使用Mysql的索引進行排序可以大大提高Mysql的效能，使用索引排序的原則是

按照建立索引的順序查詢和排序，執行如下sql

create table t1(
	id int not null auto_increment,
  uname varchar(32) not null,
  tag1 int not null,
  tag2 int not null,
  tag3 int not null,
  PRIMARY KEY(id)
)

create index idx_t1_tag1_tag2_tag3 on t1(tag1,tag2,tag3)

insert into t1(uname,tag1,tag2,tag3) 
values 
('aaa',11,23,33),
('bbb',12,2,99),
('ccc',9,3,999),
('ddd',99,99,99),
('eee',199,199,199),
('fff',299,299,299),
('ggg',99,19,9),
('hhh',1,4,12),
('jjj',1,3,121),
('kkk',1,3,16),
('lll',1,2,12)
 

可以看到t1表有一個複合索引，tag1、tag2、tag3，所謂的按照索引順序查詢和排序指在查詢即select列表中按照tag1->tag2->tag3的順序查詢，order by之後一樣

explain select tag1,tag2 from t1 ORDER BY tag1,tag2

如果是where和order by 組合起來後欄位依然是“有序”的，那麼依然會使用索引排序

explain select tag1,tag2 from t1 where tag1 = 1 ORDER BY tag2,tag3