如何避免递归CTE中禁止的外部联接？

我有一个表，它表示对象之间的非循环依赖关系：

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

我想确定修剪对象的顺序，以便从叶子开始不违反依赖关系。对于本例，结果如下所示：

每个对象在第一步中都会被修剪，在第一步，没有其他对象依赖于它。

我尝试过用递归CTE来实现这一点：

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;

但Postgres抱怨道：

[42P19]错误：对查询“deletion_order”的递归引用不能出现在外部联接中

如果没有外部连接，我如何实现这一点？请注意，如果需要更改以适应解决方案，dependencies的格式是灵活的。