Github user StephanEwen commented on the pull request:
https://github.com/apache/flink/commit/30c9e2b683bf7f4776ffc23b6a860946a4429ae5#commitcomment-21018833
Great to have this in!
I would suggest to mention in the JavaDocs of `MapState.size()` that this
can be a potentially expensive operation, because it may entail iterating over
many entries in some state backends (like RocksDB).
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