Github user chenghao-intel commented on the pull request:
https://github.com/apache/spark/pull/1430#issuecomment-49127540
The code looks good to me.
I am thinking if we could merge the ConstantFolding & NullPropagation and
make it `transformExpressionsUp` , since they kind of rely on each other, and
people may always get confused when place a Null or Constant optimization.
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