How to avoid prohibited outer join in recursive CTE? - sql

Question

I have a table which represents acyclic dependencies between objects:
CREATE TEMPORARY TABLE dependencies
(
obj_id bigint,
depended_upon bigint NULL
);
INSERT INTO dependencies
VALUES
(1, NULL), -- 1 is not depended upon by any other object
(2, 1), -- 1 depends on 2
(3, 4), -- 4 depends on 3
(3, 2), -- ... and so does 2
(4, NULL); -- 4 also is not depended upon by any other object
I'd like to determine the order in which I would have to prune objects so that no dependencies are violated, starting from the leaves. For this example, that result would look like this:
Object ID
Prune step
4
0
1
0
2
1
3
2
Each object is pruned in the first step in which no other object depends on it.
I've tried to do this with a recursive CTE:
WITH RECURSIVE deletion_order(obj_id, step) AS (
SELECT obj_id, 0
FROM dependencies
GROUP BY obj_id
HAVING COUNT(depended_upon) = 0
UNION ALL
SELECT dependencies.obj_id, step + 1
FROM dependencies
LEFT JOIN deletion_order ON dependencies.depended_upon = deletion_order.obj_id
WHERE deletion_order.obj_id IS NULL
)
SELECT *
FROM deletion_order
ORDER BY step;
But Postgres complains:
[42P19] ERROR: recursive reference to query "deletion_order" must not appear within an outer join
How can I accomplish this without an outer join? Note that the format of dependencies is flexible, if it needs to change to accommodate the solution.

Answer 1

If I understood what you are trying to achieve: you have most of the recursive queries figured out but somehow failed where it was the easiest:
The initialization of your CTE could be easier with WHERE depended_upon IS NULL.
I did not understand why you had put WHERE deletion_order.obj_id IS NULL in the recursive part of the CTE.
The JOIN should be an inner join, I did not understand why you thought you needed an outer join (I guess it is linked with the previous point but that did not help me understand).
Resulting query:
WITH RECURSIVE deletion_order(obj_id, step) AS (
SELECT obj_id, 0
FROM dependencies
WHERE depended_upon IS NULL
UNION ALL
SELECT dependencies.obj_id, step + 1
FROM dependencies
JOIN deletion_order ON dependencies.depended_upon = deletion_order.obj_id
)
SELECT obj_id, MAX(step) AS MaxStep /*, MIN(step) AS MinStep (optional, see comment below)*/
FROM deletion_order
ORDER BY MaxStep;
So I do not know how to answer your question (How to avoid outer join) because you should not have felt the need to use an outer join in the first place.
I have answered a previous question as how to build a recursive CTE here. Maybe it is worth checking.