Re: Status of DISTINCT-by-hashing work

Sounds very much like 80% 20% story. 80% that was easy to do is done and now
20% that is complex and progress is slow is left to be done. Sounds very
familiar from the comment in plan cache invalidation :)

On Tue, Aug 5, 2008 at 5:51 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> I've pretty much finished the project I got a bee in my bonnet about
> last week, which is to teach SELECT DISTINCT how to (optionally) use
> hashing for grouping in the same way that GROUP BY has been able to do
> for awhile.
>
> There are still two places in the system that hard-wire the use of
> sorting for duplicate elimination:
>
> * Set operations (UNION/INTERSECT/EXCEPT)
>
> * Aggregate functions with DISTINCT
>
> I'm thinking of trying to fix set operations before I leave this topic,
> but I'm not sure it's worth the trouble to change DISTINCT aggregates.
> They'd be a lot more work (since there's no executor infrastructure
> in place that could be used) and the return on investment seems low.
>
> Comments?
>
> regards, tom lane
>
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