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这些常被忽视的SQL错误用法,你知道吗,忽视错误的结果

这些常被忽视的SQL错误用法,你知道吗,忽视错误的结果


这些常被忽视的SQL错误用法,你知道吗

sql语句的执行顺序:

  1. FROM??
  2. ?
  3. ON??
  4. ?
  5. ?
  6. ?JOIN??
  7. ?
  8. WHERE??
  9. ?
  10. GROUP?BY??
  11. ?
  12. HAVING??
  13. ?
  14. SELECT?
  15. DISTINCT??
  16. ?
  17. ORDER?BY??
  18. ?
  19. LIMIT??
  20. ?

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。

  1. SELECT?*??
  2. FROM?operation??
  3. WHERE?type?=?'SQLStats'??
  4. ?AND?name?=?'SlowLog'??
  5. ORDER?BY?create_time??
  6. 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?

要知道永利ag手机版|首页也并不知道第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:

  1. SELECT?*??
  2. FROM?operation??
  3. WHERE?type?=?'SQLStats'??
  4. AND?name?=?'SlowLog'??
  5. AND?create_time?>?'2017-03-16?14:00:00'??
  6. 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:

  1. mysql>?explain?extended?SELECT?*??
  2. ?>?FROM?my_balance?b??
  3. ?>?WHERE?b.bpn?=?14000000123??
  4. ?>?AND?b.isverified?IS?NULL?;?
  5. mysql>?show?warnings;?
  6. |?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。

  1. UPDATE?operation?o??
  2. SET?status?=?'applying'??
  3. WHERE?o.id?IN?(SELECT?id??
  4. ?FROM?(SELECT?o.id,??
  5. ?o.status??
  6. ?FROM?operation?o??
  7. ?WHERE?o.group?=?123??
  8. ?AND?o.status?NOT?IN?(?'done'?)??
  9. ?ORDER?BY?o.parent,??
  10. ?o.id??
  11. ?LIMIT?1)?t);??

执行计划:

  1. +----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+?
  2. |?id?|?select_type?|?table?|?type?|?possible_keys?|?key?|?key_len?|?ref?|?rows?|?Extra?|?
  3. +----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+?
  4. |?1?|?PRIMARY?|?o?|?index?|?|?PRIMARY?|?8?|?|?24?|?Using?where;?Using?temporary?|?
  5. |?2?|?DEPENDENT?SUBQUERY?|?|?|?|?|?|?|?|?Impossible?WHERE?noticed?after?reading?const?tables?|?
  6. |?3?|?DERIVED?|?o?|?ref?|?idx_2,idx_5?|?idx_5?|?8?|?const?|?1?|?Using?where;?Using?filesort?|?
  7. +----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+?

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。

  1. UPDATE?operation?o??
  2. ?JOIN?(SELECT?o.id,??
  3. ?o.status??
  4. ?FROM?operation?o??
  5. ?WHERE?o.group?=?123??
  6. ?AND?o.status?NOT?IN?(?'done'?)??
  7. ?ORDER?BY?o.parent,??
  8. ?o.id??
  9. ?LIMIT?1)?t?
  10. ?ON?o.id?=?t.id??
  11. SET?status?=?'applying'??

执行计划简化为:

  1. +----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+?
  2. |?id?|?select_type?|?table?|?type?|?possible_keys?|?key?|?key_len?|?ref?|?rows?|?Extra?|?
  3. +----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+?
  4. |?1?|?PRIMARY?|?|?|?|?|?|?|?|?Impossible?WHERE?noticed?after?reading?const?tables?|?
  5. |?2?|?DERIVED?|?o?|?ref?|?idx_2,idx_5?|?idx_5?|?8?|?const?|?1?|?Using?where;?Using?filesort?|?
  6. +----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+?

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。

  1. SELECT?*??
  2. FROM?my_order?o??
  3. ?INNER?JOIN?my_appraise?a?ON?a.orderid?=?o.id??
  4. ORDER?BY?a.is_reply?ASC,??
  5. ?a.appraise_time?DESC??
  6. LIMIT?0,?20??

