Re: [PATCH] Incremental sort (was: PoC: Partial sort) - Mailing list pgsql-hackers

On Mon, Jul 08, 2019 at 12:07:06PM -0400, James Coleman wrote:
>On Mon, Jul 8, 2019 at 10:58 AM Tomas Vondra
><tomas.vondra@2ndquadrant.com> wrote:
>>
>> On Mon, Jul 08, 2019 at 10:32:18AM -0400, James Coleman wrote:
>> >On Mon, Jul 8, 2019 at 9:59 AM Tomas Vondra
>> ><tomas.vondra@2ndquadrant.com> wrote:
>> >>
>> >> On Mon, Jul 08, 2019 at 09:22:39AM -0400, James Coleman wrote:
>> >> >On Sun, Jul 7, 2019 at 5:02 PM Tomas Vondra
>> >> ><tomas.vondra@2ndquadrant.com> wrote:
>> >> >> We're running query like this:
>> >> >>
>> >> >>   SELECT a, sum(b), count(*) FROM pagg_tab_ml GROUP BY a HAVING avg(b) < 3 ORDER BY 1, 2, 3
>> >> >>
>> >> >> but we're trying to add the incremental sort *before* the aggregation,
>> >> >> because the optimizer also considers group aggregate with a sorted
>> >> >> input. And (a) is a prefix of (a,sum(b),count(b)) so we think we
>> >> >> actually can do this, but clearly that's nonsense, because we can't
>> >> >> possibly know the aggregates yet. Hence the error.
>> >> >>
>> >> >> If this is the actual issue, we need to ensure we actually can evaluate
>> >> >> all the pathkeys. I don't know how to do that yet. I thought that maybe
>> >> >> we should modify pathkeys_common_contained_in() to set presorted_keys to
>> >> >> 0 in this case.
>> >> >>
>> >> >> But then I started wondering why we don't see this issue even for
>> >> >> regular (non-incremental-sort) paths built in create_ordered_paths().
>> >> >> How come we don't see these failures there? I've modified costing to
>> >> >> make all incremental sort paths very cheap, and still nothing.
>> >> >
>> >> >I assume you mean you modified costing to make regular sort paths very cheap?
>> >> >
>> >>
>> >> No, I mean costing of incremental sort paths, so that they end up being
>> >> the cheapest ones. If some other path is cheaper, we won't see the error
>> >> because it only happens when building plan from the cheapest path.
>> >
>> >Ah, I misunderstood as you trying to figure out a way to try to cause
>> >the same problem with a regular sort.
>> >
>> >> >> So presumably there's a check elsewhere (either implicit or explicit),
>> >> >> because create_ordered_paths() uses pathkeys_common_contained_in() and
>> >> >> does not have the same issue.
>> >> >
>> >> >Given this comment in create_ordered_paths():
>> >> >
>> >> >  generate_gather_paths() will have already generated a simple Gather
>> >> >  path for the best parallel path, if any, and the loop above will have
>> >> >  considered sorting it.  Similarly, generate_gather_paths() will also
>> >> >  have generated order-preserving Gather Merge plans which can be used
>> >> >  without sorting if they happen to match the sort_pathkeys, and the loop
>> >> >  above will have handled those as well.  However, there's one more
>> >> >  possibility: it may make sense to sort the cheapest partial path
>> >> >  according to the required output order and then use Gather Merge.
>> >> >
>> >> >my understanding is that generate_gather_paths() only considers paths
>> >> >that already happen to be sorted (not explicit sorts), so I'm
>> >> >wondering if it would make more sense for the incremental sort path
>> >> >creation for this case to live alongside the explicit ordered path
>> >> >creation in create_ordered_paths() rather than in
>> >> >generate_gather_paths().
>> >> >
>> >>
>> >> How would that solve the issue? Also, we're building a gather path, so
>> >> I think generate_gather_paths() is the right place where to do it. And
>> >> we're not changing the semantics of generate_gather_paths() - the result
>> >> path should be sorted "correctly" with respect to sort_pathkeys.
>> >
>> >Does that imply what the explicit sort in create_ordered_paths() is in
>> >the wrong spot?
>> >
>>
>> I think those are essentially the right places where to do this sort of
>> stuff. Maybe there's a better place, but I don't think those places are
>> somehow wrong.
>>
>> >Or, to put it another way, do you think that both kinds of sorts
>> >should be added in the same place? It seems confusing to me that
>> >they'd be split between the two methods (unless I'm completely
>> >misunderstanding how the two work).
>> >
>>
>> The paths built in those two places were very different in one aspect:
>>
>> 1) generate_gather_paths only ever looked at pathkeys for the subpath, it
>> never even looked at ordering expected by paths above it (or the query as
>> a whole). Plain Gather ignores pathkeys entirely, Gather Merge only aims
>> to maintain ordering of the different subpaths.
>>
>> 2) create_oredered_paths is meant to enforce ordering needed by upper
>> parts of the plan - either by using a properly sorted path, or adding an
>> explicit sort.
>>
>>
>> We want to extend (1) to also look at ordering expected by the upper parts
>> of the plan, and consider incremental sort if applicable. (2) already does
>> that, and it already has the correct pathkeys to enforce.
>
>I guess I'm still not following. If (2) is responsible (currently) for
>adding an explicit sort, why wouldn't adding an incremental sort be an
>example of that responsibility? The subpath that either a Sort or
>IncrementalSort is being added on top of (to then feed into the
>GatherMerge) is the same in both cases right?
>
>Unless you're saying that the difference is that since we have a
>partial ordering already for incremental sort then incremental sort
>falls into the category of "maintaining" existing ordering of the
>subpath?
>

