jayzhan211 commented on code in PR #12623:
URL: https://github.com/apache/datafusion/pull/12623#discussion_r1776133521


##########
datafusion/physical-plan/src/aggregates/group_values/group_value_row.rs:
##########
@@ -121,37 +145,64 @@ impl<T: ArrowPrimitiveType> ArrayRowEq for 
PrimitiveGroupValueBuilder<T> {
     }
 
     fn size(&self) -> usize {
-        self.group_values.allocated_size() + self.nulls.allocated_size()
+        // BooleanBufferBuilder builder::capacity returns capacity in bits 
(not bytes)
+        let null_builder_size = self
+            .nulls
+            .as_ref()
+            .map(|nulls| nulls.capacity() / 8)
+            .unwrap_or(0);
+        self.group_values.allocated_size() + null_builder_size
     }
 
     fn build(self: Box<Self>) -> ArrayRef {
-        if self.has_null {
-            Arc::new(PrimitiveArray::<T>::new(
-                ScalarBuffer::from(self.group_values),
-                Some(NullBuffer::from(self.nulls)),
-            ))
-        } else {
-            Arc::new(PrimitiveArray::<T>::new(
-                ScalarBuffer::from(self.group_values),
-                None,
-            ))
-        }
+        let Self {
+            group_values,
+            nulls,
+            nullable: _,
+        } = *self;
+        let null_buffer = nulls.map(|mut nulls| 
NullBuffer::from(nulls.finish()));
+
+        Arc::new(PrimitiveArray::<T>::new(
+            ScalarBuffer::from(group_values),
+            null_buffer,
+        ))
     }
 
     fn take_n(&mut self, n: usize) -> ArrayRef {
-        if self.has_null {
-            let first_n = self.group_values.drain(0..n).collect::<Vec<_>>();
-            let first_n_nulls = self.nulls.drain(0..n).collect::<Vec<_>>();
-            Arc::new(PrimitiveArray::<T>::new(
-                ScalarBuffer::from(first_n),
-                Some(NullBuffer::from(first_n_nulls)),
-            ))
-        } else {
-            let first_n = self.group_values.drain(0..n).collect::<Vec<_>>();
-            self.nulls.truncate(self.nulls.len() - n);
-            Arc::new(PrimitiveArray::<T>::new(ScalarBuffer::from(first_n), 
None))
-        }
+        let first_n = self.group_values.drain(0..n).collect::<Vec<_>>();
+
+        let first_n_nulls = self.nulls.take().map(|nulls| {
+            let (first_n_nulls, remaining_nulls) = take_n_nulls(n, nulls);
+            self.nulls = Some(remaining_nulls);
+            first_n_nulls
+        });
+
+        Arc::new(PrimitiveArray::<T>::new(
+            ScalarBuffer::from(first_n),
+            first_n_nulls,
+        ))
+    }
+}
+
+/// Takes the first `n` nulls from the `nulls` buffer and
+///
+/// Returns
+/// * [`NullBuffer`] with the first `n` nulls
+/// * [`BooleanBufferBuilder`] with the remaining nulls
+fn take_n_nulls(
+    n: usize,
+    mut nulls: BooleanBufferBuilder,
+) -> (NullBuffer, BooleanBufferBuilder) {
+    // Copy over the values at  n..len-1 values to the start of a new builder
+    // (todo it would be great to use something like `set_bits` from arrow 
here.
+    let mut new_builder = BooleanBufferBuilder::new(nulls.len());
+    for i in n..nulls.len() {
+        new_builder.append(nulls.get_bit(i));

Review Comment:
   Maybe we could implement `get_bit_slice(i, j) -> &[bool]` and append with 
`append_slice`



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