `DELETE` operations when using `DATETIME` or `DATE` keys.
- Changed optimiser to make it better at deciding when to do a full join and when using keys.
- You can now use `mysqladmin proc` to display information about your own threads. Only users with the `PROCESS` privilege can get information about all threads. (In 4.0.2 one needs the `SUPER` privilege for this.)
- Added handling of formats `YYMMDD`, `YYYYMMDD`, `YYMMDDHHMMSS` for numbers when using `DATETIME` and `TIMESTAMP` types. (Formerly these formats only worked with strings.)
- Added connect option `CLIENT_IGNORE_SPACE` to allow use of spaces after function names and before ``('` (Powerbuilder requires this). This will make all function names reserved words.
- Added the `--log-long-format` option to `mysqld` to enable timestamps and INSERT_ID's in the update log.
- Added `--where` option to `mysqldump` (patch by Jim Faucette).
- The lexical analyser now uses ``perfect hashing'' for faster parsing of SQL statements.

### D.4.30 Changes in release 3.22.6

- Faster `mysqldump`.
- For the `LOAD DATA INFILE` statement, you can now use the new `LOCAL` keyword to read the file from the client. `mysqlimport` will automatically use `LOCAL` when importing with the TCP/IP protocol.
- Fixed small optimise problem when updating keys.
- Changed makefiles to support shared libraries.
- MySQL-NT can now use named pipes, which means that you can now use MySQL-NT without having to install TCP/IP.

### D.4.31 Changes in release 3.22.5

- All table lock handing is changed to avoid some very subtle deadlocks when using `DROP TABLE`, `ALTER TABLE`, `DELETE FROM TABLE` and `mysqladmin flush-tables` under heavy usage. Changed locking code to get better handling of locks of different types.
- Updated `DBI` to 1.00 and `DBD` to 1.2.0.
- Added a check that the error message file contains error messages suitable for the current version of `mysqld`. (To avoid errors if you accidentally try to use an old error message file.)
- All count structures in the client (`affected_rows()`, `insert_id()`, ...) are now of type `BIGINT` to allow 64-bit values to be used. This required a minor change in the MySQL protocol which should affect only old clients when using tables with `AUTO_INCREMENT` values > 16M.
- The return type of `mysql_fetch_lengths()` has changed from `uint *` to `ulong *`. This may give a warning for old clients but should work on most machines.
- Change `mysys` and `dbug` libraries to allocate all thread variables in one struct. This makes it easier to make a threaded ``libmysql.dll'` library.
- Use the result from `gethostname()` (instead of `uname()`) when constructing ``.pid'` file names.
- New better compressed server/client protocol.
- `COUNT()`, `STD()` and `AVG()` are extended to handle more than 4G rows.
- You can now store values in the range `-838:59:59` <= x <= `838:59:59` in a `TIME` column.
- Warning: incompatible change!! If you set a `TIME` column to too short a value, MySQL now assumes the value is given as: `[[[D ]HH:]MM:]SS` instead of `HH[:MM[:SS]]`.
- `TIME_TO_SEC()` and `SEC_TO_TIME()` can now handle negative times and hours up to 32767.
- Added new option `SET SQL_LOG_UPDATE={0|1}` to allow users with the `PROCESS` privilege to bypass the update log. (Modified patch from Sergey A Mukhin violet@rosnet.net.)
- Fixed fatal bug in `LPAD()`.
- Initialise line buffer in ``mysql.cc'` to make `BLOB` reading from pipes safer.
- Added `-O max_connect_errors=#` option to `mysqld`. Connect errors are now reset for each correct connection.
- Increased the default value of `max_allowed_packet` to `1M` in `mysqld`.
- Added `--low-priority-updates` option to `mysqld`, to give table-modifying operations (`INSERT`, `REPLACE`, `UPDATE`, `DELETE`) lower priority than retrievals. You can now use `{INSERT | REPLACE | UPDATE | DELETE} LOW_PRIORITY ...` You can also use `SET SQL_LOW_PRIORITY_UPDATES={0|1}` to change the priority for one thread. One side effect is that `LOW_PRIORITY` is now a reserved word. :(
- Add support for `INSERT INTO table ... VALUES(...),(...),(...)`, to allow inserting multiple rows with a single statement.
- `INSERT INTO tbl_name` is now also cached when used with `LOCK TABLES`. (Previously only `INSERT ... SELECT` and `LOAD DATA INFILE` were cached.)
- Allow `GROUP BY` functions with `HAVING`:

      mysql> SELECT col FROM table GROUP BY col HAVING COUNT(*)>0;

- `mysqld` will now ignore trailing ``;'` characters in queries. This is to make it easier to migrate from some other SQL servers that require the trailing ``;'`.
- Fix for corrupted fixed-format output generated by `SELECT INTO OUTFILE`.
- Warning: incompatible change! Added Oracle `GREATEST()` and `LEAST()` functions. You must now use these instead of the `MAX()` and `MIN()` functions to get the largest/smallest value from a list of values. These can now handle `REAL`, `BIGINT` and string (`CHAR` or `VARCHAR`) values.
- Warning: incompatible change! `DAYOFWEEK()` had offset 0 for Sunday. Changed the offset to 1.
- Give an error for queries that mix `GROUP BY` columns and fields when there is no `GROUP BY` specification.
- Added `--vertical` option to `mysql`, for printing results in vertical mode.
- Index-only optimisation; some queries are now resolved using only indexes. Until MySQL 4.0, this works only for numeric columns. See section 5.4.3 How MySQL Uses Indexes.
- Lots of new benchmarks.
- A new C API chapter and lots of other improvements in the manual.

### D.4.32 Changes in release 3.22.4

- Added `--tmpdir` option to `mysqld`, for specifying the location of the temporary file directory.
- MySQL now automatically changes a query from an ODBC client:

      SELECT ... FROM table WHERE auto_increment_column IS NULL

  to:

      SELECT ... FROM table WHERE auto_increment_column == LAST_INSERT_ID()

  This allows some ODBC programs (Delphi, Access) to retrieve the newly inserted row to fetch the `AUTO_INCREMENT` id.
- `DROP TABLE` now waits for all users to free a table before deleting it.
- Fixed small memory leak in the new connect protocol.
- New functions `BIN()`, `OCT()`, `HEX()` and `CONV()` for converting between different number bases.
- Added function `SUBSTRING()` with 2 arguments.
- If you created a table with a record length smaller than 5, you couldn't delete rows from the table.
- Added optimisation to remove const reference tables from `ORDER BY` and `GROUP BY`.
- `mysqld` now automatically disables system locking on Linux and Windows, and for systems that use MIT-pthreads. You can force the use of locking with the `--enable-external-locking` option.
- Added `--console` option to `mysqld`, to force a console window (for error messages) when using Windows.
- Fixed table locks for Windows.
- Allow ``$'` in identifiers.
- Changed name of user-specific configuration file from ``my.cnf'` to ``.my.cnf'` (Unix only).
- Added `DATE_ADD()` and `DATE_SUB()` functions.

### D.4.33 Changes in release 3.22.3

- Fixed a lock problem (bug in MySQL Version 3.22.1) when closing temporary tables.
- Added missing `mysql_ping()` to the client library.
- Added `--compress` option to all MySQL clients.
- Changed `byte` to `char` in ``mysql.h'` and ``mysql_com.h'`.

### D.4.34 Changes in release 3.22.2

- Searching on multiple constant keys that matched more than 30% of the rows didn't always use the best possible key.
- New functions `<<`, `>>`, `RPAD()` and `LPAD()`.
- You can now save default options (like passwords) in a configuration file (``my.cnf'`).
- Lots of small changes to get `ORDER BY` to work when no records are found when using fields that are not in `GROUP BY` (MySQL extension).
- Added `--chroot` option to `mysqld`, to start `mysqld` in a chroot environment (by Nikki Chumakov nikkic@cityline.ru).
- Trailing spaces are now ignored when comparing case-sensitive strings; this should fix some problems with ODBC and flag 512!
- Fixed a core dump bug in the range optimiser.
- Added `--one-thread` option to `mysqld`, for debugging with LinuxThreads (or `glibc`). (This replaces the `-T32` flag)
- Added `DROP TABLE IF EXISTS` to prevent an error from occurring if the table doesn't exist.
- `IF` and `EXISTS` are now reserved words (they would have to be sooner or later).
- Added lots of new options to `mysqldump`.
- Server error messages are now in ``mysqld_error.h'`.
- The server/client protocol now supports compression.
- All bug fixes from MySQL Version 3.21.32.

### D.4.35 Changes in release 3.22.1 (Jun 1998: Alpha)

- Added new C API function `mysql_ping()`.
- Added new API functions `mysql_init()` and `mysql_options()`. You now MUST call `mysql_init()` before you call `mysql_real_connect()`. You don't have to call `mysql_init()` if you only use `mysql_connect()`.
- Added `mysql_options(...,MYSQL_OPT_CONNECT_TIMEOUT,...)` so you can set a timeout for connecting to a server.
- Added `--timeout` option to `mysqladmin`, as a test of `mysql_options()`.
- Added `AFTER column` and `FIRST` options to `ALTER TABLE ... ADD columns`. This makes it possible to add a new column at some specific location within a row in an existing table.
- `WEEK()` now takes an optional argument to allow handling of weeks when the week starts on Monday (some European countries). By default, `WEEK()` assumes the week starts on Sunday.
- `TIME` columns weren't stored properly (bug in MySQL Version 3.22.0).
- `UPDATE` now returns information about how many rows were matched and updated, and how many ``warnings'' occurred when doing the update.
- Fixed incorrect result from `FORMAT(-100,2)`.
- `ENUM` and `SET` columns were compared in binary (case-sensitive) fashion; changed to be case-insensitive.

### D.4.36 Changes in release 3.22.0

- New (backward-compatible) connect protocol that allows you to specify the database to use when connecting, to get much faster connections to a specific database. The `mysql_real_connect()` call is changed to:

      mysql_real_connect(MYSQL *mysql, const char *host, const char *user,
                         const char *passwd, const char *db, uint port,
                         const char *unix_socket, uint client_flag)

- Each connection is handled by its own thread, rather than by the master `accept()` thread. This fixes permanently the telnet bug that was a topic on the mail list some time ago.
- All TCP/IP connections are now checked with backward-resolution of the hostname to get better security. `mysqld` now has a local hostname resolver cache so connections should actually be faster than before, even with this feature.
- A site automatically will be blocked from future connections if someone repeatedly connects with an ``improper header'' (like when one uses telnet).
- You can now refer to tables in different databases with references of the form `tbl_name@db_name` or `db_name.tbl_name`. This makes it possible to give a user read access to some tables and write access to others simply by keeping them in different databases!
- Added `--user` option to `mysqld`, to allow it to run as another Unix user (if it is started as the Unix `root` user).
- Added caching of users and access rights (for faster access rights checking)
- Normal users (not anonymous ones) can change their password with `mysqladmin password 'new_password'`. This uses encrypted passwords that are not logged in the normal MySQL log!
- All important string functions are now coded in assembler for x86 Linux machines. This gives a speedup of 10% in many cases.
- For tables that have many columns, the column names are now hashed for much faster column name lookup (this will speed up some benchmark tests a lot!)
- Some benchmarks are changed to get better individual timing. (Some loops were so short that a specific test took < 2 seconds. The loops have been changed to take about 20 seconds to make it easier to compare different databases. A test that took 1-2 seconds before now takes 11-24 seconds, which is much better)
- Re-arranged `SELECT` code to handle some very specific queries involving group functions (like `COUNT(*)`) without a `GROUP BY` but with `HAVING`. The following now works:

      mysql> SELECT COUNT(*) as C FROM table HAVING C > 1;

- Changed the protocol for field functions to be faster and avoid some calls to `malloc()`.
- Added `-T32` option to `mysqld`, for running all queries under the main thread. This makes it possible to debug `mysqld` under Linux with `gdb`!
- Added optimisation of `not_null_column IS NULL` (needed for some Access queries).
- Allow `STRAIGHT_JOIN` to be used between two tables to force the optimiser to join them in a specific order.
- String functions now return `VARCHAR` rather than `CHAR` and the column type is now `VARCHAR` for fields saved as `VARCHAR`. This should make the `MyODBC` driver better, but may break some old MySQL clients that don't handle `FIELD_TYPE_VARCHAR` the same way as `FIELD_TYPE_CHAR`.
- `CREATE INDEX` and `DROP INDEX` are now implemented through `ALTER TABLE`. `CREATE TABLE` is still the recommended (fast) way to create indexes.
- Added `--set-variable` option `wait_timeout` to `mysqld`.
- Added time column to `mysqladmin processlist` to show how long a query has taken or how long a thread has slept.
- Added lots of new variables to `show variables` and some new to `show status`.
- Added new type `YEAR`. `YEAR` is stored in 1 byte with allowable values of 0, and 1901 to 2155.
- Added new `DATE` type that is stored in 3 bytes rather than 4 bytes. All new tables are created with the new date type if you don't use the `--old-protocol` option to `mysqld`.
- Fixed bug in record caches; for some queries, you could get `Error from table handler: #` on some operating systems.
- Added `--enable-assembler` option to `configure`, for x86 machines (tested on Linux + `gcc`). This will enable assembler functions for the most important string functions for more speed!

## D.5 Changes in release 3.21.x

Version 3.21 is quite old now, and should be avoided if possible. This information is kept here for historical purposes only.

### D.5.1 Changes in release 3.21.33

- Fixed problem when sending `SIGHUP` to `mysqld`; `mysqld` core dumped when starting from boot on some systems.
- Fixed problem with losing a little memory for some connections.
- `DELETE FROM tbl_name` without a `WHERE` condition is now done the long way when you use `LOCK TABLES` or if the table is in use, to avoid race conditions.
- `INSERT INTO TABLE (timestamp_column) VALUES (NULL);` didn't set timestamp.

### D.5.2 Changes in release 3.21.32

- Fixed some possible race conditions when doing many reopen/close on the same tables under heavy load! This can happen if you execute `mysqladmin refresh` often. This could in some very rare cases corrupt the header of the index file and cause error 126 or 138.
- Fixed fatal bug in `refresh()` when running with the `--skip-external-locking` option. There was a ``very small'' time gap after a `mysqladmin refresh` when a table could be corrupted if one thread updated a table while another thread did `mysqladmin refresh` and another thread started a new update ont the same table before the first thread had finished. A refresh (or `--flush-tables`) will now not return until all used tables are closed!
- `SELECT DISTINCT` with a `WHERE` clause that didn't match any rows returned a row in some contexts (bug only in 3.21.31).
- `GROUP BY` + `ORDER BY` returned one empty row when no rows where found.
- Fixed a bug in the range optimiser that wrote `Use_count: Wrong count for ...` in the error log file.

### D.5.3 Changes in release 3.21.31

- Fixed a sign extension problem for the `TINYINT` type on Irix.
- Fixed problem with `LEFT("constant_string",function)`.
- Fixed problem with `FIND_IN_SET()`.
- `LEFT JOIN` core dumped if the second table is used with a constant `WHERE/ON` expression that uniquely identifies one record.
- Fixed problems with `DATE_FORMAT()` and incorrect dates. `DATE_FORMAT()` now ignores `'%'` to make it possible to extend it more easily in the future.

### D.5.4 Changes in release 3.21.30

- `mysql` now returns an exit code > 0 if the query returned an error.
- Saving of command-line history to file in `mysql` client. By Tommy Larsen tommy@mix.hive.no.
- Fixed problem with empty lines that were ignored in ``mysql.cc'`.
- Save the pid of the signal handler thread in the pid file instead of the pid of the main thread.
- Added patch by tommy@valley.ne.jp to support Japanese characters SJIS and UJIS.
- Changed `safe_mysqld` to redirect startup messages to `'hostname'.err` instead of `'hostname'.log` to reclaim file space on `mysqladmin refresh`.
- `ENUM` always had the first entry as default value.
- `ALTER TABLE` wrote two entries to the update log.
- `sql_acc()` now closes the `mysql` grant tables after a reload to save table space and memory.
- Changed `LOAD DATA` to use less memory with tables and `BLOB` columns.
- Sorting on a function which made a division / 0 produced a wrong set in some cases.
- Fixed `SELECT` problem with `LEFT()` when using the `czech` character set.
- Fixed problem in `isamchk`; it couldn't repair a packed table in a very unusual case.
- `SELECT` statements with `&` or `|` (bit functions) failed on columns with `NULL` values.
- When comparing a field = field, where one of the fields was a part key, only the length of the part key was compared.

### D.5.5 Changes in release 3.21.29

- `LOCK TABLES` + `DELETE from tbl_name` never removed locks properly.
- Fixed problem when grouping on an `OR` function.
- Fixed permission problem with `umask()` and creating new databases.
- Fixed permission problem on result file with `SELECT ... INTO OUTFILE ...`
- Fixed problem in range optimiser (core dump) for a very complex query.
- Fixed problem when using `MIN(integer)` or `MAX(integer)` in `GROUP BY`.
- Fixed bug on Alpha when using integer keys. (Other keys worked on Alpha.)
- Fixed bug in `WEEK("XXXX-xx-01")`.

### D.5.6 Changes in release 3.21.28

- Fixed socket permission (clients couldn't connect to Unix socket on Linux).
- Fixed bug in record caches; for some queries, you could get `Error from table handler: #` on some operating systems.

### D.5.7 Changes in release 3.21.27

- Added user level lock functions `GET_LOCK(string,timeout)`, `RELEASE_LOCK(string)`.
- Added `Opened_tables` to `show status`.
- Changed connect timeout to 3 seconds to make it somewhat harder for crackers to kill `mysqld` through telnet + TCP/IP.
- Fixed bug in range optimiser when using `WHERE key_part_1 >= something AND key_part_2 <= something_else`.
- Changed `configure` for detection of FreeBSD 3.0 9803xx and above
- `WHERE` with `string_col_key = constant_string` didn't always find all rows if the column had many values differing only with characters of the same sort value (like e and 'e).
- Strings keys looked up with 'ref' were not compared in case-sensitive fashion.
- Added `umask()` to make log files non-readable for normal users.
- Ignore users with old (8-byte) password on startup if not using `--old-protocol` option to `mysqld`.
- `SELECT` which matched all key fields returned the values in the case of the matched values, not of the found values. (Minor problem.)

### D.5.8 Changes in release 3.21.26

- `FROM_DAYS(0)` now returns "0000-00-00".
- In `DATE_FORMAT()`, PM and AM were swapped for hours 00 and 12.
- Extended the default maximum key size to 256.
- Fixed bug when using `BLOB`/`TEXT` in `GROUP BY` with many tables.
- An `ENUM` field that is not declared `NOT NULL` has `NULL` as the default value. (Previously, the default value was the first enumeration value.)
- Fixed bug in the join optimiser code when using many part keys on the same key: `INDEX (Organisation,Surname(35),Initials(35))`.
- Added some tests to the table order optimiser to get some cases with `SELECT ... FROM many_tables` much faster.
- Added a retry loop around `accept()` to possibly fix some problems on some Linux machines.

