On 2015/10/01 19:02, Kyotaro HORIGUCHI wrote:
At Thu, 1 Oct 2015 17:50:25 +0900, Etsuro Fujita <fujita.ets...@lab.ntt.co.jp> wrote 
in <560cf3d1.9060...@lab.ntt.co.jp>
From: pgsql-hackers-ow...@postgresql.org
[mailto:pgsql-hackers-ow...@postgresql.org] On Behalf Of Robert Haas

So, if we wanted to fix this in a way that preserves the spirit of
what's there now, it seems to me that we'd want the FDW to return
something that's like a whole row reference, but represents the output
of the foreign join rather than some underlying base table.  And then
get the EPQ machinery to have the evaluation of the ForeignScan for
the join, when it happens in an EPQ context, to return that tuple.
But I don't really have a good idea how to do that.

So, I'd like to investigate another approach that preserves the
applicability of late row locking to the join pushdown case as well as
the spirit of what's there now.  The basic idea is (1) add a new
callback routine RefetchForeignJoinRow that refetches one foreign-join
tuple from the foreign server, after locking remote tuples for the
component foreign tables if required,

It would be the case that at least one of the component relations
of a foreign join is other than ROW_MARK_COPY, which is not
possible so far on postgres_fdw.

Yes. To be exact, it's possible for the component relations to have rowmark methods other than ROW_MARK_COPY using GetForeignRowMarkType, in principle, but the server crashes ...

For the case that some of the
component relations are other than ROW_MARK_COPY, we might should
call RefetchForeignRow for such relations and construct joined
row involving ROW_MARK_COPY relations.

You are saying that we should construct the joined row using an alternative local join execution plan?

Indeed we could consider some logic for the case, it is obvious
that the case now we should focus on is a "foreign join" scan
with all underlying foreign scans are ROW_MARK_COPY, I
think. "foreign join" scan with ROW_MARK_COPY looks to be
promising (for me) and in future it would be able to coexist with
refetch mechanism maybe in your mind from this point of
view... Maybe:p

I agree that the approach "foreign-join scan with ROW_MARK_COPY" would be promising.

Best regards,
Etsuro Fujita

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