be/src/exec/parquet-column-readers.h

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.

#ifndef IMPALA_PARQUET_COLUMN_READERS_H
#define IMPALA_PARQUET_COLUMN_READERS_H

#include <boost/scoped_ptr.hpp>

#include "exec/hdfs-parquet-scanner.h"
#include "util/codec.h"
#include "util/bit-stream-utils.h"
#include "util/dict-encoding.h"
#include "util/rle-encoding.h"

namespace impala {

class Tuple;
class MemPool;

/// Decoder for all supported Parquet level encodings. Optionally reads, decodes, and
/// caches level values in batches.
/// Level values are unsigned 8-bit integers because we support a maximum nesting
/// depth of 100, as enforced by the FE. Using a small type saves memory and speeds up
/// populating the level cache (e.g., with RLE we can memset() repeated values).
///
/// TODO: expose whether we're in a run of repeated values so that callers can
/// optimise for that case.
class ParquetLevelDecoder {
 public:
  ParquetLevelDecoder(bool is_def_level_decoder)
    : decoding_error_code_(is_def_level_decoder ?
          TErrorCode::PARQUET_DEF_LEVEL_ERROR : TErrorCode::PARQUET_REP_LEVEL_ERROR) {
  }

  /// Initialize the LevelDecoder. Reads and advances the provided data buffer if the
  /// encoding requires reading metadata from the page header. 'cache_size' will be
  /// rounded up to a multiple of 32 internally.
  Status Init(const string& filename, parquet::Encoding::type encoding,
      MemPool* cache_pool, int cache_size, int max_level, int num_buffered_values,
      uint8_t** data, int* data_size);

  /// Returns the next level or INVALID_LEVEL if there was an error. Not as efficient
  /// as batched methods.
  inline int16_t ReadLevel();

  /// Decodes and caches the next batch of levels given that there are 'vals_remaining'
  /// values left to decode in the page. Resets members associated with the cache.
  /// Returns a non-ok status if there was a problem decoding a level, if a level was
  /// encountered with a value greater than max_level_, or if fewer than
  /// min(CacheSize(), vals_remaining) levels could be read, which indicates that the
  /// input did not have the expected number of values. Only valid to call when
  /// the cache has been exhausted, i.e. CacheHasNext() is false.
  Status CacheNextBatch(int vals_remaining);

  /// Functions for working with the level cache.
  inline bool CacheHasNext() const { return cached_level_idx_ < num_cached_levels_; }
  inline uint8_t CacheGetNext() {
    DCHECK_LT(cached_level_idx_, num_cached_levels_);
    return cached_levels_[cached_level_idx_++];
  }
  inline void CacheSkipLevels(int num_levels) {
    DCHECK_LE(cached_level_idx_ + num_levels, num_cached_levels_);
    cached_level_idx_ += num_levels;
  }
  inline int CacheSize() const { return num_cached_levels_; }
  inline int CacheRemaining() const { return num_cached_levels_ - cached_level_idx_; }
  inline int CacheCurrIdx() const { return cached_level_idx_; }
 private:
  /// Initializes members associated with the level cache. Allocates memory for
  /// the cache from pool, if necessary.
  Status InitCache(MemPool* pool, int cache_size);

  /// Decodes and writes a batch of levels into the cache. Returns true and sets
  /// the number of values written to the cache via *num_cached_levels if no errors
  /// are encountered. *num_cached_levels is < 'batch_size' in this case iff the
  /// end of input was hit without any other errors. Returns false if there was an
  /// error decoding a level or if there was an invalid level value greater than
  /// 'max_level_'. Only valid to call when the cache has been exhausted, i.e.
  /// CacheHasNext() is false.
  bool FillCache(int batch_size, int* num_cached_levels);

  /// RLE decoder, used if 'encoding_' is RLE.
  RleBatchDecoder<uint8_t> rle_decoder_;

  /// Buffer for a batch of levels. The memory is allocated and owned by a pool passed
  /// in Init().
  uint8_t* cached_levels_ = nullptr;

  /// Number of valid level values in the cache.
  int num_cached_levels_ = 0;

  /// Current index into cached_levels_.
  int cached_level_idx_ = 0;

  /// The parquet encoding used for the levels. Only RLE is supported for now.
  parquet::Encoding::type encoding_ = parquet::Encoding::PLAIN;

  /// For error checking and reporting.
  int max_level_ = 0;

