我正在修改我的网站的内部邮件系统,并且遇到了一些我不明白的东西。下面是表:不等于Where子句使用Filesort,但不等于。为什么?
CREATE TABLE `mails` (
`id` bigint(12) unsigned not null auto_increment,
`recipient_id` mediumint(8) unsigned not null,
`date_sent` datetime not null,
`status` enum('unread', 'read', 'deleted') default 'unread',
PRIMARY KEY(`id`),
INDEX(`recipient_id`, `status`, `date_sent`),
CONSTRAINT FOREIGN KEY (`recipient_id`) REFERENCES `members` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB;
CREATE TABLE `mail_contents` (
`mail_id` bigint(12) unsigned not null,
`sender_id` mediumint(8) unsigned not null,
`subject` varchar(150) default '',
`content` text not null,
CONSTRAINT FOREIGN KEY (`sender_id`) REFERENCES `members` (`id`) ON DELETE CASCADE,
CONSTRAINT FOREIGN KEY (`mail_id`) REFERENCES `mails` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB;
而这里的查询:
SELECT *
FROM mails AS m
LEFT JOIN mail_contents AS mc ON mc.mail_id = m.id
WHERE recipient_id = 66
AND status != 'deleted'
ORDER BY date_sent DESC
LIMIT 40\G
的EXPLAIN的查询显示“使用,其中,使用索引;使用文件排序”。但是,如果我将查询更改为:
SELECT *
FROM mails AS m
LEFT JOIN mail_contents AS mc ON mc.mail_id = m.id
WHERE recipient_id = 66
AND status = 'sent'
ORDER BY date_sent DESC
LIMIT 40\G
EXPLAIN显示“使用where;使用索引”。出于某种原因,在第一个查询中使用!=会导致一个filesort,但在第二个查询中使用=不会使用filesort。我很好奇发生了什么事情会导致差异?
我总是假设filesort是负的。我很好奇为什么包容性和排他性的运营商会做出这样的改变。 – mellowsoon 2011-05-24 01:35:54