mzabaluev commented on code in PR #19496:
URL: https://github.com/apache/datafusion/pull/19496#discussion_r2661996145


##########
datafusion/functions-window/src/nth_value.rs:
##########
@@ -519,6 +467,87 @@ impl PartitionEvaluator for NthValueEvaluator {
     }
 }
 
+impl NthValueEvaluator {
+    fn valid_index(&self, array: &ArrayRef, range: &Range<usize>) -> 
Option<usize> {
+        let n_range = range.end - range.start;
+        if self.ignore_nulls {
+            // Calculate valid indices, inside the window frame boundaries.
+            let slice = array.slice(range.start, n_range);
+            if let Some(nulls) = slice.nulls() {

Review Comment:
   Ah yes, I somehow confused the null count check with valid_indices 
retrieving no values, while it's the opposite. Also I'm a bit new to 
Arrow/Datafusion, so it's not clear to me if there are non-pathological 
scenarios that could produce an array with the null buffer present, but 
containing no nulls.



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