mysql 组合索引 or_Mysql_组合索引和单列索引

一、目标

什么时候使用组合索引，什么时候使用单独索引

组合索引、单独索引区别

组合索引：最左前缀匹配原则

二、前期数据准备

1. 建表

CREATE TABLE `user` (

`uid`int(11) NOT NULLAUTO_INCREMENT,

`name`varchar(50) DEFAULT NULL,

`pwd`varchar(50) DEFAULT NULL,

`create_time`datetime DEFAULT NULL,

`modify_time`timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,

`rids`varchar(15) DEFAULT NULL,

`nickname`varchar(45) DEFAULT NULL,

`company`varchar(15) DEFAULT NULL,PRIMARY KEY(`uid`),UNIQUE KEY`name_UNIQUE` (`name`)

) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8

2.插入数据

INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (1, 'rocker', 'rocker', NULL, '2019-10-08 11:05:02', '1', 'rocker', 'rocker');INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (2, 'danny', 'danny', NULL, '2019-10-08 11:31:36', '2', 'rocker', 'danny');INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (3, 'tom', 'tom', NULL, '2019-10-08 11:31:39', '1', 'tom', 'rocker');INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (4, 'messi', 'messi', NULL, '2019-10-08 11:31:21', '2', 'messi', 'messi');INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (5, 'wenger', 'wenger', NULL, '2019-10-08 11:29:38', '1', 'wenger', 'rocker');INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (6, 'henry', 'henry', NULL, '2019-10-08 11:30:46', '2', 'henry', 'henry');INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (7, 'ronaldo', 'ronaldo', NULL, '2019-10-08 11:30:49', '1', 'ronaldo', 'ronaldo');INSERT INTO `monitor`.`user`(`uid`, `name`, `pwd`, `create_time`, `modify_time`, `rids`, `nickname`, `company`) VALUES (8, 'kaka', 'kaka', NULL, '2019-10-08 11:29:45', '2', 'kaka', 'rocker');

三、分析索引区别

1.不加索引

首先在'nickname'和‘company’这俩字段不加索引的情况下执行一个查询语句，并分析(explain详解：传送门)

mysql> explain select * from user where nickname = 'rocker' and company = 'rocker';+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

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

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

1 row in set, 1 warning (0.00 sec)

可以看到，没有走索引，总共查询了8条数据，而表中总共也是8条数据，相当于全表扫描了。

mysql> explain select * from user where company = 'rocker' or nickname = 'rocker';+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

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

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

1 row in set, 1 warning (0.00 sec)

可以看到：

不加任何索引的情况下，不管是and还是or，都是全表扫描，没有索引。

2.单独索引

给nickname和company分别加上索引，再执行and和or的sql查询

alter table user add index`idx_nickname` (`nickname`);alter table user add index `idx_company` (`company`);

执行查询语句and

mysql> explain select * from user where nickname = 'rocker' and company = 'rocker';+----+-------------+-------+------+--------------------------+--------------+---------+-------+------+-------------+

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

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

1 row in set (0.05 sec)

执行查询语句or

mysql> explain select * from user where company = 'rocker' or nickname = 'rocker';+----+-------------+-------+------+-----------------+------+---------+------+------+-------------+

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

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

1 row in set (0.00 sec)

可以看到：

加上索引后and查询是可以走索引的，但是只有一个索引起作用，对于另一个索引字段还是要进行遍历，而且and查询会根据关联性高(符合该条件的行数少)选择具体走哪个索引

or查询不走索引

3.组合索引

删除原先的单独索引，新增组合索引

alter table user drop index`idx_nickname`alter table user drop index`idx_company`alter table user add index `idx_composition` (`nickname`,`company`);

执行查询语句and

mysql> explain select * from user where nickname = 'rocker' and company = 'rocker';+----+-------------+-------+------+-----------------+-----------------+---------+-------------+------+-------------+

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

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

1 row in set (0.00 sec)

执行查询语句or

mysql> explain select * from user where company = 'rocker' or nickname = 'rocker';+----+-------------+-------+------+-----------------+------+---------+------+------+-------------+

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

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

1 row in set (0.00 sec)

可以看到：

加上组合索引后，组合索引起作用，只需查询一条符合结果的数据，效率要比单独索引高

复合索引对于or查询不起作用

4.组合索引查询单个索引列

对于组合索引为(nickname,company)这个顺序的情况，与上面的情况顺序调换

alter table user drop index`idx_composition`;alter table user add index `idx_composition` (`company`,`nickname`);

mysql> explain select * from user where nickname = 'rocker';+----+-------------+-------+------+-----------------+-----------------+---------+-------+------+-------------+

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

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

1 row in set (0.00sec)

mysql> explain select * from user where company = 'rocker';+----+-------------+-------+------+---------------+------+---------+------+------+-------------+

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

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

1 row in set (0.00 sec)

可以看到：

组合索引中nickname在前时，单独查询nickname会走索引，单独查询compamy不会走索引

对于组合索引为(company,nickname)这个顺序的情况

alter table user drop index`idx_composition`;alter table user add index `idx_composition` (`company`,`nickname`);

mysql> explain select * from user where nickname = 'rocker';+----+-------------+-------+------+---------------+------+---------+------+------+-------------+

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

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

1 row in set (0.00sec)

mysql> explain select * from user where company = 'rocker';+----+-------------+-------+------+-----------------+-----------------+---------+-------+------+-------------+

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

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

1 row in set (0.00 sec)

可以看到：

组合索引中compamy在前时，单独查询compamy会走索引，单独查询nickname不会走索引

为什么会出现这个情况呢？

对于复合索引：Mysql从左到右的使用索引中的字段，一个查询可以只使用索引中的一部份，但只能是最左侧部分。例如索引是key index (a,b,c). 可以支持 |a | a,b | a,b,c | 3种组合进行查找，但不支持 b,c进行查找 .

转载来源：传送门