Github user hvanhovell commented on the pull request:
https://github.com/apache/spark/pull/9419#issuecomment-153146768
If I understand the problem correctly, the logical Expand operator makes
items which are not in the grouping set ```null```. This means that if a column
is both used for grouping and for aggregation and that column is not in the
grouping set, this column will be set to ```null``` and the aggregate will also
become ```null``` (which is not correct).
What I don't understand is why we are not fixing the logical Expand by
introducing two sets of expressions, one for grouping and one for aggregation.
This is much simpler and fixes the problem at its root.
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