Options that affect security: See Section 5.7.3, “Security-Related mysqld Options”.

SSL-related options: See Section 5.9.7.3, “SSL Command Options”.

Binary log control options: See Section 5.12.4, “The Binary Log”.

Replication-related options: See Section 6.9, “Replication Startup Options”.

Options specific to particular storage engines: See Section 15.1.1, “MyISAM Startup Options”, Section 15.5.3, “BDB Startup Options”, Section 15.2.4, “InnoDB Startup Options and System Variables”, and Section 17.6.5.1, “MySQL Cluster-Related Command Options for mysqld”.

--help, -?

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

--allow-suspicious-udfs

This option controls whether user-defined functions that have only an xxx 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. See Section 28.3.4.6, “User-Defined Function Security Precautions”.

--ansi

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

--basedir=path, -b path

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

--bind-address=IP

The IP address to bind to.

--binlog-format={row|statement}

Specify whether to use row-based or statement-based replication (statement-based is default). See Section 6.3, “Row-Based Replication”. This option was added in MySQL 5.1.5.

--binlog-row-event-max-size=N

Specify the maximum size of a row-based binary log event, in bytes. Rows are grouped into events smaller than this size if possible. The value should be a multiple of 256. The default is 1024. See Section 6.3, “Row-Based Replication”. This option was added in MySQL 5.1.5.

--bootstrap

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

--character-sets-dir=path

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

--character-set-client-handshake

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

--character-set-filesystem=charset_name

The filesystem character set. This option sets the character_set_filesystem system variable. It was added in MySQL 5.1.6.

--character-set-server=charset_name, -C charset_name

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

--chroot=path

Put the mysqld server in a closed environment during startup by using the chroot() system call. This is a recommended security measure. Note that use of this option somewhat limits LOAD DATA INFILE and SELECT ... INTO OUTFILE.

--collation-server=collation_name

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

--console

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

--core-file

Write a core file if mysqld dies. For some systems, you must also specify the --core-file-size 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 --user option.

--datadir=path, -h path

The path to the data directory.

--debug[=debug_options], -# [debug_options]

If MySQL is configured with --with-debug, you can use this option to get a trace file of what mysqld is doing. The debug_options string often is 'd:t:o,file_name'. The default is 'd:t:i:o,mysqld.trace'. See Section E.1.2, “Creating Trace Files”.

--default-character-set=charset_name (DEPRECATED)

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

--default-collation=collation_name

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

--default-storage-engine=type

Set the default storage engine (table type) for tables. See Chapter 15, Storage Engines and Table Types.

--default-table-type=type

This option is a synonym for --default-storage-engine.

--default-time-zone=timezone

Set the default server time zone. This option sets the global time_zone 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_time_zone system variable.

--delay-key-write[={OFF|ON|ALL}]

Specify how to use delayed key writes. Delayed key writing causes key buffers not to be flushed between writes for MyISAM tables. OFF disables delayed key writes. ON enables delayed key writes for those tables that were created with the DELAY_KEY_WRITE option. ALL delays key writes for all MyISAM tables. See Section 7.5.2, “Tuning Server Parameters”, and Section 15.1.1, “MyISAM Startup Options”.

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

--des-key-file=file_name

Read the default DES keys from this file. These keys are used by the DES_ENCRYPT() and DES_DECRYPT() functions.

--enable-named-pipe

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 server that supports named-pipe connections.

--event-scheduler

Enable the event scheduler. This option was added in MySQL 5.1.6.

--exit-info[=flags], -T [flags]

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!

--external-locking

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 lockd does not fully work (such as Linux), it is easy for mysqld to deadlock. This option previously was named --enable-locking.

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

The easiest way to ensure this is to always use --external-locking together with --delay-key-write=OFF and --query-cache-size=0. (This is not done by default because in many setups it is useful to have a mixture of the preceding options.)

--flush

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”.

--init-file=file_name

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

--innodb-xxx

The InnoDB options are listed in Section 15.2.4, “InnoDB Startup Options and System Variables”.

--language=lang_name, -L lang_name

Return client error messages in the given language. lang_name 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”.

--large-pages

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 hugetlbpage.txt file in the Linux kernel source.

This option is disabled by default.

--log[=file_name], -l [file_name]

This option enables logging to the general query log, which contains entries that record client connections and SQL statements received from clients. The log output destination can be selected with the --log-output option as of MySQL 5.1.6. Before 5.1.6, logging occurs to the general query log file. If you omit the filename, MySQL uses host_name.log as the filename. See Section 5.12.1, “Server Log Tables”, and Section 5.12.3, “The General Query Log”.

--log-bin[=base_name]

Enable binary logging. The server logs all statements that change data to the binary log, which is used for backup and replication. See Section 5.12.4, “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 host_name-bin as the basename.

--log-bin-index[=file_name]

The index file for binary log filenames. See Section 5.12.4, “The Binary Log”. If you omit the filename, and if you didn't specify one with --log-bin, MySQL uses host_name-bin.index as the filename.

--log-bin-trust-function-creators[={0|1}]

With no argument or an argument of 1, this option sets the log_bin_trust_function_creators system variable to 1. With an argument of 0, this option sets the system variable to 0. log_bin_trust_function_creators affects how MySQL enforces restrictions on stored function creation. See Section 20.4, “Binary Logging of Stored Routines and Triggers”.

--log-error[=file_name]

Log errors and startup messages to this file. See Section 5.12.2, “The Error Log”. If you omit the filename, MySQL uses host_name.err. If the filename has no extension, the server adds an extension of .err.

--log-isam[=file_name]

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

--log-long-format (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 --log-short-format.) The --log-queries-not-using-indexes option is available for the purpose of logging queries that do not use indexes to the slow query log.

--log-output[=value,...]

This option determines the destination for general query log and slow query log output. The option value can be given as one or more of the words TABLE, FILE, or NONE. If the option is given without a value, the default is TABLE (log to the general_log and slow_log tables in the mysql database). FILE causes logging to log files. (For FILE logging, the --log and -slow-log options determine the log file location.) NONE disables logging. If NONE is present in the option value, it takes precedence over any other words that are present. TABLE and FILE can both be given to select to both log output destinations.

This option selects log output destinations, but does not enable log output. To do that, use the --log and --log-slow-queries options. For more information, see Section 5.12.1, “Server Log Tables”.

The --log-output option was added in MySQL 5.1.6.

--log-queries-not-using-indexes

If you are using this option with --log-slow-queries, queries that do not use indexes are logged to the slow query log. See Section 5.12.5, “The Slow Query Log”.

--log-short-format

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-admin-statements

Log slow administrative statements such as OPTIMIZE TABLE, ANALYZE TABLE, and ALTER TABLE to the slow query log.

--log-slow-queries[=file_name]

This option enables logging to the slow query log, which contains entries for all queries that have taken more than long_query_time seconds to execute. See the descriptions of the --log-long-format and --log-short-format options for details.

The log output destination can be selected with the --log-output option as of MySQL 5.1.6. Before 5.1.6, logging occurs to the slow query log file. If you omit the filename, MySQL uses host_name-slow.log as the filename. See Section 5.12.1, “Server Log Tables”, and Section 5.12.5, “The Slow Query Log”.

--log-warnings[=level], -W [level]

Print out warnings such as Aborted connection... 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 level value if omitted is 1. To disable this option, use --log-warnings=0. 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”.

--low-priority-updates

Give table-modifying operations (INSERT, REPLACE, DELETE, UPDATE) lower priority than selects. This can also be done via {INSERT | REPLACE | DELETE | UPDATE} LOW_PRIORITY ... to lower the priority of only one query, or by SET LOW_PRIORITY_UPDATES=1 to change the priority in one thread. See Section 7.3.2, “Table Locking Issues”.

--memlock

Lock the mysqld process in memory. This works on systems such as Solaris that support the mlockall() 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 root, which is normally not a good idea for security reasons. See Section 5.7.5, “How to Run MySQL as a Normal User”.

--myisam-recover[=option[,option]...]]

Set the MyISAM storage engine recovery mode. The option value is any combination of the values of DEFAULT, BACKUP, FORCE, or QUICK. 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 MyISAM 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:

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 BACKUP,FORCE. 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 15.1.1, “MyISAM Startup Options”.

--ndb-connectstring=connect_string

When using the NDB storage engine, it is possible to point out the management server that distributes the cluster configuration by setting the connect string option. See Section 17.4.4.2, “The MySQL Cluster connectstring”, for syntax.

--ndbcluster

If the binary includes support for the NDB Cluster storage engine, this option enables the engine, which is disabled by default. See Chapter 17, MySQL Cluster.

--old-passwords

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”.

--one-thread

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”.

--open-files-limit=count

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 setrlimit(). If the value is 0, mysqld reserves max_connections×5 or max_connections + table_open_cache×2 files (whichever is larger). You should try increasing this value if mysqld gives you the error Too many open files.

--pid-file=path

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.

--port=port_num, -P port_num

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 root system user.

--port-open-timeout=num

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 5.1.5.

--safe-mode

Skip some optimization stages.

--safe-show-database (DEPRECATED)

See Section 5.8.3, “Privileges Provided by MySQL”.

