Strange behaviour ( function cachable )

I wrote some days ago about the behaviour that I notice
if I call a function cachable inside another not cachable,
I was wrong ?
I expected that if a function is cachable and is used
inside a select like:

SELECT * FROM foo WHERE id  = my_func( a_param )

then the function my_func is called one for each different value
of    a_param. But seems that this behaviour happen only in
stand alone query not if that query is executed inside another function.


Ciao
Gaetano