Re: [I] Support `Expr` creation for `ScalarUDF`: Resolve function calls by name during planning [arrow-datafusion]

["edmondop (via GitHub)" <gi...@apache.org>]

edmondop commented on issue #8157:
URL: https://github.com/apache/arrow-datafusion/issues/8157#issuecomment-1843411377

   @alamb I checked out and on main the structs and enums are already written as you recommended
   
   ```rust
   pub enum ScalarFunctionDefinition {
     /// Resolved to a built in scalar function
     /// (will be removed long term)
     BuiltIn(built_in_function::BuiltinScalarFunction),
     /// Resolved to a user defined function
     UDF(ScalarUDF),
     /// A scalar function that will be called by name
     Name(Arc<str>),
   }
   
   #[derive(Clone, PartialEq, Eq, Hash, Debug)]
   pub struct ScalarFunction {
       /// The function
       pub fun: ScalarFunctionDefinition,
       /// List of expressions to feed to the functions as arguments
       pub args: Vec<Expr>,
   }
   ``` 
   
   So I am not sure I understood correctly. I could see that `call_fn` resolves at invocation site rather than at planning site,  and changing the behavior would probably mean changing this code. What is the right type of Expr this code should return?
   ```rust
   /// Calls a named built in function
   /// ```
   /// use datafusion_expr::{col, lit, call_fn};
   ///
   /// // create the expression sin(x) < 0.2
   /// let expr = call_fn("sin", vec![col("x")]).unwrap().lt(lit(0.2));
   /// ```
   pub fn call_fn(name: impl AsRef<str>, args: Vec<Expr>) -> Result<Expr> {
       match name.as_ref().parse::<BuiltinScalarFunction>() {
           Ok(fun) => Ok(Expr::ScalarFunction(ScalarFunction::new(fun, args))),
           Err(e) => Err(e),
       }
   }
   ```


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