be/src/runtime/row-batch.h

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#ifndef IMPALA_RUNTIME_ROW_BATCH_H
#define IMPALA_RUNTIME_ROW_BATCH_H

#include <cstring>
#include <vector>

#include <boost/scoped_ptr.hpp>

#include "codegen/impala-ir.h"
#include "common/compiler-util.h"
#include "common/logging.h"
#include "gen-cpp/row_batch.pb.h"
#include "runtime/bufferpool/buffer-pool.h"
#include "runtime/descriptors.h"
#include "runtime/mem-pool.h"
#include "runtime/mem-tracker.h"

namespace kudu {
class Slice;
} // namespace kudu

namespace impala {

template <typename K, typename V> class FixedSizeHashTable;
class MemTracker;
class OutboundRowBatch;
class RowBatchHeaderPB;
class RowBatchSerializeTest;
class RuntimeState;
class Tuple;
class TupleRow;
class TupleDescriptor;

/// A RowBatch encapsulates a batch of rows, each composed of a number of tuples.
/// The maximum number of rows is fixed at the time of construction.
/// The row batch can reference various types of memory.
///   1. TupleRow ptrs - malloc'd and owned by the RowBatch. See the comment on
///      tuple_ptrs_ for more details.
///   2. Fixed and variable-length tuple data. This memory may be directly attached to
///      the batch: either in the batch's MemPool or in an attached buffer. Or it may
///      live elsewhere - either in a subsequent batch returned by an ExecNode or
///      still be owned by the ExecNode that produced the batch. In those cases the
///      owner of this RowBatch must be careful not to close the producing ExecNode
///      or free resources from trailing batches while the batch's data is still being
///      used.
///     TODO: IMPALA-4179: simplify the ownership transfer model.
///
/// In order to minimize memory allocations, RowBatches or
/// OutboundRowBatch that have been serialized and sent over the wire should be reused
/// (this prevents compression_scratch_ from being needlessly reallocated).
///
/// Row batches and memory usage: We attempt to stream row batches through the plan
/// tree without copying the data. This means that row batches are often not-compact
/// and reference memory outside of the row batch. This results in most row batches
/// having a very small memory footprint and in some row batches having a very large
/// one (it contains all the memory that other row batches are referencing). An example
/// is buffers which are only attached to one row batch. Only when the row batch reaches
/// a blocking operator or the root of the fragment is the row batch memory freed.
/// This means that in some cases (e.g. very selective queries), we still need to
/// pass the row batch through the exec nodes (even if they have no rows) to trigger
/// memory deletion. AtCapacity() encapsulates the check that the batch does not have
/// excessive memory attached to it.
///
/// A row batch is considered at capacity if all the rows are full or it has accumulated
/// auxiliary memory up to a soft cap. (See at_capacity_mem_usage_ comment).
class RowBatch {
 public:
  /// Flag indicating whether the resources attached to a RowBatch need to be flushed.
  /// Defined here as a convenience for other modules that need to communicate flushing
  /// modes.
  enum class FlushMode {
    FLUSH_RESOURCES,
    NO_FLUSH_RESOURCES,
  };

  /// Create RowBatch for a maximum of 'capacity' rows of tuples specified
  /// by 'row_desc'.
  /// tracker cannot be NULL.
  RowBatch(const RowDescriptor* row_desc, int capacity, MemTracker* tracker);

  /// Construct a row batch from a serialized protobuf input_batch and protobuf header by
  /// copying input_batch's tuple_data_ into the row batch's mempool and converting all
  /// offsets in the data back into pointers
  RowBatch(const RowDescriptor* row_desc, const OutboundRowBatch& input_batch,
      MemTracker* mem_tracker);

  /// Creates a row batch from the protobuf row batch header, decompress / copy
  /// 'input_tuple_data' into a buffer and convert all offsets in 'input_tuple_offsets'
  /// back into pointers. The tuple pointers and data's buffers are allocated from the
  /// buffer pool with 'client' as client handle. The newly created row batch is
  /// stored in 'row_batch_ptr'. Returns error status on failure. Returns ok otherwise.
  static Status FromProtobuf(const RowDescriptor* row_desc,
      const RowBatchHeaderPB& header, const kudu::Slice& input_tuple_data,
      const kudu::Slice& input_tuple_offsets, MemTracker* mem_tracker,
      BufferPool::ClientHandle* client, std::unique_ptr<RowBatch>* row_batch_ptr)
      WARN_UNUSED_RESULT;

