2020-11-23

新同事不讲“码”德,这SQL写得太野了,请耗子尾汁~

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 operationWHERE type = 'SQLStats'AND name = 'SlowLog'ORDER BY create_timeLIMIT 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 operationWHERE 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 oSET status = 'applying'WHERE o.id IN (SELECT idFROM (SELECT o.id,o.statusFROM operation oWHERE o.group = 123AND o.status NOT IN ( 'done' )ORDER BY o.parent,o.idLIMIT 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 oJOIN (SELECT o.id,o.statusFROM operation oWHERE o.group = 123AND o.status NOT IN ( 'done' )ORDER BY o.parent,o.idLIMIT 1) tON o.id = t.idSET 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 oINNER JOIN my_appraise a ON a.orderid = o.idORDER BY a.is_reply ASC,a.appraise_time DESCLIMIT 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 oINNER JOIN my_appraise aON a.orderid = o.idAND is_reply = 0ORDER BY appraise_time DESCLIMIT 0, 20)UNION ALL(SELECT *FROM my_order oINNER JOIN my_appraise aON a.orderid = o.idAND is_reply = 1ORDER BY appraise_time DESCLIMIT 0, 20)) tORDER BY is_reply ASC,appraisetime DESCLIMIT 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 nLEFT JOIN my_neighbor_apply sraON n.id = sra.neighbor_idAND sra.user_id = 'xxx'WHERE n.topic_status < 4AND EXISTS(SELECT 1FROM message_info mWHERE n.id = m.neighbor_idAND 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 nINNER JOIN message_info mON n.id = m.neighbor_idAND m.inuser = 'xxx'LEFT JOIN my_neighbor_apply sraON n.id = sra.neighbor_idAND sra.user_id = 'xxx'WHERE n.topic_status < 4AND 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:

  1. 聚合子查询;

  2. 含有LIMIT的子查询;

  3. UNION 或UNION ALL子查询;

  4. 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 operationGROUP BY target) tWHERE 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 operationWHERE 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 oLEFT JOIN my_userinfo uON o.uid = u.uidLEFT JOIN my_productinfo pON o.pid = p.pidWHERE ( o.display = 0 )AND ( o.ostaus = 1 )ORDER BY o.selltime DESCLIMIT 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 oWHERE ( o.display = 0 )AND ( o.ostaus = 1 )ORDER BY o.selltime DESCLIMIT 0, 15) oLEFT JOIN my_userinfo uON o.uid = u.uidLEFT JOIN my_productinfo pON o.pid = p.pidORDER 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 |+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+

 

看完这篇文章,你有什么收获?欢迎在留言区与30w+Java开发者一起讨论~

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原文转载:http://www.shaoqun.com/a/492866.html

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贝恩:https://www.ikjzd.com/w/1336

c2c:https://www.ikjzd.com/w/1576


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
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