Github user fhueske commented on the issue:
https://github.com/apache/flink/pull/4372
I think you are right @greghogan.
It's not about the ratio of #distinct keys to size of the dataset. But it's
also not only the ratio of #distinct keys to size of the memory. The skew of
the key distribution has an effect as well (hash-based combiners should better
handle skew than sort-based combiners).
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