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

On Mon, Feb 22, 2021 at 8:46 AM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
>
> On Fri, Feb 19, 2021 at 7:38 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).
>
> Are you saying that partitions shouldn't be added to the dependency
> list if a parallel plan was not chosen for insert into a partitioned
> table for whatever reason (parallel unsafe expressions or beaten by
> other paths in terms of cost)?
>

Right.

> If so, I am inclined to agree with
> that.
>
> I may be wrong but it doesn't seem to me that the possibility of
> constructing a better plan due to a given change is enough reason for
> plancache.c to invalidate plans that depend on that change. AIUI,
> plancache.c only considers a change interesting if it would *break* a
> Query or a plan.
>

that makes sense to me.

> So in this case, a non-parallel plan may be slower, but it isn't
> exactly rendered *wrong* by changes that make a parallel plan
> possible.
>

Right.

> > Also, if we agree that we don't have any cheap way to determine
> > parallel-safety of partitioned relations then shall we consider the
> > patch being discussed [1] to be combined here?
>
> Yes, I think it does make sense to consider the GUC patch with the
> patches on this thread.
>

Cool, Greg/Hou, can we consider this along with the next patch?

--
With Regards,
Amit Kapila.