rocksdb/table/merger.cc

//  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
//  This source code is licensed under the BSD-style license found in the
//  LICENSE file in the root directory of this source tree. An additional grant
//  of patent rights can be found in the PATENTS file in the same directory.
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#include "table/merger.h"

#include <vector>
#include <queue>

#include "rocksdb/comparator.h"
#include "rocksdb/iterator.h"
#include "rocksdb/options.h"
#include "table/iter_heap.h"
#include "table/iterator_wrapper.h"
#include "util/arena.h"
#include "util/stop_watch.h"
#include "util/perf_context_imp.h"
#include "util/autovector.h"

namespace rocksdb {
// Without anonymous namespace here, we fail the warning -Wmissing-prototypes
namespace {
typedef std::priority_queue<IteratorWrapper*, std::vector<IteratorWrapper*>,
                            MaxIteratorComparator> MergerMaxIterHeap;

typedef std::priority_queue<IteratorWrapper*, std::vector<IteratorWrapper*>,
                            MinIteratorComparator> MergerMinIterHeap;

// Return's a new MaxHeap of IteratorWrapper's using the provided Comparator.
MergerMaxIterHeap NewMergerMaxIterHeap(const Comparator* comparator) {
  return MergerMaxIterHeap(MaxIteratorComparator(comparator));
}

// Return's a new MinHeap of IteratorWrapper's using the provided Comparator.
MergerMinIterHeap NewMergerMinIterHeap(const Comparator* comparator) {
  return MergerMinIterHeap(MinIteratorComparator(comparator));
}
}  // namespace

const size_t kNumIterReserve = 4;

class MergingIterator : public Iterator {
 public:
  MergingIterator(const Comparator* comparator, Iterator** children, int n,
                  bool is_arena_mode)
      : is_arena_mode_(is_arena_mode),
        comparator_(comparator),
        current_(nullptr),
        use_heap_(true),
        direction_(kForward),
        maxHeap_(NewMergerMaxIterHeap(comparator_)),
        minHeap_(NewMergerMinIterHeap(comparator_)) {
    children_.resize(n);
    for (int i = 0; i < n; i++) {
      children_[i].Set(children[i]);
    }
    for (auto& child : children_) {
      if (child.Valid()) {
        minHeap_.push(&child);
      }
    }
  }

  virtual void AddIterator(Iterator* iter) {
    assert(direction_ == kForward);
    children_.emplace_back(iter);
    auto new_wrapper = children_.back();
    if (new_wrapper.Valid()) {
      minHeap_.push(&new_wrapper);
    }
  }

  virtual ~MergingIterator() {
    for (auto& child : children_) {
      child.DeleteIter(is_arena_mode_);
    }
  }

  virtual bool Valid() const override { return (current_ != nullptr); }

  virtual void SeekToFirst() override {
    ClearHeaps();
    for (auto& child : children_) {
      child.SeekToFirst();
      if (child.Valid()) {
        minHeap_.push(&child);
      }
    }
    FindSmallest();
    direction_ = kForward;
  }

  virtual void SeekToLast() override {
    ClearHeaps();
    for (auto& child : children_) {
      child.SeekToLast();
      if (child.Valid()) {
        maxHeap_.push(&child);
      }
    }
    FindLargest();
    direction_ = kReverse;
  }

  virtual void Seek(const Slice& target) override {
    // Invalidate the heap.
    use_heap_ = false;
    IteratorWrapper* first_child = nullptr;

    for (auto& child : children_) {
      {
        PERF_TIMER_GUARD(seek_child_seek_time);
        child.Seek(target);
      }
      PERF_COUNTER_ADD(seek_child_seek_count, 1);

      if (child.Valid()) {
        // This child has valid key
        if (!use_heap_) {
          if (first_child == nullptr) {
            // It's the first child has valid key. Only put it int
            // current_. Now the values in the heap should be invalid.
            first_child = &child;
          } else {
            // We have more than one children with valid keys. Initialize
            // the heap and put the first child into the heap.
            PERF_TIMER_GUARD(seek_min_heap_time);
            ClearHeaps();
            minHeap_.push(first_child);
          }
        }
        if (use_heap_) {
          PERF_TIMER_GUARD(seek_min_heap_time);
          minHeap_.push(&child);
        }
      }
    }
    if (use_heap_) {
      // If heap is valid, need to put the smallest key to curent_.
      PERF_TIMER_GUARD(seek_min_heap_time);
      FindSmallest();
    } else {
      // The heap is not valid, then the current_ iterator is the first
      // one, or null if there is no first child.
      current_ = first_child;
    }
    direction_ = kForward;
  }

  virtual void Next() override {
    assert(Valid());

    // Ensure that all children are positioned after key().
    // If we are moving in the forward direction, it is already
    // true for all of the non-current_ children since current_ is
    // the smallest child and key() == current_->key().  Otherwise,
    // we explicitly position the non-current_ children.
    if (direction_ != kForward) {
      ClearHeaps();
      for (auto& child : children_) {
        if (&child != current_) {
          child.Seek(key());
          if (child.Valid() &&
              comparator_->Compare(key(), child.key()) == 0) {
            child.Next();
          }
          if (child.Valid()) {
            minHeap_.push(&child);
          }
        }
      }
      direction_ = kForward;
    }

