westonpace commented on pull request #12460:
URL: https://github.com/apache/arrow/pull/12460#issuecomment-1057580161


   From a consumption standpoint the concept of "scalar expression" is pretty 
important:
   
   Oracle: 
https://docs.oracle.com/cd/B19306_01/server.102/b14200/expressions010.htm
   CockroachDb: 
https://www.cockroachlabs.com/docs/stable/scalar-expressions.html
   Substrait: https://substrait.io/expressions/scalar_functions/
   
   For example, only a scalar expression can be used in a project expression.  
These are contrasted with "table expressions" which can return multiple rows 
for each execution.
   
   If Arrow wants to have a concept of "scalar function" and "vector function" 
that is different then I think that is "ok but confusing".  For example, to 
validate a plan, we can inspect the return value of the function, and ensure 
that it is scalar, to decide if an expression is a "scalar expression" 
regardless of whether it is a "scalar function".


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