Github user marmbrus commented on the pull request:
https://github.com/apache/spark/pull/1428#issuecomment-49188774
You are right that this rule does more than null propagation now. I'm not
sure what a better name would be. `DegenerateExpressionSimplification`?
Regarding moving null propagation into the expressions, you could do it...
but what would it look like? You specify which of the children make the entire
expression null if they are null? Seems like a lot of refactoring for little
benefit...
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