mzabaluev commented on code in PR #9328:
URL: https://github.com/apache/arrow-rs/pull/9328#discussion_r2783713092


##########
arrow-avro/src/codec.rs:
##########
@@ -1533,62 +1550,35 @@ impl<'a> Maker<'a> {
                     nullable_union_variants(reader_variants)
                 {
                     let mut dt = self.resolve_type(writer_non_union, 
non_null_branch, namespace)?;
-                    let non_null_idx = match nullability {
-                        Nullability::NullFirst => 1,
-                        Nullability::NullSecond => 0,
-                    };
                     #[cfg(feature = "avro_custom_types")]
                     Self::propagate_nullability_into_ree(&mut dt, nullability);
                     dt.nullability = Some(nullability);
-                    let promotion = Self::coercion_from(&dt);
-                    dt.resolution = Some(ResolutionInfo::Union(ResolvedUnion {
-                        writer_to_reader: Arc::from(vec![Some((non_null_idx, 
promotion))]),
-                        writer_is_union: false,
-                        reader_is_union: true,
-                    }));
+                    // Ensure resolution is set to a non-Union variant to 
suppress
+                    // reading the union tag which is the default behavior.
+                    if dt.resolution.is_none() {
+                        dt.resolution = 
Some(ResolutionInfo::Promotion(Promotion::Direct));
+                    }
                     Ok(dt)
                 } else {
-                    let mut best_match: Option<(usize, AvroDataType, 
Promotion)> = None;
-                    for (i, variant) in reader_variants.iter().enumerate() {
-                        if let Ok(resolved_dt) =
-                            self.resolve_type(writer_non_union, variant, 
namespace)
-                        {
-                            let promotion = Self::coercion_from(&resolved_dt);
-                            if promotion == Promotion::Direct {
-                                best_match = Some((i, resolved_dt, promotion));
-                                break;
-                            } else if best_match.is_none() {
-                                best_match = Some((i, resolved_dt, promotion));
-                            }
-                        }
-                    }
-                    let Some((match_idx, match_dt, promotion)) = best_match 
else {
+                    let Some((match_idx, resolution)) =
+                        self.find_best_union_match(writer_non_union, 
reader_variants, namespace)
+                    else {
                         return Err(ArrowError::SchemaError(
                             "Writer schema does not match any reader union 
branch".to_string(),
                         ));
                     };
-                    let mut children = 
Vec::with_capacity(reader_variants.len());
-                    let mut match_dt = Some(match_dt);
-                    for (i, variant) in reader_variants.iter().enumerate() {
-                        if i == match_idx {
-                            if let Some(mut dt) = match_dt.take() {
-                                if matches!(dt.resolution, 
Some(ResolutionInfo::Promotion(_))) {
-                                    dt.resolution = None;
-                                }
-                                children.push(dt);
-                            }
-                        } else {
-                            children.push(self.parse_type(variant, 
namespace)?);
-                        }
-                    }
+                    let children = reader_variants
+                        .iter()
+                        .map(|variant| self.parse_type(variant, namespace))
+                        .collect::<Result<Vec<_>, _>>()?;

Review Comment:
   You are right, the test fails because inner resolution information is not 
preserved. The same should be done when resolving the union-to-union case.



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