lyne7-sc commented on code in PR #20243:
URL: https://github.com/apache/datafusion/pull/20243#discussion_r2788510095


##########
datafusion/functions-nested/src/set_ops.rs:
##########
@@ -358,69 +364,84 @@ fn generic_set_lists<OffsetSize: OffsetSizeTrait>(
         "{set_op:?} is not implemented for '{l:?}' and '{r:?}'"
     );
 
-    let mut offsets = vec![OffsetSize::usize_as(0)];
-    let mut new_arrays = vec![];
+    // Convert all values to rows in batch for performance.
     let converter = RowConverter::new(vec![SortField::new(l.value_type())])?;
-    for (l_arr, r_arr) in l.iter().zip(r.iter()) {
-        let last_offset = *offsets.last().unwrap();
-
-        let (l_values, r_values) = match (l_arr, r_arr) {
-            (Some(l_arr), Some(r_arr)) => (
-                converter.convert_columns(&[l_arr])?,
-                converter.convert_columns(&[r_arr])?,
-            ),
-            _ => {
-                offsets.push(last_offset);
-                continue;
-            }
-        };
-
-        let l_iter = l_values.iter().sorted().dedup();
-        let values_set: HashSet<_> = l_iter.clone().collect();
-        let mut rows = if set_op == SetOp::Union {
-            l_iter.collect()
-        } else {
-            vec![]
-        };
+    let rows_l = converter.convert_columns(&[Arc::clone(l.values())])?;
+    let rows_r = converter.convert_columns(&[Arc::clone(r.values())])?;
+    let l_offsets = l.value_offsets();
+    let r_offsets = r.value_offsets();
+
+    let mut result_offsets = Vec::with_capacity(l.len() + 1);
+    result_offsets.push(OffsetSize::usize_as(0));
+    let mut final_rows = Vec::with_capacity(rows_l.num_rows());
+
+    // Reuse hash sets across iterations
+    let mut seen = HashSet::new();
+    let mut r_set = HashSet::new();
+    for i in 0..l.len() {
+        let last_offset = *result_offsets.last().unwrap();
+
+        if l.is_null(i) || r.is_null(i) {
+            result_offsets.push(last_offset);
+            continue;
+        }
 
-        for r_val in r_values.iter().sorted().dedup() {
-            match set_op {
-                SetOp::Union => {
-                    if !values_set.contains(&r_val) {
-                        rows.push(r_val);
+        let l_start = l_offsets[i].as_usize();
+        let l_end = l_offsets[i + 1].as_usize();
+        let r_start = r_offsets[i].as_usize();
+        let r_end = r_offsets[i + 1].as_usize();
+
+        let mut count = 0usize;
+        // Clear sets for reuse
+        seen.clear();
+        r_set.clear();
+
+        match set_op {

Review Comment:
   Good suggestion, using const generics could definitely give us a nice 
performance boost.
   ```
   group                               optimized                              
optimized_const_generic
   -----                               ---------                              
----------
   array_intersect/high_overlap/10     1.79   800.7±51.79µs        ? ?/sec    
1.00   446.9±15.17µs        ? ?/sec
   array_intersect/high_overlap/100    1.77      8.2±0.13ms        ? ?/sec    
1.00      4.6±0.08ms        ? ?/sec
   array_intersect/high_overlap/50     1.77      4.0±0.06ms        ? ?/sec    
1.00      2.3±0.07ms        ? ?/sec
   array_intersect/low_overlap/10      1.70   570.4±53.84µs        ? ?/sec    
1.00    335.3±4.74µs        ? ?/sec
   array_intersect/low_overlap/100     1.62      6.7±0.27ms        ? ?/sec    
1.00      4.2±0.07ms        ? ?/sec
   array_intersect/low_overlap/50      1.71      3.4±0.44ms        ? ?/sec    
1.00  1993.1±23.05µs        ? ?/sec
   array_union/high_overlap/10         1.62   548.4±30.79µs        ? ?/sec    
1.00    337.6±8.12µs        ? ?/sec
   array_union/high_overlap/100        2.06      7.5±2.17ms        ? ?/sec    
1.00      3.6±0.10ms        ? ?/sec
   array_union/high_overlap/50         1.53      2.8±0.06ms        ? ?/sec    
1.00  1805.2±72.23µs        ? ?/sec
   array_union/low_overlap/10          1.88  718.8±148.49µs        ? ?/sec    
1.00   382.7±15.45µs        ? ?/sec
   array_union/low_overlap/100         1.67      6.9±0.21ms        ? ?/sec    
1.00      4.1±0.13ms        ? ?/sec
   array_union/low_overlap/50          1.70      3.5±0.10ms        ? ?/sec    
1.00      2.0±0.06ms        ? ?/sec
   
   ```



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