How to convert this code from Oracle to Postgresql - postgresql

Question

I'm trying to convert this code from Oracle database to Postgresql, can someone help me with that?
WITH cycles AS
(
SELECT SYS_CONNECT_BY_PATH (child_node_id, ',') AS path, SYS_CONNECT_BY_PATH ( CASE WHEN child_node_id < CONNECT_BY_ROOT child_node_id THEN child_node_id END, ','
) AS less_path
FROM alf_child_ASSOC
WHERE CONNECT_BY_ROOT parent_node_id = child_node_id
CONNECT BY NOCYCLE parent_node_id = PRIOR child_node_id
)
SELECT *
FROM cycles
WHERE LTRIM (less_path, ',') IS NULL ;
Thanks

Answer 1

You can convert your Oracle query from a hierarchical query to a recursive query in the form:
WITH cycles (path, root_id, child_node_id) AS (
SELECT ',' || child_node_id,
parent_node_id AS root_id,
child_node_id
FROM alf_child_assoc
WHERE parent_node_id > child_node_id
UNION ALL
SELECT c.path || ',' || a.child_node_id,
c.root_id,
a.child_node_id
FROM cycles c
INNER JOIN alf_child_assoc a
ON (c.child_node_id = a.parent_node_id)
WHERE a.parent_node_id != c.root_id
)
SELECT path
FROM cycles
WHERE child_node_id = root_id
Oracle db<>fiddle here
Then all you need to do to convert to PostgreSQL is to add the RECURSIVE keyword:
WITH RECURSIVE cycles (path, root_id, child_node_id) AS (
SELECT ',' || child_node_id,
parent_node_id AS root_id,
child_node_id
FROM alf_child_assoc
WHERE parent_node_id > child_node_id
UNION ALL
SELECT c.path || ',' || a.child_node_id,
c.root_id,
a.child_node_id
FROM cycles c
INNER JOIN alf_child_assoc a
ON (c.child_node_id = a.parent_node_id)
WHERE a.parent_node_id != c.root_id
)
SELECT path
FROM cycles
WHERE child_node_id = root_id
Which, for the sample data:
CREATE TABLE alf_child_assoc (
child_node_id NUMERIC(10,0),
parent_node_id NUMERIC(10,0)
);
INSERT INTO alf_child_assoc (child_node_id, parent_node_id) VALUES (2, 1);
INSERT INTO alf_child_assoc (child_node_id, parent_node_id) VALUES (3, 2);
INSERT INTO alf_child_assoc (child_node_id, parent_node_id) VALUES (1, 3);
Outputs:
PATH
,1,2,3
PostgreSQL db<>fiddle here

Answer 2

I found a working solution:
WITH RECURSIVE prev AS (
SELECT alf_child_ASSOC.child_node_id, 1 AS depth, array[child_node_id] as seen, false as cycle
FROM alf_child_ASSOC
UNION ALL
SELECT alf_child_ASSOC.child_node_id, prev.depth + 1, seen || alf_child_ASSOC.child_node_id as seen,
alf_child_ASSOC.child_node_id = any(seen) as cycle
FROM prev
INNER JOIN alf_child_ASSOC on prev.child_node_id = parent_node_id
AND prev.cycle = false
)
SELECT *
FROM prev
WHERE cycle = true;
Exemple of output:
child_node_id | depth | seen | cycle
---------------+-------+-------------------+-------
749254 | 2 | {749254,749254} | t
749675 | 2 | {749675,749675} | t
754208 | 2 | {754208,754208} | t