不知道大家之前对类似通过IN换INNER JOIN实现对MySQL的优化的文章有无了解，今天我在这里给大家再简单的讲讲。感兴趣的话就一起来看看正文部分吧，相信看完通过IN换INNER JOIN实现对mysql的优化你一定会有所收获的。
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SQL问题：
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 | | 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 | | 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 | | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+---------------------+--------+---------------+---------+---------+-------+------+--------------------------+
5 rows in set (0.00 sec)
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