索引對於MySQL而言,是非常重要的章節。索引知識點也巨多,要掌握透徹,需要逐一知識點一一擊破,今天來先聊聊哪些情況下會導致索引失效。
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全值符合(索引最佳)#相關免費學習推薦:##mysql影片教學
explain select * from user where name = 'zhangsan' and age = 20 and pos = 'cxy' and phone = '18730658760';
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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 、違反最左前綴法則
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、在索引列上做任何動作
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、索引範圍條件右邊的欄位
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、盡量使用覆蓋索引
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、使用不等於(! =、)
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以萬用字元開頭('�c')
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、字串不加單引號索引失效
explain select * from user where name = 2000;
8、or連接
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
使用的範例資料
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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