mingmwang opened a new issue, #2175:
URL: https://github.com/apache/arrow-datafusion/issues/2175

   **Is your feature request related to a problem or challenge? Please describe 
what you are trying to do.**
   
   In current DataFusion code base,  `Expr`, `LogicalPlan` and `ExecutionPlan` 
are represented in different ways. Actually they are all tree based structures. 
 `Expr` and `LogicalPlan` are Enums, `ExecutionPlan` is a Trait. 
   The LogicalPlan enum wrapped the different logical operator structs like 
Projection and Filter, etc. 
   
   ````
   pub enum LogicalPlan {
       Projection(Projection),
       Filter(Filter),
       ............
   }
   ````
   
   But the Expr enum doesn't wrap the expression structs and define the 
different expressions directly in the enum.
   
   ````
   pub enum Expr {
       Alias(Box<Expr>, String),
       Column(Column),
       ............
       AggregateFunction {
           fun: aggregate_function::AggregateFunction,
           args: Vec<Expr>,
           distinct: bool,
       },
       WindowFunction {
           fun: window_function::WindowFunction,
           args: Vec<Expr>,
           partition_by: Vec<Expr>,
           order_by: Vec<Expr>,
           window_frame: Option<window_frame::WindowFrame>,
       },
       ............
   }
   ````
   
   I think we should unify the coding style, at least the Expr and LogicalPlan 
representations should follow the same style. And for physical ExecutionPlan, 
it is Trait/Trait Objects, I would prefer to use Enum also.  Want to hear 
thoughts from other members.
   
   
   **Describe the solution you'd like**
   A clear and concise description of what you want to happen.
   
   **Describe alternatives you've considered**
   A clear and concise description of any alternative solutions or features 
you've considered.
   
   **Additional context**
   Add any other context or screenshots about the feature request here.
   


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