[GitHub] [arrow] jacobBaumbach commented on a change in pull request #9806: ARROW-11991: [Rust][DataFusion] Maintain partition information in Union

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jacobBaumbach commented on a change in pull request #9806:
URL: https://github.com/apache/arrow/pull/9806#discussion_r606825668



##########
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:
       Just to make sure I am on the same page: 
   
   Two `Partitioning::Hash` may only be composed if they have the same expression and number of partitions.  If the two `Partitioning::Hash` differ in expression and/or number of partitions then it becomes a `UnknownPartitioning`. Correct?




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