使用GROUP BY和ORDER BY优化对JOINed表的MySQL查询,而不使用嵌套查询

Question

这对我来说感觉像是一个初学SQL问题,但是这里有.这就是我想要做的:

• 将三个表连接在一起,产品,标签和链接表.
• 将标记聚合到一个逗号分隔的字段中(因此GROUP_CONCAT和GROUP BY)
• 限制结果(至30)
• 将结果按"创建"日期的顺序排列
• 避免在可能的情况下使用子查询,因为它们对使用Active Record框架的代码特别不愉快

我已经描述了本文底部涉及的表格,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;

有大约280,000个产品,130个标签,524,000个链接记录,我已经分析了表格.问题是它需要花费80多秒才能运行(在合适的硬件上),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执行操作,即在连接后使用大型临时表对结果进行ORDERing,然后使用LIMITing.我头脑中的查询计划会是这样的:

• 使用"已创建"键对产品表进行订购
• 逐步浏览每一行,LEFT JOIN将其与其他表格联系,直到达到LIMIT为30.

这听起来很简单,但它看起来并不像那样 - 我错过了什么？

---

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

---

更新了Salman A要求的分析信息:

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:

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

Answer 1

我会尝试先将产品数量限制为 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，而且我看不到任何其他方法来解决您的问题。

请注意，您可以通过将子选择放入视图中来消除子选择：

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

那么查询就简化为：

   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，你的n-to-n桌子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  

这应该通过以下方式使查询更快：
- 缩短所使用的密钥（在 InnoDB 上，PK 始终包含在辅助密钥中）；
- 允许您使用 PK，这应该比使用辅助密钥更快；

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