From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Shigeru HANADA <hanada(at)metrosystems(dot)co(dot)jp>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Itagaki Takahiro <itagaki(dot)takahiro(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, SAKAMOTO Masahiko <sakamoto(dot)masahiko(at)oss(dot)ntt(dot)co(dot)jp>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: patch: SQL/MED(FDW) DDL |
Date: | 2010-10-05 14:41:26 |
Message-ID: | 23958.1286289686@sss.pgh.pa.us |
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Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Oct 5, 2010 at 10:25 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> This whole discussion seems to me to be about trying to do things outside
>> the FDW that should properly be left inside the FDW. Who's to say that
>> the remote side even *has* statistics of the sort that PG creates?
>>
>> We should provide an API that lets the FDW return a cost estimate for a
>> proposed access path. Where it gets the cost estimate from is not
>> something that should be presupposed.
> Unless there's some way for the FDW to have local tables for caching
> its statistics, the chances of this having decent performance seem to
> be near-zero.
Perhaps, but that would be the FDW's problem to implement. Trying to
design such tables in advance of actually writing an FDW seems like a
completely backwards design process.
(I'd also say that your performance estimate is miles in advance of any
facts; but even if it's true, the caching ought to be inside the FDW,
because we have no clear idea of what it will need to cache.)
regards, tom lane
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