关于mysql优化之IN换INNER JOIN的实例分享

今天撸代码时，遇到SQL问题：

（相关mysql视频教程推荐：《mysql教程》）

要将A表查询的ID，匹配B表的ID，并将B表全部内容查询出来：

未优化前：

MySQL [xxuer]> SELECT 
    ->     COUNT(*)
    -> FROM
    ->     t_cmdb_app_version
    -> WHERE
    ->     id IN (SELECT 
    ->             pid
    ->         FROM
    ->             t_cmdb_app_relation UNION SELECT 
    ->             rp_id
    ->         FROM
    ->             t_cmdb_app_relation);
+----------+
| COUNT(*) |
+----------+
|      266 |
+----------+
1 row in set (0.21 sec)

优化后：

MySQL [xxuer]> SELECT 
    ->     count(*)
    -> FROM
    ->     t_cmdb_app_version a
    ->         INNER JOIN
    ->     (SELECT 
    ->         pid
    ->     FROM
    ->         t_cmdb_app_relation UNION SELECT 
    ->         rp_id
    ->     FROM
    ->         t_cmdb_app_relation) b ON a.id = b.pid;
+----------+
| count(*) |
+----------+
|      266 |
+----------+
1 row in set (0.00 sec)

查看执行计划对比：

MySQL [xxuer]> explain SELECT 
    ->     COUNT(*)
    -> FROM
    ->     t_cmdb_app_version
    -> WHERE
    ->     id IN (SELECT 
    ->             pid
    ->         FROM
    ->             t_cmdb_app_relation UNION SELECT 
    ->             rp_id
    ->         FROM
    ->             t_cmdb_app_relation);
+----+--------------------+---------------------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type        | table               | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+--------------------+---------------------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | PRIMARY            | t_cmdb_app_version  | index | NULL          | PRIMARY | 4       | NULL |  659 | Using where; Using index |
|  2 | DEPENDENT SUBQUERY | t_cmdb_app_relation | ALL   | NULL          | NULL    | NULL    | NULL |  383 | Using where              |
|  3 | DEPENDENT UNION    | t_cmdb_app_relation | ALL   | NULL          | NULL    | NULL    | NULL |  383 | Using where              |
| NULL | UNION RESULT       | <union2,3>          | ALL   | NULL          | NULL    | NULL    | NULL | NULL | Using temporary          |
+----+--------------------+---------------------+-------+---------------+---------+---------+------+------+--------------------------+
4 rows in set (0.00 sec)

MySQL [xxuer]> explain SELECT 
    ->     count(*)
    -> FROM
    ->     t_cmdb_app_version a
    ->         INNER JOIN
    ->     (SELECT 
    ->         pid
    ->     FROM
    ->         t_cmdb_app_relation UNION SELECT 
    ->         rp_id
    ->     FROM
    ->         t_cmdb_app_relation) b ON a.id = b.pid;
+----+--------------+---------------------+--------+---------------+---------+---------+-------+------+--------------------------+
| id | select_type  | table               | type   | possible_keys | key     | key_len | ref   | rows | Extra                    |
+----+--------------+---------------------+--------+---------------+---------+---------+-------+------+--------------------------+
|  1 | PRIMARY      | <derived2>          | ALL    | NULL          | NULL    | NULL    | NULL  |  766 | Using where              |
|  1 | PRIMARY      | a                   | eq_ref | PRIMARY       | PRIMARY | 4       | b.pid |    1 | Using where; Using index |
|  2 | DERIVED      | t_cmdb_app_relation | ALL    | NULL          | NULL    | NULL    | NULL  |  383 | NULL                     |
|  3 | UNION        | t_cmdb_app_relation | ALL    | NULL          | NULL    | NULL    | NULL  |  383 | NULL                     |
| NULL | UNION RESULT | <union2,3>          | ALL    | NULL          | NULL    | NULL    | NULL  | NULL | Using temporary          |
+----+--------------+---------------------+--------+---------------+---------+---------+-------+------+--------------------------+
5 rows in set (0.00 sec)

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