Filesort查詢，有索引，但沒有使用它

Question

爲什麼下面的查詢失敗它使用story_id，在story_keywords表中？

mysql> EXPLAIN SELECT `stories`.* 
    -> FROM (`stories`) 
    -> JOIN `story_keywords` ON `story_keywords`.`story_id` = `stories`.`id` 
    -> WHERE `image_full_url` != '' 
    -> AND `order` != 0 
    -> AND `news_type` IN ('movie', 'movie_review') 
    -> AND `keyword` IN ('topnews', 'toptablet') 
    -> GROUP BY `stories`.`id` 
    -> ORDER BY `created` DESC, `order` DESC 
    -> LIMIT 5 ; 
+----+-------------+----------------+--------+---------------+---------+---------+---------------------------------------+------+----------------------------------------------+ 
| id | select_type | table   | type | possible_keys | key  | key_len | ref         | rows | Extra          | 
+----+-------------+----------------+--------+---------------+---------+---------+---------------------------------------+------+----------------------------------------------+ 
| 1 | SIMPLE  | story_keywords | ALL | story_id  | NULL | NULL | NULL         | 42 | Using where; Using temporary; Using filesort | 
| 1 | SIMPLE  | stories  | eq_ref | PRIMARY  | PRIMARY | 767  | entertainment.story_keywords.story_id | 1 | Using where         | 
+----+-------------+----------------+--------+---------------+---------+---------+---------------------------------------+------+----------------------------------------------+ 
2 rows in set (0.00 sec) 


mysql> show create table stories; 
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 
| Table | 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`) 
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci | 
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 
1 row in set (0.00 sec) 

mysql> show create table story_keywords; 
+----------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 
| Table   | Create Table                                                                              | 
+----------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 
| story_keywords | CREATE TABLE `story_keywords` (
    `id` int(10) NOT NULL AUTO_INCREMENT, 
    `story_id` varchar(255) COLLATE utf8_unicode_ci NOT NULL, 
    `keyword` varchar(255) COLLATE utf8_unicode_ci NOT NULL, 
    PRIMARY KEY (`id`), 
    KEY `story_id` (`story_id`) 
) ENGINE=MyISAM AUTO_INCREMENT=85 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci | 
+----------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 
1 row in set (0.00 sec) 

Answer 1

這可能是因爲MySQL認爲它是便宜從story_keywords表中提取所有行和中，而不是使用索引加入他們的行列。這聽起來很奇怪，但是，你會發現，如果你必須在一個表上執行100次索引查找，並且這個表只有大約100行 - 讀取所有行花費會更少。其解釋很簡單：索引查找（對於BTREE索引）是O（ln N），而讀取N行是O（N）。顯然，O（N）N * O（InN）。

爲了證明這一點 - 嘗試從故事只選擇1行（和一個我的意思是一排，而不是排序整個表，限制的結果），就像：

SELECT `stories`.* 
FROM (`stories`) 
JOIN `story_keywords` ON `story_keywords`.`story_id` = `stories`.`id` 
WHERE `stories`.id = SOMETHING 

該查詢多更有可能轉向在story_keywords上編制索引。

希望這回答你的問題:)

Answer 2

安東是在正確的軌道上，但我相信還有更多的問題。正如我對OP的評論所說，id列很可能是INT類型。如解釋所示，stories上的主鍵長度爲767.通常，對於INT類型，長度將爲低位單個數字，但由於該列爲VARCHAR，因此該長度非常長。

回到主要的問題，因爲有上stories.news_type，stories.order，或story_keywords.story_keywords沒有索引，優化決定做的story_keywords一個完整的掃描，因爲它會產生最小的初始結果集。如果其中一列有索引，它可能會首先使用它。如果添加查詢可以使用的索引，則不需要執行全表掃描。