Re: Parallel INSERT (INTO ... SELECT ...)

On Fri, Feb 19, 2021 at 9:38 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Feb 18, 2021 at 11:05 AM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> >
> > > > It also occurred to me that we can avoid pointless adding of
> > > > partitions if the final plan won't use parallelism. For that, the
> > > > patch adds checking glob->parallelModeNeeded, which seems to do the
> > > > trick though I don't know if that's the correct way of doing that.
> > > >
> > >
> > > I'm not sure if's pointless adding partitions even in the case of NOT
> > > using parallelism, because we may be relying on the result of
> > > parallel-safety checks on partitions in both cases.
> > > For example, insert_parallel.sql currently includes a test (that you
> > > originally provided in a previous post) that checks a non-parallel
> > > plan is generated after a parallel-unsafe trigger is created on a
> > > partition involved in the INSERT.
> > > If I further add to that test by then dropping that trigger and then
> > > again using EXPLAIN to see what plan is generated, then I'd expect a
> > > parallel-plan to be generated, but with the setrefs-v3.patch it still
> > > generates a non-parallel plan. So I think the "&&
> > > glob->parallelModeNeeded" part of test needs to be removed.
> >
> > Ah, okay, I didn't retest my case after making that change.
> >
>
> Greg has point here but I feel something on previous lines (having a
> test of glob->parallelModeNeeded) is better. We only want to
> invalidate the plan if the prepared plan is unsafe to execute next
> time. It is quite possible that there are unsafe triggers on different
> partitions and only one of them is dropped, so next time planning will
> again yield to the same non-parallel plan. If we agree with that I
> think it is better to add this dependency in set_plan_refs (along with
> Gather node handling).
>

I think we should try to be consistent with current plan-cache
functionality, rather than possibly inventing new rules for
partitions.
I'm finding that with current Postgres functionality (master branch),
if there are two parallel-unsafe triggers defined on a normal table
and one is dropped, it results in replanning and it yields the same
(non-parallel) plan. This would seem to go against what you are
suggesting.

Regards,
Greg Nancarrow
Fujitsu Australia