Re: add_partial_path() may remove dominated path but still in use

2018年12月31日(月) 13:10 Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>:
>
> On Sun, Dec 30, 2018 at 9:01 AM Kohei KaiGai <kaigai(at)heterodb(dot)com> wrote:
> > 2018年12月30日(日) 4:12 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> > >
> > > Kohei KaiGai <kaigai(at)heterodb(dot)com> writes:
> > > > 2018年12月29日(土) 1:44 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> > > >> However, first I'd like to know why this situation is arising in the first
> > > >> place. To have the situation you're describing, we'd have to have
> > > >> attempted to make some Gather paths before we have all the partial paths
> > > >> for the relation they're for. Why is that a good thing to do? It seems
> > > >> like such Gathers are necessarily being made with incomplete information,
> > > >> and we'd be better off to fix things so that none are made till later.
> > >
> > > > Because of the hook location, Gather-node shall be constructed with built-in
> > > > and foreign partial scan node first, then extension gets a chance to add its
> > > > custom paths (partial and full).
> > > > At the set_rel_pathlist(), set_rel_pathlist_hook() is invoked next to the
> > > > generate_gather_paths().
> > >
> > > Hmm. I'm inclined to think that we should have a separate hook
> > > in which extensions are allowed to add partial paths, and that
> > > set_rel_pathlist_hook should only be allowed to add regular paths.
>
> +1. This idea sounds sensible to me.
>
> > >
> > I have almost same opinion, but the first hook does not need to be
> > dedicated for partial paths. As like set_foreign_pathlist() doing, we can
> > add both of partial and regular paths here, then generate_gather_paths()
> > may generate a Gather-path on top of the best partial-path.
> >
>
> Won't it be confusing for users if we allow both partial and full
> paths in first hook and only full paths in the second hook?
> Basically, in many cases, the second hook won't be of much use. What
> advantage you are seeing in allowing both partial and full paths in
> the first hook?
>
Two advantages. The first one is, it follows same manner of
set_foreign_pathlist()
which allows to add both of full and partial path if FDW supports parallel-scan.
The second one is practical. During the path construction, extension needs to
check availability to run (e.g, whether operators in WHERE-clause is supported
on GPU device...), calculate its estimated cost and so on. Not a small
portion of
them are common for both of full and partial path. So, if the first
hook accepts to
add both kind of paths at once, extension can share the common properties.

Probably, the second hook is only used for a few corner case where an extension
wants to manipulate path-list already built, like pg_hint_plan.

Thanks,
--
HeteroDB, Inc / The PG-Strom Project
KaiGai Kohei <kaigai(at)heterodb(dot)com>