/**
   * Looks up a field with a given name, returning null if not found.
   *
   * @param rowType Row type
   * @param columnName Field name
   * @return Field, or null if not found
   */
  public static RelDataTypeField lookupField(
      boolean caseSensitive, final RelDataType rowType, String columnName) {
    RelDataTypeField field = rowType.getField(columnName, caseSensitive);
    if (field != null) {
      return field;
    }

    // If record type is flagged as having "any field you ask for",
    // return a type. (TODO: Better way to mark accommodating types.)
    RelDataTypeField extra = RelDataTypeImpl.extra(rowType);
    if (extra != null) {
      return new RelDataTypeFieldImpl(columnName, -1, extra.getType());
    }
    return null;
  }
 public static RelDataType createTypeFromProjection(
     RelDataType type,
     List<String> columnNameList,
     RelDataTypeFactory typeFactory,
     boolean caseSensitive) {
   // If the names in columnNameList and type have case-sensitive differences,
   // the resulting type will use those from type. These are presumably more
   // canonical.
   final List<RelDataTypeField> fields = new ArrayList<RelDataTypeField>(columnNameList.size());
   for (String name : columnNameList) {
     RelDataTypeField field = type.getField(name, caseSensitive);
     fields.add(type.getFieldList().get(field.getIndex()));
   }
   return typeFactory.createStructType(fields);
 }
 /**
  * Returns the "extra" field in a row type whose presence signals that fields will come into
  * existence just by asking for them.
  *
  * @param rowType Row type
  * @return The "extra" field, or null
  */
 public static RelDataTypeField extra(RelDataType rowType) {
   // Even in a case-insensitive connection, the name must be precisely
   // "_extra".
   return rowType.getField("_extra", true);
 }