Date: Tue, 10 Aug 2010 18:11:07 +0200
From: hubert depesz lubaczewski <depesz@depesz.com>
To: Matthew Wakeling <matthew@flymine.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: Sorted group by
Message-ID: <20100810161107.GA9680@depesz.com>
Reply-To: depesz@depesz.com
References: <alpine.DEB.2.00.1008101627440.2654@aragorn.flymine.org>
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On Tue, Aug 10, 2010 at 04:40:16PM +0100, Matthew Wakeling wrote:
> 
> I'm trying to eke a little bit more performance out of an
> application, and I was wondering if there was a better way to do the
> following:
> 
> I am trying to retrieve, for many sets of rows grouped on a couple
> of fields, the value of an ungrouped field where the row has the
> highest value in another ungrouped field. For instance, I have the
> following table setup:
> 
> group  | whatever type
> value  | whatever type
> number | int
> Index: group
> 
> I then have rows like this:
> 
> group     | value         | number
> -------------------------------------
> Foo       | foo           | 1
> Foo       | turnips       | 2
> Bar       | albatross     | 3
> Bar       | monkey        | 4
> 
> I want to receive results like this:
> 
> group     | value
> -----------------------
> Foo       | turnips
> Bar       | monkey
> 
> 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?

this is trivially done when usign 'distinct on':
select distinct on (group) *
from table
order by group desc, number desc;

depesz

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