Re: DISTINCT vs. GROUP BY

Added to TODO:

* Allow DISTINCT to use hashing like GROUP BY

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Greg Stark wrote:
>
> Neil Conway <neilc(at)samurai(dot)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
>
>
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