為了演示，首先建兩個包含不良索引的表，并弄點數據。

mysql> show create table test1\G

*************************** 1. row ***************************

Table: test1

Create Table: CREATE TABLE `test1` (

`id` int(11) NOT NULL,

`f1` int(11) DEFAULT NULL,

`f2` int(11) DEFAULT NULL,

`f3` int(11) DEFAULT NULL,

PRIMARY KEY (`id`),

KEY `k1` (`f1`,`id`),

KEY `k2` (`id`,`f1`),

KEY `k3` (`f1`),

KEY `k4` (`f1`,`f3`),

KEY `k5` (`f1`,`f3`,`f2`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1

1 row in set (0.00 sec)

mysql> show create table test2\G

*************************** 1. row ***************************

Table: test2

Create Table: CREATE TABLE `test2` (

`id1` int(11) NOT NULL DEFAULT '0',

`id2` int(11) NOT NULL DEFAULT '0',

`b` int(11) DEFAULT NULL,

PRIMARY KEY (`id1`,`id2`),

KEY `k1` (`b`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1

1 row in set (0.00 sec)

mysql> select count(*) from test2 group by b;

+----------+

| count(*) |

+----------+

| 32 |

| 17 |

+----------+

2 rows in set (0.00 sec)

1. 包含主鍵的索引

innodb 本身是聚簇表，每個二級索引本身就包含主鍵，類似 f1, id 的索引實際雖然沒有害處，但反映了使用者對 mysql 索引不了解。而類似 id, f1 的是多余索引，會浪費存儲空間，并影響數據更新性能。包含主鍵的索引用這樣一句 sql 就能全部找出來。

mysql> select c.*, pk from

-> (select table_schema, table_name, index_name, concat('|', group_concat(column_name order by seq_in_index separator '|'), '|') cols

-> from INFORMATION_SCHEMA.STATISTICS

-> where index_name != 'PRIMARY' and table_schema != 'mysql'

-> group by table_schema, table_name, index_name) c,

-> (select table_schema, table_name, concat('|', group_concat(column_name order by seq_in_index separator '|'), '|') pk

-> from INFORMATION_SCHEMA.STATISTICS

-> where index_name = 'PRIMARY' and table_schema != 'mysql'

-> group by table_schema, table_name) p

-> where c.table_name = p.table_name and c.table_schema = p.table_schema and c.cols like concat('%', pk, '%');

+--------------+------------+------------+---------+------+

| table_schema | table_name | index_name | cols | pk |

+--------------+------------+------------+---------+------+

| test | test1 | k1 | |f1|id| | |id| |

| test | test1 | k2 | |id|f1| | |id| |

+--------------+------------+------------+---------+------+

2 rows in set (0.04 sec)

2. 重復索引前綴

包含重復前綴的索引，索引能由另一個包含該前綴的索引完全代替，是多余索引。多余的索引會浪費存儲空間，并影響數據更新性能。這樣的索引同樣用一句 sql 可以找出來。

mysql> select c1.table_schema, c1.table_name, c1.index_name,c1.cols,c2.index_name, c2.cols from

-> (select table_schema, table_name, index_name, concat('|', group_concat(column_name order by seq_in_index separator '|'), '|') cols

-> from INFORMATION_SCHEMA.STATISTICS

-> where table_schema != 'mysql' and index_name!='PRIMARY'

-> group by table_schema,table_name,index_name) c1,

-> (select table_schema, table_name,index_name, concat('|', group_concat(column_name order by seq_in_index separator '|'), '|') cols

-> from INFORMATION_SCHEMA.STATISTICS

-> where table_schema != 'mysql' and index_name != 'PRIMARY'

-> group by table_schema, table_name, index_name) c2

-> where c1.table_name = c2.table_name and c1.table_schema = c2.table_schema and c1.cols like concat(c2.cols, '%') and c1.index_name != c2.index_name;

+--------------+------------+------------+------------+------------+---------+

| table_schema | table_name | index_name | cols | index_name | cols |

+--------------+------------+------------+------------+------------+---------+

| test | test1 | k1 | |f1|id| | k3 | |f1| |

| test | test1 | k4 | |f1|f3| | k3 | |f1| |

| test | test1 | k5 | |f1|f3|f2| | k3 | |f1| |

| test | test1 | k5 | |f1|f3|f2| | k4 | |f1|f3| |

+--------------+------------+------------+------------+------------+---------+

4 rows in set (0.02 sec)

3. 低區分度索引

這樣的索引由于仍然會掃描大量記錄，在實際查詢時通常會被忽略。但是在某些情況下仍然是有用的。因此需要根據實際情況進一步分析。這里是區分度小于 10% 的索引，可以根據需要調整參數。

mysql> select p.table_schema, p.table_name, c.index_name, c.car, p.car total from

-> (select table_schema, table_name, index_name, max(cardinality) car

-> from INFORMATION_SCHEMA.STATISTICS

-> where index_name != 'PRIMARY'

-> group by table_schema, table_name,index_name) c,

-> (select table_schema, table_name, max(cardinality) car

-> from INFORMATION_SCHEMA.STATISTICS

-> where index_name = 'PRIMARY' and table_schema != 'mysql'

-> group by table_schema,table_name) p

-> where c.table_name = p.table_name and c.table_schema = p.table_schema and p.car > 0 and c.car / p.car < 0.1;

+--------------+------------+------------+------+-------+

| table_schema | table_name | index_name | car | total |

+--------------+------------+------------+------+-------+

| test | test2 | k1 | 4 | 49 |

+--------------+------------+------------+------+-------+

1 row in set (0.04 sec)

4. 復合主鍵

由于 innodb 是聚簇表，每個二級索引都會包含主鍵值。復合主鍵會造成二級索引龐大，而影響二級索引查詢性能，并影響更新性能。同樣需要根據實際情況進一步分析。

mysql> select table_schema, table_name, group_concat(column_name order by seq_in_index separator ',') cols, max(seq_in_index) len

-> from INFORMATION_SCHEMA.STATISTICS

-> where index_name = 'PRIMARY' and table_schema != 'mysql'

-> group by table_schema, table_name having len>1;

+--------------+------------+-----------------------------------+------+

| table_schema | table_name | cols | len |

+--------------+------------+-----------------------------------+------+

| test | test2 | id1,id2 | 2 |

+--------------+------------+-----------------------------------+------+

1 rows in set (0.01 sec)

本文來自云棲社區合作伙伴“Linux中國”，原文發布日期：2015-08-18

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