SQL优化十大法则

一、SQL优化—优化实战

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());
添加索引：

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、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。

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、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、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、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 * 使用

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、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、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

创建staffs2表，不设置name属性得默认值，如下所示：

alter table staffs2 add index idx_staffs_nameAgePos(name,age,pos);
alter table staffs2 add index idx_staffs_name(name);
执行下面查询语句：

EXPLAIN select * from staffs2 where name is null;  -- 带索引查询
EXPLAIN select * from staffs2 where name is not null;  -- 无索引
查询结果如下如所示：

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、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、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

EXPLAIN select * from staffs where name = '917';

10、OR改UNION效率更高

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

-- 解决方式：覆盖索引

EXPLAIN select name,age from staffs where name='July' or name = 'z3';

EXPLAIN select * from staffs where name='July' UNION select * from staffs where  name = 'z3';

记忆总结：

全职匹配我最爱，最左前缀要遵守；

带头大哥不能死，中间兄弟不能断；

索引列上少计算，范围之后全失效；

LIKE百分写最右，覆盖索引不写*；

不等空值还有OR，索引影响要注意；

VAR引号不可丢， SQL优化有诀窍。