Message-ID: <5A2A5E10.1040604@lab.ntt.co.jp>
Date: Fri, 08 Dec 2017 18:40:32 +0900
From: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
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Subject: Re: [HACKERS] postgres_fdw bug in 9.6
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To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>,
        Robert Haas <robertmhaas@gmail.com>,
 Jeff Janes <jeff.janes@gmail.com>,
        David Steele <david@pgmasters.net>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
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(2017/11/30 23:22), Tom Lane wrote:
> Etsuro Fujita<fujita.etsuro@lab.ntt.co.jp>  writes:
>> (2017/11/30 7:32), Tom Lane wrote:
>>> the output of the foreign join cannot change during EPQ, since the remote
>>> server already locked the rows before returning them.  The only thing that
>>> can change is the output of the local scan on public.tab.  Yes, we then
>>> need to re-verify that foo.a = tab.a ... but in this example, that's the
>>> responsibility of the NestLoop plan node, not the foreign join.
>
>> That's right, but is that true when the FDW uses late row locking?
>
> An FDW using late row locking would need to work harder, yes.  But
> that's true at the scan level as well as the join level.  We have
> already committed to using early locking in postgres_fdw, for the
> network-round-trip-cost reasons I mentioned before, and I can't see
> why we'd change that decision at the join level.

My concern is FDWs that support join pushdown in combination with late 
row locking.  I don't know whether such FDWs really exist, but if so, an 
output of a foreign join computed from re-fetched tuples might change.

> Right now we've got the worst of both worlds, in that we're actually
> doing early row locking on the remote, but we're paying (some of) the
> code complexity costs required for late locking.

Because we have provided the late row locking API, I think we should pay 
a certain degree of consideration for such FDWs.

Best regards,
Etsuro Fujita