Minor: reserve output space in ByteViewArrayDecoderDictionary

Author: alamb

parquet/src/arrow/array_reader/byte_view_array.rs

@@ -432,11 +432,13 @@ impl ByteViewArrayDecoderDictionary {
         }
     }
 
-    /// Reads the next indexes from self.decoder
-    /// the indexes are assumed to be indexes into `dict`
+    /// Reads the next `len` indexes from self.decoder
+    ///
+    /// The indexes are assumed to be indexes into `dict`
     /// the output values are written to output
     ///
-    /// Assumptions / Optimization
+    /// # Assumptions / Optimization
+    ///
     /// This function checks if dict.buffers() are the last buffers in `output`, and if so
     /// reuses the dictionary page buffers directly without copying data
     fn read(&mut self, output: &mut ViewBuffer, dict: &ViewBuffer, len: usize) -> Result<usize> {
@@ -458,6 +460,10 @@ impl ByteViewArrayDecoderDictionary {
             }
         };
 
+        // we are going to append `len` views to the output buffer so reserve
+        // the space for them to avoid reallocations
+        output.reserve_views(len);
+
         if need_to_create_new_buffer {
             for b in dict.buffers.iter() {
                 output.buffers.push(b.clone());

parquet/src/arrow/buffer/view_buffer.rs

@@ -37,6 +37,11 @@ impl ViewBuffer {
         self.views.is_empty()
     }
 
+    /// Reserve capacity for `additional` views
+    pub fn reserve_views(&mut self, additional: usize) {
+        self.views.reserve(additional);
+    }
+
     pub fn append_block(&mut self, block: Buffer) -> u32 {
         let block_id = self.buffers.len() as u32;
         self.buffers.push(block);