jgoday commented on issue #554:
URL: 
https://github.com/apache/arrow-datafusion/issues/554#issuecomment-873539365


   Hi @Jimexist, Could I take a look at this ?
   
   As I understand it:
   * a **BuiltInWindowFunctionExpr** and a **PartitionEvaluator** has to be 
implemented under _physical_plan/expressions_, **lag** and **lead** functions 
should be created with a name, an expression, and the offset/default_value 
optional arguments.
   
   * **return_type_for_built_in** and **signature_for_built_in** should be 
changed for Lag/Lead (physical_plan/window_functions)
   
   * signature has to be a **Signature::OneOf**( (expr -> result), (expr, 
offset) -> result, (expr, offset, default_value) -> result )
   
   I'm probably missing some details, but would this approach be valid ?


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