create type t_order_item as (item_number int, price numeric, quantity numeric, ... );
create type t_order as (order_id int, ..., items t_order_item[]);
function get_order(p_order_id int)
returns t_order_header

select unnest(res.items::t_order_item[]) from get_order(566) res;

select (e).*
from 
(select unnest(res.items::t_order_item[]) as e
   from get_order(566) res
) t;

select res.*
from unnest(((get_order(566)).items)) res;