8 种常见 SQL 错误用法

来源：yq.aliyun.com/articles/72501

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 |
       +----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
   
  

关于 MySQL 外部条件不能下推的详细解释说明请参考文章：

http://mysql.taobao.org/monthly/2016/07/08

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 DESC
       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。

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

       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,
                                   a
                         WHERE    r.resourcesid = a.resourcesid
                         GROUP BY resourcesid) c
       ON        a.resourceid = c.resourcesid
   
  

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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