diff --git a/src/table/src/table/scan.rs b/src/table/src/table/scan.rs
index d6ac26c0a21..ef8439bd8e9 100644
--- a/src/table/src/table/scan.rs
+++ b/src/table/src/table/scan.rs
@@ -599,9 +599,23 @@ impl RegionScanExec {
             ordering,
         )?;
         let partition_count = scanner.properties().num_partitions().max(1);
+        // Every series arrives whole in one partition, and `read_key` is its primary key, so the
+        // key says exactly that: hashing it groups the rows of one series and splits nothing. It
+        // is declared rather than left unspecified because a consumer that holds one window per
+        // series has to be told the series is whole - an optimizer free to round-robin the
+        // partition would hand it two halves of one series, each marked final by its own stream,
+        // and both would answer half the truth without saying so.
+        // The name is the one `raw_series_frame_schema` gives the column.
+        let partitioning = match Column::new_with_schema("read_key", &arrow_schema) {
+            Ok(read_key) => Partitioning::Hash(vec![Arc::new(read_key) as _], partition_count),
+            // The schema this leaf builds always has the column; losing it would mean the frame
+            // schema changed shape, and claiming a distribution over a column that is not there
+            // would be worse than saying nothing.
+            Err(_) => Partitioning::UnknownPartitioning(partition_count),
+        };
         let properties = Arc::new(PlanProperties::new(
             EquivalenceProperties::new(arrow_schema.clone()),
-            Partitioning::UnknownPartitioning(partition_count),
+            partitioning,
             EmissionType::Incremental,
             Boundedness::Bounded,
         ));
diff --git a/src/table/src/table/scan/series_tests.rs b/src/table/src/table/scan/series_tests.rs
index 19209a50af3..01c1189533c 100644
--- a/src/table/src/table/scan/series_tests.rs
+++ b/src/table/src/table/scan/series_tests.rs
@@ -699,9 +699,11 @@ async fn native_raw_frame_executes_and_decodes_bound_data() {
         })
         .unwrap();
     assert!(native_bytes > 0);
+    // One series arrives whole in one partition, and `read_key` says which series a row belongs
+    // to, so the leaf declares that as its distribution instead of leaving it unspecified.
     assert!(matches!(
         plan.properties().output_partitioning(),
-        Partitioning::UnknownPartitioning(1)
+        Partitioning::Hash(columns, 1) if columns.len() == 1
     ));
     assert!(plan.properties().output_ordering().is_none());
 }
@@ -1287,7 +1289,7 @@ async fn invalid_partition_and_nonempty_children_are_rejected() {
         .unwrap();
     assert!(matches!(
         plan.properties().output_partitioning(),
-        Partitioning::UnknownPartitioning(2)
+        Partitioning::Hash(columns, 2) if columns.len() == 1
     ));
     assert!(
         plan.execute(1, context())
@@ -1309,7 +1311,7 @@ async fn invalid_partition_and_nonempty_children_are_rejected() {
         .unwrap();
     assert!(matches!(
         changed_plan.properties().output_partitioning(),
-        Partitioning::UnknownPartitioning(3)
+        Partitioning::Hash(columns, 3) if columns.len() == 1
     ));
     assert!(plan.execute(0, context()).is_err());
     assert!(plan.execute(2, context()).is_err());