public static boolean isSupportedKey(ObjectInspector keyOi) {
    List<? extends StructField> keyFields = ((StructObjectInspector)keyOi).getAllStructFieldRefs();
    for (StructField field : keyFields) {
      if (!MapJoinKey.isSupportedField(field.getFieldObjectInspector())) {
        return false;
      }
    }
    return true;
  }
For FULL OUTER MapJoin: Iterates through the Small Table hash table and returns the key and
value rows for any non-matched keys.
ql/src/java/org/apache/hadoop/hive/ql/exec/persistence/MapJoinBytesTableContainer.java