5.2. mysqld — The MySQL Server

MySQL 5.0

5.2. mysqld — The MySQL Server

mysqld is the MySQL server. The following discussion covers these MySQL server configuration topics:

  • Startup options that the server supports

  • Server system variables

  • Server status variables

  • How to set the server SQL mode

  • The server shutdown process

5.2.1. mysqld Command Options

When you start the mysqld server, you can specify program options using any of the methods described in Section 4.3, “Specifying Program Options”. The most common methods are to provide options in an option file or on the command line. However, in most cases it is desirable to make sure that the server uses the same options each time it runs. The best way to ensure this is to list them in an option file. See Section 4.3.2, “Using Option Files”.

mysqld reads options from the and groups. mysqld_safe reads options from the , , , and groups. mysql.server reads options from the and groups.

An embedded MySQL server usually reads options from the , , and _SERVER] groups, where is the name of the application into which the server is embedded.

mysqld accepts many command options. For a brief summary, execute mysqld --help. To see the full list, use mysqld --verbose --help.

The following list shows some of the most common server options. Additional options are described in other sections:

You can also set the values of server system variables by using variable names as options, as described later in this section.

  • ,

    Display a short help message and exit. Use both the and options to see the full message.

  • This option controls whether user-defined functions that have only an symbol for the main function can be loaded. By default, the option is off and only UDFs that have at least one auxiliary symbol can be loaded; this prevents attempts at loading functions from shared object files other than those containing legitimate UDFs. This option was added in version 5.0.3. See Section 24.2.4.6, “User-Defined Function Security Precautions”.

  • Use standard (ANSI) SQL syntax instead of MySQL syntax. For more precise control over the server SQL mode, use the option instead. See Section 1.9.3, “Running MySQL in ANSI Mode”, and Section 5.2.5, “The Server SQL Mode”.

  • ,

    The path to the MySQL installation directory. All paths are usually resolved relative to this directory.

  • The IP address to bind to.

  • This option is used by the mysql_install_db script to create the MySQL privilege tables without having to start a full MySQL server.

  • The directory where character sets are installed. See Section 5.11.1, “The Character Set Used for Data and Sorting”.

  • Don't ignore character set information sent by the client. To ignore client information and use the default server character set, use ; this makes MySQL behave like MySQL 4.0.

  • The filesystem character set. This option sets the system variable. It was added in MySQL 5.0.19.

  • ,

    Use as the default server character set. See Section 5.11.1, “The Character Set Used for Data and Sorting”.

  • Put the mysqld server in a closed environment during startup by using the system call. This is a recommended security measure. Note that use of this option somewhat limits and .

  • Use as the default server collation. See Section 5.11.1, “The Character Set Used for Data and Sorting”.

  • (Windows only.) Write error log messages to and even if is specified. mysqld does not close the console window if this option is used.

  • Write a core file if mysqld dies. For some systems, you must also specify the option to mysqld_safe. See Section 5.4.1, “mysqld_safe — MySQL Server Startup Script”. Note that on some systems, such as Solaris, you do not get a core file if you are also using the option.

  • ,

    The path to the data directory.

  • ], ]

    If MySQL is configured with , you can use this option to get a trace file of what mysqld is doing. The string often is '. The default is . See Section E.1.2, “Creating Trace Files”.

  • (DEPRECATED)

    Use as the default character set. This option is deprecated in favor of . See Section 5.11.1, “The Character Set Used for Data and Sorting”.

  • Use as the default collation. This option is deprecated in favor of . See Section 5.11.1, “The Character Set Used for Data and Sorting”.

  • Set the default storage engine (table type) for tables. See Chapter 14, Storage Engines and Table Types.

  • This option is a synonym for .

  • Set the default server time zone. This option sets the global system variable. If this option is not given, the default time zone is the same as the system time zone (given by the value of the system variable.

  • Specify how to use delayed key writes. Delayed key writing causes key buffers not to be flushed between writes for tables. disables delayed key writes. enables delayed key writes for those tables that were created with the option. delays key writes for all tables. See Section 7.5.2, “Tuning Server Parameters”, and Section 14.1.1, “ Startup Options”.

    Note: If you set this variable to , you should not use tables from within another program (such as another MySQL server or myisamchk) when the tables are in use. Doing so leads to index corruption.

  • Read the default DES keys from this file. These keys are used by the and functions.

  • Enable support for named pipes. This option applies only on Windows NT, 2000, XP, and 2003 systems, and can be used only with the mysqld-nt and mysqld-max-nt servers that support named-pipe connections.

  • ], ]

    This is a bit mask of different flags that you can use for debugging the mysqld server. Do not use this option unless you know exactly what it does!

  • Enable external locking (system locking), which is disabled by default as of MySQL 4.0. Note that if you use this option on a system on which does not fully work (such as Linux), it is easy for mysqld to deadlock. This option previously was named .

    Note: If you use this option to enable updates to tables from many MySQL processes, you must ensure that the following conditions are satisfied:

    • You should not use the query cache for queries that use tables that are updated by another process.

    • You should not use or on any shared tables.

    The easiest way to ensure this is to always use together with and . (This is not done by default because in many setups it is useful to have a mixture of the preceding options.)

  • Flush (synchronize) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section A.4.2, “What to Do If MySQL Keeps Crashing”.

  • Read SQL statements from this file at startup. Each statement must be on a single line and should not include comments.

  • Adds consistency guarantees between the content of tables and the binary log. See Section 5.12.3, “The Binary Log”. This option was removed in MySQL 5.0.3, having been made obsolete by the introduction of XA transaction support.

  • The options are listed in Section 14.2.4, “ Startup Options and System Variables”.

  • , -L

    Return client error messages in the given language. can be given as the language name or as the full pathname to the directory where the language files are installed. See Section 5.11.2, “Setting the Error Message Language”.

  • Some hardware/operating system architectures support memory pages greater than the default (usually 4KB). The actual implementation of this support depends on the underlying hardware and OS. Applications that perform a lot of memory accesses may obtain performance improvements by using large pages due to reduced Translation Lookaside Buffer (TLB) misses.

    Currently, MySQL supports only the Linux implementation of large pages support (which is called HugeTLB in Linux). We have plans to extend this support to FreeBSD, Solaris and possibly other platforms.

    Before large pages can be used on Linux, it is necessary to configure the HugeTLB memory pool. For reference, consult the file in the Linux kernel source.

    This option is disabled by default. It was added in MySQL 5.0.3.

  • ], ]

    Log connections and SQL statements received from clients to this file. See Section 5.12.2, “The General Query Log”. If you omit the filename, MySQL uses .log as the filename.

  • ]

    Enable binary logging. The server logs all statements that change data to the binary log, which is used for backup and replication. See Section 5.12.3, “The Binary Log”.

    The option value, if given, is the basename for the log sequence. The server creates binary log files in sequence by adding a numeric suffix to the basename. It is recommended that you specify a basename (see Section A.8.1, “Open Issues in MySQL”, for the reason). Otherwise, MySQL uses -bin as the basename.

  • ]

    The index file for binary log filenames. See Section 5.12.3, “The Binary Log”. If you omit the filename, and if you didn't specify one with , MySQL uses -bin.index as the filename.

  • With no argument or an argument of 1, this option sets the system variable to 1. With an argument of 0, this option sets the system variable to 0. affects how MySQL enforces restrictions on stored function creation. See Section 17.4, “Binary Logging of Stored Routines and Triggers”.

    This option was added in MySQL 5.0.16.

  • This is the old name for . Before MySQL 5.0.16, it also applies to stored procedures, not just stored functions and sets the system variable. As of 5.0.16, this option is deprecated. It is recognized for backward compatibility but its use results in a warning.

    This option was added in MySQL 5.0.6.

  • ]

    Log errors and startup messages to this file. See Section 5.12.1, “The Error Log”. If you omit the filename, MySQL uses .err. If the filename has no extension, the server adds an extension of .

  • ]

    Log all changes to this file (used only when debugging ).

  • (DEPRECATED)

    Log extra information to the update log, binary update log, and slow query log, if they have been activated. For example, the username and timestamp are logged for all queries. This option is deprecated, as it now represents the default logging behavior. (See the description for .) The option is available for the purpose of logging queries that do not use indexes to the slow query log.

  • If you are using this option with , queries that do not use indexes are logged to the slow query log. See Section 5.12.4, “The Slow Query Log”.

  • Log less information to the update log, binary update log, and slow query log, if they have been activated. For example, the username and timestamp are not logged for queries.

  • Log slow administrative statements such as , , and to the slow query log.

  • ]

    Log all queries that have taken more than seconds to execute to this file. See Section 5.12.4, “The Slow Query Log”. See the descriptions of the and options for details.

  • ], ]

    Print out warnings such as to the error log. Enabling this option is recommended, for example, if you use replication (you get more information about what is happening, such as messages about network failures and reconnections). This option is enabled (1) by default, and the default value if omitted is 1. To disable this option, use . Aborted connections are not logged to the error log unless the value is greater than 1. See Section A.2.10, “Communication Errors and Aborted Connections”.

  • Give table-modifying operations (, , , ) lower priority than selects. This can also be done via to lower the priority of only one query, or by to change the priority in one thread. See Section 7.3.2, “Table Locking Issues”.

  • Lock the mysqld process in memory. This works on systems such as Solaris that support the system call. This might help if you have a problem where the operating system is causing mysqld to swap on disk. Note that use of this option requires that you run the server as , which is normally not a good idea for security reasons. See Section 5.7.5, “How to Run MySQL as a Normal User”.

  • [,]...]]

    Set the storage engine recovery mode. The option value is any combination of the values of , , , or . If you specify multiple values, separate them by commas. You can also use a value of to disable this option. If this option is used, each time mysqld opens a table, it checks whether the table is marked as crashed or wasn't closed properly. (The last option works only if you are running with external locking disabled.) If this is the case, mysqld runs a check on the table. If the table was corrupted, mysqld attempts to repair it.

    The following options affect how the repair works:

    Option Description
    The same as not giving any option to .
    If the data file was changed during recovery, save a backup of the .MYD file as .BAK.
    Run recovery even if we would lose more than one row from the file.
    Don't check the rows in the table if there aren't any delete blocks.

    Before the server automatically repairs a table, it writes a note about the repair to the error log. If you want to be able to recover from most problems without user intervention, you should use the options . This forces a repair of a table even if some rows would be deleted, but it keeps the old data file as a backup so that you can later examine what happened.

    See Section 14.1.1, “ Startup Options”.

  • When using the storage engine, it is possible to point out the management server that distributes the cluster configuration by setting the connect string option. See Section 15.4.4.2, “The Cluster , for syntax.

  • If the binary includes support for the storage engine, this option enables the engine, which is disabled by default. See Chapter 15, MySQL Cluster.

  • Force the server to generate short (pre-4.1) password hashes for new passwords. This is useful for compatibility when the server must support older client programs. See Section 5.8.9, “Password Hashing as of MySQL 4.1”.

  • Only use one thread (for debugging under Linux). This option is available only if the server is built with debugging enabled. See Section E.1, “Debugging a MySQL Server”.

  • Change the number of file descriptors available to mysqld. If this option is not set or is set to 0, mysqld uses the value to reserve file descriptors with . If the value is 0, mysqld reserves or files (whichever is larger). You should try increasing this value if mysqld gives you the error .

  • The pathname of the process ID file. This file is used by other programs such as mysqld_safe to determine the server's process ID.

  • ,

    The port number to use when listening for TCP/IP connections. The port number must be 1024 or higher unless the server is started by the system user.

  • On some systems, when the server is stopped, the TCP/IP port might not become available immediately. If the server is restarted quickly afterward, its attempt to reopen the port can fail. This option indicates how many seconds the server should wait for the TCP/IP port to become free if it cannot be opened. The default is not to wait. This option was added in MySQL 5.0.19.

