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2019
08-08

MySQL常见SQL错误用法


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'


其中字段 bpn 的定义为 varchar(20),MySQL 的策略是将字符串转换为数字之后再比较。函数作用于表字段,索引失效。上述情况可能是应用程序框架自动填入的参数,而不是程序员的原意。现在应用框架很多很繁杂,使用方便的同时也小心它可能给自己挖坑。

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。
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:
+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+| 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
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:
+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+| 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。
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:
+----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+| 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。
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:
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:
+----+--------------------+-------+------+-----+------------------------------------------+---------+-------+---------+ -----+| 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。
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:
+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+| 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:

聚合子查询;

含有 LIMIT 的子查询;

UNION 或 UNION ALL 子查询;

输出字段中的子查询;


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


执行计划变为:
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+| 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:
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。
+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+| 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。
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。
+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+| 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 |+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+

8、中间结果集下推

再来看下面这个已经初步优化过的例子(左连接中的主表优先作用查询条件):
SELECT a.*,  c.allocated FROM (  SELECT resourceid  FROM my_distribute d  WHERE isdelete = 0  AND cusmanagercode = '1234567'  ORDER BY salecode limit 20) a LEFT JOIN  (  SELECT resourcesid, sum(ifnull(allocation, 0) * 12345) allocated  FROM my_resources  GROUP BY resourcesid) c ON a.resourceid = c.resourcesid


那么该语句还存在其它问题吗?不难看出子查询 c 是全表聚合查询,在表数量特别大的情况下会导致整个语句的性能下降。
其实对于子查询 c,左连接最后结果集只关心能和主表 resourceid 能匹配的数据。因此我们可以重写语句如下,执行时间从原来的2秒下降到2毫秒。
SELECT a.*, 
       c.allocated 
FROM ( 
      SELECT resourceid 
      FROM my_distribute d 
      WHERE isdelete = 0 
      AND cusmanagercode = '1234567' 
      ORDER BY salecode limit 20) a 
LEFT JOIN 
      ( 
      SELECT resourcesid, sum(ifnull(allocation, 0) * 12345) allocated 
       FROM my_resources r, 
( 
SELECT resourceid 
FROM my_distribute d 
WHERE isdelete = 0 
AND cusmanagercode = '1234567' 
ORDER BY salecode limit 20) a 
WHERE r.resourcesid = a.resourcesid 
GROUP BY resourcesid) c 
ON a.resourceid = c.resourcesid
但是子查询 a 在我们的SQL语句中出现了多次。这种写法不仅存在额外的开销,还使得整个语句显的繁杂。使用 WITH 语句再次重写:

总结

数据库编译器产生执行计划,决定着SQL的实际执行方式。但是编译器只是尽力服务,所有数据库的编译器都不是尽善尽美的。上述提到的多数场景,在其它数据库中也存在性能问题。了解数据库编译器的特性,才能避规其短处,写出高性能的SQL语句。程序员在设计数据模型以及编写SQL语句时,要把算法的思想或意识带进来。编写复杂SQL语句要养成使用 WITH 语句的习惯。简洁且思路清晰的SQL语句也能减小数据库的负担 。


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