Github user StephanEwen commented on the issue:
https://github.com/apache/flink/pull/5236
If we want to implement something like that, we should tap into what the
data structures give us anyways, so we don't add additional overhead. Its a bit
tricky to do that, though, because especially for RocksDB, it is not easily
possible to know the number of keys at any point in time (due to the LSM model
and the lazy merging).---
