sarutak commented on PR #55912:
URL: https://github.com/apache/spark/pull/55912#issuecomment-4594268126

   @cloud-fan 
   Thank you for the feedback.
   
   > Two caveats: (a) a shared scanner base gives robustness/maintenance parity 
but not whole-stage codegen — that lives in the operator's doProduce and stays 
future work; (b) the Backward "reverse scan needs random access" concern (the 
cited blocker to spill) is avoidable — since the group is sorted ascending and 
the as-of predicate is monotone, a forward scan keeping the last 
as-of-satisfying row finds the same nearest match, so 
ExternalAppendOnlyUnsafeRowArray's forward-only generateIterator(startIndex) 
suffices. Non-blocking — an architecture direction; the null-key fix below 
should land regardless of whether this refactor happens here or in a follow-up.
   
   Let me do them in follow-up PRs.


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