why does "select * from table oder by indexed_field" not use key?

[Reindl Harald]

my reason for create a key on "qg_sort" was primary
for this query - but why is here 'filesort' used?

mysql> EXPLAIN SELECT * FROM cms1_quickbar_groups ORDER BY qg_sort ASC;
+----+-------------+----------------------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table                | type | possible_keys | key  | 
key_len | ref  | rows | Extra          |
+----+-------------+----------------------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | cms1_quickbar_groups | ALL  | NULL          | NULL | NULL  
  | NULL |    2 | Using filesort |
+----+-------------+----------------------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.01 sec)
-
cms1_quickbar_groups | CREATE TABLE `cms1_quickbar_groups` (
  `qg_id` mediumint(7) unsigned NOT NULL AUTO_INCREMENT,
  `qg_titel` varchar(255) COLLATE latin1_german1_ci NOT NULL DEFAULT '',
  `qg_titel_en` varchar(255) COLLATE latin1_german1_ci NOT NULL DEFAULT '',
  `qg_sort` smallint(3) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`qg_id`),
  KEY `qbq_key` (`qg_sort`)
) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT CHARSET=latin1 
COLLATE=latin1_german1_ci PACK_KEYS=1 DELAY_KEY_WRITE=1

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