db_iter.cc

//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root 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 "db/db_iter.h"
#include <string>
#include <limits>

#include "db/dbformat.h"
#include "db/merge_context.h"
#include "db/merge_helper.h"
#include "db/pinned_iterators_manager.h"
#include "monitoring/perf_context_imp.h"
#include "rocksdb/env.h"
#include "rocksdb/iterator.h"
#include "rocksdb/merge_operator.h"
#include "rocksdb/options.h"
#include "table/internal_iterator.h"
#include "util/arena.h"
#include "util/filename.h"
#include "util/logging.h"
#include "util/mutexlock.h"
#include "util/string_util.h"

namespace rocksdb {

#if 0
static void DumpInternalIter(Iterator* iter) {
  for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {
    ParsedInternalKey k;
    if (!ParseInternalKey(iter->key(), &k)) {
      fprintf(stderr, "Corrupt '%s'\n", EscapeString(iter->key()).c_str());
    } else {
      fprintf(stderr, "@ '%s'\n", k.DebugString().c_str());
    }
  }
}
#endif

// Memtables and sstables that make the DB representation contain
// (userkey,seq,type) => uservalue entries.  DBIter
// combines multiple entries for the same userkey found in the DB
// representation into a single entry while accounting for sequence
// numbers, deletion markers, overwrites, etc.
class DBIter final: public Iterator {
 public:
  // The following is grossly complicated. TODO: clean it up
  // Which direction is the iterator currently moving?
  // (1) When moving forward, the internal iterator is positioned at
  //     the exact entry that yields this->key(), this->value()
  // (2) When moving backwards, the internal iterator is positioned
  //     just before all entries whose user key == this->key().
  enum Direction {
    kForward,
    kReverse
  };

  // LocalStatistics contain Statistics counters that will be aggregated per
  // each iterator instance and then will be sent to the global statistics when
  // the iterator is destroyed.
  //
  // The purpose of this approach is to avoid perf regression happening
  // when multiple threads bump the atomic counters from a DBIter::Next().
  struct LocalStatistics {
    explicit LocalStatistics() { ResetCounters(); }

    void ResetCounters() {
      next_count_ = 0;
      next_found_count_ = 0;
      prev_count_ = 0;
      prev_found_count_ = 0;
      bytes_read_ = 0;
    }

    void BumpGlobalStatistics(Statistics* global_statistics) {
      RecordTick(global_statistics, NUMBER_DB_NEXT, next_count_);
      RecordTick(global_statistics, NUMBER_DB_NEXT_FOUND, next_found_count_);
      RecordTick(global_statistics, NUMBER_DB_PREV, prev_count_);
      RecordTick(global_statistics, NUMBER_DB_PREV_FOUND, prev_found_count_);
      RecordTick(global_statistics, ITER_BYTES_READ, bytes_read_);
      PERF_COUNTER_ADD(iter_read_bytes, bytes_read_);
      ResetCounters();
    }

    // Map to Tickers::NUMBER_DB_NEXT
    uint64_t next_count_;
    // Map to Tickers::NUMBER_DB_NEXT_FOUND
    uint64_t next_found_count_;
    // Map to Tickers::NUMBER_DB_PREV
    uint64_t prev_count_;
    // Map to Tickers::NUMBER_DB_PREV_FOUND
    uint64_t prev_found_count_;
    // Map to Tickers::ITER_BYTES_READ
    uint64_t bytes_read_;
  };

  DBIter(Env* _env, const ReadOptions& read_options,
         const ImmutableCFOptions& cf_options, const Comparator* cmp,
         InternalIterator* iter, SequenceNumber s, bool arena_mode,
         uint64_t max_sequential_skip_in_iterations, bool allow_blob)
      : arena_mode_(arena_mode),
        env_(_env),
        logger_(cf_options.info_log),
        user_comparator_(cmp),
        merge_operator_(cf_options.merge_operator),
        iter_(iter),
        sequence_(s),
        direction_(kForward),
        valid_(false),
        current_entry_is_merged_(false),
        statistics_(cf_options.statistics),
        iterate_upper_bound_(read_options.iterate_upper_bound),
        prefix_same_as_start_(read_options.prefix_same_as_start),
        pin_thru_lifetime_(read_options.pin_data),
        total_order_seek_(read_options.total_order_seek),
        range_del_agg_(cf_options.internal_comparator, s,
                       true /* collapse_deletions */),
        allow_blob_(allow_blob) {
    RecordTick(statistics_, NO_ITERATORS);
    prefix_extractor_ = cf_options.prefix_extractor;
    max_skip_ = max_sequential_skip_in_iterations;
    max_skippable_internal_keys_ = read_options.max_skippable_internal_keys;
    if (pin_thru_lifetime_) {
      pinned_iters_mgr_.StartPinning();
    }
    if (iter_) {
      iter_->SetPinnedItersMgr(&pinned_iters_mgr_);
    }
  }
  virtual ~DBIter() {
    // Release pinned data if any
    if (pinned_iters_mgr_.PinningEnabled()) {
      pinned_iters_mgr_.ReleasePinnedData();
    }
    RecordTick(statistics_, NO_ITERATORS, -1);
    local_stats_.BumpGlobalStatistics(statistics_);
    if (!arena_mode_) {
      delete iter_;
    } else {
      iter_->~InternalIterator();
    }
  }
  virtual void SetIter(InternalIterator* iter) {
    assert(iter_ == nullptr);
    iter_ = iter;
    iter_->SetPinnedItersMgr(&pinned_iters_mgr_);
  }
  virtual RangeDelAggregator* GetRangeDelAggregator() {
    return &range_del_agg_;
  }

  virtual bool Valid() const override { return valid_; }
  virtual Slice key() const override {
    assert(valid_);
    return saved_key_.GetUserKey();
  }
  virtual Slice value() const override {
    assert(valid_);
    if (current_entry_is_merged_) {
      // If pinned_value_ is set then the result of merge operator is one of
      // the merge operands and we should return it.
      return pinned_value_.data() ? pinned_value_ : saved_value_;
    } else if (direction_ == kReverse) {
      return pinned_value_;
    } else {
      return iter_->value();
    }
  }
  virtual Status status() const override {
    if (status_.ok()) {
      return iter_->status();
    } else {
      return status_;
    }
  }
  bool IsBlob() const {
    assert(valid_ && (allow_blob_ || !is_blob_));
    return is_blob_;
  }

