Re: Asynchronous execution on FDW

> -----Original Message-----
> From: pgsql-hackers-owner@postgresql.org
> [mailto:pgsql-hackers-owner@postgresql.org] On Behalf Of Kyotaro HORIGUCHI
> Sent: Wednesday, July 22, 2015 4:10 PM
> To: robertmhaas@gmail.com
> Cc: hlinnaka@iki.fi; pgsql-hackers@postgresql.org
> Subject: Re: [HACKERS] Asynchronous execution on FDW
>
> Hello, thank you for the comment.
>
> At Fri, 17 Jul 2015 14:34:53 -0400, Robert Haas <robertmhaas@gmail.com> wrote
> in <CA+TgmoaiJK1svzw_GkFU+zsSxciJKFELqu2AOMVUPhpSFw4BsQ@mail.gmail.com>
> > On Fri, Jul 3, 2015 at 4:41 PM, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> > > At a quick glance, I think this has all the same problems as starting the
> > > execution at ExecInit phase. The correct way to do this is to kick off the
> > > queries in the first IterateForeignScan() call. You said that "ExecProc
> > > phase does not fit" - why not?
> >
> > What exactly are those problems?
> >
> > I can think of these:
> >
> > 1. If the scan is parametrized, we probably can't do it for lack of
> > knowledge of what they will be.  This seems easy; just don't do it in
> > that case.
>
> We can put an early kick to foreign scans only for the first shot
> if we do it outside (before) ExecProc phase.
>
> Nestloop
> -> SeqScan
> -> Append
>    -> Foreign (Index) Scan
>    -> Foreign (Index) Scan
>    ..
>
> This plan premises precise (even to some extent) estimate for
> remote query but async execution within ExecProc phase would be
> in effect for this case.
>
>
> > 2. It's possible that we're down inside some subtree of the plan that
> > won't actually get executed.  This is trickier.
>
> As for current postgres_fdw, it is done simply abandoning queued
> result then close the cursor.
>
> > Consider this:
> >
> > Append
> > -> Foreign Scan
> > -> Foreign Scan
> > -> Foreign Scan
> > <repeat 17 more times>
> >
> > If we don't start each foreign scan until the first tuple is fetched,
> > we will not get any benefit here, because we won't fetch the first
> > tuple from query #2 until we finish reading the results of query #1.
> > If the result of the Append node will be needed in its entirety, we
> > really, really want to launch of those queries as early as possible.
> > OTOH, if there's a Limit node with a small limit on top of the Append
> > node, that could be quite wasteful.
>
> It's the nature of speculative execution, but the Limit will be
> pushed down onto every Foreign Scans near future.
>
> > We could decide not to care: after all, if our limit is
> > satisfied, we can just bang the remote connections shut, and if
> > they wasted some CPU, well, tough luck for them.  But it would
> > be nice to be smarter.  I'm not sure how, though.
>
> Appropriate fetch size will cap the harm and the case will be
> handled as I mentioned above as for postgres_fdw.
>
Horiguchi-san,

Let me ask an elemental question.

If we have ParallelAppend node that kicks a background worker process for
each underlying child node in parallel, does ForeignScan need to do something
special?

Expected waste of CPU or I/O is common problem to be solved, however, it does
not need to add a special case handling to ForeignScan, I think.
How about your opinion?

Thanks,
--
NEC Business Creation Division / PG-Strom Project
KaiGai Kohei <kaigai@ak.jp.nec.com>