--safe-user-create

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

--secure-auth

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

--shared-memory

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

--shared-memory-base-name=name

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

--skip-bdb

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

--skip-concurrent-insert

Turn off the ability to select and insert at the same time on MyISAM 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”.

--skip-external-locking

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 CHECK TABLE and REPAIR TABLE statements to check and repair MyISAM tables.

External locking has been disabled by default since MySQL 4.0.

--skip-grant-tables

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 FLUSH PRIVILEGES statement after connecting to the server. This option also suppresses loading of plugins and user-defined functions (UDFs).

--skip-host-cache

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”.

--skip-innodb

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

--skip-name-resolve

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

--skip-ndbcluster

Disable the NDB Cluster storage engine. This is the default for binaries that were built with NDB Cluster storage engine support; the server allocates memory and other resources for this storage engine only if the --ndbcluster option is given explicitly. See Section 17.4.3, “Quick Test Setup of MySQL Cluster”, for an example of usage.

--skip-networking

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”.

--ssl*

Options that begin with --ssl 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”.

--standalone

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

--symbolic-links, --skip-symbolic-links

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

--skip-safemalloc

If MySQL is configured with --with-debug=full, 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 --skip-safemalloc option.

--skip-show-database

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

--skip-stack-trace

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

--skip-thread-priority

Disable using thread priorities for faster response time.

--socket=path

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

--sql-mode=value[,value[,value...]]

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

--sysdate-is-now

SYSDATE() 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 NOW(). This option causes SYSDATE() to be an alias for NOW(). For information about the implications for binary logging and replication, see the description for SYSDATE() in Section 12.5, “Date and Time Functions” and for SET TIMESTAMP in Section 13.5.3, “SET Syntax”.

This option was added in MySQL 5.1.8.

--temp-pool

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.

--transaction-isolation=level

Sets the default transaction isolation level. The level value can be READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE. See Section 13.4.6, “SET TRANSACTION Syntax”.

--tmpdir=path, -t path

The path of the directory to use for creating temporary files. It might be useful if your default /tmp 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 --tmpdir 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 LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication fails.

--user={user_name|user_id}, -u {user_name|user_id}

Run the mysqld server as the user having the name user_name or the numeric user ID 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 root. The server changes its user ID during its startup sequence, causing it to run as that particular user rather than as root. See Section 5.7.1, “General Security Guidelines”.

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

--version, -V

Display version information and exit.

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 SHOW VARIABLES statement.

Section 5.2.3, “Using System Variables”, discusses the syntax for setting and displaying system variable values.

Section 5.2.3.2, “Dynamic System Variables”, lists the variables that can be set at runtime.

Information on tuning sytem variables can be found in Section 7.5.2, “Tuning Server Parameters”.

Section 15.2.4, “InnoDB Startup Options and System Variables”, lists InnoDB system variables.

auto_increment_increment

auto_increment_increment and auto_increment_offset are intended for use with master-to-master replication, and can be used to control the operation of AUTO_INCREMENT 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 auto_increment_increment or auto_increment_offset to a non-integer value gives rise to an error, and the actual value of the variable remains unchanged.

Important: auto_increment_increment and auto_increment_offset are not intended for use with MySQL Cluster replication. Attempting to set them in a Cluster replication scenario may give rise to unpredictable (and unrecoverable) errors. The use of these variables with Cluster replication is therefore not supported.

These two variables affect AUTO_INCREMENT column behavior as follows:

Should one or both of these variables be changed and then new rows inserted into a table containing an AUTO_INCREMENT column, the results may seem counterintuitive because the series of AUTO_INCREMENT 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 AUTO_INCREMENT column. In other words, the series is calculated like so:

auto_increment_offset + N × auto_increment_increment

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

mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 10    |
| auto_increment_offset    | 5     |
+--------------------------+-------+
2 rows in set (0.00 sec)

mysql> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
|   1 |
|  11 |
|  21 |
|  31 |
+-----+
4 rows in set (0.00 sec)

mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
|   1 |
|  11 |
|  21 |
|  31 |
|  35 |
|  45 |
|  55 |
|  65 |
+-----+
8 rows in set (0.00 sec)

The values shown for auto_increment_increment and auto_increment_offset generate the series 5 + N × 10, that is, [5, 15, 25, 35, 45, ...]. The greatest value present in the col column prior to the INSERT is 31, and the next available value in the AUTO_INCREMENT series is 35, so the inserted values for col begin at that point and the results are as shown for the SELECT 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 AUTO_INCREMENT 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 AUTO_INCREMENT 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 default value of auto_increment_increment is 1. See Section 6.15, “Auto-Increment in Multiple-Master Replication”.

auto_increment_offset

This variable has a default value of 1. For particulars, see the description for auto_increment_increment.

back_log

The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets very many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time.

In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen() system call should have more details. Check your OS documentation for the maximum value for this variable. back_log cannot be set higher than your operating system limit.

basedir

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

bdb_cache_parts

The number of parts to use for the BDB cache. Added in MySQL 5.1.2.

bdb_cache_size

The size of the buffer that is allocated for caching indexes and rows for BDB tables. Some systems support setting this variable larger than 4GB. If you don't use BDB tables, you should start mysqld with --skip-bdb to not allocate memory for this cache.

bdb_home

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

bdb_log_buffer_size

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

bdb_logdir

The directory where the BDB storage engine writes its log files. This variable can be set with the --bdb-logdir option.

bdb_max_lock

The maximum number of locks that can be active for a BDB 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 ...

bdb_region_size

The size of the underlying logging area of the BDB environment. This is the size of the memory pool used for storing the filenames used in a transaction. Added in MySQL 5.1.2.

bdb_shared_data

This is ON if you are using --bdb-shared-data to start Berkeley DB in multi-process mode. (Do not use DB_PRIVATE when initializing Berkeley DB.)

bdb_tmpdir

The BDB temporary file directory.

binlog_cache_size

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 (--log-bin option). If you often use large, multiple-statement transactions, you can increase this cache size to get more performance. The Binlog_cache_use and Binlog_cache_disk_use status variables can be useful for tuning the size of this variable. See Section 5.12.4, “The Binary Log”.

binlog_format

The binary logging format, either STATEMENT, ROW, or MIXED. binlog_format is set by the --binlog-format option at startup, or by the binlog_format variable at runtime (you need the SUPER privilege to set this variable on a global scope). See Section 6.3, “Row-Based Replication”. The startup variable was added in MySQL 5.1.5, and the runtime variable in MySQL 5.1.8. MIXED was added in MySQL 5.1.8.

STATEMENT is used by default. If MIXED is specified, statement-based replication is used, too, except for cases where only row-based replication is guaranteed to lead to proper results. For example, this is the case when statements contain user-defined functions (UDF) or the UUID() function. An exception to this rule is that MIXED always uses statement-based replication for stored functions and triggers.

There are exceptions when you cannot switch the replication format at runtime:

Trying to switch the format in those cases results in an error.

Before MySQL 5.1.8, switching to row-based replication format would implicitly set --log-bin-trust-function-creators=1 and --innodb_locks_unsafe_for_binlog. MySQL 5.1.8 and later no longer implicitly set these options when row-based replication is used.

bulk_insert_buffer_size

MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA INFILE 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.

character_set_client

The character set for statements that arrive from the client.

character_set_connection

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

character_set_database

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 character_set_server.

character_set_filesystem

The filesystem character set. This variable is used to interpret string literals that refer to filenames, such as in the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements and the LOAD_FILE() function. Such filenames are converted from character_set_client to character_set_filesystem before the file opening attempt occurs. The default value is binary, 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 character_set_filesytem to 'utf8'. This variable was added in MySQL 5.1.6.

character_set_results

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

character_set_server

The server's default character set.

character_set_system

The character set used by the server for storing identifiers. The value is always utf8.

character_sets_dir

The directory where character sets are installed.

collation_connection

The collation of the connection character set.

collation_database

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 collation_server.

collation_server

The server's default collation.

completion_type

The transaction completion type:

concurrent_insert

If ON (the default), MySQL allows INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle. You can turn this option off by starting mysqld with --safe or --skip-new.

This variable can take three integer values:

See also Section 7.3.3, “Concurrent Inserts”.

connect_timeout

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

datadir

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

date_format

This variable is not implemented.

datetime_format

This variable is not implemented.

default_week_format

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

delay_key_write

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

If DELAY_KEY_WRITE 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 MyISAM tables by starting the server with the --myisam-recover option (for example, --myisam-recover=BACKUP,FORCE). See Section 5.2.1, “mysqld Command Options”, and Section 15.1.1, “MyISAM Startup Options”.

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

delayed_insert_limit

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

delayed_insert_timeout

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

delayed_queue_size

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

div_precision_increment

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> SELECT 1/7;
+--------+
| 1/7    |
+--------+
| 0.1429 |
+--------+
mysql> SET div_precision_increment = 12;
mysql> SELECT 1/7;
+----------------+
| 1/7            |
+----------------+
| 0.142857142857 |
+----------------+

event_scheduler

Whether the event scheduler is disabled or enabled. By default, the scheduler is disabled. This variable was added in MySQL 5.1.6.

engine_condition_pushdown

This variable applies to NDB. By default it is 0 (OFF): If you execute a query such as SELECT * FROM t WHERE mycol = 42, where mycol 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 WHERE condition. If engine_condition_pushdown is set to 1 (ON), 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.

expire_logs_days

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.

flush

If ON, 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 ON if you start mysqld with the --flush option.

flush_time

If this is set to a non-zero value, all tables are closed every flush_time 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.

ft_boolean_syntax

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

The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:

ft_max_word_len

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

Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.

ft_min_word_len

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

Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.

ft_query_expansion_limit

The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION.

ft_stopword_file

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 myisam/ft_static.c file). Setting this variable to the empty string ('') disables stopword filtering.

