Re: DISTINCT vs. GROUP BY

Added to TODO:
* Allow DISTINCT to use hashing like GROUP BY


---------------------------------------------------------------------------

Greg Stark wrote:
> 
> Neil Conway <neilc@samurai.com> writes:
> 
> > On Mon, 2005-19-09 at 16:27 +0200, Hans-J?rgen Sch?nig wrote:
> > > I was wondering whether it is possible to teach the planner to handle 
> > > DISTINCT in a more efficient way:
> > [...]
> > > Isn't it possible to perform the same operation using a
> > > HashAggregate? 
> > 
> > One problem is that DISTINCT ON is defined to return the first unique
> > row (according to the query's ORDER BY) for the set of DISTINCT ON
> > columns, which can't easily be done via hashing.
> 
> Uhm. Sure it can.
> 
> 
> DISTINCT is really just special a case of GROUP BY. Even DISTINCT ON is just
> GROUP BY with a kind of "first()" aggregate function. What would be really
> neat would be to teach GROUP BY about first() and last() and how it can skip
> over some index entries and still satisfy the query. Then make DISTINCT and
> DISTINCT ON be handled through the exact same code path.
> 
> For bonus points teach it that min() and max() can sometimes be treated the
> same way if the path is presenting records sorted on that column.
> 
> 
> -- 
> greg
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 3: Have you checked our extensive FAQ?
> 
>                http://www.postgresql.org/docs/faq
> 

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
359-1001+  If your life is a hard drive,     |  13 Roberts Road +  Christ can be your backup.        |  Newtown Square,
Pennsylvania19073