How to merge JSONB field in a tree structure? - postgresql

Question

I have a table in Postgres which stores a tree structure. Each node has a jsonb field: params_diff:
CREATE TABLE tree (id INT, parent_id INT, params_diff JSONB);
INSERT INTO tree VALUES
(1, NULL, '{ "some_key": "some value" }'::jsonb)
, (2, 1, '{ "some_key": "other value", "other_key": "smth" }'::jsonb)
, (3, 2, '{ "other_key": "smth else" }'::jsonb);
The thing I need is to select a node by id with additional generated params field which contains the result of merging all params_diff from the whole parents chain:
SELECT tree.*, /* some magic here */ AS params FROM tree WHERE id = 3;
id | parent_id | params_diff | params
----+-----------+----------------------------+-------------------------------------------------------
3 | 2 | {"other_key": "smth else"} | {"some_key": "other value", "other_key": "smth else"}

Answer 1

Generally, a recursive CTE can do the job. Example:
Use table alias in another query to traverse a tree
We just need a more magic to decompose, process and re-assemble the JSON result. I am assuming from your example, that you want each key once only, with the first value in the search path (bottom-up):
WITH RECURSIVE cte AS (
SELECT id, parent_id, params_diff, 1 AS lvl
FROM tree
WHERE id = 3
UNION ALL
SELECT t.id, t.parent_id, t.params_diff, c.lvl + 1
FROM cte c
JOIN tree t ON t.id = c.parent_id
)
SELECT id, parent_id, params_diff
, (SELECT json_object(array_agg(key ORDER BY lvl)
, array_agg(value ORDER BY lvl))::jsonb
FROM (
SELECT key, value
FROM (
SELECT DISTINCT ON (key)
p.key, p.value, c.lvl
FROM cte c, jsonb_each_text(c.params_diff) p
ORDER BY p.key, c.lvl
) sub1
ORDER BY lvl
) sub2
) AS params
FROM cte
WHERE id = 3;
How?
Walk the tree with a classic recursive CTE.
Create a derived table with all keys and values with jsonb_each_text() in a LATERAL JOIN, remember the level in the search path (lvl).
Use DISTINCT ON to get the "first" (lowest lvl) value for each key. Details:
Select first row in each GROUP BY group?
Sort and aggregate resulting keys and values and feed the arrays to json_object() to build the final params value.
SQL Fiddle (only as far as pg 9.3 can go with json instead of jsonb).