  /// Releases all resources accumulated at this row batch.  This includes
  ///  - tuple_ptrs
  ///  - tuple mem pool data
  ///  - buffer handles from the io mgr
  ~RowBatch();

  /// AddRows() is called before adding rows to the batch. Returns the index of the next
  /// row to be added. The caller is responsible for ensuring there is enough remaining
  /// capacity: it is invalid to call AddRows when num_rows_ + n > capacity_. The rows
  /// are uninitialized and each tuple of the row must be set, after which CommitRows()
  /// can be called to update num_rows_. Two consecutive AddRow() calls without a
  /// CommitRows() call between them have the same effect as a single call.
  int ALWAYS_INLINE AddRows(int n) {
    DCHECK_LE(num_rows_ + n, capacity_);
    return num_rows_;
  }

  int ALWAYS_INLINE AddRow() { return AddRows(1); }

  void ALWAYS_INLINE CommitRows(int n) {
    DCHECK_GE(n, 0);
    DCHECK_LE(num_rows_ + n, capacity_);
    num_rows_ += n;
  }

  void ALWAYS_INLINE CommitLastRow() { CommitRows(1); }

  /// Set function can be used to reduce the number of rows in the batch. This is only
  /// used in the limit case where more rows were added than necessary.
  void set_num_rows(int num_rows) {
    DCHECK_LE(num_rows, num_rows_);
    DCHECK_GE(num_rows, 0);
    num_rows_ = num_rows;
  }

  /// Returns true if the row batch has filled rows up to its capacity or has accumulated
  /// enough memory. The memory calculation includes the tuple data pool and any
  /// auxiliary memory attached to the row batch.
  bool ALWAYS_INLINE AtCapacity() {
    DCHECK_LE(num_rows_, capacity_);
    // Check AtCapacity() condition enforced in MarkNeedsDeepCopy() and
    // MarkFlushResources().
    DCHECK((!needs_deep_copy_ && flush_mode_ == FlushMode::NO_FLUSH_RESOURCES)
        || num_rows_ == capacity_);
    int64_t mem_usage = attached_buffer_bytes_ + tuple_data_pool_.total_reserved_bytes();
    return num_rows_ == capacity_ || mem_usage >= AT_CAPACITY_MEM_USAGE;
  }

  TupleRow* ALWAYS_INLINE GetRow(int row_idx) {
    DCHECK(tuple_ptrs_ != NULL);
    DCHECK_GE(row_idx, 0);
    DCHECK_LT(row_idx, capacity_);
    return reinterpret_cast<TupleRow*>(tuple_ptrs_ + row_idx * num_tuples_per_row_);
  }

  /// An iterator for going through a row batch, starting at 'row_idx'.
  /// If 'limit' is specified, it will iterate up to row number 'row_idx + limit'
  /// or the last row, whichever comes first. Otherwise, it will iterate till the last
  /// row in the batch. This is more efficient than using GetRow() as it avoids loading
  /// the row batch state and doing multiplication on each loop with GetRow().
  class Iterator {
   public:
    Iterator(RowBatch* parent, int row_idx, int limit = -1) :
        num_tuples_per_row_(parent->num_tuples_per_row_),
        row_(parent->tuple_ptrs_ + num_tuples_per_row_ * row_idx),
        row_batch_end_(parent->tuple_ptrs_ + num_tuples_per_row_ *
            (limit == -1 ? parent->num_rows_ :
                           std::min<int>(row_idx + limit, parent->num_rows_))),
        parent_(parent) {
      DCHECK_GE(row_idx, 0);
      DCHECK_GT(num_tuples_per_row_, 0);
      /// We allow empty row batches with num_rows_ == capacity_ == 0.
      /// That's why we cannot call GetRow() above to initialize 'row_'.
      DCHECK_LE(row_idx, parent->capacity_);
    }

    /// Return the current row pointed to by the row pointer.
    TupleRow* IR_ALWAYS_INLINE Get() { return reinterpret_cast<TupleRow*>(row_); }

    /// Return the current row pointed to by the row pointer.
    const TupleRow* IR_ALWAYS_INLINE Get() const {
      return reinterpret_cast<const TupleRow*>(row_);
    }

