alamb commented on a change in pull request #9806:
URL: https://github.com/apache/arrow/pull/9806#discussion_r610912000



##########
File path: rust/datafusion/src/physical_plan/union.rs
##########
@@ -60,15 +60,31 @@ impl ExecutionPlan for UnionExec {
 
     /// Output of the union is the combination of all output partitions of the 
inputs
     fn output_partitioning(&self) -> Partitioning {
-        // Sums all the output partitions
-        let num_partitions = self
-            .inputs
+        let intial: Option<Partitioning> = None;
+        self.inputs
             .iter()
-            .map(|plan| plan.output_partitioning().partition_count())
-            .sum();
-        // TODO: this loses partitioning info in case of same partitioning 
scheme (for example `Partitioning::Hash`)
-        // https://issues.apache.org/jira/browse/ARROW-11991
-        Partitioning::UnknownPartitioning(num_partitions)
+            .fold(intial, |acc, input| {
+                match (acc, input.output_partitioning()) {
+                    (None, partition) => Some(partition),
+                    (
+                        Some(Partitioning::Hash(mut vector_acc, size_acc)),
+                        Partitioning::Hash(vector, size),
+                    ) => {
+                        vector_acc.append(&mut vector.clone());

Review comment:
       🤔 it seems 
[stackoverflow](https://stackoverflow.com/questions/25339603/how-to-test-for-equality-between-trait-objects)
 has some context
   
   Perhap we need to implement our own version of equality for `PhysicalExpr` 
or something.
   
   I am not sure how to proceed, to be honest. @Dandandan  any thoughts?




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