[GitHub] [spark] JoshRosen commented on issue #24557: [SPARK-27653][SQL] Add max_by() SQL aggregate function

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JoshRosen commented on issue #24557: [SPARK-27653][SQL] Add max_by() SQL aggregate function
URL: https://github.com/apache/spark/pull/24557#issuecomment-490549494
 
 
   Hi @viirya,
   
   Thanks for working on this! 
   
   I had a few quick questions:
   
   - Could you also implement `min_by(x, y)`?
     - It looks like you might be able to share most of the code except for replacing `GreaterThan` and `greatest`, so maybe this difference could be abstracted away via a shared abstract superclass.
   - Presto also has three-argument versions of `max_by` / `min_by`:
   
     > max_by(x, y, n) → array<[same as x]>
     > Returns n values of x associated with the n largest of all input values of y in descending order of y.
   
     I don't think we need to do this version now, especially since we can always add it in a separate followup PR (which is what Presto originally did: https://github.com/prestodb/presto/issues/3620)
   - Were there any bugs in older implementations of Presto version that we might have replicated here? Or Presto tests for edge-cases that we could emulate?
     - https://github.com/prestodb/presto/issues/7646 discusses using rows / structs as the ordering value. I suspect that would work already in your implementation, but it might be nice to check.
     - https://github.com/prestodb/presto/issues/2040 discusses a problem with null ordering values. It looks like you're testing that case, but I just wanted to double-check that we've covered the same edge-case there.

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