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

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.

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?

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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