### D.5.9 Changes in release 3.21.25

- Changed `typedef 'string'` to `typedef 'my_string'` for better portability.
- You can now kill threads that are waiting on a disk-full condition.
- Fixed some problems with UDF functions.
- Added long options to `isamchk`. Try `isamchk --help`.
- Fixed a bug when using 8 bytes long (alpha); `filesort()` didn't work. Affects `DISTINCT`, `ORDER BY` and `GROUP BY` on 64-bit processors.

### D.5.10 Changes in release 3.21.24

- Dynamic loadable functions. Based on source from Alexis Mikhailov.
- You couldn't delete from a table if no one had done a `SELECT` on the table.
- Fixed problem with range optimiser with many `OR` operators on key parts inside each other.
- Recoded `MIN()` and `MAX()` to work properly with strings and `HAVING`.
- Changed default umask value for new files from `0664` to `0660`.
- Fixed problem with `LEFT JOIN` and constant expressions in the `ON` part.
- Added Italian error messages from brenno@dewinter.com.
- `configure` now works better on OSF/1 (tested on 4.0D).
- Added hooks to allow `LIKE` optimisation with international character support.
- Upgraded `DBI` to 0.93.

### D.5.11 Changes in release 3.21.23

- The following symbols are now reserved words: `TIME`, `DATE`, `TIMESTAMP`, `TEXT`, `BIT`, `ENUM`, `NO`, `ACTION`, `CHECK`, `YEAR`, `MONTH`, `DAY`, `HOUR`, `MINUTE`, `SECOND`, `STATUS`, `VARIABLES`.
- Setting a `TIMESTAMP` to `NULL` in `LOAD DATA INFILE ...` didn't set the current time for the `TIMESTAMP`.
- Fix `BETWEEN` to recognise binary strings. Now `BETWEEN` is case-sensitive.
- Added `--skip-thread-priority` option to `mysqld`, for systems where `mysqld`'s thread scheduling doesn't work properly (BSDI 3.1).
- Added ODBC functions `DAYNAME()` and `MONTHNAME()`.
- Added function `TIME_FORMAT()`. This works like `DATE_FORMAT()`, but takes a time string (`'HH:MM:DD'`) as argument.
- Fixed unlikely(?) key optimiser bug when using `OR`s of key parts inside `AND`s.
- Added `variables` command to `mysqladmin`.
- A lot of small changes to the binary releases.
- Fixed a bug in the new protocol from MySQL Version 3.21.20.
- Changed `ALTER TABLE` to work with Windows (Windows can't rename open files). Also fixed a couple of small bugs in the Windows version.
- All standard MySQL clients are now ported to MySQL-Windows.
- MySQL can now be started as a service on NT.

### D.5.12 Changes in release 3.21.22

- Starting with this version, all MySQL distributions will be configured, compiled and tested with `crash-me` and the benchmarks on the following platforms: SunOS 5.6 sun4u, SunOS 5.5.1 sun4u, SunOS 4.14 sun4c, SunOS 5.6 i86pc, Irix 6.3 mips5k, HP-UX 10.20 hppa, AIX 4.2.1 ppc, OSF/1 V4.0 alpha, FreeBSD 2.2.2 i86pc and BSDI 3.1 i386.
- Fix `COUNT(*)` problems when the `WHERE` clause didn't match any records. (Bug from 3.21.17.)
- Removed that `NULL = NULL` is true. Now you must use `IS NULL` or `IS NOT NULL` to test whether a value is `NULL`. (This is according to ANSI SQL but may break old applications that are ported from `mSQL`.) You can get the old behaviour by compiling with `-DmSQL_COMPLIANT`.
- Fixed bug that core dumped when using many `LEFT OUTER JOIN` clauses.
- Fixed bug in `ORDER BY` on string formula with possible `NULL` values.
- Fixed problem in range optimiser when using `<=` on sub index.
- Added functions `DAYOFYEAR()`, `DAYOFMONTH()`, `MONTH()`, `YEAR()`, `WEEK()`, `QUARTER()`, `HOUR()`, `MINUTE()`, `SECOND()` and `FIND_IN_SET()`.
- Added `SHOW VARIABLES` command.
- Added support of ``long constant strings'' from ANSI SQL:

      mysql> SELECT 'first ' 'second';       -> 'first second'

- Upgraded Msql-Mysql-modules to 1.1825.
- Upgraded `mysqlaccess` to 2.02.
- Fixed problem with Russian character set and `LIKE`.
- Ported to OpenBSD 2.1.
- New Dutch error messages.

### D.5.13 Changes in release 3.21.21a

- Configure changes for some operating systems.

### D.5.14 Changes in release 3.21.21

- Fixed optimiser bug when using `WHERE data_field = date_field2 AND date_field2 = constant`.
- Added `SHOW STATUS` command.
- Removed ``manual.ps'` from the source distribution to make it smaller.

### D.5.15 Changes in release 3.21.20

- Changed the maximum table name and column name lengths from 32 to 64.
- Aliases can now be of ``any'' length.
- Fixed `mysqladmin stat` to return the right number of queries.
- Changed protocol (downward compatible) to mark if a column has the `AUTO_INCREMENT` attribute or is a `TIMESTAMP`. This is needed for the new Java driver.
- Added Hebrew sorting order by Zeev Suraski.
- Solaris 2.6: Fixed `configure` bugs and increased maximum table size from 2G to 4G.

### D.5.16 Changes in release 3.21.19

- Upgraded `DBD` to 1.1823. This version implements `mysql_use_result` in `DBD-Mysql`.
- Benchmarks updated for empress (by Luuk).
- Fixed a case of slow range searching.
- Configure fixes (``Docs'` directory).
- Added function `REVERSE()` (by Zeev Suraski).

### D.5.17 Changes in release 3.21.18

- Issue error message if client C functions are called in wrong order.
- Added automatic reconnect to the ``libmysql.c'` library. If a write command fails, an automatic reconnect is done.
- Small sort sets no longer use temporary files.
- Upgraded `DBI` to 0.91.
- Fixed a couple of problems with `LEFT OUTER JOIN`.
- Added `CROSS JOIN` syntax. `CROSS` is now a reserved word.
- Recoded `yacc`/`bison` stack allocation to be even safer and to allow MySQL to handle even bigger expressions.
- Fixed a couple of problems with the update log.
- `ORDER BY` was slow when used with key ranges.

### D.5.18 Changes in release 3.21.17

- Changed documentation string of `--with-unix-socket-path` to avoid confusion.
- Added ODBC and ANSI SQL style `LEFT OUTER JOIN`.
- The following are new reserved words: `LEFT`, `NATURAL`, `USING`.
- The client library now uses the value of the environment variable `MYSQL_HOST` as the default host if it's defined.
- `SELECT col_name, SUM(expr)` now returns `NULL` for `col_name` when there are matching rows.
- Fixed problem with comparing binary strings and `BLOB`s with ASCII characters over 127.
- Fixed lock problem: when freeing a read lock on a table with multiple read locks, a thread waiting for a write lock would have been given the lock. This shouldn't affect data integrity, but could possibly make `mysqld` restart if one thread was reading data that another thread modified.
- `LIMIT offset,count` didn't work in `INSERT ... SELECT`.
- Optimised key block caching. This will be quicker than the old algorithm when using bigger key caches.

### D.5.19 Changes in release 3.21.16

- Added ODBC 2.0 & 3.0 functions `POWER()`, `SPACE()`, `COT()`, `DEGREES()`, `RADIANS()`, `ROUND(2 arg)` and `TRUNCATE()`.
- Warning: Incompatible change! `LOCATE()` parameters were swapped according to ODBC standard. Fixed.
- Added function `TIME_TO_SEC()`.
- In some cases, default values were not used for `NOT NULL` fields.
- Timestamp wasn't always updated properly in `UPDATE SET ...` statements.
- Allow empty strings as default values for `BLOB` and `TEXT`, to be compatible with `mysqldump`.

### D.5.20 Changes in release 3.21.15

- Warning: Incompatible change! `mysqlperl` is now from Msql-Mysql-modules. This means that `connect()` now takes `host`, `database`, `user`, `password` arguments! The old version took `host`, `database`, `password`, `user`.
- Allow `DATE '1997-01-01'`, `TIME '12:10:10'` and `TIMESTAMP '1997-01-01 12:10:10'` formats required by ANSI SQL. Warning: Incompatible change! This has the unfortunate side-effect that you no longer can have columns named `DATE`, `TIME` or `TIMESTAMP`. :( Old columns can still be accessed through `tablename.columnname`!)
- Changed Makefiles to hopefully work better with BSD systems. Also, ``manual.dvi'` is now included in the distribution to avoid having stupid `make` programs trying to rebuild it.
- `readline` library upgraded to version 2.1.
- A new sortorder german-1. That is a normal ISO-Latin1 with a german sort order.
- Perl `DBI`/`DBD` is now included in the distribution. `DBI` is now the recommended way to connect to MySQL from Perl.
- New portable benchmark suite with `DBD`, with test results from `mSQL` 2.0.3, MySQL, PostgreSQL 6.2.1 and Solid server 2.2.
- `crash-me` is now included with the benchmarks; this is a Perl program designed to find as many limits as possible in a SQL server. Tested with `mSQL`, PostgreSQL, Solid and MySQL.
- Fixed bug in range-optimiser that crashed MySQL on some queries.
- Table and column name completion for `mysql` command-line tool, by Zeev Suraski and Andi Gutmans.
- Added new command `REPLACE` that works like `INSERT` but replaces conflicting records with the new record. `REPLACE INTO TABLE ... SELECT ...` works also.
- Added new commands `CREATE DATABASE db_name` and `DROP DATABASE db_name`.
- Added `RENAME` option to `ALTER TABLE`: `ALTER TABLE name RENAME TO new_name`.
- `make_binary_distribution` now includes ``libgcc.a'` in ``libmysqlclient.a'`. This should make linking work for people who don't have `gcc`.
- Changed `net_write()` to `my_net_write()` because of a name conflict with Sybase.
- New function `DAYOFWEEK()` compatible with ODBC.
- Stack checking and `bison` memory overrun checking to make MySQL safer with weird queries.

### D.5.21 Changes in release 3.21.14b

- Fixed a couple of small `configure` problems on some platforms.

### D.5.22 Changes in release 3.21.14a

- Ported to SCO Openserver 5.0.4 with FSU Pthreads.
- HP-UX 10.20 should work.
- Added new function `DATE_FORMAT()`.
- Added `NOT IN`.
- Added automatic removal of 'ODBC function conversions': `{fn now() }`
- Handle ODBC 2.50.3 option flags.
- Fixed comparison of `DATE` and `TIME` values with `NULL`.
- Changed language name from germany to german to be consistent with the other language names.
- Fixed sorting problem on functions returning a `FLOAT`. Previously, the values were converted to `INT`s before sorting.
- Fixed slow sorting when sorting on key field when using `key_column=constant`.
- Sorting on calculated `DOUBLE` values sorted on integer results instead.
- `mysql` no longer requires a database argument.
- Changed the place where `HAVING` should be. According to ANSI, it should be after `GROUP BY` but before `ORDER BY`. MySQL Version 3.20 incorrectly had it last.
- Added Sybase command `USE DATABASE` to start using another database.
- Added automatic adjusting of number of connections and table cache size if the maximum number of files that can be opened is less than needed. This should fix that `mysqld` doesn't crash even if you haven't done a `ulimit -n 256` before starting `mysqld`.
- Added lots of limit checks to make it safer when running with too little memory or when doing weird queries.

### D.5.23 Changes in release 3.21.13

- Added retry of interrupted reads and clearing of `errno`. This makes Linux systems much safer!
- Fixed locking bug when using many aliases on the same table in the same `SELECT`.
- Fixed bug with `LIKE` on number key.
- New error message so you can check whether the connection was lost while the command was running or whether the connection was down from the start.
- Added `--table` option to `mysql` to print in table format. Moved time and row information after query result. Added automatic reconnect of lost connections.
- Added `!=` as a synonym for `<>`.
- Added function `VERSION()` to make easier logs.
- New multi-user test ``tests/fork_test.pl'` to put some strain on the thread library.

### D.5.24 Changes in release 3.21.12

- Fixed `ftruncate()` call in MIT-pthreads. This made `isamchk` destroy the ``.ISM'` files on (Free)BSD 2.x systems.
- Fixed broken `__P_` patch in MIT-pthreads.
- Many memory overrun checks. All string functions now return `NULL` if the returned string should be longer than `max_allowed_packet` bytes.
- Changed the name of the `INTERVAL` type to `ENUM`, because `INTERVAL` is used in ANSI SQL.
- In some cases, doing a `JOIN` + `GROUP` + `INTO OUTFILE`, the result wasn't grouped.
- `LIKE` with `'_'` as last character didn't work. Fixed.
- Added extended ANSI SQL `TRIM()` function.
- Added `CURTIME()`.
- Added `ENCRYPT()` function by Zeev Suraski.
- Fixed better `FOREIGN KEY` syntax skipping. New reserved words: `MATCH`, `FULL`, `PARTIAL`.
- `mysqld` now allows IP number and hostname for the `--bind-address` option.
- Added `SET CHARACTER SET cp1251_koi8` to enable conversions of data to and from the `cp1251_koi8` character set.
- Lots of changes for Windows 95 port. In theory, this version should now be easily portable to Windows 95.
- Changed the `CREATE COLUMN` syntax of `NOT NULL` columns to be after the `DEFAULT` value, as specified in the ANSI SQL standard. This will make `mysqldump` with `NOT NULL` and default values incompatible with MySQL Version 3.20.
- Added many function name aliases so the functions can be used with ODBC or ANSI SQL92 syntax.
- Fixed syntax of `ALTER TABLE tbl_name ALTER COLUMN col_name SET DEFAULT NULL`.
- Added `CHAR` and `BIT` as synonyms for `CHAR(1)`.
- Fixed core dump when updating as a user who has only `SELECT` privilege.
- `INSERT ... SELECT ... GROUP BY` didn't work in some cases. An `Invalid use of group function` error occurred.
- When using `LIMIT`, `SELECT` now always uses keys instead of record scan. This will give better performance on `SELECT` and a `WHERE` that matches many rows.
- Added Russian error messages.

### D.5.25 Changes in release 3.21.11

- Configure changes.
- MySQL now works with the new thread library on BSD/OS 3.0.
- Added new group functions `BIT_OR()` and `BIT_AND()`.
- Added compatibility functions `CHECK` and `REFERENCES`. `CHECK` is now a reserved word.
- Added `ALL` option to `GRANT` for better compatibility. (`GRANT` is still a dummy function.)
- Added partly-translated Dutch error messages.
- Fixed bug in `ORDER BY` and `GROUP BY` with `NULL` columns.
- Added function `LAST_INSERT_ID()` SQL function to retrieve last `AUTO_INCREMENT` value. This is intended for clients to ODBC that can't use the `mysql_insert_id()` API function, but can be used by any client.
- Added `--flush-logs` option to `mysqladmin`.
- Added command `STATUS` to `mysql`.
- Fixed problem with `ORDER BY`/`GROUP BY` because of bug in `gcc`.
- Fixed problem with `INSERT ... SELECT ... GROUP BY`.

### D.5.26 Changes in release 3.21.10

- New `mysqlaccess`.
- `CREATE` now supports all ODBC types and the `mSQL` `TEXT` type. All ODBC 2.5 functions are also supported (added `REPEAT`). This provides better portability.
- Added text types `TINYTEXT`, `TEXT`, `MEDIUMTEXT` and `LONGTEXT`. These are actually `BLOB`types, but all searching is done in case-insensitive fashion.
- All old `BLOB` fields are now `TEXT` fields. This only changes that all searching on strings is done in case-sensitive fashion. You must do an `ALTER TABLE` and change the field type to `BLOB` if you want to have tests done in case-sensitive fashion.
- Fixed some `configure` issues.
- Made the locking code a bit safer. Fixed very unlikely deadlock situation.
- Fixed a couple of bugs in the range optimiser. Now the new range benchmark `test-select` works.

### D.5.27 Changes in release 3.21.9

- Added `--enable-unix-socket=pathname` option to `configure`.
- Fixed a couple of portability problems with include files.
- Fixed bug in range calculation that could return empty set when searching on multiple key with only one entry (very rare).
- Most things ported to FSU Pthreads, which should allow MySQL to run on Caldera (SCO). See section 2.6.6.9 Caldera (SCO) Notes.

### D.5.28 Changes in release 3.21.8

- Works now in Solaris 2.6.
- Added handling of calculation of `SUM()` functions. For example, you can now use `SUM(column)/COUNT(column)`.
- Added handling of trigometric functions: `PI()`, `ACOS()`, `ASIN()`, `ATAN()`, `COS()`, `SIN()` and `TAN()`.
- New languages: norwegian, norwegian-ny and portuguese.
- Fixed parameter bug in `net_print()` in ``procedure.cc'`.
- Fixed a couple of memory leaks.
- Now allow also the old `SELECT ... INTO OUTFILE` syntax.
- Fixed bug with `GROUP BY` and `SELECT` on key with many values.
- `mysql_fetch_lengths()` sometimes returned incorrect lengths when you used `mysql_use_result()`. This affected at least some cases of `mysqldump --quick`.
- Fixed bug in optimisation of `WHERE const op field`.
- Fixed problem when sorting on `NULL` fields.
- Fixed a couple of 64-bit (Alpha) problems.
- Added `--pid-file=#` option to `mysqld`.
- Added date formatting to `FROM_UNIXTIME()`, originally by Zeev Suraski.
- Fixed bug in `BETWEEN` in range optimiser (did only test = of the first argument).
- Added machine-dependent files for MIT-pthreads i386-SCO. There is probably more to do to get this to work on SCO 3.5.

### D.5.29 Changes in release 3.21.7

- Changed ``Makefile.am'` to take advantage of Automake 1.2.
- Added the beginnings of a benchmark suite.
- Added more secure password handling.
- Added new client function `mysql_errno()`, to get the error number of the error message. This makes error checking in the client much easier. This makes the new server incompatible with the 3.20.x server when running without `--old-protocol`. The client code is backward-compatible. More information can be found in the ``README'` file!
- Fixed some problems when using very long, illegal names.

### D.5.30 Changes in release 3.21.6

- Fixed more portability issues (incorrect `sigwait` and `sigset` defines).
- `configure` should now be able to detect the last argument to `accept()`.

### D.5.31 Changes in release 3.21.5

- Should now work with FreeBSD 3.0 if used with ``FreeBSD-3.0-libc_r-1.0.diff'`, which can be found at http://www.mysql.com/Downloads/Patches/.
- Added new `-O tmp_table_size=#` option to `mysqld`.
- New function `FROM_UNIXTIME(timestamp)` which returns a date string in `'YYYY-MM-DD HH:MM:DD'` format.
- New function `SEC_TO_TIME(seconds)` which returns a string in `'HH:MM:SS'` format.
- New function `SUBSTRING_INDEX()`, originally by Zeev Suraski.

