Mysql多列索引

表结构如下：

mysql> show create table t4\G;

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

Table: t4

Create Table: CREATE TABLE `t4` (

`c1` tinyint(1) NOT NULL DEFAULT '0',

`c2` tinyint(2) NOT NULL DEFAULT '0',

`c3` tinyint(3) NOT NULL DEFAULT '0',

`c4` tinyint(4) NOT NULL DEFAULT '0',

`c5` tinyint(5) NOT NULL DEFAULT '0',

KEY `c1234` (`c1`,`c2`,`c3`,`c4`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8

1 row in set (0.00 sec)

表数据如下：

mysql> select * from t4;

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

| c1 | c2 | c3 | c4 | c5 |

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

| 1 | 3 | 5 | 6 | 7 |

| 2 | 3 | 9 | 8 | 3 |

| 4 | 3 | 2 | 7 | 5 |

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

3 rows in set (0.00 sec)

下面几个语句，哪几个索引起作用了？为什么？

A where c1=x and c2=x and c4>x and c3=x

B where c1=x and c2=x and c4=x order by c3

C where c1=x and c4= x group by c3,c2

D where c1=x and c5=x order by c2,c3

E where c1=x and c2=x and c5=? order by c2,c3

A语句如下，我们会发现，将四列索引用上了，key_len（索引长度）为4个字节，一个数字为1个字节，可见总共用到了4个索引

mysql> explain select * from t4 where c1=1 and c2=2 and c4>3 and c3=3 \G;

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

id: 1

select_type: SIMPLE

table: t4

type: range

possible_keys: c1234

key: c1234

key_len: 4

ref: NULL

rows: 1

Extra: Using index condition

1 row in set (0.00 sec)

B语句如下，我们会发现，c1 ,c2索引用上了,在c2用到索引的基础上,c3是排好序的,因此不用额外排序，而c4没有发挥作用

mysql> explain select * from t4 where c1=1 and c2=2 and c4=3 order by c3 \G

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

id: 1

select_type: SIMPLE

table: t4

type: ref

possible_keys: c1234

key: c1234

key_len: 2

ref: const,const

rows: 1

Extra: Using index condition; Using where

1 row in set (0.00 sec)

下面的语句与上面的语句对比发现，下面的语句用到了filesort，这是因为索引c3已经帮我们排好序了，但是c5没有加索引，所以在order的时候，会使用filesort来进行排序

mysql> explain select * from t4 where c1=1 and c2=2 and c4=3 order by c5\G;

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

id: 1

select_type: SIMPLE

table: t4

type: ref

possible_keys: c1234

key: c1234

key_len: 2

ref: const,const

rows: 1

Extra: Using index condition; Using where; Using filesort

1 row in set (0.00 sec)

C语句如下，只用到c1索引,因为group by c3,c2的顺序无法利用c2,c3索引（索引的左前缀规则），因为没有利用到索引，所以用到了临时表（temporary），在mysql的group查询中，会先将查询的数据进行排序，存到临时表中，然后再进行查询。

mysql> explain select * from t4 where c1=1 and c4=2 group by c3,c2\G;

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

id: 1

select_type: SIMPLE

table: t4

type: ref

possible_keys: c1234

key: c1234

key_len: 1 #只用到c1,因为先用c3后用c2分组,导致c2,c3索引没发挥作用

ref: const

rows: 1

Extra: Using index condition; Using where; Using temporary; Using filesort

1 row in set (0.00 sec)

下面的语句与上面的相比，也是只用到了索引c1，c2和c3都用来group分组了

explain select * from t4 where c1=1 and c4=2 group by c2,c3 \G;

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

id: 1

select_type: SIMPLE

table: t4

type: ref

possible_keys: c1234

key: c1234

key_len: 1

ref: const

rows: 1

Extra: Using index condition; Using where

1 row in set (0.00 sec)

D语句如下，C1确定的基础上,c2是有序的,C2之下C3是有序的,因此c2,c3发挥的排序的作用，因此,没用到filesort

mysql> explain select * from t4 where c1=1 and c5=2 order by c2,c3\G;

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

id: 1

select_type: SIMPLE

table: t4

type: ref

possible_keys: c1234

key: c1234

key_len: 1

ref: const

rows: 1

Extra: Using where

1 row in set (0.00 sec)

下面的语句与上面相比，先用c3排序，再用c2排序，无法利用索引，索引遵循左前缀法则，所以会用到filesort

mysql> explain select * from t4 where c1=1 and c5=2 order by c3,c2\G;

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

id: 1

select_type: SIMPLE

table: t4

type: ref

possible_keys: c1234

key: c1234

key_len: 1

ref: const

rows: 1

Extra: Using where; Using filesort

1 row in set (0.00 sec)

E语句如下，这一句等价与 elect * from t4 where c1=1 and c2=3 and c5=2 order by c3; 因为c2的值既是固定的,参与排序时并不考虑

mysql> explain select * from t4 where c1=1 and c2=3 and c5=2 order by c2,c3\G;

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

id: 1

select_type: SIMPLE

table: t4

type: ref

possible_keys: c1234

key: c1234

key_len: 2

ref: const,const

rows: 1

Extra: Using where

1 row in set (0.00 sec)