Does PG cache results of an aggregate function, (and results of non-volatile functions)?

Hi,
I do have a query which make use of the results of an aggregate
function (for example bit_or) several times in the output column list
of the SELECT clause, does PostgreSQL simply execute the aggregate
function only once and provide the output to the other calls to the
same aggregate function.
How about the case of non volatile functions? Do they get executed as
many times as they occur in the select clause?

Allan.