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13.7.5.22 SHOW INDEX Statement

SHOW {INDEX | INDEXES | KEYS}    {FROM | IN}tbl_name    [{FROM | IN}db_name]    [WHEREexpr]

SHOW INDEX returns table index information. The format resembles that of theSQLStatistics call in ODBC. This statement requires some privilege for any column in the table.

mysql> SHOW INDEX FROM City\G*************************** 1. row ***************************        Table: city   Non_unique: 0     Key_name: PRIMARY Seq_in_index: 1  Column_name: ID    Collation: A  Cardinality: 4188     Sub_part: NULL       Packed: NULL         Null:   Index_type: BTREE      Comment:Index_comment:*************************** 2. row ***************************        Table: city   Non_unique: 1     Key_name: CountryCode Seq_in_index: 1  Column_name: CountryCode    Collation: A  Cardinality: 232     Sub_part: NULL       Packed: NULL         Null:   Index_type: BTREE      Comment:Index_comment:

An alternative totbl_name FROMdb_name syntax isdb_name.tbl_name. These two statements are equivalent:

SHOW INDEX FROM mytable FROM mydb;SHOW INDEX FROM mydb.mytable;

TheWHERE clause can be given to select rows using more general conditions, as discussed inSection 24.8, “Extensions to SHOW Statements”.

SHOW INDEX returns the following fields:

  • Table

    The name of the table.

  • Non_unique

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

  • Key_name

    The name of the index. If the index is the primary key, the name is alwaysPRIMARY.

  • Seq_in_index

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

  • Column_name

    The name of the column.

  • Collation

    How the column is sorted in the index. This can have valuesA (ascending) orNULL (not sorted).

  • Cardinality

    An estimate of the number of unique values in the index. To update this number, runANALYZE TABLE or (forMyISAM 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

    Prefixlimits are measured in bytes. However, prefixlengths for index specifications inCREATE TABLE,ALTER TABLE, andCREATE 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, seeSection 8.3.4, “Column Indexes”, andSection 13.1.14, “CREATE INDEX Statement”.

  • Packed

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

  • Null

    ContainsYES if the column may containNULL 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 asdisabled if the index is disabled.

  • Index_comment

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

Information about table indexes is also available from theINFORMATION_SCHEMASTATISTICS table. SeeSection 24.3.24, “The INFORMATION_SCHEMA STATISTICS Table”.

You can list a table's indexes with themysqlshow -kdb_nametbl_name command.