七个常见的SQL慢查询问题，及其解决方法

得益于摩尔定律，计算机性能已大幅提升，加上数据库的进步以及微服务所倡导的各种反模式设计。因此，我们现在编写复杂SQL查询的机会越来越少。业界(是的，甚至包括谷歌)已经开始提倡不要进行专门的SQL优化，因为节省下来的资源并不足以抵消员工的工资成本。但是，作为工程师，我们应该在技术上努力追求卓越，成为本领域的顶尖科学家。

在这里，将介绍7个常见的SQL慢查询语句，并解释如何优化它们的性能。希望这对你有所帮助。

由DALLE-3生成

1. LIMIT语句

分页是最常用的方案之一，但也容易出现问题。例如，对于以下简单的语句，DBA通常建议的解决方案是添加一个包含type、name和create_time字段的复合索引。这样，条件和排序就可以有效利用索引，从而显著提高性能。

SELECT *
FROM   operation
WHERE  type = 'SQLStats'
       AND name = 'SlowLog'
ORDER  BY create_time
LIMIT  1000, 10;

这可能会解决90%以上DBA的问题。但是，当LIMIT子句变成“LIMIT 1000000, 10”时，程序员仍会抱怨“为什么在只查询10条记录的时候，速度还这么慢?” 要知道，数据库不知道第1000000条记录从何处开始，所以即使有索引，它仍需要从头开始计算。在大多数情况下，这个性能问题是由于懒惰编程造成的。

在前端数据浏览或批量导出大量数据的场景中，可以使用上一页的最大值作为查询参数。SQL可以重新设计如下:

SELECT   *
FROM     operation
WHERE    type = 'SQLStats'
AND      name = 'SlowLog'
AND      create_time > '2017-03-16 14:00:00'
ORDER BY create_time
LIMIT    10;

采用这种新设计后，查询时间保持不变，不会随着数据量的增加而变化。

2. 隐式转换

SQL语句中另一个常见的错误是查询变量和字段定义的类型不匹配。以下面的语句为例：

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'

在这种情况下，字段bpn被定义为varchar(20)，而MySQL的策略是在比较之前将字符串转换为数字。这会导致函数被应用到表字段上，从而使索引失效。

这种情况可能是由应用程序框架自动填充参数造成的，而不是程序员的本意。如今，应用程序框架通常都很复杂，虽然它们提供了便利，但也可能带来隐患。

3. 连接更新和删除

尽管MySQL 5.6引入了物化，但它只优化了SELECT语句。对于UPDATE或DELETE语句，需要使用JOIN手动重写。

例如，请看下面的UPDATE语句。MySQL实际上执行了一个循环/嵌套子查询(DEPENDENT SUBQUERY)，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);

执行计划如下:

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

将其重写为JOIN后，子查询的选择类型从DEPENDENT SUBQUERY变为DERIVED，执行时间显著得从7秒缩短到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';

简化后的执行计划如下:

+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+
| 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无法利用索引进行混合排序。但是，在某些场景下，仍然可以使用特殊方法来提高性能。

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;

执行计划显示的是全表扫描:

+----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+
| 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，我们可以将其重写如下，从而将执行时间从1.58秒缩短到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

在处理EXISTS子句时，MySQL仍然使用嵌套子查询进行执行。以下面的SQL语句为例:

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;

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

通过删除EXISTS子句并将其更改为JOIN, 我们可以避免嵌套子查询，并将执行时间从1.93秒减少到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;

新的执行计划如下:

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

在某些情况下，外部查询条件无法下推到复杂的视图或子查询中:

请看下面的语句，其中的条件会影响聚合子查询：

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     | n          | ALL   | NULL          | NULL        | NULL    | NULL  | 1086041 | Using where |
|  1 | PRIMARY     | sra        | ref   | NULL          | idx_user_id | 123     | const |    1 | Using where |
|  2 | DEPENDENT SUBQUERY | m | ref   | NULL          | idx_message_info   | 122     | const |    1 | Using index condition; Using where |
+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+

通过删除EXISTS子句并将其更改为JOIN，我们可以避免嵌套子查询并将执行时间从1.93秒减少到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;

新的执行计划如下:

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

7. 提前缩小范围

让我们看看以下经过部分优化的示例（左连接中的主表作为主查询条件）：

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;

这条语句是否还存在其他问题？很明显，子查询c是对整个表进行聚合查询，在处理大量表时可能会导致性能下降。

事实上，对于子查询c，左连接的结果集只关心可以与主表的resourceid匹配的数据。因此，我们可以将语句重写如下，将执行时间从2秒减少到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;

然而，子查询a在我们的SQL语句中出现了多次。这种方法不仅会产生额外的成本，而且也会使语句变得更加复杂。我们可以使用WITH语句来简化它：

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;

结论

数据库编译器生成的执行计划决定了SQL语句的实际执行方式。但是，编译器只能尽力提供服务，没有一个数据库编译器是完美的。上述情况在其他数据库中也同样存在。了解了数据库编译器的特性，我们就能绕过它的限制，编写出高性能的SQL语句。

在设计数据模型和编写SQL语句时，将算法思维或算法意识引入到这个过程非常重要。在编写复杂的SQL语句时，养成使用WITH语句的习惯可以简化语句，减轻数据库的负担。

最后，下面是SQL语句的执行顺序：

FROM

ON


 JOIN

WHERE

GROUP BY

HAVING

SELECT
DISTINCT

ORDER BY

LIMIT