Oh, I think I understand what you're saying. Essentially, we should not
be making generate_gather_paths responsible for adding the incremental
sort. Instead, we should be looking at places than are adding explicit
sort (using create_sort_path) and also consider adding incremental sort.

I definitely agree with the second half - we should look at all places
that create explicit sorts and make them also consider incremental
sorts. That makes sense.

But I'm not sure it'll address all cases - the problem is that those
places add the explicit sort because they need sorted input. Gather
Merge does not do that, it only preserves existing ordering of paths.

So it's possible the path does not have an explicit sort on to, and
gather merge will not know to add it. And once we have the gather merge
in place, we can't push place "under" it.

In fact, we already have code dealing with this "issue" for a special
case - see gather_grouping_paths(). It generates plain gather merge
paths, but then also considers building one with explicit sort. But it
only does that for grouping paths (when it's clear we need to be looking
at grouping_pathkeys), and there are generate_gather_paths() that don't
have similar treatment.


>> But looking at root->sort_pathkeys in (1) seems to be the wrong thing :-(
>>
>> The thing is, we don't have just sort_pathkeys, there's distinct_pathkeys
>> and group_pathkeys too, so maybe we should be looking at those too?
>
>I don't know enough yet to answer, but I'd like to look at (in the
>debugger) the subpaths considered in each function to try to get a
>better understanding of why we don't try to explicitly sort the aggs
>(which we know we can't sort yet) but do for incremental sort. I
>assume that means a subpath has to be present in one but not the other
>since they both use the same pathkey checking function.
>

I've been wondering if we have some other code that needs to consider
interesting pathkeys "candidates" (instead of just getting the list
interesting in that place). Because then we could look at that code and
use it here ...

And guess what - postgres_fdw needs to do pretty much exactly that, when
building paths for remote relations. AFAIK we can't easily request all
plans from the remote node and then look at their pathkeys (like we'd do
with local node), so instead we deduce "interesting pathkeys" and then
request best plans for those. And deducing "interesing" pathkeys is
pretty much what get_useful_pathkeys_for_relation() is about.

