8 种常被忽视的 SQL 错误用法

Java编程指南

发布于 2019-08-02 16:25:20

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发布于 2019-08-02 16:25:20

文章被收录于专栏：Java编程指南

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

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>

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。

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。

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


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;

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

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原始发表：2019-06-03，如有侵权请联系 cloudcommunity@tencent.com 删除

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