improve comments.

Author: Rachelint

datafusion/physical-plan/src/aggregates/group_values/aggregation_hashtable.rs

@@ -101,13 +101,18 @@ impl<T: AggregationHashTableEntry> AggregationHashTable<T> {
                 let table_entry = match &table_entry {
                     AggregationHashTableEntryState::Empty => {
                         self.pending_append_indices.push(row_idx);
+                        // We place the `future group index` into `raw_table`.
+                        // And if rows with same hash values(repeated row in most situations)
+                        // exist in input `cols`, it is able to get this `future group index` here
+                        // and use compare the related `group value` after it is really inserted.
                         *table_entry = AggregationHashTableEntryState::PlaceHolder(
                             current_total_groups,
                         );
                         current_total_groups += 1;
                         continue;
                     }
                     AggregationHashTableEntryState::PlaceHolder(group_index) => {
+                        // The situation that rows with same ash values exist in `cols` mentioned above
                         let equal_to_ctx = EqualToContext {
                             row_index: row_idx,
                             group_index: *group_index,