Mysql优化

Posted 2022-04-14 lys-lyy

1. 策略1.尽量全值匹配

 

CREATE TABLE `staffs`(

id int primary key auto_increment,

name varchar(24) not null default "" comment‘姓名‘,

age int not null default 0 comment ‘年龄‘,

pos varchar(20) not null default ""  comment‘职位‘,

add_time timestamp not null default current_timestamp comment ‘入职时间‘

)charset utf8 comment ‘员工记录表‘;

insert into staffs(name,age,pos,add_time) values(‘z3‘,22,‘manage‘,now());

insert into staffs(name,age,pos,add_time) values(‘july‘,23,‘dev‘,now());

insert into staffs(name,age,pos,add_time) values(‘2000‘,23,‘dev‘,now());

 

alter table staffs add index idx_staffs_nameAgePos(name,age,pos);

 

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‘

 

当建立了索引列后，能在wherel条件中使用索引的尽量所用。

2. 策略2.最佳左前缀法则

如果索引了多列，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（和顺序无关，但是不能丢）。

EXPLAIN SELECT * FROM staffs WHERE  age = 25 AND pos = ‘dev‘

EXPLAIN SELECT * FROM staffs WHERE pos = ‘dev‘

EXPLAIN SELECT * FROM staffs WHERE NAME = ‘July‘

3. 策略3.不在索引列上做任何操作

不在索引列上做任何操作（计算、函数、(自动or手动)类型转换），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. 策略4.范围条件放最后

 

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

EXPLAIN SELECT * FROM staffs WHERE NAME = ‘July‘  and age =22;

EXPLAIN SELECT * FROM staffs WHERE NAME = ‘July‘  and age =22 and pos=‘manager‘

 

中间有范围查询会导致后面的索引列全部失效

EXPLAIN SELECT * FROM staffs WHERE NAME = ‘July‘  and age >22 and pos=‘manager‘

5. 策略5.覆盖索引尽量用

尽量使用覆盖索引(只访问索引的查询(索引列和查询列一致))，减少select *

EXPLAIN SELECT * FROM staffs WHERE NAME = ‘July‘  and age =22 and pos=‘manager‘

EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME = ‘July‘  and age =22 and pos=‘manager‘

EXPLAIN SELECT * FROM staffs WHERE NAME = ‘July‘  and age >22 and pos=‘manager‘

EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME = ‘July‘  and age >22 and pos=‘manager‘

6.策略6.不等于要甚用

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‘;

如果定要需要使用不等于,请用覆盖索引

EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME != ‘July‘;

EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME <> ‘July‘;

7. 策略7.Null/Not 有影响

 

注意null/not null对索引的可能影响

 

7.1. 自定定义为NOT NULL

EXPLAIN select * from staffs where name is null

EXPLAIN select * from staffs where name is not null

在字段为not null的情况下，使用is null 或 is not null 会导致索引失效

解决方式：覆盖索引

EXPLAIN select  name,age,pos from staffs where name is not null

7.2. 自定义为NULL或者不定义

EXPLAIN select * from staffs2 where name is null

EXPLAIN select * from staffs2 where name is not null

EXPLAIN select * from staffs2 where name is not null

Is not null 的情况会导致索引失效

解决方式：覆盖索引

EXPLAIN select  name,age,pos from staffs where name is not null

8. 策略8.Like查询要当心

like以通配符开头(‘%abc...‘)mysql索引失效会变成全表扫描的操作

EXPLAIN select * from staffs where name =‘july‘

EXPLAIN select * from staffs where name like ‘%july%‘

EXPLAIN select * from staffs where name like ‘%july‘

EXPLAIN select * from staffs where name like ‘july%‘

解决方式：覆盖索引

EXPLAIN select name,age,pos from staffs where name like ‘%july%‘

9. 策略9.字符类型加引号

字符串不加单引号索引失效

EXPLAIN select * from staffs where name = 917

解决方式：覆盖索引

EXPLAIN select  name,age,pos from staffs where name = 917

解决方式：请加引号

10. 策略10.OR改UNION效率高

EXPLAIN

select * from staffs where name=‘July‘ or name = ‘z3‘

EXPLAIN

select * from staffs where name=‘July‘

UNION

select * from staffs where  name = ‘z3‘

解决方式：覆盖索引

EXPLAIN

select name,age from staffs where name=‘July‘ or name = ‘z3‘