Re: Foreign join pushdown vs EvalPlanQual

On Wed, Oct 7, 2015 at 12:10 AM, Kyotaro HORIGUCHI
<horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>> IIUC, I think that if ROW_MARK_COPY is in use, the descriptor would
>> have 6 columns: those 4, plus a whole-row var for ft1 and another
>> whole-row bar for ft2. Maybe I'm missing something, though.
>
> You're right. The result tuple for the Robert's example has 6
> attributes in the order of [ft1.a, ft1.b, (ft1.a, ft1.b), ft2.a...]
>
> But the point of the discussion is in another point. The problem
> is when such joins are joined with another local table. For such
> case, the whole-row reference for the intermediate foreign-join
> would lose the targets in top-level tuple.

Really? Would that mean that ROW_MARK_COPY is totally broken? I bet it's not.

>> > 4, plus a whole-row reference for ROW_MARK_COPY. If we know what
>> > values we're going to store in columns 1..4, couldn't we just form
>> > them into a tuple to populate column 5? We don't actually need to be
>> > able to fetch such a tuple from the remote side because we can just
>> > construct it. I think.
>>
>> I also was thinking whether we could replace one of the whole-row vars
>> with a whole-row var that represents the scan slot of the
>> ForeignScanState node.
>
> I suppose it requires additional resjunk to be added on joinrel
> creation, which is what Kaigai-san said as overkill. But I'm
> interedted in what it looks.

I think it rather requires *replacing* two resjunk columns by one new
one. The whole-row references for the individual foreign tables are
only there to support EvalPlanQual; if we instead have a column to
populate the foreign scan's slot directly, then we can use that column
for that purpose directly and there's no remaining use for the
whole-row vars on the baserels.

--
Robert Haas
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