关于java:年轻同事不讲武德直接怼上MySQL常见SQL的七大错误用法

明天来分享几个 MySQL 常见的 SQL 谬误（不当）用法。咱们在作为一个初学者时，很有可能本人在写 SQL 时也没有留神到这些问题，导致写进去的 SQL 语句效率低下，所以咱们也能够自省自检一下。

1、LIMIT 语句

分页查问是最罕用的场景之一，但也通常也是最容易出问题的中央。比方对于上面简略的语句，个别 DBA 想到的方法是在 type, name, create_time 字段上加组合索引。这样条件排序都能无效的利用到索引，性能迅速晋升。

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，程序员依然会埋怨：我只取 10 条记录为什么还是慢？

要晓得数据库也并不知道第 1000000 条记录从什么中央开始，即便有索引也须要从头计算一次。呈现这种性能问题，少数情景下是程序员偷懒了。在前端数据浏览翻页，或者大数据分批导出等场景下，是能够将上一页的最大值当成参数作为查问条件的。SQL 从新设计如下：

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

在新设计下查问工夫根本固定，不会随着数据量的增长而发生变化。

2、隐式转换

SQL 语句中查问变量和字段定义类型不匹配是另一个常见的谬误。比方上面的语句：

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

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拟。函数作用于表字段，索引生效。

上述情况可能是应用程序框架主动填入的参数，而不是程序员的原意。当初利用框架很多很繁冗，使用方便的同时也小心它可能给本人挖坑。

3、关联更新、删除

尽管 MySQL5.6 引入了物化个性，但须要特地留神它目前仅仅针对查问语句的优化。对于更新或删除须要手工重写成 JOIN。

比方上面 UPDATE 语句，MySQL 理论执行的是循环 / 嵌套子查问（DEPENDENT SUBQUERY)，其执行工夫可想而知。

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);

执行打算：

+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+
| 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，子查问的抉择模式从 DEPENDENT SUBQUERY 变成 DERIVED, 执行速度大大放慢，从 7 秒升高到 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'

执行打算简化为：

+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+
| 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，还是有机会应用非凡办法晋升性能的。

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

执行打算显示为全表扫描：

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

因为 is_reply 只有 0 和 1 两种状态，咱们依照上面的办法重写后，执行工夫从 1.58 秒升高到 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 看待 EXISTS 子句时，依然采纳嵌套子查问的执行形式。如上面的 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

执行打算为：

+----+--------------------+-------+------+-----+------------------------------------------+---------+-------+---------+ -----+
| 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，可能防止嵌套子查问，将执行工夫从 1.93 秒升高为 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

新的执行打算：

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

内部查问条件不可能下推到简单的视图或子查问的状况有：

聚合子查问；
含有 LIMIT 的子查问；
UNION 或 UNION ALL 子查问；
输入字段中的子查问；

如上面的语句，从执行打算能够看出其条件作用于聚合子查问之后：

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

确定从语义上查问条件能够间接下推后，重写如下：

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

执行打算变为：

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
| 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、提前放大范畴

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：先做一系列的左连贯，而后排序取前 15 条记录。从执行打算也能够看出，最初一步估算排序记录数为 90 万，工夫耗费为 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) |
+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+

因为最初 WHERE 条件以及排序均针对最左主表，因而能够先对 my_order 排序提前放大数据量再做左连贯。SQL 重写后如下，执行工夫放大为 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

再查看执行打算：子查问物化后（select_type=DERIVED) 参加 JOIN。尽管估算行扫描依然为 90 万，然而利用了索引以及 LIMIT 子句后，理论执行工夫变得很小。

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

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关于java:年轻同事不讲武德直接怼上MySQL常见SQL的七大错误用法

常见 SQL 谬误用法

写在最初