[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #5973: Row `AVG` accumulator support Decimal type

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

alamb commented on code in PR #5973:
URL: https://github.com/apache/arrow-datafusion/pull/5973#discussion_r1164540227


##########
datafusion/physical-expr/src/aggregate/average.rs:
##########
@@ -298,16 +305,40 @@ impl RowAccumulator for AvgRowAccumulator {
     }
 
     fn evaluate(&self, accessor: &RowAccessor) -> Result<ScalarValue> {
-        assert_eq!(self.sum_datatype, DataType::Float64);
-        Ok(match accessor.get_u64_opt(self.state_index()) {
-            None => ScalarValue::Float64(None),
-            Some(0) => ScalarValue::Float64(None),
-            Some(n) => ScalarValue::Float64(
-                accessor
-                    .get_f64_opt(self.state_index() + 1)
-                    .map(|f| f / n as f64),
-            ),
-        })
+        match self.sum_datatype {
+            DataType::Decimal128(p, s) => {
+                match accessor.get_u64_opt(self.state_index()) {
+                    None => Ok(ScalarValue::Decimal128(None, p, s)),
+                    Some(0) => Ok(ScalarValue::Decimal128(None, p, s)),

Review Comment:
   Why do we translate `0` --> `null` here? (it is also done for Float64 below)? 
   
   I see you are just following the existing pattern, but it seems like this could be incorrect?
   
   Maybe we could add a test that calls `AVG` on (`-1` and `1`) to see if we get `0` or `NULL`



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