Github user cloud-fan commented on the issue:

    https://github.com/apache/spark/pull/20541
  
    I'm confused about why we need `PushProjectionThroughUnion`. Generally we 
only need to push down required columns, not entire project list, as there is 
no benifit of doing this. I think we just need to handle `Union` in the 
`ColumnPruning` rule, but I may miss something. cc @gatorsmile 


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