执行计划显示为全表扫描:

  1. +----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+?
  2. |?id?|?select_type?|?table?|?type?|?possible_keys?|?key?|?key_len?|?ref?|?rows?|?Extra??
  3. +----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+?
  4. |?1?|?SIMPLE?|?a?|?ALL?|?idx_orderid?|?NULL?|?NULL?|?NULL?|?1967647?|?Using?filesort?|?
  5. |?1?|?SIMPLE?|?o?|?eq_ref?|?PRIMARY?|?PRIMARY?|?122?|?a.orderid?|?1?|?NULL?|?
  6. +----+-------------+-------+--------+---------+---------+---------+-----------------+---------+-+?

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。

  1. SELECT?*??
  2. FROM?((SELECT?*?
  3. ?FROM?my_order?o??
  4. ?INNER?JOIN?my_appraise?a??
  5. ?ON?a.orderid?=?o.id??
  6. ?AND?is_reply?=?0??
  7. ?ORDER?BY?appraise_time?DESC??
  8. ?LIMIT?0,?20)??
  9. ?UNION?ALL??
  10. ?(SELECT?*?
  11. ?FROM?my_order?o??
  12. ?INNER?JOIN?my_appraise?a??
  13. ?ON?a.orderid?=?o.id??
  14. ?AND?is_reply?=?1??
  15. ?ORDER?BY?appraise_time?DESC??
  16. ?LIMIT?0,?20))?t??
  17. ORDER?BY?is_reply?ASC,??
  18. ?appraisetime?DESC??
  19. 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:

  1. SELECT?*?
  2. FROM?my_neighbor?n??
  3. ?LEFT?JOIN?my_neighbor_apply?sra??
  4. ?ON?n.id?=?sra.neighbor_id??
  5. ?AND?sra.user_id?=?'xxx'??
  6. WHERE?n.topic_status?
  7. ?AND?EXISTS(SELECT?1??
  8. ?FROM?message_info?m??
  9. ?WHERE?n.id?=?m.neighbor_id??
  10. ?AND?m.inuser?=?'xxx')??
  11. ?AND?n.topic_type?<>?5??

执行计划为:

  1. +----+--------------------+-------+------+-----+------------------------------------------+---------+-------+---------+?-----+?
  2. |?id?|?select_type?|?table?|?type?|?possible_keys?|?key?|?key_len?|?ref?|?rows?|?Extra?|?
  3. +----+--------------------+-------+------+?-----+------------------------------------------+---------+-------+---------+?-----+?
  4. |?1?|?PRIMARY?|?n?|?ALL?|?|?NULL?|?NULL?|?NULL?|?1086041?|?Using?where?|?
  5. |?1?|?PRIMARY?|?sra?|?ref?|?|?idx_user_id?|?123?|?const?|?1?|?Using?where?|?
  6. |?2?|?DEPENDENT?SUBQUERY?|?m?|?ref?|?|?idx_message_info?|?122?|?const?|?1?|?Using?index?condition;?Using?where?|?
  7. +----+--------------------+-------+------+?-----+------------------------------------------+---------+-------+---------+?-----+?

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。

  1. SELECT?*?
  2. FROM?my_neighbor?n??
  3. ?INNER?JOIN?message_info?m??
  4. ?ON?n.id?=?m.neighbor_id??
  5. ?AND?m.inuser?=?'xxx'??
  6. ?LEFT?JOIN?my_neighbor_apply?sra??
  7. ?ON?n.id?=?sra.neighbor_id??
  8. ?AND?sra.user_id?=?'xxx'??
  9. WHERE?n.topic_status?
  10. ?AND?n.topic_type?<>?5??

