Postgres recursive query with row_to_json

Question

I\'ve got a table in postgres 9.3.5 that looks like this:

CREATE TABLE customer_area_node
(
  id bigserial NOT NULL,
  customer_id integer NOT NULL,
  parent         


        

Answer 1

You cannot do that with a usual recursive CTE, because it is almost impossible to set a json value deep in its hierarchy. But you can do it reversed: build up the tree starting from its leaves, until its root:

-- calculate node levels
WITH RECURSIVE c AS (
    SELECT *, 0 as lvl
    FROM customer_area_node
    -- use parameters here, to select the root first
    WHERE customer_id = 2 AND parent_id IS NULL
  UNION ALL
    SELECT customer_area_node.*, c.lvl + 1 as lvl
    FROM customer_area_node 
    JOIN c ON customer_area_node.parent_id = c.id
),
-- select max level
maxlvl AS (
  SELECT max(lvl) maxlvl FROM c
),
-- accumulate children
j AS (
    SELECT c.*, json '[]' children -- at max level, there are only leaves
    FROM c, maxlvl
    WHERE lvl = maxlvl
  UNION ALL
    -- a little hack, because PostgreSQL doesn't like aggregated recursive terms
    SELECT (c).*, array_to_json(array_agg(j)) children
    FROM (
      SELECT c, j
      FROM j
      JOIN c ON j.parent_id = c.id
    ) v
    GROUP BY v.c
)
-- select only root
SELECT row_to_json(j) json_tree
FROM j
WHERE lvl = 0;

And this will work even with PostgreSQL 9.2+

SQLFiddle

Update: A variant, which should handle rogue leaf nodes too (which are located with a level between 1 and max-level):

WITH RECURSIVE c AS (
    SELECT *, 0 as lvl
    FROM   customer_area_node
    WHERE  customer_id = 1 AND parent_id IS NULL
  UNION ALL
    SELECT customer_area_node.*, c.lvl + 1
    FROM   customer_area_node 
    JOIN   c ON customer_area_node.parent_id = c.id
),
maxlvl AS (
  SELECT max(lvl) maxlvl FROM c
),
j AS (
    SELECT c.*, json '[]' children
    FROM   c, maxlvl
    WHERE  lvl = maxlvl
  UNION ALL
    SELECT   (c).*, array_to_json(array_agg(j) || array(SELECT r
                                                        FROM   (SELECT l.*, json '[]' children
                                                                FROM   c l, maxlvl
                                                                WHERE  l.parent_id = (c).id
                                                                AND    l.lvl < maxlvl
                                                                AND    NOT EXISTS (SELECT 1
                                                                                   FROM   c lp
                                                                                   WHERE  lp.parent_id = l.id)) r)) children
    FROM     (SELECT c, j
              FROM   c
              JOIN   j ON j.parent_id = c.id) v
    GROUP BY v.c
)
SELECT row_to_json(j) json_tree
FROM   j
WHERE  lvl = 0;

This should work too on PostgreSQL 9.2+, however, I cannot test that. (I can only test on 9.5+ right now).

These solutions can handle any column in any hierarchical table, but will always append an int typed lvl JSON property to their output.

http://rextester.com/YNU7932