Question

我有一个mysql查询，它获取私人消息列表，其中用户是发件人或接收者。

    SELECT 
    users_user1.user_name AS pm_username_1, 
    users_user1.user_avatar AS pm_username_1_avatar,
    users_user2.user_name AS pm_username_2,
    users_user2.user_avatar AS pm_username_2_avatar, 
    pms.*
FROM pm pms
LEFT JOIN users users_user1 
    ON users_user1.user_id = pms.pm_sender
LEFT JOIN users users_user2
    ON users_user2.user_id = pms.pm_receiver
WHERE pm_thread = pm_id 
    AND (pm_receiver = '1' OR pm_sender = '1')
    AND pm_delete != '1'
ORDER by pm_thread_last DESC LIMIT 0, 15

问题是......据我所知......它不能使用任何索引。

我可以用任何方式解决这个问题吗？

修改

+----+-------------+-------------+--------+---------------+---------+---------+------------------------+-------+-----------------------------+
| id | select_type | table       | type   | possible_keys | key     | key_len | ref                    | rows  | Extra                       |
+----+-------------+-------------+--------+---------------+---------+---------+------------------------+-------+-----------------------------+
|  1 | SIMPLE      | pms         | ALL    | pm_receiver   | NULL    | NULL    | NULL                   | 25354 | Using where; Using filesort |
|  1 | SIMPLE      | users_user1 | eq_ref | PRIMARY       | PRIMARY | 4       | movies.pms.pm_sender   |     1 |                             |
|  1 | SIMPLE      | users_user2 | eq_ref | PRIMARY       | PRIMARY | 4       | movies.pms.pm_receiver |     1 |                             |
+----+-------------+-------------+--------+---------------+---------+---------+------------------------+-------+-----------------------------+

将架构更改为：

(SELECT 
    users_user1.user_name AS pm_username_1, 
    users_user1.user_avatar AS pm_username_1_avatar,
    users_user2.user_name AS pm_username_2,
    users_user2.user_avatar AS pm_username_2_avatar, 
    pms.*
FROM pm pms
LEFT JOIN users users_user1 
    ON users_user1.user_id = pms.pm_sender
LEFT JOIN users users_user2
    ON users_user2.user_id = pms.pm_receiver
WHERE pm_thread = pm_id 
    AND (pm_receiver = '1')
    AND pm_delete != '1')
UNION
(SELECT 
    users_user1.user_name AS pm_username_1, 
    users_user1.user_avatar AS pm_username_1_avatar,
    users_user2.user_name AS pm_username_2,
    users_user2.user_avatar AS pm_username_2_avatar, 
    pms.*
FROM pm pms
LEFT JOIN users users_user1 
    ON users_user1.user_id = pms.pm_sender
LEFT JOIN users users_user2
    ON users_user2.user_id = pms.pm_receiver
WHERE pm_thread = pm_id 
    AND (pm_sender = '1')
    AND pm_delete != '1')
ORDER by pm_thread_last DESC LIMIT 0, 15

EXPLAIN

+----+--------------+-------------+--------+---------------+-------------+---------+------------------------+------+----------------+
| id | select_type  | table       | type   | possible_keys | key         | key_len | ref                    | rows | Extra          |
+----+--------------+-------------+--------+---------------+-------------+---------+------------------------+------+----------------+
|  1 | PRIMARY      | pms         | ref    | pm_receiver   | pm_receiver | 4       | const                  |  336 | Using where    |
|  1 | PRIMARY      | users_user1 | eq_ref | PRIMARY       | PRIMARY     | 4       | movies.pms.pm_sender   |    1 |                |
|  1 | PRIMARY      | users_user2 | eq_ref | PRIMARY       | PRIMARY     | 4       | movies.pms.pm_receiver |    1 |                |
|  2 | UNION        | pms         | ref    | pm_sender     | pm_sender   | 4       | const                  |  283 | Using where    |
|  2 | UNION        | users_user1 | eq_ref | PRIMARY       | PRIMARY     | 4       | movies.pms.pm_sender   |    1 |                |
|  2 | UNION        | users_user2 | eq_ref | PRIMARY       | PRIMARY     | 4       | movies.pms.pm_receiver |    1 |                |
| NULL | UNION RESULT | <union1,2>  | ALL    | NULL          | NULL        | NULL    | NULL                   | NULL | Using filesort |
+----+--------------+-------------+--------+---------------+-------------+---------+------------------------+------+----------------+

Answer 1

是的，MySQL可以在OR表达式中使用索引。你怎么知道它没有使用你的索引，你是否使用EXPLAIN来查看MySQL如何运行你的查询？你在那张表中有多少行？如果行数太小，那么MySQL不会使用索引，因为它可以更快地进行全表扫描。我认为阈值是100 - 如果一个表的行少于100行，那么总是进行表扫描而不是使用索引。

Answer 2

您可以使用索引提示来强制解决问题，但这样做可能不会带来效果更好的查询。

请参阅http://dev.mysql.com/doc/refman/5.0/en/index-hints.html

您使用了哪些索引定义？

Answer 3

实际上，由于它是一个OR标准，MySQL不能在所提到的任何一列上使用任何索引。那是因为索引允许您按一个或另一个列搜索，但不能同时搜索两个。

我建议将查询拆分为两个查询，这样就不必使用OR。在此之前 - 检查这是否真的给你带来了性能问题。也许你正试图解决错误的问题。

已添加：在看到完整查询后，我可以说 - 重新考虑您的数据结构。这可能对数据完整性或其他方面非常有用，但如果没有全表扫描，您就无法编写这样的查询。如果您无法对其进行重组，可能会添加另一个表，其中包含缓存的必要信息。你必须保持缓存最新。

你可以使用“col1 OR col2”在mysql中使用索引吗？

4 个答案: