Github user StefanRRichter commented on the issue:
https://github.com/apache/flink/pull/4963
@aljoscha IIRC there is no class for `StringAppendTESTOperator ` in the
RocksDB Java API. You can only pass a string that is used inside some factory
in the native RocksDB code to instantiate the append operator. I would also
suggest to simulate it, either "shallow" by adding commas or "deeper" by using
a dummy RocksDB instance internally to actually perform the merge and query for
the result.---
