Re: Fix BUG #17335: Duplicate result rows in Gather node

В Вс, 23/01/2022 в 14:56 +0300, Yura Sokolov пишет:
> В Чт, 20/01/2022 в 09:32 +1300, David Rowley пишет:
> > On Fri, 31 Dec 2021 at 00:14, Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru> wrote:
> > > Suggested quick (and valid) fix in the patch attached:
> > > - If Append has single child, then copy its parallel awareness.
> >
> > I've been looking at this and I've gone through changing my mind about
> > what's the right fix quite a number of times.
> >
> > My current thoughts are that I don't really like the fact that we can
> > have plans in the following shape:
> >
> > Finalize Aggregate
> > -> Gather
> > Workers Planned: 1
> > -> Partial Aggregate
> > -> Parallel Hash Left Join
> > Hash Cond: (gather_append_1.fk = gather_append_2.fk)
> > -> Index Scan using gather_append_1_ix on gather_append_1
> > Index Cond: (f = true)
> > -> Parallel Hash
> > -> Parallel Seq Scan on gather_append_2
> >
> > It's only made safe by the fact that Gather will only use 1 worker.
> > To me, it just seems too fragile to assume that's always going to be
> > the case. I feel like this fix just relies on the fact that
> > create_gather_path() and create_gather_merge_path() do
> > "pathnode->num_workers = subpath->parallel_workers;". If someone
> > decided that was to work a different way, then we risk this breaking
> > again. Additionally, today we have Gather and GatherMerge, but we may
> > one day end up with more node types that gather results from parallel
> > workers, or even a completely different way of executing plans.
>
> It seems strange parallel_aware and parallel_safe flags neither affect
> execution nor are properly checked.
>
> Except parallel_safe is checked in ExecSerializePlan which is called from
> ExecInitParallelPlan, which is called from ExecGather and ExecGatherMerge.
> But looks like this check doesn't affect execution as well.
>
> > I think a safer way to fix this is to just not remove the
> > Append/MergeAppend node if the parallel_aware flag of the only-child
> > and the Append/MergeAppend don't match. I've done that in the
> > attached.
> >
> > I believe the code at the end of add_paths_to_append_rel() can remain as is.
>
> I found clean_up_removed_plan_level also called from set_subqueryscan_references.
> Is there a need to patch there as well?
>
> And there is strange state:
> - in the loop by subpaths, pathnode->node.parallel_safe is set to AND of
> all its subpath's parallel_safe
> (therefore there were need to copy it in my patch version),
> - that means, our AppendPath is parallel_aware but not parallel_safe.
> It is ridiculous a bit.
>
> And it is strange AppendPath could have more parallel_workers than sum of
> its children parallel_workers.
>
> So it looks like whole machinery around parallel_aware/parallel_safe has
> no enough consistency.
>
> Either way, I attach you version of fix with my tests as new patch version.

Looks like volatile "Memory Usage:" in EXPLAIN brokes 'make check'
sporadically.

Applied replacement in style of memoize.sql test.

Why there is no way to disable "Buckets: %d Buffers: %d Memory Usage: %dkB"
output in show_hash_info?

regards,
Yura Sokolov