Re: Consider low startup cost in add_partial_path

On Fri, Oct 4, 2019 at 8:36 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Wed, Oct 2, 2019 at 10:22 AM James Coleman <jtc331(at)gmail(dot)com> wrote:
> > In all cases I've been starting with:
> >
> > set enable_hashjoin = off;
> > set enable_nestloop = off;
> > set max_parallel_workers_per_gather = 4;
> > set min_parallel_index_scan_size = 0;
> > set min_parallel_table_scan_size = 0;
> > set parallel_setup_cost = 0;
> > set parallel_tuple_cost = 0;
> >
> > I've also tried various combinations of random_page_cost,
> > cpu_index_tuple_cost, cpu_tuple_cost.
> >
> > Interestingly I've noticed plans joining two relations that look like:
> >
> > Limit
> > -> Merge Join
> > Merge Cond: (t1.pk = t2.pk)
> > -> Gather Merge
> > Workers Planned: 4
> > -> Parallel Index Scan using t_pkey on t t1
> > -> Gather Merge
> > Workers Planned: 4
> > -> Parallel Index Scan using t_pkey on t t2
> >
> > Where I would have expected a Gather Merge above a parallelized merge
> > join. Is that reasonable to expect?
>
> Well, you told the planner that parallel_setup_cost = 0, so starting
> workers is free. And you told the planner that parallel_tuple_cost =
> 0, so shipping tuples from the worker to the leader is also free. So
> it is unclear why it should prefer a single Gather Merge over two
> Gather Merges: after all, the Gather Merge is free!
>
> If you use give those things some positive cost, even if it's smaller
> than the default, you'll probably get a saner-looking plan choice.

That makes sense.

Right now I currently see trying to get this a separate test feels a
bit like a distraction.

Given there doesn't seem to be an obvious way to reproduce the issue
currently, but we know we have a reproduction example along with
incremental sort, what is the path forward for this? Is it reasonable
to try to commit it anyway knowing that it's a "correct" change and
been demonstrated elsewhere?

James