organize ExternalSorter fields (apache#13447)

datafusion/physical-plan/src/sorts/sort.rs

@@ -203,39 +203,54 @@ impl ExternalSorterMetrics {
 ///  in_mem_batches
 /// ```
 struct ExternalSorter {
-    /// schema of the output (and the input)
+    // ========================================================================
+    // PROPERTIES:
+    // Fields that define the sorter's configuration and remain constant
+    // ========================================================================
+    /// Schema of the output (and the input)
     schema: SchemaRef,
+    /// Sort expressions
+    expr: Arc<[PhysicalSortExpr]>,
+    /// If Some, the maximum number of output rows that will be produced
+    fetch: Option<usize>,
+    /// The target number of rows for output batches
+    batch_size: usize,
+    /// If the in size of buffered memory batches is below this size,
+    /// the data will be concatenated and sorted in place rather than
+    /// sort/merged.
+    sort_in_place_threshold_bytes: usize,
+
+    // ========================================================================
+    // STATE BUFFERS:
+    // Fields that hold intermediate data during sorting
+    // ========================================================================
     /// Potentially unsorted in memory buffer
     in_mem_batches: Vec<RecordBatch>,
     /// if `Self::in_mem_batches` are sorted
     in_mem_batches_sorted: bool,
+
     /// If data has previously been spilled, the locations of the
     /// spill files (in Arrow IPC format)
     spills: Vec<RefCountedTempFile>,
-    /// Sort expressions
-    expr: Arc<[PhysicalSortExpr]>,
+
+    // ========================================================================
+    // EXECUTION RESOURCES:
+    // Fields related to managing execution resources and monitoring performance.
+    // ========================================================================
     /// Runtime metrics
     metrics: ExternalSorterMetrics,
-    /// If Some, the maximum number of output rows that will be
-    /// produced.
-    fetch: Option<usize>,
+    /// A handle to the runtime to get spill files
+    runtime: Arc<RuntimeEnv>,
     /// Reservation for in_mem_batches
     reservation: MemoryReservation,
+
     /// Reservation for the merging of in-memory batches. If the sort
     /// might spill, `sort_spill_reservation_bytes` will be
     /// pre-reserved to ensure there is some space for this sort/merge.
     merge_reservation: MemoryReservation,
-    /// A handle to the runtime to get spill files
-    runtime: Arc<RuntimeEnv>,
-    /// The target number of rows for output batches
-    batch_size: usize,
     /// How much memory to reserve for performing in-memory sort/merges
     /// prior to spilling.
     sort_spill_reservation_bytes: usize,
-    /// If the in size of buffered memory batches is below this size,
-    /// the data will be concatenated and sorted in place rather than
-    /// sort/merged.
-    sort_in_place_threshold_bytes: usize,
 }
 
 impl ExternalSorter {