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I have a very simple query: SELECT comments.* FROM comments WHERE comments.imageid=46 And this is my table: CREATE TABLE IF NOT EXISTS `comments` ( `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT, `imageid` int(10) unsigned NOT NULL DEFAULT '0', `uid` bigint(20) unsigned NOT NULL DEFAULT '0', `content` text CHARACTER SET utf8, `adate` datetime DEFAULT NULL, `ip` int(10) unsigned DEFAULT NULL, PRIMARY KEY (`id`), KEY `ids` (`imageid`) USING BTREE, KEY `dt` (`adate`) USING BTREE ) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=12 ; But MySql can't use index on this simple query. here is the explain result: id select_type table type possible_keys key key_len ref rows filtered Extra 1 SIMPLE comments ALL ids NULL NULL NULL 4 75.00 Using where while I change the query to this, Mysql can use index. Why? : SELECT comments.id FROM comments WHERE comments.imageid=46 here is the explain: id select_type table type possible_keys key key_len ref rows filtered Extra 1 SIMPLE comments ref ids ids 4 const 4 100.00 Using index

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