需要一些帮助解密MYSQL EXPLAIN输出以进行复杂连接

Question

我网站的主页有一个复杂的查询，如下所示：

SELECT karmalog.*, image.title as img_title, image.date_uploaded, imagefile.file_name as img_filename, imagefile.width as img_width, imagefile.height as img_height, imagefile.transferred as img_transferred, u1.uname as usr_name1, u2.uname as usr_name2, u1.avat_url as usr_avaturl1, u2.avat_url as usr_avaturl2, class.title as class_title,forum.id as f_id, forum.name as f_name, forum.icon, forumtopic.id as ft_id, forumtopic.subject
FROM karmalog 
LEFT JOIN image on karmalog.event_type = 'image' and karmalog.object_id = image.id 
LEFT JOIN imagefile on karmalog.object_id = imagefile.image_id and imagefile.type = 'smallthumb'
LEFT JOIN class on karmalog.event_type = 'class' and karmalog.object_id = class.num
LEFT JOIN user as u1 on karmalog.user_id = u1.id
LEFT JOIN user as u2 on karmalog.user_sec_id = u2.id
LEFT JOIN forumtopic on karmalog.object_id = forumtopic.id and karmalog.event IN ('FORUM_REPLY','FORUM_CREATE')
LEFT JOIN forum on forumtopic.forum_id = forum.id
WHERE karmalog.event IN ('EDIT_PROFILE','FAV_IMG_ADD','FOLLOW','COM_POST','IMG_UPLOAD','IMG_VOTE','LIST_VOTE','JOIN','CLASS_UP','CLASS_DOWN','LIST_CREATE','FORUM_REPLY','FORUM_CREATE','FORUM_SUBSCRIBE')
  AND karmalog.delete=0
ORDER BY karmalog.date_created DESC, karmalog.id DESC 
LIMIT 0,13 

我不会向你详细说明具体细节，只是一个简短的解释：基本上这是系统中发生的事件列表，有点像流。事件可以是几种类型，并且根据其类型，它需要连接来自各种表的特定数据。

目前，此查询需要2秒才能运行，但随着条目数量的增加，它会随着时间的推移而变慢。因此，我正在寻求优化它。这是MYSQL的输出说明：

我对EXPLAIN的理解太有限，无法理解这一点。我宁愿保持这个查询不是（而不是非规范化），而是使用适当的索引或其他快速获胜来提高其性能。根据这个解释输出，你有什么看到我可以跟进吗？

编辑：根据要求提供karmalog表的定义：

CREATE TABLE `karmalog` (
  `id` int(11) NOT NULL auto_increment,
  `guid` char(36) default NULL,
  `user_id` int(11) default NULL,
  `user_sec_id` int(11) default NULL,
  `event` enum('EDIT_PROFILE','EDIT_AVATAR','EDIT_EMAIL','EDIT_PASSWORD','FAV_IMG_ADD','FAV_IMG_ADDED','FAV_IMG_REMOVE','FAV_IMG_REMOVED','FOLLOW','FOLLOWED','UNFOLLOW','UNFOLLOWED','COM_POSTED','COM_POST','COM_VOTE','COM_VOTED','IMG_VOTED','IMG_UPLOAD','LIST_CREATE','LIST_DELETE','LIST_ADMINDELETE','LIST_VOTE','LIST_VOTED','IMG_UPD','IMG_RESTORE','IMG_UPD_LIC','IMG_UPD_MOD','IMG_UPD_MODERATED','IMG_VOTE','IMG_VOTED','TAG_FAV_ADD','CLASS_DOWN','CLASS_UP','IMG_DELETE','IMG_ADMINDELETE','IMG_ADMINDELETEFAV','SET_PASSWORD','IMG_RESTORED','IMG_VIEW','FORUM_CREATE','FORUM_DELETE','FORUM_ADMINDELETE','FORUM_REPLY','FORUM_DELETEREPLY','FORUM_ADMINDELETEREPLY','FORUM_SUBSCRIBE','FORUM_UNSUBSCRIBE','TAG_INFO_EDITED','JOIN') NOT NULL,
  `event_type` enum('follow','tag','image','class','list','forum','user') NOT NULL,
  `active` bit(1) NOT NULL,
  `delete` bit(1) NOT NULL default '\0',
  `object_id` int(11) default NULL,
  `object_cache` varchar(1024) default NULL,
  `karma_delta` int(11) NOT NULL,
  `gold_delta` int(11) NOT NULL,
  `newkarma` int(11) NOT NULL,
  `newgold` int(11) NOT NULL,
  `mail_processed` bit(1) NOT NULL default '\0',
  `date_created` timestamp NOT NULL default '0000-00-00 00:00:00',
  PRIMARY KEY  (`id`),
  KEY `user_id` (`user_id`),
  KEY `user_sec_id` (`user_sec_id`),
  KEY `image_id` (`object_id`),
  CONSTRAINT `user_id` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`) ON DELETE SET NULL,
  CONSTRAINT `user_sec_id` FOREIGN KEY (`user_sec_id`) REFERENCES `user` (`id`) ON DELETE SET NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Answer 1

