private Set<RelColumnMapping> createColumnMappings(RelNode inputRel) {
    Set<RelColumnMapping> colMappings = new HashSet<>();
    for (int i = 0; i < inputRel.getRowType().getFieldCount(); ++i) {
      colMappings.add(new RelColumnMapping(i, 0, i, false));
    }
    return colMappings;
  }
get the return type for the lateral view. The return type will be a structure
where the first values are from the input RelNode. Then the output values from the
udtf will be added into the structure for the return type.
ql/src/java/org/apache/hadoop/hive/ql/parse/relnodegen/LateralViewPlan.java