Github user aalexandrov commented on the pull request:
https://github.com/apache/flink/pull/1517#issuecomment-202868272
Overall it seems that the hash-based combiner works better than the
sort-based one for (a) uniform, or normal key distribution, and (b)
fixed-length records.
For skewed key distribution (like Zipf) the two strategies are practically
equal, and for variable-length record the extra effort in compacting the record
offsets the advanges of the hash-based aggregation approach.
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