Re: BUG #19487: Error while executing SQL query involving XML parsing

Andrey Rachitskiy <pl0h0yp1(at)gmail(dot)com> writes:
> # Proposal Fix
> Fold constant `XmlExpr` in `eval_const_expressions` (same path as
> `MinMaxExpr` / `ArrayExpr`), so `IS DOCUMENT` becomes a boolean Const and
> CASE drops the unused arm before simplifying `xpath()`.

I don't object to simplifying XmlExpr where possible, but this patch
appears to believe that every variant of XmlExpr is immutable.
This is demonstrably false, eg

regression=# set timezone = 'America/New_York';
SET
regression=# SELECT xmlforest('2026-08-28 12:00 UTC'::timestamptz AS foo);
xmlforest
--------------------------------------
<foo>2026-08-28T08:00:00-04:00</foo>
(1 row)

regression=# set timezone = 'America/Chicago';
SET
regression=# SELECT xmlforest('2026-08-28 12:00 UTC'::timestamptz AS foo);
xmlforest
--------------------------------------
<foo>2026-08-28T07:00:00-05:00</foo>
(1 row)

So you need to take a closer look at which variants can be promised to
be immutable.

regards, tom lane