Mysql 优化查询--它正在使用文件排序。（包括解释、查询和显示创建表）

我想知道为什么我的查询没有使用包含查询中使用的所有字段的索引“created_2”。它似乎使用了文件排序。选择索引的规则是什么

查询：

SELECT * FROM (`stories`) WHERE `image_full_url` != '' AND `order` != 0 ORDER BY `created` DESC, `order` DESC LIMIT 5

创建表：

| 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 |

说明：

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)

完整的规则集是，并声明如果

order by

引用索引的非连续部分，则不能使用索引。如果将索引从（

created

，

image\u full\u url

，

order

）更改为（

created

，

order

，

image\u full\u url

），则可能允许使用它