MySQL版本:5.5.28-0ubuntu0.12.04.2-log

在我的印象中，JOIN总是比MySQL中的子查询更好，但EXPLAIN是更好的判断方式。下面是一个子查询比join更好的例子。

这是我的查询与3个子查询:

EXPLAIN SELECT vrl.list_id,vrl.ontology_id,vrl.position,l.name AS list_name, vrlih.position AS previous_position, vrl.moved_date 
FROM `vote-ranked-listory` vrl 
INNER JOIN lists l ON l.list_id = vrl.list_id 
INNER JOIN `vote-ranked-list-item-history` vrlih ON vrl.list_id = vrlih.list_id AND vrl.ontology_id=vrlih.ontology_id AND vrlih.type='PREVIOUS_POSITION' 
INNER JOIN list_burial_state lbs ON lbs.list_id = vrl.list_id AND lbs.burial_score < 0.5 
WHERE vrl.position <= 15 AND l.status='ACTIVE' AND l.is_public=1 AND vrl.ontology_id < 1000000000 
 AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=43) IS NULL 
 AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=55) IS NULL 
 AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=246403) IS NOT NULL 
ORDER BY vrl.moved_date DESC LIMIT 200;

解释说明:

+----+--------------------+----------+--------+-----------------------------------------------------+--------------+---------+-------------------------------------------------+------+--------------------------+
| id | select_type        | table    | type   | possible_keys                                       | key          | key_len | ref                                             | rows | Extra                    |
+----+--------------------+----------+--------+-----------------------------------------------------+--------------+---------+-------------------------------------------------+------+--------------------------+
|  1 | PRIMARY            | vrl      | index  | PRIMARY                                             | moved_date   | 8       | NULL                                            |  200 | Using where              |
|  1 | PRIMARY            | l        | eq_ref | PRIMARY,status,ispublic,idx_lookup,is_public_status | PRIMARY      | 4       | ranker.vrl.list_id                              |    1 | Using where              |
|  1 | PRIMARY            | vrlih    | eq_ref | PRIMARY                                             | PRIMARY      | 9       | ranker.vrl.list_id,ranker.vrl.ontology_id,const |    1 | Using where              |
|  1 | PRIMARY            | lbs      | eq_ref | PRIMARY,idx_list_burial_state,burial_score          | PRIMARY      | 4       | ranker.vrl.list_id                              |    1 | Using where              |
|  4 | DEPENDENT SUBQUERY | list_tag | ref    | list_tag_key,list_id,tag_id                         | list_tag_key | 9       | ranker.l.list_id,const                          |    1 | Using where; Using index |
|  3 | DEPENDENT SUBQUERY | list_tag | ref    | list_tag_key,list_id,tag_id                         | list_tag_key | 9       | ranker.l.list_id,const                          |    1 | Using where; Using index |
|  2 | DEPENDENT SUBQUERY | list_tag | ref    | list_tag_key,list_id,tag_id                         | list_tag_key | 9       | ranker.l.list_id,const                          |    1 | Using where; Using index |
+----+--------------------+----------+--------+-----------------------------------------------------+--------------+---------+-------------------------------------------------+------+--------------------------+

使用join的相同查询是:

EXPLAIN SELECT vrl.list_id,vrl.ontology_id,vrl.position,l.name AS list_name, vrlih.position AS previous_position, vrl.moved_date 
FROM `vote-ranked-listory` vrl 
INNER JOIN lists l ON l.list_id = vrl.list_id 
INNER JOIN `vote-ranked-list-item-history` vrlih ON vrl.list_id = vrlih.list_id AND vrl.ontology_id=vrlih.ontology_id AND vrlih.type='PREVIOUS_POSITION' 
INNER JOIN list_burial_state lbs ON lbs.list_id = vrl.list_id AND lbs.burial_score < 0.5 
LEFT JOIN list_tag lt1 ON lt1.list_id = vrl.list_id AND lt1.tag_id = 43 
LEFT JOIN list_tag lt2 ON lt2.list_id = vrl.list_id AND lt2.tag_id = 55 
INNER JOIN list_tag lt3 ON lt3.list_id = vrl.list_id AND lt3.tag_id = 246403 
WHERE vrl.position <= 15 AND l.status='ACTIVE' AND l.is_public=1 AND vrl.ontology_id < 1000000000 
AND lt1.list_id IS NULL AND lt2.tag_id IS NULL 
ORDER BY vrl.moved_date DESC LIMIT 200;

输出为:

