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

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?

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).

I'm not saying it would solve the issue here; just noting that the
division of labor seemed odd to me at first read through.

James Coleman