Query to get last conversations for user inbox - postgresql

Question

I need a specific SQL query to select last 10 conversations for user inbox.
Inbox shows only conversations(threads) with every user - it selects the last message from the conversation and shows it in inbox.
Edited.
Expecting result: to extract latest message from each of 10 latest conversations. Facebook shows latest conversations in the same way
And one more question. How to make a pagination to show next 10 latest messages from previous latest conversations in the next page?
Private messages in the database looks like:
| id | user_id | recipient_id | text
| 1 | 2 | 3 | Hi John!
| 2 | 3 | 2 | Hi Tom!
| 3 | 2 | 3 | How are you?
| 4 | 3 | 2 | Thanks, good! You?

Answer 1

As per my understanding, you need to get the latest message of the conversation on per-user basis (of the last 10 latest conversations)
Update: I have modified the query to get the latest_conversation_message_id for every user conversation
The below query gets the details for user_id = 2, you can modify, users.id = 2 to get it for any other user
SQLFiddle, hope this solves your purpose
SELECT
user_id,
users.name,
users2.name as sent_from_or_sent_to,
subquery.text as latest_message_of_conversation
FROM
users
JOIN
(
SELECT
text,
row_number() OVER ( PARTITION BY user_id + recipient_id ORDER BY id DESC) AS row_num,
user_id,
recipient_id,
id
FROM
private_messages
GROUP BY
id,
recipient_id,
user_id,
text
) AS subquery ON ( ( subquery.user_id = users.id OR subquery.recipient_id = users.id) AND row_num = 1 )
JOIN users as users2 ON ( users2.id = CASE WHEN users.id = subquery.user_id THEN subquery.recipient_id ELSE subquery.user_id END )
WHERE
users.id = 2
ORDER BY
subquery.id DESC
LIMIT 10
Info: The query gets the latest message of every conversation with any other user, If user_id 2, sends a message to user_id 3, that too is displayed, as it indicates the start of a conversation. The latest message of every conversation with any other user is displayed

Answer 2

To solve groupwise-max in pg you can use DISTINCT ON. Like this:
SELECT
DISTINCT ON(pm.user_id)
pm.user_id,
pm.text
FROM
private_messages AS pm
WHERE pm.recipient_id= <my user id>
ORDER BY pm.user_id, pm.id DESC;
http://sqlfiddle.com/#!12/4021d/19
To get the latest X however we will have to use it in a subselect:
SELECT
q.user_id,
q.id,
q.text
FROM
(
SELECT
DISTINCT ON(pm.user_id)
pm.user_id,
pm.id,
pm.text
FROM
private_messages AS pm
WHERE pm.recipient_id=2
ORDER BY pm.user_id, pm.id DESC
) AS q
ORDER BY q.id DESC
LIMIT 10;
http://sqlfiddle.com/#!12/4021d/28
To get both sent and recieved threads:
SELECT
q.user_id,
q.recipient_id,
q.id,
q.text
FROM
(
SELECT
DISTINCT ON(pm.user_id,pm.recipient_id)
pm.user_id,
pm.recipient_id,
pm.id,
pm.text
FROM
private_messages AS pm
WHERE pm.recipient_id=2 OR pm.user_id=2
ORDER BY pm.user_id,pm.recipient_id, pm.id DESC
) AS q
ORDER BY q.id DESC
LIMIT 10;
http://sqlfiddle.com/#!12/4021d/42

Answer 3

Paste it after your WHERE clause
ORDER BY "ColumnName" [ASC, DESC]
UNION Description at W3Schools it combines the result of this 2 statements.
SELECT "ColumnName" FROM "TableName"
UNION
SELECT "ColumnName" FROM "TableName"

Answer 4

For large data sets I think you might like to try running the two statements and then consolidating the results, as an index scan on (user_id and id) or (recipient_id and id) ought to be very efficient at getting the 10 most recent conversations of each type.
with sent_messages as (
SELECT *
FROM private_messages
WHERE user_id = my_user_id
ORDER BY id desc
LIMIT 10),
received_messages as ( SELECT *
FROM private_messages
WHERE recipient_id = my_user_id
ORDER BY id desc
LIMIT 10),
all_messages as (
select *
from sent_messages
union all
select *
from received_messages)
select *
from all_messages
order by id desc
limit 10
Edit: Actually another query worth trying might be:
select *
from private_messages
where id in (
select id
from (
SELECT id
FROM private_messages
WHERE user_id = my_user_id
ORDER BY id desc
LIMIT 10
union all
SELECT id
FROM private_messages
WHERE recipient_id = my_user_id
ORDER BY id desc
LIMIT 10) all_ids
order by id desc
limit 10) last_ten_ids
order by id desc
This might be better in 9.2+, where the indexes alone could be used to get the id's, or in cases where the most recent number to retrieve is very large. Still a bit unclear on that though. If in doubt I'd go for the former version.