feat(query): add comment

Author: sundy-li

src/query/functions-v2/src/aggregates/aggregate_distinct_state.rs

@@ -161,6 +161,7 @@ impl AggregateDistinctStringState {
             let data = unsafe { key.as_slice() };
 
             let holder = self.holders.last_mut().unwrap();
+            // TODO(sundy): may cause memory fragmentation, refactor this using arena
             if holder.may_resize(data.len()) {
                 let mut holder = StringColumnBuilder::with_capacity(
                     HOLDER_CAPACITY,

src/query/functions/src/aggregates/aggregate_distinct_state.rs

@@ -187,6 +187,7 @@ impl AggregateDistinctStringState {
             let data = unsafe { key.as_slice() };
 
             let holder = self.holders.last_mut().unwrap();
+            // TODO(sundy): may cause memory fragmentation, refactor this using arena
             if holder.may_resize(data.len()) {
                 let mut holder = MutableStringColumn::with_values_capacity(
                     HOLDER_BYTES_CAPACITY.max(data.len()),