0

为什么下面的查询使用story_keywords 表中的索引story_id 失败？

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)
4

2 回答 2

1

这可能是因为 MySQL 认为从 story_keywords 表中获取所有行并将它们加入而不是使用索引更便宜。一开始听起来很奇怪，但是，你看，如果你必须对一个表执行 100 次索引查找，而这个表只有大约 100 行——读取所有行的成本会更低。解释很简单：索引查找（对于 BTREE 索引）是 O(ln N)，而读取 N 行是 O(N)。显然，O(N) < N * O(ln N)。

为了证明这一点 - 尝试从故事中仅选择 1 行（我的意思是一行，而不是对整个表格进行排序并限制结果；），就像：

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

此查询更有可能转向 story_keywords 上的索引。

希望这能回答你的问题:)

于 2012-11-14T15:17:04.240 回答
1

安东走在正确的轨道上，但我相信问题还有更多。正如我对 OP 的评论所说，id 列很可能是INT类型。如解释所示，主键的长度stories为 767。通常对于INT类型，长度为低个位数，但由于列是 a VARCHAR，因此长度非常长。

回到主要问题，由于, 或上没有索引stories.news_type，优化器决定对 进行全面扫描，因为它将产生最小的初始结果集。如果其中一列有索引，它可能会首先使用它。如果您添加查询可以使用的索引，则不需要进行全表扫描。stories.orderstory_keywords.story_keywordsstory_keywords

于 2012-11-14T15:34:20.867 回答