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

On Sun, 17 Oct 2021 at 09:37, Jian He <hejian.mark@gmail.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
withthe 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
levelconcept?
 

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/