让 MySQL 使用主索引而不提示或强制

Question

我有以下数据库结构：

 CREATE TABLE `question` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `level_id` int(11) NOT NULL,
  `video_id` int(11) NOT NULL,
  `is_active` tinyint(1) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `question_b8f3f94a` (`level_id`),
  KEY `question_c11471f1` (`video_id`),
  CONSTRAINT `level_id_refs_id_27dd88d9` FOREIGN KEY (`level_id`) REFERENCES `level` (`id`),
  CONSTRAINT `video_id_refs_id_1c4fbe15` FOREIGN KEY (`video_id`) REFERENCES `video` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3054 DEFAULT CHARSET=utf8 

CREATE TABLE `level` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `success_rate` tinyint(3) unsigned NOT NULL,
  `name` varchar(100) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=25 DEFAULT CHARSET=utf8

CREATE TABLE `video` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `file` varchar(200) DEFAULT NULL,
  `name` longtext NOT NULL,
  `subtitle` longtext NOT NULL,
  `producer` longtext,
  `director` longtext,
  `details` longtext,
  `related_content_url` longtext,
  `counter` int(10) unsigned NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3055 DEFAULT CHARSET=utf8 

CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `password` varchar(128) NOT NULL,
  `last_login` datetime NOT NULL,
  `is_superuser` tinyint(1) NOT NULL,
  `username` varchar(82) NOT NULL,
  `first_name` varchar(30) NOT NULL,
  `last_name` varchar(30) NOT NULL,
  `email` varchar(82) NOT NULL,
  `is_staff` tinyint(1) NOT NULL,
  `is_active` tinyint(1) NOT NULL,
  `date_joined` datetime NOT NULL,
  `is_verified` tinyint(1) NOT NULL,
  `verification_code` varchar(40) DEFAULT NULL,
  `birthday` date DEFAULT NULL,
  `gender` varchar(1) DEFAULT NULL,
  `language_id` int(11) DEFAULT NULL,
  `auth_token` varchar(40) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`),
  UNIQUE KEY `verification_code` (`verification_code`),
  KEY `user_784efa61` (`language_id`),
  KEY `email_idx` (`email`),
  CONSTRAINT `language_id_refs_id_016597a8` FOREIGN KEY (`language_id`) REFERENCES `language` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=469322 DEFAULT CHARSET=utf8  


CREATE TABLE `star` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`question_id` int(11) NOT NULL,
`user_id` int(11) NOT NULL,
`counter` tinyint(3) unsigned NOT NULL,
PRIMARY KEY (`id`),
KEY `star_25110688` (`question_id`),
KEY `star_6340c63c` (`user_id`),
CONSTRAINT `question_id_refs_id_3c6023b7` FOREIGN KEY (`question_id`) REFERENCES `question` (`id`),
CONSTRAINT `user_id_refs_id_4b270cea` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3737324 DEFAULT CHARSET=utf8 

我正在使用的 orm 为特定页面生成以下查询：

SELECT *
  FROM `star` 
  INNER JOIN `question` ON (`star`.`question_id` = `question`.`id`)
  INNER JOIN `level` ON (`question`.`level_id` = `level`.`id`)
  INNER JOIN `video` ON (`question`.`video_id` = `video`.`id`)
  INNER JOIN `user` ON (`star`.`user_id` = `user`.`id`)
  ORDER BY  `star`.`id` DESC 
LIMIT 10;

这个查询运行了很长时间。

+------+-------------+----------+--------+---------------------------------------------+-------------------+---------+----------------------------+------+---------------------------------+
| id   | select_type | table    | type   | possible_keys                               | key               | key_len | ref                        | rows | Extra                           |
+------+-------------+----------+--------+---------------------------------------------+-------------------+---------+----------------------------+------+---------------------------------+
|    1 | SIMPLE      | level    | ALL    | PRIMARY                                     | NULL              | NULL    | NULL                       |   24 | Using temporary; Using filesort |
|    1 | SIMPLE      | question | ref    | PRIMARY,question_b8f3f94a,question_c11471f1 | question_b8f3f94a | 4       | level.id          |   63 |                                 |
|    1 | SIMPLE      | video    | eq_ref | PRIMARY                                     | PRIMARY           | 4       | question.video_id |    1 |                                 |
|    1 | SIMPLE      | star     | ref    | PRIMARY,star_25110688,star_6340c63c         | star_25110688     | 4       | question.id       |  631 | Using index condition           |
|    1 | SIMPLE      | user     | eq_ref | PRIMARY                                     | PRIMARY           | 4       | star.user_id      |    1 |                                 |
+------+-------------+----------+--------+---------------------------------------------+-------------------+---------+----------------------------+------+---------------------------------+

“order by star.id”不使用主键。添加 use index(primary) 解决了我的问题。但我不想手动编写查询。

有没有其他方法可以强制mysql使用主索引（使用ordr by等），而不需要手动提示或强制索引？

Answer 1

这是重写的建议。也许您的 ORM 可以被说服生成此查询。它与原版仅略有不同。唯一的变化是派生表而不是基表 star. 。由于表定义了外键并且查询从 star, ，跟随外键，结果将是相同的：

SELECT *
  FROM 
      ( SELECT * FROM star ORDER BY id DESC LIMIT 10 )                -- the only change
      AS star  
  INNER JOIN `question` ON (`star`.`question_id` = `question`.`id`)
  INNER JOIN `level` ON (`question`.`level_id` = `level`.`id`)
  INNER JOIN `video` ON (`question`.`video_id` = `video`.`id`)
  INNER JOIN `user` ON (`star`.`user_id` = `user`.`id`)
  ORDER BY  `star`.`id` DESC 
LIMIT 10 ;

另一个想法是保留您的查询但更改所有 INNER 加入到 LEFT 加入。这可能会产生一个不同的计划，利用主键 star. 。由于外键，我们再次确定查询是等效的并且会产生相同的结果。

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与效率不完全相关：

• SELECT * 不应该使用。仅添加您需要的列的列表，而不是涉及的所有表的所有列。无论如何，您将如何在结果中识别 id 专栏来自 star.id 或来自 question.id 或来自...？