  /// Number of level values cached_levels_ has memory allocated for. Always
  /// a multiple of 32 to allow reading directly from 'bit_reader_' in batches.
  int cache_size_ = 0;

  /// Number of remaining data values in the current data page.
  int num_buffered_values_ = 0;

  /// Name of the parquet file. Used for reporting level decoding errors.
  string filename_;

  /// Error code to use when reporting level decoding errors.
  TErrorCode::type decoding_error_code_;
};

/// Base class for reading a Parquet column. Reads a logical column, not necessarily a
/// column materialized in the file (e.g. collections). The two subclasses are
/// BaseScalarColumnReader and CollectionColumnReader. Column readers read one def and rep
/// level pair at a time. The current def and rep level are exposed to the user, and the
/// corresponding value (if defined) can optionally be copied into a slot via
/// ReadValue(). Can also write position slots.
class ParquetColumnReader {
 public:
  /// Creates a column reader for 'node' and associates it with the given parent scanner.
  /// Adds the new column reader to the parent's object pool.
  /// 'slot_desc' may be NULL, in which case the returned column reader can only be used
  /// to read def/rep levels.
  /// 'is_collection_field' should be set to true if the returned reader is reading a
  /// collection. This cannot be determined purely by 'node' because a repeated scalar
  /// node represents both an array and the array's items (in this case
  /// 'is_collection_field' should be true if the reader reads one value per array, and
  /// false if it reads one value per item).  The reader is added to the runtime state's
  /// object pool. Does not create child readers for collection readers; these must be
  /// added by the caller.
  static ParquetColumnReader* Create(const SchemaNode& node, bool is_collection_field,
      const SlotDescriptor* slot_desc, HdfsParquetScanner* parent);

  virtual ~ParquetColumnReader() { }

  int def_level() const { return def_level_; }
  int rep_level() const { return rep_level_; }

  const SlotDescriptor* slot_desc() const { return slot_desc_; }
  const parquet::SchemaElement& schema_element() const { return *node_.element; }
  int16_t max_def_level() const { return max_def_level_; }
  int16_t max_rep_level() const { return max_rep_level_; }
  int def_level_of_immediate_repeated_ancestor() const {
    return node_.def_level_of_immediate_repeated_ancestor;
  }
  const SlotDescriptor* pos_slot_desc() const { return pos_slot_desc_; }
  void set_pos_slot_desc(const SlotDescriptor* pos_slot_desc) {
    DCHECK(pos_slot_desc_ == NULL);
    pos_slot_desc_ = pos_slot_desc;
  }

  /// Returns true if this reader materializes collections (i.e. CollectionValues).
  virtual bool IsCollectionReader() const { return false; }

  const char* filename() const { return parent_->filename(); }

  /// Read the current value (or null) into 'tuple' for this column. This should only be
  /// called when a value is defined, i.e., def_level() >=
  /// def_level_of_immediate_repeated_ancestor() (since empty or NULL collections produce
  /// no output values), otherwise NextLevels() should be called instead.
  ///
  /// Advances this column reader to the next value (i.e. NextLevels() doesn't need to be
  /// called after calling ReadValue()).
  ///
  /// Returns false if execution should be aborted for some reason, e.g. parse_error_ is
  /// set, the query is cancelled, or the scan node limit was reached. Otherwise returns
  /// true.
  ///
  /// NextLevels() must be called on this reader before calling ReadValue() for the first
  /// time. This is to initialize the current value that ReadValue() will read.
  ///
  /// TODO: this is the function that needs to be codegen'd (e.g. CodegenReadValue())
  /// The codegened functions from all the materialized cols will then be combined
  /// into one function.
  /// TODO: another option is to materialize col by col for the entire row batch in
  /// one call.  e.g. MaterializeCol would write out 1024 values.  Our row batches
  /// are currently dense so we'll need to figure out something there.
  virtual bool ReadValue(MemPool* pool, Tuple* tuple) = 0;

  /// Same as ReadValue() but does not advance repetition level. Only valid for columns
  /// not in collections.
  virtual bool ReadNonRepeatedValue(MemPool* pool, Tuple* tuple) = 0;

