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！
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！
本文是针对mysql 5.7多列[单]索引进行验证测试!
版本：mysql 5.7
测试结果日期：2017-01-05
表结构：
mysql> show create table m_user2\G;
*************************** 1. row ***************************
Table: m_user2
Create Table: CREATE TABLE `m_user2` (
`id` int(10) unsigned NOT NULL DEFAULT '0',
`name` char(32) NOT NULL,
`age` tinyint(4) NOT NULL,
`school` char(128) NOT NULL,
`status` tinyint(4) NOT NULL DEFAULT '1',
KEY `name` (`name`),
KEY `age` (`age`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
多列[单]索引字段：
KEY name (name),
KEY age (age)
| 序号 | 字段名 | 类型 |
|---|---|---|
| 1 | name | char |
| 2 | age | tinyint |
结果：
1.[1 and 2 命中最严格的一个]
select * from m_user2 where name='feng1' and age=10
2.[1 or 2 两个都命中]
select * from m_user2 where name='feng1' or age=10
3.[1 or 2 order by 1 命中]
select * from m_user2 where name='feng1' or age=10 order by name desc
4.[between 命中]
select * from m_user2 where name between 'feng1' and 'feng123' order by name desc
select * from m_user2 where age between 10 and 20 order by name desc
5.[in 命中]
select * from m_user2 where name in ('feng1','feng123') order by name desc
6.[无where条件 直接order by 不命中]
select * from m_user2 order by name desc
测试过程
1.[1 and 2 命中最严格的一个]
mysql> explain select * from m_user2 where name='feng1' and age=10\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user2
partitions: NULL
type: ref
possible_keys: name,age
key: name
key_len: 96
ref: const
rows: 1
filtered: 5.00
Extra: Using where
1 row in set, 1 warning (0.00 sec)
2.[1 or 2 两个都命中]
mysql> explain select * from m_user2 where name='feng1' or age=10\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user2
partitions: NULL
type: index_merge
possible_keys: name,age
key: name,age
key_len: 96,1
ref: NULL
rows: 17
filtered: 100.00
Extra: Using union(name,age); Using where
1 row in set, 1 warning (0.01 sec)
3.[1 or 2 order by 1 命中]
mysql> explain select * from m_user2 where name='feng1' or age=10 order by name desc\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user2
partitions: NULL
type: index_merge
possible_keys: name,age
key: name,age
key_len: 96,1
ref: NULL
rows: 17
filtered: 100.00
Extra: Using union(name,age); Using where; Using filesort
1 row in set, 1 warning (0.00 sec)
4.[between 命中]
mysql> explain select * from m_user2 where name between 'feng1' and 'feng123' order by name desc\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user2
partitions: NULL
type: range
possible_keys: name
key: name
key_len: 96
ref: NULL
rows: 29
filtered: 100.00
Extra: Using index condition
1 row in set, 1 warning (0.00 sec)
---------------------------------------------------------------------
mysql> explain select * from m_user2 where age between 10 and 20 order by name desc\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user2
partitions: NULL
type: range
possible_keys: age
key: age
key_len: 1
ref: NULL
rows: 134
filtered: 100.00
Extra: Using index condition; Using filesort
1 row in set, 1 warning (0.01 sec)
5.[in 命中]
mysql> explain select * from m_user2 where name in ('feng1','feng123') order by name desc\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user2
partitions: NULL
type: range
possible_keys: name
key: name
key_len: 96
ref: NULL
rows: 2
filtered: 100.00
Extra: Using index condition
1 row in set, 1 warning (0.01 sec)
6.[无where条件 直接order by 不命中]
mysql> explain select * from m_user2 order by name desc\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: m_user2
partitions: NULL
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 1001
filtered: 100.00
Extra: Using filesort
1 row in set, 1 warning (0.00 sec)
