Github user cloud-fan commented on the pull request:
https://github.com/apache/spark/pull/12138#issuecomment-207689521
If my understanding is right, the problem is: when child output
changes(e.g. making them null when doing a roll up), the output of `Expand`
can't reflect it. I have a simpler idea, when set the output for `Expand`, use
a placeholder to reference to a child column, and define a `output` method in
`Expand` that replace the placeholder with child attribute. How about it?
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