### D.5.32 Changes in release 3.21.4

- Should now configure and compile on OSF/1 4.0 with the DEC compiler.
- Configuration and compilation on BSD/OS 3.0 works, but due to some bugs in BSD/OS 3.0, `mysqld` doesn't work on it yet.
- Configuration and compilation on FreeBSD 3.0 works, but I couldn't get `pthread_create` to work.

### D.5.33 Changes in release 3.21.3

- Added reverse check lookup of hostnames to get better security.
- Fixed some possible buffer overflows if filenames that are too long are used.
- `mysqld` doesn't accept hostnames that start with digits followed by a `'.'`, because the hostname may look like an IP number.
- Added `--skip-networking` option to `mysqld`, to allow only socket connections. (This will not work with MIT-pthreads!)
- Added check of too long table names for alias.
- Added check if database name is okay.
- Added check if too long table names.
- Removed incorrect `free()` that killed the server on `CREATE DATABASE` or `DROP DATABASE`.
- Changed some `mysqld` `-O` options to better names.
- Added `-O join_cache_size=#` option to `mysqld`.
- Added `-O max_join_size=#` option to `mysqld`, to be able to set a limit how big queries (in this case big = slow) one should be able to handle without specifying `SET SQL_BIG_SELECTS=1`. A # = is about 10 examined records. The default is ``unlimited''.
- When comparing a `TIME`, `DATE`, `DATETIME` or `TIMESTAMP` column to a constant, the constant is converted to a time value before performing the comparison. This will make it easier to get ODBC (particularly Access97) to work with the above types. It should also make dates easier to use and the comparisons should be quicker than before.
- Applied patch from Jochen Wiedmann that allows `query()` in `mysqlperl` to take a query with `\0` in it.
- Storing a timestamp with a 2-digit year (`YYMMDD`) didn't work.
- Fix that timestamp wasn't automatically updated if set in an `UPDATE` clause.
- Now the automatic timestamp field is the FIRST timestamp field.
- `SELECT * INTO OUTFILE`, which didn't correctly if the outfile already existed.
- `mysql` now shows the thread ID when starting or doing a reconnect.
- Changed the default sort buffer size from 2M to 1M.

### D.5.34 Changes in release 3.21.2

- The range optimiser is coded, but only 85% tested. It can be enabled with `--new`, but it crashes core a lot yet...
- More portable. Should compile on AIX and alpha-digital. At least the `isam` library should be relatively 64-bit clean.
- New `isamchk` which can detect and fix more problems.
- New options for `isamlog`.
- Using new version of Automake.
- Many small portability changes (from the AIX and alpha-digital port) Better checking of pthread(s) library.
- czech error messages by snajdr@pvt.net.
- Decreased size of some buffers to get fewer problems on systems with little memory. Also added more checks to handle ``out of memory'' problems.
- `mysqladmin`: you can now do `mysqladmin kill 5,6,7,8` to kill multiple threads.
- When the maximum connection limit is reached, one extra connection by a user with the process_acl privilege is granted.
- Added `-O backlog=#` option to `mysqld`.
- Increased maximum packet size from 512K to 1024K for client.
- Almost all of the function code is now tested in the internal test suite.
- `ALTER TABLE` now returns warnings from field conversions.
- Port changed to 3306 (got it reserved from ISI).
- Added a fix for Visual FoxBase so that any schema name from a table specification is automatically removed.
- New function `ASCII()`.
- Removed function `BETWEEN(a,b,c)`. Use the standard ANSI syntax instead: `expr BETWEEN expr AND expr`.
- MySQL no longer has to use an extra temporary table when sorting on functions or `SUM()` functions.
- Fixed bug that you couldn't use `tbl_name.field_name` in `UPDATE`.
- Fixed `SELECT DISTINCT` when using 'hidden group'. For example:

      mysql> SELECT DISTINCT MOD(some_field,10) FROM test
          ->        GROUP BY some_field;

  Note: `some_field` is normally in the `SELECT` part. ANSI SQL should require it.

### D.5.35 Changes in release 3.21.0

- New reserved words used: `INTERVAL`, `EXPLAIN`, `READ`, `WRITE`, `BINARY`.
- Added ODBC function `CHAR(num,...)`.
- New operator `IN`. This uses a binary search to find a match.
- New command `LOCK TABLES tbl_name [AS alias] {READ|WRITE} ...`
- Added `--log-update` option to `mysqld`, to get a log suitable for incremental updates.
- New command `EXPLAIN SELECT ...` to get information about how the optimiser will do the join.
- For easier client code, the client should no longer use `FIELD_TYPE_TINY_BLOB`, `FIELD_TYPE_MEDIUM_BLOB`, `FIELD_TYPE_LONG_BLOB` or `FIELD_TYPE_VAR_STRING` (as previously returned by `mysql_list_fields`). You should instead only use `FIELD_TYPE_BLOB` or `FIELD_TYPE_STRING`. If you want exact types, you should use the command `SHOW FIELDS`.
- Added varbinary syntax: `0x######` which can be used as a string (default) or a number.
- `FIELD_TYPE_CHAR` is renamed to `FIELD_TYPE_TINY`.
- Changed all fields to C++ classes.
- Removed FORM struct.
- Fields with `DEFAULT` values no longer need to be `NOT NULL`.
- New field types:
  `ENUM`
    A string which can take only a couple of defined values. The value is stored as a 1-3 byte number that is mapped automatically to a string. This is sorted according to string positions!
  `SET`
    A string which may have one or many string values separated with ','. The string is stored as a 1-, 2-, 3-, 4- or 8-byte number where each bit stands for a specific set member. This is sorted according to the unsigned value of the stored packed number.
- Now all function calculation is done with `double` or `long long`. This will provide the full 64-bit range with bit functions and fix some conversions that previously could result in precision losses. One should avoid using `unsigned long long` columns with full 64-bit range (numbers bigger than 9223372036854775807) because calculations are done with `signed long long`.
- `ORDER BY` will now put `NULL` field values first. `GROUP BY` will also work with `NULL` values.
- Full `WHERE` with expressions.
- New range optimiser that can resolve ranges when some keypart prefix is constant. Example:

      mysql> SELECT * FROM tbl_name
          ->          WHERE key_part_1="customer"
          ->          AND key_part_2>=10 AND key_part_2<=10;

## D.6 Changes in release 3.20.x

Version 3.20 is quite old now, and should be avoided if possible. This information is kept here for historical purposes only.

Changes from 3.20.18 to 3.20.32b are not documented here because the 3.21 release branched here. And the relevant changes are also documented as changes to the 3.21 version.

### D.6.1 Changes in release 3.20.18

- Added `-p#` (remove `#` directories from path) to `isamlog`. All files are written with a relative path from the database directory Now `mysqld` shouldn't crash on shutdown when using the `--log-isam` option.
- New `mysqlperl` version. It is now compatible with `msqlperl-0.63`.
- New `DBD` module available at http://www.mysql.com/Downloads/Contrib/ site.
- Added group function `STD()` (standard deviation).
- The `mysqld` server is now compiled by default without debugging information. This will make the daemon smaller and faster.
- Now one usually only has to specify the `--basedir` option to `mysqld`. All other paths are relative in a normal installation.
- `BLOB` columns sometimes contained garbage when used with a `SELECT` on more than one table and `ORDER BY`.
- Fixed that calculations that are not in `GROUP BY` work as expected (ANSI SQL extension). Example:

      mysql> SELECT id,id+1 FROM table GROUP BY id;

- The test of using `MYSQL_PWD` was reversed. Now `MYSQL_PWD` is enabled as default in the default release.
- Fixed conversion bug which caused `mysqld` to core dump with Arithmetic error on SPARC-386.
- Added `--unbuffered` option to `mysql`, for new `mysqlaccess`.
- When using overlapping (unnecessary) keys and join over many tables, the optimiser could get confused and return 0 records.

### D.6.2 Changes in release 3.20.17

- You can now use `BLOB` columns and the functions `IS NULL` and `IS NOT NULL` in the `WHERE` clause.
- All communication packets and row buffers are now allocated dynamically on demand. The default value of `max_allowed_packet` is now 64K for the server and 512K for the client. This is mainly used to catch incorrect packets that could trash all memory. The server limit may be changed when it is started.
- Changed stack usage to use less memory.
- Changed `safe_mysqld` to check for running daemon.
- The `ELT()` function is renamed to `FIELD()`. The new `ELT()` function returns a value based on an index: `FIELD()` is the inverse of `ELT()` Example: `ELT(2,"A","B","C")` returns `"B"`. `FIELD("B","A","B","C")` returns `2`.
- `COUNT(field)`, where `field` could have a `NULL` value, now works.
- A couple of bugs fixed in `SELECT ... GROUP BY`.
- Fixed memory overrun bug in `WHERE` with many unoptimisable brace levels.
- Fixed some small bugs in the grant code.
- If hostname isn't found by `get_hostname`, only the IP is checked. Previously, you got `Access denied`.
- Inserts of timestamps with values didn't always work.
- `INSERT INTO ... SELECT ... WHERE` could give the error `Duplicated field`.
- Added some tests to `safe_mysqld` to make it ``safer''.
- `LIKE` was case-sensitive in some places and case-insensitive in others. Now `LIKE` is always case-insensitive.
- ``mysql.cc'`: Allow `'#'` anywhere on the line.
- New command `SET SQL_SELECT_LIMIT=#`. See the FAQ for more details.
- New version of the `mysqlaccess` script.
- Change `FROM_DAYS()` and `WEEKDAY()` to also take a full `TIMESTAMP` or `DATETIME` as argument. Before they only took a number of type `YYYYMMDD` or `YYMMDD`.
- Added new function `UNIX_TIMESTAMP(timestamp_column)`.

### D.6.3 Changes in release 3.20.16

- More changes in MIT-pthreads to get them safer. Fixed also some link bugs at least in SunOS.
- Changed `mysqld` to work around a bug in MIT-pthreads. This makes multiple small `SELECT` operations 20 times faster. Now `lock_test.pl` should work.
- Added `mysql_FetchHash(handle)` to `mysqlperl`.
- The `mysqlbug` script is now distributed built to allow for reporting bugs that appear during the build with it.
- Changed ``libmysql.c'` to prefer `getpwuid()` instead of `cuserid()`.
- Fixed bug in `SELECT` optimiser when using many tables with the same column used as key to different tables.
- Added new `latin2` and Russian `KOI8` character tables.
- Added support for a dummy `GRANT` command to satisfy Powerbuilder.

### D.6.4 Changes in release 3.20.15

- Fixed fatal bug `packets out of order` when using MIT-pthreads.
- Removed possible loop when a thread waits for command from client and `fcntl()` fails. Thanks to Mike Bretz for finding this bug.
- Changed alarm loop in ``mysqld.cc'` because shutdown didn't always succeed in Linux.
- Removed use of `termbits` from ``mysql.cc'`. This conflicted with `glibc` 2.0.
- Fixed some syntax errors for at least BSD and Linux.
- Fixed bug when doing a `SELECT` as superuser without a database.
- Fixed bug when doing `SELECT` with group calculation to outfile.

### D.6.5 Changes in release 3.20.14

- If one gives `-p` or `--password` option to `mysql` without an argument, the user is solicited for the password from the tty.
- Added default password from `MYSQL_PWD` (by Elmar Haneke).
- Added command `kill` to `mysqladmin` to kill a specific MySQL thread.
- Sometimes when doing a reconnect on a down connection this succeeded first on second try.
- Fixed adding an `AUTO_INCREMENT` key with `ALTER_TABLE`.
- `AVG()` gave too small value on some `SELECT`s with `GROUP BY` and `ORDER BY`.
- Added new `DATETIME` type (by Giovanni Maruzzelli maruzz@matrice.it).
- Fixed that defining `DONT_USE_DEFAULT_FIELDS` works.
- Changed to use a thread to handle alarms instead of signals on Solaris to avoid race conditions.
- Fixed default length of signed numbers. (George Harvey georgeh@pinacl.co.uk.)
- Allow anything for `CREATE INDEX`.
- Add prezeros when packing numbers to `DATE`, `TIME` and `TIMESTAMP`.
- Fixed a bug in `OR` of multiple tables (gave empty set).
- Added many patches to MIT-pthreads. This fixes at least one lookup bug.

### D.6.6 Changes in release 3.20.13

- Added ANSI SQL94 `DATE` and `TIME` types.
- Fixed bug in `SELECT` with `AND`-`OR` levels.
- Added support for Slovenian characters. The ``Contrib'` directory contains source and instructions for adding other character sets.
- Fixed bug with `LIMIT` and `ORDER BY`.
- Allow `ORDER BY` and `GROUP BY` on items that aren't in the `SELECT` list. (Thanks to Wim Bonis bonis@kiss.de, for pointing this out.)
- Allow setting of timestamp values in `INSERT`.
- Fixed bug with `SELECT ... WHERE ... = NULL`.
- Added changes for `glibc` 2.0. To get `glibc` to work, you should add the ``gibc-2.0-sigwait-patch'` before compiling `glibc`.
- Fixed bug in `ALTER TABLE` when changing a `NOT NULL` field to allow `NULL` values.
- Added some ANSI92 synonyms as field types to `CREATE TABLE`. `CREATE TABLE` now allows `FLOAT(4)` and `FLOAT(8)` to mean `FLOAT` and `DOUBLE`.
- New utility program `mysqlaccess` by Yves.Carlier@rug.ac.be. This program shows the access rights for a specific user and the grant rows that determine this grant.
- Added `WHERE const op field` (by bonis@kiss.de).

### D.6.7 Changes in release 3.20.11

- When using `SELECT ... INTO OUTFILE`, all temporary tables are ISAM instead of HEAP to allow big dumps.
- Changed date functions to be string functions. This fixed some ``funny'' side effects when sorting on dates.
- Extended `ALTER TABLE` according to SQL92.
- Some minor compatibility changes.
- Added `--port` and `--socket` options to all utility programs and `mysqld`.
- Fixed MIT-pthreads `readdir_r()`. Now `mysqladmin create database` and `mysqladmin drop database` should work.
- Changed MIT-pthreads to use our `tempnam()`. This should fix the ``sort aborted'' bug.
- Added sync of records count in `sql_update`. This fixed slow updates on first connection. (Thanks to Vaclav Bittner for the test.)

### D.6.8 Changes in release 3.20.10

- New insert type: `INSERT INTO ... SELECT ...`
- `MEDIUMBLOB` fixed.
- Fixed bug in `ALTER TABLE` and `BLOB`s.
- `SELECT ... INTO OUTFILE` now creates the file in the current database directory.
- `DROP TABLE` now can take a list of tables.
- Oracle synonym `DESCRIBE` (`DESC`).
- Changes to `make_binary_distribution`.
- Added some comments to installation instructions about `configure`'s C++ link test.
- Added `--without-perl` option to `configure`.
- Lots of small portability changes.

### D.6.9 Changes in release 3.20.9

- `ALTER TABLE` didn't copy null bit. As a result, fields that were allowed to have `NULL` values were always `NULL`.
- `CREATE` didn't take numbers as `DEFAULT`.
- Some compatibility changes for SunOS.
- Removed ``config.cache'` from old distribution.

### D.6.10 Changes in release 3.20.8

- Fixed bug with `ALTER TABLE` and multi-part keys.

### D.6.11 Changes in release 3.20.7

- New commands: `ALTER TABLE`, `SELECT ... INTO OUTFILE` and `LOAD DATA INFILE`.
- New function: `NOW()`.
- Added new field `File_priv` to `mysql/user` table.
- New script `add_file_priv` which adds the new field `File_priv` to the `user` table. This script must be executed if you want to use the new `SELECT ... INTO` and `LOAD DATA INFILE ...` commands with a version of MySQL earlier than 3.20.7.
- Fixed bug in locking code, which made `lock_test.pl` test fail.
- New files ``NEW'` and ``BUGS'`.
- Changed ``select_test.c'` and ``insert_test.c'` to include ``config.h'`.
- Added `status` command to `mysqladmin` for short logging.
- Increased maximum number of keys to 16 and maximum number of key parts to 15.
- Use of sub keys. A key may now be a prefix of a string field.
- Added `-k` option to `mysqlshow`, to get key information for a table.
- Added long options to `mysqldump`.

### D.6.12 Changes in release 3.20.6

- Portable to more systems because of MIT-pthreads, which will be used automatically if `configure` cannot find a `-lpthreads` library.
- Added GNU-style long options to almost all programs. Test with ``program` --help`.
- Some shared library support for Linux.
- The FAQ is now in ``.texi'` format and is available in ``.html'`, ``.txt'` and ``.ps'` formats.
- Added new SQL function `RAND([init])`.
- Changed `sql_lex` to handle `\0` unquoted, but the client can't send the query through the C API, because it takes a str pointer. You must use `mysql_real_query()` to send the query.
- Added API function `mysql_get_client_info()`.
- `mysqld` now uses the `N_MAX_KEY_LENGTH` from ``nisam.h'` as the maximum allowable key length.
- The following now works:

      mysql> SELECT filter_nr,filter_nr FROM filter ORDER BY filter_nr;

  Previously, this resulted in the error: `Column: 'filter_nr' in order clause is ambiguous`.
- `mysql` now outputs `'\0'`, `'\t'`, `'\n'` and `'\\'` when encountering ASCII 0, tab, newline or `'\'` while writing tab-separated output. This is to allow printing of binary data in a portable format. To get the old behaviour, use `-r` (or `--raw`).
- Added german error messages (60 of 80 error messages translated).
- Added new API function `mysql_fetch_lengths(MYSQL_RES *)`, which returns an array of column lengths (of type `uint`).
- Fixed bug with `IS NULL` in `WHERE` clause.
- Changed the optimiser a little to get better results when searching on a key part.
- Added `SELECT` option `STRAIGHT_JOIN` to tell the optimiser that it should join tables in the given order.
- Added support for comments starting with `'--'` in ``mysql.cc'` (Postgres syntax).
- You can have `SELECT` expressions and table columns in a `SELECT` which are not used in the group part. This makes it efficient to implement lookups. The column that is used should be a constant for each group because the value is calculated only once for the first row that is found for a group.

      mysql> SELECT id,lookup.text,SUM(*) FROM test,lookup
          ->        WHERE test.id=lookup.id GROUP BY id;

- Fixed bug in `SUM(function)` (could cause a core dump).
- Changed `AUTO_INCREMENT` placement in the SQL query:

      INSERT INTO table (auto_field) VALUES (0);

  inserted 0, but it should insert an `AUTO_INCREMENT` value.
- ``mysqlshow.c'`: Added number of records in table. Had to change the client code a little to fix this.
- `mysql` now allows doubled `''` or `""` within strings for embedded `'` or `"`.
- New math functions: `EXP()`, `LOG()`, `SQRT()`, `ROUND()`, `CEILING()`.

