Odd behavior in foreign table modification (Was: Re: Optimization for updating foreign tables in Postgres FDW)

From: Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>
To: Thom Brown <thom(at)linux(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, Albe Laurenz <laurenz(dot)albe(at)wien(dot)gv(dot)at>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Shigeru Hanada <shigeru(dot)hanada(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Odd behavior in foreign table modification (Was: Re: Optimization for updating foreign tables in Postgres FDW)
Date: 2016-01-08 05:08:00
Message-ID: 568F4430.6060805@lab.ntt.co.jp
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On 2016/01/07 21:50, Etsuro Fujita wrote:
> On 2016/01/06 20:37, Thom Brown wrote:
>> On 25 December 2015 at 10:00, Etsuro Fujita
>> <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>>> Attached is an updated version of the patch, which is
>>> still WIP, but I'd be happy if I could get any feedback.

>> I've run into an issue:
>>
>> *# UPDATE master_customers SET id = 22 WHERE id = 16 RETURNING
>> tableoid::regclass;
>> ERROR:
>> CONTEXT: Remote SQL command: UPDATE public.customers SET id = 22
>> WHERE ((id = 16)) RETURNING NULL

> Will fix.

While working on this, I noticed that the existing postgres_fdw system
shows similar behavior, so I changed the subject.

IIUC, the reason for that is when the local query specifies "RETURNING
tableoid::regclass", the FDW has fmstate->has_returning=false while the
remote query executed at ModifyTable has "RETURNING NULL", as shown in
the above example; that would cause an abnormal exit in executing the
remote query in postgresExecForeignUpdate, since that the FDW would get
PGRES_TUPLES_OK as a result of the query while the FDW would think that
the right result to get should be PGRES_COMMAND_OK, from the flag
fmstate->has_returning=false.

Attached is a patch to fix that.

Best regards,
Etsuro Fujita

Attachment Content-Type Size
fdw-foreign-modify.patch application/x-patch 7.1 KB

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