排序分组优化

Posted 2020-12-18 juncaoit

1.说明

　　在使用order by时，经常会出现Using fileSort

　　在使用group by时，使用不当，会出现Using temporay

 

二：排序

1..Using fileSort

　　说明会对数据使用一个外部的索引排序

　　而不是内部的索引顺序进行

　　mysql无法利用索引完成排序操作称为文件排序

　　

1.索引使用情况

explain
select * from employee where name = ‘鲁班‘ and dep_id = 1 order by  age;

　　效果：

　　

 

 

explain
select name,dep_id,age from employee order by  name,dep_id,age;

　　

 

 

explain
select * from employee order by  name,dep_id,age;

　　

 

 

explain
select name,dep_id,age from employee order by  name,age;

　　

 

 

explain
select * from employee where name = ‘鲁班‘ order by  dep_id,age;

　　

 

 

explain
select name,dep_id,age from employee where name = ‘鲁班‘ order by  dep_id,age;

　　

 

 

explain
select name,dep_id,age from employee where name = ‘鲁班‘ and dep_id > 1 order by age;

　　

 

explain
select name,dep_id,age from employee where name = ‘鲁班‘  order by age,dep_id;

　　

 

 

explain
select name,dep_id,age from employee where name = ‘鲁班‘  order by dep_id asc, age desc;

　　

 

 

　　结论：

　　使用覆盖索引，同时order by符合索引顺序。也不能一升一降。

 

 

三：分组

1.说明

　　分组前必排序

 

2.