"Albe Laurenz" <laurenz(dot)albe(at)wien(dot)gv(dot)at> writes:
> I think the problem is that this + operator is implemented
> by the function "timestamptz_pl_interval", which is STABLE
> but not IMMUTABLE.
>
> I am not sure why this function cannot be IMMUTABLE, it
> seems to me that it should be.
No: the result of e.g.
SELECT TIMESTAMPTZ '2012-10-28 01:30:00' + INTERVAL '24 hours';
depends on the client's timezone and its DST rules.