pepijnve commented on code in PR #18329:
URL: https://github.com/apache/datafusion/pull/18329#discussion_r2484769130


##########
datafusion/physical-expr/src/expressions/case.rs:
##########
@@ -556,63 +651,61 @@ impl CaseExpr {
         };
 
         if when_then_expr.is_empty() {
-            exec_err!("There must be at least one WHEN clause")
-        } else {
-            let eval_method = if expr.is_some() {
-                EvalMethod::WithExpression
-            } else if when_then_expr.len() == 1
-                && is_cheap_and_infallible(&(when_then_expr[0].1))
-                && else_expr.is_none()
-            {
-                EvalMethod::InfallibleExprOrNull
-            } else if when_then_expr.len() == 1
-                && when_then_expr[0].1.as_any().is::<Literal>()
-                && else_expr.is_some()
-                && else_expr.as_ref().unwrap().as_any().is::<Literal>()
-            {
-                EvalMethod::ScalarOrScalar
-            } else if when_then_expr.len() == 1 && else_expr.is_some() {
-                EvalMethod::ExpressionOrExpression
-            } else {
-                EvalMethod::NoExpression
-            };
-
-            Ok(Self {
-                expr,
-                when_then_expr,
-                else_expr,
-                eval_method,
-            })
+            return exec_err!("There must be at least one WHEN clause");
         }
+
+        let body = CaseBody {
+            expr,
+            when_then_expr,
+            else_expr,
+        };
+
+        let eval_method = if body.expr.is_some() {
+            EvalMethod::WithExpression(body.project()?)
+        } else if body.when_then_expr.len() == 1
+            && is_cheap_and_infallible(&(body.when_then_expr[0].1))
+            && body.else_expr.is_none()
+        {
+            EvalMethod::InfallibleExprOrNull
+        } else if body.when_then_expr.len() == 1
+            && body.when_then_expr[0].1.as_any().is::<Literal>()
+            && body.else_expr.is_some()
+            && body.else_expr.as_ref().unwrap().as_any().is::<Literal>()
+        {
+            EvalMethod::ScalarOrScalar
+        } else if body.when_then_expr.len() == 1 && body.else_expr.is_some() {
+            EvalMethod::ExpressionOrExpression(body.project()?)
+        } else {
+            EvalMethod::NoExpression(body.project()?)
+        };
+
+        Ok(Self { body, eval_method })
     }
 
     /// Optional base expression that can be compared to literal values in the 
"when" expressions
     pub fn expr(&self) -> Option<&Arc<dyn PhysicalExpr>> {
-        self.expr.as_ref()
+        self.body.expr.as_ref()
     }
 
     /// One or more when/then expressions
     pub fn when_then_expr(&self) -> &[WhenThen] {
-        &self.when_then_expr
+        &self.body.when_then_expr
     }
 
     /// Optional "else" expression
     pub fn else_expr(&self) -> Option<&Arc<dyn PhysicalExpr>> {
-        self.else_expr.as_ref()
+        self.body.else_expr.as_ref()
     }
 }
 
-impl CaseExpr {
-    /// This function evaluates the form of CASE that matches an expression to 
fixed values.
-    ///
-    /// CASE expression
-    ///     WHEN value THEN result
-    ///     [WHEN ...]
-    ///     [ELSE result]
-    /// END
-    fn case_when_with_expr(&self, batch: &RecordBatch) -> 
Result<ColumnarValue> {
-        let return_type = self.data_type(&batch.schema())?;
-        let mut result_builder = ResultBuilder::new(&return_type, 
batch.num_rows());
+impl CaseBody {
+    /// See [CaseExpr::case_when_with_expr].
+    fn case_when_with_expr(
+        &self,
+        batch: &RecordBatch,
+        return_type: &DataType,
+    ) -> Result<ColumnarValue> {
+        let mut result_builder = ResultBuilder::new(return_type, 
batch.num_rows());
 
         // `remainder_rows` contains the indices of the rows that need to be 
evaluated
         let mut remainder_rows: ArrayRef =

Review Comment:
   And here it is #18444.



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