Re: Sorted group by

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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