I could not find any discussion on this but imho this seems an erroneous
error occuring now in 7.3.4 (after upgrade from 7.2.3):
select null::time, 'test'::varchar as time order by time;
ERROR: ORDER BY 'time' is ambiguous
The solution is to name the time datatype e.g.
select null::time as xyz, 'test'::varchar as time order by time;
Regards,
Alfred