Re: Custom/Foreign-Join-APIs (Re: [v9.5] Custom Plan API)

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Mar 17, 2015 at 10:11 AM, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
>> It might be an idea if foreign-scan path is not wiped out regardless of the
>> estimated cost, we will be able to construct an entirely remote-join path
>> even if intermediation path is expensive than local join.
>> A problem is, how to distinct these special paths from usual paths that are
>> eliminated on the previous stage once its path is more expensive.

> Any solution that is based on not eliminating paths that would
> otherwise be discarded based on cost seems to me to be unlikely to be
> feasible. We can't complicate the core path-cost-comparison stuff for
> the convenience of FDW or custom-scan pushdown.

I concur. I'm not even so worried about the cost of add_path as such;
the real problem with not discarding paths as aggressively as possible
is that it will result in a combinatorial explosion in the number of
path combinations that have to be examined at higher join levels.

regards, tom lane