  • Skip some optimization stages.

  • (DEPRECATED)

    See Section 5.8.3, “Privileges Provided by MySQL”.

  • If this option is enabled, a user cannot create new MySQL users by using the statement, if the user doesn't have the privilege for the table or any column in the table.

  • Disallow authentication by clients that attempt to use accounts that have old (pre-4.1) passwords.

  • Enable shared-memory connections by local clients. This option is available only on Windows.

  • The name of shared memory to use for shared-memory connections. This option is available only on Windows. The default name is . The name is case sensitive.

  • Disable the storage engine. This saves memory and might speed up some operations. Do not use this option if you require tables.

  • Turn off the ability to select and insert at the same time on tables. (This is to be used only if you think you have found a bug in this feature.) See Section 7.3.3, “Concurrent Inserts”.

  • Do not use external locking (system locking). With external locking disabled, you must shut down the server to use myisamchk. (See Section 1.4.3, “MySQL Stability”.) To avoid this requirement, use the and statements to check and repair tables.

    External locking has been disabled by default since MySQL 4.0.

  • This option causes the server not to use the privilege system at all, which gives anyone with access to the server unrestricted access to all databases. You can cause a running server to start using the grant tables again by executing mysqladmin flush-privileges or mysqladmin reload command from a system shell, or by issuing a MySQL statement after connecting to the server. This option also suppresses loading of user-defined functions (UDFs).

  • Do not use the internal hostname cache for faster name-to-IP resolution. Instead, query the DNS server every time a client connects. See Section 7.5.6, “How MySQL Uses DNS”.

  • Disable the storage engine. This saves memory and disk space and might speed up some operations. Do not use this option if you require tables.

  • Disable the storage engine. This option was added in MySQL 5.0.24. It can be used if the following behavior is undesirable: If a user has access to table , that user can create a table that accesses . However, if the user's privileges on are subsequently revoked, the user can continue to access by doing so through .

  • Do not resolve hostnames when checking client connections. Use only IP numbers. If you use this option, all column values in the grant tables must be IP numbers or . See Section 7.5.6, “How MySQL Uses DNS”.

  • Disable the storage engine. This is the default for binaries that were built with storage engine support; the server allocates memory and other resources for this storage engine only if the option is given explicitly. See Section 15.4.3, “Quick Test Setup of MySQL Cluster”, for an example of usage.

  • Don't listen for TCP/IP connections at all. All interaction with mysqld must be made via named pipes or shared memory (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are allowed. See Section 7.5.6, “How MySQL Uses DNS”.

  • Options that begin with specify whether to allow clients to connect via SSL and indicate where to find SSL keys and certificates. See Section 5.9.7.3, “SSL Command Options”.

  • Available on Windows NT-based systems only; instructs the MySQL server not to run as a service.

  • ,

    Enable or disable symbolic link support. This option has different effects on Windows and Unix:

  • If MySQL is configured with , all MySQL programs check for memory overruns during each memory allocation and memory freeing operation. This checking is very slow, so for the server you can avoid it when you don't need it by using the option.

  • With this option, the statement is allowed only to users who have the privilege, and the statement displays all database names. Without this option, is allowed to all users, but displays each database name only if the user has the privilege or some privilege for the database. Note that any global privilege is considered a privilege for the database.

  • Don't write stack traces. This option is useful when you are running mysqld under a debugger. On some systems, you also must use this option to get a core file. See Section E.1, “Debugging a MySQL Server”.

  • Disable using thread priorities for faster response time.

  • On Unix, this option specifies the Unix socket file to use when listening for local connections. The default value is . On Windows, the option specifies the pipe name to use when listening for local connections that use a named pipe. The default value is (not case sensitive).

  • [,[,...]]

    Set the SQL mode. See Section 5.2.5, “The Server SQL Mode”.

  • As of MySQL 5.0.13, by default returns the time at which it executes, not the time at which the statement in which it occurs begins executing. This differs from the behavior of . This option causes to be an alias for . For information about the implications for binary logging and replication, see the description for in Section 12.5, “Date and Time Functions” and for in Section 13.5.3, “ Syntax”.

    This option was added in MySQL 5.0.20.

  • This option causes most temporary files created by the server to use a small set of names, rather than a unique name for each new file. This works around a problem in the Linux kernel dealing with creating many new files with different names. With the old behavior, Linux seems to “leak” memory, because it is being allocated to the directory entry cache rather than to the disk cache.

  • Sets the default transaction isolation level. The value can be , , , or . See Section 13.4.6, “ Syntax”.

  • ,

    The path of the directory to use for creating temporary files. It might be useful if your default directory resides on a partition that is too small to hold temporary tables. This option accepts several paths that are used in round-robin fashion. Paths should be separated by colon characters (‘’) on Unix and semicolon characters (‘’) on Windows, NetWare, and OS/2. If the MySQL server is acting as a replication slave, you should not set to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. For more information about the storage location of temporary files, see Section A.4.4, “Where MySQL Stores Temporary Files”. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or operations. If files in the temporary file directory are lost when the server restarts, replication fails.

  • |}, |}

    Run the mysqld server as the user having the name or the numeric user ID . (“User” in this context refers to a system login account, not a MySQL user listed in the grant tables.)

    This option is mandatory when starting mysqld as . The server changes its user ID during its startup sequence, causing it to run as that particular user rather than as . See Section 5.7.1, “General Security Guidelines”.

    To avoid a possible security hole where a user adds a option to a file (thus causing the server to run as ), mysqld uses only the first option specified and produces a warning if there are multiple options. Options in and are processed before command-line options, so it is recommended that you put a option in and specify a value other than . The option in is found before any other options, which ensures that the server runs as a user other than , and that a warning results if any other option is found.

  • ,

    Display version information and exit.

You can assign a value to a server system variable by using an option of the form =. For example, sets the variable to a value of 32MB.

Note that when you assign a value to a variable, MySQL might automatically correct the value to stay within a given range, or adjust the value to the closest allowable value if only certain values are allowed.

If you want to restrict the maximum value to which a variable can be set at runtime with , you can define this by using the = command-line option.

It is also possible to set variables by using = or = syntax. This syntax is deprecated.

You can change the values of most system variables for a running server with the statement. See Section 13.5.3, “ Syntax”.

Section 5.2.2, “Server System Variables”, provides a full description for all variables, and additional information for setting them at server startup and runtime. Section 7.5.2, “Tuning Server Parameters”, includes information on optimizing the server by tuning system variables.

5.2.2. Server System Variables

The mysql server maintains many system variables that indicate how it is configured. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. Most of them can be changed dynamically while the server is running by means of the statement, which enables you to modify operation of the server without having to stop and restart it. You can refer to system variable values in expressions.

There are several ways to see the names and values of system variables:

  • To see the values that a server will use based on its compiled-in defaults and any option files that it reads, use this command:

    mysqld --verbose --help
    
  • To see the values that a server will use based on its compiled-in defaults, ignoring the settings in any option files, use this command:

    mysqld --no-defaults --verbose --help
    
  • To see the current values used by a running server, use the statement.

This section provides a description of each system variable. Variables with no version indicated are present in all MySQL 5.0 releases. For historical information concerning their implementation, please see MySQL 3.23, 4.0, 4.1 Reference Manual.

For additional system variable information, see these sections:

Note: Some of the following variable descriptions refer to “enabling” or “disabling” a variable. These variables can be enabled with the statement by setting them to or , or disabled by setting them to or . However, to set such a variable on the command line or in an option file, you must set it to or ; setting it to or will not work. For example, on the command line, works but does not.

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.

  • and are intended for use with master-to-master replication, and can be used to control the operation of columns. Both variables can be set globally or locally, and each can assume an integer value between 1 and 65,535 inclusive. Setting the value of either of these two variables to 0 causes its value to be set to 1 instead. Attempting to set the value of either of these two variables to an integer greater than 65,535 or less than 0 causes its value to be set to 65,535 instead. Attempting to set the value of or to a non-integer value gives rise to an error, and the actual value of the variable remains unchanged.

    These two variables affect column behavior as follows:

    • controls the interval between successive column values. For example:

      mysql> 
      +--------------------------+-------+
      | Variable_name            | Value |
      +--------------------------+-------+
      | auto_increment_increment | 1     |
      | auto_increment_offset    | 1     |
      +--------------------------+-------+
      2 rows in set (0.00 sec)
      
      mysql> 
          -> 
        Query OK, 0 rows affected (0.04 sec)
      
      mysql> 
      Query OK, 0 rows affected (0.00 sec)
      
      mysql> 
      +--------------------------+-------+
      | Variable_name            | Value |
      +--------------------------+-------+
      | auto_increment_increment | 10    |
      | auto_increment_offset    | 1     |
      +--------------------------+-------+
      2 rows in set (0.01 sec)
      
      mysql> 
      Query OK, 4 rows affected (0.00 sec)
      Records: 4  Duplicates: 0  Warnings: 0
      
      mysql> 
      +-----+
      | col |
      +-----+
      |   1 |
      |  11 |
      |  21 |
      |  31 |
      +-----+
      4 rows in set (0.00 sec)
      

      (Note how is used here to obtain the current values for these variables.)

    • determines the starting point for the column value. Consider the following, assuming that these statements are executed during the same session as the example given in the description for :

      mysql> 
      Query OK, 0 rows affected (0.00 sec)
      
      mysql> 
      +--------------------------+-------+
      | Variable_name            | Value |
      +--------------------------+-------+
      | auto_increment_increment | 10    |
      | auto_increment_offset    | 5     |
      +--------------------------+-------+
      2 rows in set (0.00 sec)
      
      mysql> 
          -> 
      Query OK, 0 rows affected (0.06 sec)
      
      mysql> 
      Query OK, 4 rows affected (0.00 sec)
      Records: 4  Duplicates: 0  Warnings: 0
      
      mysql> 
      +-----+
      | col |
      +-----+
      |   5 |
      |  15 |
      |  25 |
      |  35 |
      +-----+
      4 rows in set (0.02 sec)
      

      If the value of is greater than that of , the value of is ignored.

    Should one or both of these variables be changed and then new rows inserted into a table containing an column, the results may seem counterintuitive because the series of values is calculated without regard to any values already present in the column, and the next value inserted is the least value in the series that is greater than the maximum existing value in the column. In other words, the series is calculated like so:

    × auto_increment_increment

    where is a positive integer value in the series [1, 2, 3, ...]. For example:

    mysql> 
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 10    |
    | auto_increment_offset    | 5     |
    +--------------------------+-------+
    2 rows in set (0.00 sec)
    
    mysql> 
    +-----+
    | col |
    +-----+
    |   1 |
    |  11 |
    |  21 |
    |  31 |
    +-----+
    4 rows in set (0.00 sec)
    
    mysql> 
    Query OK, 4 rows affected (0.00 sec)
    Records: 4  Duplicates: 0  Warnings: 0
    
    mysql> 
    +-----+
    | col |
    +-----+
    |   1 |
    |  11 |
    |  21 |
    |  31 |
    |  35 |
    |  45 |
    |  55 |
    |  65 |
    +-----+
    8 rows in set (0.00 sec)
    

    The values shown for and generate the series 5 + × 10, that is, [5, 15, 25, 35, 45, ...]. The greatest value present in the column prior to the is 31, and the next available value in the series is 35, so the inserted values for begin at that point and the results are as shown for the query.

    It is important to remember that it is not possible to confine the effects of these two variables to a single table, and thus they do not take the place of the sequences offered by some other database management systems; these variables control the behavior of all columns in all tables on the MySQL server. If one of these variables is set globally, its effects persist until the global value is changed or overridden by setting them locally, or until mysqld is restarted. If set locally, the new value affects columns for all tables into which new rows are inserted by the current user for the duration of the session, unless the values are changed during that session.

