MySQL的使用索引优化排序

问题描述：

首先，我创建表tag：MySQL的使用索引优化排序

CREATE TABLE `tag` (
    `id` smallint(6) NOT NULL AUTO_INCREMENT, 
    `total` int(11) DEFAULT NULL, 
    `total_question` int(11) DEFAULT NULL, 
    PRIMARY KEY (`id`), 
    KEY `idx_sort` (`total`,`total_question`) USING BTREE 
) ENGINE=InnoDB DEFAULT CHARSET=gb2312;

的mysql>explain select * from tag order by total;

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+ 
| id | select_type | table | type | possible_keys | key  | key_len | ref | rows | Extra  | 
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+ 
| 1 | SIMPLE  | tag | index | NULL   | idx_sort | 10  | NULL | 1 | Using index | 
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

排序使用索引，而不是使用filesort。

当我添加列name到tag表：

CREATE TABLE `tag` (
    `id` smallint(6) NOT NULL AUTO_INCREMENT, 
    `total` int(11) DEFAULT NULL, 
    `total_question` int(11) DEFAULT NULL, 
    `name` char(20) DEFAULT NULL, 
    PRIMARY KEY (`id`), 
    KEY `idx_sort` (`total`,`total_question`) USING BTREE 
) ENGINE=InnoDB DEFAULT CHARSET=gb2312;

的MySQL>explain select * from tag order by total;

+----+-------------+-------+------+---------------+------+---------+------+------+----------------+ 
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra   | 
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+ 
| 1 | SIMPLE  | tag | ALL | NULL   | NULL | NULL | NULL | 1 | Using filesort | 
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+

排序使用文件排序，不使用索引。

当我创建的索引只total：

CREATE TABLE `tag` (
    `id` smallint(6) NOT NULL AUTO_INCREMENT, 
    `total` int(11) DEFAULT NULL, 
    `total_question` int(11) DEFAULT NULL, 
    `name` char(20) DEFAULT NULL, 
    PRIMARY KEY (`id`), 
    KEY `idx_sort` (`total`) USING BTREE 
) ENGINE=InnoDB DEFAULT CHARSET=gb2312;

的mysql>explain select * from tag order by total;

+----+-------------+-------+------+---------------+------+---------+------+------+----------------+ 
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra   | 
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+ 
| 1 | SIMPLE  | tag | ALL | NULL   | NULL | NULL | NULL | 1 | Using filesort | 
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+

排序使用文件排序！为什么？我只使用total列进行排序。

因为选择'*' - '*'就是全部，而指数里只含有总的值，因为，你是只索引总列，所以，如果你正在做'选择从标签顺序总数' ajreal 2012-03-19 11:19:38

我认为你不能如果你想在单个查询中有所有列返回 – ajreal 2012-03-19 11:49:15

什么如果你没有选择最后一个版本表中的所有列，会发生什么？从标签顺序总数中选择id，total，total_question; – 2012-03-19 12:13:48

答

可以运行desc select * from tag force index (idx_sort) order by total ; 你可以看到输出：

mysql> desc select * from tag force index (idx_sort) order by total ; 
+----+-------------+-------+-------+---------------+----------+---------+------+------+---  ----+ 
| id | select_type | table | type | possible_keys | key  | key_len | ref | rows | Extra | 
+----+-------------+-------+-------+---------------+----------+---------+------+------+--- ----+ 
| 1 | SIMPLE  | tag | index | NULL   | idx_sort | 5  | NULL | 1 |   | 
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------+

MySQL的使用索引优化排序

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