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

group_concat_max_len

The maximum allowed result length for the GROUP_CONCAT() function. The default is 1024.

have_archive

YES if mysqld supports ARCHIVE tables, NO if not.

have_bdb

YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used.

have_blackhole_engine

YES if mysqld supports BLACKHOLE tables, NO if not.

have_compress

YES if the zlib compression library is available to the server, NO if not. If not, the COMPRESS() and UNCOMPRESS() functions cannot be used.

have_crypt

YES if the crypt() system call is available to the server, NO if not. If not, the ENCRYPT() function cannot be used.

have_csv

YES if mysqld supports ARCHIVE tables, NO if not.

have_dynamic_loading

YES if mysqld supports dynamic loading of plugins, NO if not. This variable was added in MySQL 5.1.10.

have_example_engine

YES if mysqld supports EXAMPLE tables, NO if not.

have_federated_engine

YES if mysqld supports FEDERATED tables, NO if not.

have_geometry

YES if the server supports spatial data types, NO if not.

have_innodb

YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used.

have_ndbcluster

YES if mysqld supports NDB Cluster tables. DISABLED if --skip-ndbcluster is used.

have_partitioning

YES if mysqld supports partitioning. Added in MySQL 5.1.1 as have_partition_engine and renamed to have_partioning in 5.1.6.

have_openssl

YES if mysqld supports SSL connections, NO if not.

have_query_cache

YES if mysqld supports the query cache, NO if not.

have_row_based_replication

YES if the server can perform replication using row-based binary logging. If the value is NO, the server can use only statement-based logging. See Section 6.3, “Row-Based Replication”. This variable was added in MySQL 5.1.5.

have_rtree_keys

YES if RTREE indexes are available, NO if not. (These are used for spatial indexes in MyISAM tables.)

have_symlink

YES if symbolic link support is enabled, NO if not. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options, and on Windows for support of data directory symlinks.

init_connect

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 init_connect 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 init_connect is not executed for users that have the SUPER privilege. This is done so that an erroneous value for init_connect 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 init_connect for users that have the SUPER privilege enables them to open a connection and fix the init_connect value.

init_file

The name of the file specified with the --init-file 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.

init_slave

This variable is similar to init_connect, 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 init_connect variable.

innodb_xxx

InnoDB system variables are listed in Section 15.2.4, “InnoDB Startup Options and System Variables”.

interactive_timeout

The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.

join_buffer_size

The size of the buffer that is used for 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 join_buffer_size 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.

key_buffer_size

Index blocks for MyISAM tables are buffered and are shared by all threads. key_buffer_size 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 key_buffer_size 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 LOCK TABLES. See Section 7.2.16, “Speed of INSERT Statements”.

You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables. (See Section 13.5.4, “SHOW Syntax”.) The Key_reads/Key_read_requests ratio should normally be less than 0.01. The Key_writes/Key_write_requests 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 DELAY_KEY_WRITE table option.

The fraction of the key buffer in use can be determined using key_buffer_size in conjunction with the Key_blocks_unused status variable and the buffer block size, which is available from the key_cache_block_size 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 MyISAM key caches. The size limit of 4GB applies to each cache individually, not as a group. See Section 7.4.6, “The MyISAM Key Cache”.

key_cache_age_threshold

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 MyISAM Key Cache”.

key_cache_block_size

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

key_cache_division_limit

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 MyISAM Key Cache”.

language

The language used for error messages.

large_file_support

Whether mysqld was compiled with options for large file support.

large_pages

Whether large page support is enabled.

license

The type of license the server has.

local_infile

Whether LOCAL is supported for LOAD DATA INFILE statements. See Section 5.7.4, “Security Issues with LOAD DATA LOCAL”.

locked_in_memory

Whether mysqld was locked in memory with --memlock.

log

Whether logging of all statements to the general query log is enabled. See Section 5.12.3, “The General Query Log”.

log_bin

Whether the binary log is enabled. See Section 5.12.4, “The Binary Log”.

log_bin_trust_function_creators

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 routines unless they have the SUPER privilege in addition to the CREATE ROUTINE or ALTER ROUTINE privilege. A setting of 0 also enforces the restriction that a function must be declared with the DETERMINISTIC characteristic, or with the READS SQL DATA or NO SQL characteristic. If the variable is set to 1, MySQL does not enforce these restrictions on stored function creation. See Section 20.4, “Binary Logging of Stored Routines and Triggers”.

log_error

The location of the error log.

log_queries_not_using_indexes

Whether queries that do not use indexes are logged to the slow query log. See Section 5.12.5, “The Slow Query Log”. This variable was added in MySQL 5.1.11.

log_slave_updates

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 6.9, “Replication Startup Options”.

log_slow_queries

Whether slow queries should be logged. “Slow” is determined by the value of the long_query_time variable. See Section 5.12.5, “The Slow Query Log”.

log_warnings

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.

long_query_time

If a query takes longer than this many seconds, the server increments the Slow_queries status variable. If you are using the --log-slow-queries 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 5.12.5, “The Slow Query Log”.

low_priority_updates

If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. This variable previously was named sql_low_priority_updates.

lower_case_file_system

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

lower_case_table_names

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 InnoDB 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 lower_case_table_names to 2.

max_allowed_packet

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

The packet message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet 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 BLOB columns or long strings. It should be as big as the largest BLOB you want to use. The protocol limit for max_allowed_packet is 1GB.

max_binlog_cache_size

If a multiple-statement transaction requires more than this amount of memory, the server generates a Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage error.

max_binlog_size

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 max_binlog_size.

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

max_connect_errors

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 FLUSH HOSTS statement.

max_connections

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, “Too many connections”.

max_delayed_threads

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

max_error_count

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

max_heap_table_size

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

max_insert_delayed_threads

This variable is a synonym for max_delayed_threads.

max_join_size

Do not allow SELECT statements that probably need to examine more than max_join_size rows (for single-table statements) or row combinations (for multiple-table statements) or that are likely to do more than max_join_size disk seeks. By setting this value, you can catch SELECT 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 WHERE clause, that take a long time, or that return millions of rows.

Setting this variable to a value other than DEFAULT resets the value of SQL_BIG_SELECTS to 0. If you set the SQL_BIG_SELECTS value again, the max_join_size 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 sql_max_join_size.

max_length_for_sort_data

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

max_prepared_stmt_count

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 5.1.10.

max_relay_log_size

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 max_relay_log_size is 0, the server uses max_binlog_size for both the binary log and the relay log. If max_relay_log_size 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 max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. See Section 6.4, “Replication Implementation Details”.

max_seeks_for_key

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 13.5.4.17, “SHOW INDEX Syntax”). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.

max_sort_length

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

max_tmp_tables

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

max_user_connections

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

This variable has both a global scope and a (read-only) session scope. The session variable has the same value as the global variable unless the current account has a non-zero MAX_USER_CONNECTIONS resource limit. In that case, the session value reflects the account limit.

max_write_lock_count

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

myisam_data_pointer_size

The default pointer size in bytes, to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 7. The default value is 6. See Section A.2.11, “The table is full”.

myisam_max_extra_sort_file_size (DEPRECATED)

Note: This variable is not supported in MySQL 5.1. See MySQL 5.0 Reference Manual for more information.

myisam_max_sort_file_size

The maximum size of the temporary file that MySQL is allowed to use while re-creating a MyISAM index (during REPAIR TABLE, ALTER TABLE, or LOAD DATA INFILE). 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.

myisam_recover_options

The value of the --myisam-recover option. See Section 5.2.1, “mysqld Command Options”.

myisam_repair_threads

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

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

myisam_sort_buffer_size

The size of the buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE.

myisam_stats_method

How the server treats NULL values when collecting statistics about the distribution of index values for MyISAM tables. This variable has two possible values, nulls_equal and nulls_unequal. For nulls_equal, all NULL index values are considered equal and form a single value group that has a size equal to the number of NULL values. For nulls_unequal, NULL values are considered unequal, and each NULL 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, “MyISAM Index Statistics Collection”.

myisam_use_mmap

Use memory mapping for reading and writing MyISAM tables. This variable was added in MySQL 5.1.4.

multi_read_range

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 NDBCLUSTER. This engine needs to send the range requests to all nodes, and sending many of those requests at once reduces the communication costs significantly.

named_pipe

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

ndb_autoincrement_prefetch_sz

Determines the probability of gaps in an autoincremented column. Set to 1 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: 32. Mimimum value: 1.

ndb_cache_check_time

The number of milliseconds to wait before checking the NDB query cache. Setting this to 0 (the default and minimum value) means that the NDB query cache will be checked for validation on every query.

The recommended maximum value for this variable is 1000, which means that the query cache is checked once per second. A larger value means the NDB 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 2000.

ndb_extra_logging

This variable can be set to a non-zero value to enable extra NDB logging for debugging or troubleshooting purposes. The default value is 0.