    The variable was added in MySQL 5.0.2. Its default value is 1. See Section 6.13, “Auto-Increment in Multiple-Master Replication”.

  • This variable was introduced in MySQL 5.0.2. Its default value is 1. For particulars, see the description for .

  • 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 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 system call should have more details. Check your OS documentation for the maximum value for this variable. cannot be set higher than your operating system limit.

  • The MySQL installation base directory. This variable can be set with the option.

  • The size of the buffer that is allocated for caching indexes and rows for tables. If you don't use tables, you should start mysqld with to not allocate memory for this cache.

  • The base directory for tables. This should be assigned the same value as the variable.

  • The size of the buffer that is allocated for caching indexes and rows for tables. If you don't use tables, you should set this to 0 or start mysqld with to not allocate memory for this cache.

  • The directory where the storage engine writes its log files. This variable can be set with the option.

  • The maximum number of locks that can be active for a table (10,000 by default). You should increase this value if errors such as the following occur when you perform long transactions or when mysqld has to examine many rows to calculate a query:

    bdb: Lock table is out of available locks
    Got error 12 from ...
    
  • This is if you are using to start Berkeley DB in multi-process mode. (Do not use when initializing Berkeley DB.)

  • The temporary file directory.

  • The size of the cache to hold the SQL statements for the binary log during a transaction. A binary log cache is allocated for each client if the server supports any transactional storage engines and if the server has the binary log enabled ( option). If you often use large, multiple-statement transactions, you can increase this cache size to get more performance. The and status variables can be useful for tuning the size of this variable. See Section 5.12.3, “The Binary Log”.

  • uses a special tree-like cache to make bulk inserts faster for , , and when adding data to non-empty tables. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. The default value is 8MB.

  • The character set for statements that arrive from the client.

  • The character set used for literals that do not have a character set introducer and for number-to-string conversion.

  • The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as .

  • The filesystem character set. This variable is used to interpret string literals that refer to filenames, such as in the and statements and the function. Such filenames are converted from to before the file opening attempt occurs. The default value is , which means that no conversion occurs. For systems on which multi-byte filenames are allowed, a different value may be more appropriate. For example, if the system represents filenames using UTF-8, set to . This variable was added in MySQL 5.0.19.

  • The character set used for returning query results to the client.

  • The server's default character set.

  • The character set used by the server for storing identifiers. The value is always .

  • The directory where character sets are installed.

  • The collation of the connection character set.

  • The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as .

  • The server's default collation.

  • The transaction completion type:

    • If the value is 0 (the default), and are unaffected.

    • If the value is 1, and are equivalent to and , respectively. (A new transaction starts immediately with the same isolation level as the just-terminated transaction.)

    • If the value is 2, and are equivalent to and , respectively. (The server disconnects after terminating the transaction.)

    This variable was added in MySQL 5.0.3

  • If (the default), MySQL allows and statements to run concurrently for tables that have no free blocks in the middle. You can turn this option off by starting mysqld with or .

    In MySQL 5.0.6, this variable was changed to take three integer values:

    Value Description
    0 Off
    1 (Default) Enables concurrent insert for tables that don't have holes
    2 Enables concurrent inserts for all tables. If table has a hole and is in use by another thread the new row will be inserted at end of table. If table is not in use, MySQL does a normal read lock and inserts the new row into the hole.

    See also Section 7.3.3, “Concurrent Inserts”.

  • The number of seconds that the mysqld server waits for a connect packet before responding with .

  • The MySQL data directory. This variable can be set with the option.

  • This variable is not implemented.

  • This variable is not implemented.

  • The default mode value to use for the function. See Section 12.5, “Date and Time Functions”.

  • This option applies only to tables. It can have one of the following values to affect handling of the table option that can be used in statements.

    Option Description
    is ignored.
    MySQL honors any option specified in statements. This is the default value.
    All new opened tables are treated as if they were created with the option enabled.

    If is enabled for a table, the key buffer is not flushed for the table on every index update, but only when the table is closed. This speeds up writes on keys a lot, but if you use this feature, you should add automatic checking of all tables by starting the server with the option (for example, ). See Section 5.2.1, “mysqld Command Options”, and Section 14.1.1, “ Startup Options”.

    Note that enabling external locking with offers no protection against index corruption for tables that use delayed key writes.

  • After inserting delayed rows, the handler thread checks whether there are any statements pending. If so, it allows them to execute before continuing to insert delayed rows.

  • How many seconds an handler thread should wait for statements before terminating.

  • This is a per-table limit on the number of rows to queue when handling statements. If the queue becomes full, any client that issues an statement waits until there is room in the queue again.

  • This variable indicates the number of digits of precision by which to increase the result of division operations performed with the operator. The default value is 4. The minimum and maximum values are 0 and 30, respectively. The following example illustrates the effect of increasing the default value.

    mysql> 
    +--------+
    | 1/7    |
    +--------+
    | 0.1429 |
    +--------+
    mysql> 
    mysql> 
    +----------------+
    | 1/7            |
    +----------------+
    | 0.142857142857 |
    +----------------+
    

    This variable was added in MySQL 5.0.6.

  • This variable applies to NDB. By default it is 0 (): If you execute a query such as , where is a non-indexed column, the query is executed as a full table scan on every NDB node. Each node sends every row to the MySQL server, which applies the condition. If is set to 1 (), the condition is “pushed down” to the storage engine and sent to the NDB nodes. Each node uses the condition to perform the scan, and only sends back to the MySQL server the rows that match the condition.

    This variable was added in MySQL 5.0.3. Before that, the default behavior is the same as for a value of .

  • The number of days for automatic binary log removal. The default is 0, which means “no automatic removal.” Possible removals happen at startup and at binary log rotation.

  • If , the server flushes (synchronizes) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section A.4.2, “What to Do If MySQL Keeps Crashing”. This variable is set to if you start mysqld with the option.

  • If this is set to a non-zero value, all tables are closed every seconds to free up resources and synchronize unflushed data to disk. We recommend that this option be used only on Windows 9x or Me, or on systems with minimal resources.

  • The list of operators supported by boolean full-text searches performed using . See Section 12.7.1, “Boolean Full-Text Searches”.

    The default variable value is . The rules for changing the value are as follows:

    • Operator function is determined by position within the string.

    • The replacement value must be 14 characters.

    • Each character must be an ASCII non-alphanumeric character.

    • Either the first or second character must be a space.

    • No duplicates are allowed except the phrase quoting operators in positions 11 and 12. These two characters are not required to be the same, but they are the only two that may be.

    • Positions 10, 13, and 14 (which by default are set to ‘’, ‘’, and ‘’) are reserved for future extensions.

  • The maximum length of the word to be included in a index.

    Note: indexes must be rebuilt after changing this variable. Use QUICK.

  • The minimum length of the word to be included in a index.

    Note: indexes must be rebuilt after changing this variable. Use QUICK.

  • The number of top matches to use for full-text searches performed using .

  • The file from which to read the list of stopwords for full-text searches. All the words from the file are used; comments are not honored. By default, a built-in list of stopwords is used (as defined in the file). Setting this variable to the empty string () disables stopword filtering.

    Note: indexes must be rebuilt after changing this variable or the contents of the stopword file. Use QUICK.

  • The maximum allowed result length for the function. The default is 1024.

  • if mysqld supports tables, if not.

  • if mysqld supports tables. if is used.

  • if mysqld supports tables, if not.

  • if the compression library is available to the server, if not. If not, the and functions cannot be used.

  • if the system call is available to the server, if not. If not, the function cannot be used.

  • if mysqld supports tables, if not.

  • if mysqld supports tables, if not.

    if mysqld supports tables, if not. This variable was added in MySQL 5.0.3.

  • if the server supports spatial data types, if not.

  • if mysqld supports tables. if is used.

  • In MySQL 5.0, this variable appears only for reasons of backward compatibility. It is always because tables are no longer supported.

  • if mysqld supports tables. if is used.

  • if mysqld supports SSL connections, if not.

  • if mysqld supports the query cache, if not.

  • In MySQL 5.0, this variable appears only for reasons of backward compatibility. It is always because tables are no longer supported.

  • if indexes are available, if not. (These are used for spatial indexes in tables.)

  • if symbolic link support is enabled, if not. This is required on Unix for support of the and table options, and on Windows for support of data directory symlinks.

  • A string to be executed by the server for each client that connects. The string consists of one or more SQL statements. To specify multiple statements, separate them by semicolon characters. For example, each client begins by default with autocommit mode enabled. There is no global system variable to specify that autocommit should be disabled by default, but can be used to achieve the same effect:

    SET GLOBAL init_connect='SET AUTOCOMMIT=0';
    

    This variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:

    [mysqld]
    init_connect='SET AUTOCOMMIT=0'
    

    Note that the content of is not executed for users that have the privilege. This is done so that an erroneous value for does not prevent all clients from connecting. For example, the value might contain a statement that has a syntax error, thus causing client connections to fail. Not executing for users that have the privilege enables them to open a connection and fix the value.

  • The name of the file specified with the option when you start the server. This should be a file containing SQL statements that you want the server to execute when it starts. Each statement must be on a single line and should not include comments.

  • This variable is similar to , but is a string to be executed by a slave server each time the SQL thread starts. The format of the string is the same as for the variable.

  • system variables are listed in Section 14.2.4, “ Startup Options and System Variables”.

  • 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 option to . See also .

  • The size of the buffer that is used for joins that do not use indexes and thus perform full table scans. Normally, the best way to get fast joins is to add indexes. Increase the value of to get a faster full join when adding indexes is not possible. One join buffer is allocated for each full join between two tables. For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary.

  • Index blocks for tables are buffered and are shared by all threads. is the size of the buffer used for index blocks. The key buffer is also known as the key cache.

    The maximum allowable setting for is 4GB. The effective maximum size might be less, depending on your available physical RAM and per-process RAM limits imposed by your operating system or hardware platform.

    Increase the value to get better index handling (for all reads and multiple writes) to as much as you can afford. Using a value that is 25% of total memory on a machine that mainly runs MySQL is quite common. However, if you make the value too large (for example, more than 50% of your total memory) your system might start to page and become extremely slow. MySQL relies on the operating system to perform filesystem caching for data reads, so you must leave some room for the filesystem cache. Consider also the memory requirements of other storage engines.

    For even more speed when writing many rows at the same time, use . See Section 7.2.16, “Speed of Statements”.

    You can check the performance of the key buffer by issuing a statement and examining the , , , and status variables. (See Section 13.5.4, “ Syntax”.) The ratio should normally be less than 0.01. The ratio is usually near 1 if you are using mostly updates and deletes, but might be much smaller if you tend to do updates that affect many rows at the same time or if you are using the table option.

    The fraction of the key buffer in use can be determined using in conjunction with the status variable and the buffer block size, which is available from the system variable:

    1 - ((Key_blocks_unused × key_cache_block_size) / key_buffer_size)
    

    This value is an approximation because some space in the key buffer may be allocated internally for administrative structures.

    It is possible to create multiple key caches. The size limit of 4GB applies to each cache individually, not as a group. See Section 7.4.6, “The Key Cache”.

  • This value controls the demotion of buffers from the hot sub-chain of a key cache to the warm sub-chain. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. See Section 7.4.6, “The Key Cache”.

  • The size in bytes of blocks in the key cache. The default value is 1024. See Section 7.4.6, “The Key Cache”.

  • The division point between the hot and warm sub-chains of the key cache buffer chain. The value is the percentage of the buffer chain to use for the warm sub-chain. Allowable values range from 1 to 100. The default value is 100. See Section 7.4.6, “The Key Cache”.

  • The language used for error messages.

  • Whether mysqld was compiled with options for large file support.

  • Whether large page support is enabled. This variable was added in MySQL 5.0.3.

  • The type of license the server has.

