[SPARK-51720][SQL] Add Cross Join as legal in recursion of Recursive CTE

[Pajaraja]

What changes were proposed in this pull request?

Add Cross Joins as legal in recursion in Recursive CTEs.

Why are the changes needed?

Cross join is allowed in the recursion plan of Recursive CTEs in postgreSQL.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Add test to golden file cte-recursion.sql.

Was this patch authored or co-authored using generative AI tooling?

No.

[peter-toth]

Looks good and this scenario is allowed in some engines (like postgresql), but I would suggest a more intuitive test case like:

WITH RECURSIVE
    x(id) AS (SELECT 1 UNION SELECT 2),
    t(id, xid) AS (
        SELECT 0 AS id, 0 AS xid
        UNION ALL
        SELECT t.id + 1, xid * 10 + x.id FROM t CROSS JOIN x WHERE t.id < 3
    )                     
SELECT * FROM t

id | xid
-- | --
0  | 0
1  | 1
1  | 2
2  | 11
2  | 12
2  | 21
2  | 22
3  | 111
3  | 112
3  | 121
3  | 122
3  | 211
3  | 212
3  | 221
3  | 222

[Pajaraja]

Looks good and this scenario is allowed in some engines (like postgresql), but I would suggest a more intuitive test case like:

WITH RECURSIVE
    x(id) AS (SELECT 1 UNION SELECT 2),
    t(id, xid) AS (
        SELECT 0 AS id, 0 AS xid
        UNION ALL
        SELECT t.id + 1, xid * 10 + x.id FROM t CROSS JOIN x WHERE t.id < 3
    )                     
SELECT * FROM t

id | xid
-- | --
0  | 0
1  | 1
1  | 2
2  | 11
2  | 12
2  | 21
2  | 22
3  | 111
3  | 112
3  | 121
3  | 122
3  | 211
3  | 212
3  | 221
3  | 222

Added this test too. Thanks!

[MaxGekk]

@Pajaraja How about new JIRA? Specifically for the changes.

[Pajaraja]

@Pajaraja How about new JIRA? Specifically for the changes.

Added.

[peter-toth]

Thanks @Pajaraja for the fix and @MaxGekk for the review!

Merged to master (4.1.0).