MySQL 索引 - In 与 Equals 索引问题

Question

以下查询在 mysql 服务器上运行得非常快且即时:

SELECT  table_name.id 
FROM table_name 
WHERE table_name.id in (10000)

SELECT table_name.id 
from table_name 
where table_name.id = (SELECT table_name.id 
                       FROM table_name 
                       WHERE table_name.id in (10000)
                      );

但是如果我将第二个查询更改为如下，则需要超过 20 秒:

SELECT table_name.id 
from table_name 
where table_name.id in (SELECT table_name.id 
                        FROM table_name 
                        WHERE table_name.id in (10000)
                        );

在进行解释时，我得到以下输出。很明显，MySQL 在如何索引数据以及使用 in 关键字方面存在一些问题。

对于第一个查询:

+----+-------------+---------------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table         | type  | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+---------------+-------+---------------+---------+---------+-------+------+-------------+
|  1 | SIMPLE      | table_name    | const | PRIMARY       | PRIMARY | 4       | const |    1 | Using index |
+----+-------------+---------------+-------+---------------+---------+---------+-------+------+-------------+

对于第二个查询:

+----+-------------+---------------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table         | type  | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+---------------+-------+---------------+---------+---------+-------+------+-------------+
|  1 | PRIMARY     | table_name    | const | PRIMARY       | PRIMARY | 4       | const |    1 | Using index |
|  2 | SUBQUERY    | table_name    | const | PRIMARY       | PRIMARY | 4       |       |    1 | Using index |
+----+-------------+---------------+-------+---------------+---------+---------+-------+------+-------------+

对于第三个查询:

+----+--------------------+------------+-------+---------------+---------+---------+-------+---------+--------------------------+
| id | select_type        | table_name | type  | possible_keys | key     | key_len | ref   | rows    | Extra                    |
+----+--------------------+------------+-------+---------------+---------+---------+-------+---------+--------------------------+
|  1 | PRIMARY            | table_name | index | NULL          | sentTo  | 5       | NULL  | 6250751 | Using where; Using index |
|  2 | DEPENDENT SUBQUERY | table_name | const | PRIMARY       | PRIMARY | 4       | const |       1 | Using index              |
+----+--------------------+------------+-------+---------------+---------+---------+-------+---------+--------------------------+

我正在使用 InnoDB，并尝试更改第三个查询以强制使用索引，如以下类别所示。

Answer 1

在第一种情况下，您只有子查询中的第一条记录(它运行一次，因为 equals 仅适用于第一个值)

在第二个查询中，您得到了笛卡尔乘法(每个)，因为 IN 为每一行运行子查询。这对性能不利

尝试在这些情况下使用联接。