This variable was added in MySQL 5.1.6.

ndb_force_send

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

ndb_index_stat_cache_entries

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: 32.

ndb_index_stat_enable

Use NDB index statistics in query optimization. Defaults to ON.

ndb_index_stat_update_freq

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

ndb_report_thresh_binlog_epoch_slip

This is a threshold on the number of epochs to be behind before reporting binlog status. For example, a value of 3 (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.

ndb_report_thresh_binlog_mem_usage

This is a threshold on the percentage of free memory remaining before reporting binlog status. For example, a value of 10 (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.

ndb_use_copying_alter_table

Forces NDB to use copying of tables in the event of problems with online ALTER TABLE operations. The default value is OFF.

This variable was added in MySQL 5.1.12.

ndb_use_exact_count

Forces NDB to use an count of records during SELECT COUNT(*) query planning to speed up this type of query. The default value is ON. For faster queries overall, disable this feature by setting the value of ndb_use_exact_count to OFF.

ndb_use_transactions

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

net_buffer_length

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 max_allowed_packet bytes.

net_read_timeout

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, net_read_timeout is the timeout value controlling when to abort. When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort. See also slave_net_timeout.

net_retry_count

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

net_write_timeout

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 net_read_timeout.

new

This variable was used in MySQL 4.0 to turn on some 4.1 behaviors, and is retained for backward compatibility. In MySQL 5.1, its value is always OFF.

old_passwords

Whether the server should use pre-4.1-style passwords for MySQL user accounts. See Section A.2.3, “Client does not support authentication protocol”.

one_shot

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

open_files_limit

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 --open-files-limit option to mysqld or mysqld_safe. The value is 0 on systems where MySQL can't change the number of open files.

optimizer_prune_level

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.

optimizer_search_depth

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.

pid_file

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

plugin_dir

The pathname of the plugins directory. This variable was added in MySQL 5.1.2.

port

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

preload_buffer_size

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

prepared_stmt_count

The current number of prepared statements. (The maximum number of statements is given by the max_prepared_stmt_count system variable.) This variable was added in MySQL 5.1.10.

protocol_version

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

query_alloc_block_size

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.

query_cache_limit

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

query_cache_min_res_unit

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”.

query_cache_size

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 query_cache_size bytes of memory are allocated even if query_cache_type is set to 0. See Section 5.14.3, “Query Cache Configuration”, for more information.

query_cache_type

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

This variable defaults to ON.

query_cache_wlock_invalidate

Normally, when one client acquires a WRITE lock on a MyISAM 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 WRITE 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.

query_prealloc_size

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 query_prealloc_size value might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations.

range_alloc_block_size

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

read_buffer_size

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.

read_only

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

relay_log_purge

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

read_rnd_buffer_size

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 ORDER BY 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.

rpl_recovery_rank

This variable is unused.

secure_auth

If the MySQL server has been started with the --secure-auth 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 ON, otherwise it is OFF.

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, “Client does not support authentication protocol”.

server_id

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

shared_memory

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

shared_memory_base_name

(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 MYSQL. The name is case sensitive.

skip_external_locking

This is OFF if mysqld uses external locking, ON if external locking is disabled.

skip_networking

This is ON 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 ON. This variable can be set to ON with the --skip-networking option.

skip_show_database

This prevents people from using the SHOW DATABASES statement if they do not have the SHOW DATABASES 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 SHOW DATABASES privilege: If the variable value is ON, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is allowed to all users, but displays the names of only those databases for which the user has the SHOW DATABASES or other privilege.

slave_compressed_protocol

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

slave_load_tmpdir

The name of the directory where the slave creates temporary files for replicating LOAD DATA INFILE statements.

slave_net_timeout

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.

slave_skip_errors

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

slave_transaction_retries

If a replication slave SQL thread fails to execute a transaction because of an InnoDB deadlock or exceeded InnoDB's innodb_lock_wait_timeout or NDBCluster's TransactionDeadlockDetectionTimeout or TransactionInactiveTimeout, it automatically retries slave_transaction_retries times before stopping with an error. The default value is 10.

slow_launch_time

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

socket

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

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

sort_buffer_size

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

sql_mode

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

sql_slave_skip_counter

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

ssl_ca

The path to a file with a list of trusted SSL CAs. This variable was added in MySQL 5.1.11.

ssl_capath

The path to a directory that contains trusted SSL CA certificates in PEM format. This variable was added in MySQL 5.1.11.

ssl_cert

The name of the SSL certificate file to use for establishing a secure connection. This variable was added in MySQL 5.1.11.

ssl_cipher

A list of allowable ciphers to use for SSL encryption. The cipher list has the same format as the openssl ciphers command. This variable was added in MySQL 5.1.11.

ssl_key

The name of the SSL key file to use for establishing a secure connection. This variable was added in MySQL 5.1.11.

storage_engine

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

sync_binlog

If the value of this variable is positive, the MySQL server synchronizes its binary log to disk (using fdatasync()) after every sync_binlog 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 sync_binlog 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.

sync_frm

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

system_time_zone

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 system_time_zone. Typically the time zone is specified by the TZ environment variable. It also can be specified using the --timezone option of the mysqld_safe script.

The system_time_zone variable differs from time_zone. 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”.

table_cache

This is the old name of table_open_cache before MySQL 5.1.3. From 5.1.3 on, use table_open_cache instead.

table_definition_cache

The number of table definitions that can be stored in the definition cache. If you use a large number of tables, you can create a large table definition cache to speed up opening of tables. The table definition cache takes less space and does not use file descriptors, unlike the normal table cache. This variable was added in MySQL 5.1.3.

table_lock_wait_timeout

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 SUPER privilege to do this).

table_open_cache

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 Opened_tables status variable. See Section 5.2.4, “Server Status Variables”. If the value of Opened_tables is large and you don't do FLUSH TABLES often (which just forces all tables to be closed and reopened), then you should increase the value of the table_open_cache variable. For more information about the table cache, see Section 7.4.8, “How MySQL Opens and Closes Tables”. Before MySQL 5.1.3, this variable is called table_cache.

table_type

This variable is a synonym for storage_engine. In MySQL 5.1, storage_engine is the preferred name.

thread_cache_size

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 thread_cache_size 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 Connections and Threads_created status variables, you can see how efficient the thread cache is. For details, see Section 5.2.4, “Server Status Variables”.

thread_concurrency

On Solaris, mysqld calls thr_setconcurrency() 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.

thread_stack

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

time_format

This variable is not implemented.

time_zone

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 'SYSTEM' (which means, “use the value of system_time_zone”). The value can be specified explicitly at server startup with the --default-time-zone option. See Section 5.11.8, “MySQL Server Time Zone Support”.

tmp_table_size

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

tmpdir

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 tmpdir 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 LOAD DATA INFILE 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 slave_load_tmpdir variable. In that case, the slave won't use the general tmpdir value and you can set tmpdir to a non-permanent location.

transaction_alloc_block_size

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

transaction_prealloc_size

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

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

tx_isolation

The default transaction isolation level. Defaults to REPEATABLE-READ.

This variable is set by the SET TRANSACTION ISOLATION LEVEL statement. See Section 13.4.6, “SET TRANSACTION Syntax”. If you set tx_isolation 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';

updatable_views_with_limit

This variable controls whether updates can be made using a view that does not contain a primary key in the underlying table, if the update contains a LIMIT clause. (Such updates often are generated by GUI tools.) An update is an UPDATE or DELETE statement. Primary key here means a PRIMARY KEY, or a UNIQUE index in which no column can contain NULL.

The variable can have two values:

version

The version number for the server.

version_bdb

The BDB storage engine version.

version_comment

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

version_compile_machine

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

version_compile_os

The type of operating system on which MySQL was built.

wait_timeout

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 wait_timeout value is initialized from the global wait_timeout value or from the global interactive_timeout value, depending on the type of client (as defined by the CLIENT_INTERACTIVE connect option to mysql_real_connect()). See also interactive_timeout.

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 sql_mode value, which is initialized when the client connects to the value of the global sql_mode value.

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

To indicate explicitly that a variable is a session variable, precede its name by SESSION, @@session., 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.

LOCAL and @@local. are synonyms for SESSION and @@session..

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

Aborted_clients

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”.

Aborted_connects

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

Binlog_cache_disk_use

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

Binlog_cache_use

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

Bytes_received

The number of bytes received from all clients.

Bytes_sent

The number of bytes sent to all clients.

Com_xxx

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

All of the Com_stmt_xxx 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 Com_stmt_xxx status variables are as follows:

Those variables stand for prepared statement commands. Their names refer to the COM_xxx 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, Com_stmt_prepare, Com_stmt_execute and Com_stmt_close also increase for PREPARE, EXECUTE, or DEALLOCATE PREPARE, respectively. Additionally, the values of the older (available since MySQL 4.1.3) statement counter variables Com_prepare_sql, Com_execute_sql, and Com_dealloc_sql increase for the PREPARE, EXECUTE, and DEALLOCATE PREPARE statements. Com_stmt_fetch stands for the total number of network round-trips issued when fetching from cursors.

Compression

Whether the client connection uses compression in the client/server protocol. Added in MySQL 5.1.2.

