Re: group by will not use an index? - Mailing list pgsql-performance

From Steinar H. Gunderson
Subject Re: group by will not use an index?
Date
Msg-id 20070110011104.GA21351@uio.no
Whole thread Raw
In response to Re: group by will not use an index?  (Brian Herlihy <btherl@yahoo.com.au>)
List pgsql-performance
On Tue, Jan 09, 2007 at 05:07:03PM -0800, Brian Herlihy wrote:
> Actually, as I recently discovered, GROUP BY is faster than DISTINCT.  It's
> just due to how they are implemented, so don't go looking for any deep
> reason :)  The thread "GROUP BY vs DISTINCT" from 2006-12-20 discusses it.
> DISTINCT sorts the results to find the unique rows, but GROUP BY uses a
> hash.

Actually, GROUP BY _can_ use a sort too, it's just that a hash is usually
faster.

/* Steinar */
--
Homepage: http://www.sesse.net/

pgsql-performance by date:

Previous
From: Brian Herlihy
Date:
Subject: Re: group by will not use an index?
Next
From: Andrew Lazarus
Date:
Subject: Re: group by will not use an index?