  virtual Status GetProperty(std::string prop_name,
                             std::string* prop) override {
    if (prop == nullptr) {
      return Status::InvalidArgument("prop is nullptr");
    }
    if (prop_name == "rocksdb.iterator.super-version-number") {
      // First try to pass the value returned from inner iterator.
      return iter_->GetProperty(prop_name, prop);
    } else if (prop_name == "rocksdb.iterator.is-key-pinned") {
      if (valid_) {
        *prop = (pin_thru_lifetime_ && saved_key_.IsKeyPinned()) ? "1" : "0";
      } else {
        *prop = "Iterator is not valid.";
      }
      return Status::OK();
    }
    return Status::InvalidArgument("Undentified property.");
  }

  virtual void Next() override;
  virtual void Prev() override;
  virtual void Seek(const Slice& target) override;
  virtual void SeekForPrev(const Slice& target) override;
  virtual void SeekToFirst() override;
  virtual void SeekToLast() override;
  Env* env() { return env_; }
  void set_sequence(uint64_t s) { sequence_ = s; }
  void set_valid(bool v) { valid_ = v; }

 private:
  void ReverseToForward();
  void ReverseToBackward();
  void PrevInternal();
  void FindParseableKey(ParsedInternalKey* ikey, Direction direction);
  bool FindValueForCurrentKey();
  bool FindValueForCurrentKeyUsingSeek();
  void FindPrevUserKey();
  void FindNextUserKey();
  inline void FindNextUserEntry(bool skipping, bool prefix_check);
  void FindNextUserEntryInternal(bool skipping, bool prefix_check);
  bool ParseKey(ParsedInternalKey* key);
  void MergeValuesNewToOld();
  bool TooManyInternalKeysSkipped(bool increment = true);

  // Temporarily pin the blocks that we encounter until ReleaseTempPinnedData()
  // is called
  void TempPinData() {
    if (!pin_thru_lifetime_) {
      pinned_iters_mgr_.StartPinning();
    }
  }

  // Release blocks pinned by TempPinData()
  void ReleaseTempPinnedData() {
    if (!pin_thru_lifetime_ && pinned_iters_mgr_.PinningEnabled()) {
      pinned_iters_mgr_.ReleasePinnedData();
    }
  }

  inline void ClearSavedValue() {
    if (saved_value_.capacity() > 1048576) {
      std::string empty;
      swap(empty, saved_value_);
    } else {
      saved_value_.clear();
    }
  }

  inline void ResetInternalKeysSkippedCounter() {
    num_internal_keys_skipped_ = 0;
  }

  const SliceTransform* prefix_extractor_;
  bool arena_mode_;
  Env* const env_;
  Logger* logger_;
  const Comparator* const user_comparator_;
  const MergeOperator* const merge_operator_;
  InternalIterator* iter_;
  SequenceNumber sequence_;

  Status status_;
  IterKey saved_key_;
  // Reusable internal key data structure. This is only used inside one function
  // and should not be used across functions. Reusing this object can reduce
  // overhead of calling construction of the function if creating it each time.
  ParsedInternalKey ikey_;
  std::string saved_value_;
  Slice pinned_value_;
  Direction direction_;
  bool valid_;
  bool current_entry_is_merged_;
  // for prefix seek mode to support prev()
  Statistics* statistics_;
  uint64_t max_skip_;
  uint64_t max_skippable_internal_keys_;
  uint64_t num_internal_keys_skipped_;
  const Slice* iterate_upper_bound_;
  IterKey prefix_start_buf_;
  Slice prefix_start_key_;
  const bool prefix_same_as_start_;
  // Means that we will pin all data blocks we read as long the Iterator
  // is not deleted, will be true if ReadOptions::pin_data is true
  const bool pin_thru_lifetime_;
  const bool total_order_seek_;
  // List of operands for merge operator.
  MergeContext merge_context_;
  RangeDelAggregator range_del_agg_;
  LocalStatistics local_stats_;
  PinnedIteratorsManager pinned_iters_mgr_;
  bool allow_blob_;
  bool is_blob_;

  // No copying allowed
  DBIter(const DBIter&);
  void operator=(const DBIter&);
};

inline bool DBIter::ParseKey(ParsedInternalKey* ikey) {
  if (!ParseInternalKey(iter_->key(), ikey)) {
    status_ = Status::Corruption("corrupted internal key in DBIter");
    ROCKS_LOG_ERROR(logger_, "corrupted internal key in DBIter: %s",
                    iter_->key().ToString(true).c_str());
    return false;
  } else {
    return true;
  }
}

void DBIter::Next() {
  assert(valid_);

  // Release temporarily pinned blocks from last operation
  ReleaseTempPinnedData();
  ResetInternalKeysSkippedCounter();
  if (direction_ == kReverse) {
    ReverseToForward();
  } else if (iter_->Valid() && !current_entry_is_merged_) {
    // If the current value is not a merge, the iter position is the
    // current key, which is already returned. We can safely issue a
    // Next() without checking the current key.
    // If the current key is a merge, very likely iter already points
    // to the next internal position.
    iter_->Next();
    PERF_COUNTER_ADD(internal_key_skipped_count, 1);
  }

  if (statistics_ != nullptr) {
    local_stats_.next_count_++;
  }
  // Now we point to the next internal position, for both of merge and
  // not merge cases.
  if (!iter_->Valid()) {
    valid_ = false;
    return;
  }
  FindNextUserEntry(true /* skipping the current user key */, prefix_same_as_start_);
  if (statistics_ != nullptr && valid_) {
    local_stats_.next_found_count_++;
    local_stats_.bytes_read_ += (key().size() + value().size());
  }
}

