Defensively verifiying that all children plans have the same count of…

… output partitions

src/query_stage.rs

@@ -23,6 +23,7 @@ use datafusion::prelude::SessionContext;
 use datafusion_proto::bytes::physical_plan_from_bytes_with_extension_codec;
 use pyo3::prelude::*;
 use pyo3::types::PyBytes;
+use std::collections::HashSet;
 use std::sync::Arc;
 
 #[pyclass(name = "QueryStage", module = "datafusion_ray", subclass)]
@@ -99,14 +100,23 @@ impl QueryStage {
     /// Get the input partition count. This is the same as the number of concurrent tasks
     /// when we schedule this query stage for execution
     pub fn get_input_partition_count(&self) -> usize {
-        if self.plan.children().is_empty() {
-            // leaf node (file scan)
-            self.plan.output_partitioning().partition_count()
-        } else {
-            self.plan.children()[0]
-                .output_partitioning()
-                .partition_count()
+        let mut output_partition_counts = HashSet::new();
+
+        for child in self.plan.children() {
+            output_partition_counts.insert(child.output_partitioning().partition_count());
+            if output_partition_counts.len() > 1 {
+                panic!(
+                    "Children plan of {:#?} have a distinct output partitioning partition count",
+                    self.plan
+                );
+            }
         }
+        // If this stage is a leaf node (file scan), it won't have children
+        // so we return the partition count of the plan itself
+        output_partition_counts
+            .into_iter()
+            .next()
+            .unwrap_or(self.plan.output_partitioning().partition_count())
     }
 
     pub fn get_output_partition_count(&self) -> usize {