CREATE OR REPLACE FUNCTION tag_path(in_tag_id INTEGER) RETURNS text AS 
$$
DECLARE
 r text;
 id INTEGER;
BEGIN
  IF in_tag_id IS NULL THEN
    RETURN null;
  END IF;
  SELECT name, parent_id INTO r,id FROM tags WHERE tag_id=in_tag_id;
  IF (id IS NULL) THEN
     RETURN r;
  ELSE
     RETURN tag_path(id)||'->'||coalesce(r,'');
  END IF;
END;
$$ LANGUAGE plpgsql STABLE;