1.說明

　　在使用order by時，經常會出現Using fileSort

　　在使用group by時，使用不當，會出現Using temporay

二：排序

1..Using fileSort

　　說明會對資料使用一個外部的索引排序

　　而不是內部的索引順序進行

　　mysql無法利用索引完成排序操作稱為檔案排序

1.索引使用情況

explain
select * from employee where name = '魯班' 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 = '魯班' order by  dep_id,age;

explain
select name,dep_id,age from employee where name = '魯班' order by  dep_id,age;

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

explain
select name,dep_id,age from employee where name = '魯班'  order by age,dep_id;

explain
select name,dep_id,age from employee where name = '魯班'  order by dep_id asc, age desc;

　　結論：

　　使用覆蓋索引，同時order by符合索引順序。也不能一升一降。

三：分組

1.說明

　　分組前必排序

2.