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15.7.7.24 SHOW INDEX Statement

SHOW [EXTENDED] {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:      Visible: YES   Expression: NULL*************************** 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:      Visible: YES   Expression: NULL

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;

The optionalEXTENDED keyword causes the output to include information about hidden indexes that MySQL uses internally and are not accessible by users.

TheWHERE clause can be given to select rows using more general conditions, as discussed inSection 28.8, “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 column name. See also the description for theExpression column.

  • Collation

    How the column is sorted in the index. This can have valuesA (ascending),D (descending), 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 10.3.5, “Column Indexes”, andSection 15.1.18, “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.

  • Visible

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

  • Expression

    MySQL supports functional key parts (seeFunctional Key Parts); this affects both theColumn_name andExpression columns:

    • For a nonfunctional key part,Column_name indicates the column indexed by the key part andExpression isNULL.

    • For a functional key part,Column_name column isNULL andExpression indicates the expression for the key part.

Information about table indexes is also available from theINFORMATION_SCHEMASTATISTICS table. SeeSection 28.3.40, “The INFORMATION_SCHEMA STATISTICS Table”. The extended information about hidden indexes is available only usingSHOW EXTENDED INDEX; it cannot be obtained from theSTATISTICS table.

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

SHOW INDEX includes the table's generated invisible key, if it has one, by default. You can cause this information to be suppressed in the statement's output by settingshow_gipk_in_create_table_and_information_schema = OFF. For more information, seeSection 15.1.24.11, “Generated Invisible Primary Keys”.