Github user mengxr commented on the pull request:
https://github.com/apache/spark/pull/9380#issuecomment-155630541
@JihongMA Could you merge the current master? There are some merge
conflicts.
For `NaN` vs. `null`, we had some discussion in
https://issues.apache.org/jira/browse/SPARK-9079. The design is to return `NaN`
is there exist `NaN` values in the aggregation. I think we should return `NaN`
here, which is consistent with R and Python:
~~~R
> mean(c())
[1] NA
> var(c(1))
[1] NA
~~~
~~~python
> np.mean([])
Out[1] = na
> np.var([1], ddof=1)
Out[2] = nan
~~~
@marmbrus I think we can move the implementation from imperative to
declarative in 1.7. This PR is to re-use the `CentralMomentAgg` for `stddev`.
It removes 70 lines of code, which is a good sign:)
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