"Merlin Moncure" <mmoncure(at)gmail(dot)com> writes:
> select f(x) from t where id = 1 order by n;
> can cause f to execute for the entire table even if id is unique.
Really? I'd consider it a bug if so. Compare
select 1/x from t where x > 0
If the presence of zeroes in t can make this throw a zero-divide error,
the database is broken. In my mind the SQL spec is perfectly clear that
WHERE filtering occurs before evaluation of the SELECT targetlist.
(Sorting, however, occurs afterward --- so there are certainly potential
gotchas of this ilk. But the specific example you give is bogus.)
regards, tom lane