我正在修改我的網站的內部郵件系統,並且遇到了一些我不明白的東西。下面是表:不等於Where子句使用Filesort,但不等於。爲什麼?
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;
而這裏的查詢:
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的查詢顯示「使用,其中,使用索引;使用文件排序」。但是,如果我將查詢更改爲:
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顯示「使用where;使用索引」。出於某種原因,在第一個查詢中使用!=會導致一個filesort,但在第二個查詢中使用=不會使用filesort。我很好奇發生了什麼事情會導致差異?
我總是假設filesort是負的。我很好奇爲什麼包容性和排他性的運營商會做出這樣的改變。 – mellowsoon 2011-05-24 01:35:54