Rename helper for scalar subquery clarity

Update the function name to specify its relevance to scalar
subquery cross-joins. Add an intent comment for better
understanding of its purpose. Replace the old function call in
join predicate handling for improved readability.

Author: kosiew

datafusion/optimizer/src/push_down_filter.rs

@@ -308,7 +308,12 @@ fn is_derived_relation(plan: &LogicalPlan) -> bool {
     matches!(plan, LogicalPlan::SubqueryAlias(_))
 }
 
-fn should_keep_filter_above_cross_join(join: &Join, predicate: &Expr) -> bool {
+// Keep post-join filters above certain scalar-subquery cross joins to preserve
+// behavior for the window-over-scalar-subquery regression shape.
+fn should_keep_filter_above_scalar_subquery_cross_join(
+    join: &Join,
+    predicate: &Expr,
+) -> bool {
     if !join.on.is_empty() || join.filter.is_some() {
         return false;
     }
@@ -471,7 +476,7 @@ fn push_down_all_join(
         } else if right_preserved && checker.is_right_only(&predicate) {
             right_push.push(predicate);
         } else if is_inner_join
-            && !should_keep_filter_above_cross_join(&join, &predicate)
+            && !should_keep_filter_above_scalar_subquery_cross_join(&join, &predicate)
             && can_evaluate_as_join_condition(&predicate)?
         {
             // Here we do not differ it is eq or non-eq predicate, ExtractEquijoinPredicate will extract the eq predicate

datafusion/optimizer/src/utils.rs

@@ -492,29 +492,19 @@ mod tests {
     }
 
     #[test]
-    fn mixed_reference_predicate_remains_fast_pathed_in_authoritative_mode(
-    ) -> Result<()> {
+    fn mixed_reference_predicate_remains_fast_pathed_in_authoritative_mode() -> Result<()>
+    {
         let predicate = binary_expr(col("a"), Operator::Gt, col("b"));
         let column_a = Column::from_name("a");
 
         let auto_result = with_null_restriction_eval_mode_for_test(
             NullRestrictionEvalMode::Auto,
-            || {
-                is_restrict_null_predicate(
-                    predicate.clone(),
-                    std::iter::once(&column_a),
-                )
-            },
+            || is_restrict_null_predicate(predicate.clone(), std::iter::once(&column_a)),
         )?;
 
         let authoritative_only_result = with_null_restriction_eval_mode_for_test(
             NullRestrictionEvalMode::AuthoritativeOnly,
-            || {
-                is_restrict_null_predicate(
-                    predicate.clone(),
-                    std::iter::once(&column_a),
-                )
-            },
+            || is_restrict_null_predicate(predicate.clone(), std::iter::once(&column_a)),
         )?;
 
         assert!(!auto_result, "{predicate}");