    // as the current points to the current record. move the iterator forward.
    // and if it is valid add it to the heap.
    current_->Next();
    if (use_heap_) {
      if (current_->Valid()) {
        minHeap_.push(current_);
      }
      FindSmallest();
    } else if (!current_->Valid()) {
      current_ = nullptr;
    }
  }

  virtual void Prev() override {
    assert(Valid());
    // Ensure that all children are positioned before key().
    // If we are moving in the reverse direction, it is already
    // true for all of the non-current_ children since current_ is
    // the largest child and key() == current_->key().  Otherwise,
    // we explicitly position the non-current_ children.
    if (direction_ != kReverse) {
      ClearHeaps();
      for (auto& child : children_) {
        if (&child != current_) {
          child.Seek(key());
          if (child.Valid()) {
            // Child is at first entry >= key().  Step back one to be < key()
            child.Prev();
          } else {
            // Child has no entries >= key().  Position at last entry.
            child.SeekToLast();
            if (child.Valid() && comparator_->Compare(child.key(), key()) > 0) {
              // Prefix bloom or prefix hash may return !Valid() if one of the
              // following condition happens:  1. when prefix doesn't match.
              // 2. Does not exist any row larger than the key within the prefix
              // while SeekToLast() may return larger keys.
              //
              // Temporarily remove this child to avoid Prev() to return a key
              // larger than the original keys. However, this can cause missing
              // rows.
              //
              // TODO(3.13): need to fix.
              continue;
            }
          }
          if (child.Valid()) {
            maxHeap_.push(&child);
          }
        }
      }
      direction_ = kReverse;
    }

    current_->Prev();
    if (current_->Valid()) {
      maxHeap_.push(current_);
    }
    FindLargest();
  }

  virtual Slice key() const override {
    assert(Valid());
    return current_->key();
  }

  virtual Slice value() const override {
    assert(Valid());
    return current_->value();
  }

  virtual Status status() const override {
    Status s;
    for (auto& child : children_) {
      s = child.status();
      if (!s.ok()) {
        break;
      }
    }
    return s;
  }

 private:
  void FindSmallest();
  void FindLargest();
  void ClearHeaps();

  bool is_arena_mode_;
  const Comparator* comparator_;
  autovector<IteratorWrapper, kNumIterReserve> children_;
  IteratorWrapper* current_;
  // If the value is true, both of iterators in the heap and current_
  // contain valid rows. If it is false, only current_ can possibly contain
  // valid rows.
  // This flag is always true for reverse direction, as we always use heap for
  // the reverse iterating case.
  bool use_heap_;
  // Which direction is the iterator moving?
  enum Direction {
    kForward,
    kReverse
  };
  Direction direction_;
  MergerMaxIterHeap maxHeap_;
  MergerMinIterHeap minHeap_;
};

void MergingIterator::FindSmallest() {
  assert(use_heap_);
  if (minHeap_.empty()) {
    current_ = nullptr;
  } else {
    current_ = minHeap_.top();
    assert(current_->Valid());
    minHeap_.pop();
  }
}

void MergingIterator::FindLargest() {
  assert(use_heap_);
  if (maxHeap_.empty()) {
    current_ = nullptr;
  } else {
    current_ = maxHeap_.top();
    assert(current_->Valid());
    maxHeap_.pop();
  }
}

void MergingIterator::ClearHeaps() {
  use_heap_ = true;
  maxHeap_ = NewMergerMaxIterHeap(comparator_);
  minHeap_ = NewMergerMinIterHeap(comparator_);
}

Iterator* NewMergingIterator(const Comparator* cmp, Iterator** list, int n,
                             Arena* arena) {
  assert(n >= 0);
  if (n == 0) {
    return NewEmptyIterator(arena);
  } else if (n == 1) {
    return list[0];
  } else {
    if (arena == nullptr) {
      return new MergingIterator(cmp, list, n, false);
    } else {
      auto mem = arena->AllocateAligned(sizeof(MergingIterator));
      return new (mem) MergingIterator(cmp, list, n, true);
    }
  }
}

MergeIteratorBuilder::MergeIteratorBuilder(const Comparator* comparator,
                                           Arena* a)
    : first_iter(nullptr), use_merging_iter(false), arena(a) {

  auto mem = arena->AllocateAligned(sizeof(MergingIterator));
  merge_iter = new (mem) MergingIterator(comparator, nullptr, 0, true);
}

void MergeIteratorBuilder::AddIterator(Iterator* iter) {
  if (!use_merging_iter && first_iter != nullptr) {
    merge_iter->AddIterator(first_iter);
    use_merging_iter = true;
  }
  if (use_merging_iter) {
    merge_iter->AddIterator(iter);
  } else {
    first_iter = iter;
  }
}

Iterator* MergeIteratorBuilder::Finish() {
  if (!use_merging_iter) {
    return first_iter;
  } else {
    auto ret = merge_iter;
    merge_iter = nullptr;
    return ret;
  }
}

}  // namespace rocksdb