milenkovicm commented on code in PR #9333:
URL: https://github.com/apache/arrow-datafusion/pull/9333#discussion_r1508171908


##########
datafusion/core/tests/user_defined/user_defined_scalar_functions.rs:
##########
@@ -619,6 +624,117 @@ async fn verify_udf_return_type() -> Result<()> {
     Ok(())
 }
 
+#[derive(Debug, Default)]
+struct MockFunctionFactory {
+    pub captured_expr: Mutex<Option<Expr>>,
+}
+
+#[async_trait::async_trait]
+impl FunctionFactory for MockFunctionFactory {
+    #[doc = r" Crates and registers a function from [CreateFunction] 
statement"]
+    #[must_use]
+    #[allow(clippy::type_complexity, clippy::type_repetition_in_bounds)]
+    async fn create(
+        &self,
+        _config: &SessionConfig,
+        statement: CreateFunction,
+    ) -> datafusion::error::Result<RegisterFunction> {
+        // this function is a mock for testing
+        // `CreateFunction` should be used to derive this function
+
+        let mock_add = Arc::new(|args: &[datafusion_expr::ColumnarValue]| {
+            let args = datafusion_expr::ColumnarValue::values_to_arrays(args)?;
+            let base =
+                
datafusion_common::cast::as_float64_array(&args[0]).expect("cast failed");
+            let exponent =
+                
datafusion_common::cast::as_float64_array(&args[1]).expect("cast failed");
+
+            let array = base
+                .iter()
+                .zip(exponent.iter())
+                .map(|(base, exponent)| match (base, exponent) {
+                    (Some(base), Some(exponent)) => 
Some(base.add_wrapping(exponent)),
+                    _ => None,
+                })
+                .collect::<arrow_array::Float64Array>();
+            Ok(datafusion_expr::ColumnarValue::from(
+                Arc::new(array) as arrow_array::ArrayRef
+            ))
+        });
+
+        let args = statement.args.unwrap();
+        let mock_udf = create_udf(
+            &statement.name,
+            vec![args[0].data_type.clone(), args[1].data_type.clone()],
+            Arc::new(statement.return_type.unwrap()),
+            datafusion_expr::Volatility::Immutable,
+            mock_add,
+        );
+
+        // capture expression so we can verify
+        // it has been parsed
+        *self.captured_expr.lock() = statement.params.return_;
+
+        Ok(RegisterFunction::Scalar(Arc::new(mock_udf)))
+    }
+
+    async fn remove(

Review Comment:
   lets say we have a SQL UDFs which are expressed by expr, we would need to 
have some kind of registry (which is not udf_registry) which can be consulted 
by plan analyser.
   
   so in this case we would have some kind of mock function stored in function 
registry (which would do nothing, just for planning) and have an expression 
registry, which would be used to "replace" udf's in logical plan ... 
   
   btw, i believe adding sql udf would be trivial once we get this change and 
fix few issues with expression namings



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