MySQL语句性能优化之Sql错误用法

本文主要总结了慢查询优化的过程中常用的以及不合理的操作，适合所有的运维及开发人员。

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;1234567891011复制代码类型：[javascript]

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;123456789101112复制代码类型：[javascript]

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'1234567891011复制代码类型：[javascript]

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);123456789101112131415161718192021复制代码类型：[javascript]

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 |

+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+1234567891011121314复制代码类型：[javascript]

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'123456789101112131415161718192021复制代码类型：[javascript]

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 |

+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------[phone]复制代码类型：[javascript]

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, 201234567891011复制代码类型：[javascript]

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 |

+----+-------------+-------+--------+---------+---------+---------+-----------------+---------+-[phone]复制代码类型：[javascript]

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;1234567891011121314151617181920212223242526272829303132333435363738复制代码类型：[javascript]

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 <> 51234567891011121314151617181920212223复制代码类型：[javascript]

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 |

+----+--------------------+-------+------+ -----+------------------------------------------+---------+-------+---------+ -----+1234567891011121314复制代码类型：[javascript]

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 <> 512345678910111213141516171819复制代码类型：[javascript]

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 |

+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+12345678910111213复制代码类型：[javascript]

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 |

+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+1234567891011121314151617181920212223复制代码类型：[javascript]

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 target123456789复制代码类型：[javascript]

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 |

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------[phone]复制代码类型：[javascript]

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, 1512345678910111213141516171819复制代码类型：[javascript]

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) |

+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+1234567891011121314复制代码类型：[javascript]

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, 15123456789101112131415161718192021222324252627282930复制代码类型：[javascript]

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 |

+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+12345678910111213141516复制代码类型：[javascript]

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.resourcesid12345678910111213141516171819202122232425262728复制代码类型：[javascript]

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.resourcesid1234567891011121314151617181920212223242526272829303132333435363738394041复制代码类型：[javascript]

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.resourcesid1234567891011121314151617181920212223242526272829303132333435复制代码类型：[javascript]