JOIN、UNION、ORDER BYを使用したサブクエリ、filesortを使用した一時的な使用

Question

メッセージングシステムにmysqlを使用しています。私は最後のメッセージを送信するか、または他のすべてのユーザーから受信した、ユーザーが通信したメッセージをプルするクエリを作成しようとしています。

これは、すべてのメッセージを保持するテーブルである：ここでは

CREATE TABLE `privatemessages` (
    `id` int(11) NOT NULL auto_increment, 
    `recipient` int(11) NOT NULL, 
    `sender` int(11) NOT NULL, 
    `time` int(11) NOT NULL, 
    `readstatus` int(11) NOT NULL, 
    `message` varchar(255) NOT NULL, 
    `messagetype` int(11) NOT NULL, 
    `rdeleted` int(11) NOT NULL, 
    `sdeleted` int(11) NOT NULL, 
    PRIMARY KEY (`id`), 
    KEY `recipient` (`recipient`), 
    KEY `sender` (`sender`), 
    KEY `read` (`readstatus`), 
    KEY `time` (`time`), 
    KEY `openmessagingpanel` (`recipient`,`readstatus`), 
    KEY `openpmthreadrev` (`recipient`,`sender`), 
    KEY `openpmthread` (`sender`,`recipient`) 
) ENGINE=InnoDB AUTO_INCREMENT=27587533 DEFAULT CHARSET=latin1$$ 

は私に問題を与えてクエリです：

select * from 
(
select users.username, users.onlinestatus,users.profileimageid, privatemessages.id, privatemessages.time, privatemessages.message from privatemessages 
JOIN users on privatemessages.recipient=users.id WHERE sender=19 
UNION ALL 
select users.username, users.onlinestatus,users.profileimageid, privatemessages.id, privatemessages.time, privatemessages.message from privatemessages 
JOIN users on privatemessages.sender=users.id WHERE recipient=19 
ORDER BY id DESC 
) 
as testResult GROUP by testResult.username ORDER By id DESC; 

クエリは、正しい順序で、私が欲しいデータを返し、多くのメッセージを持つユーザーにとっては非常に遅いです。ここで

は説明文です：

+----+--------------+-----------------+--------+------------------------------------------------------------------+--------------------+---------+---------------------------------+------+---------------------------------+ 
| id | select_type | table   | type | possible_keys             | key    | key_len | ref        | rows | Extra       | 
+----+--------------+-----------------+--------+------------------------------------------------------------------+--------------------+---------+---------------------------------+------+---------------------------------+ 
| 1 | PRIMARY  | <derived2>  | ALL | NULL                | NULL    | NULL | NULL       | 4246 | Using temporary; Using filesort | 
| 2 | DERIVED  | privatemessages | ref | recipient,sender,openmessagingpanel,openpmthreadrev,openpmthread | sender    | 4  |         | 1076 |         | 
| 2 | DERIVED  | users   | eq_ref | PRIMARY               | PRIMARY   | 4  | chat2.privatemessages.recipient | 1 |         | 
| 3 | UNION  | privatemessages | ref | recipient,sender,openmessagingpanel,openpmthreadrev,openpmthread | openmessagingpanel | 4  |         | 6490 |         | 
| 3 | UNION  | users   | eq_ref | PRIMARY               | PRIMARY   | 4  | chat2.privatemessages.sender | 1 |         | 
| NULL | UNION RESULT | <union2,3>  | ALL | NULL                | NULL    | NULL | NULL       | NULL | Using filesort     | 
+----+--------------+-----------------+--------+------------------------------------------------------------------+--------------------+---------+---------------------------------+------+---------------------------------+ 
6 rows in set (0.08 sec) 

私が使用する必要があり、より良いクエリがありますか？ おかげ

Answer 1

は

SELECT users.username, users.onlinestatus, users.profileimageid, temp.id, temp.time, temp.message 
    FROM users 
INNER JOIN (SELECT id, time, message, recipient FROM privatemessages WHERE sender=19) temp 
    ON temp.recipient=users.id 
UNION 
SELECT users.username, users.onlinestatus,users.profileimageid, temp.id, temp.time, temp.message 
    FROM users 
INNER JOIN (SELECT id, time, message, sender FROM privatemessages WHERE sender=19) temp 
    ON temp.sender=users.id 
GROUP BY username ORDER BY id DESC