Re: Does PostgreSQL have a pseudo-column like "LEVEL" in Oracle

On Sun, 17 Oct 2021 at 09:37, Jian He <hejian(dot)mark(at)gmail(dot)com> wrote:
>
>
> https://stackoverflow.com/questions/22626394/does-postgresql-have-a-pseudo-column-like-level-in-oracle
> Wandering around, playing around, then problems come. I tried to crack the level concept . So I followed through with the most voted answer from the above link. The following is my code sample data.
>
> begin;
> create temp table tempemp (employee_id integer primary key, last_name text,manager_id integer);
> insert into tempemp values(1,'eliane',1);
> insert into tempemp values(2,'sponge',1);
> insert into tempemp values(3,'george',1);
> insert into tempemp values(4,'kramer',2);
> insert into tempemp values(5,'megan',2);
> insert into tempemp values(6,'donald',3);
> commit;
>
> WITH RECURSIVE cte AS (
> SELECT employee_id, last_name, manager_id, 1 AS level
> FROM tempemp
>
> UNION ALL
> SELECT e.employee_id, e.last_name, e.manager_id, c.level + 1
> FROM cte c
> JOIN tempemp e ON e.manager_id = c.employee_id
> )
> SELECT *
> FROM cte;
>
> But when I execute the code, It seems like an infinite loop. So, any suggestions(code sample) for me to crack the level concept?

Add a path array to track the graph as it grows, allowing you to
exclude already visited nodes.

WITH RECURSIVE cte AS (
SELECT employee_id, last_name, manager_id, 1 AS level,
ARRAY[employee_id] as path
FROM tempemp
UNION ALL
SELECT e.employee_id, e.last_name, e.manager_id, c.level + 1, path ||
e.employee_id
FROM cte c
JOIN tempemp e ON e.manager_id = c.employee_id
WHERE e.employee_id <> all (c.path)
)
SELECT employee_id, last_name, manager_id, level
FROM cte;

With the above query, it doesn't matter if the data contains loops.

This adds no appreciable execution time - the above executes in 0.2ms,
yet avoids infinite loops.

--
Simon Riggs http://www.EnterpriseDB.com/