> On Tue, Nov 17, 2015 at 6:51 PM, Kouhei Kaigai <kaigai@ak.jp.nec.com> wrote:
> > It just intends to keep code symmetry with custom-scan case, so not
> > a significant reason.
> > And, I expected ForeignScan will also need multiple sub-plans soon
> > to support more intelligent push-down like:
> >
> http://www.postgresql.org/message-id/9A28C8860F777E439AA12E8AEA7694F8010F47D
> A@BPXM15GP.gisp.nec.co.jp
>
> I might be missing something, but why would that require multiple child plans?
>
Apart from EPQ rechecks, the above aggressive push-down idea allows to send
contents of multiple relations to the remote side. In this case, ForeignScan
needs to have multiple sub-plans.
For example, please assume here is three relations; tbl_A and tbl_B are
local and small, tbl_F is remote and large.
In case when both of (tbl_A JOIN tbl_F) and (tbl_B JOIN tbl_F) produces
large number of rows thus consumes deserved amount of network traffic but
(tbl_A JOIN tbl_B JOIN tbl_F) produce small number of rows, the optimal
strategy is to send local contents to the remote side once then run
a remote query here to produce relatively smaller rows.
In the implementation level, ForeignScan shall represent (tbl_A JOIN tbl_B
JOIN tbl_F), then it returns a bunch of joined tuples. Its remote query
contains VALUES(...) clauses to pack contents of the tbl_A and tbl_B, thus,
it needs to be capable to execute underlying multiple scan plans and fetch
tuples prior to remote query execution.
So, ForeignScan may also have multiple sub-plans.
Of course, it is an independent feature from the EPQ rechecks.
It is not a matter even if we will extend this field later.
Thanks,
--
NEC Business Creation Division / PG-Strom Project
KaiGai Kohei <kaigai@ak.jp.nec.com>
