Will FUNCTION be DETERMINISTIC

Question

Hi, (my english is bad I apologize for errors)
I'd like to know if this function will really be deterministic for ORACLE?
...FUNCTION Wstart_Date(vDate DATE DEFAULT TRUNC(SYSDATE)) RETURN DATE DETERMINISTIC ...
(Returns the date of the first day of the week to which vDate belongs and if no date is given, take sysdate)
My view is:
Wstart_Date will return a different value each time I call it with NULL, because Default is applied BEFORE test for the 'DETERMINISTIC' clause of the function.
Am I right?
If I'm wrong, It should 'allways' return the same value each time I call it with NULL, because Oracle will first test the DETERMINISTIC clause, look at the known (null) parameter and 'take' the result from the cache for NULL.
Thanks.
Christian (From France)

Answer 1

As I understand it, the sole purpose of the DETEREMINISTIC clause is to tag the function as suitable for use with Function-Based Indexes and Materialized Views in query rewrite mode. As the documentation makes clear, Oracle actually has no way of telling whether the function is genuinely deterministic. If we say a function is deterministic and it ain't then it's our fault if the optimizer comes up with a cooked execution plan, but that's the only penalty.
Cheers, APC