List<Expression> mergeList(List<List<Expression>> listOfList) {
    List<Expression> result = new ArrayList<>();
    listOfList.forEach(result::addAll);
    return result;
  }
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