// PRE: saved_key_ has the current user key if skipping
// POST: saved_key_ should have the next user key if valid_,
//       if the current entry is a result of merge
//           current_entry_is_merged_ => true
//           saved_value_             => the merged value
//
// NOTE: In between, saved_key_ can point to a user key that has
//       a delete marker or a sequence number higher than sequence_
//       saved_key_ MUST have a proper user_key before calling this function
//
// The prefix_check parameter controls whether we check the iterated
// keys against the prefix of the seeked key. Set to false when
// performing a seek without a key (e.g. SeekToFirst). Set to
// prefix_same_as_start_ for other iterations.
inline void DBIter::FindNextUserEntry(bool skipping, bool prefix_check) {
  PERF_TIMER_GUARD(find_next_user_entry_time);
  FindNextUserEntryInternal(skipping, prefix_check);
}

// Actual implementation of DBIter::FindNextUserEntry()
void DBIter::FindNextUserEntryInternal(bool skipping, bool prefix_check) {
  // Loop until we hit an acceptable entry to yield
  assert(iter_->Valid());
  assert(direction_ == kForward);
  current_entry_is_merged_ = false;

  // How many times in a row we have skipped an entry with user key less than
  // or equal to saved_key_. We could skip these entries either because
  // sequence numbers were too high or because skipping = true.
  // What saved_key_ contains throughout this method:
  //  - if skipping        : saved_key_ contains the key that we need to skip,
  //                         and we haven't seen any keys greater than that,
  //  - if num_skipped > 0 : saved_key_ contains the key that we have skipped
  //                         num_skipped times, and we haven't seen any keys
  //                         greater than that,
  //  - none of the above  : saved_key_ can contain anything, it doesn't matter.
  uint64_t num_skipped = 0;

  is_blob_ = false;

  do {
    if (!ParseKey(&ikey_)) {
      // Skip corrupted keys.
      iter_->Next();
      continue;
    }

    if (iterate_upper_bound_ != nullptr &&
        user_comparator_->Compare(ikey_.user_key, *iterate_upper_bound_) >= 0) {
      break;
    }

    if (prefix_extractor_ && prefix_check &&
        prefix_extractor_->Transform(ikey_.user_key)
                .compare(prefix_start_key_) != 0) {
      break;
    }

    if (TooManyInternalKeysSkipped()) {
      return;
    }

    if (ikey_.sequence <= sequence_) {
      if (skipping && user_comparator_->Compare(ikey_.user_key,
                                                saved_key_.GetUserKey()) <= 0) {
        num_skipped++;  // skip this entry
        PERF_COUNTER_ADD(internal_key_skipped_count, 1);
      } else {
        num_skipped = 0;
        switch (ikey_.type) {
          case kTypeDeletion:
          case kTypeSingleDeletion:
            // Arrange to skip all upcoming entries for this key since
            // they are hidden by this deletion.
            saved_key_.SetUserKey(
                ikey_.user_key,
                !pin_thru_lifetime_ || !iter_->IsKeyPinned() /* copy */);
            skipping = true;
            PERF_COUNTER_ADD(internal_delete_skipped_count, 1);
            break;
          case kTypeValue:
          case kTypeBlobIndex:
            saved_key_.SetUserKey(
                ikey_.user_key,
                !pin_thru_lifetime_ || !iter_->IsKeyPinned() /* copy */);
            if (range_del_agg_.ShouldDelete(
                    ikey_, RangeDelAggregator::RangePositioningMode::
                               kForwardTraversal)) {
              // Arrange to skip all upcoming entries for this key since
              // they are hidden by this deletion.
              skipping = true;
              num_skipped = 0;
              PERF_COUNTER_ADD(internal_delete_skipped_count, 1);
            } else if (ikey_.type == kTypeBlobIndex) {
              if (!allow_blob_) {
                ROCKS_LOG_ERROR(logger_, "Encounter unexpected blob index.");
                status_ = Status::NotSupported(
                    "Encounter unexpected blob index. Please open DB with "
                    "rocksdb::blob_db::BlobDB instead.");
                valid_ = false;
              } else {
                is_blob_ = true;
                valid_ = true;
              }
              return;
            } else {
              valid_ = true;
              return;
            }
            break;
          case kTypeMerge:
            saved_key_.SetUserKey(
                ikey_.user_key,
                !pin_thru_lifetime_ || !iter_->IsKeyPinned() /* copy */);
            if (range_del_agg_.ShouldDelete(
                    ikey_, RangeDelAggregator::RangePositioningMode::
                               kForwardTraversal)) {
              // Arrange to skip all upcoming entries for this key since
              // they are hidden by this deletion.
              skipping = true;
              num_skipped = 0;
              PERF_COUNTER_ADD(internal_delete_skipped_count, 1);
            } else {
              // By now, we are sure the current ikey is going to yield a
              // value
              current_entry_is_merged_ = true;
              valid_ = true;
              MergeValuesNewToOld();  // Go to a different state machine
              return;
            }
            break;
          default:
            assert(false);
            break;
        }
      }
    } else {
      // This key was inserted after our snapshot was taken.
      PERF_COUNTER_ADD(internal_recent_skipped_count, 1);

      // Here saved_key_ may contain some old key, or the default empty key, or
      // key assigned by some random other method. We don't care.
      if (user_comparator_->Compare(ikey_.user_key, saved_key_.GetUserKey()) <=
          0) {
        num_skipped++;
      } else {
        saved_key_.SetUserKey(
            ikey_.user_key,
            !iter_->IsKeyPinned() || !pin_thru_lifetime_ /* copy */);
        skipping = false;
        num_skipped = 0;
      }
    }

    // If we have sequentially iterated via numerous equal keys, then it's
    // better to seek so that we can avoid too many key comparisons.
    if (num_skipped > max_skip_) {
      num_skipped = 0;
      std::string last_key;
      if (skipping) {
        // We're looking for the next user-key but all we see are the same
        // user-key with decreasing sequence numbers. Fast forward to
        // sequence number 0 and type deletion (the smallest type).
        AppendInternalKey(&last_key, ParsedInternalKey(saved_key_.GetUserKey(),
                                                       0, kTypeDeletion));
        // Don't set skipping = false because we may still see more user-keys
        // equal to saved_key_.
      } else {
        // We saw multiple entries with this user key and sequence numbers
        // higher than sequence_. Fast forward to sequence_.
        // Note that this only covers a case when a higher key was overwritten
        // many times since our snapshot was taken, not the case when a lot of
        // different keys were inserted after our snapshot was taken.
        AppendInternalKey(&last_key,
                          ParsedInternalKey(saved_key_.GetUserKey(), sequence_,
                                            kValueTypeForSeek));
      }
      iter_->Seek(last_key);
      RecordTick(statistics_, NUMBER_OF_RESEEKS_IN_ITERATION);
    } else {
      iter_->Next();
    }
  } while (iter_->Valid());
  valid_ = false;
}

