[GitHub] [arrow-rs] tustvold commented on a diff in pull request #2076: Support skip_values in ByteArrayColumnValueDecoder

[GitBox <gi...@apache.org>]

tustvold commented on code in PR #2076:
URL: https://github.com/apache/arrow-rs/pull/2076#discussion_r922305417


##########
parquet/src/arrow/array_reader/byte_array.rs:
##########
@@ -589,6 +680,38 @@ impl ByteArrayDecoderDictionary {
         }
         Ok(values_read)
     }
+
+    fn skip<I: OffsetSizeTrait + ScalarValue>(
+        &mut self,
+        dict: &OffsetBuffer<I>,
+        to_skip: usize,
+    ) -> Result<usize> {
+        let to_skip = to_skip.min(self.max_remaining_values);
+        // All data must be NULL
+        if dict.is_empty() {
+            return Ok(0);
+        }
+
+        let mut values_skip = 0;
+        while values_skip < to_skip {
+            if self.index_offset == self.index_buf_len {
+                let read = self.decoder.get_batch(self.index_buf.as_mut())?;

Review Comment:
   Possibly something for a follow up PR, but it would be nice if we could avoid decoding values only to dump them on the floor



##########
parquet/src/arrow/array_reader/byte_array.rs:
##########
@@ -653,6 +776,57 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_byte_array_decoder_skip() {
+        let (pages, encoded_dictionary) =
+            byte_array_all_encodings(vec!["hello", "world", "a", "b"]);
+
+        let column_desc = utf8_column();
+        let mut decoder = ByteArrayColumnValueDecoder::new(&column_desc);
+
+        decoder
+            .set_dict(encoded_dictionary, 4, Encoding::RLE_DICTIONARY, false)
+            .unwrap();
+
+        for (encoding, page) in pages {
+            let mut output = OffsetBuffer::<i32>::default();
+            decoder.set_data(encoding, page, 4, Some(4)).unwrap();
+
+            assert_eq!(decoder.read(&mut output, 0..1).unwrap(), 1);
+
+            assert_eq!(output.values.as_slice(), "hello".as_bytes());
+            assert_eq!(output.offsets.as_slice(), &[0, 5]);
+
+            assert_eq!(decoder.skip_values(1).unwrap(), 1);
+            assert_eq!(decoder.skip_values(1).unwrap(), 1);
+
+            assert_eq!(decoder.read(&mut output, 0..1).unwrap(), 1);

Review Comment:
   ```suggestion
               assert_eq!(decoder.read(&mut output, 1..2).unwrap(), 1);
   ```
   
   The decoder doesn't actually care, as it keeps track of the number of read values, but this is technically more correct



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