@Override
  public Expression visitUnaryExpression(UnaryExpression unaryExpression, C context) {
    List<Expression> childResults = getResultsFromChild(unaryExpression, context);
    return reconstructUnaryExpression(unaryExpression, childResults.get(0));
  }
Collect result from child, then reconstruct. For example, two child each give me 1 result, I
should use them to reconstruct 1 new result to upper level.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/expression/visitor/ReconstructVisitor.java