实验环境:MySQL 5.7.17
1、表结构如下,在 id,name,tx 列建立主键
- mysql> show create table txtx;
- +-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
- | Table | Create Table |
- +-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
- | txtx | CREATE TABLE `txtx` (
- `id` int(11) NOT NULL,
- `name` char(2) NOT NULL,
- `tx` char(3) NOT NULL,
- `id1` int(11) DEFAULT NULL,
- PRIMARY KEY (`id`,`name`,`tx`)
- ) ENGINE=InnoDB DEFAULT CHARSET=gbk |
- +-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
- 1 row in set (0.00 sec)
2、查看执行计划
- mysql> explain select * from txtx where id=1 and id1 =1 and tx='tx';
- +----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------------+
- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
- +----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------------+
- | 1 | SIMPLE | txtx | NULL | ref | PRIMARY | PRIMARY | 4 | const | 1 | 33.33 | Using where |
- +----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------------+
- 1 row in set, 1 warning (0.00 sec)
从上面执行计划可以看出,尽管查询在 where 语句中使用了 id 与 tx 列,但只使用了 id 列,原因是 MySQL 只能使用定义索引的最左边部分。
来源: http://www.bubuko.com/infodetail-2011206.html