这些SQL错误用法，如果经常犯，说明你的水平还很low...

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来源：developer.aliyun.com/article/72501
今天来分享几个MySQL常见的SQL错误（不当）用法。我们在作为一个初学者时，很有可能自己在写SQL时也没有注意到这些问题，导致写出来的SQL语句效率低下，所以我们也可以自省自检一下。


1、 LIMIT 语句


分页查询是最常用的场景之一，但也通常也是最容易出问题的地方。比如对于下面简单的语句，一般DBA想到的办法是在type, name, create_time字段上加组合索引。这样条件排序都能有效的利用到索引，性能迅速提升。

SELECT * FROM operation WHERE type = 'SQLStats'  AND name = 'SlowLog' ORDER BY create_time LIMIT 1000, 10;

好吧，可能90%以上的DBA解决该问题就到此为止。但当 LIMIT 子句变成 “LIMIT 1000000,10” 时，程序员仍然会抱怨：我只取10条记录为什么还是慢？


要知道数据库也并不知道第1000000条记录从什么地方开始，即使有索引也需要从头计算一次。出现这种性能问题，多数情形下是程序员偷懒了。在前端数据浏览翻页，或者大数据分批导出等场景下，是可以将上一页的最大值当成参数作为查询条件的。SQL重新设计如下：

SELECT * FROM operation WHERE type = 'SQLStats' AND name = 'SlowLog' AND create_time > '2017-03-16 14:00:00' ORDER BY create_time limit 10;

在新设计下查询时间基本固定，不会随着数据量的增长而发生变化。


2、隐式转换


SQL语句中查询变量和字段定义类型不匹配是另一个常见的错误。比如下面的语句：

mysql> explain extended SELECT *  > FROM my_balance b  > WHERE b.bpn = 14000000123  > AND b.isverified IS NULL ;mysql> show warnings;| Warning | 1739 | Cannot use ref access on index 'bpn' due to type or collation conversion on field 'bpn'
 

 


 

3、关联更新、删除
 


 

虽然MySQL5.6引入了物化特性，但需要特别注意它目前仅仅针对查询语句的优化。对于更新或删除需要手工重写成JOIN。
 


 

比如下面UPDATE语句，MySQL实际执行的是循环/嵌套子查询（DEPENDENT SUBQUERY)，其执行时间可想而知。
 

 
 
     
  
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UPDATE operation o SET status = 'applying' WHERE o.id IN (SELECT id  FROM (SELECT o.id,  o.status  FROM operation o  WHERE o.group = 123  AND o.status NOT IN ( 'done' )  ORDER BY o.parent,  o.id  LIMIT 1) t);
 

 

执行计划：
 

 
 
     
  
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+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+| 1 | PRIMARY | o | index | | PRIMARY | 8 | | 24 | Using where; Using temporary || 2 | DEPENDENT SUBQUERY | | | | | | | | Impossible WHERE noticed after reading const tables || 3 | DERIVED | o | ref | idx_2,idx_5 | idx_5 | 8 | const | 1 | Using where; Using filesort |+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+
 

 

重写为JOIN之后，子查询的选择模式从DEPENDENT SUBQUERY变成DERIVED,执行速度大大加快，从7秒降低到2毫秒。
 

 
 
     
  
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UPDATE operation o  JOIN (SELECT o.id,  o.status  FROM operation o  WHERE o.group = 123  AND o.status NOT IN ( 'done' )  ORDER BY o.parent,  o.id  LIMIT 1) t ON o.id = t.id SET status = 'applying'
 

 

执行计划简化为：
 

 
 
     
  
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+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+| 1 | PRIMARY | | | | | | | | Impossible WHERE noticed after reading const tables || 2 | DERIVED | o | ref | idx_2,idx_5 | idx_5 | 8 | const | 1 | Using where; Using filesort |+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+
 

 


 

4、混合排序
 


 

MySQL不能利用索引进行混合排序。但在某些场景，还是有机会使用特殊方法提升性能的。
 

 
 
     
  
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SELECT * FROM my_order o  INNER JOIN my_appraise a ON a.orderid = o.id ORDER BY a.is_reply ASC,  a.appraise_time DESC LIMIT 0, 20
 

 

执行计划显示为全表扫描：
 

 
 
     
  
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+----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra +----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+| 1 | SIMPLE | a | ALL | idx_orderid | NULL | NULL | NULL | 1967647 | Using filesort || 1 | SIMPLE | o | eq_ref | PRIMARY | PRIMARY | 122 | a.orderid | 1 | NULL |+----+-------------+-------+--------+---------+---------+---------+-----------------+---------+-+
 

 

由于is_reply只有0和1两种状态，我们按照下面的方法重写后，执行时间从1.58秒降低到2毫秒。
 

 
 
     
  
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SELECT * FROM ((SELECT * FROM my_order o  INNER JOIN my_appraise a  ON a.orderid = o.id  AND is_reply = 0  ORDER BY appraise_time DESC  LIMIT 0, 20)  UNION ALL  (SELECT * FROM my_order o  INNER JOIN my_appraise a  ON a.orderid = o.id  AND is_reply = 1  ORDER BY appraise_time DESC  LIMIT 0, 20)) t ORDER BY is_reply ASC,  appraisetime DESC LIMIT 20;
 

 


 

5、EXISTS语句
 


 

MySQL对待EXISTS子句时，仍然采用嵌套子查询的执行方式。如下面的SQL语句：
 

 
 
     
  
