I am wondering why my query isn't using the index "created_2" which covers all the fields used in the query. It seems to use a filesort. What are the rules for picking an index?
Query:
SELECT * FROM (`stories`) WHERE `image_full_url` != '' AND `order` != 0 ORDER BY `created` DESC, `order` DESC LIMIT 5
Create Table:
| stories | CREATE TABLE `stories` (
`id` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`news_type` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`title` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`created` datetime DEFAULT NULL,
`author` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`author_title` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`image_caption` text COLLATE utf8_unicode_ci,
`image_credit` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`image_full_url` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`body` text COLLATE utf8_unicode_ci,
`summary` text COLLATE utf8_unicode_ci,
`external_url` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`order` int(10) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `news_type` (`news_type`),
KEY `created` (`created`),
KEY `news_type_2` (`news_type`,`created`),
KEY `created_2` (`created`,`image_full_url`,`order`),
KEY `image_full_url` (`image_full_url`,`order`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |
Explain:
mysql> explain SELECT * FROM (`stories`) WHERE `image_full_url` != '' AND `order` != 0 ORDER BY `created` DESC, `order` DESC LIMIT 5;
+----+-------------+---------+-------+----------------+----------------+---------+------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------+----------------+----------------+---------+------+------+-----------------------------+
| 1 | SIMPLE | stories | range | image_full_url | image_full_url | 768 | NULL | 25 | Using where; Using filesort |
+----+-------------+---------+-------+----------------+----------------+---------+------+------+-----------------------------+
1 row in set (0.00 sec)