EXPLAIN SELECT SENDER AS PROFILEID, MESSAGE, 'R' AS SR, SEEN, DATE
FROM `MESSAGE_LOG`
JOIN MESSAGES ON ( MESSAGE_LOG.ID = MESSAGES.ID )
WHERE `RECEIVER` = '9063911' AND SENDER NOT
IN ('658', '87238', '99359', '643848', '651922', '734783', '880643'
) AND `TYPE` = 'R' AND `IS_MSG` = 'Y'
UNION SELECT RECEIVER AS PROFILEID, MESSAGE, 'S' AS SR, SEEN, DATE
FROM `MESSAGE_LOG`
JOIN MESSAGES ON ( MESSAGE_LOG.ID = MESSAGES.ID )
WHERE `SENDER` = '9063911' AND RECEIVER NOT
IN (
'658', '87238', '99359', '643848', '651922', '734783', '880643'
) AND `TYPE` = 'R' AND `IS_MSG` = 'Y'
ORDER BY DATE DESC
解释查询返回：
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY MESSAGE_LOG ref ID,SENDER,RECEIVER RECEIVER 4 const 142 Using where
1 PRIMARY MESSAGES eq_ref ID ID 4 newjs.MESSAGE_LOG.ID 1
2 UNION MESSAGE_LOG range ID,SENDER,RECEIVER SENDER 8 NULL 168 Using where
2 UNION MESSAGES eq_ref ID ID 4 newjs.MESSAGE_LOG.ID 1
NULL UNION RESULT <union1,2> ALL NULL NULL NULL NULL
查询大约需要15秒才能执行。可能是什么原因以及如何进一步优化查询。
编辑：是因为mysql无法正确使用索引吗？
MESSAGE_LOG表上的索引：
Keyname Type Cardinality Action Field
ID UNIQUE 44491833 Edit Drop ID
SENDER INDEX 44491833 Edit Drop SENDER,RECEIVER
RECEIVER INDEX 1483061 Edit Drop RECEIVER,FOLDERID,OBSCENE
MESSAGES表上的索引：
Keyname Type Cardinality Action Field
ID UNIQUE 43572638 Edit Drop ID
我可以提示mysql使用正确的索引吗？如果是，那么如何以及在哪一列？
表格结构：
CREATE TABLE `MESSAGE_LOG` (
`SENDER` int(8) unsigned NOT NULL DEFAULT '0',
`RECEIVER` int(8) unsigned NOT NULL DEFAULT '0',
`DATE` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`IP` int(10) unsigned DEFAULT NULL,
`RECEIVER_STATUS` char(1) NOT NULL DEFAULT 'U',
`FOLDERID` mediumint(9) NOT NULL DEFAULT '0',
`MSG_OBS_ID` int(11) NOT NULL DEFAULT '0',
`SENDER_STATUS` char(1) NOT NULL DEFAULT 'U',
`TYPE` char(1) NOT NULL DEFAULT 'R',
`ID` int(11) NOT NULL,
`OBSCENE` char(1) NOT NULL DEFAULT 'N',
`IS_MSG` char(1) NOT NULL DEFAULT 'N',
`SEEN` char(1) NOT NULL,
UNIQUE KEY `ID` (`ID`),
KEY `SENDER` (`SENDER`,`RECEIVER`),
KEY `RECEIVER` (`RECEIVER`,`FOLDERID`,`OBSCENE`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1
CREATE TABLE `MESSAGES` (
`ID` int(11) NOT NULL,
`MESSAGE` text NOT NULL,
UNIQUE KEY `ID` (`ID`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1