Re: [HACKERS] postgres_fdw bug in 9.6

(2017/11/30 7:32), Tom Lane wrote:
> AFAICT, [1] just demonstrates that nobody had thought about the scenario
> up to that point, not that the subsequently chosen solution was a good
> one.  In that example,
>
> LockRows  (cost=100.00..101.18 rows=4 width=70)
>     Output: tab.a, tab.b, tab.ctid, foo.*, bar.*
>     ->   Nested Loop  (cost=100.00..101.14 rows=4 width=70)
>           Output: tab.a, tab.b, tab.ctid, foo.*, bar.*
>           Join Filter: (foo.a = tab.a)
>           ->   Seq Scan on public.tab  (cost=0.00..1.01 rows=1 width=14)
>                 Output: tab.a, tab.b, tab.ctid
>           ->   Foreign Scan  (cost=100.00..100.08 rows=4 width=64)
>                 Output: foo.*, foo.a, bar.*, bar.a
>                 Relations: (public.foo) INNER JOIN (public.bar)
>                 Remote SQL: SELECT l.a1, l.a2, r.a1, r.a2 FROM (SELECT ROW(l.a9), l.a9 FROM (SELECT a a9 FROM public.foo FOR UPDATE) l) l (a1, a2) INNER JOIN (SELECT ROW(r.a9), r.a9 FROM (SELECT a a9 FROM public.bar FOR UPDATE) r) r (a1, a2) ON ((l.a2 = r.a2))
>
> 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?

Best regards,
Etsuro Fujita