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25.26 The INFORMATION_SCHEMA STATISTICS Table
The STATISTICS
table provides
information about table indexes.
Columns in STATISTICS
that represent
table statistics hold cached values. The
information_schema_stats_expiry
system variable defines the period of time before cached table
statistics expire. The default is 86400 seconds (24 hours). If
there are no cached statistics or statistics have expired,
statistics are retrieved from storage engines when querying table
statistics columns. To update cached values at any time for a
given table, use ANALYZE TABLE
. To
always retrieve the latest statistics directly from storage
engines, set
information_schema_stats_expiry=0
.
For more information, see
Section 8.2.3, “Optimizing INFORMATION_SCHEMA Queries”.
If the innodb_read_only
system
variable is enabled, ANALYZE
TABLE
may fail because it cannot update statistics
tables in the data dictionary, which use
InnoDB
. For ANALYZE
TABLE
operations that update the key distribution,
failure may occur even if the operation updates the table itself
(for example, if it is a MyISAM
table). To
obtain the updated distribution statistics, set
information_schema_stats_expiry=0
.
The STATISTICS
table has these
columns:
TABLE_CATALOG
The name of the catalog to which the table containing the index belongs. This value is always
def
.TABLE_SCHEMA
The name of the schema (database) to which the table containing the index belongs.
TABLE_NAME
The name of the table containing the index.
NON_UNIQUE
0 if the index cannot contain duplicates, 1 if it can.
INDEX_SCHEMA
The name of the schema (database) to which the index belongs.
INDEX_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.NULLABLE
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.IS_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.
Notes
There is no standard
INFORMATION_SCHEMA
table for indexes. The MySQL column list is similar to what SQL Server 2000 returns forsp_statistics
, except thatQUALIFIER
andOWNER
are replaced withCATALOG
andSCHEMA
, respectively.
Information about table indexes is also available from the
SHOW INDEX
statement. See
Section 13.7.6.22, “SHOW INDEX Syntax”. The following statements are
equivalent:
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