mysql視頻教程欄目索引失效的情況。

索引對于MySQL而言，是非常重要的篇章。索引知識點(diǎn)也巨多，要想掌握透徹，需要逐個知識點(diǎn)一一擊破，今天來先來聊聊哪些情況下會導(dǎo)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';

和索引順序無關(guān)，MySQL底層的優(yōu)化器會進(jìn)行優(yōu)化，調(diào)整索引的順序

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，要遵守最左前綴法則

即查詢從索引的最左前列開始并且不跳過索引中的列

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計(jì)算、函數(shù)、(自動or手動)類型轉(zhuǎn)換等操作，會導(dǎo)致索引失效從而全表掃描

explain select * from user where left(name,5) = 'zhangsan' and age = 20 and phone = '18730658760';

3、索引范圍條件右邊的列索引范圍條件右邊的索引列會失效

explain select * from user where name = 'zhangsan' and age > 20 and pos = 'cxy';

4、盡量使用覆蓋索引只訪問索引查詢(索引列和查詢列一致)，減少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(!=、<>)的時(shí)候無法使用索引會導(dǎo)致全表掃描(除覆蓋索引外)

explain select * from user where age != 20;

explain select * from user where age <> 20;

6、like以通配符開頭('%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(索引參與了排序)

explain select * from user where name = 'zhangsan' and age = 20 order by age,pos;

備注：索引有兩個作用：排序和查找導(dǎo)致額外的文件排序(會降低性能)

explain select name,age from user where name = 'zhangsan' order by pos;//違反最左前綴法則

explain select name,age from user where name = 'zhangsan' order by pos,age;//違反最左前綴法則

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(索引參與了排序)

explain select name,age from user where name = 'zhangsan' group by age;

備注：分組之前必排序(排序同order by)

導(dǎo)致產(chǎn)生臨時(shí)表(會降低性能)

explain select name,pos from user where name = 'zhangsan' group by pos;//違反最左前綴法則

explain select name,age from user where name = 'zhangsan' group by pos,age;//違反最左前綴法則

explain select name,age from user where name = 'zhangsan' group by age,created_time;//含非索引字段

使用的示例數(shù)據(jù)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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