8种经常被忽视的SQL错误用法，你有没有踩过坑？

1，LIMIT语句

分页查询是最常用的场景之一，但也通常通常也是最容易出问题的地方。针对下面简单的语句，一般DBA认为的方法是在类型，名称，create_time上下上加组合索引。这样的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;复制代码

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

要知道数据库也不一定知道第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，实际上是下一个是程序员偷懒了。

在重新数据浏览翻页，或者大数据分批添加等场景下，是可以将上一页的完全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;复制代码

在新设计下查询时间基本固定，不会传递数据量的增长而发生变化。

2，隐式转换

SQL语句中查询变量和变量定义类型不匹配是另一个常见的错误。

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'复制代码

其中片段bpn的定义为varchar（20），MySQL的策略是将串行转换为数字之后再比较。函数作用于表变量，索引无效。

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约会了物化特性，但需要特别注意它总体上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。

例如下面的一条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);复制代码

执行计划：

+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+

+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+

| 1 | PRIMARY | o | index | | PRIMARY | 8 | | 24 | Using where; Using temporary |

| 3 | DERIVED | o | ref | idx_2,idx_5 | idx_5 | 8 | const | 1 | Using where; Using filesort |

+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+复制代码

变为为JOIN之后，子查询的选择模式从DEPENDENT子查询转换为DERIVED，执行速度大大加快，从7秒降低到2秒钟

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' 复制代码

执行计划简化为：

+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+

+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+

| 2 | DERIVED | o | ref | idx_2,idx_5 | idx_5 | 8 | const | 1 | Using where; Using filesort |

+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+复制代码

4，混合排序

MySQL不能利用索引进行混合排序。但在某些场景下，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 复制代码

执行计划显示为全表扫描：

+----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+

+----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+

+----+-------------+-------+--------+---------+---------+---------+-----------------+---------+-+复制代码

由于is_reply只有0和1两个状态，我们按照下面的方法重写后，执行时间从1.58秒降低到2秒钟。

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;复制代码

5，EXISTS语句

MySQL处理EXISTS子句时，仍采用交替子查询的执行方式。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 复制代码

执行计划为：

+----+--------------------+-------+------+-----+------------------------------------------+---------+-------+---------+ -----+

+----+--------------------+-------+------+ -----+------------------------------------------+---------+-------+---------+ -----+

| 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 |

+----+--------------------+-------+------+ -----+------------------------------------------+---------+-------+---------+ -----+ 复制代码

去掉存在更改为join，能够避免重叠子查询，将执行时间从1.93秒降低为1秒钟。

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 复制代码

新的执行计划：

+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+

+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+

| 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 |

+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+复制代码

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；

包含LIMIT的子查询；

UNION或UNION ALL子查询；

输出细分中的子查询；

如下面的语句，从执行计划可以抛光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' 复制代码

+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+

+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+

| 1 | PRIMARY | | ref | | | 514 | const | 2 | Using where |

| 2 | DERIVED | operation | index | idx_4 | idx_4 | 519 | NULL | 20 | Using index |

+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+复制代码

确定从语义上查询条件可以直接下推后，改为如下：

SELECT target,

Count(*)

FROM operation

WHERE target = 'rm-xxxx'

GROUP BY target复制代码

执行计划体现：

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+

| 1 | SIMPLE | operation | ref | idx_4 | idx_4 | 514 | const | 1 | Using where; Using index |

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+复制代码

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 复制代码

该SQL语句原意是：先做一段的左连接，然后排序取前15条记录。从执行计划也可以裁剪，最后一步将记录数设为90万，时间消耗为12秒。

+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+

+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+

+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+复制代码

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左右左右。

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 DESC

limit 0, 15复制代码

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

+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+

+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+

| 2 | DERIVED | o | index | NULL | idx_1 | 5 | NULL | 909112 | Using where |

+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+复制代码

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复制代码

那么该语句还存在其他问题吗？不严重出子查询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。因此我们可以重新编写语句如下，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复制代码

但是子查询在我们的SQL语句中出现了多次。这种写法已经存在额外的开销，还整个整个语句显的繁杂。使用WITH语句再次替换：

WITH a AS

(

SELECT resourceid

FROM my_distribute d

WHERE isdelete = 0

AND cusmanagercode = '1234567'

ORDER BY salecode limit 20)

SELECT a.*,

c.allocated

FROM a

LEFT JOIN

(

SELECT resourcesid， sum(ifnull(allocation, 0) * 12345) allocated

FROM my_resources r,

WHERE r.resourcesid = a.resourcesid

GROUP BY resourcesid) c

ON a.resourceid = c.resourcesid复制代码

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。

了解数据库编译器的特性，能够避免法规其短处，写出高效的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。

编写且复杂的SQL语句也能解析数据库的负担。

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8种经常被忽视的SQL错误用法，你有没有踩过坑？

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