### D.6.13 Changes in release 3.20.3

- The `configure` source now compiles a thread-free client library `-lmysqlclient`. This is the only library that needs to be linked with client applications. When using the binary releases, you must link with `-lmysql -lmysys -ldbug -lmystrings` as before.
- New `readline` library from `bash-2.0`.
- LOTS of small changes to `configure` and makefiles (and related source).
- It should now be possible to compile in another directory using `VPATH`. Tested with GNU Make 3.75.
- `safe_mysqld` and `mysql.server` changed to be more compatible between the source and the binary releases.
- `LIMIT` now takes one or two numeric arguments. If one argument is given, it indicates the maximum number of rows in a result. If two arguments are given, the first argument indicates the offset of the first row to return, the second is the maximum number of rows. With this it's easy to do a poor man's next page/previous page WWW application.
- Changed name of SQL function `FIELDS()` to `ELT()`. Changed SQL function `INTERVALL()` to `INTERVAL()`.
- Made `SHOW COLUMNS` a synonym for `SHOW FIELDS`. Added compatibility syntax `FRIEND KEY` to `CREATE TABLE`. In MySQL, this creates a non-unique key on the given columns.
- Added `CREATE INDEX` and `DROP INDEX` as compatibility functions. In MySQL, `CREATE INDEX` only checks if the index exists and issues an error if it doesn't exist. `DROP INDEX` always succeeds.
- ``mysqladmin.c'`: added client version to version information.
- Fixed core dump bug in `sql_acl` (core on new connection).
- Removed `host`, `user` and `db` tables from database `test` in the distribution.
- `FIELD_TYPE_CHAR` can now be signed (-128 to 127) or unsigned (0 to 255) Previously, it was always unsigned.
- Bug fixes in `CONCAT()` and `WEEKDAY()`.
- Changed a lot of source to get `mysqld` to be compiled with SunPro compiler.
- SQL functions must now have a `'('` immediately after the function name (no intervening space). For example, `'USER('` is regarded as beginning a function call, and `'USER ('` is regarded as an identifier `USER` followed by a `'('`, not as a function call.

### D.6.14 Changes in release 3.20.0

- The source distribution is done with `configure` and Automake. It will make porting much easier. The `readline` library is included in the distribution.
- Separate client compilation: the client code should be very easy to compile on systems which don't have threads.
- The old Perl interface code is automatically compiled and installed. Automatic compiling of `DBD` will follow when the new `DBD` code is ported.
- Dynamic language support: `mysqld` can now be started with Swedish or English (default) error messages.
- New functions: `INSERT()`, `RTRIM()`, `LTRIM()` and `FORMAT()`.
- `mysqldump` now works correctly for all field types (even `AUTO_INCREMENT`). The format for `SHOW FIELDS FROM tbl_name` is changed so the `Type` column contains information suitable for `CREATE TABLE`. In previous releases, some `CREATE TABLE` information had to be patched when re-creating tables.
- Some parser bugs from 3.19.5 (`BLOB` and `TIMESTAMP`) are corrected. `TIMESTAMP` now returns different date information depending on its create length.
- Changed parser to allow a database, table or field name to start with a number or `'_'`.
- All old C code from Unireg changed to C++ and cleaned up. This makes the daemon a little smaller and easier to understand.
- A lot of small bug fixes done.
- New ``INSTALL'` files (not final version) and some information regarding porting.

## D.7 Changes in release 3.19.x

Version 3.19 is quite old now, and should be avoided if possible. This information is kept here for historical purposes only.

### D.7.1 Changes in release 3.19.5

- Some new functions, some more optimisation on joins.
- Should now compile clean on Linux (2.0.x).
- Added functions `DATABASE()`, `USER()`, `POW()`, `LOG10()` (needed for ODBC).
- In a `WHERE` with an `ORDER BY` on fields from only one table, the table is now preferred as first table in a multi-join.
- `HAVING` and `IS NULL` or `IS NOT NULL` now works.
- A group on one column and a sort on a group function (`SUM()`, `AVG()`...) didn't work together. Fixed.
- `mysqldump`: Didn't send password to server.

### D.7.2 Changes in release 3.19.4

- Fixed horrible locking bug when inserting in one thread and reading in another thread.
- Fixed one-off decimal bug. 1.00 was output as 1.0.
- Added attribute `'Locked'` to process list as info if a query is locked by another query.
- Fixed full magic timestamp. Timestamp length may now be 14, 12, 10, 8, 6, 4 or 2 bytes.
- Sort on some numeric functions could sort incorrectly on last number.
- `IF(arg,syntax_error,syntax_error)` crashed.
- Added functions `CEILING()`, `ROUND()`, `EXP()`, `LOG()` and `SQRT()`.
- Enhanced `BETWEEN` to handle strings.

### D.7.3 Changes in release 3.19.3

- Fixed `SELECT` with grouping on `BLOB` columns not to return incorrect `BLOB` info. Grouping, sorting and distinct on `BLOB` columns will not yet work as expected (probably it will group/sort by the first 7 characters in the `BLOB`). Grouping on formulas with a fixed string size (use `MID()` on a `BLOB`) should work.
- When doing a full join (no direct keys) on multiple tables with `BLOB` fields, the `BLOB` was garbage on output.
- Fixed `DISTINCT` with calculated columns.

# E Porting to Other Systems

This appendix will help you port MySQL to other operationg systems. Do check the list of currently supported operating systems first. See section 2.2.2 Operating Systems Supported by MySQL. If you have created a new port of MySQL, please let us know so that we can list it here and on our web site (http://www.mysql.com/), recommending it to other users.

Note: If you create a new port of MySQL, you are free to copy and distribute it under the GPL license, but it does not make you a copyright holder of MySQL.

A working Posix thread library is needed for the server. On Solaris 2.5 we use Sun PThreads (the native thread support in 2.4 and earlier versions are not good enough) and on Linux we use LinuxThreads by Xavier Leroy, Xavier.Leroy@inria.fr.

The hard part of porting to a new Unix variant without good native thread support is probably to port MIT-pthreads. See ``mit-pthreads/README'` and Programming POSIX Threads (http://www.humanfactor.com/pthreads/).

The MySQL distribution includes a patched version of Provenzano's Pthreads from MIT (see the MIT Pthreads web page at http://www.mit.edu:8001/people/proven/pthreads.html). This can be used for some operating systems that do not have POSIX threads.

It is also possible to use another user level thread package named FSU Pthreads (see FSU Pthreads home page). This implementation is being used for the SCO port.

See the ``thr_lock.c'` and ``thr_alarm.c'` programs in the ``mysys'` directory for some tests/examples of these problems.

Both the server and the client need a working C++ compiler (we use `gcc` and have tried SPARCworks). Another compiler that is known to work is the Irix `cc`.

To compile only the client use `./configure --without-server`.

There is currently no support for only compiling the server, nor is it likly to be added unless someone has a good reason for it.

If you want/need to change any ``Makefile'` or the configure script you must get Automake and Autoconf. We have used the `automake-1.2` and `autoconf-2.12` distributions.

All steps needed to remake everything from the most basic files.

    /bin/rm */.deps/*.P
    /bin/rm -f config.cache
    aclocal
    autoheader
    aclocal
    automake
    autoconf
    ./configure --with-debug=full --prefix='your installation directory'

    # The makefiles generated above need GNU make 3.75 or newer.
    # (called gmake below)
    gmake clean all install init-db

If you run into problems with a new port, you may have to do some debugging of MySQL! See section E.1 Debugging a MySQL server.

Note: before you start debugging `mysqld`, first get the test programs `mysys/thr_alarm` and `mysys/thr_lock` to work. This will ensure that your thread installation has even a remote chance to work!

## E.1 Debugging a MySQL server

If you are using some functionality that is very new in MySQL, you can try to run `mysqld` with the `--skip-new` (which will disable all new, potentially unsafe functionality) or with `--safe-mode` which disables a lot of optimisation that may cause problems. See section A.4.1 What To Do If MySQL Keeps Crashing.

If `mysqld` doesn't want to start, you should check that you don't have any ``my.cnf'` files that interfere with your setup! You can check your ``my.cnf'` arguments with `mysqld --print-defaults` and avoid using them by starting with `mysqld --no-defaults ...`.

If `mysqld` starts to eat up CPU or memory or if it ``hangs'', you can use `mysqladmin processlist status` to find out if someone is executing a query that takes a long time. It may be a good idea to run `mysqladmin -i10 processlist status` in some window if you are experiencing performance problems or problems when new clients can't connect.

The command `mysqladmin debug` will dump some information about locks in use, used memory and query usage to the mysql log file. This may help solve some problems. This command also provides some useful information even if you haven't compiled MySQL for debugging!

If the problem is that some tables are getting slower and slower you should try to optimise the table with `OPTIMIZE TABLE` or `myisamchk`. See section 4 Database Administration. You should also check the slow queries with `EXPLAIN`.

You should also read the OS-specific section in this manual for problems that may be unique to your environment. See section 2.6 Operating System Specific Notes.

### E.1.1 Compiling MYSQL for Debugging

If you have some very specific problem, you can always try to debug MySQL. To do this you must configure MySQL with the `--with-debug` or the `--with-debug=full` option. You can check whether MySQL was compiled with debugging by doing: `mysqld --help`. If the `--debug` flag is listed with the options then you have debugging enabled. `mysqladmin ver` also lists the `mysqld` version as `mysql ... --debug` in this case.

If you are using gcc or egcs, the recommended configure line is:

    CC=gcc CFLAGS="-O2" CXX=gcc CXXFLAGS="-O2 -felide-constructors \
       -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql \
       --with-debug --with-extra-charsets=complex

This will avoid problems with the `libstdc++` library and with C++ exceptions (many compilers have problems with C++ exceptions in threaded code) and compile a MySQL version with support for all character sets.

If you suspect a memory overrun error, you can configure MySQL with `--with-debug=full`, which will install a memory allocation (`SAFEMALLOC`) checker. Running with `SAFEMALLOC` is however quite slow, so if you get performance problems you should start `mysqld` with the `--skip-safemalloc` option. This will disable the memory overrun checks for each call to `malloc` and `free`.

If `mysqld` stops crashing when you compile it with `--with-debug`, you have probably found a compiler bug or a timing bug within MySQL. In this case you can try to add `-g` to the `CFLAGS` and `CXXFLAGS` variables above and not use `--with-debug`. If `mysqld` now dies, you can at least attach to it with `gdb` or use `gdb` on the core file to find out what happened.

When you configure MySQL for debugging you automatically enable a lot of extra safety check functions that monitor the health of `mysqld`. If they find something ``unexpected,'' an entry will be written to `stderr`, which `safe_mysqld` directs to the error log! This also means that if you are having some unexpected problems with MySQL and are using a source distribution, the first thing you should do is to configure MySQL for debugging! (The second thing, of course, is to send mail to mysql@lists.mysql.com and ask for help. Please use the `mysqlbug` script for all bug reports or questions regarding the MySQL version you are using!

In the Windows MySQL distribution, `mysqld.exe` is by default compiled with support for trace files.

### E.1.2 Creating Trace Files

If the `mysqld` server doesn't start or if you can cause the `mysqld` server to crash quickly, you can try to create a trace file to find the problem.

To do this you have to have a `mysqld` that is compiled for debugging. You can check this by executing `mysqld -V`. If the version number ends with `-debug`, it's compiled with support for trace files.

Start the `mysqld` server with a trace log in ``/tmp/mysqld.trace'` (or ``C:\mysqld.trace'` on Windows):

`mysqld --debug`

On Windows you should also use the `--standalone` flag to not start `mysqld` as a service:

In a DOS window do:

    mysqld --debug --standalone

After this you can use the `mysql.exe` command-line tool in a second DOS window to reproduce the problem. You can take down the above `mysqld` server with `mysqladmin shutdown`.

Note that the trace file will get very big! If you want to have a smaller trace file, you can use something like:

`mysqld --debug=d,info,error,query,general,where:O,/tmp/mysqld.trace`

which only prints information with the most interesting tags in ``/tmp/mysqld.trace'`.

If you make a bug report about this, please only send the lines from the trace file to the appropriate mailing list where something seems to go wrong! If you can't locate the wrong place, you can ftp the trace file, together with a full bug report, to ftp://support.mysql.com/pub/mysql/secret/ so that a MySQL developer can take a look a this.

The trace file is made with the DBUG package by Fred Fish. See section E.3 The DBUG Package.

### E.1.3 Debugging mysqld under gdb

On most systems you can also start `mysqld` from `gdb` to get more information if `mysqld` crashes.

With some older `gdb` versions on Linux you must use `run --one-thread` if you want to be able to debug `mysqld` threads. In this case you can only have one thread active at a time. We recommend you to upgrade to gdb 5.1 ASAP as thread debugging works much better with this version!

When running `mysqld` under gdb, you should disable the stack trace with `--skip-stack-trace` to be able to catch segfaults within gdb.

It's very hard to debug MySQL under `gdb` if you do a lot of new connections the whole time as `gdb` doesn't free the memory for old threads. You can avoid this problem by starting `mysqld` with `-O thread_cache_size= 'max_connections +1'`. In most cases just using `-O thread_cache_size=5'` will help a lot!

If you want to get a core dump on Linux if `mysqld` dies with a SIGSEGV signal, you can start `mysqld` with the `--core-file` option. This core file can be used to make a backtrace that may help you find out why `mysqld` died:

    shell> gdb mysqld core
    gdb>   backtrace full
    gdb>   exit

See section A.4.1 What To Do If MySQL Keeps Crashing.

If you are using gdb 4.17.x or above on Linux, you should install a ``.gdb'` file, with the following information, in your current directory:

    set print sevenbit off
    handle SIGUSR1 nostop noprint
    handle SIGUSR2 nostop noprint
    handle SIGWAITING nostop noprint
    handle SIGLWP nostop noprint
    handle SIGPIPE nostop
    handle SIGALRM nostop
    handle SIGHUP nostop
    handle SIGTERM nostop noprint

If you have problems debugging threads with gdb, you should download gdb 5.x and try this instead. The new gdb version has very improved thread handling!

Here is an example how to debug mysqld:

    shell> gdb /usr/local/libexec/mysqld
    gdb> run
    ...
    backtrace full # Do this when mysqld crashes

Include the above output in a mail generated with `mysqlbug` and mail this to `mysql@lists.mysql.com`.

If `mysqld` hangs you can try to use some system tools like `strace` or `/usr/proc/bin/pstack` to examine where `mysqld` has hung.

    strace /tmp/log libexec/mysqld

If you are using the Perl `DBI` interface, you can turn on debugging information by using the `trace` method or by setting the `DBI_TRACE` environment variable. See section 8.2.2 The `DBI` Interface.

### E.1.4 Using a Stack Trace

On some operating systems, the error log will contain a stack trace if `mysqld` dies unexpectedly. You can use this to find out where (and maybe why) `mysqld` died. See section 4.9.1 The Error Log. To get a stack trace, you must not compile `mysqld` with the `-fomit-frame-pointer` option to gcc. See section E.1.1 Compiling MYSQL for Debugging.

If the error file contains something like the following:

    mysqld got signal 11;
    The manual section 'Debugging a MySQL server' tells you how to use a
    stack trace and/or the core file to produce a readable backtrace that may
    help in finding out why mysqld died
    Attemping backtrace. You can use the following information to find out
    where mysqld died.  If you see no messages after this, something went
    terribly wrong
    stack range sanity check, ok, backtrace follows
    0x40077552
    0x81281a0
    0x8128f47
    0x8127be0
    0x8127995
    0x8104947
    0x80ff28f
    0x810131b
    0x80ee4bc
    0x80c3c91
    0x80c6b43
    0x80c1fd9
    0x80c1686

you can find where `mysqld` died by doing the following:

1. Copy the above numbers to a file, for example ``mysqld.stack'`.
2. Make a symbol file for the `mysqld` server:

       nm -n libexec/mysqld > /tmp/mysqld.sym

   Note that many MySQL binary distributions comes with the above file, named `mysqld.sym.gz`. In this case you must unpack this by doing:

       gunzip < bin/mysqld.sym.gz > /tmp/mysqld.sym

3. Execute `resolve_stack_dump -s /tmp/mysqld.sym -n mysqld.stack`. This will print out where `mysqld` died. If this doesn't help you find out why `mysqld` died, you should make a bug report and include the output from the above commend with the bug report. Note however that in most cases it will not help us to just have a stack trace to find the reason for the problem. To be able to locate the bug or provide a workaround, we would in most cases need to know the query that killed `mysqld` and preferable a test case so that we can repeat the problem! See section 1.6.2.3 How to Report Bugs or Problems.

### E.1.5 Using Log Files to Find Cause of Errors in mysqld

Note that before starting `mysqld` with `--log` you should check all your tables with `myisamchk`. See section 4 Database Administration.

If `mysqld` dies or hangs, you should start `mysqld` with `--log`. When `mysqld` dies again, you can examine the end of the log file for the query that killed `mysqld`.

If you are using `--log` without a file name, the log is stored in the database directory as 'hostname'.log In most cases it's the last query in the log file that killed `mysqld`, but if possible you should verify this by restarting `mysqld` and executing the found query from the `mysql` command-line tools. If this works, you should also test all complicated queries that didn't complete.

You can also try the command `EXPLAIN` on all `SELECT` statements that takes a long time to ensure that `mysqld` is using indexes properly. See section 5.2.1 `EXPLAIN` Syntax (Get Information About a `SELECT`).

You can find the queries that take a long time to execute by starting `mysqld` with `--log-slow-queries`. See section 4.9.5 The Slow Query Log.

If you find the text `mysqld restarted` in the error log file (normally named ``hostname.err'`) you have probably found a query that causes `mysqld` to fail. If this happens you should check all your tables with `myisamchk` (see section 4 Database Administration), and test the queries in the MySQL log files to see if one doesn't work. If you find such a query, try first upgrading to the newest MySQL version. If this doesn't help and you can't find anything in the `mysql` mail archive, you should report the bug to mysql@lists.mysql.com. Links to mail archives are available online at http://lists.mysql.com/.

If you have started `mysqld` with `myisam-recover`, MySQL will automatically check and try to repair `MyISAM` tables if they are marked as 'not closed properly' or 'crashed'. If this happens, MySQL will write an entry in the `hostname.err` file `'Warning: Checking table ...'` which is followed by `Warning: Repairing table` if the table needs to be repaired. If you get a lot of these errors, without `mysqld` having died unexpectedly just before, then something is wrong and needs to be investigated further. See section 4.1.1 `mysqld` Command-line Options.

It's of course not a good sign if `mysqld` did died unexpectedly, but in this case one shouldn't investigate the `Checking table...` messages but instead try to find out why `mysqld` died.