  • Whether is supported for statements. See Section 5.7.4, “Security Issues with .

  • Whether mysqld was locked in memory with .

  • Whether logging of all statements to the general query log is enabled. See Section 5.12.2, “The General Query Log”.

  • Whether the binary log is enabled. See Section 5.12.3, “The Binary Log”.

  • This variable applies when binary logging is enabled. It controls whether stored function creators can be trusted not to create stored functions that will cause unsafe events to be written to the binary log. If set to 0 (the default), users are not allowed to create or alter stored functions unless they have the privilege in addition to the or privilege. A setting of 0 also enforces the restriction that a function must be declared with the characteristic, or with the or characteristic. If the variable is set to 1, MySQL does not enforce these restrictions on stored function creation. See Section 17.4, “Binary Logging of Stored Routines and Triggers”.

    This variable was added in MySQL 5.0.16.

  • This is the old name for . Before MySQL 5.0.16, it also applies to stored procedures, not just stored functions. As of 5.0.16, this variable is deprecated. It is recognized for backward compatibility but its use results in a warning.

    This variable was added in MySQL 5.0.6.

  • The location of the error log.

  • Whether queries that do not use indexes are logged to the slow query log. See Section 5.12.4, “The Slow Query Log”. This variable was added in MySQL 5.0.23.

  • Whether updates received by a slave server from a master server should be logged to the slave's own binary log. Binary logging must be enabled on the slave for this variable to have any effect. See Section 6.8, “Replication Startup Options”.

  • Whether slow queries should be logged. “Slow” is determined by the value of the variable. See Section 5.12.4, “The Slow Query Log”.

  • Whether to produce additional warning messages. It is enabled (1) by default. Aborted connections are not logged to the error log unless the value is greater than 1.

  • If a query takes longer than this many seconds, the server increments the status variable. If you are using the option, the query is logged to the slow query log file. This value is measured in real time, not CPU time, so a query that is under the threshold on a lightly loaded system might be above the threshold on a heavily loaded one. The minimum value is 1. The default is 10. See Section 5.12.4, “The Slow Query Log”.

  • If set to , all , , , and statements wait until there is no pending or on the affected table. This variable previously was named .

  • This variable describes the case sensitivity of filenames on the filesystem where the data directory is located. means filenames are case sensitive, means they are not case sensitive.

  • If set to 1, table names are stored in lowercase on disk and table name comparisons are not case sensitive. If set to 2 table names are stored as given but compared in lowercase. This option also applies to database names and table aliases. See Section 9.2.2, “Identifier Case Sensitivity”.

    If you are using tables, you should set this variable to 1 on all platforms to force names to be converted to lowercase.

    You should not set this variable to 0 if you are running MySQL on a system that does not have case-sensitive filenames (such as Windows or Mac OS X). If this variable is not set at startup and the filesystem on which the data directory is located does not have case-sensitive filenames, MySQL automatically sets to 2.

  • The maximum size of one packet or any generated/intermediate string.

    The packet message buffer is initialized to bytes, but can grow up to bytes when needed. This value by default is small, to catch large (possibly incorrect) packets.

    You must increase this value if you are using large columns or long strings. It should be as big as the largest you want to use. The protocol limit for is 1GB.

  • If a multiple-statement transaction requires more than this amount of memory, the server generates a error.

  • If a write to the binary log causes the current log file size to exceed the value of this variable, the server rotates the binary logs (closes the current file and opens the next one). You cannot set this variable to more than 1GB or to less than 4096 bytes. The default value is 1GB.

    A transaction is written in one chunk to the binary log, so it is never split between several binary logs. Therefore, if you have big transactions, you might see binary logs larger than .

    If is 0, the value of applies to relay logs as well.

  • If there are more than this number of interrupted connections from a host, that host is blocked from further connections. You can unblock blocked hosts with the statement.

  • The number of simultaneous client connections allowed. Increasing this value increases the number of file descriptors that mysqld requires. See Section 7.4.8, “How MySQL Opens and Closes Tables”, for comments on file descriptor limits. See also Section A.2.6, “.

  • Do not start more than this number of threads to handle statements. If you try to insert data into a new table after all threads are in use, the row is inserted as if the attribute wasn't specified. If you set this to 0, MySQL never creates a thread to handle rows; in effect, this disables entirely.

  • The maximum number of error, warning, and note messages to be stored for display by the and statements.

  • This variable sets the maximum size to which tables are allowed to grow. The value of the variable is used to calculate table values. Setting this variable has no effect on any existing table, unless the table is re-created with a statement such as or altered with or .

  • This variable is a synonym for .

  • Do not allow statements that probably need to examine more than rows (for single-table statements) or row combinations (for multiple-table statements) or that are likely to do more than disk seeks. By setting this value, you can catch statements where keys are not used properly and that would probably take a long time. Set it if your users tend to perform joins that lack a clause, that take a long time, or that return millions of rows.

    Setting this variable to a value other than resets the value of to . If you set the value again, the variable is ignored.

    If a query result is in the query cache, no result size check is performed, because the result has previously been computed and it does not burden the server to send it to the client.

    This variable previously was named .

  • The cutoff on the size of index values that determines which algorithm to use. See Section 7.2.12, “ Optimization”.

  • This variable limits the total number of prepared statements in the server. It can be used in environments where there is the potential for denial-of-service attacks based on running the server out of memory by preparing huge numbers of statements. The default value is 16,382. The allowable range of values is from 0 to 1 milliion. If the value is set lower than the current number of prepared statements, existing statements are not affected and can be used, but no new statements can be prepared until the current number drops below the limit. This variable was added in MySQL 5.0.21.

  • If a write by a replication slave to its relay log causes the current log file size to exceed the value of this variable, the slave rotates the relay logs (closes the current file and opens the next one). If is 0, the server uses for both the binary log and the relay log. If is greater than 0, it constrains the size of the relay log, which enables you to have different sizes for the two logs. You must set to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. See Section 6.3, “Replication Implementation Details”.

  • Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer assumes that no more than this number of key seeks are required when searching for matching rows in a table by scanning an index, regardless of the actual cardinality of the index (see Section 13.5.4.13, “ Syntax”). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.

  • The number of bytes to use when sorting or values. Only the first bytes of each value are used; the rest are ignored.

  • The number of times that a stored procedure may call itself. The default value for this option is 0, which completely disallows recursion in stored procedures. The maximum value is 255.

    This variable can be set globally and per session.

  • The maximum number of temporary tables a client can keep open at the same time. (This option does not yet do anything.)

  • The maximum number of simultaneous connections allowed to any given MySQL account. A value of 0 means “no limit.

    Before MySQL 5.0.3, this variable has only global scope. Beginning with MySQL 5.0.3, it also has a read-only session scope. The session variable has the same value as the global variable unless the current account has a non-zero resource limit. In that case, the session value reflects the account limit.

  • After this many write locks, allow some pending read lock requests to be processed in between.

  • The default pointer size in bytes, to be used by for tables when no option is specified. This variable cannot be less than 2 or larger than 7. The default value is 6 (4 before MySQL 5.0.6). This variable was added in MySQL 4.1.2. See Section A.2.11, “.

  • (DEPRECATED)

    If the temporary file used for fast index creation would be larger than using the key cache by the amount specified here, 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. The value is given in bytes.

    Note: This variable was removed in MySQL 5.0.6.

  • The maximum size of the temporary file that MySQL is allowed to use while re-creating a index (during , , or ). If the file size would be larger than this value, the index is created using the key cache instead, which is slower. The value is given in bytes.

  • The value of the option. See Section 5.2.1, “mysqld Command Options”.

  • If this value is greater than 1, table indexes are created in parallel (each index in its own thread) during the process. The default value is 1.

    Note: Multi-threaded repair is still beta-quality code.

  • The size of the buffer that is allocated when sorting indexes during a or when creating indexes with or .

  • How the server treats values when collecting statistics about the distribution of index values for tables. This variable has two possible values, and . For , all index values are considered equal and form a single value group that has a size equal to the number of values. For , values are considered unequal, and each forms a distinct value group of size 1.

    The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 7.4.7, “ Index Statistics Collection”.

    This variable was added in MySQL 5.0.14. For older versions, the statistics collection method is equivalent to .

  • Specifies the maximum number of ranges to send to a storage engine during range selects. The default value is 256. Sending multiple ranges to an engine is a feature that can improve the performance of certain selects dramatically, particularly for . This engine needs to send the range requests to all nodes, and sending many of those requests at once reduces the communication costs significantly. This variable was added in MySQL 5.0.3.

  • (Windows only.) Indicates whether the server supports connections over named pipes.

  • Determines the probability of gaps in an autoincremented column. Set to to minimize this. Set to a high value for optimization — makes inserts faster, but decreases the likelihood that consecutive autoincrement numbers will be used in a batch of inserts. Default value: . Mimimum value: .

  • The number of milliseconds to wait before checking the query cache. Setting this to (the default and minimum value) means that the query cache will be checked for validation on every query.

    The recommended maximum value for this variable is , which means that the query cache is checked once per second. A larger value means the query cache is less often checked and invalidated due to updates on a different mysqld. It is generally not desirable to set this to a value greater than .

  • Forces sending of buffers to immediately, without waiting for other threads. Defaults to .

  • Sets the granularity of the statistics by determining the number of starting and ending keys to store in the statistics memory cache. Zero means no caching takes place; in this case, the data nodes are always queries directly. Default value: .

  • Use index statistics in query optimization. Defaults to .

  • How often to query data nodes instead of the statistics cache. For example, a value of (the default) means to direct every 20th query to the data nodes.

  • This is a threshold on the number of epochs to be behind before reporting binlog status. For example, a value of (the default) means that if the difference between which epoch has been received from the storage nodes and which epoch has been applied to the binlog is 3 or more, a status message will be sent to the cluster log.

  • This is a threshold on the percentage of free memory remaining before reporting binlog status. For example, a value of (the default) means that if the amount of available memory for receiving binlog data from the data nodes falls below 10%, a status message will be sent to the cluster log.

  • Forces to use an count of records during query planning to speed up this type of query. The default value is . For faster queries overall, disable this feature by setting the value of to .

  • You can disable transaction support by setting this variable's values to (not recommended). The default is .

  • The communication buffer is reset to this size between SQL statements. This variable should not normally be changed, but if you have very little memory, you can set it to the expected length of statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to bytes.

  • The number of seconds to wait for more data from a connection before aborting the read. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory. When the server is reading from the client, is the timeout value controlling when to abort. When the server is writing to the client, is the timeout value controlling when to abort. See also .

  • If a read on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads.

  • The number of seconds to wait for a block to be written to a connection before aborting the write. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory. See also .

  • This variable was used in MySQL 4.0 to turn on some 4.1 behaviors, and is retained for backward compatibility. In MySQL 5.0, its value is always .

  • Whether the server should use pre-4.1-style passwords for MySQL user accounts. See Section A.2.3, “.

  • This is not a variable, but it can be used when setting some variables. It is described in Section 13.5.3, “ Syntax”.

  • The number of files that the operating system allows mysqld to open. This is the real value allowed by the system and might be different from the value you gave using the option to mysqld or mysqld_safe. The value is 0 on systems where MySQL can't change the number of open files.

  • Controls the heuristics applied during query optimization to prune less-promising partial plans from the optimizer search space. A value of 0 disables heuristics so that the optimizer performs an exhaustive search. A value of 1 causes the optimizer to prune plans based on the number of rows retrieved by intermediate plans. This variable was added in MySQL 5.0.1.

  • The maximum depth of search performed by the query optimizer. Values larger than the number of relations in a query result in better query plans, but take longer to generate an execution plan for a query. Values smaller than the number of relations in a query return an execution plan quicker, but the resulting plan may be far from being optimal. If set to 0, the system automatically picks a reasonable value. If set to the maximum number of tables used in a query plus 2, the optimizer switches to the algorithm used in MySQL 5.0.0 (and previous versions) for performing searches. This variable was added in MySQL 5.0.1.

