mysql - 在不使用嵌套查询的情况下使用 GROUP BY 和 ORDER BY 优化 JOINed 表上的 MySQL 查询

Question

对我来说，这有点像是一个初学者 SQL 问题，但这里是。这就是我想要做的:

• 将三个表连接在一起，产品、标签和一个链接表。
• 将标签聚合到一个逗号分隔的字段中(因此是 GROUP_CONCAT 和 GROUP BY)
• 限制结果(最多 30 个)
• 按照“创建”日期的顺序排列结果
• 尽可能避免使用子查询，因为使用 Active Record 框架编写代码特别不愉快

我已经在这篇文章的底部描述了所涉及的表，但这是我正在执行的查询

   SELECT p.*, GROUP_CONCAT(pt.name) 
     FROM products p
LEFT JOIN product_tags_for_products pt4p ON (pt4p.product_id = p.id)
LEFT JOIN product_tags pt ON (pt.id = pt4p.product_tag_id)
 GROUP BY p.id
 ORDER BY p.created 
    LIMIT 30;

大约有 280,000 种产品、130 个标签、524,000 条链接记录，我已经分析了这些表。问题是它需要超过 80 秒才能运行(在像样的硬件上)，这对我来说是错误的。

这是 EXPLAIN 的结果:

id   select_type    table    type    possible_keys                    key                              key_len    ref                   rows  Extra
1    SIMPLE         p        index   NULL                             created                          4          NULL                  30    "Using temporary"
1    SIMPLE         pt4p     ref     idx_product_tags_for_products    idx_product_tags_for_products    3          s.id                  1     "Using index"
1    SIMPLE         pt       eq_ref  PRIMARY                          PRIMARY                          4          pt4p.product_tag_id   1    

我认为它以错误的顺序做事，即在连接后对结果进行排序，使用一个大的临时表，然后进行限制。我脑海中的查询计划是这样的:

• 使用“created”键对产品表进行排序
• 遍历每一行，将其与其他表进行左连接，直到达到 30 的限制。

这听起来很简单，但它似乎并不像那样工作——我是不是漏掉了什么？

---

CREATE TABLE `products` (
  `id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
  `title` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  `rating` float NOT NULL,
  `created` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `last_modified` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `active` tinyint(1) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `created` (`created`),
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

CREATE TABLE `product_tags_for_products` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `product_id` mediumint(8) unsigned NOT NULL,
  `product_tag_id` int(10) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `idx_product_tags_for_products` (`product_id`,`product_tag_id`),
  KEY `product_tag_id` (`product_tag_id`),
  CONSTRAINT `product_tags_for_products_ibfk_1` FOREIGN KEY (`product_id`) REFERENCES `products` (`id`),
  CONSTRAINT `product_tags_for_products_ibfk_2` FOREIGN KEY (`product_tag_id`) REFERENCES `product_tags` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci


CREATE TABLE `product_tags` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(100) COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

---

应 Salman A 的要求更新了分析信息:

