Re: postgres_fdw returns wrong rows for age(timestamptz, timestamptz) filters

On Fri, 2026-10-30 at 02:30 +0530, Gaurav Singh wrote:
> age(timestamptz, timestamptz) is marked IMMUTABLE, but its result
> depends on the TimeZone setting, because it converts both values to
> local time before subtracting:
>
> SET TimeZone = 'UTC';
> SELECT age('2026-03-01 00:00+00'::timestamptz, '2026-01-31 00:00+00'::timestamptz);
> -- 1 mon 1 day
>
> SET TimeZone = 'America/New_York';
> SELECT age('2026-03-01 00:00+00'::timestamptz, '2026-01-31 00:00+00'::timestamptz);
> -- 29 days
>
> Because of the IMMUTABLE label, postgres_fdw considers age() safe to
> send to the remote server. The remote session runs with
> timezone = 'GMT', so the filter is evaluated in GMT instead of the
> local session's time zone, and the query returns wrong rows.
>
> The mislabel itself was raised recently in the BUG #19644 thread,
> where the concern was that relabeling it STABLE would break existing
> generated columns and indexes.
>
> Would it make sense to mark age(timestamptz, timestamptz) STABLE on
> master, or, if that is too disruptive, to stop postgres_fdw from
> shipping it?

I think that the best solution would be to change the result of the
age() function: if it didn't try to convert to months and just
returned days and smaller, the result should be immutable.

Yours,
Laurenz Albe