Postgresql recursive CTE results ordering - postgresql

Question

I'm working on a query to pull data out of a hierarchy
e.g.
CREATE table org (
id INT PRIMARY KEY,
name TEXT NOT NULL,
parent_id INT);
INSERT INTO org (id, name) VALUES (0, 'top');
INSERT INTO org (id, name, parent_id) VALUES (1, 'middle1', 0);
INSERT INTO org (id, name, parent_id) VALUES (2, 'middle2', 0);
INSERT INTO org (id, name, parent_id) VALUES (3, 'bottom3', 1);
WITH RECURSIVE parent_org (id, parent_id, name) AS (
SELECT id, parent_id, name
FROM org
WHERE id = 3
UNION ALL
SELECT o.id, o.parent_id, o.name
FROM org o, parent_org po
WHERE po.parent_id = o.id)
SELECT id, parent_id, name
FROM parent_org;
It works as expected.
3 1 "bottom3"
1 0 "middle1"
0 "top"
It's also returning the data in the order that I expect, and it makes sense to me that it would do this because of the way that the results would be discovered.
The question is, can I count on the order being like this?

Answer 1

Yes, there is a defined order. In the Postgres WITH doc, they give the following example:
WITH RECURSIVE search_graph(id, link, data, depth, path, cycle) AS (
SELECT g.id, g.link, g.data, 1,
ARRAY[ROW(g.f1, g.f2)],
false
FROM graph g
UNION ALL
SELECT g.id, g.link, g.data, sg.depth + 1,
path || ROW(g.f1, g.f2),
ROW(g.f1, g.f2) = ANY(path)
FROM graph g, search_graph sg
WHERE g.id = sg.link AND NOT cycle
)
SELECT * FROM search_graph;
About which they say in a Tip box (formatting mine):
The recursive query evaluation algorithm produces its output in
breadth-first search order. You can display the results in depth-first
search order by making the outer query ORDER BY a "path" column
constructed in this way.
You do appear to be getting breadth-first output in your case above based on the INSERT statements, so I would say you could, if you wanted, modify your outer SELECT to order it in another fashion.
I believe the analog for depth-first in your case would probably be this:
WITH RECURSIVE parent_org (id, parent_id, name) AS (
SELECT id, parent_id, name
FROM org
WHERE id = 3
UNION ALL
SELECT o.id, o.parent_id, o.name
FROM org o, parent_org po
WHERE po.parent_id = o.id)
SELECT id, parent_id, name
FROM parent_org
ORDER BY id;
As I would expect (running things through in my head) that to yield this:
0 "top"
1 0 "middle1"
3 1 "bottom3"