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

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)? 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.

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.

> 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.

> I feel we should focus on getting the first patch of Greg
> (v18-0001-Enable-parallel-SELECT-for-INSERT-INTO-.-SELECT, along with
> a test case patch) and Hou-San's patch discussed at [1] ready. Then we
> can focus on the
> v18-0003-Enable-parallel-INSERT-and-or-SELECT-for-INSERT-INTO. Because
> even if we get the first patch that is good enough for some users.

+1.
--
Amit Langote
EDB: http://www.enterprisedb.com