  • The pathname of the process ID (PID) file. This variable can be set with the option.

  • The number of the port on which the server listens for TCP/IP connections. This variable can be set with the option.

  • The size of the buffer that is allocated when preloading indexes.

  • The current number of prepared statements. (The maximum number of statements is given by the system variable.) This variable was added in MySQL 5.0.21.

  • The version of the client/server protocol used by the MySQL server.

  • The allocation size of memory blocks that are allocated for objects created during statement parsing and execution. If you have problems with memory fragmentation, it might help to increase this a bit.

  • Don't cache results that are larger than this number of bytes. The default value is 1MB.

  • The minimum size (in bytes) for blocks allocated by the query cache. The default value is 4096 (4KB). Tuning information for this variable is given in Section 5.14.3, “Query Cache Configuration”.

  • The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. The allowable values are multiples of 1024; other values are rounded down to the nearest multiple. Note that bytes of memory are allocated even if is set to 0. See Section 5.14.3, “Query Cache Configuration”, for more information.

  • Set the query cache type. Setting the value sets the type for all clients that connect thereafter. Individual clients can set the value to affect their own use of the query cache. Possible values are shown in the following table:

    Option Description
    or Don't cache results in or retrieve results from the query cache. Note that this does not deallocate the query cache buffer. To do that, you should set to 0.
    or Cache all query results except for those that begin with .
    or Cache results only for queries that begin with .

    This variable defaults to .

  • Normally, when one client acquires a lock on a table, other clients are not blocked from issuing statements that read from the table if the query results are present in the query cache. Setting this variable to 1 causes acquisition of a lock for a table to invalidate any queries in the query cache that refer to the table. This forces other clients that attempt to access the table to wait while the lock is in effect.

  • The size of the persistent buffer used for statement parsing and execution. This buffer is not freed between statements. If you are running complex queries, a larger value might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations.

  • The size of blocks that are allocated when doing range optimization.

  • Each thread that does a sequential scan allocates a buffer of this size (in bytes) for each table it scans. If you do many sequential scans, you might want to increase this value, which defaults to 131072.

  • When the variable is set to for a replication slave server, it causes the slave to allow no updates except from slave threads or from users that have the privilege. This can be useful to ensure that a slave server accepts updates only from its master server and not from clients. As of MySQL 5.0.16, this variable does not apply to tables.

  • Disables or enables automatic purging of relay log files as soon as they are not needed any more. The default value is 1 ().

  • When reading rows in sorted order following a key-sorting operation, the rows are read through this buffer to avoid disk seeks. Setting the variable to a large value can improve performance by a lot. However, this is a buffer allocated for each client, so you should not set the global variable to a large value. Instead, change the session variable only from within those clients that need to run large queries.

  • This variable is unused.

  • If the MySQL server has been started with the option, it blocks connections from all accounts that have passwords stored in the old (pre-4.1) format. In that case, the value of this variable is , otherwise it is .

    You should enable this option if you want to prevent all use of passwords employing the old format (and hence insecure communication over the network).

    Server startup fails with an error if this option is enabled and the privilege tables are in pre-4.1 format. See Section A.2.3, “.

  • The server ID. This value is set by the option. It is used for replication to enable master and slave servers to identify themselves uniquely.

  • (Windows only.) Whether the server allows shared-memory connections.

  • (Windows only.) The name of shared memory to use for shared-memory connections. This is useful when running multiple MySQL instances on a single physical machine. The default name is . The name is case sensitive.

  • This is if mysqld uses external locking, if external locking is disabled.

  • This is if the server allows only local (non-TCP/IP) connections. On Unix, local connections use a Unix socket file. On Windows, local connections use a named pipe or shared memory. On NetWare, only TCP/IP connections are supported, so do not set this variable to . This variable can be set to with the option.

  • This prevents people from using the statement if they do not have the privilege. This can improve security if you have concerns about users being able to see databases belonging to other users. Its effect depends on the privilege: If the variable value is , the statement is allowed only to users who have the privilege, and the statement displays all database names. If the value is , is allowed to all users, but displays the names of only those databases for which the user has the or other privilege.

  • Whether to use compression of the slave/master protocol if both the slave and the master support it.

  • The name of the directory where the slave creates temporary files for replicating statements.

  • The number of seconds to wait for more data from a master/slave connection before aborting the read. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory.

  • The replication errors that the slave should skip (ignore).

  • If a replication slave SQL thread fails to execute a transaction because of an deadlock or exceeded 's or NDBCluster's or , it automatically retries times before stopping with an error. The default priot to MySQL 4.0.3 is 0. You must explicitly set the value greater than 0 to enable the “retry” behavior, which is probably a good idea. In MySQL 5.0.3 or newer, the default is 10.

  • If creating a thread takes longer than this many seconds, the server increments the status variable.

  • On Unix platforms, this variable is the name of the socket file that is used for local client connections. The default is . (For some distribution formats, the directory might be different, such as for RPMs.)

    On Windows, this variable is the name of the named pipe that is used for local client connections. The default value is (not case sensitive).

  • Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster or operations. See Section A.4.4, “Where MySQL Stores Temporary Files”.

  • The current server SQL mode, which can be set dynamically. See Section 5.2.5, “The Server SQL Mode”.

  • The number of events from the master that a slave server should skip. See Section 13.6.2.6, “ Syntax”.

  • The path to a file with a list of trusted SSL CAs. This variable was added in MySQL 5.0.23.

  • The path to a directory that contains trusted SSL CA certificates in PEM format. This variable was added in MySQL 5.0.23.

  • The name of the SSL certificate file to use for establishing a secure connection. This variable was added in MySQL 5.0.23.

  • A list of allowable ciphers to use for SSL encryption. The cipher list has the same format as the command. This variable was added in MySQL 5.0.23.

  • The name of the SSL key file to use for establishing a secure connection. This variable was added in MySQL 5.0.23.

  • The default storage engine (table type). To set the storage engine at server startup, use the option. See Section 5.2.1, “mysqld Command Options”.

  • If the value of this variable is positive, the MySQL server synchronizes its binary log to disk (using ) after every writes to the binary log. Note that there is one write to the binary log per statement if autocommit is enabled, and one write per transaction otherwise. The default value is 0, which does no synchronizing to disk. A value of 1 is the safest choice, because in the event of a crash you lose at most one statement or transaction from the binary log. However, it is also the slowest choice (unless the disk has a battery-backed cache, which makes synchronization very fast).

    If the value of is 0 (the default), no extra flushing is done. The server relies on the operating system to flush the file contents occasionaly as for any other file.

  • If this variable is set to 1, when any non-temporary table is created its file is synchronized to disk (using ). This is slower but safer in case of a crash. The default is 1.

  • The server system time zone. When the server begins executing, it inherits a time zone setting from the machine defaults, possibly modified by the environment of the account used for running the server or the startup script. The value is used to set . Typically the time zone is specified by the environment variable. It also can be specified using the option of the mysqld_safe script.

    The variable differs from . Although they might have the same value, the latter variable is used to initialize the time zone for each client that connects. See Section 5.11.8, “MySQL Server Time Zone Support”.

  • The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. You can check whether you need to increase the table cache by checking the status variable. See Section 5.2.4, “Server Status Variables”. If the value of is large and you don't do often (which just forces all tables to be closed and reopened), then you should increase the value of the variable. For more information about the table cache, see Section 7.4.8, “How MySQL Opens and Closes Tables”.

  • Specifies a wait timeout for table-level locks, in seconds. The default timeout is 50 seconds. The timeout is active only if the connection has open cursors. This variable can also be set globally at runtime (you need the privilege to do this). It's available as of MySQL 5.0.10.

  • This variable is a synonym for . In MySQL 5.0, is the preferred name.

  • How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there are fewer than threads there. Requests for threads are satisfied by reusing threads taken from the cache if possible, 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 provide a notable performance improvement if you have a good thread implementation.) By examining the difference between the and status variables, you can see how efficient the thread cache is. For details, see Section 5.2.4, “Server Status Variables”.

  • On Solaris, mysqld calls with this value. This function enables applications to give the threads system a hint about the desired number of threads that should be run at the same time.

  • The stack size for each thread. Many of the limits detected by the test are dependent on this value. The default is large enough for normal operation. See Section 7.1.4, “The MySQL Benchmark Suite”. The default is 192KB.

  • This variable is not implemented.

  • The current time zone. This variable is used to initialize the tome zone for each client that connects. By default, the initial value of this is (which means, “use the value of ”). The value can be specified explicitly at server startup with the option. See Section 5.11.8, “MySQL Server Time Zone Support”.

  • The maximum size of in-memory temporary tables. (The actual limit is determined as the smaller of and .) If an in-memory temporary table exceeds the limit, MySQL automatically converts it to an on-disk table. Increase the value of (and if necessary) if you do many advanced queries and you have lots of memory.

  • The directory used for temporary files and temporary tables. This variable can be set to a list of several paths that are used in round-robin fashion. Paths should be separated by colon characters (‘’) on Unix and semicolon characters (‘’) on Windows, NetWare, and OS/2.

    The multiple-directory feature can be used to spread the load between several physical disks. If the MySQL server is acting as a replication slave, you should not set to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or operations. If files in the temporary file directory are lost when the server restarts, replication fails. However, if you are using MySQL 4.0.0 or later, you can set the slave's temporary directory using the variable. In that case, the slave won't use the general value and you can set to a non-permanent location.

  • The amount in bytes by which to increase a per-transaction memory pool which needs memory. See the description of .

  • There is a per-transaction memory pool from which various transaction-related allocations take memory. The initial size of the pool in bytes is . For every allocation that cannot be satisfied from the pool because it has insufficient memory available, the pool is increased by bytes. When the transaction ends, the pool is truncated to bytes.

    By making sufficiently large to contain all statements within a single transaction, you can avoid many calls.

  • The default transaction isolation level. Defaults to .

    This variable is set by the statement. See Section 13.4.6, “ Syntax”. If you set directly to an isolation level name that contains a space, the name should be enclosed within quotes, with the space replaced by a dash. For example:

    SET tx_isolation = 'READ-COMMITTED';
    
  • This variable controls whether updates to a view can be made when the view does not contain all columns of the primary key defined in the underlying table, if the update statement contains a clause. (Such updates often are generated by GUI tools.) An update is an or statement. Primary key here means a , or a index in which no column can contain .

    The variable can have two values:

    • or : Issue a warning only (not an error message). This is the default value.

    • or : Prohibit the update.

    This variable was added in MySQL 5.0.2.

  • The version number for the server.

  • The storage engine version.

  • The configure script has a option that allows a comment to be specified when building MySQL. This variable contains the value of that comment.

  • The type of machine or architecture on which MySQL was built.

  • The type of operating system on which MySQL was built.

  • The number of seconds the server waits for activity on a non-interactive connection before closing it. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory.

    On thread startup, the session value is initialized from the global value or from the global value, depending on the type of client (as defined by the connect option to ). See also .

5.2.3. Using System Variables

The mysql server maintains many system variables that indicate how it is configured. Section 5.2.2, “Server System Variables”, describes the meaning of these variables. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. Most of them can be changed dynamically while the server is running by means of the statement, which enables you to modify operation of the server without having to stop and restart it. You can refer to system variable values in expressions.

The server maintains two kinds of system variables. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections. A given system variable can have both a global and a session value. Global and session system variables are related as follows:

  • When the server starts, it initializes all global variables to their default values. These defaults can be changed by options specified on the command line or in an option file. (See Section 4.3, “Specifying Program Options”.)

