diff --git a/src/promql/src/extension_plan/series_divide.rs b/src/promql/src/extension_plan/series_divide.rs
index 9d1d8f03e72e..064fb824bd2e 100644
--- a/src/promql/src/extension_plan/series_divide.rs
+++ b/src/promql/src/extension_plan/series_divide.rs
@@ -283,6 +283,7 @@ impl SeriesDivideStream {
     }
 
     fn find_first_diff_row(&self, batch: &RecordBatch) -> usize {
+        // fast path: no tag columns means all data belongs to the same series.
         if self.tag_indices.is_empty() {
             return batch.num_rows();
         }