Re: Foreign join pushdown vs EvalPlanQual

On 2015/10/07 6:19, Robert Haas wrote:
> On Fri, Oct 2, 2015 at 4:26 AM, Kyotaro HORIGUCHI
> <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
>> During join search, a joinrel should be comptible between local
>> joins and remote joins, of course target list also should be
>> so. So it is quite difficult to add wholerow resjunk for joinrels
>> before whole join tree is completed even if we allow row marks
>> that are not bound to base RTEs.

> Suppose ROW_MARK_COPY is in use, and suppose the query is: SELECT
> ft1.a, ft1.b, ft2.a, ft2.b FROM ft1, ft2 WHERE ft1.x = ft2.x;
>
> When the foreign join is executed, there's going to be a slot that
> needs to be populated with ft1.a, ft1.b, ft2.a, ft2.b, and a whole row
> reference. Now, let's suppose the slot descriptor has 5 columns: those
> 4, plus a whole-row reference for ROW_MARK_COPY.

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.

> 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.

Best regards,
Etsuro Fujita