// Merge values of the same user key starting from the current iter_ position
// Scan from the newer entries to older entries.
// PRE: iter_->key() points to the first merge type entry
//      saved_key_ stores the user key
// POST: saved_value_ has the merged value for the user key
//       iter_ points to the next entry (or invalid)
void DBIter::MergeValuesNewToOld() {
  if (!merge_operator_) {
    ROCKS_LOG_ERROR(logger_, "Options::merge_operator is null.");
    status_ = Status::InvalidArgument("merge_operator_ must be set.");
    valid_ = false;
    return;
  }

  // Temporarily pin the blocks that hold merge operands
  TempPinData();
  merge_context_.Clear();
  // Start the merge process by pushing the first operand
  merge_context_.PushOperand(iter_->value(),
                             iter_->IsValuePinned() /* operand_pinned */);

  ParsedInternalKey ikey;
  Status s;
  for (iter_->Next(); iter_->Valid(); iter_->Next()) {
    if (!ParseKey(&ikey)) {
      // skip corrupted key
      continue;
    }

    if (!user_comparator_->Equal(ikey.user_key, saved_key_.GetUserKey())) {
      // hit the next user key, stop right here
      break;
    } else if (kTypeDeletion == ikey.type || kTypeSingleDeletion == ikey.type ||
               range_del_agg_.ShouldDelete(
                   ikey, RangeDelAggregator::RangePositioningMode::
                             kForwardTraversal)) {
      // hit a delete with the same user key, stop right here
      // iter_ is positioned after delete
      iter_->Next();
      break;
    } else if (kTypeValue == ikey.type) {
      // hit a put, merge the put value with operands and store the
      // final result in saved_value_. We are done!
      // ignore corruption if there is any.
      const Slice val = iter_->value();
      s = MergeHelper::TimedFullMerge(
          merge_operator_, ikey.user_key, &val, merge_context_.GetOperands(),
          &saved_value_, logger_, statistics_, env_, &pinned_value_, true);
      if (!s.ok()) {
        status_ = s;
      }
      // iter_ is positioned after put
      iter_->Next();
      return;
    } else if (kTypeMerge == ikey.type) {
      // hit a merge, add the value as an operand and run associative merge.
      // when complete, add result to operands and continue.
      merge_context_.PushOperand(iter_->value(),
                                 iter_->IsValuePinned() /* operand_pinned */);
      PERF_COUNTER_ADD(internal_merge_count, 1);
    } else if (kTypeBlobIndex == ikey.type) {
      if (!allow_blob_) {
        ROCKS_LOG_ERROR(logger_, "Encounter unexpected blob index.");
        status_ = Status::NotSupported(
            "Encounter unexpected blob index. Please open DB with "
            "rocksdb::blob_db::BlobDB instead.");
      } else {
        status_ =
            Status::NotSupported("Blob DB does not support merge operator.");
      }
      valid_ = false;
      return;
    } else {
      assert(false);
    }
  }

  // we either exhausted all internal keys under this user key, or hit
  // a deletion marker.
  // feed null as the existing value to the merge operator, such that
  // client can differentiate this scenario and do things accordingly.
  s = MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetUserKey(),
                                  nullptr, merge_context_.GetOperands(),
                                  &saved_value_, logger_, statistics_, env_,
                                  &pinned_value_, true);
  if (!s.ok()) {
    status_ = s;
  }
}

void DBIter::Prev() {
  assert(valid_);
  ReleaseTempPinnedData();
  ResetInternalKeysSkippedCounter();
  if (direction_ == kForward) {
    ReverseToBackward();
  }
  PrevInternal();
  if (statistics_ != nullptr) {
    local_stats_.prev_count_++;
    if (valid_) {
      local_stats_.prev_found_count_++;
      local_stats_.bytes_read_ += (key().size() + value().size());
    }
  }
}

void DBIter::ReverseToForward() {
  if (prefix_extractor_ != nullptr && !total_order_seek_) {
    IterKey last_key;
    last_key.SetInternalKey(ParsedInternalKey(
        saved_key_.GetUserKey(), kMaxSequenceNumber, kValueTypeForSeek));
    iter_->Seek(last_key.GetInternalKey());
  }
  FindNextUserKey();
  direction_ = kForward;
  if (!iter_->Valid()) {
    iter_->SeekToFirst();
    range_del_agg_.InvalidateTombstoneMapPositions();
  }
}

void DBIter::ReverseToBackward() {
  if (prefix_extractor_ != nullptr && !total_order_seek_) {
    IterKey last_key;
    last_key.SetInternalKey(ParsedInternalKey(saved_key_.GetUserKey(), 0,
                                              kValueTypeForSeekForPrev));
    iter_->SeekForPrev(last_key.GetInternalKey());
  }
  if (current_entry_is_merged_) {
    // Not placed in the same key. Need to call Prev() until finding the
    // previous key.
    if (!iter_->Valid()) {
      iter_->SeekToLast();
      range_del_agg_.InvalidateTombstoneMapPositions();
    }
    ParsedInternalKey ikey;
    FindParseableKey(&ikey, kReverse);
    while (iter_->Valid() &&
           user_comparator_->Compare(ikey.user_key, saved_key_.GetUserKey()) >
               0) {
      assert(ikey.sequence != kMaxSequenceNumber);
      if (ikey.sequence > sequence_) {
        PERF_COUNTER_ADD(internal_recent_skipped_count, 1);
      } else {
        PERF_COUNTER_ADD(internal_key_skipped_count, 1);
      }
      iter_->Prev();
      FindParseableKey(&ikey, kReverse);
    }
  }
#ifndef NDEBUG
  if (iter_->Valid()) {
    ParsedInternalKey ikey;
    assert(ParseKey(&ikey));
    assert(user_comparator_->Compare(ikey.user_key, saved_key_.GetUserKey()) <=
           0);
  }
#endif