  • The server also maintains a set of session variables for each client that connects. The client's session variables are initialized at connect time using the current values of the corresponding global variables. For example, the client's SQL mode is controlled by the session value, which is initialized when the client connects to the value of the global value.

System variable values can be set globally at server startup by using options on the command line or in an option file. When you use a startup option to set a variable that takes a numeric value, the value can be given with a suffix of , , or (either uppercase or lowercase) to indicate a multiplier of 1024, 10242 or 10243; that is, units of kilobytes, megabytes, or gigabygtes, respectively. Thus, the following command starts the server with a query cache size of 16 megabytes and a maximum packet size of one gigabyte:

mysqld --query_cache_size=16M --max_allowed_packet=1G

Within an option file, those variables are set like this:

[mysqld]
query_cache_size=16M
max_allowed_packet=1G

The lettercase of suffix letters does not matter; and are equivalent, as are and .

If you want to restrict the maximum value to which a system variable can be set at runtime with the statement, you can specify this maximum by using an option of the form = at server startup. For example, to prevent the value of from being increased to more than 32MB at runtime, use the option .

Many system variables are dynamic and can be changed while the server runs by using the statement. For a list, see Section 5.2.3.2, “Dynamic System Variables”. To change a system variable with , refer to it as , optionally preceded by a modifier:

  • To indicate explicitly that a variable is a global variable, precede its name by or . The privilege is required to set global variables.

  • To indicate explicitly that a variable is a session variable, precede its name by , , or . Setting a session variable requires no special privilege, but a client can change only its own session variables, not those of any other client.

  • and are synonyms for and .

  • If no modifier is present, changes the session variable.

A statement can contain multiple variable assignments, separated by commas. If you set several system variables, the most recent or modifier in the statement is used for following variables that have no modifier specified.

Examples:

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;

When you assign a value to a system variable with , you cannot use suffix letters in the value (as can be done with startup options). However, the value can take the form of an expression:

SET sort_buffer_size = 10 * 1024 * 1024;

The syntax for system variables is supported for compatibility with some other database systems.

If you change a session system variable, the value remains in effect until your session ends or until you change the variable to a different value. The change is not visible to other clients.

If you change a global system variable, the value is remembered and used for new connections until the server restarts. (To make a global system variable setting permanent, you should set it in an option file.) The change is visible to any client that accesses that global variable. However, the change affects the corresponding session variable only for clients that connect after the change. The global variable change does not affect the session variable for any client that is currently connected (not even that of the client that issues the statement).

To prevent incorrect usage, MySQL produces an error if you use with a variable that can only be used with or if you do not specify (or ) when setting a global variable.

To set a variable to the value or a value to the compiled-in MySQL default value, use the keyword. For example, the following two statements are identical in setting the session value of to the global value:

SET max_join_size=DEFAULT;
SET @@[email protected]@global.max_join_size;

Not all system variables can be set to . In such cases, use of results in an error.

You can refer to the values of specific global or sesson system variables in expressions by using one of the -modifiers. For example, you can retrieve values in a statement like this:

SELECT @@global.sql_mode, @@session.sql_mode, @@sql_mode;

When you refer to a system variable in an expression as (that is, when you do not specify or ), MySQL returns the session value if it exists and the global value otherwise. (This differs from = , which always refers to the session value.)

Note: Some system variables can be enabled with the statement by setting them to or , or disabled by setting them to or . However, to set such a variable on the command line or in an option file, you must set it to or ; setting it to or will not work. For example, on the command line, works but does not.

To display system variable names and values, use the statement.

mysql> 
+--------+--------------------------------------------------------------+
| Variable_name                   | Value                               |
+--------+--------------------------------------------------------------+
| auto_increment_increment        | 1                                   |
| auto_increment_offset           | 1                                   |
| automatic_sp_privileges         | ON                                  |
| back_log                        | 50                                  |
| basedir                         | /                                   |
| bdb_cache_size                  | 8388600                             |
| bdb_home                        | /var/lib/mysql/                     |
| bdb_log_buffer_size             | 32768                               |
| bdb_logdir                      |                                     |
| bdb_max_lock                    | 10000                               |
| bdb_shared_data                 | OFF                                 |
| bdb_tmpdir                      | /tmp/                               |
| binlog_cache_size               | 32768                               |
| bulk_insert_buffer_size         | 8388608                             |
| character_set_client            | latin1                              |
| character_set_connection        | latin1                              |
| character_set_database          | latin1                              |
| character_set_results           | latin1                              |
| character_set_server            | latin1                              |
| character_set_system            | utf8                                |
| character_sets_dir              | /usr/share/mysql/charsets/          |
| collation_connection            | latin1_swedish_ci                   |
| collation_database              | latin1_swedish_ci                   |
| collation_server                | latin1_swedish_ci                   |
...
| innodb_additional_mem_pool_size | 1048576                             |
| innodb_autoextend_increment     | 8                                   |
| innodb_buffer_pool_awe_mem_mb   | 0                                   |
| innodb_buffer_pool_size         | 8388608                             |
| innodb_checksums                | ON                                  |
| innodb_commit_concurrency       | 0                                   |
| innodb_concurrency_tickets      | 500                                 |
| innodb_data_file_path           | ibdata1:10M:autoextend              |
| innodb_data_home_dir            |                                     |
...
| version                         | 5.0.19-Max                          |
| version_comment                 | MySQL Community Edition - Max (GPL) |
| version_compile_machine         | i686                                |
| version_compile_os              | pc-linux-gnu                        |
| wait_timeout                    | 28800                               |
+--------+--------------------------------------------------------------+

With a clause, the statement displays only those variables that match the pattern. To obtain a specific variable name, use a clause as shown:

SHOW VARIABLES LIKE 'max_join_size';
SHOW SESSION VARIABLES LIKE 'max_join_size';

To get a list of variables whose name match a pattern, use the ‘’ wildcard character in a clause:

SHOW VARIABLES LIKE '%size%';
SHOW GLOBAL VARIABLES LIKE '%size%';

Wildcard characters can be used in any position within the pattern to be matched. Strictly speaking, because ‘’ is a wildcard that matches any single character, you should escape it as ‘’ to match it literally. In practice, this is rarely necessary.

For , if you specify neither nor , MySQL returns values.

The reason for requiring the keyword when setting -only variables but not when retrieving them is to prevent problems in the future. If we were to remove a variable that has the same name as a variable, a client with the privilege might accidentally change the variable rather than just the variable for its own connection. If we add a variable with the same name as a variable, a client that intends to change the variable might find only its own variable changed.

5.2.3.1. Structured System Variables

A structured variable differs from a regular system variable in two respects:

  • Its value is a structure with components that specify server parameters considered to be closely related.

  • There might be several instances of a given type of structured variable. Each one has a different name and refers to a different resource maintained by the server.

MySQL 5.0 supports one structured variable type, which specifies parameters governing the operation of key caches. A key cache structured variable has these components:

This section describes the syntax for referring to structured variables. Key cache variables are used for syntax examples, but specific details about how key caches operate are found elsewhere, in Section 7.4.6, “The Key Cache”.

To refer to a component of a structured variable instance, you can use a compound name in format. Examples:

hot_cache.key_buffer_size
hot_cache.key_cache_block_size
cold_cache.key_cache_block_size

For each structured system variable, an instance with the name of is always predefined. If you refer to a component of a structured variable without any instance name, the instance is used. Thus, and both refer to the same system variable.

Structured variable instances and components follow these naming rules:

  • For a given type of structured variable, each instance must have a name that is unique within variables of that type. However, instance names need not be unique across structured variable types. For example, each structured variable has an instance named , so is not unique across variable types.

  • The names of the components of each structured variable type must be unique across all system variable names. If this were not true (that is, if two different types of structured variables could share component member names), it would not be clear which default structured variable to use for references to member names that are not qualified by an instance name.

  • If a structured variable instance name is not legal as an unquoted identifier, refer to it as a quoted identifier using backticks. For example, is not legal, but is.

  • , , and are not legal instance names. This avoids a conflict with notation such as for referring to non-structured system variables.

Currently, the first two rules have no possibility of being violated because the only structured variable type is the one for key caches. These rules will assume greater significance if some other type of structured variable is created in the future.

With one exception, you can refer to structured variable components using compound names in any context where simple variable names can occur. For example, you can assign a value to a structured variable using a command-line option:

shell> 

In an option file, use this syntax:

[mysqld]
hot_cache.key_buffer_size=64K

If you start the server with this option, it creates a key cache named with a size of 64KB in addition to the default key cache that has a default size of 8MB.

Suppose that you start the server as follows:

shell> 
         
         

In this case, the server sets the size of the default key cache to 256KB. (You could also have written .) In addition, the server creates a second key cache named that has a size of 128KB, with the size of block buffers for caching table index blocks set to 2048 bytes.

The following example starts the server with three different key caches having sizes in a 3:1:1 ratio:

shell> 
         
         

Structured variable values may be set and retrieved at runtime as well. For example, to set a key cache named to a size of 10MB, use either of these statements:

mysql> 
mysql> 

To retrieve the cache size, do this:

mysql> 

However, the following statement does not work. The variable is not interpreted as a compound name, but as a simple string for a pattern-matching operation:

mysql> 

This is the exception to being able to use structured variable names anywhere a simple variable name may occur.

5.2.3.2. Dynamic System Variables

Many server system variables are dynamic and can be set at runtime using or . You can also obtain their values using . See Section 5.2.3, “Using System Variables”.

The following table shows the full list of all dynamic system variables. The last column indicates for each variable whether or (or both) apply. The table also lists session options that can be set with the statement. Section 13.5.3, “ Syntax”, discusses these options.

Variables that have a type of “string” take a string value. Variables that have a type of “numeric” take a numeric value. Variables that have a type of “boolean” can be set to 0, 1, or . (If you set them on the command line or in an option file, use the numeric values.) Variables that are marked as “enumeration” normally should be set to one of the available values for the variable, but can also be set to the number that corresponds to the desired enumeration value. For enumerated system variables, the first enumeration value corresponds to 0. This differs from columns, for which the first enumeration value corresponds to 1.

Variable Name Value Type Type
boolean
boolean
numeric
numeric |
string |
string |
string |
string |
string |
string |
string |
numeric |
numeric
numeric
numeric |
| |
numeric
numeric
numeric
numeric |
boolean |
numeric
numeric
boolean
numeric
boolean
numeric
numeric |
numeric
numeric
numeric
numeric
numeric
numeric
boolean |
numeric
boolean |
numeric
numeric
boolean
numeric |
numeric |
numeric
numeric
boolean
boolean
numeric
numeric |
boolean |
numeric |
numeric
numeric
numeric
numeric
numeric
numeric |
numeric |
numeric
numeric |
numeric
numeric
numeric |
numeric |
numeric |
numeric
numeric
enum |
numeric |
numeric
boolean
numeric |
numeric |
numeric |
numeric |
numeric |
numeric |
numeric |
numeric |
numeric |
numeric |
numeric |
numeric |
numeric
numeric
enumeration |
boolean |
numeric |
numeric |
numeric |
numeric
numeric |
numeric
boolean
boolean
numeric
boolean
numeric
numeric
numeric
numeric |
boolean
boolean
boolean
boolean
boolean
boolean
boolean
boolean |
numeric |
enumeration |
boolean
boolean
boolean
numeric
numeric
enumeration |
boolean
numeric
boolean
enumeration |
numeric
enumeration |
numeric
string |
boolean
enumeration |
numeric |
numeric |
enumeration |
boolean
numeric |
numeric

5.2.4. Server Status Variables

The server maintains many status variables that provide information about its operation. You can view these variables and their values by using the statement:

mysql> 
+-----------------------------------+------------+
| Variable_name                     | Value      |
+-----------------------------------+------------+
| Aborted_clients                   | 0          |
| Aborted_connects                  | 0          |
| Bytes_received                    | 155372598  |
| Bytes_sent                        | 1176560426 |
...
| Connections                       | 30023      |
| Created_tmp_disk_tables           | 0          |
| Created_tmp_files                 | 3          |
| Created_tmp_tables                | 2          |
...
| Threads_created                   | 217        |
| Threads_running                   | 88         |
| Uptime                            | 1389872    |
+-----------------------------------+------------+

Many status variables are reset to 0 by the statement.

