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8种常见SQL错误用法

时间:2019-11-22 01:11:22      阅读:91      评论:0      收藏:0      [点我收藏+]

标签:select   and   匹配   uid   提升性能   应用   list   notice   stat   

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 。

 

来源:https://yq.aliyun.com/articles/72501

8种常见SQL错误用法

标签:select   and   匹配   uid   提升性能   应用   list   notice   stat   

原文地址:https://www.cnblogs.com/vitowang/p/11909328.html

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