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SELECT *FROM my_neighbor n  LEFT JOIN my_neighbor_apply sra  ON n.id = sra.neighbor_id  AND sra.user_id = 'xxx' WHERE n.topic_status < 4  AND EXISTS(SELECT 1  FROM message_info m  WHERE n.id = m.neighbor_id  AND m.inuser = 'xxx')  AND n.topic_type <> 5
 

 

执行计划为：
 

 
 
     
  
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+----+--------------------+-------+------+-----+------------------------------------------+---------+-------+---------+ -----+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+--------------------+-------+------+ -----+------------------------------------------+---------+-------+---------+ -----+| 1 | PRIMARY | n | ALL | | NULL | NULL | NULL | 1086041 | Using where || 1 | PRIMARY | sra | ref | | idx_user_id | 123 | const | 1 | Using where || 2 | DEPENDENT SUBQUERY | m | ref | | idx_message_info | 122 | const | 1 | Using index condition; Using where |+----+--------------------+-------+------+ -----+------------------------------------------+---------+-------+---------+ -----+
 

 

去掉exists更改为join，能够避免嵌套子查询，将执行时间从1.93秒降低为1毫秒。
 

 
 
     
  
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SELECT *FROM my_neighbor n  INNER JOIN message_info m  ON n.id = m.neighbor_id  AND m.inuser = 'xxx'  LEFT JOIN my_neighbor_apply sra  ON n.id = sra.neighbor_id  AND sra.user_id = 'xxx' WHERE n.topic_status < 4  AND n.topic_type <> 5
 

 

新的执行计划：
 

 
 
     
  
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+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+| 1 | SIMPLE | m | ref | | idx_message_info | 122 | const | 1 | Using index condition || 1 | SIMPLE | n | eq_ref | | PRIMARY | 122 | ighbor_id | 1 | Using where || 1 | SIMPLE | sra | ref | | idx_user_id | 123 | const | 1 | Using where |+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+
 

 


 

6、条件下推
 


 

外部查询条件不能够下推到复杂的视图或子查询的情况有：
 

     
 
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    聚合子查询； 
  
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    含有LIMIT的子查询； 
  
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    UNION 或UNION ALL子查询； 
  
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    输出字段中的子查询； 
  
    5.  

 


 

如下面的语句，从执行计划可以看出其条件作用于聚合子查询之后：
 

 
 
     
  
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SELECT * FROM (SELECT target,  Count(*)  FROM operation  GROUP BY target) t WHERE target = 'rm-xxxx' +----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+| 1 | PRIMARY | <derived2> | ref | <auto_key0> | <auto_key0> | 514 | const | 2 | Using where || 2 | DERIVED | operation | index | idx_4 | idx_4 | 519 | NULL | 20 | Using index |+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+
 

 

确定从语义上查询条件可以直接下推后，重写如下：
 

 
 
     
  
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SELECT target,  Count(*) FROM operation WHERE target = 'rm-xxxx' GROUP BY target
 

 

执行计划变为：
 

 
 
     
  
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+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+| 1 | SIMPLE | operation | ref | idx_4 | idx_4 | 514 | const | 1 | Using where; Using index |+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
 

 


 

7、提前缩小范围
 


 

先上初始SQL语句：
 

 
 
     
  
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SELECT * FROM my_order o  LEFT JOIN my_userinfo u  ON o.uid = u.uid LEFT JOIN my_productinfo p  ON o.pid = p.pid WHERE ( o.display = 0 )  AND ( o.ostaus = 1 ) ORDER BY o.selltime DESC LIMIT 0, 15
 

 

该SQL语句原意是：先做一系列的左连接，然后排序取前15条记录。从执行计划也可以看出，最后一步估算排序记录数为90万，时间消耗为12秒。
 

 
 
     
  
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+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+| 1 | SIMPLE | o | ALL | NULL | NULL | NULL | NULL | 909119 | Using where; Using temporary; Using filesort || 1 | SIMPLE | u | eq_ref | PRIMARY | PRIMARY | 4 | o.uid | 1 | NULL || 1 | SIMPLE | p | ALL | PRIMARY | NULL | NULL | NULL | 6 | Using where; Using join buffer (Block Nested Loop) |+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+
 

 

由于最后WHERE条件以及排序均针对最左主表，因此可以先对my_order排序提前缩小数据量再做左连接。SQL重写后如下，执行时间缩小为1毫秒左右。
 

 
 
     
  
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SELECT * FROM (SELECT * FROM my_order o WHERE ( o.display = 0 )  AND ( o.ostaus = 1 ) ORDER BY o.selltime DESC LIMIT 0, 15) o  LEFT JOIN my_userinfo u  ON o.uid = u.uid  LEFT JOIN my_productinfo p  ON o.pid = p.pid ORDER BY o.selltime DESClimit 0, 15
 

 

再检查执行计划：子查询物化后（select_type=DERIVED)参与JOIN。虽然估算行扫描仍然为90万，但是利用了索引以及LIMIT 子句后，实际执行时间变得很小。
 

 
 
     
  
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+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 15 | Using temporary; Using filesort || 1 | PRIMARY | u | eq_ref | PRIMARY | PRIMARY | 4 | o.uid | 1 | NULL || 1 | PRIMARY | p | ALL | PRIMARY | NULL | NULL | NULL | 6 | Using where; Using join buffer (Block Nested Loop) || 2 | DERIVED | o | index | NULL | idx_1 | 5 | NULL | 909112 | Using where |+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+
 

 

 
 
 
  
这些SQL错误用法，如果经常犯，说明你的水平还很low...
 
  
 
   
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