今天这篇主要讲order by 语句中的多个字段asc desc的问题。mysql5中，索引存储的排序方式是ASC的，没有DESC的索引。现在能够理解为啥order by 默认是按照ASC来排序的了吧?虽然索引是ASC的，但是也可以反向进行检索，就相当于DESC了。如果您在ORDER BY 语句中使用了 DESC排序，mysql确实会反向进行检索。在理论上，反向检索与正向检索的速度一样的快。但是在某些操作系统上面，并不支持反向的read-ahead预读，所以反向检索会略慢。由于设计的原因，在myisam引擎中，反向的检索速度比正向检索要慢得多。如果ORDER BY 子句中同时出现ASC和DESC，会是怎样的情况呢?

OEDER BY price ASC, date DESC LIMIT 0,10;

而且在 (price,date)上有一个组合索引。

explain之后可以发现，虽然用到了这个索引，但是仍然会用到filesort，说明只是使用到了索引中price的ASC排序。

看一个实际的例子吧：

discuz 7.2 gbk版，主题列表：cdb_threads。

mysql> SHOW CREATE TABLE cdb_threads;

| cdb_threads | CREATE TABLE `cdb_threads` (

`tid` mediumint(8) UNSIGNED NOT NULL AUTO_INCREMENT,

`fid` smallint(6) UNSIGNED NOT NULL DEFAULT '0',

`iconid` smallint(6) UNSIGNED NOT NULL DEFAULT '0',

`typeid` smallint(6) UNSIGNED NOT NULL DEFAULT '0',

`sortid` smallint(6) UNSIGNED NOT NULL DEFAULT '0',

`readperm` tinyint(3) UNSIGNED NOT NULL DEFAULT '0',

`price` smallint(6) NOT NULL DEFAULT '0',

`author` char(15) NOT NULL,

`authorid` mediumint(8) UNSIGNED NOT NULL DEFAULT '0',

`subject` char(80) NOT NULL,

`dateline` int(10) UNSIGNED NOT NULL DEFAULT '0',

`lastpost` int(10) UNSIGNED NOT NULL DEFAULT '0',

`lastposter` char(15) NOT NULL,

`views` int(10) UNSIGNED NOT NULL DEFAULT '0',

`replies` mediumint(8) UNSIGNED NOT NULL DEFAULT '0',

`displayorder` tinyint(1) NOT NULL DEFAULT '0',

`highlight` tinyint(1) NOT NULL DEFAULT '0',

`digest` tinyint(1) NOT NULL DEFAULT '0',

`rate` tinyint(1) NOT NULL DEFAULT '0',

`special` tinyint(1) NOT NULL DEFAULT '0',

`attachment` tinyint(1) NOT NULL DEFAULT '0',

`moderated` tinyint(1) NOT NULL DEFAULT '0',

`closed` mediumint(8) UNSIGNED NOT NULL DEFAULT '0',

`itemid` mediumint(8) UNSIGNED NOT NULL DEFAULT '0',

`supe_pushstatus` tinyint(1) NOT NULL DEFAULT '0',

`sgid` mediumint(8) UNSIGNED NOT NULL DEFAULT '0',

`recommends` smallint(6) NOT NULL,

`recommend_add` smallint(6) NOT NULL,

`recommend_sub` smallint(6) NOT NULL,

`heats` int(10) UNSIGNED NOT NULL DEFAULT '0',

`status` smallint(6) UNSIGNED NOT NULL DEFAULT '0',

PRIMARY KEY (`tid`),

KEY `digest` (`digest`),

KEY `displayorder` (`fid`,`displayorder`,`lastpost`),

KEY `typeid` (`fid`,`typeid`,`displayorder`,`lastpost`),

KEY `sgid` (`fid`,`sgid`),

KEY `sortid` (`sortid`),

KEY `recommends` (`recommends`),

KEY `heats` (`heats`),

KEY `authorid` (`authorid`)

) ENGINE=InnoDB AUTO_INCREMENT=330109 DEFAULT CHARSET=gbk |

fid开头的组合索引有三个：

KEY `displayorder` (`fid`,`displayorder`,`lastpost`),

KEY `typeid` (`fid`,`typeid`,`displayorder`,`lastpost`),

KEY `sgid` (`fid`,`sgid`),

我们用fid和displayorder字段来做排序。

先看order by fid ASC,displayorder ASC的情况：

mysql> EXPLAIN SELECT * FROM cdb_threads WHERE fid IN(1,3,5) ORDER BY fid ASC,displayorder ASC;

