create table objects (
    id int primary key,
    parent_id int,
    label text,
    content text);

insert into objects values
(1, 4, 'f', 'data for f'),
(2, 5, 'e', 'data for e'),
(3, 5, 'd', 'data for d'),
(4, 6, 'c', ''),
(5, 6, 'b', ''),
(6, 0, 'a', '');

with recursive nodes(id, path, content) as (
    select id, label, content
    from objects
    where parent_id = 0
union all
    select o.id, concat(path, '->', label), o.content
    from objects o
    join nodes n on n.id = o.parent_id
)
select *
from nodes
order by id desc;

 id |  path   |  content   
----+---------+------------
  6 | a       | 
  5 | a->b    | 
  4 | a->c    | 
  3 | a->b->d | data for d
  2 | a->b->e | data for e
  1 | a->c->f | data for f
(6 rows)