[GitHub] [spark] cloud-fan commented on pull request #38034: [SPARK-40599][SQL] Add multiTransform methods to TreeNode to generate alternatives

[GitBox <gi...@apache.org>]

cloud-fan commented on PR #38034:
URL: https://github.com/apache/spark/pull/38034#issuecomment-1382041199

   I'm a bit hesitant to touch the core `TreeNode` library as these things can be hard to generalize. For example, for output ordering `a + b`, if the project list is `a, a as x, b as y1, b as y2`, we should produce `a + y1, a + y2, x + y2, x + y2`. Note that:
   1. sometimes we only want to replace one alias as the original attribute `a` is still in the output
   2. We might do some early pruning to avoid exponential growth
   3. We can strip some expression from the project list, e.g. `empty2null(a)` -> `a`
   
   Are you sure the `multiTransform` can meet all these requirements? Can you give an example about how to use it to implement the algorithm in https://github.com/apache/spark/pull/39556#discussion_r1069575600 ?


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