westonpace commented on PR #34576:
URL: https://github.com/apache/arrow/pull/34576#issuecomment-1470912007

   I'm fine with this change so don't let this discussion block merging.
   
   > It would, the join code in R ensures that the join keys are present at the 
join step, then the select of left.a happens after
   
   Right.  But I'm more worried about `left.b`.  Maybe the case I came up with 
isn't good enough but I know there are some queries where you will inevitably 
end up with:
   
   ```
   Scan -> Project -> Join -> Project
   ```
   
   What expression is assigned to the scan in that case?  Is it the expression 
from the first projection (would be wrong), the expression from the second 
projection (would be wrong), or are you actually doing some kind of pushdown 
logic in R?  Or maybe you have some way of preventing this case?


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