The status variables have the following meanings. Variables with no version indicated were already present prior to MySQL 5.0. For information regarding their implementation history, see MySQL 3.23, 4.0, 4.1 Reference Manual.

  • The number of connections that were aborted because the client died without closing the connection properly. See Section A.2.10, “Communication Errors and Aborted Connections”.

  • The number of failed attempts to connect to the MySQL server. See Section A.2.10, “Communication Errors and Aborted Connections”.

  • The number of transactions that used the temporary binary log cache but that exceeded the value of and used a temporary file to store statements from the transaction.

  • The number of transactions that used the temporary binary log cache.

  • The number of bytes received from all clients.

  • The number of bytes sent to all clients.

  • The statement counter variables indicate the number of times each statement has been executed. There is one status variable for each type of statement. For example, and count and statements, respectively.

    All of the variables are increased even if a prepared statement argument is unknown or an error occurred during execution. In other words, their values correspond to the number of requests issued, not to the number of requests successfully completed.

    The status variables were added in 5.0.8:

    Those variables stand for prepared statement commands. Their names refer to the command set used in the network layer. In other words, their values increase whenever prepared statement API calls such as mysql_stmt_prepare(), mysql_stmt_execute(), and so forth are executed. However, , and also increase for , , or , respectively. Additionally, the values of the older (available since MySQL 4.1.3) statement counter variables , , and increase for the , , and statements. stands for the total number of network round-trips issued when fetching from cursors.

  • Whether the client connection uses compression in the client/server protocol. Added in MySQL 5.0.16.

  • The number of connection attempts (successful or not) to the MySQL server.

  • The number of temporary tables on disk created automatically by the server while executing statements.

  • How many temporary files mysqld has created.

  • The number of in-memory temporary tables created automatically by the server while executing statements. If is large, you may want to increase the value to cause temporary tables to be memory-based instead of disk-based.

  • The number of rows written with for which some error occurred (probably ).

  • The number of handler threads in use.

  • The number of rows written.

  • The number of executed statements.

  • The number of internal statements.

  • The number of times that rows have been deleted from tables.

  • The MySQL server can ask the storage engine if it knows about a table with a given name. This is called discovery. indicates the number of times that tables have been discovered via this mechanism.

  • A counter for the prepare phase of two-phase commit operations. Added in MySQL 5.0.3.

  • The number of times the first entry was read from an index. If this value is high, it suggests that the server is doing a lot of full index scans; for example, , assuming that is indexed.

  • The number of requests to read a row based on a key. If this value is high, it is a good indication that your tables are properly indexed for your queries.

  • The number of requests to read the next row in key order. This value is incremented if you are querying an index column with a range constraint or if you are doing an index scan.

  • The number of requests to read the previous row in key order. This read method is mainly used to optimize .

  • The number of requests to read a row based on a fixed position. This value is high if you are doing a lot of queries that require sorting of the result. You probably have a lot of queries that require MySQL to scan entire tables or you have joins that don't use keys properly.

  • The number of requests to read the next row in the data file. This value is 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.

  • The number of requests for a storage engine to perform a rollback operation.

  • The number of requests for a storage engine to place a savepoint. Added in MySQL 5.0.3.

  • The number of requests for a storage engine to roll back to a savepoint. Added in MySQL 5.0.3.

  • The number of requests to update a row in a table.

  • The number of requests to insert a row in a table.

  • The number of pages containing data (dirty or clean). Added in MySQL 5.0.2.

  • The number of pages currently dirty. Added in MySQL 5.0.2.

  • The number of buffer pool page-flush requests. Added in MySQL 5.0.2.

  • The number of free pages. Added in MySQL 5.0.2.

  • The number of latched pages in buffer pool. These are pages currently being read or written or that cannot be flushed or removed for some other reason. Added in MySQL 5.0.2.

  • The number of pages that are busy because they have been allocated for administrative overhead such as row locks or the adaptive hash index. This value can also be calculated as . Added in MySQL 5.0.2.

  • The total size of buffer pool, in pages. Added in MySQL 5.0.2.

  • The number of “random” read-aheads initiated by . This happens when a query scans a large portion of a table but in random order. Added in MySQL 5.0.2.

  • The number of sequential read-aheads initiated by . This happens when does a sequential full table scan. Added in MySQL 5.0.2.

  • The number of logical read requests has done. Added in MySQL 5.0.2.

  • The number of logical reads that could not satisfy from the buffer pool and had to do a single-page read. Added in MySQL 5.0.2.

  • Normally, writes to the buffer pool happen in the background. However, if it is necessary to read or create a page and no clean pages are available, it is also necessary to wait for pages to be flushed first. This counter counts instances of these waits. If the buffer pool size has been set properly, this value should be small. Added in MySQL 5.0.2.

  • The number writes done to the buffer pool. Added in MySQL 5.0.2.

  • The number of operations so far. Added in MySQL 5.0.2.

  • The current number of pending operations. Added in MySQL 5.0.2.

  • The current number of pending reads. Added in MySQL 5.0.2.

  • The current number of pending writes. Added in MySQL 5.0.2.

  • The amount of data read so far, in bytes. Added in MySQL 5.0.2.

  • The total number of data reads. Added in MySQL 5.0.2.

  • The total number of data writes. Added in MySQL 5.0.2.

  • The amount of data written so far, in bytes. Added in MySQL 5.0.2.

  • ,

    The number of doublewrite operations that have been performed and the number of pages that have been written for this purpose. Added in MySQL 5.0.2. See Section 14.2.14.1, “ Disk I/O”.

  • The number of times that the log buffer was too small and a wait was required for it to be flushed before continuing. Added in MySQL 5.0.2.

  • The number of log write requests. Added in MySQL 5.0.2.

  • The number of physical writes to the log file. Added in MySQL 5.0.2.

  • The number of writes done to the log file. Added in MySQL 5.0.2.

  • The number of pending log file operations. Added in MySQL 5.0.2.

  • The number of pending log file writes. Added in MySQL 5.0.2.

  • The number of bytes written to the log file. Added in MySQL 5.0.2.

  • The compiled-in page size (default 16KB). Many values are counted in pages; the page size allows them to be easily converted to bytes. Added in MySQL 5.0.2.

  • The number of pages created. Added in MySQL 5.0.2.

  • The number of pages read. Added in MySQL 5.0.2.

  • The number of pages written. Added in MySQL 5.0.2.

  • The number of row locks currently being waited for. Added in MySQL 5.0.3.

  • The total time spent in acquiring row locks, in milliseconds. Added in MySQL 5.0.3.

  • The average time to acquire a row lock, in milliseconds. Added in MySQL 5.0.3.

  • The maximum time to acquire a row lock, in milliseconds. Added in MySQL 5.0.3.

  • The number of times a row lock had to be waited for. Added in MySQL 5.0.3.

  • The number of rows deleted from tables. Added in MySQL 5.0.2.

  • The number of rows inserted into tables. Added in MySQL 5.0.2.

  • The number of rows read from tables. Added in MySQL 5.0.2.

  • The number of rows updated in tables. Added in MySQL 5.0.2.

  • The number of key blocks in the key cache that have changed but have not yet been flushed to disk.

  • The number of unused blocks in the key cache. You can use this value to determine how much of the key cache is in use; see the discussion of in Section 5.2.2, “Server System Variables”.

  • The number of used blocks in the key cache. This value is a high-water mark that indicates the maximum number of blocks that have ever been in use at one time.

  • The number of requests to read a key block from the cache.

  • The number of physical reads of a key block from disk. If is large, then your value is probably too small. The cache miss rate can be calculated as /.

  • The number of requests to write a key block to the cache.

  • The number of physical writes of a key block to disk.

  • The total cost of the last compiled query as computed by the query optimizer. This is useful for comparing the cost of different query plans for the same query. The default value of 0 means that no query has been compiled yet. This variable was added in MySQL 5.0.1, with a default value of -1. In MySQL 5.0.7, the default was changed to 0; also in version 5.0.7, the scope of was changed to session rather than global.

    Prior to MySQL 5.0.16, this variable was not updated for queries served from the query cache.

  • The maximum number of connections that have been in use simultaneously since the server started.

  • If the server is acting as a MySQL Cluster node, then the value of this variable its node ID in the cluster.

    If the server is not part of of a MySQL Cluster, then the value of this variable is 0.

  • If the server is part of a MySQL Cluster, the value of this variable is the hostname or IP address of the Cluster management server from which it gets its configuration data.

    If the server is not part of of a MySQL Cluster, then the value of this variable is an empty string.

    Prior to MySQL 5.0.23, this variable was named .

  • If the server is part of a MySQL Cluster, the value of this variable is the number of the port through which it is connected to the CLuster management server from which it gets its configuration data.

    If the server is not part of of a MySQL Cluster, then the value of this variable is 0.

    Prior to MySQL 5.0.23, this variable was named .

  • If the server is part of a MySQL Cluster, the value of this variable is the number of data nodes in the cluster.

    If the server is not part of of a MySQL Cluster, then the value of this variable is 0.

  • The number of rows waiting to be written in queues.

  • The number of files that are open.

  • The number of streams that are open (used mainly for logging).

  • The number of tables that are open.

  • The number of tables that have been opened. If is big, your value is probably too small.

  • The number of free memory blocks in the query cache.

  • The amount of free memory for the query cache.

  • The number of query cache hits.

  • The number of queries added to the query cache.

  • The number of queries that were deleted from the query cache because of low memory.

  • The number of non-cached queries (not cacheable, or not cached due to the setting).

  • The number of queries registered in the query cache.

  • The total number of blocks in the query cache.

  • The number of statements that clients have sent to the server.

  • The status of fail-safe replication (not yet implemented).

  • The number of joins that perform table scans because they do not use indexes. If this value is not 0, you should carefully check the indexes of your tables.

  • The number of joins that used a range search on a reference table.

  • The number of joins that used ranges on the first table. This is normally not a critical issue even if the value is quite large.

  • The number of joins without keys that check for key usage after each row. If this is not 0, you should carefully check the indexes of your tables.

  • The number of joins that did a full scan of the first table.

  • The number of temporary tables that the slave SQL thread currently has open.

  • This is if this server is a slave that is connected to a master.

  • The total number of times since startup that the replication slave SQL thread has retried transactions. This variable was added in version 5.0.4.

  • The number of threads that have taken more than seconds to create.

  • The number of queries that have taken more than seconds. See Section 5.12.4, “The Slow Query Log”.

  • The number of merge passes that the sort algorithm has had to do. If this value is large, you should consider increasing the value of the system variable.

  • The number of sorts that were done using ranges.

  • The number of sorted rows.

  • The number of sorts that were done by scanning the table.

  • Variables used for SSL connections.

  • The number of times that a table lock was acquired immediately.

  • The number of times that 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 optimize your queries, and then either split your table or tables or use replication.

  • The number of threads in the thread cache.

  • The number of currently open connections.

  • The number of threads created to handle connections. If is big, you may want to increase the value. The cache miss rate can be calculated as /.

  • The number of threads that are not sleeping.

  • The number of seconds that the server has been up.

5.2.5. The Server SQL Mode

The MySQL server can operate in different SQL modes, and can apply these modes differently for different clients. This capability enables each application to tailor the server's operating mode to its own requirements.

Modes define what SQL syntax MySQL should support and what kind of data validation checks it should perform. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers.