### E.1.6 Making a Test Case When You Experience Table Corruption

If you get corrupted tables or if `mysqld` always fails after some update commands, you can test if this bug is reproducible by doing the following:

- Take down the MySQL daemon (with `mysqladmin shutdown`).
- Make a backup of the tables (to guard against the very unlikely case that the repair will do something bad).
- Check all tables with `myisamchk -s database/*.MYI`. Repair any wrong tables with `myisamchk -r database/table.MYI`.
- Make a second backup of the tables.
- Remove (or move away) any old log files from the MySQL data directory if you need more space.
- Start `mysqld` with `--log-bin`. See section 4.9.4 The Binary Update Log. If you want to find a query that crashes `mysqld`, you should use `--log --log-bin`.
- When you have gotten a crashed table, stop the `mysqld server`.
- Restore the backup.
- Restart the `mysqld` server without `--log-bin`
- Re-execute the commands with `mysqlbinlog update-log-file | mysql`. The update log is saved in the MySQL database directory with the name `hostname-bin.#`.
- If the tables are corrupted again or you can get `mysqld` to die with the above command, you have found reproducible bug that should be easy to fix! FTP the tables and the binary log to ftp://support.mysql.com/pub/mysql/secret/ and send a mail to bugs@lists.mysql.com or (if you are a support customer) to support@mysql.com about the problem and the MySQL team will fix it as soon as possible.

You can also use the script `mysql_find_rows` to just execute some of the update statements if you want to narrow down the problem.

## E.2 Debugging a MySQL client

To be able to debug a MySQL client with the integrated debug package, you should configure MySQL with `--with-debug` or `--with-debug=full`. See section 2.3.3 Typical `configure` Options.

Before running a client, you should set the `MYSQL_DEBUG` environment variable:

    shell> MYSQL_DEBUG=d:t:O,/tmp/client.trace
    shell> export MYSQL_DEBUG

This causes clients to generate a trace file in ``/tmp/client.trace'`.

If you have problems with your own client code, you should attempt to connect to the server and run your query using a client that is known to work. Do this by running `mysql` in debugging mode (assuming you have compiled MySQL with debugging on):

    shell> mysql --debug=d:t:O,/tmp/client.trace

This will provide useful information in case you mail a bug report. See section 1.6.2.3 How to Report Bugs or Problems.

If your client crashes at some 'legal' looking code, you should check that your ``mysql.h'` include file matches your mysql library file. A very common mistake is to use an old ``mysql.h'` file from an old MySQL installation with new MySQL library.

## E.3 The DBUG Package

The MySQL server and most MySQL clients are compiled with the DBUG package originally made by Fred Fish. When one has configured MySQL for debugging, this package makes it possible to get a trace file of what the program is debugging. See section E.1.2 Creating Trace Files.

One uses the debug package by invoking the program with the `--debug="..."` or the `-#...` option.

Most MySQL programs has a default debug string that will be used if you don't specify an option to `--debug`. The default trace file is usually `/tmp/programname.trace` on Unix and `\programname.trace` on Windows.

The debug control string is a sequence of colon separated fields as follows:

    <field_1>:<field_2>:...:<field_N>

Each field consists of a mandatory flag character followed by an optional "," and comma-separated list of modifiers:

    flag[,modifier,modifier,...,modifier]

The currently recognised flag characters are:

Flag | Description
d | Enable output from DBUG_<N> macros for the current state. May be followed by a list of keywords which selects output only for the DBUG macros with that keyword. An empty list of keywords implies output for all macros.
D | Delay after each debugger output line. The argument is the number of tenths of seconds to delay, subject to machine capabilities. That is, `-#D,20` is delay two seconds.
f | Limit debugging and/or tracing, and profiling to the list of named functions. Note that a null list will disable all functions. The appropriate "d" or "t" flags must still be given, this flag only limits their actions if they are enabled.
F | Identify the source file name for each line of debug or trace output.
i | Identify the process with the pid or thread id for each line of debug or trace output.
g | Enable profiling. Create a file called 'dbugmon.out' containing information that can be used to profile the program. May be followed by a list of keywords that select profiling only for the functions in that list. A null list implies that all functions are considered.
L | Identify the source file line number for each line of debug or trace output.
n | Print the current function nesting depth for each line of debug or trace output.
N | Number each line of dbug output.
o | Redirect the debugger output stream to the specified file. The default output is stderr.
O | As `O` but the file is really flushed between each write. When needed the file is closed and reopened between each write.
p | Limit debugger actions to specified processes. A process must be identified with the DBUG_PROCESS macro and match one in the list for debugger actions to occur.
P | Print the current process name for each line of debug or trace output.
r | When pushing a new state, do not inherit the previous state's function nesting level. Useful when the output is to start at the left margin.
S | Do function _sanity(_file_,_line_) at each debugged function until _sanity() returns something that differs from 0. (Mostly used with safemalloc to find memory leaks)
t | Enable function call/exit trace lines. May be followed by a list (containing only one modifier) giving a numeric maximum trace level, beyond which no output will occur for either debugging or tracing macros. The default is a compile time option.

Some examples of debug control strings which might appear on a shell command-line (the "-#" is typically used to introduce a control string to an application program) are:

    -#d:t
    -#d:f,main,subr1:F:L:t,20
    -#d,input,output,files:n
    -#d:t:i:O,\\mysqld.trace

In MySQL, common tags to print (with the `d` option) are: `enter`,`exit`,`error`,`warning`,`info` and `loop`.

## E.4 Locking methods

Currently MySQL only supports table locking for `ISAM`/`MyISAM` and `HEAP` tables, page-level locking for `BDB` tables and row-level locking for `InnoDB` tables. See section 5.3.1 How MySQL Locks Tables. With `MyISAM` tables one can freely mix `INSERT` and `SELECT` without locks (`Versioning`).

Starting in version 3.23.33, you can analyse the table lock contention on your system by checking `Table_locks_waited` and `Table_locks_immediate` environment variables.

To decide if you want to use a table type with row-level locking, you will want to look at what the application does and what the select/update pattern of the data is.

Pros for row locking:

- Fewer lock conflicts when accessing different rows in many threads.
- Fewer changes for rollbacks.
- Makes it possible to lock a single row a long time.

Cons:

- Takes more memory than page level or table locks.
- Is slower than page level or table locks when used on a big part of the table, because one has to do many more locks.
- Is definitely much worse than other locks if you do often do `GROUP BY` on a large part of the data or if one has to often scan the whole table.
- With higher level locks one can also more easily support locks of different types to tune the application as the lock overhead is less notable as for row level locks.

Table locks are superior to page level / row level locks in the following cases:

- Mostly reads
- Read and updates on strict keys; this is where one updates or deletes a row that can be fetched with one key read:

      UPDATE table_name SET column=value WHERE unique_key#
      DELETE FROM table_name WHERE unique_key=#

- `SELECT` combined with `INSERT` (and very few `UPDATE`s and `DELETE`s).
- Many scans / `GROUP BY` on the whole table without any writers.

Other options than row / page level locking:

Versioning (like we use in MySQL for concurrent inserts) where you can have one writer at the same time as many readers. This means that the database/table supports different views for the data depending on when one started to access it. Other names for this are time travel, copy on write or copy on demand.

Copy on demand is in many case much better than page or row level locking; the worst case does, however, use much more memory than when using normal locks.

Instead of using row level locks one can use application level locks (like get_lock/release_lock in MySQL). This works of course only in well-behaved applications.

In many cases one can do an educated guess which locking type is best for the application, but generally it's very hard to say that a given lock type is better than another; everything depends on the application and different part of the application may require different lock types.

Here are some tips about locking in MySQL:

Most web applications do lots of selects, very few deletes, updates mainly on keys, and inserts in some specific tables. The base MySQL setup is very well tuned for this.

Concurrent users are not a problem if one doesn't mix updates with selects that need to examine many rows in the same table.

If one mixes inserts and deletes on the same table then `INSERT DELAYED` may be of great help.

One can also use `LOCK TABLES` to speed up things (many updates within a single lock is much faster than updates without locks). Splitting thing to different tables will also help.

If you get speed problems with the table locks in MySQL, you may be able to solve these by converting some of your tables to `InnoDB` or `BDB` tables. See section 7.5 `InnoDB` Tables. See section 7.6 `BDB` or `BerkeleyDB` Tables.

The optimisation section in the manual covers a lot of different aspects of how to tune applications. See section 5.2.12 Other Optimisation Tips.

## E.5 Comments about RTS threads

I have tried to use the RTS thread packages with MySQL but stumbled on the following problems:

They use an old version of a lot of POSIX calls and it is very tedious to make wrappers for all functions. I am inclined to think that it would be easier to change the thread libraries to the newest POSIX specification.

Some wrappers are already written. See ``mysys/my_pthread.c'` for more info.

At least the following should be changed:

`pthread_get_specific` should use one argument. `sigwait` should take two arguments. A lot of functions (at least `pthread_cond_wait`, `pthread_cond_timedwait`) should return the error code on error. Now they return -1 and set `errno`.

Another problem is that user-level threads use the `ALRM` signal and this aborts a lot of functions (`read`, `write`, `open`...). MySQL should do a retry on interrupt on all of these but it is not that easy to verify it.

The biggest unsolved problem is the following:

To get thread-level alarms I changed ``mysys/thr_alarm.c'` to wait between alarms with `pthread_cond_timedwait()`, but this aborts with error `EINTR`. I tried to debug the thread library as to why this happens, but couldn't find any easy solution.

If someone wants to try MySQL with RTS threads I suggest the following:

- Change functions MySQL uses from the thread library to POSIX. This shouldn't take that long.
- Compile all libraries with the `-DHAVE_rts_threads`.
- Compile `thr_alarm`.
- If there are some small differences in the implementation, they may be fixed by changing ``my_pthread.h'` and ``my_pthread.c'`.
- Run `thr_alarm`. If it runs without any ``warning'', ``error'' or aborted messages, you are on the right track. Here is a successful run on Solaris:

      Main thread: 1
      Thread 0 (5) started
      Thread: 5  Waiting
      process_alarm
      Thread 1 (6) started
      Thread: 6  Waiting
      process_alarm
      process_alarm
      thread_alarm
      Thread: 6  Slept for 1 (1) sec
      Thread: 6  Waiting
      process_alarm
      process_alarm
      thread_alarm
      Thread: 6  Slept for 2 (2) sec
      Thread: 6  Simulation of no alarm needed
      Thread: 6  Slept for 0 (3) sec
      Thread: 6  Waiting
      process_alarm
      process_alarm
      thread_alarm
      Thread: 6  Slept for 4 (4) sec
      Thread: 6  Waiting
      process_alarm
      thread_alarm
      Thread: 5  Slept for 10 (10) sec
      Thread: 5  Waiting
      process_alarm
      process_alarm
      thread_alarm
      Thread: 6  Slept for 5 (5) sec
      Thread: 6  Waiting
      process_alarm
      process_alarm

      ...
      thread_alarm
      Thread: 5  Slept for 0 (1) sec
      end

## E.6 Differences between different thread packages

MySQL is very dependent on the thread package used. So when choosing a good platform for MySQL, the thread package is very important.

There are at least three types of thread packages:

- User threads in a single process. Thread switching is managed with alarms and the threads library manages all non-thread-safe functions with locks. Read, write and select operations are usually managed with a thread-specific select that switches to another thread if the running threads have to wait for data. If the user thread packages are integrated in the standard libs (FreeBSD and BSDI threads) the thread package requires less overhead than thread packages that have to map all unsafe calls (MIT-pthreads, FSU Pthreads and RTS threads). In some environments (for example, SCO), all system calls are thread-safe so the mapping can be done very easily (FSU Pthreads on SCO). Downside: All mapped calls take a little time and it's quite tricky to be able to handle all situations. There are usually also some system calls that are not handled by the thread package (like MIT-pthreads and sockets). Thread scheduling isn't always optimal.
- User threads in separate processes. Thread switching is done by the kernel and all data are shared between threads. The thread package manages the standard thread calls to allow sharing data between threads. LinuxThreads is using this method. Downside: Lots of processes. Thread creating is slow. If one thread dies the rest are usually left hanging and you must kill them all before restarting. Thread switching is somewhat expensive.
- Kernel threads. Thread switching is handled by the thread library or the kernel and is very fast. Everything is done in one process, but on some systems, `ps` may show the different threads. If one thread aborts, the whole process aborts. Most system calls are thread-safe and should require very little overhead. Solaris, HP-UX, AIX and OSF/1 have kernel threads.

In some systems kernel threads are managed by integrating user level threads in the system libraries. In such cases, the thread switching can only be done by the thread library and the kernel isn't really ``thread aware''.

# F Environment Variables

Here is a list of all the environment variables that are used directly or indirectly by MySQL. Most of these can also be found in other places in this manual.

Note that any options on the command-line will take precedence over values specified in configuration files and environment variables, and values in configuration files take precedence over values in environment variables.

In many cases it's preferable to use a configure file instead of environment variables to modify the behaviour of MySQL. See section 4.1.2 ``my.cnf'` Option Files.

Variable | Description
`CCX` | Set this to your C++ compiler when running configure.
`CC` | Set this to your C compiler when running configure.
`CFLAGS` | Flags for your C compiler when running configure.
`CXXFLAGS` | Flags for your C++ compiler when running configure.
`DBI_USER` | The default user name for Perl DBI.
`DBI_TRACE` | Used when tracing Perl DBI.
`HOME` | The default path for the `mysql` history file is ``$HOME/.mysql_history'`.
`LD_RUN_PATH` | Used to specify where your ``libmysqlclient.so'` is.
`MYSQL_DEBUG` | Debug-trace options when debugging.
`MYSQL_HISTFILE` | The path to the `mysql` history file.
`MYSQL_HOST` | Default host name used by the `mysql` command-line client.
`MYSQL_PS1` | Command prompt to use in the `mysql` command-line client. See section 4.8.2 `mysql`, The Command-line Tool.
`MYSQL_PWD` | The default password when connecting to `mysqld`. Note that use of this is insecure!
`MYSQL_TCP_PORT` | The default TCP/IP port.
`MYSQL_UNIX_PORT` | The default socket; used for connections to `localhost`.
`PATH` | Used by the shell to finds the MySQL programs.
`TMPDIR` | The directory where temporary tables/files are created.
`TZ` | This should be set to your local time zone. See section A.4.6 Time Zone Problems.
`UMASK_DIR` | The user-directory creation mask when creating directories. Note that this is ANDed with `UMASK`!
`UMASK` | The user-file creation mask when creating files.
`USER` | The default user on Windows to use when connecting to `mysqld`.

# G MySQL Regular Expressions

A regular expression (regex) is a powerful way of specifying a complex search.

MySQL uses Henry Spencer's implementation of regular expressions, which is aimed at conformance with POSIX 1003.2. MySQL uses the extended version.

This is a simplistic reference that skips the details. To get more exact information, see Henry Spencer's `regex(7)` manual page that is included in the source distribution. See section C Credits.

A regular expression describes a set of strings. The simplest regexp is one that has no special characters in it. For example, the regexp `hello` matches `hello` and nothing else.

Non-trivial regular expressions use certain special constructs so that they can match more than one string. For example, the regexp `hello|word` matches either the string `hello` or the string `word`.

As a more complex example, the regexp `B[an]*s` matches any of the strings `Bananas`, `Baaaaas`, `Bs`, and any other string starting with a `B`, ending with an `s`, and containing any number of `a` or `n` characters in between.

A regular expression may use any of the following special characters/constructs:

`^`
  Match the beginning of a string.

      mysql> SELECT "fo\nfo" REGEXP "^fo$";           -> 0
      mysql> SELECT "fofo" REGEXP "^fo";              -> 1

`$`
  Match the end of a string.

      mysql> SELECT "fo\no" REGEXP "^fo\no$";         -> 1
      mysql> SELECT "fo\no" REGEXP "^fo$";            -> 0

`.`
  Match any character (including newline).

      mysql> SELECT "fofo" REGEXP "^f.*";             -> 1
      mysql> SELECT "fo\nfo" REGEXP "^f.*";           -> 1

`a*`
  Match any sequence of zero or more `a` characters.

      mysql> SELECT "Ban" REGEXP "^Ba*n";             -> 1
      mysql> SELECT "Baaan" REGEXP "^Ba*n";           -> 1
      mysql> SELECT "Bn" REGEXP "^Ba*n";              -> 1

`a+`
  Match any sequence of one or more `a` characters.

      mysql> SELECT "Ban" REGEXP "^Ba+n";             -> 1
      mysql> SELECT "Bn" REGEXP "^Ba+n";              -> 0

`a?`
  Match either zero or one `a` character.

      mysql> SELECT "Bn" REGEXP "^Ba?n";              -> 1
      mysql> SELECT "Ban" REGEXP "^Ba?n";             -> 1
      mysql> SELECT "Baan" REGEXP "^Ba?n";            -> 0

`de|abc`
  Match either of the sequences `de` or `abc`.

      mysql> SELECT "pi" REGEXP "pi|apa";             -> 1
      mysql> SELECT "axe" REGEXP "pi|apa";            -> 0
      mysql> SELECT "apa" REGEXP "pi|apa";            -> 1
      mysql> SELECT "apa" REGEXP "^(pi|apa)$";        -> 1
      mysql> SELECT "pi" REGEXP "^(pi|apa)$";         -> 1
      mysql> SELECT "pix" REGEXP "^(pi|apa)$";        -> 0

`(abc)*`
  Match zero or more instances of the sequence `abc`.

      mysql> SELECT "pi" REGEXP "^(pi)*$";            -> 1
      mysql> SELECT "pip" REGEXP "^(pi)*$";           -> 0
      mysql> SELECT "pipi" REGEXP "^(pi)*$";          -> 1

`{1}`
`{2,3}`
  The is a more general way of writing regexps that match many occurrences of the previous atom.
  `a*`
    Can be written as `a{0,}`.
  `a+`
    Can be written as `a{1,}`.
  `a?`
    Can be written as `a{0,1}`.
  To be more precise, an atom followed by a bound containing one integer `i` and no comma matches a sequence of exactly `i` matches of the atom. An atom followed by a bound containing one integer `i` and a comma matches a sequence of `i` or more matches of the atom. An atom followed by a bound containing two integers `i` and `j` matches a sequence of `i` through `j` (inclusive) matches of the atom. Both arguments must be in the range from `0` to `RE_DUP_MAX` (default 255), inclusive. If there are two arguments, the second must be greater than or equal to the first.
`[a-dX]`
`[^a-dX]`
  Matches any character which is (or is not, if ^ is used) either `a`, `b`, `c`, `d` or `X`. To include a literal `]` character, it must immediately follow the opening bracket `[`. To include a literal `-` character, it must be written first or last. So `[0-9]` matches any decimal digit. Any character that does not have a defined meaning inside a `[]` pair has no special meaning and matches only itself.

      mysql> SELECT "aXbc" REGEXP "[a-dXYZ]";         -> 1
      mysql> SELECT "aXbc" REGEXP "^[a-dXYZ]$";       -> 0
      mysql> SELECT "aXbc" REGEXP "^[a-dXYZ]+$";      -> 1
      mysql> SELECT "aXbc" REGEXP "^[^a-dXYZ]+$";     -> 0
      mysql> SELECT "gheis" REGEXP "^[^a-dXYZ]+$";    -> 1
      mysql> SELECT "gheisa" REGEXP "^[^a-dXYZ]+$";   -> 0

`[[.characters.]]`
  The sequence of characters of that collating element. The sequence is a single element of the bracket expression's list. A bracket expression containing a multi-character collating element can thus match more than one character, for example, if the collating sequence includes a `ch` collating element, then the regular expression `[[.ch.]]*c` matches the first five characters of `chchcc`.
`[=character_class=]`
  An equivalence class, standing for the sequences of characters of all collating elements equivalent to that one, including itself. For example, if `o` and `(+)` are the members of an equivalence class, then `[[=o=]]`, `[[=(+)=]]`, and `[o(+)]` are all synonymous. An equivalence class may not be an endpoint of a range.
`[:character_class:]`
  Within a bracket expression, the name of a character class enclosed in `[:` and `:]` stands for the list of all characters belonging to that class. Standard character class names are:

  Name | Name | Name
  alnum | digit | punct
  alpha | graph | space
  blank | lower | upper
  cntrl | print | xdigit

  These stand for the character classes defined in the `ctype(3)` manual page. A locale may provide others. A character class may not be used as an endpoint of a range.

      mysql> SELECT "justalnums" REGEXP "[[:alnum:]]+";       -> 1
      mysql> SELECT "!!" REGEXP "[[:alnum:]]+";               -> 0

`[[:<:]]`
`[[:>:]]`
  These match the null string at the beginning and end of a word respectively. A word is defined as a sequence of word characters which is neither preceded nor followed by word characters. A word character is an alnum character (as defined by `ctype(3)`) or an underscore (`_`).

      mysql> SELECT "a word a" REGEXP "[[:<:]]word[[:>:]]";      -> 1
      mysql> SELECT "a xword a" REGEXP "[[:<:]]word[[:>:]]";     -> 0

    mysql> SELECT "weeknights" REGEXP "^(wee|week)(knights|nights)$"; -> 1

# H GNU General Public License

Version 2, June 1991

    Copyright © 1989, 1991 Free Software Foundation, Inc.
    59 Temple Place - Suite 330, Boston, MA  02111-1307, USA

    Everyone is permitted to copy and distribute verbatim copies
    of this license document, but changing it is not allowed.