+----+-------------+-------------+-------+--------------------------+--------------+---------+------+-------+-------------+

| id | select_type | TABLE | type | possible_keys | KEY | key_len | ref | rows | Extra |

+----+-------------+-------------+-------+--------------------------+--------------+---------+------+-------+-------------+

| 1 | SIMPLE | cdb_threads | range | displayorder,typeid,sgid | displayorder | 2 | NULL | 12728 | USING WHERE |

+----+-------------+-------------+-------+--------------------------+--------------+---------+------+-------+-------------+

1 row IN SET (0.00 sec)

再看ORDER BY fid DESC, displayorder DESC的情况：

mysql> EXPLAIN SELECT * FROM cdb_threads WHERE fid IN(1,3,5) ORDER BY fid DESC,displayorder DESC;

+----+-------------+-------------+-------+--------------------------+--------------+---------+------+-------+-------------+

| id | select_type | TABLE | type | possible_keys | KEY | key_len | ref | rows | Extra |

+----+-------------+-------------+-------+--------------------------+--------------+---------+------+-------+-------------+

| 1 | SIMPLE | cdb_threads | range | displayorder,typeid,sgid | displayorder | 2 | NULL | 12728 | USING WHERE |

+----+-------------+-------------+-------+--------------------------+--------------+---------+------+-------+-------------+

1 row IN SET (0.00 sec)

这两种情况下，使用到的KEY都是 KEY `displayorder` (`fid`,`displayorder`,`lastpost`), 没有进行filesort,很完美。

再来看一个DESC,另外一个ASC的情况：

mysql> EXPLAIN SELECT * FROM cdb_threads WHERE fid IN(1,3,5) ORDER BY fid DESC,displayorder ASC;

+----+-------------+-------------+-------+--------------------------+------+---------+------+------+-----------------------------+

| id | select_type | TABLE | type | possible_keys | KEY | key_len | ref | rows | Extra |

+----+-------------+-------------+-------+--------------------------+------+---------+------+------+-----------------------------+

| 1 | SIMPLE | cdb_threads | range | displayorder,typeid,sgid | sgid | 2 | NULL | 6512 | USING WHERE; USING filesort |

+----+-------------+-------------+-------+--------------------------+------+---------+------+------+-----------------------------+

1 row IN SET (0.00 sec)

mysql> EXPLAIN SELECT * FROM cdb_threads WHERE fid IN(1,3,5) ORDER BY fid ASC,displayorder DESC;

+----+-------------+-------------+-------+--------------------------+------+---------+------+------+-----------------------------+

| id | select_type | TABLE | type | possible_keys | KEY | key_len | ref | rows | Extra |

+----+-------------+-------------+-------+--------------------------+------+---------+------+------+-----------------------------+

| 1 | SIMPLE | cdb_threads | range | displayorder,typeid,sgid | sgid | 2 | NULL | 6512 | USING WHERE; USING filesort |

+----+-------------+-------------+-------+--------------------------+------+---------+------+------+-----------------------------+

1 row IN SET (0.00 sec)

这两次使用到的key是 KEY `sgid` (`fid`,`sgid`), 由于我们并没有涉及到sgid，所以只用到了fid的索引。。。 至于displayorder字段怎样排序，用的是filesort。肯定比直接使用索引要慢多了。

如果可以搞一个fid ASC, displayorder DESC的组合索引，那就方便多了。事实上mysql不支持这么做啦。

既然mysql不支持这种方式，那我们只好用其它方法解决这个问题。

创建一个新的字段，叫做reverse_displayorder。 此字段中保存的值为 displayorder字段的值乘以-1。

于是 order by fid ASC, displayorder DESC 就可以转化成 order by fid ASC, reverse_displayorder ASC了。

如果是mysql 5.0或之后的版本，只要创建一个触发器(trigger)来自动更新reverse_displayorder的值就可以了，程序都不用大改。

虽然discuz没有这样做，但是MediaWiki确实是这样设计的。