Sounds good. I think this would need to be restricted by operator and datatype, since in general you won't know if the datatype functions need a snapshot or not. Immutable functions for the operators ought to do it, but I think that might not be enough.
It requires introduction of new "safe" functions (& operators). Immutable functions are not enough safe.
CREATE OR REPLACE FUNCTION public.fx() RETURNS integer LANGUAGE plpgsql IMMUTABLE AS $function$ BEGIN RETURN (SELECT count(*) FROM pg_class); END; $function$
That function is incorrectly marked as IMMUTABLE. In that situation, it's enough that we throw a sane error like "ERROR: no snapshot available".
Yes, it is incorrect mark. Unfortunately - this is often workaround for wrong estimations - so I afraid, in this case, your proposed fix breaks lot of applications.
