【MYSQL优化13】索引长度与区分度

理想的索引
1.查询频繁
2.区分度高
3.长度小
4.尽量能覆盖常用查询字段

测试区分度和长度的比值，取合适的索引长度

mysql> select uid,name,length(name) from user order by length(name) desc limit 1;
+---------+-------------------------------------------------------------------+--------------+
| uid     | name                                                              | length(name) |
+---------+-------------------------------------------------------------------+--------------+
| 1733863 | 贵州运满来物流有限公司，蔡回筠，电话13608541671                      |           65 |
+---------+-------------------------------------------------------------------+--------------+

mysql> select count(distinct left(name,1)) from user;
+------------------------------+
| count(distinct left(name,1)) |
+------------------------------+
|                         5638 |
+------------------------------+

mysql> select count(*) from user;
+----------+
| count(*) |
+----------+
|   305188 |
+----------+

mysql> select count(distinct left(name,1))/count(*) from user;
+---------------------------------------+
| count(distinct left(name,1))/count(*) |
+---------------------------------------+
|                                0.0185 |
+---------------------------------------+

mysql> select count(distinct left(name,2))/count(*) from user;
+---------------------------------------+
| count(distinct left(name,2))/count(*) |
+---------------------------------------+
|                                0.2153 |
+---------------------------------------+

mysql> select count(distinct left(name,3))/count(*) from user;
+---------------------------------------+
| count(distinct left(name,3))/count(*) |
+---------------------------------------+
|                                0.3611 |
+---------------------------------------+

...

mysql> select count(distinct left(name,65))/count(*) from user;
+---------------------------------------+
| count(distinct left(name,3))/count(*) |
+---------------------------------------+
|                                1      |
+---------------------------------------+

根据图表可知，索引长度取10左右即可

mysql> alter table user add index name(name(10));

mysql> show create table user \G;
*************************** 1. row ***************************
       Table: user
Create Table: CREATE TABLE `user` (
  `uid` int(11) NOT NULL AUTO_INCREMENT COMMENT '用户id',
  `name` varchar(32) DEFAULT NULL COMMENT '姓名',
  ...
  PRIMARY KEY (`uid`) USING BTREE,
  KEY `name` (`name`(10))
) ENGINE=InnoDB AUTO_INCREMENT=1850120 DEFAULT CHARSET=utf8 COMMENT='用户表'

mysql> explain select * from user where name='中' \G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: user
         type: ref
possible_keys: name
          key: name
      key_len: 33
          ref: const
         rows: 3
        Extra: Using where