Обсуждение: Consider low startup cost in add_partial_path

Over in the incremental sort patch discussion we found [1] a case
where a higher cost plan ends up being chosen because a low startup
cost partial path is ignored in favor of a lower total cost partial
path and a limit is a applied on top of that which would normal favor
the lower startup cost plan.

45be99f8cd5d606086e0a458c9c72910ba8a613d originally added
`add_partial_path` with the comment:

> Neither do we need to consider startup costs:
> parallelism is only used for plans that will be run to completion.
> Therefore, this routine is much simpler than add_path: it needs to
> consider only pathkeys and total cost.

I'm not entirely sure if that is still true or not--I can't easily
come up with a scenario in which it's not, but I also can't come up
with an inherent reason why such a scenario cannot exist.

We could just continue to include this change as part of the
incremental sort patch itself, but it seemed worth it to me to break
it out for some more targeted discussion, and also include Robert as
the initial author of add_partial_path in the hopes that maybe we
could retrieve some almost 4-year-old memories on why this was
inherently true then, and maybe that would shed some light on whether
it's still inherently true.

I've attached a patch (by Tomas Vondra, also cc'd) to consider startup
cost in add_partial_path, but should we apply the patch we'll also
likely need to apply the same kind of change to
add_partial_path_precheck.

James Coleman

[1]: https://www.postgresql.org/message-id/20190720132244.3vgg2uynfpxh3me5%40development

On Fri, Sep 27, 2019 at 2:24 PM James Coleman <jtc331@gmail.com> wrote:
> Over in the incremental sort patch discussion we found [1] a case
> where a higher cost plan ends up being chosen because a low startup
> cost partial path is ignored in favor of a lower total cost partial
> path and a limit is a applied on top of that which would normal favor
> the lower startup cost plan.
>
> 45be99f8cd5d606086e0a458c9c72910ba8a613d originally added
> `add_partial_path` with the comment:
>
> > Neither do we need to consider startup costs:
> > parallelism is only used for plans that will be run to completion.
> > Therefore, this routine is much simpler than add_path: it needs to
> > consider only pathkeys and total cost.
>
> I'm not entirely sure if that is still true or not--I can't easily
> come up with a scenario in which it's not, but I also can't come up
> with an inherent reason why such a scenario cannot exist.

I think I just didn't think carefully about the Limit case.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Sat, Sep 28, 2019 at 12:16:05AM -0400, Robert Haas wrote:
>On Fri, Sep 27, 2019 at 2:24 PM James Coleman <jtc331@gmail.com> wrote:
>> Over in the incremental sort patch discussion we found [1] a case
>> where a higher cost plan ends up being chosen because a low startup
>> cost partial path is ignored in favor of a lower total cost partial
>> path and a limit is a applied on top of that which would normal favor
>> the lower startup cost plan.
>>
>> 45be99f8cd5d606086e0a458c9c72910ba8a613d originally added
>> `add_partial_path` with the comment:
>>
>> > Neither do we need to consider startup costs:
>> > parallelism is only used for plans that will be run to completion.
>> > Therefore, this routine is much simpler than add_path: it needs to
>> > consider only pathkeys and total cost.
>>
>> I'm not entirely sure if that is still true or not--I can't easily
>> come up with a scenario in which it's not, but I also can't come up
>> with an inherent reason why such a scenario cannot exist.
>
>I think I just didn't think carefully about the Limit case.
>

Thanks! In that case I suggest we treat it as a separate patch/fix,
independent of the incremental sort patch. I don't want to bury it in
that patch series, it's already pretty large.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On Sat, Sep 28, 2019 at 12:16:05AM -0400, Robert Haas wrote:

On Fri, Sep 27, 2019 at 2:24 PM James Coleman <jtc331@gmail.com> wrote:

Over in the incremental sort patch discussion we found [1] a case
where a higher cost plan ends up being chosen because a low startup
cost partial path is ignored in favor of a lower total cost partial
path and a limit is a applied on top of that which would normal favor
the lower startup cost plan.

45be99f8cd5d606086e0a458c9c72910ba8a613d originally added
`add_partial_path` with the comment:

> Neither do we need to consider startup costs:
> parallelism is only used for plans that will be run to completion.
> Therefore, this routine is much simpler than add_path: it needs to
> consider only pathkeys and total cost.

I'm not entirely sure if that is still true or not--I can't easily
come up with a scenario in which it's not, but I also can't come up
with an inherent reason why such a scenario cannot exist.

I think I just didn't think carefully about the Limit case.

Thanks! In that case I suggest we treat it as a separate patch/fix,
independent of the incremental sort patch. I don't want to bury it in
that patch series, it's already pretty large.

On Sat, Sep 28, 2019 at 7:21 PM James Coleman <jtc331@gmail.com> wrote:
> Now the trick is to figure out a way to demonstrate it in test :)
>
> Basically we need:
> Path A: Can short circuit with LIMIT but has high total cost
> Path B: Can’t short circuit with LIMIT but has lower total cost
>
> (Both must be parallel aware of course.)

I'm adding one requirement, or clarifying it anyway: the above paths
must be partial paths, and can't just apply at the top level of the
parallel part of the plan. I.e., the lower startup cost has to matter
at a subtree of the parallel portion of the plan.

> Maybe ordering in B can be a sort node and A can be an index scan (perhaps with very high random page cost?) and
forcechoosing a parallel plan? 
>
> I’m trying to describe this to jog my thoughts (not in front of my laptop right now so can’t try it out).
>
> Any other ideas?

