MySQL：子查询还是联合？

我不解释查询的逻辑，因为它并不简单。如果我详细解释的话，没有人愿意阅读和深入探究这个问题的本质

有两个查询。他们工作做得很好。结果：相同。在手动模式下检查，在纸上和数据库上使用铅笔

哪个查询将减少服务器的负载？还是只有在实际生产服务器上工作一段时间后才能发现

操作UNION很难加载服务器？我应该在querys的解释中寻找什么

1 q带++

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问：工会在哪里

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)

---

看起来您正在查找表的一系列不同查询的记录计数之和

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？它有更少的工作要做。您的第二个备选方案必须对一组comid值进行争论，然后对它们进行计数。这需要时间

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

这两种方法的性能取决于每个子查询的索引选择。

是否必须使用COUNTcomid？或者你能用COUNT*？也就是说，是否存在任何空comid值？计数*稍微快一点。在第二个查询中，联合的每个部分返回的comid值集是否不相交？如果是，请使用UNION ALL。同样，如果可以，请使用COUNT*。不可以为null，好的，我使用COUNT*duolicates不可能，索引存在。任何变量：union、+++、countcomid、count*queryS的运行时间都小于0.08秒。测试了大约70000-100000条记录，一切正常。将来我需要监视生产服务器。