Re: enable_incremental_sort changes query behavior

On Fri, Oct 2, 2020 at 11:16 PM James Coleman <jtc331(at)gmail(dot)com> wrote:
>
> On Fri, Oct 2, 2020 at 7:07 PM James Coleman <jtc331(at)gmail(dot)com> wrote:
> >
> > On Fri, Oct 2, 2020 at 6:28 PM Tomas Vondra
> > <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> > >
> > > On Fri, Oct 02, 2020 at 05:45:52PM -0400, James Coleman wrote:
> > > >On Fri, Oct 2, 2020 at 4:56 PM Tomas Vondra
> > > ><tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> > > >>
> > > >> ...
> > > >>
> > > >> More importanly, it does not actually fix the issue - it does fix that
> > > >> particular query, but just replacing the DISTINCT with either ORDER BY
> > > >> or GROUP BY make it fail again :-(
> > > >>
> > > >> Attached is a simple script I used, demonstrating these issues for the
> > > >> three cases that expect to have ressortgroupref != 0 (per the comment
> > > >> before TargetEntry in plannodes.h).
> > > >
> > > >So does checking for volatile expressions (if you happened to test
> > > >that) solve all the cases? If you haven't tested that yet, I can try
> > > >to do that this evening.
> > > >
> > >
> > > Yes, it does fix all the three queries in the SQL script.
> > >
> > > The question however is whether this is the root issue, and whether it's
> > > the right way to fix it. For example - volatility is not the only reason
> > > that may block qual pushdown. If you look at qual_is_pushdown_safe, it
> > > also blocks pushdown of leaky functions in security_barrier views. So I
> > > wonder if that could cause failures too, somehow. But I haven't managed
> > > to create such example.
> >
> > I was about to say that the issue here is slightly different from qual
> > etc. pushdown, since we're not concerned about quals here, and so I
> > wonder where we determine what target list entries to put in a given
> > scan path, but then I realized that implies (maybe!) a simpler
> > solution. Instead of duplicating checks on target list entries would
> > be safe, why not check directly in get_useful_pathkeys_for_relation()
> > whether or not the pathkey has a target list entry?
> >
> > I haven't been able to try that out yet, and so maybe I'm missing
> > something, but I need to step out for a bit, so I'll have to look at
> > it later.
>
> I've started poking at this, but haven't yet found a workable
> solution. See the attached patch which "solves" the problem by
> breaking putting the sort under the gather merge, but it at least
> demonstrates conceptually what I think we're interested in doing.
>
> The issue currently is that the comparison of expressions fails -- in
> our query, the first column selected shows up as a Var (with the same
> contents) but is a different pointer in the em_expr and the reltarget
> exprs list. I don't yet see a good way to get the equivalence class
> for a PathTarget entry.

Hmm, I think I was looking to do is the attached patch. I didn't
realize until I did a lot of reading through source that we have an
equal() function that can compare exprs. That (plus the realization in
[1] the originally reported backtrace wasn't where the error actually
came from) convinced me that what we need is to confirm not only that
the all of the vars in the ec member come from the relids in the rel
but also that the expr is actually represented in the target of the
rel.

With the gucs changed as I mentioned earlier both of the plans (with
and without a volatile call in the 2nd select entry) now look like:

Unique
-> Gather Merge
Workers Planned: 2
-> Sort
Sort Key: ref_0.i, (md5(ref_0.t))
-> Nested Loop
-> Parallel Index Scan using ref_0_i_idx on ref_0
-> Function Scan on generate_series ref_1

Without the gucs changed the minimal repro case now doesn't error, but
results in this plan:

HashAggregate
Group Key: ref_0.i, CASE WHEN pg_rotate_logfile_old() THEN ref_0.t
ELSE ref_0.t END
-> Nested Loop
-> Seq Scan on ref_0
-> Function Scan on generate_series ref_1

Similarly in your six queries I now only see parallel query showing up
in the last one.

I created an entirely new function because adding the target expr
lookup to the existing find_em_expr_for_rel() function broke a bunch
of postgres_fdw tests. That maybe raises questions about whether that
code also could have problems in theory/in the future, but I didn't
investigate further. In any case we already know it excludes
volatile...so maybe it's fine because in practice that's actually a
broader exclusion than what we're doing here.

This seems to fix the issue, but I'd like feedback on whether it's too
strict. We could of course just check em_has_volatile, but I'm
wondering if that's simultaneously too strict (by not allowing the
volatile expression to be computed in the gather merge supposing
there's no join) and too loose (maybe there are other cases we should
care about?). It also just strikes me as re-encoding rules that should
have already been applied (and thus we should be able to look up in
the data we have if it's safe to use the expr/pathkey). Those are
mostly intuitions though.

James

1: https://www.postgresql.org/message-id/CAAaqYe8zqDAv0Sfak5Riu%2BDKsm-i3ARPursn5v6qTwiCXmkXKQ%40mail.gmail.com