Connections

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

Created_tmp_disk_tables

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

Created_tmp_files

How many temporary files mysqld has created.

Created_tmp_tables

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

Delayed_errors

The number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key).

Delayed_insert_threads

The number of INSERT DELAYED handler threads in use.

Delayed_writes

The number of INSERT DELAYED rows written.

Flush_commands

The number of executed FLUSH statements.

Handler_commit

The number of internal COMMIT statements.

Handler_discover

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

Handler_delete

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

Handler_read_first

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, SELECT col1 FROM foo, assuming that col1 is indexed.

Handler_read_key

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.

Handler_read_next

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.

Handler_read_prev

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

Handler_read_rnd

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.

Handler_read_rnd_next

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.

Handler_rollback

The number of internal ROLLBACK statements.

Handler_update

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

Handler_write

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

Innodb_buffer_pool_pages_data

The number of pages containing data (dirty or clean).

Innodb_buffer_pool_pages_dirty

The number of pages currently dirty.

Innodb_buffer_pool_pages_flushed

The number of buffer pool page-flush requests.

Innodb_buffer_pool_pages_free

The number of free pages.

Innodb_buffer_pool_pages_latched

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

Innodb_buffer_pool_pages_misc

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 Innodb_buffer_pool_pages_total – Innodb_buffer_pool_pages_free – Innodb_buffer_pool_pages_data.

Innodb_buffer_pool_pages_total

The total size of the buffer pool, in pages.

Innodb_buffer_pool_read_ahead_rnd

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

Innodb_buffer_pool_read_ahead_seq

The number of sequential read-aheads initiated by InnoDB. This happens when InnoDB does a sequential full table scan.

Innodb_buffer_pool_read_requests

The number of logical read requests InnoDB has done.

Innodb_buffer_pool_reads

The number of logical reads that InnoDB could not satisfy from the buffer pool and had to do a single-page read.

Innodb_buffer_pool_wait_free

Normally, writes to the InnoDB 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.

Innodb_buffer_pool_write_requests

The number writes done to the InnoDB buffer pool.

Innodb_data_fsyncs

The number of fsync() operations so far.

Innodb_data_pending_fsyncs

The current number of pending fsync() operations.

Innodb_data_pending_reads

The current number of pending reads.

Innodb_data_pending_writes

The current number of pending writes.

Innodb_data_read

The amount of data read so far, in bytes.

Innodb_data_reads

The total number of data reads.

Innodb_data_writes

The total number of data writes.

Innodb_data_written

The amount of data written so far, in bytes.

Innodb_dblwr_writes, Innodb_dblwr_pages_written

The number of doublewrite operations that have been performed and the number of pages that have been written for this purpose. See Section 15.2.14.1, “InnoDB Disk I/O”.

Innodb_log_waits

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

Innodb_log_write_requests

The number of log write requests.

Innodb_log_writes

The number of physical writes to the log file.

Innodb_os_log_fsyncs

The number of fsync() writes done to the log file.

Innodb_os_log_pending_fsyncs

The number of pending log file fsync() operations.

Innodb_os_log_pending_writes

The number of pending log file writes.

Innodb_os_log_written

The number of bytes written to the log file.

Innodb_page_size

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

Innodb_pages_created

The number of pages created.

Innodb_pages_read

The number of pages read.

Innodb_pages_written

The number of pages written.

Innodb_row_lock_current_waits

The number of row locks currently being waited for.

Innodb_row_lock_time

The total time spent in acquiring row locks, in milliseconds.

Innodb_row_lock_time_avg

The average time to acquire a row lock, in milliseconds.

Innodb_row_lock_time_max

The maximum time to acquire a row lock, in milliseconds.

Innodb_row_lock_waits

The number of times a row lock had to be waited for.

Innodb_rows_deleted

The number of rows deleted from InnoDB tables.

Innodb_rows_inserted

The number of rows inserted into InnoDB tables.

Innodb_rows_read

The number of rows read from InnoDB tables.

Innodb_rows_updated

The number of rows updated in InnoDB tables.

Key_blocks_not_flushed

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

Key_blocks_unused

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 key_buffer_size in Section 5.2.2, “Server System Variables”.

Key_blocks_used

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.

Key_read_requests

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

Key_reads

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

Key_write_requests

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

Key_writes

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

Last_query_cost

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. The default value is 0. Last_query_cost has session scope.

Max_used_connections

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

Not_flushed_delayed_rows

The number of rows waiting to be written in INSERT DELAY queues.

Open_files

The number of files that are open.

Open_streams

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

Open_tables

The number of tables that are open.

Opened_tables

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

Qcache_free_blocks

The number of free memory blocks in the query cache.

Qcache_free_memory

The amount of free memory for the query cache.

Qcache_hits

The number of query cache hits.

Qcache_inserts

The number of queries added to the query cache.

Qcache_lowmem_prunes

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

Qcache_not_cached

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

Qcache_queries_in_cache

The number of queries registered in the query cache.

Qcache_total_blocks

The total number of blocks in the query cache.

Questions

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

Rpl_status

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

Select_full_join

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.

Select_full_range_join

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

Select_range

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.

Select_range_check

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.

Select_scan

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

Slave_open_temp_tables

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

Slave_running

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

Slave_retried_transactions

The total number of times since startup that the replication slave SQL thread has retried transactions.

Slow_launch_threads

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

Slow_queries

The number of queries that have taken more than long_query_time seconds. See Section 5.12.5, “The Slow Query Log”.

Sort_merge_passes

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 sort_buffer_size system variable.

Sort_range

The number of sorts that were done using ranges.

Sort_rows

The number of sorted rows.

Sort_scan

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

Ssl_xxx

Variables used for SSL connections.

Table_locks_immediate

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

Table_locks_waited

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.

Threads_cached

The number of threads in the thread cache.

Threads_connected

The number of currently open connections.

Threads_created

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

Threads_running

The number of threads that are not sleeping.

Uptime

The number of seconds that the server has been up.

ANSI

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

STRICT_TRANS_TABLES

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.

TRADITIONAL

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 INSERT/UPDATE 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.

ALLOW_INVALID_DATES

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 DATE and DATETIME columns. It does not apply TIMESTAMP columns, which always require a valid date.

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 '2004-04-31' are converted to '0000-00-00' and a warning is generated. With strict mode enabled, invalid dates generate an error. To allow such dates, enable ALLOW_INVALID_DATES.

ANSI_QUOTES

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 ANSI_QUOTES enabled, you cannot use double quotes to quote literal strings, because it is interpreted as an identifier.

ERROR_FOR_DIVISION_BY_ZERO

Produce an error in strict mode (otherwise a warning) when a division by zero (or MOD(X,0)) occurs during an INSERT or UPDATE. If this mode is not enabled, MySQL instead returns NULL for divisions by zero. For INSERT IGNORE or UPDATE IGNORE, MySQL generates a warning for divisions by zero, but the result of the operation is NULL.

HIGH_NOT_PRECEDENCE

The precedence of the NOT operator is such that expressions such as NOT a BETWEEN b AND c are parsed as NOT (a BETWEEN b AND c). In some older versions of MySQL, the expression was parsed as (NOT a) BETWEEN b AND c. The old higher-precedence behavior can be obtained by enabling the HIGH_NOT_PRECEDENCE SQL mode.

mysql> SET sql_mode = '';
mysql> SELECT NOT 1 BETWEEN -5 AND 5;
        -> 0
mysql> SET sql_mode = 'broken_not';
mysql> SELECT NOT 1 BETWEEN -5 AND 5;
        -> 1

IGNORE_SPACE

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 USER() function, the name of the user table in the mysql database and the User column in that table become reserved, so you must quote them:

SELECT "User" FROM mysql."user";

The IGNORE_SPACE 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 IGNORE_SPACE is enabled.

NO_AUTO_CREATE_USER

Prevent GRANT from automatically creating new users if it would otherwise do so, unless a non-empty password also is specified.

NO_AUTO_VALUE_ON_ZERO

NO_AUTO_VALUE_ON_ZERO affects handling of AUTO_INCREMENT columns. Normally, you generate the next sequence number for the column by inserting either NULL or 0 into it. NO_AUTO_VALUE_ON_ZERO suppresses this behavior for 0 so that only NULL generates the next sequence number.

This mode can be useful if 0 has been stored in a table's AUTO_INCREMENT column. (Storing 0 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 0 values, resulting in a table with contents different from the one that was dumped. Enabling NO_AUTO_VALUE_ON_ZERO before reloading the dump file solves this problem. mysqldump now automatically includes in its output a statement that enables NO_AUTO_VALUE_ON_ZERO, to avoid this problem.

NO_BACKSLASH_ESCAPES

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

NO_DIR_IN_CREATE

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

NO_ENGINE_SUBSTITUTION

Prevents automatic substitution of the default storage engine when a statement such as CREATE TABLE specifies a storage engine that is disabled or not compiled in.