You can set the default SQL mode by starting mysqld with the " option. is a list of different modes separated by comma (‘’) characters. The default value is empty (no modes set). The value also can be empty () if you want to clear it explicitly.

You can change the SQL mode at runtime by using a ' statement to set the system value. Setting the variable requires the privilege and affects the operation of all clients that connect from that time on. Setting the variable affects only the current client. Any client can change its own session value at any time.

You can retrieve the current global or session value with the following statements:

SELECT @@global.sql_mode;
SELECT @@session.sql_mode;

The most important values are probably these:

  • Change syntax and behavior to be more conformant to standard SQL.

  • If a value could not be inserted as given into a transactional table, abort the statement. For a non-transactional table, abort the statement if the value occurs in a single-row statement or the first row of a multiple-row statement. More detail is given later in this section. (Implemented in MySQL 5.0.2)

  • Make MySQL behave like a “traditional” SQL database system. A simple description of this mode is “give an error instead of a warning” when inserting an incorrect value into a column. Note: The / aborts as soon as the error is noticed. This may not be what you want if you are using a non-transactional storage engine, because data changes made prior to the error are not be rolled back, resulting in a “partially done” update. (Added in MySQL 5.0.2)

When this manual refers to “strict mode,” it means a mode where at least one of or is enabled.

The following list describes all supported modes:

  • Don't do full checking of dates. Check only that the month is in the range from 1 to 12 and the day is in the range from 1 to 31. This is very convenient for Web applications where you obtain year, month, and day in three different fields and you want to store exactly what the user inserted (without date validation). This mode applies to and columns. It does not apply columns, which always require a valid date.

    This mode is implemented in MySQL 5.0.2. Before 5.0.2, this was the default MySQL date-handling mode. As of 5.0.2, the server requires that month and day values be legal, and not merely in the range 1 to 12 and 1 to 31, respectively. With strict mode disabled, invalid dates such as are converted to and a warning is generated. With strict mode enabled, invalid dates generate an error. To allow such dates, enable .

  • Treat ‘’ as an identifier quote character (like the ‘’ quote character) and not as a string quote character. You can still use ‘’ to quote identifiers with this mode enabled. With enabled, you cannot use double quotes to quote literal strings, because it is interpreted as an identifier.

  • Produce an error in strict mode (otherwise a warning) when a division by zero (or ) occurs during an or . If this mode is not enabled, MySQL instead returns for divisions by zero. For or , MySQL generates a warning for divisions by zero, but the result of the operation is . (Implemented in MySQL 5.0.2)

  • From MySQL 5.0.2 on, the precedence of the operator is such that expressions such as are parsed as . Before MySQL 5.0.2, the expression is parsed as . The old higher-precedence behavior can be obtained by enabling the SQL mode. (Added in MySQL 5.0.2)

    mysql> 
    mysql> 
            -> 0
    mysql> 
    mysql> 
            -> 1
    
  • Allow spaces between a function name and the ‘’ character. This forces all function names to be treated as reserved words. As a result, if you want to access any database, table, or column name that is a reserved word, you must quote it. For example, because there is a function, the name of the table in the database and the column in that table become reserved, so you must quote them:

    SELECT "User" FROM mysql."user";
    

    The SQL mode applies to built-in functions, not to stored routines. it is always allowable to have spaces after a routine name, regardless of whether is enabled.

  • Prevent from automatically creating new users if it would otherwise do so, unless a non-empty password also is specified. (Added in MySQL 5.0.2)

  • affects handling of columns. Normally, you generate the next sequence number for the column by inserting either or into it. suppresses this behavior for so that only generates the next sequence number.

    This mode can be useful if has been stored in a table's column. (Storing is not a recommended practice, by the way.) For example, if you dump the table with mysqldump and then reload it, MySQL normally generates new sequence numbers when it encounters the values, resulting in a table with contents different from the one that was dumped. Enabling before reloading the dump file solves this problem. mysqldump now automatically includes in its output a statement that enables , to avoid this problem.

  • Disable the use of the backslash character (‘’) as an escape character within strings. With this mode enabled, backslash becomes an ordinary character like any other. (Implemented in MySQL 5.0.1)

  • When creating a table, ignore all and directives. This option is useful on slave replication servers.

  • Prevents automatic substitution of the default storage engine when a statement such as specifies a storage engine that is disabled or not compiled in. (Implemented in MySQL 5.0.8)

  • Do not print MySQL-specific column options in the output of . This mode is used by mysqldump in portability mode.

  • Do not print MySQL-specific index options in the output of . This mode is used by mysqldump in portability mode.

  • Do not print MySQL-specific table options (such as ) in the output of . This mode is used by mysqldump in portability mode.

  • In integer subtraction operations, do not mark the result as if one of the operands is unsigned. Note that this makes not 100% usable in all contexts. See Section 12.8, “Cast Functions and Operators”.

    mysql>t; 
    mysql>t; 
    +-------------------------+
    | CAST(0 AS UNSIGNED) - 1 |
    +-------------------------+
    |    18446744073709551615 |
    +-------------------------+
    mysql>t; 
    mysql>t; 
    +-------------------------+
    | CAST(0 AS UNSIGNED) - 1 |
    +-------------------------+
    |                      -1 |
    +-------------------------+
    
  • In strict mode, don't allow as a valid date. You can still insert zero dates with the option. When not in strict mode, the date is accepted but a warning is generated. (Added in MySQL 5.0.2)

  • In strict mode, don't accept dates where the month or day part is 0. If used with the option, MySQL inserts a date for any such date. When not in strict mode, the date is accepted but a warning is generated. (Added in MySQL 5.0.2)

  • Do not allow queries for which the list refers to non-aggregated columns that are not named in the clause. The following query is invalid with this mode enabled because is not named in the clause:

    SELECT name, address, MAX(age) FROM t GROUP BY name;
    

    As of MySQL 5.0.23, this mode also restricts references to non-aggregated columns in the clause that are not named in the clause.

  • Treat as a string concatenation operator (same as ) rather than as a synonym for .

  • Treat as a synonym for . By default, MySQL treats as a synonym for .

  • Enable strict mode for all storage engines. Invalid data values are rejected. Additional detail follows. (Added in MySQL 5.0.2)

  • Enable strict mode for transactional storage engines, and when possible for non-transactional storage engines. Additional details follow. (Implemented in MySQL 5.0.2)

Strict mode controls how MySQL handles input values that are invalid or missing. A value can be invalid for several reasons. For example, it might have the wrong data type for the column, or it might be out of range. A value is missing when a new row to be inserted does not contain a value for a column that has no explicit clause in its definition.

For transactional tables, an error occurs for invalid or missing values in a statement when either of the or modes are enabled. The statement is aborted and rolled back.

For non-transactional tables, the behavior is the same for either mode, if the bad value occurs in the first row to be inserted or updated. The statement is aborted and the table remains unchanged. If the statement inserts or modifies multiple rows and the bad value occurs in the second or later row, the result depends on which strict option is enabled:

  • For , MySQL returns an error and ignores the rest of the rows. However, in this case, the earlier rows still have been inserted or updated. This means that you might get a partial update, which might not be what you want. To avoid this, it's best to use single-row statements because these can be aborted without changing the table.

  • For , MySQL converts an invalid value to the closest valid value for the column and insert the adjusted value. If a value is missing, MySQL inserts the implicit default value for the column data type. In either case, MySQL generates a warning rather than an error and continues processing the statement. Implicit defaults are described in Section 11.1.4, “Data Type Default Values”.

Strict mode disallows invalid date values such as . It does not disallow dates with zero parts such as or “zero” dates. To disallow these as well, enable the and SQL modes in addition to strict mode.

If you are not using strict mode (that is, neither nor is enabled), MySQL inserts adjusted values for invalid or missing values and produces warnings. In strict mode, you can produce this behavior by using or . See Section 13.5.4.25, “ Syntax”.

The following special modes are provided as shorthand for combinations of mode values from the preceding list. All are available in MySQL 5.0 beginning with version 5.0.0, except for , which was implemented in MySQL 5.0.2.

The descriptions include all mode values that are available in the most recent version of MySQL. For older versions, a combination mode does not include individual mode values that are not available except in newer versions.

  • Equivalent to , , , . Before MySQL 5.0.3, also includes . See Section 1.9.3, “Running MySQL in ANSI Mode”.

  • Equivalent to , , , , , .

  • Equivalent to , , , , , , .

  • Equivalent to , , , , , .

  • Equivalent to , .

  • Equivalent to , .

  • Equivalent to , , , , , , .

  • Equivalent to , , , , , .

  • Equivalent to , , , , , .

5.2.6. The MySQL Server Shutdown Process

The server shutdown process takes place as follows:

  1. The shutdown process is initiated.

    Server shutdown can be initiated several ways. For example, a user with the privilege can execute a mysqladmin shutdown command. mysqladmin can be used on any platform supported by MySQL. Other operating system-specific shutdown initiation methods are possible as well: The server shuts down on Unix when it receives a signal. A server running as a service on Windows shuts down when the services manager tells it to.

  2. The server creates a shutdown thread if necessary.

    Depending on how shutdown was initiated, the server might create a thread to handle the shutdown process. If shutdown was requested by a client, a shutdown thread is created. If shutdown is the result of receiving a signal, the signal thread might handle shutdown itself, or it might create a separate thread to do so. If the server tries to create a shutdown thread and cannot (for example, if memory is exhausted), it issues a diagnostic message that appears in the error log:

    Error: Can't create thread to kill server
    
  3. The server stops accepting new connections.

    To prevent new activity from being initiated during shutdown, the server stops accepting new client connections. It does this by closing the network connections to which it normally listens for connections: the TCP/IP port, the Unix socket file, the Windows named pipe, and shared memory on Windows.

  4. The server terminates current activity.

    For each thread that is associated with a client connection, the connection to the client is broken and the thread is marked as killed. Threads die when they notice that they are so marked. Threads for idle connections die quickly. Threads that currently are processing statements check their state periodically and take longer to die. For additional information about thread termination, see Section 13.5.5.3, “ Syntax”, in particular for the instructions about killed or operations on tables.

    For threads that have an open transaction, the transaction is rolled back. Note that if a thread is updating a non-transactional table, an operation such as a multiple-row or may leave the table partially updated, because the operation can terminate before completion.

    If the server is a master replication server, threads associated with currently connected slaves are treated like other client threads. That is, each one is marked as killed and exits when it next checks its state.

    If the server is a slave replication server, the I/O and SQL threads, if active, are stopped before client threads are marked as killed. The SQL thread is allowed to finish its current statement (to avoid causing replication problems), and then stops. If the SQL thread was in the middle of a transaction at this point, the transaction is rolled back.

  5. Storage engines are shut down or closed.

    At this stage, the table cache is flushed and all open tables are closed.

    Each storage engine performs any actions necessary for tables that it manages. For example, flushes any pending index writes for a table. flushes its buffer pool to disk (starting from 5.0.5: unless is 2), writes the current LSN to the tablespace, and terminates its own internal threads.

  6. The server exits.

5.2.7. MySQL Server-Side Help Support

MySQL Server supports a statement that returns online information from the MySQL Reference manual (see Section 13.3.2, “ Syntax”). The proper operation of this statement requires that the help tables in the database be initialized with help topic information, which is done by processing the contents of the script.

For a MySQL binary distribution on Unix, help table setup occurs when you run mysql_install_db. For an RPM distribution on Linux or binary distribution on Windows, help table setup occurs as part of the MySQL installation process.

For a MySQL source distribution, you can find the file in the directory. To load the file manually, make sure that you have initialized the database by running mysql_install_db, and then process the file with the mysql client as follows:

shell> 

If you are working with BitKeeper and a MySQL development source tree, the tree doesn't contain . You can download the proper file for your version of MySQL from http://dev.mysql.com/doc/. After downloading and uncompressing the file, process it with mysql as just described.