fix comments

datafusion/physical-expr/src/array_expressions.rs

@@ -570,13 +570,16 @@ fn general_array_pop(
 ) -> Result<(Vec<i64>, Vec<i64>)> {
     if from_back {
         let key = vec![0; list_array.len()];
+        // Atttetion: `arr.len() - 1` in extra key defines the last element position (position = index + 1, not inclusive) we want in the new array.
         let extra_key: Vec<_> = list_array
             .iter()
             .map(|x| x.map_or(0, |arr| arr.len() as i64 - 1))
             .collect();
         Ok((key, extra_key))
     } else {
-        let key: Vec<_> = list_array.iter().map(|x| x.map_or(0, |_arr| 2)).collect();
+        // Atttetion: 2 in the `key`` defines the first element position (position = index + 1) we want in the new array.
+        // We only handle two cases of the first element index: if the old array has any elements, starts from 2 (index + 1), or starts from initial.
+        let key: Vec<_> = list_array.iter().map(|x| x.map_or(0, |_| 2)).collect();
         let extra_key: Vec<_> = list_array
             .iter()
             .map(|x| x.map_or(0, |arr| arr.len() as i64))