NO_FIELD_OPTIONS

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

NO_KEY_OPTIONS

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

NO_TABLE_OPTIONS

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

NO_UNSIGNED_SUBTRACTION

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

mysql>t; SET sql_mode = '';
mysql>t; SELECT CAST(0 AS UNSIGNED) - 1;
+-------------------------+
| CAST(0 AS UNSIGNED) - 1 |
+-------------------------+
|    18446744073709551615 |
+-------------------------+
mysql>t; SET sql_mode = 'NO_UNSIGNED_SUBTRACTION';
mysql>t; SELECT CAST(0 AS UNSIGNED) - 1;
+-------------------------+
| CAST(0 AS UNSIGNED) - 1 |
+-------------------------+
|                      -1 |
+-------------------------+

NO_ZERO_DATE

In strict mode, don't allow '0000-00-00' as a valid date. You can still insert zero dates with the IGNORE option. When not in strict mode, the date is accepted but a warning is generated.

NO_ZERO_IN_DATE

In strict mode, don't accept dates where the month or day part is 0. If used with the IGNORE option, MySQL inserts a '0000-00-00' date for any such date. When not in strict mode, the date is accepted but a warning is generated.

ONLY_FULL_GROUP_BY

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

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

As of MySQL 5.1.11, this mode also restricts references to non-aggregated columns in the HAVING clause that are not named in the GROUP BY clause.

PIPES_AS_CONCAT

Treat || as a string concatenation operator (same as CONCAT()) rather than as a synonym for OR.

REAL_AS_FLOAT

Treat REAL as a synonym for FLOAT. By default, MySQL treats REAL as a synonym for DOUBLE.

STRICT_ALL_TABLES

Enable strict mode for all storage engines. Invalid data values are rejected. Additional detail follows.

STRICT_TRANS_TABLES

Enable strict mode for transactional storage engines, and when possible for non-transactional storage engines. Additional details follow.

For STRICT_ALL_TABLES, 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 STRICT_TRANS_TABLES, 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”.

ANSI

Equivalent to REAL_AS_FLOAT, PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE. See Section 1.9.3, “Running MySQL in ANSI Mode”.

DB2

Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.

MAXDB

Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS, NO_AUTO_CREATE_USER.

MSSQL

Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.

MYSQL323

Equivalent to NO_FIELD_OPTIONS, HIGH_NOT_PRECEDENCE.

MYSQL40

Equivalent to NO_FIELD_OPTIONS, HIGH_NOT_PRECEDENCE.

ORACLE

Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS, NO_AUTO_CREATE_USER.

POSTGRESQL

Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.

TRADITIONAL

Equivalent to STRICT_TRANS_TABLES, STRICT_ALL_TABLES, NO_ZERO_IN_DATE, NO_ZERO_DATE, ERROR_FOR_DIVISION_BY_ZERO, NO_AUTO_CREATE_USER.

--help

Display a help message and exit.

--autoclose

(NetWare only) On NetWare, mysqld_safe provides a screen presence. When you unload (shut down) the mysqld_safe NLM, the screen does not by default go away. Instead, it prompts for user input:

*<NLM has terminated; Press any key to close the screen>*

If you want NetWare to close the screen automatically instead, use the --autoclose option to mysqld_safe.

--basedir=path

The path to the MySQL installation directory.

--core-file-size=size

The size of the core file that mysqld should be able to create. The option value is passed to ulimit -c.

--datadir=path

The path to the data directory.

--defaults-extra-file=path

The name of an option file to be read in addition to the usual option files. This must be the first option on the command line if it is used. If the file does not exist or is otherwise inaccessible, the server will exit with an error.

--defaults-file=file_name

The name of an option file to be read instead of the usual option files. This must be the first option on the command line if it is used.

--ledir=path

If mysqld_safe cannot find the server, use this option to indicate the pathname to the directory where the server is located.

--log-error=file_name

Write the error log to the given file. See Section 5.12.2, “The Error Log”.

--mysqld=prog_name

The name of the server program (in the ledir directory) that you want to start. This option is needed if you use the MySQL binary distribution but have the data directory outside of the binary distribution. If mysqld_safe cannot find the server, use the --ledir option to indicate the pathname to the directory where the server is located.

--mysqld-version=suffix

This option is similar to the --mysqld option, but you specify only the suffix for the server program name. The basename is assumed to be mysqld. For example, if you use --mysqld-version=debug, mysqld_safe starts the mysqld-debug program in the ledir directory. If the argument to --mysqld-version is empty, mysqld_safe uses mysqld in the ledir directory.

--nice=priority

Use the nice program to set the server's scheduling priority to the given value.

--no-defaults

Do not read any option files. This must be the first option on the command line if it is used.

--open-files-limit=count

The number of files that mysqld should be able to open. The option value is passed to ulimit -n. Note that you need to start mysqld_safe as root for this to work properly!

--pid-file=file_name

The pathname of the process ID file.

--port=port_num

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

--socket=path

The Unix socket file that the server should use when listening for local connections.

--timezone=timezone

Set the TZ time zone environment variable to the given option value. Consult your operating system documentation for legal time zone specification formats.

--user={user_name|user_id}

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

The server and databases can be found relative to the working directory (the directory from which mysqld_safe is invoked). For binary distributions, mysqld_safe looks under its working directory for bin and data directories. For source distributions, it looks for libexec and var directories. This condition should be met if you execute mysqld_safe from your MySQL installation directory (for example, /usr/local/mysql for a binary distribution).

If the server and databases cannot be found relative to the working directory, mysqld_safe attempts to locate them by absolute pathnames. Typical locations are /usr/local/libexec and /usr/local/var. The actual locations are determined from the values configured into the distribution at the time it was built. They should be correct if MySQL is installed in the location specified at configuration time.

--help

Display a help message and exit.

--config-file=file_name

Specify the name of an alternative option file. This affects where mysqld_multi looks for [mysqldN] option groups. Without this option, all options are read from the usual my.cnf file. The option does not affect where mysqld_multi reads its own options, which are always taken from the [mysqld_multi] group in the usual my.cnf file.

--example

Display a sample option file.

--log=file_name

Specify the name of the log file. If the file exists, log output is appended to it.

--mysqladmin=prog_name

The mysqladmin binary to be used to stop servers.

--mysqld=prog_name

The mysqld binary to be used. Note that you can specify mysqld_safe as the value for this option also. If you use mysqld_safe to start the server, you can include the mysqld or ledir options in the corresponding [mysqldN] option group. These options indicate the name of the server that mysqld_safe should start and the pathname of the directory where the server is located. (See the descriptions for these options in Section 5.4.1, “mysqld_safe — MySQL Server Startup Script”.) Example:

[mysqld38]
mysqld = mysqld-debug
ledir  = /opt/local/mysql/libexec

--no-log

Print log information to stdout rather than to the log file. By default, output goes to the log file.

--password=password

The password of the MySQL account to use when invoking mysqladmin. Note that the password value is not optional for this option, unlike for other MySQL programs.

--silent

Silent mode; disable warnings.

--tcp-ip

Connect to each MySQL server via the TCP/IP port instead of the Unix socket file. (If a socket file is missing, the server might still be running, but accessible only via the TCP/IP port.) By default, connections are made using the Unix socket file. This option affects stop and report operations.

--user=user_name

The username of the MySQL account to use when invoking mysqladmin.

--verbose

Be more verbose.

--version

Display version information and exit.

Most important: Before using mysqld_multi be sure that you understand the meanings of the options that are passed to the mysqld servers and why you would want to have separate mysqld processes. Beware of the dangers of using multiple mysqld servers with the same data directory. Use separate data directories, unless you know what you are doing. Starting multiple servers with the same data directory does not give you extra performance in a threaded system. See Section 5.13, “Running Multiple MySQL Servers on the Same Machine”.

Important: Make sure that the data directory for each server is fully accessible to the Unix account that the specific mysqld process is started as. Do not use the Unix root account for this, unless you know what you are doing. See Section 5.7.5, “How to Run MySQL as a Normal User”.

Make sure that the MySQL account used for stopping the mysqld servers (with the mysqladmin program) has the same username and password for each server. Also, make sure that the account has the SHUTDOWN privilege. If the servers that you want to manage have different usernames or passwords for the administrative accounts, you might want to create an account on each server that has the same username and password. For example, you might set up a common multi_admin account by executing the following commands for each server:

shell> mysql -u root -S /tmp/mysql.sock -p
Enter password:
mysql> GRANT SHUTDOWN ON *.*
    -> TO 'multi_admin'@'localhost' IDENTIFIED BY 'multipass';

See Section 5.8.2, “How the Privilege System Works”. You have to do this for each mysqld server. Change the connection parameters appropriately when connecting to each one. Note that the hostname part of the account name must allow you to connect as multi_admin from the host where you want to run mysqld_multi.

The Unix socket file and the TCP/IP port number must be different for every mysqld.

The --pid-file option is very important if you are using mysqld_safe to start mysqld (for example, --mysqld=mysqld_safe) Every mysqld should have its own process ID file. The advantage of using mysqld_safe instead of mysqld is that mysqld_safe monitors its mysqld process and restarts it if the process terminates due to a signal sent using kill -9 or for other reasons, such as a segmentation fault. Please note that the mysqld_safe script might require that you start it from a certain place. This means that you might have to change location to a certain directory before running mysqld_multi. If you have problems starting, please see the mysqld_safe script. Check especially the lines:

----------------------------------------------------------------
MY_PWD=`pwd`
# Check if we are starting this relative (for the binary release)
if test -d $MY_PWD/data/mysql -a -f ./share/mysql/english/errmsg.sys -a \
 -x ./bin/mysqld
----------------------------------------------------------------

The test performed by these lines should be successful, or you might encounter problems. See Section 5.4.1, “mysqld_safe — MySQL Server Startup Script”.

