下面的查询返回一组用户，对于每一行，从正在搜索的用户 (id = 4) 角度来看，有一些关系

SELECT `users`.`firstname` AS firstname,
       `users`.`lastname` AS lastname,
       COUNT(`trusted_users`.`id`) AS number_of_friend_in_common,
       CASE ... AS friend,
       CASE ... AS facebook_invitable,
       CASE ... AS address_book_invitable,
       CASE ... AS virtual_user,
FROM `users`

LEFT OUTER JOIN `trusted_users` 
  ON `trusted_users`.`user_id` = 4 AND `trusted_users`.`trust_user_id` = `users`.`id`

LEFT OUTER JOIN `facebook_friends` 
  ON (`facebook_friends`.`user_id` = 4 AND `facebook_friends`.`friend_user_id` = `users`.`id`
  OR `facebook_friends`.`user_id` = `users`.`id` AND `facebook_friends`.`friend_user_id` = 4)

LEFT OUTER JOIN `address_book_contacts` 
  ON `address_book_contacts`.`owner_id` = 4 AND `address_book_contacts`.`email_digest` = `users`.`email_digest`

LEFT OUTER JOIN `friends` 
  ON (`friends`.`me_id` = `users`.`id` AND `friends`.`him_id` = 4
  OR `friends`.`me_id` = 4 AND `friends`.`him_id` = `users`.`id`)

WHERE `users`.`id` NOT IN
    (SELECT CASE
                WHEN `friends`.`me_id` = 4 THEN `friends`.`him_id`
                ELSE `friends`.`me_id`
            END
     FROM `friends`
     WHERE (`friends`.`status` = 0
            AND `friends`.`him_id` = 4
            AND `friends`.`him_status` = 7
            OR `friends`.`status` = 0
            AND `friends`.`me_id` = 4
            AND `friends`.`me_status` = 7))
  AND (`users`.`firstname` LIKE '%a%' OR `users`.`lastname` LIKE '%a%')
GROUP BY `users`.`id`
ORDER BY friend DESC,
         facebook_invitable DESC,
         address_book_invitable DESC,
         number_of_friend_in_common DESC,
         virtual_user DESC,
         firstname,
         lastname LIMIT 0, 20

每个表的行数:

trusted_users:255k

facebook_friends:1k

address_book_contacts:1.5M

friend :70k

用户:32k

join的所有字段都有索引。查询需要 1.1 秒，这对于我们拥有的数据量来说是 Not Acceptable 。 我究竟做错了什么？我应该拆分多个查询并自己分页吗？

编辑 1: EXPLAIN 结果

+----+-------------+-----------------------+-------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------+---------+----------------------------------+-------+----------------------------------------------------------------------------------------------------------------------------+
| id | select_type | table                 | type        | possible_keys                                                                                                                                                               | key                                                       | key_len | ref                              | rows  | Extra                                                                                                                      |
+----+-------------+-----------------------+-------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------+---------+----------------------------------+-------+----------------------------------------------------------------------------------------------------------------------------+
|  1 | PRIMARY     | users                 | ALL         | PRIMARY,index_users_on_chat_id,index_users_login_facebook_id,index_users_on_login,index_users_on_parent_id,index_users_account_type,index_users_email_digest,index_users_id | NULL                                                      | NULL    | NULL                             | 31847 | Using where; Using temporary; Using filesort                                                                               |
|  1 | PRIMARY     | trusted_users         | ref         | index_trusted_users_user,index_trusted_users_trust_user                                                                                                                     | index_trusted_users_trust_user                            | 5       | messenger_dev.users.id           |     6 | Using where                                                                                                                |
|  1 | PRIMARY     | facebook_friends      | index_merge | index_facebook_friends_user,index_facebook_friends_friend                                                                                                                   | index_facebook_friends_user,index_facebook_friends_friend | 5,5     | NULL                             |     2 | Using union(index_facebook_friends_user,index_facebook_friends_friend); Using where; Using join buffer (Block Nested Loop) |
|  1 | PRIMARY     | address_book_contacts | ref         | index_address_book_contacts_owner_id,index_address_book_contacts_email                                                                                                      | index_address_book_contacts_email                         | 767     | messenger_dev.users.email_digest |     1 | Using where                                                                                                                |
|  1 | PRIMARY     | friends               | index_merge | index_friends_me_him,index_friends_me,index_friends_him                                                                                                                     | index_friends_him,index_friends_me                        | 5,5     | NULL                             |    18 | Using union(index_friends_him,index_friends_me); Using where; Using join buffer (Block Nested Loop)                        |
|  2 | SUBQUERY    | friends               | index_merge | index_friends_me_him,index_friends_me,index_friends_him                                                                                                                     | index_friends_him,index_friends_me                        | 5,5     | NULL                             |    18 | Using union(index_friends_him,index_friends_me); Using where                                                               |
+----+-------------+-----------------------+-------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------------------+---------+----------------------------------+-------+----------------------------------------------------------------------------------------------------------------------------+
6 rows in set (0,00 sec)