So I've copied this function (and two more, called from it), whacked it
a bit until it removed (shakespeare-writing chimp comes to mind) and
voila, it seems to be working. The errors you reported are gone, and the
plans seems to be reasonable.

Attached is a sequence of 4 patches:


0001-fix-pathkey-processing-in-generate_gather_paths.patch
----------------------------------------------------------
This is the fixed version of my previous patch, with the stuff stolen
from postgres_fdw.


0002-fix-costing-in-cost_incremental_sort.patch
-----------------------------------------------
This is the costing fix, I mentioned before.


0003-fix-explain-in-parallel-mode.patch
---------------------------------------
Minor bug in explain, when incremental sort ends up being in the
parallel part of the plan (missing newline on per-worker line)


0004-rework-where-incremental-sort-paths-are-created.patch
----------------------------------------------------------
This undoes the generate_gather_paths() changes from 0001, and instead
modifies a bunch of places that call create_sort_path() to also consider
incremental sorts. There are a couple remaining, but those should not be
relevant to the queries I've been looking at.


Essentially, 0002 and 0003 are bugfixes. 0001 and 0004 are the two
different aproaches to building incremental sort + gather merge.

Now, consider this example:

  create table t (a int, b int, c int);
  insert into t select mod(i,100),mod(i,100),i from generate_series(1,10000000) s(i);
  create index on t (a);
  analyze t;
  explain select a,b,sum(c) from t group by 1,2 order by 1,2,3 limit 1;

With 0001+0002+0003 pathes, I get a plan like this:

                                                     QUERY PLAN                                                     
--------------------------------------------------------------------------------------------------------------------
 Limit  (cost=10375.39..10594.72 rows=1 width=16)
   ->  Incremental Sort  (cost=10375.39..2203675.71 rows=10000 width=16)
         Sort Key: a, b, (sum(c))
         Presorted Key: a, b
         ->  GroupAggregate  (cost=10156.07..2203225.71 rows=10000 width=16)
               Group Key: a, b
               ->  Gather Merge  (cost=10156.07..2128124.39 rows=10000175 width=12)
                     Workers Planned: 2
                     ->  Incremental Sort  (cost=9156.04..972856.05 rows=4166740 width=12)
                           Sort Key: a, b
                           Presorted Key: a
                           ->  Parallel Index Scan using t_a_idx on t  (cost=0.43..417690.30 rows=4166740 width=12)
(12 rows)


and with 0004, I get this:

                                              QUERY PLAN                                              
------------------------------------------------------------------------------------------------------
 Limit  (cost=20443.84..20665.32 rows=1 width=16)
   ->  Incremental Sort  (cost=20443.84..2235250.05 rows=10000 width=16)
         Sort Key: a, b, (sum(c))
         Presorted Key: a, b
         ->  GroupAggregate  (cost=20222.37..2234800.05 rows=10000 width=16)
               Group Key: a, b
               ->  Incremental Sort  (cost=20222.37..2159698.74 rows=10000175 width=12)
                     Sort Key: a, b
                     Presorted Key: a
                     ->  Index Scan using t_a_idx on t  (cost=0.43..476024.65 rows=10000175 width=12)
(10 rows)

Notice that cost of the second plan is almost double the first one. That
means 0004 does not even generate the first plan, i.e. there are cases
where we don't try to add the explicit sort before passing the path to
generate_gather_paths().

And I think I know why is that - while gather_grouping_paths() tries to
add explicit sort below the gather merge, there are other places that
call generate_gather_paths() that don't do that. In this case it's
probably apply_scanjoin_target_to_paths() which simply builds

   parallel (seq|index) scan + gather merge

and that's it. The problem is likely the same - the code does not know
which pathkeys are "interesting" at that point. We probably need to
teach planner to do this.


FWIW tweaking all the create_sort_path() places to also consider adding
incremental sort is a bit tedious and invasive, and it almost doubles
the amount of repetitive code. It's OK for experiment like this, but we
should try handling this in a nicer way (move to a separate function
that does both, or something like that).


regards

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