viirya commented on code in PR #8626:
URL: https://github.com/apache/arrow-datafusion/pull/8626#discussion_r1436026666


##########
datafusion/optimizer/src/push_down_filter.rs:
##########
@@ -955,6 +965,36 @@ impl PushDownFilter {
     }
 }
 
+/// Convert cross join to join by pushing down filter predicate to the join 
condition
+fn convert_cross_join_to_inner_join(cross_join: CrossJoin) -> Result<Join> {
+    let CrossJoin { left, right, .. } = cross_join;
+    let join_schema = build_join_schema(left.schema(), right.schema(), 
&JoinType::Inner)?;
+    // predicate is given
+    Ok(Join {
+        left,
+        right,
+        join_type: JoinType::Inner,
+        join_constraint: JoinConstraint::On,
+        on: vec![],
+        filter: None,
+        schema: DFSchemaRef::new(join_schema),
+        null_equals_null: true,
+    })
+}
+
+/// Converts the inner join with empty equality predicate and empty filter 
condition to the cross join
+fn convert_to_cross_join_if_beneficial(plan: LogicalPlan) -> 
Result<LogicalPlan> {
+    if let LogicalPlan::Join(join) = &plan {
+        // Can be converted back to cross join
+        if join.on.is_empty() && join.filter.is_none() {
+            return LogicalPlanBuilder::from(join.left.as_ref().clone())
+                .cross_join(join.right.as_ref().clone())?
+                .build();
+        }
+    }
+    Ok(plan)
+}

Review Comment:
   And, the reason that the predicate is kept in Filter in `push_down_all_join` 
is because it cannot be pushed into Join filter or pushed down through Join. 
How does it push into/push down later?



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