Re: PassDownLimitBound for ForeignScan/CustomScan [take-2]

> -----Original Message-----
> From: pgsql-hackers-owner@postgresql.org
> [mailto:pgsql-hackers-owner@postgresql.org] On Behalf Of Robert Haas
> Sent: Thursday, November 10, 2016 3:08 AM
> To: Kaigai Kouhei(海外 浩平)
> Cc: pgsql-hackers@postgresql.org; Jeevan Chalke; Etsuro Fujita; Andres Freund
> Subject: Re: [HACKERS] PassDownLimitBound for ForeignScan/CustomScan [take-2]
> 
> On Mon, Oct 31, 2016 at 10:20 AM, Kouhei Kaigai <kaigai@ak.jp.nec.com> wrote:
> > As an example, I enhanced postgres_fdw to understand the ps_numTuples
> > if it is set. If and when remote ORDER BY is pushed down, the latest
> > code tries to sort the entire remote table because it does not know
> > how many rows to be returned. Thus, it took larger execution time.
> > On the other hands, the patched one runs the remote query with LIMIT
> > clause according to the ps_numTuples; which is informed by the Limit
> > node on top of the ForeignScan node.
> 
> So there are two cases here.  If the user says LIMIT 12, we could in
> theory know that at planner time and optimize accordingly.  If the
> user says LIMIT twelve(), however, we will need to wait until
> execution time unless twelve() happens to be capable of being
> simplified to a constant by the planner.
> 
> Therefore, it's possible to imagine having two mechanisms here. In the
> simple case where the LIMIT and OFFSET values are constants, we could
> implement a system to get hold of that information during planning and
> use it for whatever we like.   In addition, we can have an
> execution-time system that optimizes based on values available at
> execution (regardless of whether those values were also available
> during planning).  Those are, basically, two separate things, and this
> patch has enough to do just focusing on one of them.
>
OK, we need to have a private value of postgres_fdw to indicate whether
LIMIT and OFFSET were supplied at the planner stage. If any, it has to
be matched with the ps_numTuples informed at the executor stage.

I'll revise the patch.
--
NEC OSS Promotion Center / PG-Strom Project
KaiGai Kohei <kaigai@ak.jp.nec.com>