MySQL 性能分析 之 联合索引(复合索引)实践分析
作为开发者，大家都知道，一个服务器、一个数据库的性能是项目的重中之重，后台架构、写法与数据库设计的好坏往往直接影响到整个项目的性能。
索引:是当你的业务完成后，跟据查询条件来建立的。当你的数据量大（一般是10万条数据）了之后，我们会再把普通索引删除，使用自建索引表。因为数据量大的时候你要批量修改（索引表也会修改）会变的非常的慢！
这里给分析一下MySQL的索引；索引分：普通索引和联合索引，而索引的关键相信很多人也知道，用非主键字段进行查询的时候MySQL在查询的时候就是扫表行为，如果有索引的话，情况就得到了大大的改善。
加索引的时候，先建议使用单列索引一个一个加！然后再改进使用联合索引！而且最好是根据自己的项目业务查询来统计 哪些字段的使用率是多少 哪些字段组合在一次使用了多少次，因为今天分析的就是联合索引并不是在任何情况下都命中的
表结构
mysql> show create table m_userG;
*************************** 1. row ***************************
Table: m_user
Create Table: CREATE TABLE `m_user` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` char(32) NOT NULL,
`age` tinyint(4) NOT NULL,
`school` char(128) NOT NULL,
`status` tinyint(4) NOT NULL DEFAULT '1',
PRIMARY KEY (`id`),
KEY `name` (`name`,`age`,`status`)
) ENGINE=InnoDB AUTO_INCREMENT=1001 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
联合索引字段： KEY name (name,age,status)
模拟了数据后 我们来查看结果
- [1 3 命中]
select * from m_user where name='leal' and status=1
mysql> desc select * from m_user where name='leal' and status=1 G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user
partitions: NULL
type: ref
possible_keys: name
key: name
key_len: 96
ref: const
rows: 1
filtered: 10.00
Extra: Using index condition
1 row in set, 1 warning (0.00 sec)
- [1 3 order by 3 命中]
select * from m_user where name='leal' and status=1 order by status desc
mysql> desc select * from m_user where name='leal' and status=1 order by status desc G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user
partitions: NULL
type: ref
possible_keys: name
key: name
key_len: 96
ref: const
rows: 1
filtered: 10.00
Extra: Using index condition
1 row in set, 1 warning (0.00 sec)
- [2 3 不命中]
select * from m_user where age=10 and status=1
mysql> desc select * from m_user where age=10 and status=1 G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user
partitions: NULL
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 1000
filtered: 1.00
Extra: Using where
1 row in set, 1 warning (0.00 sec)
- [1 in 命中]
select * from m_user where name in ('leal') and age<10
select * from m_user where name in ('leal') and age<10 order by school
mysql> desc select * from m_user where name in ('leal') and age<10 order by school
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user
partitions: NULL
type: range
possible_keys: name
key: name
key_len: 97
ref: NULL
rows: 1
filtered: 100.00
Extra: Using index condition; Using filesort
1 row in set, 1 warning (0.00 sec)
- [1 between 不命中]
select * from m_user where name between 'leal' and 'tom'
select * from m_user where name between 'leal' and 'tom' order by school desc
mysql> desc select * from m_user where name between 'leal' and 'tom' order by school desc G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user
partitions: NULL
type: ALL
possible_keys: name
key: NULL
key_len: NULL
ref: NULL
rows: 1000
filtered: 22.38
Extra: Using where; Using filesort
1 row in set, 1 warning (0.00 sec)
- [1 <> 不命中]
select * from m_user where name<>'leal'
select * from m_user where name<>'leal' and age<10
select * from m_user where name<>'leal' and age<10 order by school desc
mysql> desc select * from m_user where name<>'leal' and age<10 order by school desc G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user
partitions: NULL
type: ALL
possible_keys: name
key: NULL
key_len: NULL
ref: NULL
rows: 1000
filtered: 33.30
Extra: Using where; Using filesort
1 row in set, 1 warning (0.00 sec)
--------------------------------------------------------------
mysql> desc select * from m_user where name<>'leal'G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user
partitions: NULL
type: ALL
possible_keys: name
key: NULL
key_len: NULL
ref: NULL
rows: 1000
filtered: 99.90
Extra: Using where
1 row in set, 1 warning (0.00 sec)
- [1 < 或 <= 命中]
select * from m_user where name < 'leal'
select * from m_user where name <= 'leal'
select * from m_user where name <= 'leal' and age<10
select * from m_user where name <= 'leal' and age<10 order by school desc
mysql> desc select * from m_user where name < 'leal' and age<10 order by school desc G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user
partitions: NULL
type: range
possible_keys: name
key: name
key_len: 96
ref: NULL
rows: 1
filtered: 33.33
Extra: Using index condition; Using filesort
1 row in set, 1 warning (0.00 sec)
- [1 > 或 >= 不命中]
select * from m_user where name>'leal'
select * from m_user where name>='leal'
mysql> desc select * from m_user where name >= 'leal' and age<10 order by school desc G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user
partitions: NULL
type: ALL
possible_keys: name
key: NULL
key_len: NULL
ref: NULL
rows: 1000
filtered: 33.30
Extra: Using where; Using filesort
1 row in set, 1 warning (0.00 sec)
- [无where条件 直接order by 不命中]
select * from m_user order by name desc
mysql> explain select * from m_user order by name descG;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user
partitions: NULL
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 1000
filtered: 100.00
Extra: Using filesort
1 row in set, 1 warning (0.13 sec)
结束语
一般人和一个项目都是从小到大逐渐健壮的，前期一般的项目 直接加索引就够了，保证命中率；当项目业务复杂到一定程度或者负载到一定程度的时候，就得删除索引，自建索引表来管理数据库的索引了。
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