    /// Increment the row pointer and return the next row.
    TupleRow* IR_ALWAYS_INLINE Next() {
      row_ += num_tuples_per_row_;
      DCHECK_LE((row_ - parent_->tuple_ptrs_) / num_tuples_per_row_, parent_->capacity_);
      return Get();
    }

    void IR_ALWAYS_INLINE Advance(int n) {
      row_ += n * num_tuples_per_row_;
      DCHECK_LE((row_ - parent_->tuple_ptrs_) / num_tuples_per_row_, parent_->capacity_);
    }

    /// Returns the index in the RowBatch of the current row. This does an integer
    /// division and so should not be used in hot inner loops.
    int RowNum() { return (row_ - parent_->tuple_ptrs_) / num_tuples_per_row_; }

    /// Returns number of rows remaining.
    int RemainingRows() { return (row_batch_end_ - row_) / num_tuples_per_row_; }

    /// Returns true if the iterator is beyond the last row for read iterators.
    /// Useful for read iterators to determine the limit. Write iterators should use
    /// RowBatch::AtCapacity() instead.
    bool IR_ALWAYS_INLINE AtEnd() const { return row_ >= row_batch_end_; }

    /// Returns the row batch which this iterator is iterating through.
    RowBatch* parent() { return parent_; }

   private:
    /// Number of tuples per row.
    const int num_tuples_per_row_;

    /// Pointer to the current row.
    Tuple** row_;

    /// Pointer to the row after the last row for read iterators.
    Tuple** const row_batch_end_;

    /// The row batch being iterated on.
    RowBatch* const parent_;
  };

  int num_tuples_per_row() { return num_tuples_per_row_; }
  MemPool* tuple_data_pool() { return &tuple_data_pool_; }
  int num_buffers() const { return buffers_.size(); }

  MemPoolCounters GetMemPoolCounters() { return tuple_data_pool_.GetMemPoolCounters(); }

  /// Resets the row batch, returning all resources it has accumulated.
  void Reset();

  /// Adds a buffer to this row batch. The buffer is deleted when freeing resources.
  /// The buffer's memory remains accounted against the original owner, even when the
  /// ownership of batches is transferred. If the original owner wants the memory to be
  /// released, it should call this with 'mode' FLUSH_RESOURCES (see MarkFlushResources()
  /// for further explanation).
  /// TODO: IMPALA-4179: simplify the ownership transfer model.
  void AddBuffer(BufferPool::ClientHandle* client, BufferPool::BufferHandle&& buffer,
      FlushMode flush);

  /// Used by an operator to indicate that it cannot produce more rows until the
  /// resources that it has attached to the row batch are freed or acquired by an
  /// ancestor operator. After this is called, the batch is at capacity and no more rows
  /// can be added. The "flush" mark is transferred by TransferResourceOwnership(). This
  /// ensures that batches are flushed by streaming operators all the way up the operator
  /// tree. Blocking operators can still accumulate batches with this flag.
  /// TODO: IMPALA-4179: blocking operators should acquire all memory resources including
  /// attached buffers, so that MarkFlushResources() can guarantee that the
  /// resources will not be accounted against the original operator (this is currently
  /// not true for buffers, which aren't transferred).
  void MarkFlushResources() {
    DCHECK_LE(num_rows_, capacity_);
    capacity_ = num_rows_;
    flush_mode_ = FlushMode::FLUSH_RESOURCES;
  }

  /// Set the capacity of the row batch to 'limit' so that only that many rows can be
  /// appended to it. The capacity will be restored to its original value when Reset()
  /// is called.
  void limit_capacity(int limit) {
    DCHECK_LE(num_rows_, limit);
    capacity_ = limit;
  }

  FlushMode flush_mode() const { return flush_mode_; }

  /// Called to indicate that some resources backing this batch were not attached and
  /// will be cleaned up after the next GetNext() call. This means that the batch must
  /// be returned up the operator tree. Blocking operators must deep-copy any rows from
  /// this batch or preceding batches.
  ///
  /// This is a stronger version of MarkFlushResources(), because blocking operators
  /// are not allowed to accumulate batches with the 'needs_deep_copy' flag.
  /// TODO: IMPALA-4179: always attach backing resources and remove this flag.
  void MarkNeedsDeepCopy() {
    MarkFlushResources(); // No more rows should be added to the batch.
    needs_deep_copy_ = true;
  }

  bool needs_deep_copy() { return needs_deep_copy_; }

  /// Transfer ownership of resources to dest.  This includes tuple data in mem
  /// pool and buffers.
  void TransferResourceOwnership(RowBatch* dest);

