MySQL常见的SQL错误

作者: 羋学僧 | 来源:发表于2020-11-29 21:30 被阅读0次

今天在分享一些MySQL 常见的SQL错误（不当）的情况下。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。

先列出sql语句的执行顺序：

FROM
<left_table>

ON
<join_condition>

<join_type>
 JOIN
<right_table>

WHERE
<where_condition>

GROUP BY
<group_by_list>

HAVING
<having_condition>

SELECT

DISTINCT
<select_list>

ORDER BY
<order_by_condition>

LIMIT
<limit_number>

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;

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，程序员仍然会休息：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，实际上就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，是可以将上一页的逐步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'

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

改为为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'

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

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;

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

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

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，包含LIMIT的子查询； 3，UNION或UNION ALL子查询； 4，输出替换中的子查询；

如以下的语句，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'
+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+
| 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，改为如下：

SELECT target,
       Count(*)
FROM   operation
WHERE  target = 'rm-xxxx'
GROUP  BY target

执行计划体现：

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
| 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以前的文章：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：

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：先做一段的左连接，然后排序取前15条记录。从执行计划也可以修剪，最后一步将记录数为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。

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

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,
                            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。

了解数据库编译器的特性，能够避免法规其短处，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。

编写且复杂的SQL语句也能解析数据库的负担。

MySQL常见的SQL错误

1，LIMIT语句

2，隐式转换

3，关联更新，删除

4，混合排序

5，EXISTS语句

6，条件下推

7，提前缩小范围

8，中间结果集下推

总结

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