db_iter.cc

//  Copyright (c) 2011-present, 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 "db/db_iter.h"
#include <stdexcept>
#include <deque>
#include <string>
#include <limits>

#include "db/dbformat.h"
#include "db/filename.h"
#include "db/merge_context.h"
#include "db/merge_helper.h"
#include "db/pinned_iterators_manager.h"
#include "port/port.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/logging.h"
#include "util/mutexlock.h"
#include "util/perf_context_imp.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: 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_);
      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 ImmutableCFOptions& ioptions, const Comparator* cmp,
         InternalIterator* iter, SequenceNumber s, bool arena_mode,
         uint64_t max_sequential_skip_in_iterations, uint64_t version_number,
         const Slice* iterate_upper_bound = nullptr,
         bool prefix_same_as_start = false, bool pin_data = false)
      : arena_mode_(arena_mode),
        env_(env),
        logger_(ioptions.info_log),
        user_comparator_(cmp),
        merge_operator_(ioptions.merge_operator),
        iter_(iter),
        sequence_(s),
        direction_(kForward),
        valid_(false),
        current_entry_is_merged_(false),
        statistics_(ioptions.statistics),
        version_number_(version_number),
        iterate_upper_bound_(iterate_upper_bound),
        prefix_same_as_start_(prefix_same_as_start),
        pin_thru_lifetime_(pin_data) {
    RecordTick(statistics_, NO_ITERATORS);
    prefix_extractor_ = ioptions.prefix_extractor;
    max_skip_ = max_sequential_skip_in_iterations;
    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 bool Valid() const override { return valid_; }
  virtual Slice key() const override {
    assert(valid_);
    return saved_key_.GetKey();
  }
  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_;
    }
  }

  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.
      if (!iter_->GetProperty(prop_name, prop).ok()) {
        *prop = ToString(version_number_);
      }
      return Status::OK();
    } 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;

 private:
  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();

  // 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();
    }
  }

  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 const sequence_;

  Status status_;
  IterKey saved_key_;
  std::string saved_value_;
  Slice pinned_value_;
  Direction direction_;
  bool valid_;
  bool current_entry_is_merged_;
  Statistics* statistics_;
  uint64_t max_skip_;
  uint64_t version_number_;
  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_;
  // List of operands for merge operator.
  MergeContext merge_context_;
  LocalStatistics local_stats_;
  PinnedIteratorsManager pinned_iters_mgr_;

  // 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");
    Log(InfoLogLevel::ERROR_LEVEL,
        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();
  if (direction_ == kReverse) {
    FindNextUserKey();
    direction_ = kForward;
    if (!iter_->Valid()) {
      iter_->SeekToFirst();
    }
  } 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
//
// 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;
  uint64_t num_skipped = 0;
  do {
    ParsedInternalKey ikey;

    if (ParseKey(&ikey)) {
      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 (ikey.sequence <= sequence_) {
        if (skipping &&
           user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) <= 0) {
          num_skipped++;  // skip this entry
          PERF_COUNTER_ADD(internal_key_skipped_count, 1);
        } else {
          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_.SetKey(
                  ikey.user_key,
                  !iter_->IsKeyPinned() || !pin_thru_lifetime_ /* copy */);
              skipping = true;
              num_skipped = 0;
              PERF_COUNTER_ADD(internal_delete_skipped_count, 1);
              break;
            case kTypeValue:
              valid_ = true;
              saved_key_.SetKey(
                  ikey.user_key,
                  !iter_->IsKeyPinned() || !pin_thru_lifetime_ /* copy */);
              return;
            case kTypeMerge:
              // By now, we are sure the current ikey is going to yield a value
              saved_key_.SetKey(
                  ikey.user_key,
                  !iter_->IsKeyPinned() || !pin_thru_lifetime_ /* copy */);
              current_entry_is_merged_ = true;
              valid_ = true;
              MergeValuesNewToOld();  // Go to a different state machine
              return;
            default:
              assert(false);
              break;
          }
        }
      }
    }
    // If we have sequentially iterated via numerous keys and still not
    // found the next user-key, then it is better to seek so that we can
    // avoid too many key comparisons. We seek to the last occurrence of
    // our current key by looking for sequence number 0 and type deletion
    // (the smallest type).
    if (skipping && num_skipped > max_skip_) {
      num_skipped = 0;
      std::string last_key;
      AppendInternalKey(&last_key, ParsedInternalKey(saved_key_.GetKey(), 0,
                                                     kTypeDeletion));
      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_) {
    Log(InfoLogLevel::ERROR_LEVEL,
        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;
  for (iter_->Next(); iter_->Valid(); iter_->Next()) {
    if (!ParseKey(&ikey)) {
      // skip corrupted key
      continue;
    }

