mysql高效查询left join和group by(加索引)

mysql高效查询

mysql牺牲了group by来增加left join的速度（前提是加了索引）。

user表：10万数据

实例1： 200秒左右

				?

									SELECT U.id, A.favorite_count FROM (SELECT id from user) U

									LEFT JOIN (

									  -- 点赞数

									  SELECT favorite_by AS user_id, SUM(favorite_count) AS favorite_count

									  FROM favorite

									  GROUP BY favorite_by

									)  A ON U.id=A.user_id

									LEFT JOIN (

									  -- 评论数量

									  SELECT user_id, COUNT(*) AS comment_count

									  FROM photo_comment

									  GROUP BY user_id

									) B ON U.id=B.user_id

实例2：1秒多

				?

									select uf.user_id , uf.favorite_count, COUNT(pc.id) as comment_count from (

									select u.id as user_id , SUM(f.favorite_count) as favorite_count from (SELECT id from user) u 

									LEFT JOIN favorite f on f.favorite_by = u.id  

									GROUP BY u.id

									) uf 

									LEFT JOIN photo_comment pc on pc.user_id = uf.user_id

									GROUP BY uf.user_id

附：如何在MySQL高效的join3个表

对于下面这个三个表的join语句

				?

									select * 

									from t1 

									join t2 on(t1.a=t2.a) 

									join t3 on (t2.b=t3.b) 

									where t1.c>=X and t2.c>=Y and t3.c>=Z;