## H.1 Preamble

The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.

When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.

To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.

For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.

We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.

Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.

Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.

The precise terms and conditions for copying, distribution and modification follow.

## H.2 TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

1. This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License. The ``Program'', below, refers to any such program or work, and a ``work based on the Program'' means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another language. (Hereinafter, translation is included without limitation in the term ``modification''.) Each licensee is addressed as ``you''. Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
2. You may copy and distribute verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program. You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
3. You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:
   1. You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.
   2. You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.
   3. If the modified program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.)
   These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it. Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program. In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.
4. You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:
   1. Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
   2. Accompany it with a written offer, valid for at least three years, to give any third-party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
   3. Accompany it with the information you received as to the offer to distribute corresponding source code. (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.)
   The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable. If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code.
5. You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
6. You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it.
7. Each time you redistribute the Program (or any work based on the Program), the recipient automatically receives a license from the original licensor to copy, distribute or modify the Program subject to these terms and conditions. You may not impose any further restrictions on the recipients' exercise of the rights granted herein. You are not responsible for enforcing compliance by third parties to this License.
8. If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (not limited to patent issues), conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not distribute the Program at all. For example, if a patent license would not permit royalty-free redistribution of the Program by all those who receive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Program. If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances. It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system, which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice. This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.
9. If the distribution and/or use of the Program is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Program under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License.
10. The Free Software Foundation may publish revised and/or new versions of the General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and ``any later version'', you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.
11. If you wish to incorporate parts of the Program into other free programs whose distribution conditions are different, write to the author to ask for permission. For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally.

    9.4 NO WARRANTY

12. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
13. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

## 9.5 END OF TERMS AND CONDITIONS

## H.3 How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.

To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the ``copyright'' line and a pointer to where the full notice is found.

    one line to give the program's name and a brief idea of what it does.
    Copyright (C) yyyy  name of author

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this when it starts in an interactive mode:

    Gnomovision version 69, Copyright (C) 19yy name of author
    Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
    This is free software, and you are welcome to redistribute it
    under certain conditions; type `show c' for details.

The hypothetical commands ``show w'` and ``show c'` should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than ``show w'` and ``show c'`; they could even be mouse-clicks or menu items--whatever suits your program.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a ``copyright disclaimer'' for the program, if necessary. Here is a sample; alter the names:

    Yoyodyne, Inc., hereby disclaims all copyright interest in the program
    `Gnomovision' (which makes passes at compilers) written by James Hacker.

    signature of Ty Coon, 1 April 1989
    Ty Coon, President of Vice

This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.

# I GNU Lesser General Public License

Version 2.1, February 1999

    Copyright © 1991, 1999 Free Software Foundation, Inc.
    59 Temple Place -- Suite 330, Boston, MA 02111-1307, USA

    Everyone is permitted to copy and distribute verbatim copies
    of this license document, but changing it is not allowed.

    [This is the first released version of the Lesser GPL.  It also counts
    as the successor of the GNU Library Public License, version 2, hence the
    version number 2.1.]

## I.1 Preamble

The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public Licenses are intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users.

This license, the Lesser General Public License, applies to some specially designated software--typically libraries--of the Free Software Foundation and other authors who decide to use it. You can use it too, but we suggest you first think carefully about whether this license or the ordinary General Public License is the better strategy to use in any particular case, based on the explanations below.

When we speak of free software, we are referring to freedom of use, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish); that you receive source code or can get it if you want it; that you can change the software and use pieces of it in new free programs; and that you are informed that you can do these things.

To protect your rights, we need to make restrictions that forbid distributors to deny you these rights or to ask you to surrender these rights. These restrictions translate to certain responsibilities for you if you distribute copies of the library or if you modify it.

For example, if you distribute copies of the library, whether gratis or for a fee, you must give the recipients all the rights that we gave you. You must make sure that they, too, receive or can get the source code. If you link other code with the library, you must provide complete object files to the recipients, so that they can relink them with the library after making changes to the library and recompiling it. And you must show them these terms so they know their rights.

We protect your rights with a two-step method: (1) we copyright the library, and (2) we offer you this license, which gives you legal permission to copy, distribute and/or modify the library.

To protect each distributor, we want to make it very clear that there is no warranty for the free library. Also, if the library is modified by someone else and passed on, the recipients should know that what they have is not the original version, so that the original author's reputation will not be affected by problems that might be introduced by others.

Finally, software patents pose a constant threat to the existence of any free program. We wish to make sure that a company cannot effectively restrict the users of a free program by obtaining a restrictive license from a patent holder. Therefore, we insist that any patent license obtained for a version of the library must be consistent with the full freedom of use specified in this license.

Most GNU software, including some libraries, is covered by the ordinary GNU General Public License. This license, the GNU Lesser General Public License, applies to certain designated libraries, and is quite different from the ordinary General Public License. We use this license for certain libraries in order to permit linking those libraries into non-free programs.

When a program is linked with a library, whether statically or using a shared library, the combination of the two is legally speaking a combined work, a derivative of the original library. The ordinary General Public License therefore permits such linking only if the entire combination fits its criteria of freedom. The Lesser General Public License permits more lax criteria for linking other code with the library.

We call this license the Lesser General Public License because it does Less to protect the user's freedom than the ordinary General Public License. It also provides other free software developers Less of an advantage over competing non-free programs. These disadvantages are the reason we use the ordinary General Public License for many libraries. However, the Lesser license provides advantages in certain special circumstances.

For example, on rare occasions, there may be a special need to encourage the widest possible use of a certain library, so that it becomes a de-facto standard. To achieve this, non-free programs must be allowed to use the library. A more frequent case is that a free library does the same job as widely used non-free libraries. In this case, there is little to gain by limiting the free library to free software only, so we use the Lesser General Public License.

In other cases, permission to use a particular library in non-free programs enables a greater number of people to use a large body of free software. For example, permission to use the GNU C Library in non-free programs enables many more people to use the whole GNU operating system, as well as its variant, the GNU/Linux operating system.

Although the Lesser General Public License is Less protective of the users' freedom, it does ensure that the user of a program that is linked with the Library has the freedom and the wherewithal to run that program using a modified version of the Library.

The precise terms and conditions for copying, distribution and modification follow. Pay close attention to the difference between a ``work based on the library'' and a ``work that uses the library''. The former contains code derived from the library, whereas the latter must be combined with the library in order to run.

## I.2 TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

1. This License Agreement applies to any software library or other program which contains a notice placed by the copyright holder or other authorized party saying it may be distributed under the terms of this Lesser General Public License (also called ``this License''). Each licensee is addressed as ``you''. A ``library'' means a collection of software functions and/or data prepared so as to be conveniently linked with application programs (which use some of those functions and data) to form executables. The ``Library'', below, refers to any such software library or work which has been distributed under these terms. A ``work based on the Library'' means either the Library or any derivative work under copyright law: that is to say, a work containing the Library or a portion of it, either verbatim or with modifications and/or translated straightforwardly into another language. (Hereinafter, translation is included without limitation in the term ``modification''.) ``Source code'' for a work means the preferred form of the work for making modifications to it. For a library, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the library. Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running a program using the Library is not restricted, and output from such a program is covered only if its contents constitute a work based on the Library (independent of the use of the Library in a tool for writing it). Whether that is true depends on what the Library does and what the program that uses the Library does.
2. You may copy and distribute verbatim copies of the Library's complete source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and distribute a copy of this License along with the Library. You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
3. You may modify your copy or copies of the Library or any portion of it, thus forming a work based on the Library, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:
   1. The modified work must itself be a software library.
   2. You must cause the files modified to carry prominent notices stating that you changed the files and the date of any change.
   3. You must cause the whole of the work to be licensed at no charge to all third parties under the terms of this License.
   4. If a facility in the modified Library refers to a function or a table of data to be supplied by an application program that uses the facility, other than as an argument passed when the facility is invoked, then you must make a good faith effort to ensure that, in the event an application does not supply such function or table, the facility still operates, and performs whatever part of its purpose remains meaningful. (For example, a function in a library to compute square roots has a purpose that is entirely well-defined independent of the application. Therefore, Subsection 2d requires that any application-supplied function or table used by this function must be optional: if the application does not supply it, the square root function must still compute square roots.)
   These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Library, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Library, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it. Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Library. In addition, mere aggregation of another work not based on the Library with the Library (or with a work based on the Library) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.
4. You may opt to apply the terms of the ordinary GNU General Public License instead of this License to a given copy of the Library. To do this, you must alter all the notices that refer to this License, so that they refer to the ordinary GNU General Public License, version 2, instead of to this License. (If a newer version than version 2 of the ordinary GNU General Public License has appeared, then you can specify that version instead if you wish.) Do not make any other change in these notices. Once this change is made in a given copy, it is irreversible for that copy, so the ordinary GNU General Public License applies to all subsequent copies and derivative works made from that copy. This option is useful when you wish to copy part of the code of the Library into a program that is not a library.
5. You may copy and distribute the Library (or a portion or derivative of it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange. If distribution of object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place satisfies the requirement to distribute the source code, even though third parties are not compelled to copy the source along with the object code.
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    9.6 NO WARRANTY

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## 9.7 END OF TERMS AND CONDITIONS

## I.3 How to Apply These Terms to Your New Libraries

If you develop a new library, and you want it to be of the greatest possible use to the public, we recommend making it free software that everyone can redistribute and change. You can do so by permitting redistribution under these terms (or, alternatively, under the terms of the ordinary General Public License).

To apply these terms, attach the following notices to the library. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the ``copyright'' line and a pointer to where the full notice is found.

    one line to give the library's name and an idea of what it does.
    Copyright (C) year  name of author

    This library is free software; you can redistribute it and/or modify it
    under the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation; either version 2.1 of the License, or (at
    your option) any later version.

    This library is distributed in the hope that it will be useful, but
    WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307,
    USA.

Also add information on how to contact you by electronic and paper mail.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a ``copyright disclaimer'' for the library, if necessary. Here is a sample; alter the names:

    Yoyodyne, Inc., hereby disclaims all copyright interest in the library
    `Frob' (a library for tweaking knobs) written by James Random Hacker.

    signature of Ty Coon, 1 April 1990
    Ty Coon, President of Vice

That's all there is to it!

# SQL command, type and function index

##

- ! (logical NOT)
- != (not equal)
- "
- % (modulo)
- % (wildcard character)
- & (bitwise AND)
- && (logical AND)
- () (parentheses)
- (Control-Z) \z
- * (multiplication)
- + (addition)
- - (subtraction)
- - (unary minus)
- -p option
- -password option
- .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file
- .mysql_history file
- .pid (process ID) file
- / (division)
- ``/etc/passwd'`
- /etc/passwd
- < (less than)
- <<
- << (left shift)
- <= (less than or equal)
- <=> (Equal to)
- <> (not equal)
- = (equal)
- > (greater than)
- >= (greater than or equal)
- >> (right shift)
- \" (double quote)
- \' (single quote)
- \0 (ASCII 0)
- \\ (escape)
- \b (backspace)
- \n (newline)
- \r (carriage return)
- \t (tab)
- \z (Control-Z) ASCII(26)
- ^ (bitwise XOR)
- _ (wildcard character)
- `

## A

- ABS()
- ACOS()
- ADDDATE()
- addition (+)
- AES_DECRYPT()
- AES_ENCRYPT()
- alias
- ALTER COLUMN
- ALTER TABLE, ALTER TABLE, ALTER TABLE
- ANALYZE TABLE
- AND, bitwise
- AND, logical
- arithmetic functions
- AS, AS
- ASCII()
- ASIN()
- ATAN()
- ATAN2()
- AUTO_INCREMENT, using with DBI
- AVG()

## B

- backspace (\b)
- BACKUP TABLE
- BEGIN
- BENCHMARK()
- BETWEEN ... AND
- BIGINT
- BIN()
- BINARY
- BIT
- bit functions
- BIT_AND()
- BIT_COUNT
- BIT_COUNT()
- BIT_LENGTH()
- BIT_OR
- BIT_OR()
- BLOB, BLOB
- BOOL

## C

- carriage return (\r)
- CASE
- CAST
- casts
- CC environment variable, CC environment variable
- `CC` environment variable
- CCX environment variable
- CEILING()
- CFLAGS environment variable, CFLAGS environment variable
- CHAR, CHAR
- CHAR VARYING
- CHAR()
- CHAR_LENGTH()
- CHARACTER
- CHARACTER VARYING
- CHARACTER_LENGTH()
- CHECK TABLE
- ChopBlanks DBI method
- COALESCE()
- command-line options
- Comment syntax
- COMMIT, COMMIT
- comparison operators
- CONCAT()
- CONCAT_WS()
- configure option, --with-charset
- configure option, --with-extra-charsets
- connect() DBI method
- CONNECTION_ID()
- control flow functions
- CONV()
- CONVERT
- COS()
- COT()
- COUNT()
- COUNT(DISTINCT)
- CREATE DATABASE
- CREATE FUNCTION
- CREATE INDEX
- CREATE TABLE
- CROSS JOIN
- CURDATE()
- CURRENT_DATE
- CURRENT_TIME
- CURRENT_TIMESTAMP
- CURRENT_USER()
- CURTIME()
- `CXX` environment variable
- CXX environment variable, CXX environment variable, CXX environment variable
- CXXFLAGS environment variable, CXXFLAGS environment variable, CXXFLAGS environment variable, CXXFLAGS environment variable

## D

- data_sources() DBI method
- DATABASE()
- DATE, DATE, DATE
- date and time functions
- DATE_ADD()
- DATE_FORMAT()
- DATE_SUB()
- DATETIME, DATETIME
- DAYNAME()
- DAYOFMONTH()
- DAYOFWEEK()
- DAYOFYEAR()
- DBI->connect()
- DBI->data_sources()
- DBI->disconnect
- DBI->do()
- DBI->execute
- DBI->fetchall_arrayref
- DBI->fetchrow_array
- DBI->fetchrow_arrayref
- DBI->fetchrow_hashref
- DBI->finish
- DBI->prepare()
- DBI->quote
- DBI->quote()
- DBI->rows
- DBI->trace, DBI->trace
- DBI->{ChopBlanks}
- DBI->{insertid}
- DBI->{is_blob}
- DBI->{is_key}
- DBI->{is_not_null}
- DBI->{is_num}
- DBI->{is_pri_key}
- DBI->{length}
- DBI->{max_length}
- DBI->{NAME}
- DBI->{NULLABLE}
- DBI->{NUM_OF_FIELDS}
- DBI->{table}
- DBI->{type}
- DBI_TRACE environment variable, DBI_TRACE environment variable
- `DBI_TRACE` environment variable
- DBI_USER environment variable
- DEC
- DECIMAL
- DECODE()
- DEGREES()
- DELAYED
- DELETE
- DES_DECRYPT()
- DES_ENCRYPT()
- DESC
- DESCRIBE, DESCRIBE
- disconnect DBI method
- DISTINCT, DISTINCT, DISTINCT
- division (/)
- DO
- do() DBI method
- DOUBLE
- DOUBLE PRECISION
- double quote (\")
- DROP DATABASE
- DROP FUNCTION
- DROP INDEX, DROP INDEX
- DROP PRIMARY KEY
- DROP TABLE
- DUMPFILE