You might want to use the --user option for mysqld, but to do this you need to run the mysqld_multi script as the Unix root user. Having the option in the option file doesn't matter; you just get a warning if you are not the superuser and the mysqld processes are started under your own Unix account.

--help, -?

Display a help message and exit.

--angel-pid-file=file_name

The file in which the angel process records its process ID when mysqlmanager runs in daemon mode. By default, this file is named mysqlmanager.angel.pid. If the --pid-file option is given, the default angel PID file becomes the same except that any extension is replaced with an extension of .angel.pid. This option was added in MySQL 5.1.11.

--bind-address=IP

The IP address to bind to.

--debug=debug_options, -# debug_options

Write a debugging log. The debug_options string often is 'd:t:o,file_name'. This option was added in MySQL 5.1.10.

--default-mysqld-path=path

On Unix, the pathname of the MySQL Server binary, if no path was provided in the instance section. Example: --default-mysqld-path=/usr/sbin/mysqld

--defaults-file=file_name

Read Instance Manager and MySQL Server settings from the given file. All configuration changes by the Instance Manager will be made to this file. This must be the first option on the command line if it is used.

--install

On Windows, install Instance Manager as a Windows service.

--log=file_name

The path to the IM log file. This is used with the --run-as-service option.

--monitoring-interval=seconds

The interval in seconds for monitoring instances. The default value is 20 seconds. Instance Manager tries to connect to each monitored instance using the non-existing MySQL_Instance_Manager user account to check whether it is alive/not hanging. In the case of a failure to connect, IM performs several attempts to restart the instance. The nonguarded option in the appropriate instance section disables this behavior for a particular instance. The monitoring process will produce messages in the general query log similar to the following:

Access denied for user 'MySQL_Instance_M'@'localhost' (using password: YES)

--passwd, -P

Prepare an entry for the password file and exit.

--password-file=file_name

Look for the Instance Manager users and passwords in this file. The default file is /etc/mysqlmanager.passwd.

--pid-file=file_name

The process ID file to use. By default, this file is named mysqlmanager.pid.

--port=port_num

The TCP/IP port number to use for incoming connections. (The default port number assigned by IANA is 2273).

--print-defaults

Print the current defaults and exit. This must be the first option on the command line if it is used.

--remove

On Windows, removes Instance Manager as a Windows service. This assumes that Instance Manager has been run with --install previously.

--run-as-service

On Unix, daemonize and start the angel process. The angel process is simple and unlikely to crash. It will restart the Instance Manager itself in case of a failure.

--socket=path

On Unix, the socket file to use for incoming connections. By default, the file is named /tmp/mysqlmanager.sock.

--standalone

On Windows, run Instance Manager in standalone mode.

--user=user_name

On Unix, the username to start and run the mysqlmanager under. It is recommended to run mysqlmanager under the same user account used to run the mysqld server. (“User” in this context refers to a system login account, not a MySQL user listed in the grant tables.)

--version, -V

Output version information and exit.

--wait-timeout=N

The number of seconds to wait for activity on a connection befoe closing it. The default is 28800 seconds (8 hours).

This option was added in MySQL 5.1.7. Before that, the timeout is 30 seconds and cannot be changed.

mysqld-path = path

The pathname to the mysqld server binary.

shutdown-delay = seconds

The number of seconds IM should wait for the instance to shut down. The default value is 35 seconds. After the delay expires, the IM assumes that the instance is hanging and attempts to terminate it. If you use InnoDB with large tables, you should increase this value.

nonguarded

This option should be specified if you want to disable IM monitoring functionality for a certain instance.

START INSTANCE instance_name

This command attempts to start an instance.

mysql> START INSTANCE mysqld4;
Query OK, 0 rows affected (0,00 sec)

STOP INSTANCE instance_name

This command attempts to stop an instance.

mysql> STOP INSTANCE mysqld4;
Query OK, 0 rows affected (0,00 sec)

SHOW INSTANCES

Shows the names of all loaded instances.

mysql> SHOW INSTANCES;
+---------------+---------+
| instance_name | status  |
+---------------+---------+
| mysqld3       | offline |
| mysqld4       | online  |
| mysqld2       | offline |
+---------------+---------+
3 rows in set (0,04 sec)

SHOW INSTANCE STATUS instance_name

Shows the status and the version information for an instance.

mysql> SHOW INSTANCE STATUS mysqld3;
+---------------+--------+---------+
| instance_name | status | version |
+---------------+--------+---------+
| mysqld3       | online | unknown |
+---------------+--------+---------+
1 row in set (0.00 sec)

SHOW INSTANCE OPTIONS instance_name

Shows the options used by an instance.

mysql> SHOW INSTANCE OPTIONS mysqld3;
+---------------+---------------------------------------------------+
| option_name   | value                                             |
+---------------+---------------------------------------------------+
| instance_name | mysqld3                                           |
| mysqld-path   | /home/cps/mysql/trees/mysql-4.1/sql/mysqld        |
| port          | 3309                                              |
| socket        | /tmp/mysql.sock3                                  |
| pid-file      | hostname.pid3                                     |
| datadir       | /home/cps/mysql_data/data_dir1/                   |
| language      | /home/cps/mysql/trees/mysql-4.1/sql/share/english |
+---------------+---------------------------------------------------+
7 rows in set (0.01 sec)

SHOW instance_name LOG FILES

The command lists all log files used by the instance. The result set contains the path to the log file and the log file size. If no log file path is specified in the configuration file (for example, log=/var/mysql.log), the Instance Manager tries to guess its placement. If the IM is unable to guess the logfile placement you should specify the log file location explicitly by using the appropriate log option in the instance section of the configuration file.

mysql> SHOW mysqld LOG FILES;
+-------------+------------------------------------+----------+
| Logfile     | Path                               | Filesize |
+-------------+------------------------------------+----------+
| ERROR LOG   | /home/cps/var/mysql/owlet.err      | 9186     |
| GENERAL LOG | /home/cps/var/mysql/owlet.log      | 471503   |
| SLOW LOG    | /home/cps/var/mysql/owlet-slow.log | 4463     |
+-------------+------------------------------------+----------+
3 rows in set (0.01 sec)

SHOW instance_name LOG {ERROR | SLOW | GENERAL} size[,offset_from_end]

This command retrieves a portion of the specified log file. Because most users are interested in the latest log messages, the size parameter defines the number of bytes you would like to retrieve starting from the log end. You can retrieve data from the middle of the log file by specifying the optional offset_from_end parameter. The following example retrieves 21 bytes of data, starting 23 bytes from the end of the log file and ending 2 bytes from the end of the log file:

mysql> SHOW mysqld LOG GENERAL 21, 2;
+---------------------+
| Log                 |
+---------------------+
| using password: YES |
+---------------------+
1 row in set (0.00 sec)

SET instance_name.option_name=option_value

This command edits the specified instance's configuration file to change or add instance options. The IM assumes that the configuration file is located at /etc/my.cnf. You should check that the file exists and has appropriate permissions.

mysql> SET mysqld2.port=3322;
Query OK, 0 rows affected (0.00 sec)

Changes made to the configuration file do not take effect until the MySQL server is restarted. In addition, these changes are not stored in the instance manager's local cache of instance settings until a FLUSH INSTANCES command is executed.

UNSET instance_name.option_name

This command removes an option from an instance's configuration file.

mysql> UNSET mysqld2.port;
Query OK, 0 rows affected (0.00 sec)

Changes made to the configuration file do not take effect until the MySQL server is restarted. In addition, these changes are not stored in the instance manager's local cache of instance settings until a FLUSH INSTANCES command is executed.

FLUSH INSTANCES

This command forces IM to reread the configuration file and to refresh internal structures. This command should be performed after editing the configuration file. The command does not restart instances.

mysql> FLUSH INSTANCES;
Query OK, 0 rows affected (0.04 sec)

--help

Display a short help message and exit.

--basedir=path

The path to the MySQL installation directory.

--datadir=path

The path to the data directory.

--force

Force execution of mysqlcheck even if mysql_upgrade has already been executed for the current version of MySQL. (In other words, this option causes the mysql_upgrade.info file to be ignored.)

--user=user_name, -u user_name

The MySQL username to use when connecting to the server. The default username is root.

--verbose

Verbose mode. Print more information about what the program does.

Do not ever give anyone (except MySQL root accounts) access to the user table in the mysql database! This is critical.

Learn the MySQL access privilege system. The GRANT and REVOKE statements are used for controlling access to MySQL. Do not grant more privileges than necessary. Never grant privileges to all hosts.

Checklist:

Do not store any plain-text passwords in your database. If your computer becomes compromised, the intruder can take the full list of passwords and use them. Instead, use MD5(), SHA1(), or some other one-way hashing function and store the hash value.

Do not choose passwords from dictionaries. Special programs exist to break passwords. Even passwords like “xfish98” are very bad. Much better is “duag98” which contains the same word “fish” but typed one key to the left on a standard QWERTY keyboard. Another method is to use a password that is taken from the first characters of each word in a sentence (for example, “Mary had a little lamb” results in a password of “Mhall”). The password is easy to remember and type, but difficult to guess for someone who does not know the sentence.

