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From: Thom Brown <thom@linux.com>
Date: Tue, 7 Jul 2026 15:20:14 +0100
Message-ID: 
 <CAA-aLv6F_ebaU6Czuht5NTRQog0r-TpR9emV-qDKUnGe=eR8Lw@mail.gmail.com>
Subject: Re: EXPLAIN (VERBOSE) fails with for JSON_ARRAYAGG/JSON_OBJECTAGG +
 window function
To: Richard Guo <guofenglinux@gmail.com>
Cc: pgsql-bugs <pgsql-bugs@postgresql.org>
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On Tue, 7 Jul 2026 at 14:49, Thom Brown <thom@linux.com> wrote:
>
> On Mon, 6 Jul 2026 at 03:17, Richard Guo <guofenglinux@gmail.com> wrote:
> >
> > On Fri, Jul 3, 2026 at 5:36=E2=80=AFPM Thom Brown <thom@linux.com> wrot=
e:
> > > Thanks for taking a look at this. It's unfortunate that reconstructin=
g
> > > the syntax is problematic, because in the case of the original query
> > > that tripped on this bug, I lose the following from my original query=
:
> > > ...
> > > I guess the relevant information can be gleaned from this, but not fo=
r
> > > JSON_ARRAYAGG(i RETURNING text) because we get the same output whethe=
r
> > > we're returning text or json.
> >
> > Yeah, the v1 patch is lossy, and the lost information cannot be
> > recovered from the plan at all.
> >
> > So I'm switching back to the resolve_special_varno() reconstruction I
> > had tried and set aside.  My earlier objection was that it makes the
> > WindowAgg look like it computes a JSON aggregate when it only passes
> > through a value computed below.  But the lower node still prints
> > jsonb_agg_strict(name), so the plan does show where the aggregate is
> > computed, and I think that is a smaller cost than losing the
> > information outright.  Also, partial aggregation already deparses this
> > way.  The combining aggregate's argument is a Var referencing the
> > partial aggregate's output, and get_agg_expr() resolves it back with
> > resolve_special_varno() to reprint the aggregate at the finalizing
> > node.
> >
> > I still don't want to hack the planner to keep the JsonConstructorExpr
> > with its Aggref in make_window_input_target(), because I still think
> > that that is too invasive and changes which node evaluates the
> > wrapper.
> >
> > Hence, I end up with the attached v2 patch.
>
> I've tested it against the output from the 1st patch.
>
> Patch 1:
> ((jsonb_agg_strict(e.emp_name ORDER BY e.emp_name)))
>
> Patch 2:
> (JSON_ARRAYAGG(e.emp_name ORDER BY e.emp_name RETURNING jsonb))
>
> 1:
> ((jsonb_object_agg_strict(e.emp_name, e.salary)))
>
> 2:
> (JSON_OBJECTAGG(e.emp_name : e.salary ABSENT ON NULL RETURNING jsonb))
>
> With that last one, why is that colon there?

Actually, I don't know what I was thinking. This is clearly a
key:value pairing, so ignore that remark. But we do get more info with
this line in the 2nd patch.

So all looks good to me.

Thanks

Thom