  /// Batched version of ReadValue() that reads up to max_values at once and materializes
  /// them into tuples in tuple_mem. Returns the number of values actually materialized
  /// in *num_values. The return value, error behavior and state changes are generally
  /// the same as in ReadValue(). For example, if an error occurs in the middle of
  /// materializing a batch then false is returned, and num_values, tuple_mem, as well as
  /// this column reader are left in an undefined state, assuming that the caller will
  /// immediately abort execution. NextLevels() does *not* need to be called before
  /// ReadValueBatch(), unlike ReadValue().
  virtual bool ReadValueBatch(MemPool* pool, int max_values, int tuple_size,
      uint8_t* tuple_mem, int* num_values);

  /// Batched version of ReadNonRepeatedValue() that reads up to max_values at once and
  /// materializes them into tuples in tuple_mem.
  /// The return value and error behavior are the same as in ReadValueBatch().
  virtual bool ReadNonRepeatedValueBatch(MemPool* pool, int max_values, int tuple_size,
      uint8_t* tuple_mem, int* num_values);

  /// Advances this column reader's def and rep levels to the next logical value, i.e. to
  /// the next scalar value or the beginning of the next collection, without attempting to
  /// read the value. This is used to skip past def/rep levels that don't materialize a
  /// value, such as the def/rep levels corresponding to an empty containing collection.
  ///
  /// NextLevels() must be called on this reader before calling ReadValue() for the first
  /// time. This is to initialize the current value that ReadValue() will read.
  ///
  /// Returns false if execution should be aborted for some reason, e.g. parse_error_ is
  /// set, the query is cancelled, or the scan node limit was reached. Otherwise returns
  /// true.
  virtual bool NextLevels() = 0;

  /// Writes pos_current_value_ (i.e. "reads" the synthetic position field of the
  /// parent collection) to 'pos' and increments pos_current_value_. Only valid to
  /// call when doing non-batched reading, i.e. NextLevels() must have been called
  /// before each call to this function to advance to the next element in the
  /// collection.
  void ReadPositionNonBatched(int64_t* pos);

  /// Returns true if this column reader has reached the end of the row group.
  inline bool RowGroupAtEnd() {
    DCHECK_EQ(rep_level_ == HdfsParquetScanner::ROW_GROUP_END,
              def_level_ == HdfsParquetScanner::ROW_GROUP_END);
    return rep_level_ == HdfsParquetScanner::ROW_GROUP_END;
  }

  /// If 'row_batch' is non-NULL, transfers the remaining resources backing tuples to it,
  /// and frees up other resources. If 'row_batch' is NULL frees all resources instead.
  virtual void Close(RowBatch* row_batch) = 0;

 protected:
  HdfsParquetScanner* parent_;
  const SchemaNode& node_;
  const SlotDescriptor* const slot_desc_;

  /// The slot descriptor for the position field of the tuple, if there is one. NULL if
  /// there's not. Only one column reader for a given tuple desc will have this set.
  const SlotDescriptor* pos_slot_desc_;

  /// The next value to write into the position slot, if there is one. 64-bit int because
  /// the pos slot is always a BIGINT Set to INVALID_POS when this column reader does not
  /// have a current rep and def level (i.e. before the first NextLevels() call or after
  /// the last value in the column has been read).
  int64_t pos_current_value_;

  /// The current repetition and definition levels of this reader. Advanced via
  /// ReadValue() and NextLevels(). Set to INVALID_LEVEL before the first NextLevels()
  /// call for a row group or if an error is encountered decoding a level. Set to
  /// ROW_GROUP_END after the last value in the column has been read). If this is not
  /// inside a collection, rep_level_ is always 0, INVALID_LEVEL or ROW_GROUP_END.
  /// int16_t is large enough to hold the valid levels 0-255 and negative sentinel values
  /// INVALID_LEVEL and ROW_GROUP_END. The maximum values are cached here because they
  /// are accessed in inner loops.
  int16_t rep_level_;
  const int16_t max_rep_level_;
  int16_t def_level_;
  const int16_t max_def_level_;

  // Cache frequently accessed members of slot_desc_ for perf.

