How does one print depth-level in a Postgres query that uses RECURSIVE to select descendants?

Question

I have a table persons that contains a column for parent_id, which refers to another row in the same table. Assume this is the logical hierarchy:

          P1
  P2      P3      P4
P5  P6  P7  P8  P9  P10

I have written a query that prints all parents of a given node, along with the height above the node, and it seems to work fine:

WITH
RECURSIVE ancestors AS (
  SELECT id, parent_id
    FROM persons
    WHERE id = 8
  UNION
    SELECT p.id, p.parent_id
      FROM persons p
      INNER JOIN ancestors
        ON
          p.id = ancestors.parent_id
  )
SELECT persons.id, persons.name,
      ROW_NUMBER() over () as height
  FROM ancestors
  INNER JOIN persons
  ON
    ancestors.id = persons.id
  WHERE
    persons.id <> 8

Result:

  id   |    name     | height 
-------+-------------+---------
    3  | P3          |      1
    1  | P1          |      2
(2 rows)

I now want to write a query that similarly prints all descendants, along with depth. Here's the query so far (same as above with id and parent_id swapped in the UNION join):

WITH
RECURSIVE descendants AS (
  SELECT id, parent_id
    FROM persons
    WHERE id = 1
  UNION
    SELECT p.id, p.parent_id
      FROM persons p
      INNER JOIN descendants
        ON
          p.parent_id = descendants.id
  )
SELECT persons.id, persons.name,
      ROW_NUMBER() over () as depth
  FROM descendants
  INNER JOIN persons
  ON
    descendants.id = persons.id
  WHERE
    persons.id <> 1

This gives the following result:

  id   |    name     | depth
-------+-------------+---------
    2  | P2          |      1
    3  | P3          |      2
    4  | P4          |      3
    5  | P5          |      4
    6  | P6          |      5
    7  | P7          |      6
    8  | P8          |      7
    9  | P9          |      8
    10 | P10         |      9
(9 rows)

Clearly, the depth is all wrong. ROW_NUMBER() isn't doing what I want. How do I go about this?

I've thought about using a counter within the recursive part of the query itself, which increments every time it is run, but I'm not sure if there's a way to achieve that.

Answer 1

Use an additional integer column with values incremented at each recursive step.

WITH RECURSIVE descendants AS (
    SELECT id, parent_id, 0 AS depth
    FROM persons
    WHERE id = 1
UNION
    SELECT p.id, p.parent_id, d.depth+ 1
    FROM persons p
    INNER JOIN descendants d
    ON p.parent_id = d.id
)
SELECT p.id, p.name, depth
FROM descendants d
INNER JOIN persons p
ON d.id = p.id
WHERE p.id <> 1;

 id | name | depth 
----+------+-------
  2 | P2   |     1
  3 | P3   |     1
  4 | P4   |     1
  5 | P5   |     2
  6 | P6   |     2
  7 | P7   |     2
  8 | P8   |     2
  9 | P9   |     2
 10 | P10  |     2
(9 rows)

Db<>fiddle.