Postgresql收件箱查询

Question

我有一个如下所示的Messages表:

                    Messages
+-----+------------+-------------+--------------+
|  id |  sender_id | receiver_id |  created_at  |
+-----------------------------------------------+
|  1  |      1     |      2      |   1/1/2013   |
|  2  |      1     |      2      |   1/1/2013   |
|  3  |      2     |      1      |   1/2/2013   |
|  4  |      3     |      2      |   1/2/2013   |
|  5  |      3     |      2      |   1/3/2013   |
|  6  |      5     |      4      |   1/4/2013   |
+-----------------------------------------------+

如果'thread'是给定sender_id和receiver_id之间的一组消息,我希望查询返回最近10个线程的最新10条消息,其中sender_id或receiver_id是给定的id.

给定user_id为5的预期输出:

+-----+------------+-------------+--------------+
|  id |  sender_id | receiver_id |  created_at  |
+-----------------------------------------------+
|  1  |      5     |      2      |   1/4/2013   |
|  2  |      5     |      2      |   1/4/2013   |
|  3  |      2     |      5      |   1/4/2013   |
|  4  |      3     |      5      |   1/4/2013   |
|  5  |      5     |      2      |   1/3/2013   |
|  6  |      5     |      4      |   1/3/2013   |
+-----------------------------------------------+

例如,用户5和2之间(上面有4个)和10个线程的限制(上面有3个)之间的最多10个消息的限制.

我一直在使用子查询尝试这种查询,但没有设法获得不同线程数量的第二个限制.

SELECT * FROM (SELECT DISTINCT ON (sender_id, receiver_id) messages.* 
FROM messages 
WHERE (receiver_id = 5 OR sender_id = 5) ORDER BY sender_id, receiver_id, 
created_at DESC)   
q ORDER BY created_at DESC 
LIMIT 10 OFFSET 0;

我正在考虑创建一个新的Thread表,其中包含一个thread_id字段,该字段是sender_id + receiver_id的串联,然后只是加入Messages,但我有一个偷偷摸摸的怀疑,它应该只用一个表来实现.

Answer 1

我还没有对此进行测试，但看起来您忘记了子LIMIT 10查询中的 ，它为您提供了 10 个最新线程：

SELECT
  *
FROM
  (SELECT DISTINCT ON
     (sender_id, receiver_id) messages.* 
   FROM
     messages 
   WHERE
     (receiver_id = 5 OR sender_id = 5)
   ORDER BY
     sender_id, receiver_id, created_at DESC
   LIMIT
     10)   
  q
ORDER BY
  created_at DESC 
LIMIT
  10
OFFSET
  0;

（我已经漂亮地打印了 SQL，因此更容易知道发生了什么。）