jecsand838 commented on code in PR #8349:
URL: https://github.com/apache/arrow-rs/pull/8349#discussion_r2369923968


##########
arrow-avro/src/reader/record.rs:
##########
@@ -1518,19 +1104,340 @@ impl Decoder {
                     .map_err(|e| ArrowError::ParseError(e.to_string()))?;
                 Arc::new(vals)
             }
-            Self::Union(fields, type_ids, offsets, encodings, _, None) => {
-                flush_union!(fields, type_ids, offsets, encodings)
-            }
-            Self::Union(fields, type_ids, offsets, encodings, _, 
Some(union_resolution)) => {
-                match &mut union_resolution.kind {
-                    UnionResolvedKind::Both { .. } | 
UnionResolvedKind::FromSingle { .. } => {
-                        flush_union!(fields, type_ids, offsets, encodings)
-                    }
-                    UnionResolvedKind::ToSingle { target } => 
target.flush(nulls)?,
+            Self::Union(u) => u.flush(nulls)?,
+        })
+    }
+}
+
+#[derive(Debug)]
+struct DispatchLut {
+    to_reader: Box<[i16]>,
+    promotion: Box<[Promotion]>,
+}
+
+impl DispatchLut {
+    fn from_writer_to_reader(promotion_map: &[Option<(usize, Promotion)>]) -> 
Self {
+        let mut to_reader = Vec::with_capacity(promotion_map.len());
+        let mut promotion = Vec::with_capacity(promotion_map.len());
+        for map in promotion_map {
+            match *map {
+                Some((idx, promo)) => {
+                    debug_assert!(idx <= i16::MAX as usize);
+                    to_reader.push(idx as i16);
+                    promotion.push(promo);
+                }
+                None => {
+                    to_reader.push(-1);
+                    promotion.push(Promotion::Direct);
                 }
             }
+        }
+        Self {
+            to_reader: to_reader.into_boxed_slice(),
+            promotion: promotion.into_boxed_slice(),
+        }
+    }
+
+    // Resolve a writer branch index to (reader_idx, promotion)
+    #[inline]
+    fn resolve(&self, writer_idx: usize) -> Option<(usize, Promotion)> {
+        if writer_idx >= self.to_reader.len() {
+            return None;
+        }
+        let reader_index = self.to_reader[writer_idx];
+        if reader_index < 0 {
+            None
+        } else {
+            Some((reader_index as usize, self.promotion[writer_idx]))
+        }
+    }
+}
+
+#[derive(Debug)]
+struct UnionDecoder {
+    fields: UnionFields,
+    type_ids: Vec<i8>,
+    offsets: Vec<i32>,
+    branches: Vec<Decoder>,
+    counts: Vec<i32>,
+    type_id_by_reader_idx: Arc<[i8]>,
+    null_branch: Option<usize>,
+    default_emit_idx: usize,
+    null_emit_idx: usize,
+    plan: UnionReadPlan,
+}
+
+impl Default for UnionDecoder {
+    fn default() -> Self {
+        Self {
+            fields: UnionFields::empty(),
+            type_ids: Vec::new(),
+            offsets: Vec::new(),
+            branches: Vec::new(),
+            counts: Vec::new(),
+            type_id_by_reader_idx: Arc::from([]),
+            null_branch: None,
+            default_emit_idx: 0,
+            null_emit_idx: 0,
+            plan: UnionReadPlan::Passthrough,

Review Comment:
   I tried that and `Default` is not implemented on `UnionFields`,
   
   ```
   Trait `Default` is not implemented for `UnionFields` [E0277]
   ```



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