be/src/runtime/sorted-run-merger.h

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#ifndef IMPALA_RUNTIME_SORTED_RUN_MERGER_H_
#define IMPALA_RUNTIME_SORTED_RUN_MERGER_H_

#include <boost/scoped_ptr.hpp>
#include <boost/thread/mutex.hpp>

#include "common/object-pool.h"
#include "util/tuple-row-compare.h"

namespace impala {

class RowBatch;
class RowDescriptor;
class RuntimeProfile;

/// SortedRunMerger is used to merge multiple sorted runs of tuples. A run is a sorted
/// sequence of row batches, which are fetched from a RunBatchSupplierFn function object.
/// Merging is implemented using a binary min-heap that maintains the run with the next
/// tuple in sorted order at the top of the heap.
///
/// Merged batches of rows are retrieved from SortedRunMerger via calls to GetNext().
/// The merger is constructed with a boolean flag deep_copy_input.
/// If true, sorted output rows are deep copied into the data pool of the output batch.
/// If false, GetNext() only copies tuple pointers (TupleRows) into the output batch,
/// and transfers resource ownership from the input batches to the output batch when
/// an input batch is processed.
///
/// SortedRunMerger cannot handle "flushing resources" so if the RunBatchSupplierFn
/// can return batches with FLUSH_RESOURCES set, the merger must have 'deep_copy_input'
/// set. This is because AdvanceMinRow() gets the next batch before freeing resources
/// from the previous batch.
/// TODO: it would be nice to fix this to avoid unnecessary copies.
class SortedRunMerger {
 public:
  /// Function that returns the next batch of rows from an input sorted run. The batch
  /// is owned by the supplier (i.e. not SortedRunMerger). eos is indicated by a NULL
  /// batch being returned. The returned batch can have any number of rows (including
  /// zero).
  typedef boost::function<Status (RowBatch**)> RunBatchSupplierFn;

  SortedRunMerger(const TupleRowComparator& comparator, const RowDescriptor* row_desc,
      RuntimeProfile* profile, bool deep_copy_input);

  /// Prepare this merger to merge and return rows from the sorted runs in 'input_runs'.
  /// Retrieves the first batch from each run and sets up the binary heap implementing
  /// the priority queue.
  Status Prepare(const std::vector<RunBatchSupplierFn>& input_runs);

  /// Return the next batch of sorted rows from this merger.
  Status GetNext(RowBatch* output_batch, bool* eos);

 private:
  class SortedRunWrapper;

  /// Remove the current row from the current min RunBatchSupplierFn and try to advance to
  /// the next row. If 'deep_copy_input_' is false, 'transfer_batch' must be supplied to
  /// attach resources to.
  ///
  /// When AdvanceMinRow returns, the previous min is advanced to the next row and the
  /// heap is reordered accordingly. The RunBatchSupplierFn is removed from the heap if
  /// this was its last row. Any completed resources are transferred to the batch.
  Status AdvanceMinRow(RowBatch* transfer_batch);

  /// Assuming the element at parent_index is the only out of place element in the heap,
  /// restore the heap property (i.e. swap elements so parent <= children).
  void Heapify(int parent_index);

  /// The binary min-heap used to merge rows from the sorted input runs. Since the heap is
  /// stored in a 0-indexed array, the 0-th element is the minimum element in the heap,
  /// and the children of the element at index i are 2*i+1 and 2*i+2. The heap property is
  /// that row of the parent element is <= the rows of the child elements according to the
  /// comparator comparator_.
  /// The SortedRunWrapper objects used in the min_heap_ are owned by this
  /// SortedRunMerger instance.
  std::vector<SortedRunWrapper*> min_heap_;

  /// Row comparator. Returns true if lhs < rhs.
  const TupleRowComparator& comparator_;

  /// Descriptor for the rows provided by the input runs. Owned by the exec-node through
  /// which this merger was created.
  const RowDescriptor* input_row_desc_;

  /// True if rows must be deep copied into the output batch.
  bool deep_copy_input_;

  /// Pool of SortedRunWrapper instances.
  ObjectPool pool_;

  /// Times calls to GetNext().
  RuntimeProfile::Counter* get_next_timer_;

  /// Times calls to get the next batch of rows from the input run.
  RuntimeProfile::Counter* get_next_batch_timer_;
};

}

#endif