Problem with timestamp

Hello,

I try to check the time needed by a function.
I would like to :
. select current_timestamp,
. execute 5000 times a function
. select the current_timestamp and return the interval.
Problem : the interval is always '00:00', the two timestamps have always
the same value.
Could you help me ? have you any suggestions for testing sql functions
speed?
Thank you in advance,
Béatrice

create function test_function() returns interval
as '
declare
j integer;
t1 timestamp;
t2 timestamp;
t3 interval;
x integer;
begin
select into t1 now();
FOR j in 0..5000 loop
select into x get_function_to_test();
end loop;
select into t2 now();
t3 := t2 - t1;
RAISE NOTICE '' from % to % = %'',t2,t1,t3;
return t3;

end;
' language 'plpgsql';