MySQL：子查询或联合？

Question

I do not explain the logic of the query，因为不简单。如果我详细解释 - 没有人愿意阅读和深入研究查询的本质。

有两个查询。他们做得很好。结果：相同。在手动模式下，用铅笔在纸上（和数据库）检查。

哪个查询会减少服务器的负载？还是只有在实际（生产）服务器上工作一段时间后才能发现？

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操作UNION很难加载服务器？ What should I look for in the explanations of the querys?

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1 q whith + + +

select (select count(comid)  from coms join posts on pid=pid_coms where uid_posts=8888 and uid_coms=8888) 

+
(select count(comid)  from frends join posts on sl_frend=uid_posts join coms on pid=pid_coms 
where uid_coms=8888 and m_frend=8888 and ((postacc=1 and postcomacc=2) or (postacc=2 and postcomacc=2) or (postacc=2 and postcomacc=1))) 

+ 
(select count(comid)  from frends join posts on m_frend=uid_posts join coms on pid=pid_coms
where uid_coms=8888 and sl_frend=8888 and ((postacc=1 and postcomacc=2) or (postacc=2 and postcomacc=2) or (postacc=2 and postcomacc=1)))

+
(select count(comid)  from coms join posts on pid_coms=pid where uid_posts != 8888 and uid_coms=8888 and postacc=1 and postcomacc=1) CountMyComms;

EXPLAIN
+----+-------------+--------+--------+-----------------------+----------+---------+---------------------+------+----------------+
| id | select_type | table  | type   | possible_keys         | key      | key_len | ref                 | rows | Extra          |
+----+-------------+--------+--------+-----------------------+----------+---------+---------------------+------+----------------+
|  1 | PRIMARY     | NULL   | NULL   | NULL                  | NULL     | NULL    | NULL                | NULL | No tables used |
|  5 | SUBQUERY    | coms   | ref    | uid_coms,pid_coms     | uid_coms | 4       | const               |    7 |                |
|  5 | SUBQUERY    | posts  | eq_ref | PRIMARY,pid,uid_posts | PRIMARY  | 4       | mbs.coms.pid_coms   |    1 | Using where    |
|  4 | SUBQUERY    | frends | ref    | m_frend,sl_frend      | sl_frend | 4       | const               |    1 |                |
|  4 | SUBQUERY    | coms   | ref    | uid_coms,pid_coms     | uid_coms | 4       | const               |    7 |                |
|  4 | SUBQUERY    | posts  | eq_ref | PRIMARY,pid,uid_posts | PRIMARY  | 4       | mbs.coms.pid_coms   |    1 | Using where    |
|  3 | SUBQUERY    | coms   | ref    | uid_coms,pid_coms     | uid_coms | 4       | const               |    7 |                |
|  3 | SUBQUERY    | posts  | eq_ref | PRIMARY,pid,uid_posts | PRIMARY  | 4       | mbs.coms.pid_coms   |    1 | Using where    |
|  3 | SUBQUERY    | frends | ref    | m_frend,sl_frend      | sl_frend | 4       | mbs.posts.uid_posts |    1 | Using where    |
|  2 | SUBQUERY    | coms   | ref    | uid_coms,pid_coms     | uid_coms | 4       | const               |    7 |                |
|  2 | SUBQUERY    | posts  | eq_ref | PRIMARY,pid,uid_posts | PRIMARY  | 4       | mbs.coms.pid_coms   |    1 | Using where    |
+----+-------------+--------+--------+-----------------------+----------+---------+---------------------+------+----------------+
11 rows in set (0.00 sec)

2 q 带工会

select count(comid) from (select comid from coms join posts on pid=pid_coms where uid_posts=8888 and uid_coms=8888 

union
select comid from frends join posts on sl_frend=uid_posts join coms on pid=pid_coms 
where uid_coms=8888 and m_frend=8888 and ((postacc=1 and postcomacc=2) or (postacc=2 and postcomacc=2) or (postacc=2 and postcomacc=1)) 

union 
select comid from frends join posts on m_frend=uid_posts join coms on pid=pid_coms 
where uid_coms=8888 and sl_frend=8888 and ((postacc=1 and postcomacc=2) or (postacc=2 and postcomacc=2) or (postacc=2 and postcomacc=1)) 

union 
select comid from coms join posts on pid_coms=pid 
where uid_posts != 8888 and uid_coms=8888 and postacc=1 and postcomacc=1) a;

EXPLAIN
+----+--------------+----------------+--------+-----------------------+----------+---------+---------------------+------+------------------------------+
| id | select_type  | table          | type   | possible_keys         | key      | key_len | ref                 | rows | Extra                        |
+----+--------------+----------------+--------+-----------------------+----------+---------+---------------------+------+------------------------------+
|  1 | PRIMARY      | NULL           | NULL   | NULL                  | NULL     | NULL    | NULL                | NULL | Select tables optimized away |
|  2 | DERIVED      | coms           | ref    | uid_coms,pid_coms     | uid_coms | 4       |                     |    7 |                              |
|  2 | DERIVED      | posts          | eq_ref | PRIMARY,pid,uid_posts | PRIMARY  | 4       | mbs.coms.pid_coms   |    1 | Using where                  |
|  3 | UNION        | coms           | ref    | uid_coms,pid_coms     | uid_coms | 4       |                     |    7 |                              |
|  3 | UNION        | posts          | eq_ref | PRIMARY,pid,uid_posts | PRIMARY  | 4       | mbs.coms.pid_coms   |    1 | Using where                  |
|  3 | UNION        | frends         | ref    | m_frend,sl_frend      | sl_frend | 4       | mbs.posts.uid_posts |    1 | Using where                  |
|  4 | UNION        | frends         | ref    | m_frend,sl_frend      | sl_frend | 4       |                     |    1 |                              |
|  4 | UNION        | coms           | ref    | uid_coms,pid_coms     | uid_coms | 4       |                     |    7 |                              |
|  4 | UNION        | posts          | eq_ref | PRIMARY,pid,uid_posts | PRIMARY  | 4       | mbs.coms.pid_coms   |    1 | Using where                  |
|  5 | UNION        | coms           | ref    | uid_coms,pid_coms     | uid_coms | 4       |                     |    7 |                              |
|  5 | UNION        | posts          | eq_ref | PRIMARY,pid,uid_posts | PRIMARY  | 4       | mbs.coms.pid_coms   |    1 | Using where                  |
| NULL | UNION RESULT | <union2,3,4,5> | ALL    | NULL                  | NULL     | NULL    | NULL                | NULL |                              |
+----+--------------+----------------+--------+-----------------------+----------+---------+---------------------+------+------------------------------+
12 rows in set (0.00 sec)

Answer 1

您似乎正在查找对表的一系列不同查询的记录总数。

第一种选择...计算每个查询的结果，然后添加它们...会更快。为什么？它要做的工作更少。您的第二种选择必须整理一组comid 值，然后计算它们。这需要时间。

如果可以，请使用COUNT(*)。这个更便宜。尽可能使用UNION ALL 而不是UNION； UNION 删除重复项，UNION ALL 不删除。删除重复项需要时间。

任一替代方案的性能取决于为每个子查询选择好的索引。