spurious function execution in prepared statements.

OK, I have a situation that might be a performance problem, a bug, or an
unavoidable consequence of using prepared statements.  The short version
is that I am getting function executions for rows not returned in a
result set when they are in a prepared statement.

In other words, I have a query:
select f(t.c) from t where [boolean expr on t] limit 1;

because of the limit phrase, obviously, at most one record is returned
and f executes at most once regardless of the plan used (in practice,
sometimes index, sometimes seq_scan.

Now, if the same query is executed as a prepared statement,
prepare ps(...) as select f(t.c) from t where [expr] limit 1;
execute ps;

now, if ps ends up using a index scan on t, everything is ok.  However,
if ps does a seqscan, f executes for every row on t examined until the
[expr] criteria is met.  Is this a bug?  If necessary I should be able
to set up a reproducible example.  The easy workaround is to not use
prepared statements in these situations, but I need to be able to
guarantee that f only executes once (even if that means exploring
subqueries).

Merlin