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 | Sub Practice | 30

I've written a function as this

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