Method simplify

datafusion-examples/examples/udf/advanced_udwf.rs:191–208

this function will simplify `SimplifySmoothItUdf` to `AggregateUDF` for `Avg` default implementation will not be called (left as `todo!()`)

(&self)

    /// this function will simplify `SimplifySmoothItUdf` to `AggregateUDF` for `Avg`
    /// default implementation will not be called (left as `todo!()`)
    fn simplify(&self) -> Option<WindowFunctionSimplification> {
        let simplify = |window_function: WindowFunction, _: &SimplifyContext| {
            Ok(Expr::from(WindowFunction {
                fun: WindowFunctionDefinition::AggregateUDF(avg_udaf()),
                params: WindowFunctionParams {
                    args: window_function.params.args,
                    partition_by: window_function.params.partition_by,
                    order_by: window_function.params.order_by,
                    window_frame: window_function.params.window_frame,
                    null_treatment: window_function.params.null_treatment,
                    distinct: window_function.params.distinct,
                    filter: window_function.params.filter,
                },
            }))
        };

        Some(Box::new(simplify))
    }

    fn field(&self, field_args: WindowUDFFieldArgs) -> Result<FieldRef> {
        Ok(Field::new(field_args.name(), DataType::Float64, true).into())