code: public SortPartitionOperator<T> sortPartition(int field, Order order) {
  		if (useKeySelector) {
  			throw new InvalidProgramException("Expression keys cannot be appended after a KeySelector");
  		}
  
  		ensureSortableKey(field);
  		keys.add(new Keys.ExpressionKeys<>(field, getType()));
  		orders.add(order);
  
  		return this;
  	}
docs: Appends an additional sort order with the specified field in the specified order to the
  local partition sorting of the DataSet.
  
  @param field The field index of the additional sort order of the local partition sorting.
  @param order The order of the additional sort order of the local partition sorting.
  @return The DataSet with sorted local partitions.