5.3 The mysql System Database

The mysql database is the system database. It contains tables that store information required by the MySQL server as it runs.

Tables in the mysql database fall into these categories:

The remainder of this section enumerates the tables in each category, with cross references for additional information. System tables use the MyISAM storage engine unless otherwise indicated.

Warning

Do not convert MySQL system tables in the mysql database from MyISAM to InnoDB tables. This is an unsupported operation. If you do this, MySQL does not restart until you restore the old system tables from a backup or regenerate them by reinitializing the data directory (see Section 2.9.1, “Initializing the Data Directory”).

Grant System Tables

These system tables contain grant information about user accounts and the privileges held by them:

user: User accounts, global privileges, and other nonprivilege columns.
db: Database-level privileges.
tables_priv: Table-level privileges.
columns_priv: Column-level privileges.
procs_priv: Stored procedure and function privileges.
proxies_priv: Proxy-user privileges.

For more information about the structure, contents, and purpose of the grant tables, see Section 6.2.3, “Grant Tables”.

Object Information System Tables

These system tables contain information about stored programs, loadable functions, and server-side plugins:

event: The registry for Event Scheduler events installed using CREATE EVENT. If the server is started with the --skip-grant-tables option, the event scheduler is disabled and events registered in the table do not run. See Section 23.4.2, “Event Scheduler Configuration”.
func: The registry for loadable functions installed using CREATE FUNCTION. During the normal startup sequence, the server loads functions registered in this table. If the server is started with the --skip-grant-tables option, functions registered in the table are not loaded and are unavailable. See Section 5.6.1, “Installing and Uninstalling Loadable Functions”.
plugin: The registry for server-side plugins installed using INSTALL PLUGIN. During the normal startup sequence, the server loads plugins registered in this table. If the server is started with the --skip-grant-tables option, plugins registered in the table are not loaded and are unavailable. See Section 5.5.1, “Installing and Uninstalling Plugins”.

The plugin table uses the InnoDB storage engine as of MySQL 5.7.6, MyISAM before that.
proc: Information about stored procedures and functions. See Section 23.2, “Using Stored Routines”.

Log System Tables

The server uses these system tables for logging:

general_log: The general query log table.
slow_log: The slow query log table.

Log tables use the CSV storage engine.

For more information, see Section 5.4, “MySQL Server Logs”.

Server-Side Help System Tables

These system tables contain server-side help information:

help_category: Information about help categories.
help_keyword: Keywords associated with help topics.
help_relation: Mappings between help keywords and topics.
help_topic: Help topic contents.

These tables use the InnoDB storage engine as of MySQL 5.7.5, MyISAM before that.

For more information, see Section 5.1.14, “Server-Side Help Support”.

Time Zone System Tables

These system tables contain time zone information:

time_zone: Time zone IDs and whether they use leap seconds.
time_zone_leap_second: When leap seconds occur.
time_zone_name: Mappings between time zone IDs and names.
time_zone_transition, time_zone_transition_type: Time zone descriptions.

These tables use the InnoDB storage engine as of MySQL 5.7.5, MyISAM before that.

For more information, see Section 5.1.13, “MySQL Server Time Zone Support”.

Replication System Tables

The server uses these system tables to support replication:

gtid_executed: Table for storing GTID values. See mysql.gtid_executed Table.

The gtid_executed table uses the InnoDB storage engine.
ndb_binlog_index: Binary log information for NDB Cluster replication. See Section 21.7.4, “NDB Cluster Replication Schema and Tables”.

Prior to NDB 7.5.2, this table employed the MyISAM storage engine. In NDB 7.5.2 and later, it uses InnoDB. If you are planning an upgrade from a NDB Cluster previous release to NDB 7.5.2 or later, see Section 21.3.7, “Upgrading and Downgrading NDB Cluster”, for important information relating to this change.
slave_master_info, slave_relay_log_info, slave_worker_info: Used to store replication information on replica servers. See Section 16.2.4, “Relay Log and Replication Metadata Repositories”.

All three of these tables use the InnoDB storage engine.

Optimizer System Tables

These system tables are for use by the optimizer:

innodb_index_stats, innodb_table_stats: Used for InnoDB persistent optimizer statistics. See Section 14.8.11.1, “Configuring Persistent Optimizer Statistics Parameters”.
server_cost, engine_cost: The optimizer cost model uses tables that contain cost estimate information about operations that occur during query execution. server_cost contains optimizer cost estimates for general server operations. engine_cost contains estimates for operations specific to particular storage engines. See Section 8.9.5, “The Optimizer Cost Model”.

These tables use the InnoDB storage engine.

Miscellaneous System Tables

Other system tables do not fall into the preceding categories:

audit_log_filter, audit_log_user: If MySQL Enterprise Audit is installed, these tables provide persistent storage of audit log filter definitions and user accounts. See Audit Log Tables.
firewall_users, firewall_whitelist: If MySQL Enterprise Firewall is installed, these tables provide persistent storage for information used by the firewall. See Section 6.4.6, “MySQL Enterprise Firewall”.
servers: Used by the FEDERATED storage engine. See Section 15.8.2.2, “Creating a FEDERATED Table Using CREATE SERVER”.

The servers table uses the InnoDB storage engine as of MySQL 5.7.6, MyISAM before that.