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MySQL 5.5 Reference Manual  /  INFORMATION_SCHEMA Tables  /  The INFORMATION_SCHEMA STATISTICS Table

21.21 The INFORMATION_SCHEMA STATISTICS Table

The STATISTICS table provides information about table indexes.

The SHOW Name values in the following table correspond to column names of the SHOW INDEX statement.

INFORMATION_SCHEMA Name SHOW Name Remarks
TABLE_CATALOG
TABLE_SCHEMA
TABLE_NAME Table
NON_UNIQUE Non_unique
INDEX_SCHEMA
INDEX_NAME Key_name
SEQ_IN_INDEX Seq_in_index
COLUMN_NAME Column_name
COLLATION Collation
CARDINALITY Cardinality
SUB_PART Sub_part MySQL extension
PACKED Packed MySQL extension
NULLABLE Null MySQL extension
INDEX_TYPE Index_type MySQL extension
COMMENT Comment MySQL extension
INDEX_COMMENT Index_comment MySQL extension

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 8.3.4, “Column Indexes”, and Section 13.1.13, “CREATE INDEX Syntax”.

  • 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.

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 13.7.5.23, “SHOW INDEX Syntax”. 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

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