houqp commented on pull request #55:
URL: https://github.com/apache/arrow-datafusion/pull/55#issuecomment-841614960


   After a lot of back and forth with @jorgecarleitao, I have updated the 
invariant doc 
(https://docs.google.com/document/d/1dbK-3eaTHlzZcHzpTk1h-LA3b7dcxsVBcoZeVKYIPwI)
 to enforce strict equality in all optimization invariants. As a result, 
optimizations like reordering join sides need to add an extra projection to 
maintain the field order. We think the extra projection should have minimal 
overhead, but helps keep the field order semantic simple and easy for end users 
to understand.
   
   This doc is ready for final review. In the mean time, I am going to work on 
fixing up the ballista builds.


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