Recursive function

Hi,
I have a table with the following details.
section_type_id | section_type_name | parent_section_type_id
-----------------+-------------------+------------------------             10 | Unit              |             20 |
Block            |                     10             30 | Practice          |                     20             40 |
SubPractice      |                     30 

I've written a function as this

CREATE or replace FUNCTION get_child_section_types(int) RETURNS setof int   AS $$    declare        v_section_type_id
aliasfor $1;        v_rec record;       begin        for v_rec in select section_type_id from master_section_type where 
parent_section_type_id=v_section_type_id loop            return next v_rec.section_type_id;        end loop;
return;      end; 
$$   LANGUAGE plpgsql;

which returns output like,

select * from get_child_section_types(10);get_child_section_types
-------------------------                     20

but I need the function to return all section types under the child nodes
too.
So, how to make the function to be recursive.

with thanks & regards,
S.Gnanavel