分析mysql索引信息及空间占用

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 25.49, “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), or NULL (not sorted).

• Cardinality

  An 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_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

  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.

  For additional information about index prefixes, see Section 8.3.5, “Column Indexes”, and Section 13.1.15, “CREATE INDEX Statement”.

• Packed

  Indicates how the key is packed. NULL if it is not.

• Null

  Contains YES if the column may contain NULL 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 and Expression columns:

  • For 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.

Information about table indexes is also available from the INFORMATION_SCHEMA STATISTICS table. See Section 25.32, “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.

https://dev.mysql.com/doc/refman/8.0/en/optimizer-statistics.html

25.32 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), or NULL (not sorted).

• CARDINALITY

  An 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_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

  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.

  For additional information about index prefixes, see Section 8.3.5, “Column Indexes”, and Section 13.1.15, “CREATE INDEX Statement”.

• PACKED

  Indicates how the key is packed. NULL if it is not.

• NULLABLE

  Contains YES if the column may contain NULL 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 and EXPRESSION columns:

  • For 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.

Notes

• There is no standard INFORMATION_SCHEMA table for indexes. The MySQL column list is similar to what SQL Server 2000 returns for sp_statistics, except that QUALIFIER and OWNER are replaced with CATALOG and SCHEMA, respectively.

Information about table indexes is also available from the SHOW INDEX statement. See Section 13.7.7.22, “SHOW INDEX Statement”. The following statements are equivalent:

SELECT * FROM INFORMATION_SCHEMA.STATISTICS
  WHERE table_name = ‘tbl_name‘
  AND table_schema = ‘db_name‘

SHOW INDEX
  FROM tbl_name
  FROM db_name

分析mysql索引信息及空间占用

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本文系统来源：https://www.cnblogs.com/bensonwei/p/12665928.html

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