慢加入？ NULL ref哪个更好

Question

我有两张桌子：

CREATE TABLE `bb_index` (
  `bb_id` smallint(5) unsigned NOT NULL,
  `object_id` bigint(10) unsigned NOT NULL,
  `in_object_id` bigint(10) unsigned NOT NULL,
  `f` int(10) unsigned DEFAULT NULL,
  `length` decimal(10,5) DEFAULT NULL,
  PRIMARY KEY (`bb_id`,`object_id`,`in_object_id`),
  KEY `in_obj_key` (`bb_id`,`in_object_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;


CREATE TABLE `obj_bb_relation` (`bb_id` smallint(5) unsigned NOT NULL DEFAULT '0',
  `object_id` bigint(10) unsigned NOT NULL DEFAULT '0',
  `object_bb_f` int(11) unsigned DEFAULT NULL,
  PRIMARY KEY (`bb_id`,`object_id`),
  KEY `object_id` (`object_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

哪个更好： 选项1：

SELECT object_id, f, length , object_bb_freq as C 
  FROM (select object_id, f, length 
          from bb_index use index (in_obj_key) 
         where bb_id = 14 
           and in_object_id = XXX
       ) as a join obj_bb_relation as b using (object_id)

说明：

选项2：

SELECT object_id, f, length, object_bb_f AS C 
  FROM bb_index AS a 
  JOIN obj_bb_relation AS b USING ( object_id ) 
 WHERE in_object_id = XXX 
   AND a.bb_id =YY 
   AND b.bb_id =YY 

OR？

另一种选择..？

Answer 1

在大多数情况下两者都是相同的。大多数数据库系统现在都有查询优化，它会自动从查询2，查询1进行查询。甚至更好。但是如果你使用一些非常旧的或简化的数据库，选项1会更好。除了一些非常特殊的情况，加入应该是最后的。

Answer 2

首先，您的查询几乎不相似！

SELECT object_id, f, length , object_bb_freq as C
FROM (select object_id, f, length
      from bb_index use index (in_obj_key)
      where bb_id = 14 and in_object_id = XXX) as a
join obj_bb_relation as b
     using (object_id)

SELECT object_id, f, length, object_bb_f AS C
FROM bb_index AS a
JOIN obj_bb_relation AS b
     USING ( object_id )
WHERE in_object_id = XXX AND a.bb_id =YY AND b.bb_id =YY 

翻译中缺少非常重要的位，即

• bb_index上的强制索引
• b.bb_id上的过滤器

除此之外，因为您使用了ANSI INNER JOIN表单而不是逗号形式（FROM bb_index, obj_bb_relation WHERE ..），所以MySQL会根据需要随机播放WHERE和ON / USING子句，因为它们是在语义上是一样的。这与你已经subqueried a的事实无关，因为MySQL非常聪明，可以展开它。

就个人而言，由于你在a.bb_id和b.bb_id上都有过滤器，如果你知道一个表只返回另一个表的1％的记录，那么你可以使用经常被诽谤的逗号。表示法（FROM bb_index, obj_bb_relation WHERE ..）因为MySQL会在加入bb_index之前处理obj_bb_relation上的过滤器 - 否则我通常会使用展平的JOIN表单（选项2），因为它比子查询更清晰在选项1）