Re: [PATCH] Lazy hashaggregate when no aggregation is needed

Hi,

> -----Original Message-----
> From: Robert Haas [mailto:robertmhaas(at)gmail(dot)com]
> Sent: Tuesday, June 19, 2012 3:12 AM
> To: Ants Aasma
> Cc: Etsuro Fujita; Jay Levitt; Tom Lane; PostgreSQL-development; Francois
> Deliege
> Subject: Re: [HACKERS] [PATCH] Lazy hashaggregate when no aggregation is
> needed
>
> On Fri, Jun 15, 2012 at 9:22 AM, Ants Aasma <ants(at)cybertec(dot)at> wrote:
> > Exactly. I think the first question for this patch should be whether
> > this use-case is worth the complexity of the patch. I can't imagine
> > any really compelling use cases that need an arbitrary distinct subset
> > of results.
>
> Me neither.
>
> > The original complaint on -performance [1], didn't specify a real
> > world use case, but it seemed to be a case of an ORM generating
> > suboptimal queries. On the other hand, the patch itself is in my
> > opinion rather simple, so it might be worth it.
>
> Yeah.
>
> > It has one outstanding issue, query_planner chooses the cheapest path
> > based on total cost. This can be suboptimal when that path happens to
> > have high startup cost. It seems to me that enabling the query_planner
> > to find the cheapest unsorted path returning a limited amount of
> > tuples would require some major surgery to the planner. To be clear,
> > this is only a case of missed optimization, not a regression.
>
> I'm confused by this remark, because surely the query planner does it this
> way only if there's no LIMIT. When there is a LIMIT, we choose based on
> the startup cost plus the estimated fraction of the total cost we expect
> to pay based on dividing the LIMIT by the overall row count estimate. Or
> is this not what you're talking about?

I think that Ants is pointing the way of estimating costs in
choose_hashed_grouping()/choose_hashed_distinct(), ie cost_agg() for
cheapest_path + hashagg, where the costs are calculated based on the total
cost only of cheapest_path. I think that it might be good to do cost_agg()
for the discussed case with the AGG_SORTED strategy, not the AGG_HASHED
strategy.

> > It won't help set returning functions because the tuplestore for those
> > is fully materialized when the first row is fetched.
>
> That's surely a problem for another day.

OK

Best regards,
Etsuro Fujita