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

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.etsuro@lab.ntt.co.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