新的执行计划:

  1. +----+-------------+-------+--------+?-----+------------------------------------------+---------+?-----+------+?-----+?
  2. |?id?|?select_type?|?table?|?type?|?possible_keys?|?key?|?key_len?|?ref?|?rows?|?Extra?|?
  3. +----+-------------+-------+--------+?-----+------------------------------------------+---------+?-----+------+?-----+?
  4. |?1?|?SIMPLE?|?m?|?ref?|?|?idx_message_info?|?122?|?const?|?1?|?Using?index?condition?|?
  5. |?1?|?SIMPLE?|?n?|?eq_ref?|?|?PRIMARY?|?122?|?ighbor_id?|?1?|?Using?where?|?
  6. |?1?|?SIMPLE?|?sra?|?ref?|?|?idx_user_id?|?123?|?const?|?1?|?Using?where?|?
  7. +----+-------------+-------+--------+?-----+------------------------------------------+---------+?-----+------+?-----+?

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:

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:

  1. SELECT?*??
  2. FROM?(SELECT?target,??
  3. ?Count(*)??
  4. ?FROM?operation??
  5. ?GROUP?BY?target)?t??
  6. WHERE?target?=?'rm-xxxx'??
    ?
  1. +----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+?
  2. |?id?|?select_type?|?table?|?type?|?possible_keys?|?key?|?key_len?|?ref?|?rows?|?Extra?|?
  3. +----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+?
  4. |?1?|?PRIMARY?|??|?ref?|??|??|?514?|?const?|?2?|?Using?where?|?
  5. |?2?|?DERIVED?|?operation?|?index?|?idx_4?|?idx_4?|?519?|?NULL?|?20?|?Using?index?|?
  6. +----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+?

确定从语义上查询条件可以直接下推后,重写如下:

  1. SELECT?target,??
  2. ?Count(*)??
  3. FROM?operation??
  4. WHERE?target?=?'rm-xxxx'??
  5. GROUP?BY?target?

执行计划变为:

  1. +----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+?
  2. |?id?|?select_type?|?table?|?type?|?possible_keys?|?key?|?key_len?|?ref?|?rows?|?Extra?|?
  3. +----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+?
  4. |?1?|?SIMPLE?|?operation?|?ref?|?idx_4?|?idx_4?|?514?|?const?|?1?|?Using?where;?Using?index?|?
  5. +----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+?

关于 MySQL 外部条件不能下推的详细解释说明请参考以前文章:MySQL · 性能优化 · 条件下推到物化表 http://mysql.taobao.org/monthly/2016/07/08

7、提前缩小范围**

先上初始 SQL 语句:

  1. SELECT?*??
  2. FROM?my_order?o??
  3. ?LEFT?JOIN?my_userinfo?u??
  4. ?ON?o.uid?=?u.uid?
  5. ?LEFT?JOIN?my_productinfo?p??
  6. ?ON?o.pid?=?p.pid??
  7. WHERE?(?o.display?=?0?)??
  8. ?AND?(?o.ostaus?=?1?)??
  9. ORDER?BY?o.selltime?DESC??
  10. 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。

  1. +----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+?
  2. |?id?|?select_type?|?table?|?type?|?possible_keys?|?key?|?key_len?|?ref?|?rows?|?Extra?|?
  3. +----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+?
  4. |?1?|?SIMPLE?|?o?|?ALL?|?NULL?|?NULL?|?NULL?|?NULL?|?909119?|?Using?where;?Using?temporary;?Using?filesort?|?
  5. |?1?|?SIMPLE?|?u?|?eq_ref?|?PRIMARY?|?PRIMARY?|?4?|?o.uid?|?1?|?NULL?|?
  6. |?1?|?SIMPLE?|?p?|?ALL?|?PRIMARY?|?NULL?|?NULL?|?NULL?|?6?|?Using?where;?Using?join?buffer?(Block?Nested?Loop)?|?
  7. +----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+?

