postgres group by aggregate function

Question

I have a messages table which looks like this:

+------------+-------------+----------+
| sender_id  |  created_at | message  |
+------------+-------------+----------+
|      1     | 2010-06-14  | the msg  |
|      1     | 2010-06-15  | the msg  |
|      2     | 2010-06-16  | the msg  |
|      3     | 2010-06-14  | the msg  |
+------------+-------------+----------|

I want to select the single most recent message for each sender.

This seems like a GROUP BY sender_id and ORDER BY created_at but I'm having trouble getting the most recent message selected.

I'm using postgres so need an aggregate function on the created_at field in the SELECT statement if I want to order by that field so I was looking at doing something like this as an initial test

SELECT messages.sender_id, MAX(messages.created_at) as the_date 
FROM messages 
GROUP BY sender_id 
ORDER BY the_date DESC 
LIMIT 10;

This seems to work but when I want to select 'message' as well I have no idea what aggregate function to use on it. I basically just want the message that corresponds to the MAX created_at.

Is there some way of getting at this or am I approaching it the wrong way?

Answer 1

This:

SELECT  *
FROM    (
        SELECT  DISTINCT ON (sender_id) *
        FROM    messages 
        ORDER BY
                sender_id, created_at DESC 
        ) q
ORDER BY
        created_at DESC
LIMIT 5

or this:

SELECT  (mi).*
FROM    (
        SELECT  (
                SELECT  mi
                FROM    messages mi
                WHERE   mi.sender_id = m.sender_id
                ORDER BY
                        created_at DESC
                LIMIT 1
                ) AS mi
        FROM    messages m
        GROUP BY
                sender_id
        ) q
ORDER BY
        (mi).created_at  DESC
LIMIT 5

Create an index on (sender_id, created_at) for this to work fast.

You may find this article interesting:

• PostgreSQL: Selecting records holding group-wise maximum

Answer 2

Use a correlated sub query:

select * from messages m1 
where m1.created_at = (
    select max(m2.create_at) 
    from messages m2 
    where m1.sender_id = m2.sender_id
);

The sub query is reevaluated for each row processed by the upper query.