Re: CustomScan under the Gather node?

> -----Original Message-----
> From: Amit Kapila [mailto:amit(dot)kapila16(at)gmail(dot)com]
> Sent: Wednesday, January 27, 2016 2:30 PM
> To: Kaigai Kouhei(海外 浩平)
> Cc: pgsql-hackers(at)postgresql(dot)org
> Subject: ##freemail## Re: [HACKERS] CustomScan under the Gather node?
>
> On Tue, Jan 26, 2016 at 12:00 PM, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
> >
> > Hello,
> >
> > What enhancement will be necessary to implement similar feature of
> > partial seq-scan using custom-scan interface?
> >
> > It seems to me callbacks on the three points below are needed.
> > * ExecParallelEstimate
> > * ExecParallelInitializeDSM
> > * ExecParallelInitializeWorker
> >
> > Anything else?
>
> I don't think so.
>
> > Does ForeignScan also need equivalent enhancement?
>
> I think this depends on the way ForeignScan is supposed to be
> parallelized, basically if it needs to coordinate any information
> with other set of workers, then it will require such an enhancement.
>
After the post yesterday, I reminded an possible scenario around FDW
if it manages own private storage, like cstore_fdw.

Probably, ForeignScan node performing on columnar store (for example)
will need a coordination information like as partial seq-scan doing.
It is a case very similar to the implementation on local storage.

On the other hands, if we try postgres_fdw (or others) to get parallelized
with background worker, I doubt whether we need this coordination information
on local side. Remote query will have an additional qualifier to skip blocks
already fetched for this purpose.
At least, it does not needs something special enhancement.

Thanks,
--
NEC Business Creation Division / PG-Strom Project
KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com>