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。

  1. SELECT?*??
  2. FROM?(?
  3. SELECT?*??
  4. FROM?my_order?o??
  5. WHERE?(?o.display?=?0?)??
  6. ?AND?(?o.ostaus?=?1?)??
  7. ORDER?BY?o.selltime?DESC??
  8. LIMIT?0,?15?
  9. )?o??
  10. ?LEFT?JOIN?my_userinfo?u??
  11. ?ON?o.uid?=?u.uid??
  12. ?LEFT?JOIN?my_productinfo?p??
  13. ?ON?o.pid?=?p.pid??
  14. ORDER?BY?o.selltime?DESC?
  15. limit?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。

  1. +----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+?
  2. |?id?|?select_type?|?table?|?type?|?possible_keys?|?key?|?key_len?|?ref?|?rows?|?Extra?|?
  3. +----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+?
  4. |?1?|?PRIMARY?|??|?ALL?|?NULL?|?NULL?|?NULL?|?NULL?|?15?|?Using?temporary;?Using?filesort?|?
  5. |?1?|?PRIMARY?|?u?|?eq_ref?|?PRIMARY?|?PRIMARY?|?4?|?o.uid?|?1?|?NULL?|?
  6. |?1?|?PRIMARY?|?p?|?ALL?|?PRIMARY?|?NULL?|?NULL?|?NULL?|?6?|?Using?where;?Using?join?buffer?(Block?Nested?Loop)?|?
  7. |?2?|?DERIVED?|?o?|?index?|?NULL?|?idx_1?|?5?|?NULL?|?909112?|?Using?where?|?
  8. +----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+?

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):

  1. SELECT?a.*,??
  2. ?c.allocated??
  3. FROM?(??
  4. ?SELECT?resourceid??
  5. ?FROM?my_distribute?d??
  6. ?WHERE?isdelete?=?0??
  7. ?AND?cusmanagercode?=?'1234567'??
  8. ?ORDER?BY?salecode?limit?20)?a??
  9. LEFT?JOIN??
  10. ?(??
  11. ?SELECT?resourcesid,?sum(ifnull(allocation,?0)?*?12345)?allocated??
  12. ?FROM?my_resources??
  13. ?GROUP?BY?resourcesid)?c??
  14. 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。

  1. SELECT?a.*,??
  2. ?c.allocated??
  3. FROM?(??
  4. ?SELECT?resourceid??
  5. ?FROM?my_distribute?d??
  6. ?WHERE?isdelete?=?0??
  7. ?AND?cusmanagercode?=?'1234567'??
  8. ?ORDER?BY?salecode?limit?20)?a??
  9. LEFT?JOIN??
  10. ?(??
  11. ?SELECT?resourcesid,?sum(ifnull(allocation,?0)?*?12345)?allocated??
  12. ?FROM?my_resources?r,??
  13. ?(??
  14. ?SELECT?resourceid??
  15. ?FROM?my_distribute?d??
  16. ?WHERE?isdelete?=?0??
  17. ?AND?cusmanagercode?=?'1234567'??
  18. ?ORDER?BY?salecode?limit?20)?a??
  19. ?WHERE?r.resourcesid?=?a.resourcesid??
  20. ?GROUP?BY?resourcesid)?c??
  21. 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:

  1. WITH?a?AS??
  2. (??
  3. ?SELECT?resourceid??
  4. ?FROM?my_distribute?d??
  5. ?WHERE?isdelete?=?0??
  6. ?AND?cusmanagercode?=?'1234567'??
  7. ?ORDER?BY?salecode?limit?20)?
  8. SELECT?a.*,??
  9. ?c.allocated??
  10. FROM?a??
  11. LEFT?JOIN??
  12. ?(??
  13. ?SELECT?resourcesid,?sum(ifnull(allocation,?0)?*?12345)?allocated??
  14. ?FROM?my_resources?r,??
  15. ?a??
  16. ?WHERE?r.resourcesid?=?a.resourcesid??
  17. ?GROUP?BY?resourcesid)?c??
  18. ON?a.resourceid?=?c.resourcesid?

总结

永利ag手机版|首页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,所有永利ag手机版|首页的编译器都不是尽善尽美的。

上述提到的多数场景,在其它永利ag手机版|首页中也存在性能问题。了解永利ag手机版|首页编译器的特性,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语句也能减小永利ag手机版|首页的负担 。

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