Mysql 选择需要20秒，删除仍在运行@30分钟

完成此选择查询大约需要20秒

 select Count(*)
   from products as bad_rows
   inner join (
   select pid, MAX(last_updated_date) as maxdate
      from products
       group by pid
         having count(*) > 1
      ) as good_rows on good_rows.pid= bad_rows.pid
        and good_rows.maxdate <> bad_rows.last_updated_date
        where bad_rows.available = 0

另一方面，删除操作在30分钟后仍在运行

  delete bad_rows
     from products as bad_rows
      inner join (
       select pid, MAX(last_updated_date) as maxdate
          from products
           group by pid
             having count(*) > 1
          ) as good_rows on good_rows.pid= bad_rows.pid
            and good_rows.maxdate <> bad_rows.last_updated_date
            where bad_rows.available = 0

为什么?

表模式如下所示：

+----+-------------+------------+------+---------------+------+---------+------+-------+--------------------------------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows  | Extra                          |
+----+-------------+------------+------+---------------+------+---------+------+-------+--------------------------------+
|  1 | PRIMARY     | <derived2> | ALL  | NULL          | NULL | NULL    | NULL |  6253 |                                |
|  1 | PRIMARY     | bad_rows   | ALL  | NULL          | NULL | NULL    | NULL | 34603 | Using where; Using join buffer |
|  2 | DERIVED     | products   | ALL  | NULL          | NULL | NULL    | NULL | 34603 | Using temporary; Using filesort|
+----+-------------+------------+------+---------------+------+---------+------+-------+--------------------------------

选择的解释如下：

+----+-------------+------------+------+---------------+------+---------+------+-------+--------------------------------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows  | Extra                          |
+----+-------------+------------+------+---------------+------+---------+------+-------+--------------------------------+
|  1 | PRIMARY     | <derived2> | ALL  | NULL          | NULL | NULL    | NULL |  6253 |                                |
|  1 | PRIMARY     | bad_rows   | ALL  | NULL          | NULL | NULL    | NULL | 34603 | Using where; Using join buffer |
|  2 | DERIVED     | products   | ALL  | NULL          | NULL | NULL    | NULL | 34603 | Using temporary; Using filesort|
+----+-------------+------------+------+---------------+------+---------+------+-------+--------------------------------

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好的，我只是用谷歌搜索结果解释，这暗示我的查询可能会很慢，因为pid上没有索引。它实际上并没有这么说，但我只是从阅读解释的结果中有一种预感。

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所以我在pid上添加了一个索引。在1分钟内删除

您的查询的解释计划是什么样的？有多少行？基于表的索引或函数有多少？由于管理索引的开销，当索引存在时，删除需要更长的时间。或基于函数的索引等功能。。。33000行。和1个索引id。尝试解释添加解释的结果是如何使用索引的极好介绍。