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Comments on How to break infinite loop in CTE
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How to break infinite loop in CTE
I have a parent-child relation in my table, with possibly circular cases. Is it possible to break the infinite recursion in CTE checking values of all previous rows? I would need something like this:
with my_cte(childId, parentId, col1)
AS (
SELECT r.childId, r.parentId, r.col1
FROM My_Table r WHERE r.childId = 'x' AND (r.col1 = 1 or r.col1 = 2)
UNION ALL
SELECT rel.childId, rel.parentId, rel.col1
FROM My_Table rel INNER JOIN my_cte sd ON rel.childId = sd.parentId where (rel.col1 = 1 OR rel.col1 = 2) AND ...
-- check that rel.childId, rel.parentId not in previous rows
)
SELECT DISTINCT * from shared_documents
How can I achieve this? Or how to approach this?
Example table with records would be:
| childId | parentId | col1 |
|---|---|---|
| 1 | 3 | 1 |
| 1 | 4 | 1 |
| 3 | 5 | 1 |
| 5 | 1 | 1 -- circular case |
| 6 | 2 | 1 |
| 5 | 7 | 1 |
If I ask for hierarchy related to 1, I should get
| childId | parentId | col1 |
|---|---|---|
| 1 | 3 | 1 |
| 1 | 4 | 1 |
| 3 | 5 | 1 |
| 5 | 1 | 1 -- circular case |
| 5 | 7 | 1 |
- 1 has two parents: 3 and 4.
- 3 has 1 parent: 5
- 5 has 2 parents: 1 and 7. But 1 brings the infinite loop, so the query should ignore this. And continue to the next row.
Post
For small tables you can bound the recursion depth:
WITH my_cte(childId, parentId, depth, max_depth)
AS (
SELECT r.childId, r.parentId, 1, (SELECT COUNT(*) FROM My_Table)
FROM My_Table r
WHERE r.childId = 1
UNION ALL
SELECT rel.childId, rel.parentId, sd.depth + 1, sd.max_depth
FROM My_Table rel INNER JOIN my_cte sd ON rel.childId = sd.parentId
WHERE sd.depth < sd.max_depth
)
SELECT DISTINCT childId, parentId
FROM my_cte
This approach is limited by the bound that MAXRECURSION <= 32767.
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