  /// Update accounting so that attached memory is accounted against 'new_tracker'.
  void SetMemTracker(MemTracker* new_tracker);

  void CopyRow(TupleRow* src, TupleRow* dest) {
    memcpy(dest, src, num_tuples_per_row_ * sizeof(Tuple*));
  }

  /// Copies tuple pointers from another row batch. It is allowed for 'src' to have
  /// less tuples per row - in this case the prefix is copied and remaining tuples are
  /// initialized to 0. The caller must ensure that src has enough rows / dst has enough
  /// capacity.
  void CopyRows(RowBatch* src, int num_rows, int src_offset, int dst_offset);

  /// Copy 'num_rows' rows from 'src' to 'dest' within the batch. Useful for exec
  /// nodes that skip an offset and copied more than necessary.
  void CopyRows(int64_t dest, int64_t src, int64_t num_rows) {
    DCHECK_LE(dest, src);
    DCHECK_LE(src + num_rows, capacity_);
    memmove(tuple_ptrs_ + num_tuples_per_row_ * dest,
        tuple_ptrs_ + num_tuples_per_row_ * src,
        num_rows * num_tuples_per_row_ * sizeof(Tuple*));
  }

  void ClearTuplePointers() {
    memset(tuple_ptrs_, 0, capacity_ * num_tuples_per_row_ * sizeof(Tuple*));
  }

  void ClearRow(TupleRow* row) {
    memset(row, 0, num_tuples_per_row_ * sizeof(Tuple*));
  }

  /// Acquires state from the 'src' row batch into this row batch. This includes all
  /// buffers and tuple data.
  /// This row batch must be empty and have the same row descriptor as the src batch.
  /// This is used for scan nodes which produce RowBatches asynchronously.  Typically,
  /// an ExecNode is handed a row batch to populate (pull model) but ScanNodes have
  /// multiple threads which push row batches.
  void AcquireState(RowBatch* src);

  /// Deep copy all rows this row batch into dst, using memory allocated from
  /// dst's tuple_data_pool_. Only valid when dst is empty.
  /// TODO: the current implementation of deep copy can produce an oversized
  /// row batch if there are duplicate tuples in this row batch.
  void DeepCopyTo(RowBatch* dst);

  /// Create a serialized version of this row batch in output_batch, attaching all of the
  /// data it references to output_batch.tuple_data. This function attempts to detect
  /// duplicate tuples in the row batch to reduce the serialized size.
  /// output_batch.tuple_data will be LZ4-compressed unless the compressed data is larger
  /// larger than the uncompressed data. Use output_batch.compression_type to determine
  /// whether tuple_data is compressed. If an in-flight row is present in this row batch,
  /// it is ignored. This function does not Reset().
  Status Serialize(OutboundRowBatch* output_batch, TrackedString* compression_scratch);

  /// Utility function: returns total byte size of a batch in either serialized or
  /// deserialized form. If a row batch is compressed, its serialized size can be much
  /// less than the deserialized size.
  static int64_t GetSerializedSize(const OutboundRowBatch& batch);
  static int64_t GetDeserializedSize(const OutboundRowBatch& batch);
  static int64_t GetDeserializedSize(const RowBatchHeaderPB& header,
      const kudu::Slice& tuple_offsets);

  int ALWAYS_INLINE num_rows() const { return num_rows_; }
  int ALWAYS_INLINE capacity() const { return capacity_; }

  // The maximum value that capacity_ ever took, before MarkCapacity() might have changed
  // it.
  int ALWAYS_INLINE InitialCapacity() const {
    return tuple_ptrs_size_ / (num_tuples_per_row_ * sizeof(Tuple*));
  }

  const RowDescriptor* row_desc() const { return row_desc_; }

  /// Max memory that this row batch can accumulate before it is considered at capacity.
  /// This is a soft capacity: row batches may exceed the capacity, preferably only by a
  /// row's worth of data.
  static const int AT_CAPACITY_MEM_USAGE = 8 * 1024 * 1024;

  // Max memory out of AT_CAPACITY_MEM_USAGE that should be used for fixed-length data,
  // in order to leave room for variable-length data.
  static const int FIXED_LEN_BUFFER_LIMIT = AT_CAPACITY_MEM_USAGE / 2;

