default UTF8String stringValueAt(int index) {
        throw new UnsupportedOperationException();
    }
The user should <b>NOT</b> cache the returned {@link UTF8String} instance,
normally it just a pointer pointed to some memory which could be freed or changed at any time.
The right way to hold those value is using {@link UTF8String#clone()} to create a new one.
indexr-segment/src/main/java/io/indexr/segment/DPValues.java