From: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
To: 'Robert Haas' <robertmhaas@gmail.com>
Cc: 'Ants Aasma' <ants@cybertec.at>
Cc: 'Jay Levitt' <jay.levitt@gmail.com>
Cc: 'Tom Lane' <tgl@sss.pgh.pa.us>
Cc: 'PostgreSQL-development' <pgsql-hackers@postgresql.org>
Cc: 'Francois Deliege' <fdeliege@gmail.com>
Subject: Re: [PATCH] Lazy hashaggregate when no aggregation is needed
Date: Wed, 27 Jun 2012 12:53:40 +0900
Message-ID: <003e01cd5418$706dba40$51492ec0$@lab.ntt.co.jp> (raw)
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Hi,
> -----Original Message-----
> From: Robert Haas [mailto:robertmhaas@gmail.com]
> Sent: Wednesday, June 27, 2012 5:09 AM
> To: Etsuro Fujita
> Cc: Ants Aasma; Jay Levitt; Tom Lane; PostgreSQL-development; Francois Deliege
> Subject: Re: [HACKERS] [PATCH] Lazy hashaggregate when no aggregation is
needed
>
> On Fri, Jun 22, 2012 at 10:12 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> > On Tue, Jun 19, 2012 at 5:41 AM, Etsuro Fujita
> > <fujita.etsuro@lab.ntt.co.jp> wrote:
> >>> 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.
> >
> > Well, Ants already made some adjustments to those functions; not sure
> > if this means they need some more adjustment, but I don't see that
> > there's a general problem with the costing algorithm around LIMIT.
>
> Ants, do you intend to update this patch for this CommitFest? Or at
> all? It seems nobody's too excited about this, so I'm not sure
> whether it makes sense for you to put more work on it. But please
> advise as to your plans.
Please excuse my slow response, I would also like to know your plan.
Best regards,
Etsuro Fujita
> Thanks,
>
> --
> Robert Haas
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>
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To: pgsql-hackers@postgresql.org
Cc: fujita.etsuro@lab.ntt.co.jp, robertmhaas@gmail.com, ants@cybertec.at, jay.levitt@gmail.com, tgl@sss.pgh.pa.us, fdeliege@gmail.com
Subject: Re: [PATCH] Lazy hashaggregate when no aggregation is needed
In-Reply-To: <003e01cd5418$706dba40$51492ec0$@lab.ntt.co.jp>
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