    if (!user_comparator_->Equal(ikey.user_key, saved_key_.GetKey())) {
      // hit the next user key, stop right here
      break;
    } else if (kTypeDeletion == ikey.type || kTypeSingleDeletion == ikey.type) {
      // 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();
      MergeHelper::TimedFullMerge(merge_operator_, ikey.user_key, &val,
                                  merge_context_.GetOperands(), &saved_value_,
                                  logger_, statistics_, env_, &pinned_value_);
      // 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 */);
    } 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.
  MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetKey(), nullptr,
                              merge_context_.GetOperands(), &saved_value_,
                              logger_, statistics_, env_, &pinned_value_);
}

void DBIter::Prev() {
  assert(valid_);
  ReleaseTempPinnedData();
  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::ReverseToBackward() {
  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();
    }
    ParsedInternalKey ikey;
    FindParseableKey(&ikey, kReverse);
    while (iter_->Valid() &&
           user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) > 0) {
      iter_->Prev();
      FindParseableKey(&ikey, kReverse);
    }
  }
#ifndef NDEBUG
  if (iter_->Valid()) {
    ParsedInternalKey ikey;
    assert(ParseKey(&ikey));
    assert(user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) <= 0);
  }
#endif

  FindPrevUserKey();
  direction_ = kReverse;
}

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

  ParsedInternalKey ikey;
  bool match_prefix = true;

  while (iter_->Valid()) {
    saved_key_.SetKey(ExtractUserKey(iter_->key()),
                      !iter_->IsKeyPinned() || !pin_thru_lifetime_ /* copy */);
    if (FindValueForCurrentKey()) {
      valid_ = true;
      if (!iter_->Valid()) {
        return;
      }
      FindParseableKey(&ikey, kReverse);
      if (user_comparator_->Equal(ikey.user_key, saved_key_.GetKey())) {
        FindPrevUserKey();
      }
      if (valid_ && prefix_extractor_ && prefix_same_as_start_ &&
          prefix_extractor_->Transform(saved_key_.GetKey())
                  .compare(prefix_start_key_) != 0) {
        match_prefix = false;
        break;
      }
      return;
    }
    if (!iter_->Valid()) {
      break;
    }
    FindParseableKey(&ikey, kReverse);
    if (user_comparator_->Equal(ikey.user_key, saved_key_.GetKey())) {
      FindPrevUserKey();
    }
  }
  // We haven't found any key - iterator is not valid
  // Or the prefix is different than start prefix
  assert(!iter_->Valid() || !match_prefix);
  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_.GetKey())) {
    // 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:
        merge_context_.Clear();
        assert(iter_->IsValuePinned());
        pinned_value_ = iter_->value();
        last_not_merge_type = kTypeValue;
        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:
        assert(merge_operator_ != nullptr);
        merge_context_.PushOperandBack(
            iter_->value(), iter_->IsValuePinned() /* operand_pinned */);
        break;
      default:
        assert(false);
    }

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

  switch (last_key_entry_type) {
    case kTypeDeletion:
    case kTypeSingleDeletion:
      valid_ = false;
      return false;
    case kTypeMerge:
      current_entry_is_merged_ = true;
      if (last_not_merge_type == kTypeDeletion) {
        MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetKey(),
                                    nullptr, merge_context_.GetOperands(),
                                    &saved_value_, logger_, statistics_, env_,
                                    &pinned_value_);
      } else {
        assert(last_not_merge_type == kTypeValue);
        MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetKey(),
                                    &pinned_value_,
                                    merge_context_.GetOperands(), &saved_value_,
                                    logger_, statistics_, env_, &pinned_value_);
      }
      break;
    case kTypeValue:
      // do nothing - we've already has value in saved_value_
      break;
    default:
      assert(false);
      break;
  }
  valid_ = true;
  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_.GetKey(), 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 == kTypeValue || ikey.type == kTypeDeletion ||
      ikey.type == kTypeSingleDeletion) {
    if (ikey.type == kTypeValue) {
      assert(iter_->IsValuePinned());
      pinned_value_ = iter_->value();
      valid_ = true;
      return true;
    }
    valid_ = false;
    return false;
  }

  // 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_.GetKey()) &&
         ikey.type == kTypeMerge) {
    merge_context_.PushOperand(iter_->value(),
                               iter_->IsValuePinned() /* operand_pinned */);
    iter_->Next();
    FindParseableKey(&ikey, kForward);
  }

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

  const Slice& val = iter_->value();
  MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetKey(), &val,
                              merge_context_.GetOperands(), &saved_value_,
                              logger_, statistics_, env_, &pinned_value_);
  valid_ = true;
  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_.GetKey())) {
    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_.GetKey())) == 0 ||
                            (cmp > 0 && ikey.sequence > sequence_))) {
    if (cmp == 0) {
      if (num_skipped >= max_skip_) {
        num_skipped = 0;
        IterKey last_key;
        last_key.SetInternalKey(ParsedInternalKey(
            saved_key_.GetKey(), kMaxSequenceNumber, kValueTypeForSeek));
        iter_->Seek(last_key.GetKey());
        RecordTick(statistics_, NUMBER_OF_RESEEKS_IN_ITERATION);
      } else {
        ++num_skipped;
      }
    }
    iter_->Prev();
    FindParseableKey(&ikey, kReverse);
  }
}

