【数据库】之MySQL索引优化

DROP TABLE IF EXISTS `staffs`;
CREATE TABLE `staffs` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(24) NOT NULL,
  `age` int(11) NOT NULL DEFAULT '0',
  `pos` varchar(20) NOT NULL,
  `add_time` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
 
insert into staffs(name,age,pos,add_time) values ('z3',22,'manager',NOW());
insert into staffs(name,age,pos,add_time) values ('july',23,'dex',NOW());
insert into staffs(name,age,pos,add_time) values ('2000',23,'manager',NOW());
 
alter table staffs add index idx_staffs_nap(name,age,pos);

1.全值匹配我最爱

EXPLAIN SELECT * FROM staffs WHERE NAME = 'July';

EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' AND age = 25;

EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' AND age = 25 AND pos = 'dev';

2.最佳左前缀法则

如果索引了多例，要遵守最左前缀法则。指的是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 staffs WHERE NAME = 'July' AND age = 23 AND pos = 'dev';

最左前列索引不能丢

EXPLAIN SELECT * FROM staffs WHERE age = 23 AND pos = 'dev';

不能跳过中间列索引

只用到了第一个索引

EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' AND pos = 'dev';

EXPLAIN SELECT * FROM staffs WHERE NAME = 'July';

3.不在索引列上做任何操作

计算、函数、（自动or手动）类型转换等，会导致索引失效而转向全表扫描

EXPLAIN SELECT * FROM staffs WHERE NAME = 'July';

EXPLAIN SELECT * FROM staffs WHERE left(NAME,4) = 'July';

4.存储引擎不能使用索引中范围条件右边的列

范围之后全失效

EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' AND age = 25 AND pos = 'manager';

EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' AND age > 25 AND pos = 'manager';

5.尽量使用覆盖索引

只访问索引的查询即索引列和查询列一致或在索引列内，减少select *

EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' AND age = 25 AND pos = 'manager';

EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME = 'July' AND age = 25 AND pos = 'manager';

6.MySQL在使用不等于（！=或者<>）的时候无法使用索引会导致全表扫描

explain select * from staffs where name = 'July';

explain select * from staffs where name != 'July';

explain select * from staffs where name <> 'July';

7.is null，is not null 也无法使用索引

explain select * from staffs where name is null;

explain select * from staffs where name is not null;

8.like以通配符开头（’$abc…’）MySQL 索引失效会变成全表扫描操作。

like百分号写右边

explain select * from staffs where name like '%July';

explain select * from staffs where name like '%July%';

explain select * from staffs where name like 'July%';

问题：解决like’%字符串%'时索引不被使用的方法

实验证明，覆盖索引，可以解决左右百分号全表扫描的问题。

CREATE TABLE `tbl_user`(
  `id` INT(11) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(30) DEFAULT NULL,
  `age` INT(11) DEFAULT NULL,
  `email` VARCHAR(20)  DEFAULT NULL,
  PRIMARY KEY (`id`)
)ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
 
insert into tbl_user(name,age,email) values ('1aa1',21,'[email protected]');
insert into tbl_user(name,age,email) values ('2aa2',222,'[email protected]');
insert into tbl_user(name,age,email) values ('3aa3',265,'[email protected]');
insert into tbl_user(name,age,email) values ('4aa4',21,'[email protected]');

select * from tbl_user;

select * from tbl_user where name like '%aa%';

select * from tbl_user where name like 'aa%';
# Empty set

select * from tbl_user where name like '%aa';
# Empty set

建索引前，全表扫描，建立索引后用覆盖索引

索引建立前后对比

create index idx_user_name_age on tbl_user(name,age);

explain select name,age from tbl_user where name like '%aa%'; 

explain select age from tbl_user where name like '%aa%';

explain select id from tbl_user where name like '%aa%';   

explain select name from tbl_user where name like '%aa%';  

explain select id, name, age from tbl_user where name like '%aa%';

explain select id, name, age,email from tbl_user where name like '%aa%';   

explain select * from tbl_user where name like '%aa%'; 

9.字符串不加单引号索引失效

desc staffs;

name是varchar类型，但是有个值是2000。条件查询的时候，这个2000的值可以有单引号也可以没有。接下来我们看两种情况的区别。

select * from staffs where name = '2000';

select * from staffs where name = 2000;

如果不用单引号，就会把2000从整型自动转换成字符型。对应上面的第3条规则（不能有隐式的类型转换）。

explain select * from staffs where name = '2000';

explain select * from staffs where name = 2000;

10.少用or，用它连接时会索引失效

select * from staffs where name = 'July' or name = 'z3';

explain select * from staffs where name = 'July' or name = 'z3';

总结

index(a,b,c)

WHERE语句	索引是否被使用
where a = 3	Y，使用到 a
where a = 3 and b = 5	Y，使用到 a，b
where a = 3 and b = 5 and c = 4	Y，使用到 a，b，c
where a = 3 或者 where b = 3 and c = 4 或者 where c = 4	N
where a = 3 and c = 5	使用到 a，但是 c 不可以，b 中间断了
where a = 3 and b > 4 and c = 5	使用到 a 和 b，c 不能用在范围之后，b 断了
where a = 3 and b like ‘kk%’ and c = 4	a 能用，b 不能用，c 不能用