[GitHub] [arrow-datafusion] yahoNanJing commented on a diff in pull request #6065: Optimize row hash

["yahoNanJing (via GitHub)" <gi...@apache.org>]

yahoNanJing commented on code in PR #6065:
URL: https://github.com/apache/arrow-datafusion/pull/6065#discussion_r1174503169


##########
datafusion/core/src/physical_plan/aggregates/row_hash.rs:
##########
@@ -772,22 +773,22 @@ fn slice_and_maybe_filter(
     filter_opt: Option<&Arc<dyn Array>>,
     offsets: &[usize],
 ) -> Result<Vec<ArrayRef>> {
-    let sliced_arrays: Vec<ArrayRef> = aggr_array
-        .iter()
-        .map(|array| array.slice(offsets[0], offsets[1] - offsets[0]))
-        .collect();
+    let null_array = Arc::new(NullArray::new(0)) as ArrayRef;
+    let mut sliced_arrays: Vec<ArrayRef> = vec![null_array; aggr_array.len()];
 
-    let filtered_arrays = match filter_opt.as_ref() {
-        Some(f) => {
-            let sliced = f.slice(offsets[0], offsets[1] - offsets[0]);
-            let filter_array = as_boolean_array(&sliced)?;
+    if let Some(f) = filter_opt {
+        let sliced = f.slice(offsets[0], offsets[1] - offsets[0]);
+        let filter_array = as_boolean_array(&sliced)?;
 
-            sliced_arrays
-                .iter()
-                .map(|array| filter(array, filter_array).unwrap())
-                .collect::<Vec<ArrayRef>>()
+        for (i, arr) in aggr_array.iter().enumerate() {
+            let sliced = &arr.slice(offsets[0], offsets[1] - offsets[0]);
+            sliced_arrays[i] = filter(sliced, filter_array).unwrap();

Review Comment:
   Actually, I don't think it's a good idea to do more than one thing in a loop, especially the `slice` of Array is not so light weight behavior.



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