Сэйлормунне.
такое
create function aaa (p1 int default null)

или

create function aaa (p1 int = null)

35.11. Porting from Oracle PL/SQL
The main differences you should keep in mind when porting from PL/SQL to PL/pgSQL are:
There are no default values for parameters in PostgreSQL.

You can overload function names in PostgreSQL. This is often used to work around the lack of default parameters.

No need for cursors in PL/pgSQL, just put the query in the FOR statement. (See Example 35.5, “Porting a Function that Creates Another Function from PL/SQL to PL/pgSQL”.)

In PostgreSQL you need to use dollar quoting or escape single quotes in the function body. See Section 35.2.1, “Handling of Quotation Marks”.

Instead of packages, use schemas to organize your functions into groups.

Since there are no packages, there are no package-level variables either. This is somewhat annoying. You may be able to keep per-session state in temporary tables, instead.