MySQL- SQL优化通过

MySQL-- SQL优化经过

背景：有一个SQL如下

SELECT ut.* 
 FROM user_transaction ut 
 WHERE tx_type IN (1, 4, 8) AND tx_status = 1
 AND share_public = 1 
 AND user_id != 103 AND (peer_user_id IS NULL OR peer_user_id != 103) 
 AND (
 user_id IN (SELECT DISTINCT friend_user_id from user_friend uf WHERE uf.user_id = 103 AND uf.status = 1) 
AND
 peer_user_id IN (SELECT DISTINCT friend_user_id from user_friend uf WHERE uf.user_id = 103 AND uf.status = 1) 
 ) 
 ORDER BY tx_date DESC 
 LIMIT 1, 10;


--也有下面OR的情况，业务需要
SELECT ut.* 
 FROM user_transaction ut 
 WHERE tx_type IN (1, 4, 8) AND tx_status = 1
 AND share_public = 1 
 AND user_id != 103 AND (peer_user_id IS NULL OR peer_user_id != 103) 
 AND (
 user_id IN (SELECT DISTINCT friend_user_id from user_friend uf WHERE uf.user_id = 103 AND uf.status = 1) 
OR
 peer_user_id IN (SELECT DISTINCT friend_user_id from user_friend uf WHERE uf.user_id = 103 AND uf.status = 1) 
 ) 
 ORDER BY tx_date DESC 
 LIMIT 1, 10;

有索引，速度还算可以

问题所在

1. 这里用了in，如果in里面的数据比较多，索引就可能会没有（InnoDB几十条数据就会出现），影响执行速度。
2. 这里条件用了OR，想要改SQL不容易。

修改前的思考，解决方法

1. 用Between代替IN，不过这里是IN里面用了子查询，所以不能用。
2. 用Exists代替IN，试了下，结果反而没了索引，速度更慢。
3. 用Union All代替IN或者AND或者OR，这种方法在这里的IN行不通，因为IN里面的ID很多。
4. 用Inner join代替IN，这是网上的方法

实际修改

用Between代替IN

    这个方法直接就没有试，因为不合适。

用Exists代替IN

修改如下：

SELECT distinct pub_name
  FROM publishers
  WHERE pub_id IN
    (SELECT pub_id
      FROM titles
      WHERE type = 'business')

SELECT DISTINCT pub_name
  FROM publishers
  WHERE EXISTS
  (SELECT *
    FROM titles
    WHERE pub_id = publishers.pub_id
    AND type = 'business')

 但是实际情况反而索引没有了，执行速度更慢。

用Inner join代替IN

写法如下：

 SELECT ut.* 
 FROM user_transaction ut 
INNER JOIN user_friend uf ON uf.friend_user_id = ut.user_id AND uf.user_id = 103 AND uf.status = 1
INNER JOIN user_friend uf2 ON uf2.friend_user_id = ut.peer_user_id AND uf2.user_id = 103 AND uf2.status = 1
 WHERE tx_type IN (1, 4, 8) AND tx_status = 1
 AND share_public = 1 
 AND ut.user_id != 103 AND (peer_user_id IS NULL OR peer_user_id != 103)
 ORDER BY tx_date DESC 
 LIMIT 1, 10;

 这样写，既保证了索引，对SQL的影响也不大，但是还剩最后一个问题，就是怎么将OR的数据合在一起。

使用Union All 或Union

写法如下：

(
 SELECT ut.* 
 FROM user_transaction ut 
INNER JOIN user_friend uf ON uf.friend_user_id = ut.user_id AND uf.user_id = 103 AND uf.status = 1
 WHERE tx_type IN (1, 4, 8) AND tx_status = 1
 AND share_public = 1 
 AND ut.user_id != 103 AND (peer_user_id IS NULL OR peer_user_id != 103)
)
UNION
(
 SELECT ut.* 
 FROM user_transaction ut 
INNER JOIN user_friend uf ON uf.friend_user_id = ut.peer_user_id AND uf.user_id = 103 AND uf.status = 1
 WHERE tx_type IN (1, 4, 8) AND tx_status = 1
 AND share_public = 1 
 AND ut.user_id != 103 AND (peer_user_id IS NULL OR peer_user_id != 103)
)
ORDER BY tx_date DESC 
 LIMIT 0, 10;

这样就将OR条件的数据合并在一起了，而可以思考是否过滤重复。