Full values match (best index)
explain select * from user where name = 'zhangsan' and age = 20 and pos = 'cxy' and phone = '18730658760';
And index order independent, MySQL underlying optimization will be optimized, the adjustment of the index sequence explain select * from user where name = 'zhangsan' and age = 20 and pos = 'cxy' and phone = '18730658760';
1, the most left-prefix rule violation
If the index multiple columns, the most left-prefix to comply with the law that is most queries from the start and left the forefront of the index does not skip columns in the index explain select * from user where age = 20 and phone = '18730658760' and pos = 'cxy' ;
2, do anything in the index column
Computing, function (automatic or manual) type conversion and other operations, will cause the index fail so full table scan explain select * from user where left ( name, 5) = 'zhangsan' and age = 20 and phone = '18730658760';
Column 3, the right conditions index range
Index Index range condition rightmost column fail explain select * from user where name = 'zhangsan' and age> 20 and pos = 'cxy';
4、尽量使用覆盖索引
只访问索引查询(索引列和查询列一致),减少select* explain select name,age,pos,phone from user where age = 20;
5、使用不等于(!=、<>)
mysql在使用不等于(!=、<>)的时候无法使用索引会导致全表扫描(除覆盖索引外) explain select * from user where age != 20; explain select * from user where age <> 20;
6、like以通配符开头('%abc')
索引失效 explain select * from user where name like '%zhangsan';
索引生效 explain select * from user where name like 'zhangsan%';
7、字符串不加单引号索引失效
explain select * from user where name = 2000;
8、or连接
少用or explain select * from user where name = '2000' or age = 20 or pos ='cxy';
9、order by
正常(索引参与了排序) explain select * from user where name = 'zhangsan' and age = 20 order by age,pos; 备注:索引有两个作用:排序和查找
导致额外的文件排序(会降低性能) explain select name,age from user where name = 'zhangsan' order by pos;//违反最左前缀法则 explain select name,age from user where name = 'zhangsan' order by pos,age;//违反最左前缀法则 explain select * from user where name = 'zhangsan' and age = 20 order by created_time,age;//含非索引字段
10、group by
正常(索引参与了排序) explain select name,age from user where name = 'zhangsan' group by age; 备注:分组之前必排序(排序同order by)
导致产生临时表(会降低性能) explain select name,pos from user where name = 'zhangsan' group by pos;//违反最左前缀法则 explain select name,age from user where name = 'zhangsan' group by pos,age;//违反最左前缀法则 explain select name,age from user where name = 'zhangsan' group by age,created_time;//含非索引字段
使用的示例数据
mysql> show create table user \G ****************************************************** Table: user Create Table: CREATE TABLE `user` ( `id` int(10) NOT NULL AUTO_INCREMENT, `name` varchar(20) DEFAULT NULL, `age` int(10) DEFAULT '0', `pos` varchar(30) DEFAULT NULL, `phone` varchar(11) DEFAULT NULL, `created_time` datetime DEFAULT NULL, PRIMARY KEY (`id`), KEY `idx_name_age_pos_phone` (`name`,`age`,`pos`,`phone`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
了解更多SQL常见问题及解决方式请查看原文:https://developer.aliyun.com/article/741403?utm_content=g_1000097870