Status,  
  Duration,CPU_user,CPU_system,Context_voluntary,Context_involuntary,Block_ops_in,Block_ops_out,Messages_sent,Messages_received,Page_faults_major,Page_faults_minor,Swaps,Source_function,Source_file,Source_line
starting,              
  0.000124,0.000106,0.000015,0,0,0,0,0,0,0,0,0,NULL,NULL,NULL
"Opening tables",      
  0.000022,0.000020,0.000003,0,0,0,0,0,0,0,0,0,"unknown function",sql_base.cc,4519
"System lock",   
  0.000007,0.000004,0.000002,0,0,0,0,0,0,0,0,0,"unknown function",lock.cc,258
"Table lock",   
  0.000011,0.000009,0.000002,0,0,0,0,0,0,0,0,0,"unknown function",lock.cc,269
init,           
  0.000055,0.000054,0.000001,0,0,0,0,0,0,0,0,0,"unknown function",sql_select.cc,2524
optimizing,       
  0.000008,0.000006,0.000002,0,0,0,0,0,0,0,0,0,"unknown function",sql_select.cc,833
statistics,      
  0.000116,0.000051,0.000066,0,0,0,0,0,0,0,1,0,"unknown function",sql_select.cc,1024
preparing,       
  0.000027,0.000023,0.000003,0,0,0,0,0,0,0,0,0,"unknown function",sql_select.cc,1046
"Creating tmp table",
  0.000054,0.000053,0.000002,0,0,0,0,0,0,0,0,0,"unknown function",sql_select.cc,1546
"Sorting for group", 
  0.000018,0.000015,0.000003,0,0,0,0,0,0,0,0,0,"unknown function",sql_select.cc,1596
executing,       
  0.000004,0.000002,0.000001,0,0,0,0,0,0,0,0,0,"unknown function",sql_select.cc,1780
"Copying to tmp table", 
  0.061716,0.049455,0.013560,0,18,0,0,0,0,0,3680,0,"unknown function",sql_select.cc,1927
"converting HEAP to MyISAM",
  0.046731,0.006371,0.017543,3,5,0,3,0,0,0,32,0,"unknown function",sql_select.cc,10980
"Copying to tmp table on disk", 
 10.700166,3.038211,1.191086,538,1230,1,31,0,0,0,65,0,"unknown function",sql_select.cc,11045
"Sorting result", 
  0.777887,0.155327,0.618896,2,137,0,1,0,0,0,634,0,"unknown function",sql_select.cc,2201
"Sending data", 
  0.000336,0.000159,0.000178,0,0,0,0,0,0,0,1,0,"unknown function",sql_select.cc,2334
end, 
  0.000005,0.000003,0.000002,0,0,0,0,0,0,0,0,0,"unknown function",sql_select.cc,2570
"removing tmp table", 
  0.106382,0.000058,0.080105,4,9,0,11,0,0,0,0,0,"unknown function",sql_select.cc,10912
end, 
  0.000015,0.000007,0.000007,0,0,0,0,0,0,0,0,0,"unknown function",sql_select.cc,10937
"query end", 
  0.000004,0.000002,0.000001,0,0,0,0,0,0,0,0,0,"unknown function",sql_parse.cc,5083
"freeing items", 
  0.000012,0.000012,0.000001,0,0,0,0,0,0,0,0,0,"unknown function",sql_parse.cc,6107
"removing tmp table", 
  0.000010,0.000009,0.000001,0,0,0,0,0,0,0,0,0,"unknown function",sql_select.cc,10912
"freeing items", 
  0.000084,0.000022,0.000057,0,1,0,0,1,0,0,0,0,"unknown function",sql_select.cc,10937
"logging slow query", 
  0.000004,0.000001,0.000001,0,0,0,0,0,0,0,0,0,"unknown function",sql_parse.cc,1723
"logging slow query", 
  0.000049,0.000031,0.000018,0,0,0,0,0,0,0,0,0,"unknown function",sql_parse.cc,1733
"cleaning up", 
  0.000007,0.000005,0.000002,0,0,0,0,0,0,0,0,0,"unknown function",sql_parse.cc,1691

表格是:

Products = 84.1MiB(产品表中有额外的字段，为清楚起见我省略了) 标签 = 32KiB 链接表 = 46.6MiB

Answer 1

我会尝试将产品数量首先限制在 30 个，然后仅加入 30 个产品:

   SELECT p.*, GROUP_CONCAT(pt.name) as tags
     FROM (SELECT p30.* FROM products p30 ORDER BY p30.created LIMIT 30) p 
LEFT JOIN product_tags_for_products pt4p ON (pt4p.product_id = p.id) 
LEFT JOIN product_tags pt ON (pt.id = pt4p.product_tag_id) 
 GROUP BY p.id 
 ORDER BY p.created  

我知道您说过没有子查询，但您没有解释原因，而且我看不出有任何其他方法可以解决您的问题。

请注意，您可以通过将子选择放在 View 中来消除子选择:

CREATE VIEW v_last30products AS 
  SELECT p30.* FROM products p30 ORDER BY p30.created LIMIT 30; 

然后查询被简化为:

   SELECT p.*, GROUP_CONCAT(pt.name) as tags
     FROM v_last30products p 
LEFT JOIN product_tags_for_products pt4p ON (pt4p.product_id = p.id) 
LEFT JOIN product_tags pt ON (pt.id = pt4p.product_tag_id) 
 GROUP BY p.id 
 ORDER BY p.created  

其他问题，您的n-to-n 表product_tags_for_products

没有意义，我会像这样重组它:

CREATE TABLE `product_tags_for_products` (    
  `product_id` mediumint(8) unsigned NOT NULL,    
  `product_tag_id` int(10) unsigned NOT NULL,    
  PRIMARY KEY (`product_id`,`product_tag_id`),       
  CONSTRAINT `product_tags_for_products_ibfk_1` FOREIGN KEY (`product_id`) REFERENCES `products` (`id`),    
  CONSTRAINT `product_tags_for_products_ibfk_2` FOREIGN KEY (`product_tag_id`) REFERENCES `product_tags` (`id`)    
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci  

这应该使查询更快:
- 缩短使用的 key (在 InnoDB 上，PK 始终包含在辅助 key 中)；
- 允许您使用比使用辅助 key 更快的 PK；

更多速度问题
如果您仅将 select * 替换为您需要的字段 select p.title, p.rating, ... FROM 这也会稍微加快速度。