这是我的表格：

CREATE TABLE `person` (

`id` bigint(10) NOT NULL AUTO_INCREMENT,

`name` varchar(20) DEFAULT NULL,

`age` int(10) NOT NULL DEFAULT '0',

PRIMARY KEY (`id`),

KEY `age` (`age`)

) ENGINE=InnoDB AUTO_INCREMENT=10000 DEFAULT CHARSET=latin1;

这是解释的输出：

mysql> explain select * from person order by age\G

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: person

type: ALL

possible_keys: NULL

key: NULL

key_len: NULL

ref: NULL

rows: 10367

Extra: Using filesort

1 row in set (0.00 sec)

这是怎么回事？为什么MySQL不使用年龄指数进行排序？我试过doind analyze table,但它没有任何区别.

仅供参考,这是表中数据的分布：

mysql> select age, count(*) from person group by age;

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

| age | count(*) |

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

| 21 | 1250 |

| 22 | 1216 |

| 23 | 1278 |

| 24 | 1262 |

| 25 | 1263 |

| 26 | 1221 |

| 27 | 1239 |

| 28 | 1270 |

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

8 rows in set (0.04 sec)

UPDATE

@grisha似乎认为你不能选择一个不在索引中的字段.这似乎没有任何意义,但是,它看起来像下面的工作：

mysql> explain select age from person order by age \G

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: person

type: index

possible_keys: NULL

key: age

key_len: 4

ref: NULL

rows: 10367

Extra: Using index

1 row in set (0.00 sec)

而且,如果我添加一个涵盖所有字段的索引,它也会起作用：

mysql> alter table person add key `idx1` (`age`, `id`, `name`);

Query OK, 0 rows affected (0.29 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> explain select * from person order by age\G

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: person

type: index

possible_keys: NULL

key: idx1

key_len: 35

ref: NULL

rows: 10367

Extra: Using index

1 row in set (0.00 sec)

@eggyal建议使用索引提示.这似乎也有效,可能是正确的答案：

mysql> explain select * from person force key for order by (age) order by age\G

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: person

type: index

possible_keys: NULL

key: age

key_len: 4

ref: NULL

rows: 10367

Extra:

1 row in set (0.02 sec)