• 首先，您可能错过(event_type, object_id)上的复合索引。二读时，不要理会。您可能需要这样的索引用于其他查询，但不能用于此查询（因为ORDER BY ... LIMIT）。

• 其次，您在date_created和ORDER BY此列中没有索引。在此添加索引。考虑到WHERE条件，最佳索引可能是(delete, date_created)或(event, date_created)或（可能最佳）：(event, delete, date_created)。

• 第三，尝试像这样重写：

先限制，然后加入（更正）：

SELECT karmalog.*, image.title as img_title, image.date_uploaded, imagefile.file_name as img_filename, imagefile.width as img_width, imagefile.height as img_height, imagefile.transferred as img_transferred, u1.uname as usr_name1, u2.uname as usr_name2, u1.avat_url as usr_avaturl1, u2.avat_url as usr_avaturl2, class.title as class_title,forum.id as f_id, forum.name as f_name, forum.icon, forumtopic.id as ft_id, forumtopic.subject
FROM 
    ( SELECT *
      FROM karmalog
      WHERE karmalog.event IN ('EDIT_PROFILE','FAV_IMG_ADD','FOLLOW','COM_POST','IMG_UPLOAD','IMG_VOTE','LIST_VOTE','JOIN','CLASS_UP','CLASS_DOWN','LIST_CREATE','FORUM_REPLY','FORUM_CREATE','FORUM_SUBSCRIBE')
        AND karmalog.delete=0
      ORDER BY karmalog.date_created DESC, karmalog.id DESC 
      LIMIT 0,13  
    ) AS karmalog 
LEFT JOIN image on karmalog.event_type = 'image' and karmalog.object_id = image.id 
LEFT JOIN imagefile on karmalog.object_id = imagefile.image_id and imagefile.type = 'smallthumb'
LEFT JOIN class on karmalog.event_type = 'class' and karmalog.object_id = class.num
LEFT JOIN user as u1 on karmalog.user_id = u1.id
LEFT JOIN user as u2 on karmalog.user_sec_id = u2.id
LEFT JOIN forumtopic on karmalog.object_id = forumtopic.id and karmalog.event IN ('FORUM_REPLY','FORUM_CREATE')
LEFT JOIN forum on forumtopic.forum_id = forum.id
ORDER BY karmalog.date_created DESC, karmalog.id DESC 

Answer 2

解释的重要部分是：possible keys，keys和rows。

如果没有可能的键，则需要创建索引 如果没有使用密钥，则可能是由于：

• 密钥的基数低;
• 使用功能;

将您的精力集中在具有最多行数的表格上。即karmalog。

请记住，MySQL每个表每个选择只能使用一个索引 连接都是左连接，因此它们不限制karmalog索引中的行数，这对您没有帮助。
查看where部分，deleted的基数较低（只有2个值，其中90％为=0）。因此，只有字段event + date_created才有资格获得索引，将索引放在：

ALTER TABLE karmalog ADD INDEX date_event (event, date_created);

Answer 3

尝试在表karmalog肯定事件（可能是delete和object_id）上放一个索引，因为这会使它更快，并为第一次连接提供一个键。

再看一下这个表，看看你是否可以通过某种联接来做到这一点，以便将来更轻松。但这可能意味着你的数据库发生了变化

karmalog.event IN ('EDIT_PROFILE','FAV_IMG_ADD','FOLLOW','COM_POST','IMG_UPLOAD','IMG_VOTE','LIST_VOTE','JOIN','CLASS_UP','CLASS_DOWN','LIST_CREATE','FORUM_REPLY','FORUM_CREATE','FORUM_SUBSCRIBE')