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MySQL 8.0 Reference Manual  /  MySQL Server Administration  /  The mysql System Database

Pre-General Availability Draft: 2017-05-23

6.3 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. A broad categorization is that the mysql database contains data dictionary tables that store database object metadata, and system tables used for other operational purposes. The following discussion further subdivides the set of system tables into smaller categories.

The remainder of this section enumerates the tables in each category, with cross references for additional information. Data dictionary tables and system tables use the InnoDB storage engine unless otherwise indicated.

Data Dictionary Tables

These tables comprise the data dictionary, which contains metadata about database objects. For additional information, see Chapter 15, MySQL Data Dictionary.

Important

The data dictionary is new in MySQL 8.0. A data dictionary-enabled server entails some general operational differences compared to previous MySQL releases. For details, see Section 15.6, “Data Dictionary Usage Differences”. Also, for upgrades to MySQL 8.0 from MySQL 5.7, the upgrade procedure differs somewhat from previous MySQL releases and requires that you verify the upgrade readiness of your installation by checking specific prerequisites. For more information, see Section 2.10.1, “Upgrading MySQL”, particularly Verifying Upgrade Prerequisites for Your MySQL 5.7 Installation.

  • catalogs: Catalog information.

  • character_sets: Information about available character sets.

  • collations: Information about collations for each character set.

  • column_type_elements: Information about types used by columns.

  • columns: Information about columns in tables.

  • events: Information about Event Scheduler events. See Section 22.4, “Using the Event Scheduler”. The server loads events listed in this table during its startup sequence, unless started with the --skip-grant-tables option.

  • foreign_keys, foreign_key_column_usage: Information about foreign keys.

  • index_column_usage: Information about columns used by indexes.

  • index_partitions: Information about partitions used by indexes.

  • index_stats: Used to store dynamic index statistics generated when ANALYZE TABLE is executed.

  • indexes: Information about table indexes.

  • parameter_type_elements: Information about stored procedure and function parameters, and about return values for stored functions.

  • parameters: Information about stored procedures and functions. See Section 22.2, “Using Stored Routines (Procedures and Functions)”.

  • routines: Information about stored procedures and functions. See Section 22.2, “Using Stored Routines (Procedures and Functions)”.

  • schemata: Information about schemata. In MySQL, a schema is a database, so this table provides information about databases.

  • st_spatial_reference_systems: Information about available spatial reference systems for spatial data.

  • table_partition_values: Information about values used by table partitions.

  • table_partitions: Information about partitions used by tables.

  • table_stats: Information about dynamic table statistics generated when ANALYZE TABLE is executed.

  • tables: Information about tables in databases.

  • tablespace_files: Information about files used by tablespaces.

  • tablespaces: Information about active tablespaces.

  • triggers: Information about triggers.

  • version: A single-row table that identifies the data dictionary version. The server uses this to determine whether the data dictionary must be upgraded to a newer version.

  • view_routine_usage: Information about dependencies between views and stored functions used by them.

  • view_table_usage: Used to track dependencies between views and their underlying tables.

Data dictionary tables are invisible. They cannot be read with SELECT, do not appear in the output of SHOW TABLES, are not listed in the INFORMATION_SCHEMA.TABLES table, and so forth. However, in most cases there are corresponding INFORMATION_SCHEMA tables that can be queried. Conceptually, the INFORMATION_SCHEMA provides a view through which MySQL exposes data dictionary metadata. For example, you cannot select from the mysql.schemata table directly:

mysql> SELECT * FROM mysql.schemata;
ERROR 3554 (HY000): Access to data dictionary table 'mysql.schemata' is rejected.

Instead, select that information from the corresponding INFORMATION_SCHEMA table:

mysql> SELECT * FROM INFORMATION_SCHEMA.SCHEMATA\G
*************************** 1. row ***************************
              CATALOG_NAME: def
               SCHEMA_NAME: mysql
DEFAULT_CHARACTER_SET_NAME: utf8mb4
    DEFAULT_COLLATION_NAME: utf8mb4_0900_ai_ci
                  SQL_PATH: NULL
*************************** 2. row ***************************
              CATALOG_NAME: def
               SCHEMA_NAME: information_schema
DEFAULT_CHARACTER_SET_NAME: utf8
    DEFAULT_COLLATION_NAME: utf8_general_ci
                  SQL_PATH: NULL
...

There is no INFORMATION_SCHEMA table that corresponds exactly to mysql.indexes, but INFORMATION_SCHEMA.STATISTICS contains much of the same information.

As of yet, there are no INFORMATION_SCHEMA tables that correspond exactly to mysql.foreign_keys, mysql.foreign_key_column_usage. The standard SQL way to obtain foreign key information is by using the INFORMATION_SCHEMA REFERENTIAL_CONSTRAINTS and KEY_COLUMN_USAGE tables; these tables are now implemented as views on the foreign_keys, foreign_key_column_usage, and other data dictionary tables.

Some system tables from before MySQL 8.0 have been replaced by data dictionary tables and are no longer present in the mysql system database:

  • The events data dictionary table supersedes the event table from before MySQL 8.0.

  • The parameters and routines data dictionary tables together supersede the proc table from before MySQL 8.0.

Grant System Tables

These system tables contain grant information about user accounts and the privileges held by them. For additional information about the structure, contents, and purpose of the these tables, see Section 7.2.3, “Grant Tables”.

As of MySQL 8.0, the grant tables are InnoDB (transactional) tables. Previously, these were MyISAM (nontransactional) tables. The change of grant-table storage engine underlies an accompanying change in MySQL 8.0 to the behavior of account-management statements such as CREATE USER and GRANT. Previously, an account-management statement that named multiple users could succeed for some users and fail for others. The statements are now transactional and either succeed for all named users or roll back and have no effect if any error occurs.

Note

If MySQL is upgraded from an older version but the grant tables have not been upgraded from MyISAM to InnoDB, the server considers them read only and account-management statements produce an error. For upgrade instructions, see Section 2.10.1, “Upgrading MySQL”.

  • user: User accounts, global privileges, and other non-privilege columns.

  • global_grants: Assignments of dynamic global privileges to users; see Section 7.2.2, “Static Versus Dynamic Privileges”.

  • 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.

  • default_roles: This table lists default roles to be activated after a user connects and authenticates, or executes SET ROLE DEFAULT.

  • role_edges: This table lists edges for role subgraphs.

    A given user table row might refer to a user account or a role. The server can distinquish whether a row represents a user account, a role, or both by consulting the role_edges table for information about relations between authentication IDs.

Object Information System Tables

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

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 6.4, “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.

For more information, see Section 6.1.10, “Server-Side Help”.

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.

For more information, see Section 11.6, “MySQL Server Time Zone Support”.

Replication System Tables

The server uses these system tables to support replication:

Optimizer System Tables

These system tables are for use by the optimizer:

Miscellaneous System Tables

Other system tables do not fit the preceding categories:


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