To: pgsql-performance@postgresql.org
From: Thomas Kellerer <spam_eater@gmx.net>
Subject: Re: Sorted group by
Date: Tue, 10 Aug 2010 17:56:46 +0200
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Matthew Wakeling wrote on 10.08.2010 17:40:
> Currently, I do this in my application by ordering by the number and
> only using the last value. I imagine that this is something that can be
> done in the new Postgres 9, with a sorted group by - something like this:
>
> SELECT group, LAST(value, ORDER BY number) FROM table GROUP BY group
>
> Is this something that is already built in, or would I have to write my
> own LAST aggregate function?

No. It's built in (8.4) and it's called Windowing functions:
http://www.postgresql.org/docs/8.4/static/tutorial-window.html
http://www.postgresql.org/docs/8.4/static/functions-window.html

SELECT group, last_value(value) over(ORDER BY number)
FROM table

You don't need the group by then (but you can apply e.g. an ORDER BY GROUP)

Thomas