mysql视频教程栏目索引失效的情况。
索引对于MySQL而言，是非常重要的篇章。索引知识点也巨多，要想掌握透彻，需要逐个知识点一一击破，今天来先来聊聊哪些情况下会导致索引失效。
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全值匹配(索引最佳)explain select * from user where name = 'zhangsan' and age = 20 and pos = 'cxy' and phone = '18730658760';
和索引顺序无关，MySQL底层的优化器会进行优化，调整索引的顺序
explain select * from user where name = 'zhangsan' and age = 20 and pos = 'cxy' and phone = '18730658760';
1、违反最左前缀法则如果索引有多列，要遵守最左前缀法则
即查询从索引的最左前列开始并且不跳过索引中的列
explain select * from user where age = 20 and phone = '18730658760' and pos = 'cxy';
2、在索引列上做任何操作如计算、函数、(自动or手动)类型转换等操作，会导致索引失效从而全表扫描
explain select * from user where left(name,5) = 'zhangsan' and age = 20 and phone = '18730658760';
3、索引范围条件右边的列索引范围条件右边的索引列会失效
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
