test=*# select * from frosch ;
 id | parent_id | t
----+-----------+---
  1 |           | a
  2 |         1 | b
  3 |         2 | c
  4 |         3 | d
  5 |         4 | e
  6 |           | f
  7 |         6 | g
  8 |         7 | h
  9 |         8 | i
(9 rows)

Time: 0,274 ms
test=*# with recursive r as (select id, parent_id, t, 0::int as c from frosch where parent_id is null and id=1 union all select f.id, f.parent_id, r.t || f.t, c+1 from frosch f inner join r on (f.parent_id=r.id)) select * from r;
 id | parent_id |   t   | c
----+-----------+-------+---
  1 |           | a     | 0
  2 |         1 | ab    | 1
  3 |         2 | abc   | 2
  4 |         3 | abcd  | 3
  5 |         4 | abcde | 4
(5 rows)

test=*# with recursive r as (select id, parent_id, t, 0::int as c from frosch where parent_id is null union all select f.id, f.parent_id, r.t || f.t, c+1 from frosch f inner join r on (f.parent_id=r.id)) select * from r order by id;
 id | parent_id |   t   | c
----+-----------+-------+---
  1 |           | a     | 0
  2 |         1 | ab    | 1
  3 |         2 | abc   | 2
  4 |         3 | abcd  | 3
  5 |         4 | abcde | 4
  6 |           | f     | 0
  7 |         6 | fg    | 1
  8 |         7 | fgh   | 2
  9 |         8 | fghi  | 3
(9 rows)