Minor refactor and comments

src/arrow_reader/decoder/mod.rs

@@ -52,10 +52,10 @@ impl<T: ArrowPrimitiveType> PrimitiveArrayDecoder<T> {
     ) -> Result<PrimitiveArray<T>> {
         let present = derive_present_vec(&mut self.present, parent_present, batch_size);
 
-        match &present {
+        match present {
             Some(present) => {
                 let mut builder = PrimitiveBuilder::<T>::with_capacity(batch_size);
-                for &is_present in present {
+                for is_present in present {
                     if is_present {
                         // TODO: return as error instead
                         let val = self
@@ -128,10 +128,10 @@ impl ArrayBatchDecoder for BooleanArrayDecoder {
     ) -> Result<ArrayRef> {
         let present = derive_present_vec(&mut self.present, parent_present, batch_size);
 
-        match &present {
+        match present {
             Some(present) => {
                 let mut builder = BooleanBuilder::with_capacity(batch_size);
-                for &is_present in present {
+                for is_present in present {
                     if is_present {
                         // TODO: return as error instead
                         let val = self

src/arrow_reader/decoder/string.rs

@@ -173,7 +173,13 @@ impl ArrayBatchDecoder for DictionaryStringArrayDecoder {
         let keys = self
             .indexes
             .next_primitive_batch(batch_size, parent_present)?;
+        // TODO: ORC spec states: For dictionary encodings the dictionary is sorted
+        //       (in lexicographical order of bytes in the UTF-8 encodings).
+        //       So we can set the is_ordered property here?
         let array = DictionaryArray::try_new(keys, self.dictionary.clone()).context(ArrowSnafu)?;
+        // Cast back to StringArray to ensure all stripes have consistent datatype
+        // TODO: Is there anyway to preserve the dictionary encoding?
+        //       This costs performance.
         let array = cast(&array, &DataType::Utf8).context(ArrowSnafu)?;
 
         let array = Arc::new(array);