// 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();
  saved_key_.Clear();
  // now savved_key is used to store internal key.
  saved_key_.SetInternalKey(target, sequence_);

  {
    PERF_TIMER_GUARD(seek_internal_seek_time);
    iter_->Seek(saved_key_.GetKey());
  }

  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());
      }
    }
  } else {
    valid_ = false;
  }
  if (valid_ && prefix_extractor_ && prefix_same_as_start_) {
    prefix_start_buf_.SetKey(prefix_start_key_);
    prefix_start_key_ = prefix_start_buf_.GetKey();
  }
}

void DBIter::SeekForPrev(const Slice& target) {
  StopWatch sw(env_, statistics_, DB_SEEK);
  ReleaseTempPinnedData();
  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_.GetKey());
  }

  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());
      }
    }
  } else {
    valid_ = false;
  }
  if (valid_ && prefix_extractor_ && prefix_same_as_start_) {
    prefix_start_buf_.SetKey(prefix_start_key_);
    prefix_start_key_ = prefix_start_buf_.GetKey();
  }
}

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();
  ClearSavedValue();

  {
    PERF_TIMER_GUARD(seek_internal_seek_time);
    iter_->SeekToFirst();
  }

  RecordTick(statistics_, NUMBER_DB_SEEK);
  if (iter_->Valid()) {
    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());
      }
    }
  } else {
    valid_ = false;
  }
  if (valid_ && prefix_extractor_ && prefix_same_as_start_) {
    prefix_start_buf_.SetKey(prefix_extractor_->Transform(saved_key_.GetKey()));
    prefix_start_key_ = prefix_start_buf_.GetKey();
  }
}

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();
  ClearSavedValue();

  {
    PERF_TIMER_GUARD(seek_internal_seek_time);
    iter_->SeekToLast();
  }
  // 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) {
    saved_key_.SetKey(*iterate_upper_bound_, false /* copy */);
    std::string last_key;
    AppendInternalKey(&last_key,
                      ParsedInternalKey(saved_key_.GetKey(), kMaxSequenceNumber,
                                        kValueTypeForSeek));

    iter_->Seek(last_key);

    if (!iter_->Valid()) {
      iter_->SeekToLast();
    } else {
      iter_->Prev();
      if (!iter_->Valid()) {
        valid_ = false;
        return;
      }
    }
  }
  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());
    }
  }
  if (valid_ && prefix_extractor_ && prefix_same_as_start_) {
    prefix_start_buf_.SetKey(prefix_extractor_->Transform(saved_key_.GetKey()));
    prefix_start_key_ = prefix_start_buf_.GetKey();
  }
}

Iterator* NewDBIterator(Env* env, const ImmutableCFOptions& ioptions,
                        const Comparator* user_key_comparator,
                        InternalIterator* internal_iter,
                        const SequenceNumber& sequence,
                        uint64_t max_sequential_skip_in_iterations,
                        uint64_t version_number,
                        const Slice* iterate_upper_bound,
                        bool prefix_same_as_start, bool pin_data) {
  DBIter* db_iter =
      new DBIter(env, ioptions, user_key_comparator, internal_iter, sequence,
                 false, max_sequential_skip_in_iterations, version_number,
                 iterate_upper_bound, prefix_same_as_start, pin_data);
  return db_iter;
}

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

void ArenaWrappedDBIter::SetDBIter(DBIter* iter) { db_iter_ = iter; }

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(); }
inline Status ArenaWrappedDBIter::GetProperty(std::string prop_name,
                                              std::string* prop) {
  return db_iter_->GetProperty(prop_name, prop);
}
void ArenaWrappedDBIter::RegisterCleanup(CleanupFunction function, void* arg1,
                                         void* arg2) {
  db_iter_->RegisterCleanup(function, arg1, arg2);
}

ArenaWrappedDBIter* NewArenaWrappedDbIterator(
    Env* env, const ImmutableCFOptions& ioptions,
    const Comparator* user_key_comparator, const SequenceNumber& sequence,
    uint64_t max_sequential_skip_in_iterations, uint64_t version_number,
    const Slice* iterate_upper_bound, bool prefix_same_as_start,
    bool pin_data) {
  ArenaWrappedDBIter* iter = new ArenaWrappedDBIter();
  Arena* arena = iter->GetArena();
  auto mem = arena->AllocateAligned(sizeof(DBIter));
  DBIter* db_iter =
      new (mem) DBIter(env, ioptions, user_key_comparator, nullptr, sequence,
                       true, max_sequential_skip_in_iterations, version_number,
                       iterate_upper_bound, prefix_same_as_start, pin_data);

  iter->SetDBIter(db_iter);

  return iter;
}

}  // namespace rocksdb