Correctness issue with unparsing UNION & UNION ALL #24446

Describe the bug

Consider a UNION ALL (b UNION a), that becomes a plan

Union
    a
    Distinct::All
      Union
        b
        a

This will get incorrectly unparsed as SELECT a UNION SELECT b UNION SELECT a.

All union operations are now distinct, instead of correctly preserving the true per node nature. Root cause: traversing the inner Distinct::All(Union) sets QueryBuilder::distinct_union = true globally. All subsequent Unions are considered distinct during unpausing the query. A single, query-level boolean is insufficient to track ast::SetQuantifier for each UNION operation.

To Reproduce

Add this example case to datafusion/sql/tests/cases/plan_to_sql.rs and rerun.

r#"SELECT j1_string FROM j1 UNION ALL (SELECT j2_string FROM j2 UNION SELECT j1_string FROM j1)"#,

Expected behavior

Actual: the outer ALL is dropped.

SELECT a.x FROM a UNION SELECT b.x FROM b UNION SELECT a.x FROM a

Expected

SELECT a.x FROM a UNION ALL (SELECT b.x FROM b UNION SELECT a.x FROM a)

Additional context

No response