@Override
  public long calculateMaxPeekMemory() {
    // while doing linear fill, we may need to copy the corresponding column if there exists null
    // values, and we may also need to cache next TsBlock to get next not null value
    // so the max peek memory may be triple or more, here we just use 3 as the estimated factor
    // because in most cases, we will get next not null value in next TsBlock
    return 3 * child.calculateMaxPeekMemory() + child.calculateRetainedSizeAfterCallingNext();
  }
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