  /// slot_desc_->tuple_offset(). -1 if slot_desc_ is NULL.
  const int tuple_offset_;

  /// slot_desc_->null_indicator_offset(). Invalid if slot_desc_ is NULL.
  const NullIndicatorOffset null_indicator_offset_;

  ParquetColumnReader(HdfsParquetScanner* parent, const SchemaNode& node,
      const SlotDescriptor* slot_desc)
    : parent_(parent),
      node_(node),
      slot_desc_(slot_desc),
      pos_slot_desc_(NULL),
      pos_current_value_(HdfsParquetScanner::INVALID_POS),
      rep_level_(HdfsParquetScanner::INVALID_LEVEL),
      max_rep_level_(node_.max_rep_level),
      def_level_(HdfsParquetScanner::INVALID_LEVEL),
      max_def_level_(node_.max_def_level),
      tuple_offset_(slot_desc == NULL ? -1 : slot_desc->tuple_offset()),
      null_indicator_offset_(slot_desc == NULL ? NullIndicatorOffset() :
          slot_desc->null_indicator_offset()) {
    DCHECK_GE(node_.max_rep_level, 0);
    DCHECK_LE(node_.max_rep_level, std::numeric_limits<int16_t>::max());
    DCHECK_GE(node_.max_def_level, 0);
    DCHECK_LE(node_.max_def_level, std::numeric_limits<int16_t>::max());
    // rep_level_ is always valid and equal to 0 if col not in collection.
    if (max_rep_level() == 0) rep_level_ = 0;
  }

  /// Called in the middle of creating a scratch tuple batch to simulate failures
  /// such as exceeding memory limit or cancellation. Returns false if the debug
  /// action deems that the parquet column reader should halt execution. 'val_count'
  /// is the counter which the column reader uses to track the number of tuples
  /// produced so far. If the column reader should halt execution, 'parse_status_'
  /// is updated with the error status and 'val_count' is set to 0.
  bool ColReaderDebugAction(int* val_count);
};

/// Reader for a single column from the parquet file.  It's associated with a
/// ScannerContext::Stream and is responsible for decoding the data.  Super class for
/// per-type column readers. This contains most of the logic, the type specific functions
/// must be implemented in the subclass.
class BaseScalarColumnReader : public ParquetColumnReader {
 public:
  BaseScalarColumnReader(HdfsParquetScanner* parent, const SchemaNode& node,
      const SlotDescriptor* slot_desc)
    : ParquetColumnReader(parent, node, slot_desc),
      data_page_pool_(new MemPool(parent->scan_node_->mem_tracker())) {
    DCHECK_GE(node_.col_idx, 0) << node_.DebugString();
  }

  virtual ~BaseScalarColumnReader() { }

  /// Resets the reader for each row group in the file and creates the scan
  /// range for the column, but does not start it. To start scanning,
  /// set_io_reservation() must be called to assign reservation to this
  /// column, followed by StartScan().
  Status Reset(const HdfsFileDesc& file_desc, const parquet::ColumnChunk& col_chunk,
    int row_group_idx);

  /// Starts the column scan range. The reader must be Reset() and have a
  /// reservation assigned via set_io_reservation(). This must be called
  /// before any of the column data can be read (including dictionary and
  /// data pages). Returns an error status if there was an error starting the
  /// scan or allocating buffers for it.
  Status StartScan();

  /// Helper to start scans for multiple columns at once.
  static Status StartScans(const std::vector<BaseScalarColumnReader*> readers) {
    for (BaseScalarColumnReader* reader : readers) RETURN_IF_ERROR(reader->StartScan());
    return Status::OK();
  }

  virtual void Close(RowBatch* row_batch) {
    if (row_batch != nullptr && PageContainsTupleData(page_encoding_)) {
      row_batch->tuple_data_pool()->AcquireData(data_page_pool_.get(), false);
    } else {
      data_page_pool_->FreeAll();
    }
    if (decompressor_ != nullptr) decompressor_->Close();
    DictDecoderBase* dict_decoder = GetDictionaryDecoder();
    if (dict_decoder != nullptr) dict_decoder->Close();
  }

  io::ScanRange* scan_range() const { return scan_range_; }
  int64_t total_len() const { return metadata_->total_compressed_size; }
  int col_idx() const { return node_.col_idx; }
  THdfsCompression::type codec() const {
    if (metadata_ == NULL) return THdfsCompression::NONE;
    return ConvertParquetToImpalaCodec(metadata_->codec);
  }
  void set_io_reservation(int bytes) { io_reservation_ = bytes; }

  /// Reads the next definition and repetition levels for this column. Initializes the
  /// next data page if necessary.
  virtual bool NextLevels() { return NextLevels<true>(); }

  /// Check the data stream to see if there is a dictionary page. If there is,
  /// use that page to initialize dict_decoder_ and advance the data stream
  /// past the dictionary page.
  Status InitDictionary();

  /// Convenience function to initialize multiple dictionaries.
  static Status InitDictionaries(const std::vector<BaseScalarColumnReader*> readers);

