小编给大家分享一下mysql索引失效的原因,希望大家阅读完这篇文章后大所收获,下面让我们一起去探讨吧!
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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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';
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';
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';
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';
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;
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;
索引失效 explain select * from user where name like '%zhangsan';
索引生效 explain select * from user where name like 'zhangsan%';
explain select * from user where name = 2000;
少用or explain select * from user where name = '2000' or age = 20 or pos ='cxy';
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
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
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
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