Robert Haas <robertmhaas@gmail.com> writes:
> Both you and Etsuro Fujita are proposing to fix this problem by
> somehow making it the FDW's problem to reconstruct the tuple
> previously produced by the join from whole-row images of the baserels.
> But that's not looking back far enough: why are we asking for
> whole-row images of the baserels when what we really want is a
> whole-row image of the output of the join? The output of the join is
> what we need to re-return.
There are multiple components to the requirement though:
1. Recheck the rows that were in the baserels and possibly fetch updated
versions of them. (Once we're in EPQ, we want the most recent row
versions, not what the query snapshot can see.)
2. Apply relevant restriction clauses and see if the updated row versions
still pass the clauses.
3. If so, form a join row and return that. Else return NULL.
One way or another, the FDW has to do all of the above, or as much of it
as it can't fob off on the remote server, once it's decided to bypass
local implementation of the join. Just recomputing the original join
row is *not* good enough.
I think what Kaigai-san and Etsuro-san are after is trying to find a way
to reuse some of the existing EPQ machinery to help with that. This may
not be practical, or it may end up being messier than a standalone
implementation; but it's not silly on its face to want to reuse some of
that code.
regards, tom lane