enhance the document for column-reader

cpp/src/parquet/column_reader.cc

@@ -2050,6 +2050,14 @@ class TypedRecordReader : public TypedColumnReaderImpl<DType>,
   LevelInfo leaf_info_;
 };
 
+/// In FLBARecordReader, we read fixed length byte array values.
+///
+/// Unlike other fixed length types, the `values_` buffer is not used to store
+/// values, instead we use `data_builder_` to store the values, and `null_bitmap_builder_`
+/// is used to store the null bitmap.
+///
+/// The `values_` buffer is used to store the temporary values for `Decode`, and it would
+/// be Reset after each `Decode` call. The `valid_bits_` buffer is never used.
 class FLBARecordReader final : public TypedRecordReader<FLBAType>,
                                virtual public BinaryRecordReader {
  public:
@@ -2134,6 +2142,13 @@ class FLBARecordReader final : public TypedRecordReader<FLBAType>,
   ::arrow::BufferBuilder data_builder_;
 };
 
+/// ByteArrayRecordReader read variable length byte array values.
+///
+/// It only calls `DecodeArrowNonNull` and `DecodeArrow` to read values, and
+/// `Decode` and `DecodeSpaced` are not used.
+///
+/// The `values_` buffers are never used, and the `accumulator_`
+/// is used to store the values.
 class ByteArrayChunkedRecordReader final : public TypedRecordReader<ByteArrayType>,
                                            virtual public BinaryRecordReader {
  public:
@@ -2147,7 +2162,7 @@ class ByteArrayChunkedRecordReader final : public TypedRecordReader<ByteArrayTyp
 
   ::arrow::ArrayVector GetBuilderChunks() override {
     ::arrow::ArrayVector result = accumulator_.chunks;
-    if (result.size() == 0 || accumulator_.builder->length() > 0) {
+    if (result.empty() || accumulator_.builder->length() > 0) {
       std::shared_ptr<::arrow::Array> last_chunk;
       PARQUET_THROW_NOT_OK(accumulator_.builder->Finish(&last_chunk));
       result.push_back(std::move(last_chunk));
@@ -2176,6 +2191,11 @@ class ByteArrayChunkedRecordReader final : public TypedRecordReader<ByteArrayTyp
   typename EncodingTraits<ByteArrayType>::Accumulator accumulator_;
 };
 
+/// ByteArrayDictionaryRecordReader read ::arrow::dictionary(index: int32, values:
+/// binary).
+///
+/// If underlying column is dictionary encoded, it will call `DecodeIndices` to read,
+/// otherwise it will call `DecodeArrowNonNull` to read.
 class ByteArrayDictionaryRecordReader final : public TypedRecordReader<ByteArrayType>,
                                               virtual public DictionaryRecordReader {
  public:
@@ -2225,10 +2245,9 @@ class ByteArrayDictionaryRecordReader final : public TypedRecordReader<ByteArray
     } else {
       num_decoded = this->current_decoder_->DecodeArrowNonNull(
           static_cast<int>(values_to_read), &builder_);
-
-      /// Flush values since they have been copied into the builder
-      ResetValues();
     }
+    // Flush values since they have been copied into the builder
+    ResetValues();
     CheckNumberDecoded(num_decoded, values_to_read);
   }
 
@@ -2244,11 +2263,10 @@ class ByteArrayDictionaryRecordReader final : public TypedRecordReader<ByteArray
       num_decoded = this->current_decoder_->DecodeArrow(
           static_cast<int>(values_to_read), static_cast<int>(null_count),
           valid_bits_->mutable_data(), values_written_, &builder_);
-
-      /// Flush values since they have been copied into the builder
-      ResetValues();
     }
     ARROW_DCHECK_EQ(num_decoded, values_to_read - null_count);
+    // Flush values since they have been copied into the builder
+    ResetValues();
   }
 
  private:

cpp/src/parquet/column_reader.h

@@ -446,7 +446,9 @@ class PARQUET_EXPORT RecordReader {
   int64_t null_count_;
 
   /// \brief Each bit corresponds to one element in 'values_' and specifies if it
-  /// is null or not null. Not set if read_dense_for_nullable_ is true.
+  /// is null or not null.
+  ///
+  /// Not set if leaf type is not nullable or read_dense_for_nullable_ is true.
   std::shared_ptr<::arrow::ResizableBuffer> valid_bits_;
 
   /// \brief Buffer for definition levels. May contain more levels than
@@ -471,7 +473,10 @@ class PARQUET_EXPORT RecordReader {
 
   bool read_dictionary_ = false;
   // If true, we will not leave any space for the null values in the values_
-  // vector.
+  // vector or fill nulls values in BinaryRecordReader/DictionaryRecordReader.
+  //
+  // If read_dense_for_nullable_ is true, the BinaryRecordReader/DictionaryRecordReader
+  // might still populate the validity bitmap buffer.
   bool read_dense_for_nullable_ = false;
 };
 

cpp/src/parquet/decoder.cc

@@ -2047,6 +2047,8 @@ class DeltaByteArrayDecoderImpl : public DecoderImpl, public TypedDecoderImpl<DT
   int num_valid_values_{0};
   uint32_t prefix_len_offset_{0};
   std::shared_ptr<ResizableBuffer> buffered_prefix_length_;
+  // buffer for decoded strings, which gurantees the lifetime of the decoded strings
+  // until the next call of Decode.
   std::shared_ptr<ResizableBuffer> buffered_data_;
 };