Support convert_to_state for AVG accumulator

Author: alamb

datafusion/functions-aggregate/src/average.rs

@@ -19,8 +19,9 @@
 
 use arrow::array::{
     self, Array, ArrayRef, ArrowNativeTypeOp, ArrowNumericType, ArrowPrimitiveType,
-    AsArray, PrimitiveArray, PrimitiveBuilder, UInt64Array,
+    AsArray, BooleanArray, Int64Array, PrimitiveArray, PrimitiveBuilder, UInt64Array,
 };
+use arrow::buffer::NullBuffer;
 use arrow::compute::sum;
 use arrow::datatypes::{
     i256, ArrowNativeType, DataType, Decimal128Type, Decimal256Type, DecimalType, Field,
@@ -554,8 +555,51 @@ where
         Ok(())
     }
 
+    fn convert_to_state(
+        &self,
+        values: &[ArrayRef],
+        opt_filter: Option<&BooleanArray>,
+    ) -> Result<Vec<ArrayRef>> {
+        let counts = Arc::new(Int64Array::from_value(1, values.len()));
+        let sums = values[0].as_primitive::<T>();
+
+        let nulls = filtered_null_mask(opt_filter, sums);
+        let sums = PrimitiveArray::<T>::new(sums.values().clone(), nulls)
+            .with_data_type(self.sum_data_type.clone());
+
+        Ok(vec![counts, Arc::new(sums)])
+    }
+
+    fn convert_to_state_supported(&self) -> bool {
+        true
+    }
+
     fn size(&self) -> usize {
         self.counts.capacity() * std::mem::size_of::<u64>()
             + self.sums.capacity() * std::mem::size_of::<T>()
     }
 }
+
+/// Converts a `BooleanBuffer` representing a filter to a `NullBuffer`
+/// where the NullBuffer is true for all values that were true
+/// in the filter and `null` for any values that were false or null
+fn filter_to_nulls(filter: &BooleanArray) -> Option<NullBuffer> {
+    let (filter_bools, filter_nulls) = filter.clone().into_parts();
+    // Only keep values where the filter was true
+    // convert all false to null
+    let filter_bools = NullBuffer::from(filter_bools);
+    NullBuffer::union(Some(&filter_bools), filter_nulls.as_ref())
+}
+
+/// Compute the final null mask for an array
+///
+/// The output null mask :
+/// * is true (non null) for all values that were true in the filter and non null in the input
+/// * is false (null) for all values that were false in the filter or null in the input
+fn filtered_null_mask(
+    opt_filter: Option<&BooleanArray>,
+    input: &dyn Array,
+) -> Option<NullBuffer> {
+    let opt_filter = opt_filter.and_then(filter_to_nulls);
+    NullBuffer::union(opt_filter.as_ref(), input.nulls())
+}