  // Returns the dictionary or NULL if the dictionary doesn't exist
  virtual DictDecoderBase* GetDictionaryDecoder() { return nullptr; }

  // Returns whether the datatype for this column requires conversion from the on-disk
  // format for correctness. For example, timestamps can require an offset to be
  // applied.
  virtual bool NeedsConversion() { return false; }

  // Returns whether the datatype for this column requires validation. For example,
  // the timestamp format has certain bit combinations that are invalid, and these
  // need to be validated when read from disk.
  virtual bool NeedsValidation() { return false; }

  // TODO: Some encodings might benefit a lot from a SkipValues(int num_rows) if
  // we know this row can be skipped. This could be very useful with stats and big
  // sections can be skipped. Implement that when we can benefit from it.

 protected:
  // Friend parent scanner so it can perform validation (e.g. ValidateEndOfRowGroup())
  friend class HdfsParquetScanner;

  // Class members that are accessed for every column should be included up here so they
  // fit in as few cache lines as possible.

  /// Pointer to start of next value in data page
  uint8_t* data_ = nullptr;

  /// End of the data page.
  const uint8_t* data_end_ = nullptr;

  /// Decoder for definition levels.
  ParquetLevelDecoder def_levels_{true};

  /// Decoder for repetition levels.
  ParquetLevelDecoder rep_levels_{false};

  /// Page encoding for values of the current data page. Cached here for perf. Set in
  /// InitDataPage().
  parquet::Encoding::type page_encoding_ = parquet::Encoding::PLAIN_DICTIONARY;

  /// Num values remaining in the current data page
  int num_buffered_values_ = 0;

  // Less frequently used members that are not accessed in inner loop should go below
  // here so they do not occupy precious cache line space.

  /// The number of values seen so far. Updated per data page.
  int64_t num_values_read_ = 0;

  /// Metadata for the column for the current row group.
  const parquet::ColumnMetaData* metadata_ = nullptr;

  boost::scoped_ptr<Codec> decompressor_;

  /// The scan range for the column's data. Initialized for each row group by Reset().
  io::ScanRange* scan_range_ = nullptr;

  // Stream used to read data from 'scan_range_'. Initialized by StartScan().
  ScannerContext::Stream* stream_ = nullptr;

  /// Reservation in bytes to use for I/O buffers in 'scan_range_'/'stream_'. Must be set
  /// with set_io_reservation() before 'stream_' is initialized. Reset for each row group
  /// by Reset().
  int64_t io_reservation_ = 0;

  /// Pool to allocate storage for data pages from - either decompression buffers for
  /// compressed data pages or copies of the data page with var-len data to attach to
  /// batches.
  boost::scoped_ptr<MemPool> data_page_pool_;

  /// Header for current data page.
  parquet::PageHeader current_page_header_;

  /// Reads the next page header into next_page_header/next_header_size.
  /// If the stream reaches the end before reading a complete page header,
  /// eos is set to true. If peek is false, the stream position is advanced
  /// past the page header. If peek is true, the stream position is not moved.
  /// Returns an error status if the next page header could not be read.
  Status ReadPageHeader(bool peek, parquet::PageHeader* next_page_header,
      uint32_t* next_header_size, bool* eos);

  /// Read the next data page. If a dictionary page is encountered, that will be read and
  /// this function will continue reading the next data page.
  Status ReadDataPage();

  /// Try to move the the next page and buffer more values. Return false and sets rep_level_,
  /// def_level_ and pos_current_value_ to -1 if no more pages or an error encountered.
  bool NextPage();

  /// Implementation for NextLevels().
  template <bool ADVANCE_REP_LEVEL>
  bool NextLevels();

  /// Creates a dictionary decoder from values/size. 'decoder' is set to point to a
  /// dictionary decoder stored in this object. Subclass must implement this. Returns
  /// an error status if the dictionary values could not be decoded successfully.
  virtual Status CreateDictionaryDecoder(uint8_t* values, int size,
      DictDecoderBase** decoder) = 0;

  /// Return true if the column has a dictionary decoder. Subclass must implement this.
  virtual bool HasDictionaryDecoder() = 0;

  /// Clear the dictionary decoder so HasDictionaryDecoder() will return false. Subclass
  /// must implement this.
  virtual void ClearDictionaryDecoder() = 0;

