Re: Foreign join pushdown vs EvalPlanQual - Mailing list pgsql-hackers

On 2015/11/03 22:15, Kouhei Kaigai wrote:
> A challenge is that junk wholerow references on behalf of ROW_MARK_COPY
> are injected by preprocess_targetlist(). It is earlier than the main path
> consideration by query_planner(), thus, it is not predictable how remote
> query shall be executed at this point.
> If ROW_MARK_COPY, base tuple image is fetched using this junk attribute.
> So, here is two options if we allow to put joined tuple on either of
> es_epqTuple[].
>
> options-1) We ignore record type definition. FDW returns a joined tuple
> towards the whole-row reference of either of the base relations in this
> join. The junk attribute shall be filtered out eventually and only FDW
> driver shall see, so it is harmless to do (probably).
> This option takes no big changes, however, we need a little brave to adopt.
>
> options-2) We allow FDW/CSP to adjust target-list of the relevant nodes
> after these paths get chosen by planner. It enables to remove whole-row
> reference of base relations and add alternative whole-row reference instead
> if FDW/CSP can support it.
> This feature can be relevant to target-list push-down to the remote side,
> not only EPQ rechecks, because adjustment of target-list means we allows
> FDW/CSP to determine which expression shall be executed locally, or shall
> not be.
> I think, this option is more straightforward, however, needs a little bit
> deeper consideration, because we have to design the best hook point and
> need to ensure how path-ification will perform.
>
> Therefore, I think we need two steps towards the entire solution.
> Step-1) FDW/CSP will recheck base EPQ tuples and support local
> reconstruction on the fly. It does not need something special
> enhancement on the planner - so we can fix up by v9.5 release.
> Step-2) FDW/CSP will support adjustment of target-list to add whole-row
> reference of joined tuple instead of multiple base relations, then FDW/CSP
> will be able to put a joined tuple on either of EPQ slot if it wants - it
> takes a new feature enhancement, so v9.6 is a suitable timeline.
>
> How about your opinion towards the direction?
> I don't want to drop extra optimization opportunity, however, we are now in
> November. I don't have enough brave to add none-obvious new feature here.

I think we need to consider a general solution that can be applied not 
only to the case where the component tables in a foreign join all use 
ROW_MARK_COPY but to the case where those tables use different rowmark 
types such as ROW_MARK_COPY and ROW_MARK_EXCLUSIVE, as I pointed out 
upthread.

Best regards,
Etsuro Fujita