  FindPrevUserKey();
  direction_ = kReverse;
}

void DBIter::PrevInternal() {
  if (!iter_->Valid()) {
    valid_ = false;
    return;
  }

  ParsedInternalKey ikey;

  while (iter_->Valid()) {
    saved_key_.SetUserKey(
        ExtractUserKey(iter_->key()),
        !iter_->IsKeyPinned() || !pin_thru_lifetime_ /* copy */);

    if (FindValueForCurrentKey()) {
      if (!iter_->Valid()) {
        return;
      }
      FindParseableKey(&ikey, kReverse);
      if (user_comparator_->Equal(ikey.user_key, saved_key_.GetUserKey())) {
        FindPrevUserKey();
      }
      if (valid_ && prefix_extractor_ && prefix_same_as_start_ &&
          prefix_extractor_->Transform(saved_key_.GetUserKey())
                  .compare(prefix_start_key_) != 0) {
        valid_ = false;
      }
      return;
    }

    if (TooManyInternalKeysSkipped(false)) {
      return;
    }

    if (!iter_->Valid()) {
      break;
    }
    FindParseableKey(&ikey, kReverse);
    if (user_comparator_->Equal(ikey.user_key, saved_key_.GetUserKey())) {
      FindPrevUserKey();
    }
  }
  // We haven't found any key - iterator is not valid
  // Or the prefix is different than start prefix
  assert(!iter_->Valid());
  valid_ = false;
}

// This function checks, if the entry with biggest sequence_number <= sequence_
// is non kTypeDeletion or kTypeSingleDeletion. If it's not, we save value in
// saved_value_
bool DBIter::FindValueForCurrentKey() {
  assert(iter_->Valid());
  merge_context_.Clear();
  current_entry_is_merged_ = false;
  // last entry before merge (could be kTypeDeletion, kTypeSingleDeletion or
  // kTypeValue)
  ValueType last_not_merge_type = kTypeDeletion;
  ValueType last_key_entry_type = kTypeDeletion;

  ParsedInternalKey ikey;
  FindParseableKey(&ikey, kReverse);

  // Temporarily pin blocks that hold (merge operands / the value)
  ReleaseTempPinnedData();
  TempPinData();
  size_t num_skipped = 0;
  while (iter_->Valid() && ikey.sequence <= sequence_ &&
         user_comparator_->Equal(ikey.user_key, saved_key_.GetUserKey())) {
    if (TooManyInternalKeysSkipped()) {
      return false;
    }

    // We iterate too much: let's use Seek() to avoid too much key comparisons
    if (num_skipped >= max_skip_) {
      return FindValueForCurrentKeyUsingSeek();
    }

    last_key_entry_type = ikey.type;
    switch (last_key_entry_type) {
      case kTypeValue:
      case kTypeBlobIndex:
        if (range_del_agg_.ShouldDelete(
                ikey,
                RangeDelAggregator::RangePositioningMode::kBackwardTraversal)) {
          last_key_entry_type = kTypeRangeDeletion;
          PERF_COUNTER_ADD(internal_delete_skipped_count, 1);
        } else {
          assert(iter_->IsValuePinned());
          pinned_value_ = iter_->value();
        }
        merge_context_.Clear();
        last_not_merge_type = last_key_entry_type;
        break;
      case kTypeDeletion:
      case kTypeSingleDeletion:
        merge_context_.Clear();
        last_not_merge_type = last_key_entry_type;
        PERF_COUNTER_ADD(internal_delete_skipped_count, 1);
        break;
      case kTypeMerge:
        if (range_del_agg_.ShouldDelete(
                ikey,
                RangeDelAggregator::RangePositioningMode::kBackwardTraversal)) {
          merge_context_.Clear();
          last_key_entry_type = kTypeRangeDeletion;
          last_not_merge_type = last_key_entry_type;
          PERF_COUNTER_ADD(internal_delete_skipped_count, 1);
        } else {
          assert(merge_operator_ != nullptr);
          merge_context_.PushOperandBack(
              iter_->value(), iter_->IsValuePinned() /* operand_pinned */);
          PERF_COUNTER_ADD(internal_merge_count, 1);
        }
        break;
      default:
        assert(false);
    }

    PERF_COUNTER_ADD(internal_key_skipped_count, 1);
    assert(user_comparator_->Equal(ikey.user_key, saved_key_.GetUserKey()));
    iter_->Prev();
    ++num_skipped;
    FindParseableKey(&ikey, kReverse);
  }

  Status s;
  is_blob_ = false;
  switch (last_key_entry_type) {
    case kTypeDeletion:
    case kTypeSingleDeletion:
    case kTypeRangeDeletion:
      valid_ = false;
      return false;
    case kTypeMerge:
      current_entry_is_merged_ = true;
      if (last_not_merge_type == kTypeDeletion ||
          last_not_merge_type == kTypeSingleDeletion ||
          last_not_merge_type == kTypeRangeDeletion) {
        s = MergeHelper::TimedFullMerge(
            merge_operator_, saved_key_.GetUserKey(), nullptr,
            merge_context_.GetOperands(), &saved_value_, logger_, statistics_,
            env_, &pinned_value_, true);
      } else if (last_not_merge_type == kTypeBlobIndex) {
        if (!allow_blob_) {
          ROCKS_LOG_ERROR(logger_, "Encounter unexpected blob index.");
          status_ = Status::NotSupported(
              "Encounter unexpected blob index. Please open DB with "
              "rocksdb::blob_db::BlobDB instead.");
        } else {
          status_ =
              Status::NotSupported("Blob DB does not support merge operator.");
        }
        valid_ = false;
        return true;
      } else {
        assert(last_not_merge_type == kTypeValue);
        s = MergeHelper::TimedFullMerge(
            merge_operator_, saved_key_.GetUserKey(), &pinned_value_,
            merge_context_.GetOperands(), &saved_value_, logger_, statistics_,
            env_, &pinned_value_, true);
      }
      break;
    case kTypeValue:
      // do nothing - we've already has value in saved_value_
      break;
    case kTypeBlobIndex:
      if (!allow_blob_) {
        ROCKS_LOG_ERROR(logger_, "Encounter unexpected blob index.");
        status_ = Status::NotSupported(
            "Encounter unexpected blob index. Please open DB with "
            "rocksdb::blob_db::BlobDB instead.");
        valid_ = false;
        return true;
      }
      is_blob_ = true;
      break;
    default:
      assert(false);
      break;
  }
  valid_ = true;
  if (!s.ok()) {
    status_ = s;
  }
  return true;
}

