Du skulle använda ett rekursivt vanligt tabelluttryck som helt enkelt "bär" roten genom rekursionsnivåerna:
with recursive fg_tree as (
select fg_id,
fg_id as fg_clasifier_id -- <<< this is the "root"
from fg
where parent_fg_id is null -- <<< this is the "start with" part
union all
select c.fg_id,
p.fg_clasifier_id
from fg c
join fg_tree p on p.fg_id = c.parent_fg_id -- <<< this is the "connect by" part
)
select *
from fg_tree;
Mer information om rekursiva vanliga tabelluttryck i manualen:http:// www.postgresql.org/docs/current/static/queries-with.html