@Override
  public long ramBytesUsed() {
    return INSTANCE_SIZE
        + MemoryEstimationHelper.getEstimatedSizeOfAccountableObject(child)
        + MemoryEstimationHelper.getEstimatedSizeOfAccountableObject(operatorContext)
        + RamUsageEstimator.sizeOf(nextTsBlockIndex);
  }
indicate whether we can call child.next(). it's used to make sure that child.next() will only
be called once in AbstractLinearFillOperator.next().
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/process/AbstractLinearFillOperator.java