// This function is used in FindValueForCurrentKey.
// We use Seek() function instead of Prev() to find necessary value
bool DBIter::FindValueForCurrentKeyUsingSeek() {
  // FindValueForCurrentKey will enable pinning before calling
  // FindValueForCurrentKeyUsingSeek()
  assert(pinned_iters_mgr_.PinningEnabled());
  std::string last_key;
  AppendInternalKey(&last_key, ParsedInternalKey(saved_key_.GetUserKey(),
                                                 sequence_, kValueTypeForSeek));
  iter_->Seek(last_key);
  RecordTick(statistics_, NUMBER_OF_RESEEKS_IN_ITERATION);

  // assume there is at least one parseable key for this user key
  ParsedInternalKey ikey;
  FindParseableKey(&ikey, kForward);

  if (ikey.type == kTypeDeletion || ikey.type == kTypeSingleDeletion ||
      range_del_agg_.ShouldDelete(
          ikey, RangeDelAggregator::RangePositioningMode::kBackwardTraversal)) {
    valid_ = false;
    return false;
  }
  if (ikey.type == kTypeBlobIndex && !allow_blob_) {
    ROCKS_LOG_ERROR(logger_, "Encounter unexpected blob index.");
    status_ = Status::NotSupported(
        "Encounter unexpected blob index. Please open DB with "
        "rocksdb::blob_db::BlobDB instead.");
    valid_ = false;
    return true;
  }
  if (ikey.type == kTypeValue || ikey.type == kTypeBlobIndex) {
    assert(iter_->IsValuePinned());
    pinned_value_ = iter_->value();
    valid_ = true;
    return true;
  }

  // kTypeMerge. We need to collect all kTypeMerge values and save them
  // in operands
  current_entry_is_merged_ = true;
  merge_context_.Clear();
  while (
      iter_->Valid() &&
      user_comparator_->Equal(ikey.user_key, saved_key_.GetUserKey()) &&
      ikey.type == kTypeMerge &&
      !range_del_agg_.ShouldDelete(
          ikey, RangeDelAggregator::RangePositioningMode::kBackwardTraversal)) {
    merge_context_.PushOperand(iter_->value(),
                               iter_->IsValuePinned() /* operand_pinned */);
    PERF_COUNTER_ADD(internal_merge_count, 1);
    iter_->Next();
    FindParseableKey(&ikey, kForward);
  }

  Status s;
  if (!iter_->Valid() ||
      !user_comparator_->Equal(ikey.user_key, saved_key_.GetUserKey()) ||
      ikey.type == kTypeDeletion || ikey.type == kTypeSingleDeletion ||
      range_del_agg_.ShouldDelete(
          ikey, RangeDelAggregator::RangePositioningMode::kBackwardTraversal)) {
    s = MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetUserKey(),
                                    nullptr, merge_context_.GetOperands(),
                                    &saved_value_, logger_, statistics_, env_,
                                    &pinned_value_, true);
    // Make iter_ valid and point to saved_key_
    if (!iter_->Valid() ||
        !user_comparator_->Equal(ikey.user_key, saved_key_.GetUserKey())) {
      iter_->Seek(last_key);
      RecordTick(statistics_, NUMBER_OF_RESEEKS_IN_ITERATION);
    }
    valid_ = true;
    if (!s.ok()) {
      status_ = s;
    }
    return true;
  }

  const Slice& val = iter_->value();
  s = MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetUserKey(),
                                  &val, merge_context_.GetOperands(),
                                  &saved_value_, logger_, statistics_, env_,
                                  &pinned_value_, true);
  valid_ = true;
  if (!s.ok()) {
    status_ = s;
  }
  return true;
}

// Used in Next to change directions
// Go to next user key
// Don't use Seek(),
// because next user key will be very close
void DBIter::FindNextUserKey() {
  if (!iter_->Valid()) {
    return;
  }
  ParsedInternalKey ikey;
  FindParseableKey(&ikey, kForward);
  while (iter_->Valid() &&
         !user_comparator_->Equal(ikey.user_key, saved_key_.GetUserKey())) {
    iter_->Next();
    FindParseableKey(&ikey, kForward);
  }
}

// Go to previous user_key
void DBIter::FindPrevUserKey() {
  if (!iter_->Valid()) {
    return;
  }
  size_t num_skipped = 0;
  ParsedInternalKey ikey;
  FindParseableKey(&ikey, kReverse);
  int cmp;
  while (iter_->Valid() &&
         ((cmp = user_comparator_->Compare(ikey.user_key,
                                           saved_key_.GetUserKey())) == 0 ||
          (cmp > 0 && ikey.sequence > sequence_))) {
    if (TooManyInternalKeysSkipped()) {
      return;
    }

    if (cmp == 0) {
      if (num_skipped >= max_skip_) {
        num_skipped = 0;
        IterKey last_key;
        last_key.SetInternalKey(ParsedInternalKey(
            saved_key_.GetUserKey(), kMaxSequenceNumber, kValueTypeForSeek));
        iter_->Seek(last_key.GetInternalKey());
        RecordTick(statistics_, NUMBER_OF_RESEEKS_IN_ITERATION);
      } else {
        ++num_skipped;
      }
    }
    assert(ikey.sequence != kMaxSequenceNumber);
    if (ikey.sequence > sequence_) {
      PERF_COUNTER_ADD(internal_recent_skipped_count, 1);
    } else {
      PERF_COUNTER_ADD(internal_key_skipped_count, 1);
    }
    iter_->Prev();
    FindParseableKey(&ikey, kReverse);
  }
}

bool DBIter::TooManyInternalKeysSkipped(bool increment) {
  if ((max_skippable_internal_keys_ > 0) &&
      (num_internal_keys_skipped_ > max_skippable_internal_keys_)) {
    valid_ = false;
    status_ = Status::Incomplete("Too many internal keys skipped.");
    return true;
  } else if (increment) {
    num_internal_keys_skipped_++;
  }
  return false;
}

