add more comment

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

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

@@ -98,6 +98,20 @@ pub(crate) struct SortPreservingMergeStream<C: CursorValues> {
     cursors: Vec<Option<Cursor<C>>>,
 
     /// Configuration parameter to enable round-robin selection of tied winners of loser tree.
+    /// 
+    /// To address the issue of unbalanced polling between partitions due to tie-breakers being based 
+    /// on partition index, especially in cases of low cardinality, we are making changes to the winner 
+    /// selection mechanism. Previously, partitions with smaller indices were consistently chosen as the winners,
+    /// leading to an uneven distribution of polling. This caused upstream operator buffers for the other partitions
+    /// to grow excessively, as they continued receiving data without consuming it.
+    /// 
+    /// For example, an upstream operator like a repartition execution would keep sending data to certain partitions, 
+    /// but those partitions wouldn't consume the data if they weren't selected as winners. This resulted in inefficient buffer usage.
+    /// 
+    /// To resolve this, we are modifying the tie-breaking logic. Instead of always choosing the partition with the smallest index, 
+    /// we now select the partition that has the fewest poll counts for the same value. 
+    /// This ensures that multiple partitions with the same value are chosen equally, distributing the polling load in a round-robin fashion. 
+    /// This approach balances the workload more effectively across partitions and avoids excessive buffer growth.Round robin tie breaker
     enable_round_robin_tie_breaker: bool,
 
     /// Flag indicating whether we are in the mode of round-robin