Re: fdw_scan_tlist for foreign table scans breaks EPQ testing, doesn't it? - Mailing list pgsql-hackers

> -----Original Message-----
> From: Etsuro Fujita [mailto:fujita.etsuro@lab.ntt.co.jp]
> Sent: Wednesday, July 22, 2015 7:05 PM
> To: Kaigai Kouhei(海外 浩平)
> Cc: pgsql-hackers@postgresql.org
> Subject: Re: [HACKERS] fdw_scan_tlist for foreign table scans breaks EPQ testing,
> doesn't it?
>
> Hi KaiGai-san,
>
> On 2015/07/22 16:44, Kouhei Kaigai wrote:
> >> The latest foreign-join pushdown patch allows fdw_scan_tlist to be set
> >> to a targetlist even for simple foreign table scans.  However, since I
> >> think we assume that the test tuple of a foreign table for an EPQ
> >> testing, whether it may be copied from the whole-row var or returned by
> >> the RefetchForeignRow routine, has the rowtype declared for the foreign
> >> table, ISTM that EPQ testing doesn't work properly in such a case since
> >> that the targetlist and qual are adjusted to reference fdw_scan_tlist in
> >> such a case.  Maybe I'm missing something though.
> >>
> > Let me confirm step-by-step.
> > For EPQ testing, whole-row-reference or RefetchForeignRow pulls a record
> > with row-type compatible to the base foreign table. Then, this record
> > is stored in the es_epqTuple[] indexed by the base relation.
> >
> > According to the previous discussion, I expect these tuples are re-checked
> > by built-in execution plan, but equivalent to the sub-plan entirely pushed
> > out to the remote side.
> > Do we see the same assumption?
>
> No, what I'm concerned about is the case when scanrelid > 0.
>
Hmm. if scanrelid > 0, then fdw_scan_tlist should be NIL.
I want to put Assert(scanrelid==0 || fdw_scan_tlist == NIL) just after
the GetForeignPlan() in createplan.c.

I'm curious why you tried to put valid fdw_scan_tlist for scanrelid > 0.
It's unusual.

> > If so, next step is enhancement of ExecScanFetch() to run the alternative
> > built-in plans towards each es_epqTuple[] records, if given scanrelid==0.
> > In this case, expression nodes adjusted to fdw_scan_tlist never called,
> > so it should not lead any problems...?
>
> When scanrelid = 0, I think we should run the alternative plans in
> ExecScanFetch or somewhere, as you mentioned.
>
OK,

> >> I don't understand custom scans/joins exactly, but I have a similar
> >> concern for the simple-custom-scan case too.
> >>
> > In case of custom scan/join, it fetches a record using heap_fetch()
> > identified by ctid, and saved to es_epqTuple[].
> > Then, EvalPlanQual() walks down the plan-tree. Once it appears a node
> > of custom-join (scanrelid==0), it shall call the equivalent alternatives
> > if possible, or calls ExecProcNode() towards the underlying nodes then
> > re-construct its result according to the custom_scan_tlist definition.
> >
> > It does not look to me problematic.
>
> Sorry, I don't understand what you mean.  Maybe I have to learn more
> about custom scans/joins, but thanks for the explanation!
>
--
NEC Business Creation Division / PG-Strom Project
KaiGai Kohei <kaigai@ak.jp.nec.com>