Github user srowen commented on the pull request:
https://github.com/apache/spark/pull/13403
Yeah I find this surprising too. As far as I know, databases tend to treat
stddev as the sample stddev -- except Hive for some reason, AFAIK. I've never
quite understood the theoretical motivation for that. Maybe the idea is that
the aggregate is typically over some projection, or subset, of all data. But to
me, the default should logically be population stdev, since there's no inherent
reason to believe the result set is not the entire population of interest.
For RDDs, it seems even clearer that the behavior should be population
stddev.
For Datasets, maybe it's less surprising to do what databases usually do.
It does bear documentation for sure, but maybe not changing the behavior.
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