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SQL优化

作者:互联网

1.1. 优化实战

1.1.1. 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。

 

 

1.1.2. 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。

 

 

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

EXPLAIN SELECT * FROM staffs WHERE pos = 'dev'

EXPLAIN SELECT * FROM staffs WHERE NAME = 'July'

 

1.1.3. 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';

 

1.1.4. 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'

 

1.1.5. 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'

 

1.1.6. 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';

 

1.1.7. 策略7.Null/Not 有影响

注意null/not null对索引的可能影响

 

1.1.7.1. 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

 

 

1.1.7.2. 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

 

 

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

 

 

 

1.1.8. 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%'

 

1.1.9. 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

 

1.1.10. 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'

 

 

 

1.1.11. 例子

 

 

 

 

答案:

 

 

 

记忆总结:

l 全职匹配我最爱,最左前缀要遵守;

 

l 带头大哥不能死,中间兄弟不能断;

 

l 索引列上少计算,范围之后全失效;

 

l LIKE百分写最右,覆盖索引不写*;

 

l 不等空值还有OR,索引影响要注意;

 

l VAR引号不可丢, SQL优化有诀窍。

 

1.2. 批量导入

 

 

 

1.2.1. insert语句优化;

l   提交前关闭自动提交

  尽量使用批量insert语句

  可以使用MyISAM存储引擎

 

 

1.2.2. LOAD DATA INFLIE

LOAD DATA INFLIE;

使用LOAD DATA INFLIE ,比一般的insert语句快20倍

 

select * into OUTFILE 'D:\\product.txt' from product_info

 

load data INFILE 'D:\\product.txt' into table product_info

 

 

load data INFILE '/soft/product3.txt' into table product_info

 

 

 

查看当前数据库是否支持load data方式导入导出:

show VARIABLES like 'secure_file_priv'

 

 

 解释:

l secure_file_priv 为 NULL 时,表示限制mysqld不允许导入或导出。

 

l secure_file_priv 为 /tmp 时,表示限制mysqld只能在/tmp目录中执行导入导出,其他目录不能执行。

 

l secure_file_priv 没有值时,表示不限制mysqld在任意目录的导入导出。

 

配置当前数据库支持load data方式导入导出:

  在my.ini文件中添加:secure_file_priv=''

 

标签:name,age,pos,SQL,July,staffs,优化,EXPLAIN
来源: https://www.cnblogs.com/Soy-technology/p/11089325.html