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Re: FDW for PostgreSQL

From: Shigeru HANADA <shigeru(dot)hanada(at)gmail(dot)com>
To: Kohei KaiGai <kaigai(at)kaigai(dot)gr(dot)jp>
Cc: Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: FDW for PostgreSQL
Date: 2012-11-06 09:49:24
Message-ID: 5098DD24.6070008@gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Sorry for delayed response.

On Sun, Oct 21, 2012 at 3:16 AM, Kohei KaiGai <kaigai(at)kaigai(dot)gr(dot)jp> wrote:
> 2012/10/11 Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>:
>> I've reviewed your patch quickly.  I noticed that the patch has been created in
>> a slightly different way from the guidelines:
>> http://www.postgresql.org/docs/devel/static/fdw-planning.html  The guidelines
>> say the following, but the patch identifies the clauses in
>> baserel->baserestrictinfo in GetForeignRelSize, not GetForeignPaths.  Also, it
>> has been implemented so that most sub_expressions are evaluated at the remote
>> end, not the local end, though I'm missing something.  For postgresql_fdw to be
>> a good reference for FDW developers, ISTM it would be better that it be
>> consistent with the guidelines.  I think it would be needed to update the
>> following document or redesign the function to be consistent with the following
>> document.
>>
> Hmm. It seems to me Fujita-san's comment is right.

Indeed postgresql_fdw touches baserestrictinfo in GetForeignRelSize, but 
it's because of optimization for better width estimate.  The doc 
Fujita-san pointed says that:

> The actual identification of such a clause should happen during
> GetForeignPaths, since it would affect the cost estimate for the
> path.

I understood this description says that "you need to touch baserestrict 
info *before* GetForeignPlan to estimate costs of optimized path".  I 
don't feel that this description prohibits FDW to touch baserestrictinfo 
in GetForeignRelSize, but mentioning it clearly might be better.

Regards,
--
Shigeru HANADA


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