postgresで同等のconnect_by_root

再帰レベルを介してルートを単純に「運ぶ」再帰共通テーブル式を使用します。

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;

マニュアルの再帰的な共通テーブル式の詳細： http：// www.postgresql.org/docs/current/static/queries-with.html