Functions can be declared with parameters with default values or
expressions.  The default expressions are used as parameter value
if the parameter is not explicitly specified in a function call.
All parameters after a parameter with default value have to be
parameters with default values as well.

It allows user to invoke a UDF without setting all the parameters.
Two examples to demo its usage:

    CREATE FUNCTION dfunc1(text DEFAULT 'Hello', text DEFAULT 'World')
        RETURNS text AS $$
        SELECT $1 || ', ' || $2;
        $$ LANGUAGE SQL;
    SELECT dfunc1();  -- 'Hello, World'
    SELECT dfunc1('Hi');  -- 'Hi, World'
    SELECT dfunc1('Hi', 'Beijing');  -- 'Hi, Beijing'

    CREATE FUNCTION dfunc2(id int4, t timestamp DEFAULT now())
        RETURNS text AS $$
        SELECT 'Time for id:' || $1 || ' is ' || $2;
        $$ LANGUAGE SQL;
    SELECT dfunc2(24);  -- 'Time for id:24 is 2016-01-07 14:38'

NOTE: The default change set is ported from from PostgreSQL 8.4,
    original commits:
    '517ae403'
    '455dffbb'