// Skip all unparseable keys
void DBIter::FindParseableKey(ParsedInternalKey* ikey, Direction direction) {
  while (iter_->Valid() && !ParseKey(ikey)) {
    if (direction == kReverse) {
      iter_->Prev();
    } else {
      iter_->Next();
    }
  }
}

void DBIter::Seek(const Slice& target) {
  StopWatch sw(env_, statistics_, DB_SEEK);
  ReleaseTempPinnedData();
  ResetInternalKeysSkippedCounter();
  saved_key_.Clear();
  saved_key_.SetInternalKey(target, sequence_);

  {
    PERF_TIMER_GUARD(seek_internal_seek_time);
    iter_->Seek(saved_key_.GetInternalKey());
    range_del_agg_.InvalidateTombstoneMapPositions();
  }
  RecordTick(statistics_, NUMBER_DB_SEEK);
  if (iter_->Valid()) {
    if (prefix_extractor_ && prefix_same_as_start_) {
      prefix_start_key_ = prefix_extractor_->Transform(target);
    }
    direction_ = kForward;
    ClearSavedValue();
    FindNextUserEntry(false /* not skipping */, prefix_same_as_start_);
    if (!valid_) {
      prefix_start_key_.clear();
    }
    if (statistics_ != nullptr) {
      if (valid_) {
        RecordTick(statistics_, NUMBER_DB_SEEK_FOUND);
        RecordTick(statistics_, ITER_BYTES_READ, key().size() + value().size());
        PERF_COUNTER_ADD(iter_read_bytes, key().size() + value().size());
      }
    }
  } else {
    valid_ = false;
  }

  if (valid_ && prefix_extractor_ && prefix_same_as_start_) {
    prefix_start_buf_.SetUserKey(prefix_start_key_);
    prefix_start_key_ = prefix_start_buf_.GetUserKey();
  }
}

void DBIter::SeekForPrev(const Slice& target) {
  StopWatch sw(env_, statistics_, DB_SEEK);
  ReleaseTempPinnedData();
  ResetInternalKeysSkippedCounter();
  saved_key_.Clear();
  // now saved_key is used to store internal key.
  saved_key_.SetInternalKey(target, 0 /* sequence_number */,
                            kValueTypeForSeekForPrev);

  {
    PERF_TIMER_GUARD(seek_internal_seek_time);
    iter_->SeekForPrev(saved_key_.GetInternalKey());
    range_del_agg_.InvalidateTombstoneMapPositions();
  }

  RecordTick(statistics_, NUMBER_DB_SEEK);
  if (iter_->Valid()) {
    if (prefix_extractor_ && prefix_same_as_start_) {
      prefix_start_key_ = prefix_extractor_->Transform(target);
    }
    direction_ = kReverse;
    ClearSavedValue();
    PrevInternal();
    if (!valid_) {
      prefix_start_key_.clear();
    }
    if (statistics_ != nullptr) {
      if (valid_) {
        RecordTick(statistics_, NUMBER_DB_SEEK_FOUND);
        RecordTick(statistics_, ITER_BYTES_READ, key().size() + value().size());
        PERF_COUNTER_ADD(iter_read_bytes, key().size() + value().size());
      }
    }
  } else {
    valid_ = false;
  }
  if (valid_ && prefix_extractor_ && prefix_same_as_start_) {
    prefix_start_buf_.SetUserKey(prefix_start_key_);
    prefix_start_key_ = prefix_start_buf_.GetUserKey();
  }
}

void DBIter::SeekToFirst() {
  // Don't use iter_::Seek() if we set a prefix extractor
  // because prefix seek will be used.
  if (prefix_extractor_ != nullptr) {
    max_skip_ = std::numeric_limits<uint64_t>::max();
  }
  direction_ = kForward;
  ReleaseTempPinnedData();
  ResetInternalKeysSkippedCounter();
  ClearSavedValue();

  {
    PERF_TIMER_GUARD(seek_internal_seek_time);
    iter_->SeekToFirst();
    range_del_agg_.InvalidateTombstoneMapPositions();
  }

  RecordTick(statistics_, NUMBER_DB_SEEK);
  if (iter_->Valid()) {
    saved_key_.SetUserKey(
        ExtractUserKey(iter_->key()),
        !iter_->IsKeyPinned() || !pin_thru_lifetime_ /* copy */);
    FindNextUserEntry(false /* not skipping */, false /* no prefix check */);
    if (statistics_ != nullptr) {
      if (valid_) {
        RecordTick(statistics_, NUMBER_DB_SEEK_FOUND);
        RecordTick(statistics_, ITER_BYTES_READ, key().size() + value().size());
        PERF_COUNTER_ADD(iter_read_bytes, key().size() + value().size());
      }
    }
  } else {
    valid_ = false;
  }
  if (valid_ && prefix_extractor_ && prefix_same_as_start_) {
    prefix_start_buf_.SetUserKey(
        prefix_extractor_->Transform(saved_key_.GetUserKey()));
    prefix_start_key_ = prefix_start_buf_.GetUserKey();
  }
}

void DBIter::SeekToLast() {
  // Don't use iter_::Seek() if we set a prefix extractor
  // because prefix seek will be used.
  if (prefix_extractor_ != nullptr) {
    max_skip_ = std::numeric_limits<uint64_t>::max();
  }
  direction_ = kReverse;
  ReleaseTempPinnedData();
  ResetInternalKeysSkippedCounter();
  ClearSavedValue();