## E

- ELT()
- ENCODE()
- ENCRYPT()
- ENUM, ENUM
- Environment variable, CC
- environment variable, `CC`
- environment variable, CC
- Environment variable, CCX
- Environment variable, CFLAGS
- environment variable, CFLAGS
- environment variable, `CXX`
- Environment variable, CXX
- environment variable, CXX
- Environment variable, CXXFLAGS
- environment variable, CXXFLAGS, environment variable, CXXFLAGS, environment variable, CXXFLAGS
- Environment variable, DBI_TRACE, Environment variable, DBI_TRACE
- environment variable, `DBI_TRACE`
- Environment variable, DBI_USER
- Environment variable, HOME
- environment variable, `HOME`
- Environment variable, LD_RUN_PATH, Environment variable, LD_RUN_PATH, Environment variable, LD_RUN_PATH
- environment variable, LD_RUN_PATH
- Environment variable, MYSQL_DEBUG, Environment variable, MYSQL_DEBUG
- environment variable, `MYSQL_DEBUG`
- Environment variable, MYSQL_HISTFILE
- environment variable, `MYSQL_HISTFILE`
- Environment variable, MYSQL_HOST
- environment variable, `MYSQL_HOST`
- Environment variable, MYSQL_PS1
- Environment variable, MYSQL_PWD
- environment variable, `MYSQL_PWD`, environment variable, `MYSQL_PWD`
- environment variable, MYSQL_TCP_PORT
- Environment variable, MYSQL_TCP_PORT
- environment variable, `MYSQL_TCP_PORT`, environment variable, `MYSQL_TCP_PORT`
- environment variable, MYSQL_UNIX_PORT
- Environment variable, MYSQL_UNIX_PORT, Environment variable, MYSQL_UNIX_PORT
- environment variable, `MYSQL_UNIX_PORT`, environment variable, `MYSQL_UNIX_PORT`
- environment variable, PATH
- Environment variable, PATH
- Environment variable, TMPDIR, Environment variable, TMPDIR
- Environment variable, TZ, Environment variable, TZ
- Environment variable, UMASK, Environment variable, UMASK
- Environment variable, UMASK_DIR, Environment variable, UMASK_DIR
- environment variable, `USER`
- Environment variable, USER
- Environment variables, CXX
- equal (=)
- escape (\\)
- execute DBI method
- EXP()
- EXPLAIN
- EXPORT_SET()
- EXTRACT(), EXTRACT()

## F

- fetchall_arrayref DBI method
- fetchrow_array DBI method
- fetchrow_arrayref DBI method
- fetchrow_hashref DBI method
- FIELD()
- FILE
- FIND_IN_SET()
- finish DBI method
- FLOAT, FLOAT
- FLOAT(M,D)
- FLOAT(precision), FLOAT(precision)
- FLOOR()
- FLUSH
- FORMAT()
- FOUND_ROWS()
- FROM_DAYS()
- FROM_UNIXTIME(), FROM_UNIXTIME()
- functions, arithmetic
- functions, bit
- functions, control flow
- functions, date and time
- functions, GROUP BY
- functions, mathematical
- functions, miscellaneous
- functions, string
- functions, string comparison
- Functions, user-defined

## G

- GET_LOCK()
- GRANT
- GRANT statement, GRANT statement
- greater than (>)
- greater than or equal (>=)
- GREATEST()
- GROUP BY functions

## H

- HANDLER
- HEX()
- hexadecimal values
- `HOME` environment variable
- HOME environment variable
- host.frm, problems finding
- HOUR()

## I

- identifiers, quoting
- IF()
- IFNULL()
- IGNORE INDEX, IGNORE INDEX
- IGNORE KEY, IGNORE KEY
- IN
- INET_ATON()
- INET_NTOA()
- INNER JOIN
- INSERT, INSERT
- INSERT ... SELECT
- INSERT DELAYED
- INSERT statement, grant privileges
- INSERT()
- insertid DBI method
- INSTR()
- INT
- INTEGER
- INTERVAL()
- IS NOT NULL
- IS NULL
- IS NULL, and indexes
- is_blob DBI method
- IS_FREE_LOCK()
- is_key DBI method
- is_not_null DBI method
- is_num DBI method
- is_pri_key DBI method
- ISNULL()
- ISOLATION LEVEL

## J

- JOIN

## K

- KILL

## L

- LAST_INSERT_ID()
- LAST_INSERT_ID([expr])
- LCASE()
- LD_RUN_PATH environment variable, LD_RUN_PATH environment variable, LD_RUN_PATH environment variable, LD_RUN_PATH environment variable
- LEAST()
- LEFT JOIN, LEFT JOIN
- LEFT OUTER JOIN
- LEFT()
- length DBI method
- LENGTH()
- less than (<)
- less than or equal (<=)
- LIKE
- LIKE, and indexes
- LIKE, and wildcards
- LIMIT, LIMIT
- LN()
- LOAD DATA INFILE, LOAD DATA INFILE
- LOAD_FILE()
- LOCATE(), LOCATE()
- LOCK TABLES
- LOG()
- LOG10()
- LOG2()
- Logical operators
- LONGBLOB
- LONGTEXT
- LOWER()
- LPAD()
- LTRIM()

## M

- MAKE_SET()
- MASTER_POS_WAIT()
- MATCH ... AGAINST()
- mathematical functions
- MAX()
- max_length DBI method
- MD5()
- MEDIUMBLOB
- MEDIUMINT
- MEDIUMTEXT
- MID()
- MIN()
- minus, unary (-)
- MINUTE()
- miscellaneous functions
- MOD()
- modulo (%)
- MONTH()
- MONTHNAME()
- multiplication (*)
- `my_init()`
- my_ulonglong C type
- my_ulonglong values, printing
- MYSQL C type
- `mysql_affected_rows()`
- `mysql_change_user()`
- `mysql_character_set_name()`
- `mysql_close()`
- `mysql_connect()`
- `mysql_create_db()`
- `mysql_data_seek()`
- MYSQL_DEBUG environment variable, MYSQL_DEBUG environment variable
- `MYSQL_DEBUG` environment variable
- `mysql_debug()`
- `mysql_drop_db()`
- `mysql_dump_debug_info()`
- `mysql_eof()`
- `mysql_errno()`
- `mysql_error()`
- `mysql_escape_string()`
- `mysql_fetch_field()`
- `mysql_fetch_field_direct()`
- `mysql_fetch_fields()`
- `mysql_fetch_lengths()`
- `mysql_fetch_row()`
- MYSQL_FIELD C type
- `mysql_field_count()`, `mysql_field_count()`
- MYSQL_FIELD_OFFSET C type
- `mysql_field_seek()`
- `mysql_field_tell()`
- `mysql_free_result()`
- `mysql_get_client_info()`
- `mysql_get_host_info()`
- `mysql_get_proto_info()`
- `mysql_get_server_info()`
- MYSQL_HISTFILE environment variable
- `MYSQL_HISTFILE` environment variable
- `MYSQL_HOST` environment variable
- MYSQL_HOST environment variable
- mysql_info(), mysql_info(), mysql_info(), mysql_info()
- `mysql_info()`
- `mysql_init()`
- `mysql_insert_id()`
- mysql_insert_id()
- `mysql_kill()`
- `mysql_list_dbs()`
- `mysql_list_fields()`
- `mysql_list_processes()`
- `mysql_list_tables()`
- `mysql_num_fields()`
- `mysql_num_rows()`
- `mysql_options()`
- `mysql_ping()`
- MYSQL_PS1 environment variable
- `MYSQL_PWD` environment variable, `MYSQL_PWD` environment variable
- MYSQL_PWD environment variable
- `mysql_query()`, `mysql_query()`
- `mysql_real_connect()`
- mysql_real_escape_string()
- `mysql_real_escape_string()`
- `mysql_real_query()`
- `mysql_reload()`
- MYSQL_RES C type
- MYSQL_ROW C type
- `mysql_row_seek()`
- `mysql_row_tell()`
- `mysql_select_db()`
- `mysql_server_end()`
- `mysql_server_init()`
- `mysql_shutdown()`
- `mysql_stat()`
- `mysql_store_result()`, `mysql_store_result()`
- MYSQL_TCP_PORT environment variable, MYSQL_TCP_PORT environment variable
- `MYSQL_TCP_PORT` environment variable, `MYSQL_TCP_PORT` environment variable
- `mysql_thread_end()`
- `mysql_thread_id()`
- `mysql_thread_init()`
- `mysql_thread_safe()`
- `MYSQL_UNIX_PORT` environment variable, `MYSQL_UNIX_PORT` environment variable
- MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable
- `mysql_use_result()`

## N

- NAME DBI method
- NATIONAL CHAR
- NATURAL LEFT JOIN
- NATURAL LEFT OUTER JOIN
- NATURAL RIGHT JOIN
- NATURAL RIGHT OUTER JOIN
- NCHAR
- newline (\n)
- NOT BETWEEN
- not equal (!=)
- not equal (<>)
- NOT IN
- NOT LIKE
- NOT REGEXP
- NOT, logical
- NOW()
- NUL
- NULL, NULL
- NULL value
- NULLABLE DBI method
- NULLIF()
- NUM_OF_FIELDS DBI method
- NUMERIC

## O

- OCT()
- OCTET_LENGTH()
- Operators, logical
- OPTIMIZE TABLE
- OR, bitwise
- OR, logical
- ORD()
- ORDER BY

## P

- parentheses ( and )
- PASSWORD(), PASSWORD(), PASSWORD(), PASSWORD()
- PATH environment variable, PATH environment variable
- PERIOD_ADD()
- PERIOD_DIFF()
- PI()
- POSITION()
- POW()
- POWER()
- prepare() DBI method
- PRIMARY KEY, PRIMARY KEY
- PROCESSLIST

## Q

- QUARTER()
- QUOTE()
- quote() DBI method
- quoting of identifiers

## R

- RADIANS()
- RAND()
- REAL
- REGEXP
- RELEASE_LOCK()
- RENAME TABLE
- REPAIR TABLE
- REPEAT()
- REPLACE
- REPLACE ... SELECT
- REPLACE()
- REQUIRE GRANT option
- RESTORE TABLE
- return (\r)
- REVERSE()
- REVOKE
- RIGHT JOIN
- RIGHT OUTER JOIN
- RIGHT()
- RLIKE
- ROLLBACK, ROLLBACK
- ROUND(), ROUND()
- rows DBI method
- RPAD()
- RTRIM()

## S

- SEC_TO_TIME()
- SECOND()
- SELECT
- SELECT INTO TABLE
- SELECT speed
- SELECT, optimising
- SESSION_USER()
- SET, SET
- SET OPTION
- SET PASSWORD statement
- SET TRANSACTION
- SHA()
- SHA1()
- SHOW COLUMNS
- SHOW CREATE TABLE
- SHOW DATABASE INFO
- SHOW DATABASES
- SHOW FIELDS
- SHOW GRANTS
- SHOW INDEX
- SHOW KEYS
- SHOW MASTER LOGS
- SHOW MASTER STATUS
- SHOW PROCESSLIST
- SHOW SLAVE STATUS
- SHOW STATUS
- SHOW TABLE STATUS
- SHOW TABLES
- SHOW VARIABLES
- SIGN()
- SIN()
- single quote (\')
- SMALLINT
- SOUNDEX()
- SOUNDS LIKE
- SPACE()
- SQL_CACHE
- SQL_NO_CACHE
- SQRT()
- statements, GRANT
- statements, INSERT
- STD()
- STDDEV()
- STRAIGHT_JOIN
- STRCMP()
- string comparison functions
- string functions
- SUBDATE()
- SUBSTRING(), SUBSTRING()
- SUBSTRING_INDEX()
- subtraction (-)
- SUM()
- SYSDATE()
- SYSTEM_USER()

## T

- tab (\t)
- table DBI method
- table_cache
- TAN()
- TEXT, TEXT
- threads
- TIME, TIME
- TIME_FORMAT()
- TIME_TO_SEC()
- TIMESTAMP, TIMESTAMP
- TINYBLOB
- TINYINT
- TINYTEXT
- TMPDIR environment variable, TMPDIR environment variable
- TO_DAYS()
- trace DBI method, trace DBI method
- TRIM()
- TRUNCATE
- TRUNCATE()
- type DBI method
- Types
- TZ environment variable, TZ environment variable

## U

- UCASE()
- UDF functions
- ulimit
- UMASK environment variable, UMASK environment variable
- UMASK_DIR environment variable, UMASK_DIR environment variable
- unary minus (-)
- UNION, UNION
- UNIQUE
- UNIX_TIMESTAMP()
- UNLOCK TABLES
- UPDATE
- UPPER()
- USE
- USE INDEX, USE INDEX
- USE KEY, USE KEY
- USER environment variable
- `USER` environment variable
- USER()
- User-defined functions

## V

- VARCHAR, VARCHAR
- VARIANCE()
- VERSION()

## W

- WEEK()
- WEEKDAY()
- WHERE
- Wildcard character (%)
- Wildcard character (_)
- without-server option

## X

- XOR, bitwise
- XOR, logical

## Y

- YEAR, YEAR
- YEAR()

##

- | (bitwise OR)
- || (logical OR)
- ~

# Concept Index

## A

- aborted clients
- aborted connection
- access control
- access denied errors
- access privileges
- Access program
- `ACID`
- ACID
- ACLs
- ActiveState Perl
- adding, character sets
- adding, native functions
- adding, new functions
- adding, new user privileges
- adding, new users
- adding, procedures
- adding, user-definable functions
- administration, server
- ADO program
- age, calculating
- alias names, case-sensitivity
- aliases, for expressions
- aliases, for tables
- aliases, in `GROUP BY` clauses
- aliases, in `ORDER BY` clauses
- aliases, names
- aliases, on expressions
- anonymous user, anonymous user, anonymous user, anonymous user
- ANSI mode, running
- ANSI SQL, differences from
- ANSI SQL92, extensions to
- answering questions, etiquette
- Apache
- APIs
- APIs, Perl
- applying, patches
- argument processing
- arithmetic expressions
- AUTO-INCREMENT, ODBC
- AUTO_INCREMENT
- `AUTO_INCREMENT`, and `NULL` values

## B

- backing up, databases, backing up, databases
- backslash, escape character
- backups
- backups, database
- batch mode
- `batch`, `mysql` option
- `BDB` table type
- `BDB` tables
- benchmark suite
- benchmarks
- `BerkeleyDB` table type
- Big5 Chinese character encoding
- binary distributions
- binary distributions, installing
- binary distributions, on HP-UX
- binary distributions, on Linux
- binary log
- `Binlog_Dump`
- bit_functions, example
- BitKeeper tree
- `BLOB` columns, default values
- `BLOB` columns, indexing
- `BLOB`, inserting binary data
- `BLOB`, size
- books, about MySQL
- Borland Builder 4 program
- Borland C++ compiler
- brackets, square
- buffer sizes, client
- buffer sizes, `mysqld` server
- bug reports, criteria for
- bug reports, e-mail address
- bugs, known
- bugs, reporting
- building, client programs

## C

- C API, datatypes
- C API, functions
- C API, linking problems
- C++
- C++ APIs
- C++ Builder
- C++ compiler cannot create executables
- C++ compiler, `gcc`
- caches, clearing
- calculating, dates
- calling sequences for aggregate functions, UDF
- calling sequences for simple functions, UDF
- can't create/write to file
- case-sensitivity, in access checking
- case-sensitivity, in names
- case-sensitivity, in searches
- case-sensitivity, in string comparisons
- case-sensitivity, of database names
- case-sensitivity, of table names
- cast operators
- casts
- `cc1plus` problems
- certification
- ChangeLog
- changes to privileges
- changes, log
- changes, version 3.19
- changes, version 3.20
- changes, version 3.21
- changes, version 3.22
- changes, version 3.23
- changes, version 4.0
- changes, version 4.1
- changing socket location, changing socket location, changing socket location
- changing, column
- changing, column order
- changing, field
- changing, table, changing, table, changing, table
- character sets, character sets
- character sets, adding
- `character-sets-dir`, `mysql` option
- characters, multi-byte
- check options, myisamchk
- checking, tables for errors
- checksum errors
- Chinese
- choosing types
- choosing, a MySQL version
- clearing, caches
- client programs, building
- client tools
- clients, debugging
- clients, threaded
- closing, tables
- ColdFusion program
- collating, strings
- column names, case-sensitivity
- column, changing
- columns, changing
- columns, displaying
- columns, indexes
- columns, names
- columns, other types
- columns, selecting
- columns, storage requirements
- columns, types
- command syntax
- command-line history
- command-line options, `mysql`
- command-line tool
- commands out of sync
- commands, for binary distribution
- commands, list of
- commands, replication
- comments, adding
- comments, starting
- commercial support, types
- communications protocols
- comparisons, MySQL vs. others
- compatibility, between MySQL versions, compatibility, between MySQL versions, compatibility, between MySQL versions, compatibility, between MySQL versions
- compatibility, with ANSI SQL
- compatibility, with mSQL
- compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC
- compatibility, with Oracle, compatibility, with Oracle, compatibility, with Oracle
- compatibility, with PostgreSQL
- compatibility, with Sybase
- compiler, C++ `gcc`
- compiling, on Windows
- compiling, optimising
- compiling, problems
- compiling, speed
- compiling, statically
- compiling, user-defined functions
- compliance, Y2K
- `compress`, `mysql` option
- compressed tables
- config-file option
- config.cache
- `config.cache` file
- configuration files
- configuration options
- configure option, --with-low-memory
- `configure` script
- `configure`, running after prior invocation
- `connect_timeout` variable
- connecting, remotely with SSH
- connecting, to the server, connecting, to the server
- connecting, verification
- connection, aborted
- constant table, constant table
- consultants, list of
- consulting
- contact information
- `Contrib` directory
- contributed programs
- contributing companies, list of
- contributors, list of
- control access
- conventions, typographical
- converters
- converting, tools
- copyrights
- costs, support
- counting, table rows
- crackers, security against
- crash
- crash, recovery
- crash, repeated
- crash-me
- crash-me program, crash-me program
- creating, bug reports
- creating, databases
- creating, default startup options
- creating, tables
- customer support, mailing address
- customers, of MySQL
- CVS tree

## D

- data, character sets
- data, importing
- data, loading into tables
- data, retrieving
- data, size
- database design
- database names, case-sensitivity, database names, case-sensitivity
- database, deleting
- `database`, `mysql` option
- databases, backups
- databases, creating
- databases, defined
- databases, displaying
- databases, dumping, databases, dumping
- databases, information about
- databases, MySQL vs. others
- databases, names
- databases, replicating
- databases, selecting
- databases, symbolic links
- databases, using
- DataJunction
- datatypes, C API
- Date and Time types
- date calculations
- DATE columns, problems
- date functions, Y2K compliance
- date types
- date types, Y2K issues
- date values, problems
- `db` table, sorting
- `DBI` interface
- `DBI` Perl module
- `DBI/DBD`
- DBUG package
- `debug-info`, `mysql` option
- `debug`, `mysql` option
- debugging, client
- debugging, server
- decimal point
- default hostname
- default installation location
- default options
- default values, default values, default values, default values
- default values, `BLOB` and `TEXT` columns
- default values, suppression
- default, privileges
- `default-character-set`, `mysql` option
- defaults, embedded
- delayed_insert_limit
- deleting, database
- deleting, function
- deleting, index, deleting, index
- deleting, primary key
- deleting, rows
- deleting, table
- deletion, `mysql.sock`
- Delphi
- Delphi program
- design, choices
- design, issues
- design, limitations
- developers, list of
- development source tree
- digits
- directory structure, default
- disconnecting, from the server
- disk full
- disk issues
- disks, splitting data across
- display size
- displaying, database information
- displaying, information, `SHOW`
- displaying, table status
- DNS
- downgrading
- downloading
- dumping, databases, dumping, databases
- dynamic table characteristics

