Improve performance reading ByteViewArray from parquet by removing …

…an implicit copy (#6031)

* update byte view array to not implicit copy

* Add small comments

parquet/src/arrow/array_reader/byte_view_array.rs

@@ -71,7 +71,6 @@ struct ByteViewArrayReader {
 }
 
 impl ByteViewArrayReader {
-    #[allow(unused)]
     fn new(
         pages: Box<dyn PageIterator>,
         data_type: ArrowType,
@@ -316,7 +315,10 @@ impl ByteViewArrayDecoderPlain {
     }
 
     pub fn read(&mut self, output: &mut ViewBuffer, len: usize) -> Result<usize> {
-        let block_id = output.append_block(self.buf.clone().into());
+        // Here we convert `bytes::Bytes` into `arrow_buffer::Bytes`, which is zero copy
+        // Then we convert `arrow_buffer::Bytes` into `arrow_buffer:Buffer`, which is also zero copy
+        let buf = arrow_buffer::Buffer::from_bytes(self.buf.clone().into());
+        let block_id = output.append_block(buf);
 
         let to_read = len.min(self.max_remaining_values);
 
@@ -546,7 +548,10 @@ impl ByteViewArrayDecoderDeltaLength {
 
         let src_lengths = &self.lengths[self.length_offset..self.length_offset + to_read];
 
-        let block_id = output.append_block(self.data.clone().into());
+        // Here we convert `bytes::Bytes` into `arrow_buffer::Bytes`, which is zero copy
+        // Then we convert `arrow_buffer::Bytes` into `arrow_buffer:Buffer`, which is also zero copy
+        let bytes = arrow_buffer::Buffer::from_bytes(self.data.clone().into());
+        let block_id = output.append_block(bytes);
 
         let mut current_offset = self.data_offset;
         let initial_offset = current_offset;

parquet/src/arrow/buffer/view_buffer.rs

@@ -68,7 +68,6 @@ impl ViewBuffer {
     }
 
     /// Converts this into an [`ArrayRef`] with the provided `data_type` and `null_buffer`
-    #[allow(unused)]
     pub fn into_array(self, null_buffer: Option<Buffer>, data_type: &ArrowType) -> ArrayRef {
         let len = self.views.len();
         let views = Buffer::from_vec(self.views);