Github user mengxr commented on the pull request:
https://github.com/apache/spark/pull/9003#issuecomment-150939324
@sethah It seems that `VARIANCE` in HIVE returns the population variance.
It is the same in MySQL:
https://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_variance.
So let's keep the same behavior. Then it is a little awkward that `stddev` is
sample variance in our implementation. HIVE doesn't have `stddev`, while MySQL
uses population stddev for `stddev`. I think it is more important to be
compatible with existing SQL engines. We can address the `stddev` in a
follow-up PR.
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