这几个SQL语法的坑，你踩过吗

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今天给大家分享几个SQL常见的"坏毛病"及优化技巧.

SQL语句的执行顺序：

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;

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 *



 >       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'

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.

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3、关联更新、删除

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

UPDATE operation o
     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
      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           |
+----+-------------+-------+--------+---------+---------+---------+-----------------+---------+-+

SELECT *
FROM   ((SELECT *
      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
             AND sra.user_id = 'xxx'
WHERE  n.topic_status < 4
   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 |
+----+--------------------+-------+------+ -----+------------------------------------------+---------+-------+---------+ -----+

SELECT *
FROM   my_neighbor n
             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；

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,
    GROUP  BY target) t
WHERE  target = 'rm-xxxx'
+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+
| id | select_type | table      | type  | possible_keys | key         | key_len | ref   | rows | Extra       |
+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+
|  1 | PRIMARY     |  | ref   |    |  | 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 |
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+

7、提前缩小范围

先上初始 SQL 语句：

+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+
| 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,所以呢可以先对 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.

+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows   | Extra                                              |
+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+
|  1 | PRIMARY     |  | 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.*,
               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.

SELECT    a.*,
               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
(
               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 .

以上就是土嘎嘎小编为大家整理的这几个SQL语法的坑,你踩过吗相关主题介绍,如果您觉得小编更新的文章只要能对粉丝们有用,就是我们最大的鼓励和动力,不要忘记讲本站分享给您身边的朋友哦！！