I've been playing with this a good bit, and I'm struggling to come up
with a test case. Because the issue only manifests in a subtree of the
parallel portion of the plan, a scan on a single relation won't do.
Merge join seems like a good area to look at because it requires
ordering, and that ordering can be either the result of an index scan
(short-circuit-able) or an explicit sort (not short-circuit-able). But
I've been unable to make that result in any different plans with
either 2 or 3 relations joined together, ordered, and a limit applied.

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?

If 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

On Wed, Oct 2, 2019 at 10:22 AM James Coleman <jtc331@gmail.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.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Fri, Oct 4, 2019 at 8:36 AM Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Wed, Oct 2, 2019 at 10:22 AM James Coleman <jtc331@gmail.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

Hi,

For the record, here is the relevant part of the Incremental Sort patch
series, updating add_partial_path and add_partial_path_precheck to also
consider startup cost.

The changes in the first two patches are pretty straight-forward, plus
there's a proposed optimization in the precheck function to only run
compare_pathkeys if entirely necessary. I'm currently evaluating those
changes and I'll post the results to the incremental sort thread.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

[ reviving an old thread ]

On Sun, Apr 5, 2020 at 10:14 AM Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
> For the record, here is the relevant part of the Incremental Sort patch
> series, updating add_partial_path and add_partial_path_precheck to also
> consider startup cost.
>
> The changes in the first two patches are pretty straight-forward, plus
> there's a proposed optimization in the precheck function to only run
> compare_pathkeys if entirely necessary. I'm currently evaluating those
> changes and I'll post the results to the incremental sort thread.

I rediscovered this problem while testing pg_plan_advice, and I think
we should commit something to fix it. Consider the following query
from the regression tests:

explain (costs off) select a,b,sum(c) from t group by 1,2 order by
1,2,3 limit 1;

With the settings in use at the time the plan is executed, the plan
has a cost of 68.11. If you rerun the same query with
enable_seqscan=off, the estimated cost drops to 65.28. This is
obviously a bit of a surprising outcome, since we should be
considering a subset of the original possibilities and should
therefore not be able to achieve a better outcome. What happens is
that the planner is deciding between Finalize GroupAggregate => Gather
Merge => Partial GroupAggregate => Incremental Sort => Parallel Index
Scan and GroupAggregate => Gather Merge => Incremental Sort =>
Parallel Index Scan. When we're computing partial paths for
partially_grouped_rel, we normally construct two: one that does
Incremental Sort => Parallel Index Scan and another that does Sort =>
Parallel Seq Scan. Because add_partial_path() doesn't consider startup
costs, the second path causes the first one to be discarded. Once we
add the Gather Merge node, we do consider startup costs (since it's
not a partial path any more) and now the high startup cost of Sort =>
Parallel Seq Scan makes it lose out. Instead, we abandon the idea of
two-step aggregation altogether and pick a plan that involves one step
aggregation. To me, this is a pretty clear illustration that the
comments that I wrote here are just wrong, the reasoning is wrong, and
what we're doing doesn't make a whole lot of sense. We're throwing
away paths that would turn out to be the winning option if we kept
them on the basis of a specious argument that startup cost doesn't
matter.

So, I started working on Tomas's patches from April 5, 2020. They
don't apply any more, so I had to rebase them. That resulted in a
bunch of regression test failures, and some of those plan changes
didn't make a whole lot of sense. At that point, I realized that when
I added the disabled_nodes field to the Path structure, I failed to
properly adjust the logic in add_partial_path() for the fact that we
now keep path lists sorted first by number of disabled nodes and then
by total cost. After fixing that, there were still two tests failing.
One was the query above, where the plan improves. In the other case, a
Hash Left Join became a Hash Right Join, and the reason that happened
is because the two paths had exactly equivalent total costs, but the
Hash Right Join has a cheaper startup cost. Arguably that's an
improvement, too, but it defeated the purpose of the test case, so
that will need to be adjusted. On further study, I found yet another
bug, which is that add_partial_path_precheck() *also* wasn't properly
adjusted to deal with the disabled_nodes field. In other words,
everything that is wrong here is 100% my fault: some of it I did wrong
when I added parallel query, and the rest of it I did wrong when I
added disabled_nodes.

So attached are a couple of patches to try to improve things. 0001
fixes the lack of a disabled_nodes test in add_partial_path. 0002
rewrites teaches add_partial_path about startup cost, and also
rewrites add_partial_path_precheck to do the cost comparisons in the
same manner as we do in compare_path_costs_fuzzily. This fixes both
the failure of that function to consider disabled_nodes, and also the
failure of that function to consider startup_cost. These are formally
two separate bugs, but I do not really relish the idea of fixing them
separately, because it means rewriting add_partial_path_precheck()
twice, and I think the ending state will be the same as what I have
here, and the intermediate state will be of no use to anyone and
possibly buggy as well. The only argument I can see for trying to
separate the two fixes to add_partial_path_precheck() is of one of the
following two things is true:

(1) We want to back-patch the disable_cost portion of the fix. This
has the possibility of destabilizing plans in released branches, so I
assume this is not going to be very appealing.

(2) We don't actually want the changes to consider startup cost. In
that case, this will definitely need to be changed. There's certainly
an argument that considering the startup cost increases the number of
paths we retain and therefore increases planning cost, but I feel like
that argument should lose out to the counter-argument that what we're
doing now is based on the fiction that the startup cost *can't* matter
in the case of a partial path, which we now know is not true: James
Coleman found that originally when working on Incremental Sort, and we
now have a regression test query that demonstrates it. If we're going
to skip considering startup cost in certain cases to reduce path
explosion, it should be based on a principled design choice, which
doesn't appear to be the case here. Rather, it appears to be based on
vintage-2016 Robert writing some stuff that wasn't true, and then we
kinda just rolled with it.

--
Robert Haas
EDB: http://www.enterprisedb.com