PostgreSQL - retrieve all IDs in a tree for a given subnode

Question

I have a non-binary tree of customer, and I need to obtain all the IDs in a tree for the given node.

The table is very simple, just an join table with a parent id and child id. This is a representation of the tree I stored in my db.

In this example if I search for node 17 I need in return 14-17. If I search for 11 I need in return 1-6-5-4-8-11-12-7-2-10-3.

The order is not important. I only need the ID to avoid circularity when adding children to a node.

I created this query. The ancestor part works fine, I retrieve all parent nodes, but for the descendants I have some trouble. I'm only able to retrieve some part of the tree. For example, with node 11 I retrieve 4-10-6-11-7-8, so all right part of the tree is missing.

WITH RECURSIVE
-- starting node(s)
starting (parent, child) AS
(
  SELECT t.parent, t.child
  FROM public.customerincustomer AS t
  WHERE t.child = :node or t.parent = :node
)
,
ancestors (parent, child) AS
(
  SELECT t.parent, t.child
  FROM public.customerincustomer AS t
  WHERE t.parent IN (SELECT parent FROM starting)
  UNION ALL
  SELECT t.parent, t.child
  FROM public.customerincustomer AS t JOIN ancestors AS a ON t.child = a.parent
),
descendants (parent, child) AS
(
  SELECT t.parent, t.child
  FROM public.customerincustomer AS t
  WHERE t.parent IN (SELECT parent FROM starting) or t.child in (select child from starting)
  UNION ALL
  SELECT t.parent, t.child
  FROM public.customerincustomer AS t JOIN ancestors AS a ON t.parent = a.child
)

table ancestors
union all
table descendants

UPDATE

I see that many examples included in the tree table also the root in form (root_id, null).

In my case I don't have this record.

For example, taking the smallest tree 14->17, in my table I have only one record parent, child

14 17

Answer 1

A very primitive implementation:

It basically divides the problem into two subproblems:

• First find all the ancestors of the node in question (including the node itself). If the node has no parents, then this would be just itself.
• Then find the descendants of all those ancestors (including themselves). We may have several nodes in the ancestors result set, we may get duplicates here, so we use UNION (and not UNION ALL) to remove them.
• Note that the query will work even if the input node is a root with has no children.
• It will also work if the data set is not a forest of trees but an arbitrary directed graph (where nodes can have more than one parent).

The query:

WITH RECURSIVE 
ancestors (parent) AS
(
  SELECT :node                      -- start with the given node
  UNION ALL
  SELECT t.parent                   -- and find all its ancestors
  FROM public.customerincustomer AS t JOIN ancestors AS a ON t.child = a.parent
),
descendants (customer) AS
(
  SELECT parent AS customer       -- now start with all the ancestors
  FROM ancestors
  UNION 
  SELECT t.child                  -- and find all their descendants
  FROM public.customerincustomer AS t JOIN descendants AS d ON t.parent = d.customer
)
SELECT customer
FROM descendants ;

Answer 2

This function returns the parent level of node_id:

_{There is a 'level' row due there isn't a row (id, null) for parent row.}

CREATE FUNCTION get_parent(node_id int)
RETURNS integer AS
$$
  WITH RECURSIVE get_parent AS
  (
      SELECT 
          t1.id, 
          t1.parent_id, 
          t1.name, 
          0 AS level
      FROM 
          tree t1
      WHERE 
          t1.id = node_id
      UNION ALL
      SELECT 
          t2.id, 
          t2.parent_id, 
          t2.name,
          level+1
      FROM 
          tree t2 
      INNER JOIN 
          get_parent ON get_parent.parent_id = t2.id
  )
  SELECT
      id
  FROM
      get_parent
  ORDER BY
      level DESC
  LIMIT 1 ;
$$
LANGUAGE SQL;

select get_parent(7);

| get_parent |
| ---------: |
|          6 |

Now, next query returns the whole tree structure based on a parent node.

WITH RECURSIVE childs AS
(
    SELECT 
        t1.id, 
        t1.parent_id, 
        t1.name
    FROM 
        tree t1
    WHERE 
        t1.id = get_parent(7)
    UNION ALL
    SELECT 
        t2.id, 
        t2.parent_id, 
        t2.name
    FROM 
        tree t2 
    INNER JOIN 
        childs ON childs.id = t2.parent_id
)
SELECT
    id,
    parent_id,
    name
 FROM
    childs;

id | parent_id | name   
-: | --------: | :------
 6 |         1 | Node 6 
 4 |         6 | Node 4 
 8 |         6 | Node 8 
11 |         6 | Node 11
 7 |        11 | Node 7 
10 |         7 | Node 10

db<>fiddle here