Retrieving full hierarchy sorted by a column under PostgreSQL's Ltree module

Question

I\'m using PostgreSQL\'s Ltree module for storing hierarchical data. I\'m looking to retrieve the full hierarchy sorted by a particular column.

Consider the following ta

Answer 1

create table comments (
  id serial,
  parent_id int,
  msg text,
  primary key (id)
);

insert into comments (id, parent_id, msg) values (1, null, 'msg 1');
insert into comments (id, parent_id, msg) values (2, null, 'msg 2');
insert into comments (id, parent_id, msg) values (3, 1, 'msg 1 / ans 1');
insert into comments (id, parent_id, msg) values (4, null, 'msg 3');
insert into comments (id, parent_id, msg) values (5, 2, 'msg 2 / ans 1');
insert into comments (id, parent_id, msg) values (6, 2, 'msg 2 / ans 2');
insert into comments (id, parent_id, msg) values (7, 2, 'msg 2 / ans 3');

---

desc

WITH RECURSIVE q AS
(
  SELECT  id, msg, 1 as level, ARRAY[id] as path
  FROM  comments c
  WHERE parent_id is null
  UNION ALL
  SELECT  sub.id, sub.msg, level + 1, path || sub.id
  FROM  q
    JOIN  comments sub
      ON  sub.parent_id = q.id
)
SELECT  id, msg, level
FROM    q
order by path || array_fill(100500, ARRAY[8 - level]) desc;

results in

4,"msg 3",1
2,"msg 2",1
7,"msg 2 / ans 3",2
6,"msg 2 / ans 2",2
5,"msg 2 / ans 1",2
1,"msg 1",1
3,"msg 1 / ans 1",2

---

asc

WITH RECURSIVE q AS
(
  SELECT  id, msg, 1 as level, ARRAY[id] as path
  FROM  comments c
  WHERE parent_id is null
  UNION ALL
  SELECT  sub.id, sub.msg, level + 1, path || sub.id
  FROM  q
    JOIN  comments sub
      ON  sub.parent_id = q.id
)
SELECT  id, msg, level
FROM    q
--order by path || array_fill(100500, ARRAY[8 - level]) desc;
order by path;

results in

1,"msg 1",1
3,"msg 1 / ans 1",2
2,"msg 2",1
5,"msg 2 / ans 1",2
6,"msg 2 / ans 2",2
7,"msg 2 / ans 3",2
4,"msg 3",1