编辑2:表结构

CREATE TABLE `address_book_contacts` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `email_digest` varchar(191) DEFAULT NULL,
  `code` varchar(191) DEFAULT NULL,
  `created_at` datetime NOT NULL,
  `updated_at` datetime NOT NULL,
  `owner_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `index_address_book_contacts_owner_id` (`owner_id`),
  KEY `index_address_book_contacts_email` (`email_digest`)
) ENGINE=InnoDB AUTO_INCREMENT=1598109 DEFAULT CHARSET=utf8mb4;

CREATE TABLE `trusted_users` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `friend_id` int(11) DEFAULT NULL,
  `user_id` int(11) DEFAULT NULL,
  `trust_user_id` int(11) DEFAULT NULL,
  `created_at` datetime NOT NULL,
  `updated_at` datetime NOT NULL,
  PRIMARY KEY (`id`),
  KEY `index_trusted_users_on_friend_id` (`friend_id`),
  KEY `index_trusted_users_user` (`user_id`),
  KEY `index_trusted_users_trust_user` (`trust_user_id`),
  CONSTRAINT `fk_rails_007c31c802` FOREIGN KEY (`trust_user_id`) REFERENCES `users` (`id`),
  CONSTRAINT `fk_rails_ca24cb4e23` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=275576 DEFAULT CHARSET=utf8mb4;

CREATE TABLE `facebook_friends` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) DEFAULT NULL,
  `friend_user_id` int(11) DEFAULT NULL,
  `created_at` datetime NOT NULL,
  `updated_at` datetime NOT NULL,
  `code` varchar(191) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `index_facebook_friends_user` (`user_id`),
  KEY `index_facebook_friends_friend` (`friend_user_id`),
  KEY `index_facebook_friends_code` (`code`(5)),
  CONSTRAINT `fk_rails_78285a074e` FOREIGN KEY (`friend_user_id`) REFERENCES `users` (`id`),
  CONSTRAINT `fk_rails_aa3ac53a81` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1149 DEFAULT CHARSET=utf8mb4;

CREATE TABLE `friends` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `me_id` int(11) DEFAULT NULL,
  `him_id` int(11) DEFAULT NULL,
  `owner_id` int(11) DEFAULT NULL,
  `status` int(11) DEFAULT NULL,
  `created_at` datetime NOT NULL,
  `updated_at` datetime NOT NULL,
  `me_status` int(11) DEFAULT '0',
  `him_status` int(11) DEFAULT '0',
  PRIMARY KEY (`id`),
  UNIQUE KEY `index_friends_me_him` (`me_id`,`him_id`),
  KEY `index_friends_me` (`me_id`),
  KEY `index_friends_him` (`him_id`),
  KEY `index_friends_owner` (`owner_id`),
  CONSTRAINT `fk_rails_9fa3474d31` FOREIGN KEY (`owner_id`) REFERENCES `users` (`id`),
  CONSTRAINT `fk_rails_d3ebb6657f` FOREIGN KEY (`him_id`) REFERENCES `users` (`id`),
  CONSTRAINT `fk_rails_fccfd1b821` FOREIGN KEY (`me_id`) REFERENCES `users` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=95724 DEFAULT CHARSET=utf8mb4;

CREATE TABLE `users` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `account_type` varchar(191) NOT NULL,
  `firstname` varchar(191) DEFAULT NULL,
  `lastname` varchar(191) DEFAULT NULL,
  `login` varchar(191) DEFAULT NULL,
  `avatar` varchar(191) DEFAULT NULL,
  `gender` varchar(1) DEFAULT NULL,
  `locale` varchar(191) DEFAULT NULL,
  `birthdate` date DEFAULT NULL,
  `password_digest` varchar(191) DEFAULT NULL,
  `email_digest` varchar(191) DEFAULT NULL,
  `created_at` datetime NOT NULL,
  `updated_at` datetime NOT NULL,
  PRIMARY KEY (`id`),
  KEY `index_users_on_login` (`login`),
  KEY `index_users_account_type` (`account_type`),
  KEY `index_users_email_digest` (`email_digest`),
  KEY `index_uses_firstname` (`firstname`),
  KEY `index_users_lastname` (`lastname`)
) ENGINE=InnoDB AUTO_INCREMENT=32516 DEFAULT CHARSET=utf8mb4;

最佳答案

看起来 MySQL 没有为 users 表选择任何可用的索引。

首先，运行ANALYZE TABLE users;，然后重新运行EXPLAIN命令。第一个 rows 单元格的值现在是否大大低于 31847？如果是这样，您的问题就应该解决了!

如果没有，运行OPTIMIZE TABLE users;，然后重新运行EXPLAIN命令。第一个 rows 单元格的值现在是否大大低于 31847？如果是这样，您的问题就应该解决了!

如果这些步骤都没有帮助，请尝试在 FROM users 部分之后立即添加 USE INDEX (PRIMARY) 或 USE INDEX (users_id)您的查询。

希望这对您有所帮助!

关于mysql - 如何优化左连接上大数据的MySQL查询？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/42093480/