private void writeSortKey(
      SortKey sortKey, ColumnBuilder[] columnBuilders, ColumnBuilder timeColumnBuilder) {
    appendTime(timeColumnBuilder, sortKey.tsBlock.getTimeByIndex(sortKey.rowIndex));
    for (int i = 0; i < columnBuilders.length; i++) {
      if (sortKey.tsBlock.getColumn(i).isNull(sortKey.rowIndex)) {
        columnBuilders[i].appendNull();
      } else {
        columnBuilders[i].write(sortKey.tsBlock.getColumn(i), sortKey.rowIndex);
      }
    }
  }
todo: directly serialize the sorted line instead of copy into a new tsBlock.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/utils/sort/DiskSpiller.java