(Postgres) Query in a tree table in ascending and descending mode - postgresql

Question

I'm having some issues with two queries to search in a "tree" table.
So, my table is represented by the following code, and it has one only direction. However, I need to get data in both directions, ascending and descending mode.
create table graph_examle (input int null, output int );
insert into graph_examle (input, output) values
(null, 1),
(1, 2),
(2, 3 ),
(3, 4 ),
(null, 7 ),
(7,8),
(8, 4 ),
(null, 10 ),
(10, 11 ),
(11, 4),
(3, 15),
(25, 15),
(26, 15),
(15, 4 );
The ascending query has some issues. If I search by id 1, I'm expecting to see the relations:
1, 1->2, 1->2->3, 1->2->3->4, but the results are:
WITH recursive cte (initial_id, level, path, loop, input, output) AS
(
SELECT input, 1, ':' ||input || ':' , 0, input, output
FROM graph_examle WHERE input = 1
UNION ALL
SELECT
c.initial_id,
c.level + 1,
c.path ||ur.input|| ':' ,
CASE WHEN c.path LIKE '%:' ||ur.input || ':%' THEN 1 ELSE 0 END,
ur.*
FROM graph_examle ur
INNER JOIN cte c ON c.output = ur.input AND c.loop = 0
)
SELECT *
FROM cte
ORDER BY initial_id, level;
The descending query does not work as expected. If I search by id 4, I'm expecting to see the relations:
4, 4->3, 4->3->2, 4->3->2->1
4->8, 4->8->7
4->11,4->11>10
4->15, (...)
But I'm only getting:
WITH RECURSIVE cte (input, output, level, real_parent_id, path) AS
(
SELECT
ur.input, ur.input, 1, output, ( ur.input|| ' -> ' || ur.output)
FROM graph_examle ur
WHERE ur.output = 4
UNION ALL
SELECT
ur_cte.input, ur.input, level + 1, ur.output, (ur_cte.path || '->' || ur.output)
FROM cte ur_cte
INNER JOIN graph_examle ur on ur.input = ur_cte.real_parent_id
)
SELECT *
FROM cte
ORDER BY path
Note that in my queries I'm trying to solve circular dependencies

Answer 1

The ascending query sounds good ... maybe you can concatenate the path and output columns.
For the descending query, you can try this :
WITH RECURSIVE cte (input, output, level, path, loop) AS
(
SELECT
ur.input, ur.output, 1, ( ur.output|| ' -> ' || ur.input), 0
FROM graph_examle ur
WHERE ur.output = 4
UNION ALL
SELECT
ur.input, ur_cte.output, level + 1, (ur_cte.path || '->' || ur.input),
CASE WHEN ur_cte.path LIKE '%->' || ur.input THEN 1 ELSE 0 END
FROM cte ur_cte
INNER JOIN graph_examle ur on ur.output = ur_cte.input
WHERE ur_cte.loop = 0
AND ur.input IS NOT NULL
)
SELECT *
FROM cte
ORDER BY path
see dbfiddle