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13.7.6.22 SHOW INDEX Syntax
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.
- *************************** 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
- 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
- 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:
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 25.41, “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 8.3.5, “Column Indexes”, and Section 13.1.15, “CREATE INDEX Syntax”.
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 8.3.12, “Invisible Indexes”.
Expression
MySQL 8.0.13 and higher supports functional key parts (see Functional Key Parts), which 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 25.25, “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.
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Document créé le 26/06/2006, dernière modification le 26/10/2018
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