Re: enable_incremental_sort changes query behavior

On Fri, Oct 2, 2020 at 10:53 AM James Coleman <jtc331@gmail.com> wrote:
>
> On Fri, Oct 2, 2020 at 10:32 AM Tomas Vondra
> <tomas.vondra@2ndquadrant.com> wrote:
> >
> > On Fri, Oct 02, 2020 at 09:19:44AM -0400, James Coleman wrote:
> > >
> > > ...
> > >
> > >I've been able to confirm that the problem goes away if we stop adding
> > >the gather merge paths in generate_useful_gather_paths().
> > >
> > >I'm not sure yet what conclusion that leads us to. It seems to be that
> > >the biggest clue remains that all of this works correctly unless one
> > >of the selected columns (which happens to be a pathkey at this point
> > >because it's a DISTINCT query) contains a volatile expression.
> > >
> >
> > Yeah. It seems to me this is a bug in get_useful_pathkeys_for_relation,
> > which is calling find_em_expr_for_rel and is happy with anything it
> > returns. But this was copied from postgres_fdw, which however does a bit
> > more here:
> >
> >      if (pathkey_ec->ec_has_volatile ||
> >          !(em_expr = find_em_expr_for_rel(pathkey_ec, rel)) ||
> >          !is_foreign_expr(root, rel, em_expr))
> >
> > So not only does it explicitly check volatility of the pathkey, it also
> > calls is_foreign_expr which checks the expression for mutable functions.
> >
> > The attached patch seems to fix this, but it only adds the check for
> > mutable functions. Maybe it should check ec_has_volatile too ...
>
> We actually discussed the volatility check in that function back on
> the original thread [1], and we'd concluded that was specifically
> necessary for the fdw code because the function would execute on two
> different servers (and thus produce different results), but that in a
> local server only scenario it should be fine.
>
> My understanding (correct me if I'm wrong) is that the volatile
> function should only be executed once (at the scan level?) to build
> the tuple and from then on the datum isn't going to change, so I'm not
> sure why the volatility would matter here.
>
> James
>
> 1: https://www.postgresql.org/message-id/20200328025830.6v6ogkseohakc32q%40development

Oh, hmm, could what I said all be true, but there still be some rule
that you shouldn't compare datums generated from volatile expressions
in different backends (i.e., parallel query)?

James