Your First PL/pgSQL Function - PostgreSQL Server Programming - page 69

DECLARE a decimal(1000,0) := 0;
DECLARE b decimal(1000,0) := 1;
BEGIN
IF (n < 1) THEN
RETURN 0;
END IF;
LOOP
EXIT WHEN counter = n;
counter := counter + 1;
SELECT b,a+b INTO a,b;
END LOOP;
RETURN a;
END;
$$
LANGUAGE plpgsql;
SELECT fib(4);