  /// Initializes the reader with the data contents. This is the content for the entire
  /// decompressed data page. Decoders can initialize state from here. The caller must
  /// validate the input such that 'size' is non-negative and that 'data' has at least
  /// 'size' bytes remaining.
  virtual Status InitDataPage(uint8_t* data, int size) = 0;

  /// Allocate memory for the uncompressed contents of a data page of 'size' bytes from
  /// 'data_page_pool_'. 'err_ctx' provides context for error messages. On success, 'buffer'
  /// points to the allocated memory. Otherwise an error status is returned.
  Status AllocateUncompressedDataPage(
      int64_t size, const char* err_ctx, uint8_t** buffer);

  /// Returns true if a data page for this column with the specified 'encoding' may
  /// contain strings referenced by returned batches. Cases where this is not true are:
  /// * Dictionary-compressed pages, where any string data lives in 'dictionary_pool_'.
  /// * Fixed-length slots, where there is no string data.
  bool PageContainsTupleData(parquet::Encoding::type page_encoding) {
    return page_encoding != parquet::Encoding::PLAIN_DICTIONARY
        && slot_desc_ != nullptr && slot_desc_->type().IsVarLenStringType();
  }

  /// Slow-path status construction code for def/rep decoding errors. 'level_name' is
  /// either "rep" or "def", 'decoded_level' is the value returned from
  /// ParquetLevelDecoder::ReadLevel() and 'max_level' is the maximum allowed value.
  void __attribute__((noinline)) SetLevelDecodeError(const char* level_name,
      int decoded_level, int max_level);

  // Returns a detailed error message about unsupported encoding.
  Status GetUnsupportedDecodingError();
};

/// Collections are not materialized directly in parquet files; only scalar values appear
/// in the file. CollectionColumnReader uses the definition and repetition levels of child
/// column readers to figure out the boundaries of each collection in this column.
class CollectionColumnReader : public ParquetColumnReader {
 public:
  CollectionColumnReader(HdfsParquetScanner* parent, const SchemaNode& node,
      const SlotDescriptor* slot_desc)
    : ParquetColumnReader(parent, node, slot_desc) {
    DCHECK(node_.is_repeated());
    if (slot_desc != NULL) DCHECK(slot_desc->type().IsCollectionType());
  }

  virtual ~CollectionColumnReader() { }

  vector<ParquetColumnReader*>* children() { return &children_; }

  virtual bool IsCollectionReader() const { return true; }

  /// The repetition level indicating that the current value is the first in a new
  /// collection (meaning the last value read was the final item in the previous
  /// collection).
  int new_collection_rep_level() const { return max_rep_level() - 1; }

  /// Materializes CollectionValue into tuple slot (if materializing) and advances to next
  /// value.
  virtual bool ReadValue(MemPool* pool, Tuple* tuple);

  /// Same as ReadValue but does not advance repetition level. Only valid for columns not
  /// in collections.
  virtual bool ReadNonRepeatedValue(MemPool* pool, Tuple* tuple);

  /// Advances all child readers to the beginning of the next collection and updates this
  /// reader's state.
  virtual bool NextLevels();

  /// This is called once for each row group in the file.
  void Reset() {
    def_level_ = HdfsParquetScanner::INVALID_LEVEL;
    rep_level_ = HdfsParquetScanner::INVALID_LEVEL;
    pos_current_value_ = HdfsParquetScanner::INVALID_POS;
  }

  virtual void Close(RowBatch* row_batch) {
    for (ParquetColumnReader* child_reader: children_) {
      child_reader->Close(row_batch);
    }
  }

 private:
  /// Column readers of fields contained within this collection. There is at least one
  /// child reader per collection reader. Child readers either materialize slots in the
  /// collection item tuples, or there is a single child reader that does not materialize
  /// any slot and is only used by this reader to read def and rep levels.
  vector<ParquetColumnReader*> children_;

  /// Updates this reader's def_level_, rep_level_, and pos_current_value_ based on child
  /// reader's state.
  void UpdateDerivedState();

  /// Recursively reads from children_ to assemble a single CollectionValue into
  /// 'slot'. Also advances rep_level_ and def_level_ via NextLevels().
  ///
  /// Returns false if execution should be aborted for some reason, e.g. parse_error_ is
  /// set, the query is cancelled, or the scan node limit was reached. Otherwise returns
  /// true.
  inline bool ReadSlot(CollectionValue* slot, MemPool* pool);
};

}

#endif