Re: Incremental sorts and EXEC_FLAG_REWIND

On Wed, Apr 15, 2020 at 2:04 PM James Coleman <jtc331(at)gmail(dot)com> wrote:
>
> On Wed, Apr 15, 2020 at 11:02 AM James Coleman <jtc331(at)gmail(dot)com> wrote:
> >
> > On Tue, Apr 14, 2020 at 2:53 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> > >
> > > Hi,
> > >
> > > When initializing an incremental sort node, we have the following as
> > > of ExecInitIncrementalSort():
> > > /*
> > > * Incremental sort can't be used with either EXEC_FLAG_REWIND,
> > > * EXEC_FLAG_BACKWARD or EXEC_FLAG_MARK, because we only one of many sort
> > > * batches in the current sort state.
> > > */
> > > Assert((eflags & (EXEC_FLAG_BACKWARD |
> > > EXEC_FLAG_MARK)) == 0);
> > > While I don't quite follow why EXEC_FLAG_REWIND should be allowed here
> > > to begin with (because incremental sorts don't support rescans without
> > > parameter changes, right?), the comment and the assertion are telling
> > > a different story.
> >
> > I remember changing this assertion in response to an issue I'd found
> > which led to rewriting the rescan implementation, but I must have
> > missed updating the comment.
>
> All right, here are the most relevant messages:
>
> [1]: Here I'd said:
> ----------
> While working on finding a test case to show rescan isn't implemented
> properly yet, I came across a bug. At the top of
> ExecInitIncrementalSort, we assert that eflags does not contain
> EXEC_FLAG_REWIND. But the following query (with merge and hash joins
> disabled) breaks that assertion:
>
> select * from t join (select * from t order by a, b) s on s.a = t.a
> where t.a in (1,2);
>
> The comments about this flag in src/include/executor/executor.h say:
>
> * REWIND indicates that the plan node should try to efficiently support
> * rescans without parameter changes. (Nodes must support ExecReScan calls
> * in any case, but if this flag was not given, they are at liberty to do it
> * through complete recalculation. Note that a parameter change forces a
> * full recalculation in any case.)
>
> Now we know that except in rare cases (as just discussed recently up
> thread) we can't implement rescan efficiently.
>
> So is this a planner bug (i.e., should we try not to generate
> incremental sort plans that require efficient rewind)? Or can we just
> remove that part of the assertion and know that we'll implement the
> rescan, albeit inefficiently? We already explicitly declare that we
> don't support backwards scanning, but I don't see a way to declare the
> same for rewind.
> ----------
>
> So it seems to me that we can't disallow REWIND, and we have to
> support rescan, but, we could try to mitigate the effects (without a
> param change) with a materialize node, as noted below.
>
> [2]: Here, in response to my questioning above if this was a planner
> bug, I'd said:
> ----------
> Other nodes seem to get a materialization node placed above them to
> support this case "better". Is that something we should be doing?
> ----------
>
> I never got any reply on this point; if we _did_ introduce a
> materialize node here, then it would mean we could start disallowing
> REWIND again. See the email for full details of a specific plan that I
> encountered that reproduced this.
>
> Thoughts?
>
> > In the meantime, your question is primarily about making sure the
> > code/comments/etc. are consistent and not a behavioral problem or
> > failure you've seen in testing?
>
> Still want to confirm this is the case.
>
> James
>
> [1]: https://www.postgresql.org/message-id/CAAaqYe9%2Bap2SbU_E2WaC4F9ZMF4oa%3DpJZ1NBwaKDMP6GFUA77g%40mail.gmail.com
> [2]: https://www.postgresql.org/message-id/CAAaqYe-sOp2o%3DL7nvGZDJ6GsL9%3Db_ztrGE1rhyi%2BF82p3my2bQ%40mail.gmail.com

Looking at this more, I think this is definitely suspect. The current
code shields lower nodes from EXEC_FLAG_BACKWARD and EXEC_FLAG_MARK --
the former is definitely fine because we declare that we don't support
backwards scans. The latter seems like the same reasoning would apply,
but unfortunately we didn't add it to ExecSupportsMarkRestore, so I've
attached a patch to do that.

The EXEC_FLAG_REWIND situation though I'm still not clear on -- given
the comments/docs seem to suggest it's a hint for efficiency rather
than something that has to work or be declared as not implemented, so
it seems like one of the following should be the outcome:

1. "Disallow" it by only generating materialize nodes above the
incremental sort node if REWIND will be required. I'm not sure if this
would mean that incremental sort just wouldn't be useful in that case?
2. Keep the existing implementation where we basically ignore REWIND
and use our more inefficient implementation. In this case, I believe
we need to stop shielding child nodes from REWIND, though, since we we
aren't actually storing the full result set and will instead be
re-executing the child nodes.

I've attached a patch to take course (2), since it's the easiest to
implement. But I'd still like feedback on what we should do here,
because I don't feel like I actually know what the semantics expected
of the executor/planner are on this point. If we do go with this
approach, someone should verify my comments additions about
materialize nodes is correct.

James