query to select a linked list - Mailing list pgsql-sql

Hi,

To build a threaded forum application I came up the following schema:

forum
------
id_forum | integer| not null  default nextval('forum_id_forum_seq'::regclass)
id_parent| integer| 
subject  | text   | not null
message  | text   | 

Each message a unique id_forum and an id_parent pointing to the replied 
post (empty if first post).

How can I build an elegant query to select all messages in a thread?

Thanks,