## E

- e-mail lists
- Eiffel Wrapper
- embedded MySQL server library
- employment with MySQL
- employment, contact information
- `enable-named-commands`, `mysql` option
- entering, queries
- ENUM, size
- environment variables, environment variables, environment variables, environment variables
- environment variables, list of
- Errcode
- errno
- error mesaages, can't find file
- error messages, displaying
- error messages, languages
- errors, access denied
- errors, checking tables for
- errors, common
- errors, directory checksum
- errors, handling for UDFs
- errors, known
- errors, linking
- errors, list of
- errors, reporting, errors, reporting, errors, reporting
- escape characters
- estimating, query performance
- example option
- examples, compressed tables
- examples, `myisamchk` output
- examples, queries
- Excel
- `execute`, `mysql` option
- expression aliases, expression aliases
- expressions, extended
- extensions, to ANSI SQL
- extracting, dates

## F

- `fatal signal 11`
- features of MySQL
- field, changing
- files, binary log
- files, `config.cache`
- files, error messages
- files, log, files, log
- files, not found message
- files, permissions
- files, query log
- files, repairing
- files, script
- files, size limits
- files, slow query log
- files, text
- files, `tmp`
- files, update log
- files,``my.cnf'`
- floating-point number
- floats
- flush tables
- `force`, `mysql` option
- foreign keys, foreign keys, foreign keys
- free licensing
- FreeBSD troubleshooting
- full disk
- full-text search
- FULLTEXT
- function, deleting
- functions for `SELECT` and `WHERE` clauses
- functions, C API
- functions, grouping
- functions, native, adding
- functions, new
- functions, user-definable, adding
- functions, user-defined

## G

- `gcc`
- gdb, using
- general information
- General Public License
- General Public License, MySQL
- getting MySQL
- global privileges
- goals of MySQL
- GPL, General Public License
- GPL, GNU General Public License
- GPL, MySQL
- grant tables
- grant tables, re-creating
- grant tables, sorting, grant tables, sorting
- granting, privleges
- `GROUP BY`, aliases in
- `GROUP BY`, extensions to ANSI SQL, `GROUP BY`, extensions to ANSI SQL
- grouping, expressions

## H

- handling, errors
- `HEAP` table type
- help option
- `help`, `mysql` option
- hints, hints, hints, hints, hints
- history file
- history of MySQL
- `host` table
- `host` table, sorting
- `host`, `mysql` option
- hostname caching
- hostname, default
- HP-UX, binary distribution
- `html`, `mysql` option

## I

- ID, unique
- `ignore-space`, `mysql` option
- importing, data
- increasing, performance
- increasing, speed
- index, deleting, index, deleting
- indexes
- indexes, and `BLOB` columns
- indexes, and `IS NULL`
- indexes, and `LIKE`
- indexes, and `NULL` values
- indexes, and `TEXT` columns
- indexes, block size
- indexes, columns
- indexes, leftmost prefix of
- indexes, multi-column
- indexes, multi-part
- indexes, names
- indexes, use of
- `InnoDB` table type
- `InnoDB` tables
- INSERT DELAYED
- inserting, speed of
- installation layouts
- installation overview
- Installing many servers
- installing, binary distribution
- installing, overview
- installing, Perl
- installing, Perl on Windows
- installing, source distribution
- installing, user-defined functions
- integers
- internal compiler errors
- internal locking
- internals
- Internet Service Providers
- `ISAM` table type
- ISP services

## J

- Java connectivity
- JDBC
- jobs at MySQL

## K

- key space, MyISAM
- keys
- keys, foreign, keys, foreign
- keys, multi-column
- keys, searching on two
- keywords
- known errors

## L

- language support
- last row, unique ID
- layout of installation
- leftmost prefix of indexes
- legal names
- LGPL, GNU Lesser General Public License
- LGPL, GNU Library General Public License
- LGPL, Lesser General Public License
- LGPL, Library General Public License
- libmysqld
- library, `mysqlclient`
- licenses
- licensing costs
- licensing policy
- licensing terms
- licensing, contact information
- licensing, examples
- licensing, free
- limitations, design
- limits, file-size
- linking
- linking, errors
- linking, problems
- linking, speed
- links, symbolic
- Linux, binary distribution
- literals
- loading, tables
- locking
- locking methods
- locking, row-level
- locking, tables
- Log files
- log files
- log files, maintaining
- log files, names
- log option
- log, changes
- logos

## M

- magazines, online
- mailing address, for customer support
- mailing list address
- mailing lists
- mailing lists, archive location
- mailing lists, guidelines
- main features of MySQL
- maintaining, log files
- maintaining, tables
- `make_binary_distribution`
- manual, available formats
- manual, online location
- manual, typographical conventions
- manuals, about MySQL
- master-slave setup
- matching, patterns
- max memory used
- `max_allowed_packet`
- `max_join_size`
- memory usage, myisamchk
- memory use, memory use
- `MERGE` table type
- MERGE tables, defined
- messages, languages
- methods, locking
- mirror sites
- MIT-pthreads
- modes, batch
- modules, list of
- monitor, terminal
- mSQL compatibility
- mSQL vs. MySQL, protocol
- mSQL, MySQL vs mSQL, overview
- `msql2mysql`
- multi mysqld
- multi-byte character sets
- multi-byte characters
- multi-column indexes
- multi-part index
- multiple servers
- My, derivation
- ``my.cnf'` file
- `MyISAM` table type
- MyISAM, compressed tables
- `myisamchk`, `myisamchk`
- `myisamchk`, example output
- `myisamchk`, options
- `myisampack`, `myisampack`
- MyODBC
- MyODBC, reporting problems
- `mysql`
- MySQL AB, defined
- MySQL binary distribution
- MySQL certification
- `mysql` command-line options
- MySQL consulting
- MySQL history
- MySQL mailing lists
- MySQL name
- MySQL Portals
- MySQL related information URLs
- MySQL source distribution
- MySQL table types
- MySQL Testimonials
- MySQL tools, conversion
- MySQL training
- MySQL version
- MySQL, defined
- MySQL, introduction
- MySQL, pronunciation
- mysql.sock, changing location of
- `mysql.sock`, protection
- `mysql_fix_privilege_tables`
- `mysql_install_db`
- `mysql_install_db` script
- `mysqlaccess`
- `mysqladmin`, `mysqladmin`, `mysqladmin`, `mysqladmin`, `mysqladmin`, `mysqladmin`, `mysqladmin`
- mysqladmin option
- `mysqlbinlog`, `mysqlbinlog`
- `mysqlbug`
- `mysqlbug` script
- `mysqlbug` script, location
- `mysqlclient` library
- `mysqld`
- mysqld option
- `mysqld` options
- mysqld options
- `mysqld` server, buffer sizes
- `mysqld`, starting
- `mysqld-max`
- `mysqld_multi`
- `mysqldump`, `mysqldump`, `mysqldump`
- `mysqlimport`, `mysqlimport`, `mysqlimport`, `mysqlimport`
- `mysqlshow`
- mysqltest, MySQL Test Suite

## N

- named pipes
- names
- names, case-sensitivity
- names, variables
- naming, releases of MySQL
- native functions, adding
- native thread support
- negative values
- net etiquette, net etiquette
- `net_buffer_length`
- netmask notation, in `mysql.user` table
- new procedures, adding
- new users, adding
- news sites
- no matching rows
- `no-auto-rehash`, `mysql` option
- `no-beep`, `mysql` option
- no-log option
- `no-named-commands`, `mysql` option
- `no-pager`, `mysql` option
- `no-tee`, `mysql` option
- non-delimited strings
- Non-transactional tables
- NULL value
- `NULL` values, and indexes
- `NULL` values, vs. empty values
- `NULL`, testing for null, `NULL`, testing for null, `NULL`, testing for null, `NULL`, testing for null
- `NULL` values, and `AUTO_INCREMENT` columns
- `NULL` values, and `TIMESTAMP` columns
- numbers
- numeric types

## O

- ODBC
- ODBC compatibility, ODBC compatibility, ODBC compatibility, ODBC compatibility, ODBC compatibility, ODBC compatibility, ODBC compatibility
- ODBC, administrator
- odbcadmin program
- OLEDB
- `one-database`, `mysql` option
- online location of manual
- online magazines
- `Open Source`, defined
- open tables, open tables
- opening, tables
- opens
- OpenSSL
- operating systems, file-size limits
- operating systems, supported
- operating systems, Windows versus Unix
- operations, arithmetic
- operators, cast, operators, cast
- optimisation, tips
- optimisations
- optimising, DISTINCT
- optimising, LEFT JOIN
- optimising, LIMIT
- optimising, tables
- option files
- options, command-line
- options, command-line, `mysql`
- options, configure
- options, `myisamchk`
- options, provided by MySQL
- options, replication
- Oracle compatibility, Oracle compatibility, Oracle compatibility
- `ORDER BY`, aliases in
- overview

## P

- `pack_isam`
- `pager`, `mysql` option
- parameters, server
- partnering with MySQL AB
- password encryption, reversibility of
- password option
- password, root user
- `password`, `mysql` option
- passwords, for users
- passwords, forgotten
- passwords, resetting
- passwords, security
- passwords, setting, passwords, setting, passwords, setting
- patches, applying
- pattern matching
- performance, benchmarks
- performance, disk issues
- performance, estimating
- performance, improving, performance, improving
- Perl API
- Perl DBI/DBD, installation problems
- Perl, installing
- Perl, installing on Windows
- Perl, modules
- permission checks, effect on speed
- perror
- PHP API
- PHP, web sites
- `port`, `mysql` option
- portability
- portability, types
- porting, to other systems
- post-install, many servers
- post-installation, setup and testing
- PostgreSQL compatibility
- PostgreSQL vs. MySQL, benchmarks
- PostgreSQL vs. MySQL, features
- PostgreSQL vs. MySQL, overview
- PostgreSQL vs. MySQL, strategies
- prices, support
- primary key, deleting
- privilege information, location
- privilege system
- privilege system, described
- privilege, changes
- privileges, access
- privileges, adding
- privileges, default
- privileges, display
- privileges, granting
- privileges, revoking
- problems, access denied errors
- problems, common errors
- problems, compiling
- problems, `DATE` columns
- problems, date values
- problems, installing on IBM-AIX
- problems, installing on Solaris
- problems, installing Perl
- problems, linking
- problems, ODBC
- problems, reporting
- problems, starting the server
- problems, table locking
- problems, timezone
- procedures, adding
- procedures, stored
- process support
- processes, display
- processing, arguments
- products, selling
- programs, client
- programs, contributed
- programs, crash-me
- programs, list of, programs, list of
- `prompt` command
- `prompt`, `mysql` option
- prompts, meanings
- pronunciation, MySQL
- Protocol mismatch
- Python APIs

## Q

- queries, entering
- queries, estimating performance
- queries, examples
- queries, speed of
- queries, Twin Studeis project
- Query Cache
- query log
- questions
- questions, answering
- `quick`, `mysql` option
- quotes, in strings
- quoting
- quoting binary data
- quoting strings

## R

- `raw`, `mysql` option
- re-creating, grant tables
- reconfiguring, reconfiguring
- recovery, from crash
- reducing, data size
- references
- regex
- regular expression syntax, described
- relational databases, defined
- release numbers
- releases, naming scheme
- releases, testing
- releases, updating
- reordering, columns
- repair options, myisamchk
- repairing, tables
- `replace`
- replication
- replication, commands
- replication, two-way
- reporting, bugs
- reporting, errors, reporting, errors
- reporting, MyODBC problems
- reserved words, exceptions
- restarting, the server
- retrieving, data from tables
- return values, UDFs
- revoking, privleges
- root password
- root user, password resetting
- rounding errors, rounding errors
- rows, counting
- rows, deleting
- rows, locking
- rows, matching problems
- rows, selecting
- rows, sorting
- RPM file
- RPM Package Manager, RPM Package Manager
- RPM, defined
- RTS-threads
- running a web server
- running `configure` after prior invocation
- running, ANSI mode
- running, batch mode
- running, multiple servers
- running, queries

## S

- `safe-mode` command
- `safe-updates`, `mysql` option
- `safe_mysqld`
- script files
- scripts, scripts, scripts
- scripts, `mysql_install_db`
- scripts, `mysqlbug`
- search engines, web
- searching, and case-sensitivity
- searching, full-text
- searching, MySQL web pages
- searching, two keys
- security system
- security, against crackers
- `SELECT`, Query Cache
- `select_limit`
- selecting, databases
- selling products
- sequence emulation
- server administration
- server, connecting, server, connecting
- server, debugging
- server, disconnecting
- server, restart
- server, shutdown
- server, starting
- server, starting and stopping
- server, starting problems
- servers, multiple
- services
- services, ISP
- services, web
- SET, size
- `set-variable`, `mysql` option
- setting, passwords
- setup, post-installation
- shell syntax
- showing, database information
- shutting down, the server
- silent column changes
- `silent`, `mysql` option
- size of tables
- sizes, display
- `skip-column-names`, `mysql` option
- `skip-line-numbers`, `mysql` option
- slow queries
- slow query log
- socket location, changing
- `socket`, `mysql` option
- Solaris installation problems
- Solaris troubleshooting
- sorting, character sets
- sorting, data
- sorting, grant tables, sorting, grant tables
- sorting, table rows
- source distribution, installing
- speed, compiling
- speed, increasing
- speed, inserting
- speed, linking
- speed, of queries, speed, of queries
- SQL commands, replication
- SQL, defined
- `sql_yacc.cc` problems
- square brackets
- SSH
- SSL and X509 Basics
- SSL related options
- stability
- standards compatibility
- Starting many servers
- starting, comments
- starting, `mysqld`
- starting, the server
- starting, the server automatically
- startup options, default
- startup parameters
- startup parameters, `mysql`
- startup parameters, tuning
- statically, compiling
- `status` command
- status command, results
- status, tables
- stopping, the server
- storage of data
- storage requirements, column type
- storage space, minimising
- stored procedures and triggers, defined
- string collating
- string comparisons, case-sensitivity
- string types
- strings, defined
- strings, escaping characters
- strings, non-delimited
- strings, quoting
- striping, defined
- sub`SELECT`s
- superuser
- support costs
- support terms
- support, for operating systems
- support, licensing
- support, mailing address
- support, types
- suppression, default values
- Sybase compatibility
- symbolic links, symbolic links
- syntax, regular expression
- system optimisation
- system table
- system variables
- system, privilege
- system, security

## T

- table aliases
- table cache
- table is full, table is full
- table names, case-sensitivity, table names, case-sensitivity
- table types, choosing
- table, changing, table, changing, table, changing
- table, deleting
- `table`, `mysql` option
- tables, `BDB`
- tables, `Berkeley DB`
- tables, changing column order
- tables, checking
- tables, closing
- tables, compressed
- tables, compressed format
- tables, constant, tables, constant
- tables, counting rows
- tables, creating
- tables, defragment, tables, defragment
- tables, defragmenting
- tables, deleting rows
- tables, displaying
- tables, displaying status
- tables, dumping, tables, dumping
- tables, dynamic
- tables, error checking
- tables, flush
- tables, fragmentation
- tables, grant
- tables, `HEAP`
- tables, `host`
- tables, improving performance
- tables, information
- tables, information about
- tables, ISAM
- tables, loading data
- tables, locking
- tables, maintenance regimen
- tables, maximum size
- tables, merging
- tables, multiple
- tables, names
- tables, open
- tables, opening
- tables, optimising
- tables, repairing
- tables, retrieving data
- tables, selecting columns
- tables, selecting rows
- tables, sorting rows
- tables, symbolic links
- tables, system
- tables, too many
- tables, unique ID for last row
- tables, updating
- tar, problems on Solaris
- Tcl APIs
- tcp-ip option
- TCP/IP
- technical support, licensing
- technical support, mailing address
- `tee`, `mysql` option
- temporary file, write access
- temporary tables, problems
- terminal monitor, defined
- testing mysqld, mysqltest
- testing, connection to the server
- testing, installation
- testing, of MySQL releases
- testing, post-installation
- testing, the server
- Texinfo
- `TEXT` columns, default values
- `TEXT` columns, indexing
- text files, importing
- `TEXT`, size
- thread packages, differences between
- thread support
- thread support, non-native
- threaded clients
- threads, threads
- threads, display
- threads, RTS
- time types
- timeout, timeout, timeout
- timeout, `connect_timeout` variable
- `TIMESTAMP`, and `NULL` values
- timezone problems
- tips, optimisation
- ToDo list for MySQL
- TODO, embedded server
- TODO, symlinks
- tools, command-line
- tools, converting
- tools, mysqld_multi
- tools, safe_mysqld
- trademarks
- training
- transaction-safe tables, transaction-safe tables
- transactions, support, transactions, support
- triggers, stored
- troubleshooting, FreeBSD
- troubleshooting, Solaris
- tutorial
- Twin Studies, queries
- type conversions
- types of support
- types, columns, types, columns
- types, date
- types, Date and Time
- types, numeric
- types, of tables
- types, portability
- types, strings
- types, time
- typographical conventions

## U

- UDFs, compiling
- UDFs, defined
- UDFs, return values
- `unbuffered`, `mysql` option
- unique ID
- unloading, tables
- update log
- updating, releases of MySQL
- updating, tables
- upgrading
- upgrading, 3.20 to 3.21
- upgrading, 3.21 to 3.22
- upgrading, 3.22 to 3.23
- upgrading, 3.23 to 4.0
- upgrading, 4.0 to 4.1
- upgrading, different architecture
- uptime
- URLS for downloading MySQL
- URLs to MySQL information
- user names, and passwords
- user option
- user privileges, adding
- `user` table, sorting
- user variables
- user-defined functions, adding, user-defined functions, adding
- `user`, `mysql` option
- users, adding
- users, of MySQL
- users, root
- uses, of MySQL
- using multiple disks to start data
- utilities

## V

- valid numbers, examples
- `VARCHAR`, size
- variables, `mysqld`
- variables, status
- variables, System
- variables, user
- variables, values
- `verbose`, `mysql` option
- version option
- version, choosing
- version, latest
- `version`, `mysql` option
- `vertical`, `mysql` option
- views
- virtual memory, problems while compiling
- Visual Basic

## W

- `wait`, `mysql` option
- web pages, miscellaneous
- web search engines
- web server, running
- web sites
- What is encryption
- What is X509/Certificate?
- wildcards, and `LIKE`
- wildcards, in `mysql.columns_priv` table
- wildcards, in `mysql.db` table
- wildcards, in `mysql.host` table
- wildcards, in `mysql.tables_priv` table
- wildcards, in `mysql.user` table
- Windows
- Windows, compiling on
- Windows, open issues
- Windows, versus Unix
- Word program
- wrappers, Eiffel
- write access, tmp

## X

- `xml`, `mysql` option

## Y

- Year 2000 compliance
- Year 2000 issues

This document was generated on 16 December 2002 using the texi2html translator version 1.52 (extended by davida@detron.se).