  {
    PERF_TIMER_GUARD(seek_internal_seek_time);
    iter_->SeekToLast();
    range_del_agg_.InvalidateTombstoneMapPositions();
  }
  // When the iterate_upper_bound is set to a value,
  // it will seek to the last key before the
  // ReadOptions.iterate_upper_bound
  if (iter_->Valid() && iterate_upper_bound_ != nullptr) {
    SeekForPrev(*iterate_upper_bound_);
    range_del_agg_.InvalidateTombstoneMapPositions();
    if (!Valid()) {
      return;
    } else if (user_comparator_->Equal(*iterate_upper_bound_, key())) {
      Prev();
    }
  } else {
    PrevInternal();
  }
  if (statistics_ != nullptr) {
    RecordTick(statistics_, NUMBER_DB_SEEK);
    if (valid_) {
      RecordTick(statistics_, NUMBER_DB_SEEK_FOUND);
      RecordTick(statistics_, ITER_BYTES_READ, key().size() + value().size());
      PERF_COUNTER_ADD(iter_read_bytes, key().size() + value().size());
    }
  }
  if (valid_ && prefix_extractor_ && prefix_same_as_start_) {
    prefix_start_buf_.SetUserKey(
        prefix_extractor_->Transform(saved_key_.GetUserKey()));
    prefix_start_key_ = prefix_start_buf_.GetUserKey();
  }
}

Iterator* NewDBIterator(Env* env, const ReadOptions& read_options,
                        const ImmutableCFOptions& cf_options,
                        const Comparator* user_key_comparator,
                        InternalIterator* internal_iter,
                        const SequenceNumber& sequence,
                        uint64_t max_sequential_skip_in_iterations,
                        bool allow_blob) {
  DBIter* db_iter = new DBIter(
      env, read_options, cf_options, user_key_comparator, internal_iter,
      sequence, false, max_sequential_skip_in_iterations, allow_blob);
  return db_iter;
}

ArenaWrappedDBIter::~ArenaWrappedDBIter() { db_iter_->~DBIter(); }

RangeDelAggregator* ArenaWrappedDBIter::GetRangeDelAggregator() {
  return db_iter_->GetRangeDelAggregator();
}

void ArenaWrappedDBIter::SetIterUnderDBIter(InternalIterator* iter) {
  static_cast<DBIter*>(db_iter_)->SetIter(iter);
}

inline bool ArenaWrappedDBIter::Valid() const { return db_iter_->Valid(); }
inline void ArenaWrappedDBIter::SeekToFirst() { db_iter_->SeekToFirst(); }
inline void ArenaWrappedDBIter::SeekToLast() { db_iter_->SeekToLast(); }
inline void ArenaWrappedDBIter::Seek(const Slice& target) {
  db_iter_->Seek(target);
}
inline void ArenaWrappedDBIter::SeekForPrev(const Slice& target) {
  db_iter_->SeekForPrev(target);
}
inline void ArenaWrappedDBIter::Next() { db_iter_->Next(); }
inline void ArenaWrappedDBIter::Prev() { db_iter_->Prev(); }
inline Slice ArenaWrappedDBIter::key() const { return db_iter_->key(); }
inline Slice ArenaWrappedDBIter::value() const { return db_iter_->value(); }
inline Status ArenaWrappedDBIter::status() const { return db_iter_->status(); }
bool ArenaWrappedDBIter::IsBlob() const { return db_iter_->IsBlob(); }
inline Status ArenaWrappedDBIter::GetProperty(std::string prop_name,
                                              std::string* prop) {
  if (prop_name == "rocksdb.iterator.super-version-number") {
    // First try to pass the value returned from inner iterator.
    if (!db_iter_->GetProperty(prop_name, prop).ok()) {
      *prop = ToString(sv_number_);
    }
    return Status::OK();
  }
  return db_iter_->GetProperty(prop_name, prop);
}

void ArenaWrappedDBIter::Init(Env* env, const ReadOptions& read_options,
                              const ImmutableCFOptions& cf_options,
                              const SequenceNumber& sequence,
                              uint64_t max_sequential_skip_in_iteration,
                              uint64_t version_number, bool allow_blob) {
  auto mem = arena_.AllocateAligned(sizeof(DBIter));
  db_iter_ = new (mem)
      DBIter(env, read_options, cf_options, cf_options.user_comparator, nullptr,
             sequence, true, max_sequential_skip_in_iteration, allow_blob);
  sv_number_ = version_number;
}

Status ArenaWrappedDBIter::Refresh() {
  if (cfd_ == nullptr || db_impl_ == nullptr) {
    return Status::NotSupported("Creating renew iterator is not allowed.");
  }
  assert(db_iter_ != nullptr);
  SequenceNumber latest_seq = db_impl_->GetLatestSequenceNumber();
  uint64_t cur_sv_number = cfd_->GetSuperVersionNumber();
  if (sv_number_ != cur_sv_number) {
    Env* env = db_iter_->env();
    db_iter_->~DBIter();
    arena_.~Arena();
    new (&arena_) Arena();

    SuperVersion* sv = cfd_->GetReferencedSuperVersion(db_impl_->mutex());
    Init(env, read_options_, *(cfd_->ioptions()), latest_seq,
         sv->mutable_cf_options.max_sequential_skip_in_iterations,
         cur_sv_number, allow_blob_);

    InternalIterator* internal_iter = db_impl_->NewInternalIterator(
        read_options_, cfd_, sv, &arena_, db_iter_->GetRangeDelAggregator());
    SetIterUnderDBIter(internal_iter);
  } else {
    db_iter_->set_sequence(latest_seq);
    db_iter_->set_valid(false);
  }
  return Status::OK();
}

ArenaWrappedDBIter* NewArenaWrappedDbIterator(
    Env* env, const ReadOptions& read_options,
    const ImmutableCFOptions& cf_options, const SequenceNumber& sequence,
    uint64_t max_sequential_skip_in_iterations, uint64_t version_number,
    DBImpl* db_impl, ColumnFamilyData* cfd, bool allow_blob) {
  ArenaWrappedDBIter* iter = new ArenaWrappedDBIter();
  iter->Init(env, read_options, cf_options, sequence,
             max_sequential_skip_in_iterations, version_number, allow_blob);
  if (db_impl != nullptr && cfd != nullptr) {
    iter->StoreRefreshInfo(read_options, db_impl, cfd, allow_blob);
  }

  return iter;
}

}  // namespace rocksdb