SHOW INDEX

Description

SHOW INDEX returns table index information.

SHOW INDEX returns the following fields:

FieldsDescription
TableThe name of the table.
Non_unique0 if the index cannot contain duplicates, 1 if it can.
Key_nameThe name of the index. If the index is the primary key, the name is always PRIMARY.
Seq_in_indexThe column sequence number in the index, starting with 1.
Column_nameThe column name. See also the description for the Expression column.
CollationHow the column is sorted in the index. This can have values A (ascending), D (descending), or NULL (not sorted).
CardinalityAn 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_partThe 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.
PackedIndicates how the key is packed. NULL if it is not.
NullContains YES if the column may contain NULL values and ‘’ if not.
Index_typeThe index method used (BTREE, FULLTEXT, HASH, RTREE).
CommentInformation about the index not described in its own column, such as disabled if the index is disabled.
VisibleWhether the index is visible to the optimizer.
ExpressionFor 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.

Syntax

  1. > SHOW {INDEX | INDEXES}
  2.     {FROM | IN} tbl_name
  3.     [{FROM | IN} db_name]

Explanations

An alternative to tbl_name FROM db_name syntax is db_name.tbl_name.

Examples

  1. CREATE TABLE show_01(sname varchar(30),id int);
  2. mysql> show INDEX FROM show_01;
  3. +---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+------------------+---------+------------+
  4. | Table   | Non_unique | Key_name   | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment          | Visible | Expression |
  5. +---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+------------------+---------+------------+
  6. | show_01 |          0 | id         |            1 | id          | A         |           0 | NULL     | NULL   | YES  |            |                  | YES     | NULL       |
  7. | show_01 |          0 | sname      |            1 | sname       | A         |           0 | NULL     | NULL   | YES  |            |                  | YES     | NULL       |
  8. | show_01 |          0 | __mo_rowid |            1 | __mo_rowid  | A         |           0 | NULL     | NULL   | NO   |            | Physical address | NO      | NULL       |
  9. +---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+------------------+---------+------------+
  10. 3 rows in set (0.02 sec)