pepijnve commented on code in PR #23407:
URL: https://github.com/apache/datafusion/pull/23407#discussion_r3554178502


##########
datafusion/physical-plan/src/sorts/merge.rs:
##########
@@ -171,24 +152,40 @@ impl<C: CursorValues> SortPreservingMergeStream<C> {
             in_progress: BatchBuilder::new(schema, stream_count, batch_size, 
reservation),
             streams,
             metrics,
-            done: false,
-            drain_in_progress_on_done: false,
             cursors: (0..stream_count).map(|_| None).collect(),
             prev_cursors: (0..stream_count).map(|_| None).collect(),
             round_robin_tie_breaker_mode: false,
             num_of_polled_with_same_value: vec![0; stream_count],
             current_reset_epoch: 0,
             poll_reset_epochs: vec![0; stream_count],
             loser_tree: vec![],
-            loser_tree_adjusted: false,
             batch_size,
             fetch,
             produced: 0,
-            uninitiated_partitions: (0..stream_count).collect(),
             enable_round_robin_tie_breaker,
         }
     }
 
+    pub(crate) fn into_stream(self) -> SendableRecordBatchStream
+    where
+        C: 'static,
+    {
+        let schema_clone = Arc::clone(self.in_progress.schema());
+
+        let cloned_metrics = self.metrics.clone();
+
+        let mut this = self;
+        let stream = Gen::new(|co| async move {

Review Comment:
   Looking at the implementation of `async-stream` and comparing to 
`sync::Gen`, the `async-stream` implementation uses a thread local `Cell` while 
`sync::Gen` uses a mutex. It's only one mutex acquire per batch of course, but 
this does incur some cost compared to the code it's replacing. Is this a 
concern if the intention would be to apply this pattern more generally?



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