pepijnve commented on code in PR #18152:
URL: https://github.com/apache/datafusion/pull/18152#discussion_r2466398955


##########
datafusion/physical-expr/src/expressions/case.rs:
##########
@@ -283,70 +717,81 @@ impl CaseExpr {
     /// END
     fn case_when_no_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());
 
-        // start with nulls as default output
-        let mut current_value = new_null_array(&return_type, batch.num_rows());
-        let mut remainder = BooleanArray::from(vec![true; batch.num_rows()]);
-        let mut remainder_count = batch.num_rows();
-        for i in 0..self.when_then_expr.len() {
-            // If there are no rows left to process, break out of the loop 
early
-            if remainder_count == 0 {
-                break;
-            }
+        // `remainder_rows` contains the indices of the rows that need to be 
evaluated
+        let mut remainder_rows: ArrayRef =
+            Arc::new(UInt32Array::from_iter(0..batch.num_rows() as u32));
+        // `remainder_batch` contains the rows themselves that need to be 
evaluated
+        let mut remainder_batch = Cow::Borrowed(batch);
 
+        for i in 0..self.when_then_expr.len() {
+            // Evaluate the 'when' predicate for the remainder batch
+            // This results in a boolean array with the same length as the 
remaining number of rows
             let when_predicate = &self.when_then_expr[i].0;
-            let when_value = when_predicate.evaluate_selection(batch, 
&remainder)?;
-            let when_value = when_value.into_array(batch.num_rows())?;
+            let when_value = when_predicate
+                .evaluate(&remainder_batch)?
+                .into_array(remainder_batch.num_rows())?;

Review Comment:
   I had did a quick attempt at this, but it would make already complicated 
code even more complicated. Going to leave this for a future PR.



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