@Override
  public List<Expression> visitUnaryExpression(UnaryExpression unaryExpression, Void context) {
    return mergeList(getResultsFromChild(unaryExpression, null));
  }
Simply collects result from child-expression. For example, two child give me 3 and 4 results, I
will return 3+4 = 7 results to upper level.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/expression/visitor/CollectVisitor.java