@Override
  public String toString() {
    return toStringHelper(this).addValue(relationId).addValue(fieldIndex).toString();
  }
Returns {@link RelationType#indexOf(Field) field index} of the field in the containing
relation.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/analyzer/FieldId.java