SHOW [EXTENDED] {INDEX | INDEXES | KEYS}
{FROM | IN} tbl_name
[{FROM | IN} db_name]
[WHERE expr]
SHOW INDEX
returns table index
information. The format resembles that of the
SQLStatistics
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 to
syntax is
tbl_name
FROM db_name
db_name
.tbl_name
.
These two statements are equivalent:
SHOW INDEX FROM mytable FROM mydb;
SHOW INDEX FROM mydb.mytable;
The optional EXTENDED
keyword causes the
output to include information about hidden indexes that MySQL
uses internally and are not accessible by users.
The WHERE
clause can be given to select rows
using more general conditions, as discussed in
Section 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 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), orNULL
(not sorted).Cardinality
An estimate of the number of unique values in the index. To update this number, run
ANALYZE 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.NotePrefix limits are measured in bytes. However, prefix lengths for index specifications in
CREATE 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, 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.Null
Contains
YES
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 as
disabled
if the index is disabled.Index_comment
Any comment provided for the index with a
COMMENT
attribute when the index was created.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); this affects both the
Column_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 the
INFORMATION_SCHEMA
STATISTICS
table. See
Section 28.3.34, “The INFORMATION_SCHEMA STATISTICS Table”. The
extended information about hidden indexes is available only
using SHOW EXTENDED INDEX
; it cannot be
obtained from the STATISTICS
table.
You can list a table's indexes with the mysqlshow -k
db_name
tbl_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 setting
show_gipk_in_create_table_and_information_schema
= OFF
. For more information, see
Section 15.1.20.11, “Generated Invisible Primary Keys”.