+----+-------------+-------+--------+-----------------------------------------------------+--------------+---------+---------------------------------------------+------+----------------------------------------------+
| id | select_type | table | type   | possible_keys                                       | key          | key_len | ref                                         | rows | Extra                                        |
+----+-------------+-------+--------+-----------------------------------------------------+--------------+---------+---------------------------------------------+------+----------------------------------------------+
|  1 | SIMPLE      | lt3   | ref    | list_tag_key,list_id,tag_id                         | tag_id       | 5       | const                                       | 2386 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | l     | eq_ref | PRIMARY,status,ispublic,idx_lookup,is_public_status | PRIMARY      | 4       | ranker.lt3.list_id                          |    1 | Using where                                  |
|  1 | SIMPLE      | vrlih | ref    | PRIMARY                                             | PRIMARY      | 4       | ranker.lt3.list_id                          |  103 | Using where                                  |
|  1 | SIMPLE      | vrl   | ref    | PRIMARY                                             | PRIMARY      | 8       | ranker.lt3.list_id,ranker.vrlih.ontology_id |   65 | Using where                                  |
|  1 | SIMPLE      | lt1   | ref    | list_tag_key,list_id,tag_id                         | list_tag_key | 9       | ranker.lt3.list_id,const                    |    1 | Using where; Using index; Not exists         |
|  1 | SIMPLE      | lbs   | eq_ref | PRIMARY,idx_list_burial_state,burial_score          | PRIMARY      | 4       | ranker.vrl.list_id                          |    1 | Using where                                  |
|  1 | SIMPLE      | lt2   | ref    | list_tag_key,list_id,tag_id                         | list_tag_key | 9       | ranker.lt3.list_id,const                    |    1 | Using where; Using index                     |
+----+-------------+-------+--------+-----------------------------------------------------+--------------+---------+---------------------------------------------+------+----------------------------------------------+

rows列的比较表明了差异，使用join的查询使用的是using temporary;使用filesort。

当然，当我运行这两个查询时，第一个查询在0.02秒内完成，第二个查询甚至在1分钟后都没有完成，所以EXPLAIN正确地解释了这些查询。

如果我在list_tag表上没有INNER JOIN，即如果我删除

AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=246403) IS NOT NULL  

从第一个查询和相应的:

INNER JOIN list_tag lt3 ON lt3.list_id = vrl.list_id AND lt3.tag_id = 246403

从第二个查询开始，那么EXPLAIN为两个查询返回相同的行数，并且这两个查询的运行速度相同。

摘自MySQL手册(13.2.10.11将子查询重写为连接):

LEFT [OUTER] JOIN可以比等效的子查询更快，因为服务器可以更好地优化它——这不是MySQL服务器独有的事实。

所以子查询可能比LEFT [OUTER] JOIN慢，但在我看来，它们的优势是可读性略高。

我只是在考虑同样的问题，但我在FROM部分使用子查询。 我需要连接和查询大表，“从”表有2800万条记录，但结果只有128个这样小的结果大数据!我在它上面使用MAX()函数。

首先，我使用LEFT JOIN，因为我认为这是正确的方式，mysql可以优化等。 第二次只是为了测试，我重写了针对JOIN的子选择。

LEFT JOIN运行时:1.12s SUB-SELECT运行时间:0.06秒

子选择比连接快18倍!只是在chokito广告。subselect看起来很糟糕，但结果…

在旧Mambo CMS的一个非常大的数据库上运行:

SELECT id, alias
FROM
  mos_categories
WHERE
  id IN (
    SELECT
      DISTINCT catid
    FROM mos_content
  );

0秒

SELECT
  DISTINCT mos_content.catid,
  mos_categories.alias
FROM
  mos_content, mos_categories
WHERE
  mos_content.catid = mos_categories.id;

~ 3秒

EXPLAIN说明它们检查的行数完全相同，但其中一个需要3秒，另一个几乎是即时的。这个故事的寓意?如果性能很重要(什么时候不重要?)，尝试多种方法，看看哪一种最快。

和…

SELECT
  DISTINCT mos_categories.id,
  mos_categories.alias
FROM
  mos_content, mos_categories
WHERE
  mos_content.catid = mos_categories.id;

0秒

同样，结果相同，检查的行数相同。我猜是DISTINCT mos_content。catid比DISTINCT mos_categories需要更长的时间来计算。id。

A general rule is that joins are faster in most cases (99%). The more data tables have, the subqueries are slower. The less data tables have, the subqueries have equivalent speed as joins. The subqueries are simpler, easier to understand, and easier to read. Most of the web and app frameworks and their "ORM"s and "Active record"s generate queries with subqueries, because with subqueries are easier to split responsibility, maintain code, etc. For smaller web sites or apps subqueries are OK, but for larger web sites and apps you will often have to rewrite generated queries to join queries, especial if a query uses many subqueries in the query.

有人说“一些RDBMS可以将子查询重写为连接，或将连接重写为子查询，当它认为其中一个比另一个快时”，但这句话适用于简单的情况，当然不适用于带有子查询的复杂查询，这实际上会导致性能问题。

这取决于几个因素，包括正在运行的特定查询、数据库中的数据量。子查询首先运行内部查询，然后从结果集中再次过滤出实际结果。而在join中运行并产生结果。

最好的策略是同时测试连接解决方案和子查询解决方案，以获得优化的解决方案。