PL/PGSQL Function Overloading

PostgreSQL allows more than one function to have the same name, so long as the arguments are different. If more than one function has the same name, we say those functions are overloaded. When a function is called, PostgreSQL determines the exact function is being called based on the input arguments.
Let’s take a look at the following get_rental_duration() function.

Input:

CREATE OR REPLACE FUNCTION get_rental_duration(p_customer_id INTEGER)
RETURNS INTEGER AS $$
DECLARE
rental_duration INTEGER;
BEGIN
SELECT INTO rental_duration SUM( EXTRACT( DAY FROM return_date - rental_date))
FROM rental
WHERE customer_id=p_customer_id;
RETURN rental_duration;
END; $$
LANGUAGE plpgsql;

Output:

SELECT get_rental_duration(232);

Input:

CREATE OR REPLACE FUNCTION get_rental_duration(p_customer_id INTEGER, p_from_date DATE)
RETURNS INTEGER AS $$
DECLARE
rental_duration integer;
BEGIN
SELECT INTO rental_duration
SUM( EXTRACT( DAY FROM return_date + '12:00:00' - rental_date))
FROM rental
WHERE customer_id= p_customer_id AND
rental_date >= p_from_date;
RETURN rental_duration;
END; $$
LANGUAGE plpgsql;

Output:

SELECT get_rental_duration(232,'2005-07-01');