为什么我的索引不用于文本列上的此select distinct查询?

我希望在下面的select distinct查询中使用其中一个索引:

CREATE TABLE test(
  value TEXT
);

INSERT INTO test (value) VALUES ('a');
INSERT INTO test (value) VALUES ('b');
INSERT INTO test (value) VALUES ('c');

CREATE INDEX value_i ON test(value(32));
CREATE FULLTEXT INDEX value_i_ft ON test(value);

SELECT DISTINCT value FROM test;

EXPLAIN SELECT DISTINCT value FROM test;

然而,似乎不是:

--------------
EXPLAIN SELECT DISTINCT value FROM test
--------------

+----+-------------+-------+------+---------------+------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra           |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------+
|  1 | SIMPLE      | test  | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using temporary |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------+
1 row in set (0.00 sec)

我使用的是MySQLver14.12distrib 5.0.77,用于RedhatLinuxGNU(i686),使用的是readline 5.1。这种查询在70000行上取1.5秒,其中有250个不同的值,而在索引整数列的同一个表上取约10毫秒。