Mysql exists 与 in

今天公司同事反馈一个SQL语句删除数据删除了一个小时，还没有删除完，强制中断。第一眼看到 exists 的时候，脑子里要有这么个概念：

Oracle exists 的效率比in 高。而Mysql 则不一定。 Mysql 使用eixsts 与使用in的规则为：

子查询的表大的时候，使用EXISTS可以有效减少总的循环次数来提升速度；
外查询的表大的时候，使用IN可以有效减少对外查询表循环遍历来提升速度。
从本质上讲，exists 是以外查询为驱动表，而in 是以子查询为驱动表(驱动表决定了以哪个结果集作为nestloop的对比依据)。

3.1.1 SQL

DELETE t FROM   o.`AI_AD_U_L` t   WHERE EXISTS (SELECT     1   FROM     o.`AI_AD_U_L_TEMP`  AS a   WHERE a.`ca_id`=t.`ca_id`);

3.1.2 分析过程

查看表上的索引

mysql> show index from AI_AD_U_L;

+-----------+------------+---------------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| Table     | Non_unique | Key_name                        | Seq_in_index | Column_name  | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |

+-----------+------------+---------------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| AI_AD_U_L |          0 | PRIMARY                         |            1 | prod_inst_id | A         |    21162012 |     NULL | NULL   |      | BTREE      |         |               |

| AI_AD_U_L |          1 | ai_sync_prod_level_cust_addr_id |            1 | cust_addr_id | A         |     8266746 |     NULL | NULL   | YES  | BTREE      |         |               |

| AI_AD_U_L |          1 | ai_sync_prod_level_mac          |            1 | mac          | A         |    12227460 |     NULL | NULL   | YES  | BTREE      |         |               |

+-----------+------------+---------------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

3 rows in set (0.00 sec)

mysql> show index from AI_AD_U_L_TEMP;

+----------------+------------+-------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| Table          | Non_unique | Key_name          | Seq_in_index | Column_name  | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |

+----------------+------------+-------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| AI_AD_U_L_TEMP |          1 | idx_cust_addr_id2 |            1 | cust_addr_id | A         |        2366 |     NULL | NULL   | YES  | BTREE      |         |               |

| AI_AD_U_L_TEMP |          1 | idx_prod_inst_id  |            1 | prod_inst_id | A         |        3791 |     NULL | NULL   |      | BTREE      |         |               |

+----------------+------------+-------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

2 rows in set (0.00 sec)

此时表上是有对应字段的索引的，如果索引不存在，需要创建索引。

查看执行计划

mysql> explain DELETE   t FROM   o.`AI_AD_U_L` t WHERE EXISTS   (SELECT     1   FROM     o.`AI_AD_U_L_TEMP` AS a   WHERE a.prod_inst_id = t.prod_inst_id);

+----+--------------------+-------+------------+------+------------------+------------------+---------+-----------------------+----------+----------+-------------+

| id | select_type        | table | partitions | type | possible_keys    | key              | key_len | ref                   | rows     | filtered | Extra       |

+----+--------------------+-------+------------+------+------------------+------------------+---------+-----------------------+----------+----------+-------------+

|  1 | DELETE             | t     | NULL       | ALL  | NULL             | NULL             | NULL    | NULL                  | 21162122 |   100.00 | Using where |

|  2 | DEPENDENT SUBQUERY | a     | NULL       | ref  | idx_prod_inst_id | idx_prod_inst_id | 8       | o.t.prod_inst_id      |        1 |   100.00 | Using index |

+----+--------------------+-------+------------+------+------------------+------------------+---------+-----------------------+----------+----------+-------------+

2 rows in set, 1 warning (0.01 sec)

通过执行计划发现两点问题：

外查询表数据量大,21162122,也就是访问了21162122次，而子查询通过索引只访问了一次。
发现子查询使用了索引，而外查询表上没有使用索引。

从以上两点发现，说明外查询作为了驱动表。

查看子查询中表的数据量

mysql> select count(*) from AI_AD_U_L_TEMP;

+----------+

| count(*) |

+----------+

|     3791 |

+----------+

1 row in set (0.00 sec)

子查询中数据量小，应以子查询为驱动表。应该用exists 应换成in。

调整SQL语句并查看执行计划 将exists 改为in 的用法。

mysql> explain DELETE   t FROM   o.`AI_AD_U_L` t WHERE t.prod_inst_id in  (SELECT prod_inst_id FROM     o.`AI_AD_U_L_TEMP` AS a   );

+----+-------------+-------+------------+--------+------------------+------------------+---------+-----------------------+------+----------+------------------------+

| id | select_type | table | partitions | type   | possible_keys    | key              | key_len | ref                   | rows | filtered | Extra                  |

+----+-------------+-------+------------+--------+------------------+------------------+---------+-----------------------+------+----------+------------------------+

|  1 | SIMPLE      | a     | NULL       | index  | idx_prod_inst_id | idx_prod_inst_id | 8       | NULL                  | 3791 |   100.00 | Using index; LooseScan |

|  1 | DELETE      | t     | NULL       | eq_ref | PRIMARY          | PRIMARY          | 8       | o.a.prod_inst_id |    1 |   100.00 | NULL                   |

+----+-------------+-------+------------+--------+------------------+------------------+---------+-----------------------+------+----------+------------------------+

2 rows in set (0.00 sec)

从执行计划中可以看到,两张表都在使用索引。而外表的访问次数也明显下降为子查询表中的行数。大量减少了循环访问外表的次数。

执行SQL语句

mysql> DELETE   t FROM   o.`AI_AD_U_L` t WHERE t.prod_inst_id in  (SELECT prod_inst_id FROM     o.`AI_AD_U_L_TEMP` AS a   );

Query OK, 3525 rows affected (0.44 sec)

我们看到效果明显, 原来1小时都无法执行完成的SQL，现在只需要0.44秒。