  // Batch size to compute hash, keep it small to avoid large stack allocations.
  // 16 provided the same speedup compared to operating over a full batch.
  static const int HASH_BATCH_SIZE = 16;

  /// Allocates a buffer large enough for the fixed-length portion of 'capacity_' rows in
  /// this batch from 'tuple_data_pool_'. 'capacity_' is reduced if the allocation would
  /// exceed FIXED_LEN_BUFFER_LIMIT. Always returns enough space for at least one row.
  /// Returns Status::MEM_LIMIT_EXCEEDED and sets 'buffer' to NULL if a memory limit would
  /// have been exceeded. 'state' is used to log the error.
  /// On success, sets 'buffer_size' to the size in bytes and 'buffer' to the buffer.
  Status ResizeAndAllocateTupleBuffer(
      RuntimeState* state, int64_t* buffer_size, uint8_t** buffer);

  /// Same as above except allocates buffer for 'capacity' rows with fixed-length portions
  /// of 'row_size' bytes each from 'pool', instead of using RowBatch's member variables.
  static Status ResizeAndAllocateTupleBuffer(RuntimeState* state, MemPool* pool,
      int row_size, int* capacity, int64_t* buffer_size, uint8_t** buffer);

  /// Helper function to log the batch's rows if VLOG_ROW is enabled. 'context' is a
  /// string to prepend to the log message.
  void VLogRows(const std::string& context);

 private:
  friend class RowBatchSerializeBaseline;
  friend class RowBatchSerializeBenchmark;
  friend class RowBatchSerializeTest;
  friend class SimpleTupleStreamTest;

  /// Creates an empty row batch based on the serialized row batch header. Called from
  /// FromProtobuf() above before desrialization of a protobuf row batch.
  RowBatch(const RowDescriptor* row_desc, const RowBatchHeaderPB& header,
      MemTracker* mem_tracker);

  /// Allocate from buffer pool a buffer of 'len' using the client handle 'client'.
  /// The actual buffer size is 'len' rounded up to power of 2 or minimum buffer size,
  /// whichever is larger. The reservation of 'client' may be increased. On success,
  /// the newly allocated buffer is returned in 'buffer_handle'. Return error status
  /// if allocation failed. In which case, 'buffer_handle' is not opened.
  Status AllocateBuffer(BufferPool::ClientHandle* client, int64_t len,
      BufferPool::BufferHandle* buffer_handle);

  /// Free all BufferInfo and the associated buffers in 'buffers_'.
  void FreeBuffers();

  /// Decide whether to do full tuple deduplication based on row composition. Full
  /// deduplication is enabled only when there is risk of the serialized size being
  /// much larger than in-memory size due to non-adjacent duplicate tuples.
  bool UseFullDedup();

  /// Overload for testing that allows the test to force the deduplication level.
  Status Serialize(OutboundRowBatch* output_batch, bool full_dedup,
      TrackedString* compression_scratch);

  typedef FixedSizeHashTable<Tuple*, int> DedupMap;

  /// Implementation for protobuf to serialize this row batch.
  ///
  /// 'distinct_tuples': pointer to an empty DedupMap. Should not be null if full
  ///                    deduplication is used.
  /// 'output_batch': output_batch's tuple_offsets and tuple_data will be modified in the
  ///                 following ways:
  ///                 - 'tuple_data': Updated to hold the serialized tuples'
  ///                                 data. If 'is_compressed' is true, this is LZ4
  ///                                 compressed. The tuple_data_length_ is also updated
  ///                                 accordingly.
  ///                 - 'tuple_offsets': Updated to contain offsets of all tuples into
  ///                                    'tuple_data' upon return. There are a total of
  ///                                    num_rows * num_tuples_per_row offsets. An offset
  ///                                    of -1 records a NULL.
  ///
  /// 'is_compressed': true if compression is applied on 'tuple_data'.
  /// 'size': Expected size of serialized row batch data.
  ///
  /// Returns error status if serialization failed. Returns OK otherwise.
  Status Serialize(DedupMap* distinct_tuples, OutboundRowBatch* output_batch,
      bool* is_compressed, int64_t size, TrackedString* compression_scratch);

