Re: BUG #19418: SQL/JSON JSON_VALUE() does not conform to ISO/IEC 9075-2:2023(E) 6.34 <JSON value constructor>

Hi Richard,

On Thu, Apr 16, 2026 at 3:05 PM Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
> On Tue, Mar 3, 2026 at 11:32 PM Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
> > On Tue, Mar 3, 2026 at 10:03 AM Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
> > > That is a good point I hadn't considered. So I think the ideal fix is
> > > to have the parser preserve the user's original JSON_ARRAY(query)
> > > syntax as much as possible, and then defer the JSON_ARRAYAGG rewrite
> > > trick to the planner, perhaps during expression preprocessing.
>
> > I tried hacking on this idea to see how it would look in practice, and
> > here is what I got.
>
> After a second look at this approach, I don't like it very much. It
> manually constructed the new querytree, including Aggref,
> RangeTblEntry, and JsonConstructorExpr nodes, during planning,
> bypassing parse analysis entirely. This is essentially repeating the
> parser's work by hand in the planner, which is fragile and prone to
> failing to handle all cases correctly.
>
> Maybe a simpler way is to keep the JSON_ARRAYAGG rewrite trick in the
> parser, as the current master does, but wrap the result in a COALESCE
> to handle the empty-set case. We can preserve a copy of the user's
> original subquery in a new field of JsonConstructorExpr, and then
> ruleutils.c can use this field to deparse the original
> JSON_ARRAY(SELECT ...) syntax for view definitions. You may think
> this would introduce extra transform work, but it wouldn't: the
> current master already transforms the original subquery to validate
> the single-column constraint, then throws the result away. We simply
> keep it instead.
>
> I tried this idea and ended up with the attached.

Agreed that v4 is the better direction.

A couple of minor nits.

The comment on orig_query could say "not walked" a bit more helpfully, e.g.

Node *orig_query; /* for deparse only; not walked (func is) */

I also noticed that the comment for 'func' is incomplete as it is and
this change warrants an update. Maybe a bit long, but how about:

Expr *func; /* expression producing the result:
* Aggref/WindowFunc for *AGG,
* CoalesceExpr for ARRAY_QUERY,
* json[b]_xxx() call for remaining types */

--
Thanks, Amit Langote