[GitHub] [arrow] alamb commented on a change in pull request #8034: ARROW-9464: [Rust] [DataFusion] Physical plan optimization rule to insert MergeExec when needed

[GitBox <gi...@apache.org>]

alamb commented on a change in pull request #8034:
URL: https://github.com/apache/arrow/pull/8034#discussion_r475537669



##########
File path: rust/datafusion/src/execution/physical_plan/mod.rs
##########
@@ -74,6 +85,15 @@ impl Partitioning {
     }
 }
 
+/// Distribution schemes
+#[derive(Debug, Clone)]
+pub enum Distribution {
+    /// Unspecified distribution

Review comment:
       I think "unspecified" is more accurate -- specifically the data is distributed around all the partitions, but the optimizer/planner doesn't know *how* it is distributed

##########
File path: rust/datafusion/src/execution/physical_plan/parquet.rs
##########
@@ -91,11 +91,26 @@ impl ExecutionPlan for ParquetExec {
         self.schema.clone()
     }
 
+    fn children(&self) -> Vec<Arc<dyn ExecutionPlan>> {
+        // this is a leaf node and has no children
+        vec![]
+    }
+
     /// Get the output partitioning of this plan
     fn output_partitioning(&self) -> Partitioning {
         Partitioning::UnknownPartitioning(self.filenames.len())
     }
 
+    fn with_new_children(
+        &self,
+        _: Vec<Arc<dyn ExecutionPlan>>,
+    ) -> Result<Arc<dyn ExecutionPlan>> {
+        Err(ExecutionError::General(format!(

Review comment:
       This is fine for this PR, but I think this implementation could actually check that children was empty and then return `self.clone()`.

##########
File path: rust/datafusion/src/execution/physical_plan/mod.rs
##########
@@ -50,6 +50,17 @@ pub trait ExecutionPlan: Debug + Send + Sync {
     fn schema(&self) -> SchemaRef;
     /// Specifies the output partitioning scheme of this plan
     fn output_partitioning(&self) -> Partitioning;
+    /// Specifies the data distribution requirements of all the children for this operator
+    fn required_child_distribution(&self) -> Distribution {
+        Distribution::UnspecifiedDistribution
+    }
+    /// Get the children of this plan

Review comment:
       Or maybe
   
   ```
   /// `children` of this plan. This corresponds to all plans that provide input to this plan. 
   /// This function should return an empty vector for leaf nodes such as `scans`.
   ```




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