alamb commented on issue #9972:
URL: 
https://github.com/apache/arrow-datafusion/issues/9972#issuecomment-2041024779

   > First / Last aggregate function has the method fn reverse_expr(&self) -> 
Option<Arc<dyn AggregateExpr>> that returns another new AggregateExpr. We can't 
support this method since AggregateExpr is in functions-aggregate (I plan to 
move it here in https://github.com/apache/arrow-datafusion/pull/9960, it 
doesn't work too if we keep it in physical-expr-common). I propose that we move 
AggregateUDFImpl to functions-aggregate.
   
   I don't understand why it wouldn't work if we kept `AggregateExpr` in 
`physical-expr-common`?
   
   Since `functions-aggregate` depends on `physical-expr-common` then I think 
it would be reasonable for `FirstValueUDAF` that is defined 
`functions-aggregate` to return  a `LastValueUDAF` (wrapped in an 
`AggregateExpr`) as the result of `reverse_expr`
   
   I may be missing something though


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