Invest in a firewall. This protects you from at least 50% of all types of exploits in any software. Put MySQL behind the firewall or in a demilitarized zone (DMZ).

Checklist:

Do not trust any data entered by users of your applications. They can try to trick your code by entering special or escaped character sequences in Web forms, URLs, or whatever application you have built. Be sure that your application remains secure if a user enters something like “; DROP DATABASE mysql;”. This is an extreme example, but large security leaks and data loss might occur as a result of hackers using similar techniques, if you do not prepare for them.

A common mistake is to protect only string data values. Remember to check numeric data as well. If an application generates a query such as SELECT * FROM table WHERE ID=234 when a user enters the value 234, the user can enter the value 234 OR 1=1 to cause the application to generate the query SELECT * FROM table WHERE ID=234 OR 1=1. As a result, the server retrieves every row in the table. This exposes every row and causes excessive server load. The simplest way to protect from this type of attack is to use single quotes around the numeric constants: SELECT * FROM table WHERE ID='234'. If the user enters extra information, it all becomes part of the string. In a numeric context, MySQL automatically converts this string to a number and strips any trailing non-numeric characters from it.

Sometimes people think that if a database contains only publicly available data, it need not be protected. This is incorrect. Even if it is allowable to display any row in the database, you should still protect against denial of service attacks (for example, those that are based on the technique in the preceding paragraph that causes the server to waste resources). Otherwise, your server becomes unresponsive to legitimate users.

Checklist:

Many application programming interfaces provide a means of escaping special characters in data values. Properly used, this prevents application users from entering values that cause the application to generate statements that have a different effect than you intend:

Other programming interfaces might have similar capabilities.

Do not transmit plain (unencrypted) data over the Internet. This information is accessible to everyone who has the time and ability to intercept it and use it for their own purposes. Instead, use an encrypted protocol such as SSL or SSH. MySQL supports internal SSL connections as of version 4.0. Another technique is to use SSH port-forwarding to create an encrypted (and compressed) tunnel for the communication.

Learn to use the tcpdump and strings utilities. In most cases, you can check whether MySQL data streams are unencrypted by issuing a command like the following:

shell> tcpdump -l -i eth0 -w - src or dst port 3306 | strings

(This works under Linux and should work with small modifications under other systems.) Warning: If you do not see plaintext data, this doesn't always mean that the information actually is encrypted. If you need high security, you should consult with a security expert.

Require all MySQL accounts to have a password. A client program does not necessarily know the identity of the person running it. It is common for client/server applications that the user can specify any username to the client program. For example, anyone can use the mysql program to connect as any other person simply by invoking it as mysql -u other_user db_name if other_user has no password. If all account have a password, connecting using another user's account becomes much more difficult.

For a discussion of methods for setting passwords, see Section 5.9.5, “Assigning Account Passwords”.

Never run the MySQL server as the Unix root user. This is extremely dangerous, because any user with the FILE privilege is able to cause the server to create files as root (for example, ~root/.bashrc). To prevent this, mysqld refuses to run as root unless that is specified explicitly using the --user=root option.

mysqld can (and should) be run as an ordinary, unprivileged user instead. You can create a separate Unix account named mysql to make everything even more secure. Use this account only for administering MySQL. To start mysqld as a different Unix user, add a user option that specifies the username in the [mysqld] group of the my.cnf option file where you specify server options. For example:

[mysqld]
user=mysql

This causes the server to start as the designated user whether you start it manually or by using mysqld_safe or mysql.server. For more details, see Section 5.7.5, “How to Run MySQL as a Normal User”.

Running mysqld as a Unix user other than root does not mean that you need to change the root username in the user table. Usernames for MySQL accounts have nothing to do with usernames for Unix accounts.

Do not allow the use of symlinks to tables. (This capability can be disabled with the --skip-symbolic-links option.) This is especially important if you run mysqld as root, because anyone that has write access to the server's data directory then could delete any file in the system! See Section 7.6.1.2, “Using Symbolic Links for Tables on Unix”.

Make sure that the only Unix user with read or write privileges in the database directories is the user that mysqld runs as.

Do not grant the PROCESS or SUPER privilege to non-administrative users. The output of mysqladmin processlist and SHOW PROCESSLIST shows the text of any statements currently being executed, so any user who is allowed to see the server process list might be able to see statements issued by other users such as UPDATE user SET password=PASSWORD('not_secure').

mysqld reserves an extra connection for users who have the SUPER privilege, so that a MySQL root user can log in and check server activity even if all normal connections are in use.

The SUPER privilege can be used to terminate client connections, change server operation by changing the value of system variables, and control replication servers.

Do not grant the FILE privilege to non-administrative users. Any user that has this privilege can write a file anywhere in the filesystem with the privileges of the mysqld daemon. To make this a bit safer, files generated with SELECT ... INTO OUTFILE do not overwrite existing files and are writable by everyone.

The FILE privilege may also be used to read any file that is world-readable or accessible to the Unix user that the server runs as. With this privilege, you can read any file into a database table. This could be abused, for example, by using LOAD DATA to load /etc/passwd into a table, which then can be displayed with SELECT.

If you do not trust your DNS, you should use IP numbers rather than hostnames in the grant tables. In any case, you should be very careful about creating grant table entries using hostname values that contain wildcards.

If you want to restrict the number of connections allowed to a single account, you can do so by setting the max_user_connections variable in mysqld. The GRANT statement also supports resource control options for limiting the extent of server use allowed to an account. See Section 13.5.1.3, “GRANT Syntax”.

--allow-suspicious-udfs

This option controls whether user-defined functions that have only an xxx 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. See Section 28.3.4.6, “User-Defined Function Security Precautions”.

--local-infile[={0|1}]

If you start the server with --local-infile=0, clients cannot use LOCAL in LOAD DATA statements. See Section 5.7.4, “Security Issues with LOAD DATA LOCAL”.

--old-passwords

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”.

--safe-show-database (OBSOLETE)

In previous versions of MySQL, this option caused the SHOW DATABASES statement to display the names of only those databases for which the user had some kind of privilege. In MySQL 5.1, this option is no longer available as this is now the default behavior, and there is a SHOW DATABASES privilege that can be used to control access to database names on a per-account basis. See Section 13.5.1.3, “GRANT Syntax”.

--safe-user-create

If this option is enabled, a user cannot create new MySQL users by using the GRANT statement unless the user has the INSERT privilege for the mysql.user table. If you want a user to have the ability to create new users that have those privileges that the user has right to grant, you should grant the user the following privilege:

GRANT INSERT(user) ON mysql.user TO 'user_name'@'host_name';

This ensures that the user cannot change any privilege columns directly, but has to use the GRANT statement to give privileges to other users.

--secure-auth

Disallow authentication for accounts that have old (pre-4.1) passwords.

The mysql client also has a --secure-auth option, which prevents connections to a server if the server requires a password in old format for the client account.

--skip-grant-tables

This option causes the server not to use the privilege system at all. This 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 FLUSH PRIVILEGES statement. This option also suppresses loading of plugins and user-defined functions (UDFs).

--skip-name-resolve

Hostnames are not resolved. All Host column values in the grant tables must be IP numbers or localhost.

--skip-networking

Do not allow TCP/IP connections over the network. All connections to mysqld must be made via Unix socket files.

--skip-show-database

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

--ssl*

Options that begin with --ssl 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”.

The transfer of the file from the client host to the server host is initiated by the MySQL server. In theory, a patched server could be built that would tell the client program to transfer a file of the server's choosing rather than the file named by the client in the LOAD DATA statement. Such a server could access any file on the client host to which the client user has read access.

In a Web environment where the clients are connecting from a Web server, a user could use LOAD DATA LOCAL to read any files that the Web server process has read access to (assuming that a user could run any command against the SQL server). In this environment, the client with respect to the MySQL server actually is the Web server, not the remote program being run by the user who connects to the Web server.

By default, all MySQL clients and libraries in binary distributions are compiled with the --enable-local-infile option, to be compatible with MySQL 3.23.48 and before.

If you build MySQL from source but do not invoke configure with the --enable-local-infile option, LOAD DATA LOCAL cannot be used by any client unless it is written explicitly to invoke mysql_options(... MYSQL_OPT_LOCAL_INFILE, 0). See Section 26.2.3.48, “mysql_options()”.

You can disable all LOAD DATA LOCAL commands from the server side by starting mysqld with the --local-infile=0 option.

For the mysql command-line client, LOAD DATA LOCAL can be enabled by specifying the --local-infile[=1] option, or disabled with the --local-infile=0 option. Similarly, for mysqlimport, the --local or -L option enables local data file loading. In any case, successful use of a local loading operation requires that the server is enabled to allow it.

If you use LOAD DATA LOCAL in Perl scripts or other programs that read the [client] group from option files, you can add the local-infile=1 option to that group. However, to keep this from causing problems for programs that do not understand local-infile, specify it using the loose- prefix:

[client]
loose-local-infile=1

If LOAD DATA LOCAL INFILE is disabled, either in the server or the client, a client that attempts to issue such a statement receives the following error message:

ERROR 1148: The used command is not allowed with this MySQL version