28.3.34 The INFORMATION_SCHEMA STATISTICS Table

The STATISTICS table provides information about table indexes.

Columns in STATISTICS that represent table statistics hold cached values. The information_schema_stats_expiry system variable defines the period of time before cached table statistics expire. The default is 86400 seconds (24 hours). If there are no cached statistics or statistics have expired, statistics are retrieved from storage engines when querying table statistics columns. To update cached values at any time for a given table, use ANALYZE TABLE. To always retrieve the latest statistics directly from storage engines, set information_schema_stats_expiry=0. For more information, see Section 10.2.3, “Optimizing INFORMATION_SCHEMA Queries”.

Note

If the innodb_read_only system variable is enabled, ANALYZE TABLE may fail because it cannot update statistics tables in the data dictionary, which use InnoDB. For ANALYZE TABLE operations that update the key distribution, failure may occur even if the operation updates the table itself (for example, if it is a MyISAM table). To obtain the updated distribution statistics, set information_schema_stats_expiry=0.

The STATISTICS table has these columns:

• TABLE_CATALOG

  The name of the catalog to which the table containing the index belongs. This value is always def.

• TABLE_SCHEMA

  The name of the schema (database) to which the table containing the index belongs.

• TABLE_NAME

  The name of the table containing the index.

• NON_UNIQUE

  0 if the index cannot contain duplicates, 1 if it can.

• INDEX_SCHEMA

  The name of the schema (database) to which the index belongs.

• INDEX_NAME

  The name of the index. If the index is the primary key, the name is always PRIMARY.

• SEQ_IN_INDEX

  The column sequence number in the index, starting with 1.

• COLUMN_NAME

  The column name. See also the description for the EXPRESSION column.

• COLLATION

  How the column is sorted in the index. This can have values A (ascending), D (descending), or NULL (not sorted).

• CARDINALITY

  An estimate of the number of unique values in the index. To update this number, run ANALYZE TABLE or (for MyISAM tables) myisamchk -a.

  CARDINALITY is counted based on statistics stored as integers, so the value is not necessarily exact even for small tables. The higher the cardinality, the greater the chance that MySQL uses the index when doing joins.

• SUB_PART

  The index prefix. That is, the number of indexed characters if the column is only partly indexed, NULL if the entire column is indexed.

  Note

  Prefix limits are measured in bytes. However, prefix lengths for index specifications in CREATE TABLE, ALTER TABLE, and CREATE INDEX statements are interpreted as number of characters for nonbinary string types (CHAR, VARCHAR, TEXT) and number of bytes for binary string types (BINARY, VARBINARY, BLOB). Take this into account when specifying a prefix length for a nonbinary string column that uses a multibyte character set.

  For additional information about index prefixes, see Section 10.3.5, “Column Indexes”, and Section 15.1.15, “CREATE INDEX Statement”.

• PACKED

  Indicates how the key is packed. NULL if it is not.

• NULLABLE

  Contains YES if the column may contain NULL values and '' if not.

• INDEX_TYPE

  The index method used (BTREE, FULLTEXT, HASH, RTREE).

• COMMENT

  Information about the index not described in its own column, such as disabled if the index is disabled.

• INDEX_COMMENT

  Any comment provided for the index with a COMMENT attribute when the index was created.

• IS_VISIBLE

  Whether the index is visible to the optimizer. See Section 10.3.12, “Invisible Indexes”.

• EXPRESSION

  MySQL supports functional key parts (see Functional Key Parts), which affects both the COLUMN_NAME and EXPRESSION columns:

  • For a nonfunctional key part, COLUMN_NAME indicates the column indexed by the key part and EXPRESSION is NULL.

  • For a functional key part, COLUMN_NAME column is NULL and EXPRESSION indicates the expression for the key part.

Notes

• There is no standard INFORMATION_SCHEMA table for indexes. The MySQL column list is similar to what SQL Server 2000 returns for sp_statistics, except that QUALIFIER and OWNER are replaced with CATALOG and SCHEMA, respectively.

Information about table indexes is also available from the SHOW INDEX statement. See Section 15.7.7.23, “SHOW INDEX Statement”. The following statements are equivalent:

SELECT * FROM INFORMATION_SCHEMA.STATISTICS
  WHERE table_name = 'tbl_name'
  AND table_schema = 'db_name'

SHOW INDEX
  FROM tbl_name
  FROM db_name

Information about generated invisible primary key columns is visible in this table by default. You can cause such information to be hidden by setting show_gipk_in_create_table_and_information_schema = OFF. For more information, see Section 15.1.20.11, “Generated Invisible Primary Keys”.