Dandandan commented on code in PR #15380:
URL: https://github.com/apache/datafusion/pull/15380#discussion_r2050297829


##########
datafusion/physical-plan/src/sorts/sort.rs:
##########
@@ -662,53 +665,152 @@ impl ExternalSorter {
         let elapsed_compute = metrics.elapsed_compute().clone();
         let _timer = elapsed_compute.timer();
 
-        // Please pay attention that any operation inside of 
`in_mem_sort_stream` will
-        // not perform any memory reservation. This is for avoiding the need 
of handling
-        // reservation failure and spilling in the middle of the sort/merge. 
The memory
-        // space for batches produced by the resulting stream will be reserved 
by the
-        // consumer of the stream.
+        // Note, in theory in memory batches should have limited size, but 
some testing
+        // cases testing the memory limit use `sort_in_place_threshold_bytes` 
to, so here we
+        // set a larger limit to avoid testing failure.
+        if self.expr.len() <= 2
+            && self.reservation.size() < 1000 * 
self.sort_in_place_threshold_bytes
+        {
+            let interleave_indices = self.build_sorted_indices(
+                self.in_mem_batches.as_slice(),
+                Arc::clone(&self.expr),
+            )?;
 
-        if self.in_mem_batches.len() == 1 {
-            let batch = self.in_mem_batches.swap_remove(0);
-            let reservation = self.reservation.take();
-            return self.sort_batch_stream(batch, metrics, reservation);
-        }
+            let batches: Vec<&RecordBatch> = 
self.in_mem_batches.iter().collect();
+            let sorted_batch = interleave_record_batch(&batches, 
&interleave_indices)?;
 
-        // If less than sort_in_place_threshold_bytes, concatenate and sort in 
place
-        if self.reservation.size() < self.sort_in_place_threshold_bytes {
-            // Concatenate memory batches together and sort
-            let batch = concat_batches(&self.schema, &self.in_mem_batches)?;
             self.in_mem_batches.clear();
             self.reservation
-                .try_resize(get_reserved_byte_for_record_batch(&batch))
+                .try_resize(get_reserved_byte_for_record_batch(&sorted_batch))
                 .map_err(Self::err_with_oom_context)?;
-            let reservation = self.reservation.take();
-            return self.sort_batch_stream(batch, metrics, reservation);
+
+            metrics.record_output(sorted_batch.num_rows());
+
+            Ok(Box::pin(RecordBatchStreamAdapter::new(
+                Arc::clone(&self.schema),
+                futures::stream::once(async { Ok(sorted_batch) }),
+            )) as SendableRecordBatchStream)
+        } else {
+            // Please pay attention that any operation inside of 
`in_mem_sort_stream` will
+            // not perform any memory reservation. This is for avoiding the 
need of handling
+            // reservation failure and spilling in the middle of the 
sort/merge. The memory
+            // space for batches produced by the resulting stream will be 
reserved by the
+            // consumer of the stream.
+
+            if self.in_mem_batches.len() == 1 {
+                let batch = self.in_mem_batches.swap_remove(0);
+                let reservation = self.reservation.take();
+                return self.sort_batch_stream(batch, metrics, reservation);
+            }
+
+            // If less than sort_in_place_threshold_bytes, concatenate and 
sort in place
+            if self.reservation.size() < self.sort_in_place_threshold_bytes {
+                // Concatenate memory batches together and sort
+                let batch = concat_batches(&self.schema, 
&self.in_mem_batches)?;

Review Comment:
   Here we still can avoid concat_batches (only concat sort keys like for the 
other case)



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