message: Add comment to sort_state
diff: @@ -325,6 +325,9 @@ void GlobalSortState::AddLocalState(LocalSortState &local_sort_state) {
  }
  // Sort accumulated data
  + // we only re-order the heap when the data is expected to not fit in memory
  + // re-ordering the heap avoids random access when reading/merging but incurs a significant cost of shuffling data
  + // when data fits in memory, doing random access on reads is cheaper than re-shuffling
  local_sort_state.Sort(*this, external || !local_sort_state.sorted_blocks.empty());
  // Append local state sorted data to this global state
  