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.
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.
regards, tom lane