rluvaton commented on code in PR #23407:
URL: https://github.com/apache/datafusion/pull/23407#discussion_r3570273331


##########
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:
   I tried `try_unfold`/`fold` the problem with that is that it keep the flow 
of code the same - that is - first check if you are finished, then have code to 
reconstruct where you last left off - which is both bad for perf and making it 
harder to understand.
   
   having generator like you are able to keep the same flow in context.



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