Desafío de consulta recursiva:ejemplo simple de padre/hijo

Con la ayuda de RhodiumToad en #postgresql, llegué a esta solución:

WITH RECURSIVE node_graph AS (
    SELECT ancestor_node_id as path_start, descendant_node_id as path_end,
           array[ancestor_node_id, descendant_node_id] as path 
    FROM node_relations

    UNION ALL 

    SELECT ng.path_start, nr.descendant_node_id as path_end,
           ng.path || nr.descendant_node_id as path
    FROM node_graph ng
    JOIN node_relations nr ON ng.path_end = nr.ancestor_node_id
) 
SELECT * from node_graph order by path_start, array_length(path,1);

El resultado es exactamente el esperado.