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Signed-off-by: jayzhan211 <[email protected]>

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

@@ -445,9 +445,14 @@ impl<C: CursorValues> SortPreservingMergeStream<C> {
                 self.update_winner(cmp_node, winner, challenger);
             }
         } else if challenger < *winner {
-            // If the winner doesn't survive in the final match, it means the value has changed.
-            // The polls count are outdated (because the value advanced) but not yet cleaned-up at this point.
-            // Given the value is equal, we choose the smaller index if the value is the same.
+            // If the winner doesn’t survive in the final match, it indicates that the original winner
+            // has moved up in value, so the challenger now becomes the new winner.
+            // This also means that we’re in a new round of the tie breaker,
+            // and the polls count is outdated (though not yet cleaned up).
+            //
+            // By the time we reach this code, both the new winner and the current challenger
+            // have the same value, and neither has an updated polls count.
+            // Therefore, we simply select the one with the smaller index.
             self.update_winner(cmp_node, winner, challenger);
         }
     }

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

@@ -357,6 +357,8 @@ mod tests {
     use futures::{FutureExt, Stream, StreamExt};
     use tokio::time::timeout;
 
+    // The number in the function is highly related to the memory limit we are testing
+    // any change of the constant should be aware of
     fn generate_task_ctx_for_round_robin_tie_breaker() -> Result<Arc<TaskContext>> {
         let runtime = RuntimeEnvBuilder::new()
             .with_memory_limit(20_000_000, 1.0)
@@ -367,6 +369,8 @@ mod tests {
             .with_session_config(config);
         Ok(Arc::new(task_ctx))
     }
+    // The number in the function is highly related to the memory limit we are testing,
+    // any change of the constant should be aware of
     fn generate_spm_for_round_robin_tie_breaker(
         enable_round_robin_repartition: bool,
     ) -> Result<Arc<SortPreservingMergeExec>> {