  /// Implementation for protobuf to deserialize a row batch.
  ///
  /// 'input_tuple_offsets': an int32_t array of tuples; offsets into 'input_tuple_data'.
  /// Used for populating the tuples in the row batch with actual pointers.
  ///
  /// 'input_tuple_data': contains pointer and size of tuples' data buffer.
  /// If 'is_compressed' is true, the data is compressed.
  ///
  /// 'uncompressed_size': the uncompressed size of 'input_tuple_data' if it's compressed.
  ///
  /// 'is_compressed': True if 'input_tuple_data' is compressed.
  ///
  /// 'tuple_data': buffer of 'uncompressed_size' bytes for holding tuple data.
  void Deserialize(const kudu::Slice& input_tuple_offsets,
      const kudu::Slice& input_tuple_data, int64_t uncompressed_size, bool is_compressed,
      uint8_t* tuple_data);

  /// The total size of all data represented in this row batch (tuples and referenced
  /// string and collection data). This is the size of the row batch after removing all
  /// gaps in the auxiliary and deduplicated tuples (i.e. the smallest footprint for the
  /// row batch). If the distinct_tuples argument is non-null, full deduplication is
  /// enabled. The distinct_tuples map must be empty.
  int64_t TotalByteSize(DedupMap* distinct_tuples);

  Status SerializeInternal(int64_t size, DedupMap* distinct_tuples,
      vector<int32_t>* tuple_offsets, char* tuple_data);

  /// All members below need to be handled in RowBatch::AcquireState()

  // Class members that are accessed on performance-critical paths should appear
  // up the top to fit in as few cache lines as possible.

  int num_rows_;  // # of committed rows
  int capacity_;  // the value of num_rows_ at which batch is considered full.

  /// If FLUSH_RESOURCES, the resources attached to this batch should be freed or
  /// acquired by a new owner as soon as possible. See MarkFlushResources(). If
  /// FLUSH_RESOURCES, AtCapacity() is also true.
  FlushMode flush_mode_;

  /// If true, this batch references unowned memory that will be cleaned up soon.
  /// See MarkNeedsDeepCopy(). If true, 'flush_mode_' is FLUSH_RESOURCES and
  /// AtCapacity() is true.
  bool needs_deep_copy_;

  const int num_tuples_per_row_;

  /// Array of pointers with InitialCapacity() * num_tuples_per_row_ elements.
  /// The memory ownership depends on whether legacy joins and aggs are enabled.
  ///
  /// Memory is malloc'd and owned by RowBatch and is freed upon its destruction. This is
  /// more performant that allocating the pointers from 'tuple_data_pool_' especially
  /// with SubplanNodes in the ExecNode tree because the tuple pointers are not
  /// transferred and do not have to be re-created in every Reset().
  const int tuple_ptrs_size_;
  Tuple** tuple_ptrs_ = nullptr;

  /// Total bytes of BufferPool buffers attached to this batch.
  int64_t attached_buffer_bytes_;

  /// holding (some of the) data referenced by rows
  MemPool tuple_data_pool_;

  // Less frequently used members that are not accessed on performance-critical paths
  // should go below here.

  /// Full row descriptor for rows in this batch. Owned by the exec node that produced
  /// this batch.
  const RowDescriptor* row_desc_;

  MemTracker* mem_tracker_;  // not owned

  struct BufferInfo {
    BufferPool::ClientHandle* client = nullptr;
    BufferPool::BufferHandle buffer;
  };

  /// Pages attached to this row batch. See AddBuffer() for ownership semantics.
  std::vector<BufferInfo> buffers_;

  /// The BufferInfo for the 'tuple_ptrs_' which are allocated from the buffer pool.
  std::unique_ptr<BufferInfo> tuple_ptrs_info_;
};
}

/// Macros for iterating through '_row_batch', starting at '_start_row_idx'.
/// '_row_batch' is the row batch to iterate through.
/// '_start_row_idx' is the starting row index.
/// '_iter' is the iterator.
/// '_limit' is the max number of rows to iterate over.
#define FOREACH_ROW(_row_batch, _start_row_idx, _iter)                  \
    for (RowBatch::Iterator _iter(_row_batch, _start_row_idx);          \
         !_iter.AtEnd(); _iter.Next())

#define FOREACH_ROW_LIMIT(_row_batch, _start_row_idx, _limit, _iter)    \
    for (RowBatch::Iterator _iter(_row_batch, _start_row_idx, _limit);  \
         !_iter.AtEnd(); _iter.Next())

#endif