Adding upstream version 14.2.21.upstream/14.2.21upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1592 insertions, 0 deletions

diff --git a/src/rocksdb/db/db_iter.cc b/src/rocksdb/db/db_iter.cc
new file mode 100644
index 00000000..541a5fbe
--- /dev/null
+++ b/src/rocksdb/db/db_iter.cc
@@ -0,0 +1,1592 @@
+//  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 <iostream>
+#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"
+#include "util/trace_replay.h"
+#include "util/user_comparator_wrapper.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:
+  //   (1a) if current_entry_is_merged_ = false, the internal iterator is
+  //        positioned at the exact entry that yields this->key(), this->value()
+  //   (1b) if current_entry_is_merged_ = true, the internal iterator is
+  //        positioned immediately after the last entry that contributed to the
+  //        current this->value(). That entry may or may not have key equal to
+  //        this->key().
+  // (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;
+      skip_count_ = 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_);
+      RecordTick(global_statistics, NUMBER_ITER_SKIP, skip_count_);
+      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_;
+    // Map to Tickers::NUMBER_ITER_SKIP
+    uint64_t skip_count_;
+  };
+
+  DBIter(Env* _env, const ReadOptions& read_options,
+         const ImmutableCFOptions& cf_options,
+         const MutableCFOptions& mutable_cf_options, const Comparator* cmp,
+         InternalIterator* iter, SequenceNumber s, bool arena_mode,
+         uint64_t max_sequential_skip_in_iterations,
+         ReadCallback* read_callback, DBImpl* db_impl, ColumnFamilyData* cfd,
+         bool allow_blob)
+      : env_(_env),
+        logger_(cf_options.info_log),
+        user_comparator_(cmp),
+        merge_operator_(cf_options.merge_operator),
+        iter_(iter),
+        read_callback_(read_callback),
+        sequence_(s),
+        statistics_(cf_options.statistics),
+        num_internal_keys_skipped_(0),
+        iterate_lower_bound_(read_options.iterate_lower_bound),
+        iterate_upper_bound_(read_options.iterate_upper_bound),
+        direction_(kForward),
+        valid_(false),
+        current_entry_is_merged_(false),
+        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),
+        allow_blob_(allow_blob),
+        is_blob_(false),
+        arena_mode_(arena_mode),
+        range_del_agg_(&cf_options.internal_comparator, s),
+        db_impl_(db_impl),
+        cfd_(cfd),
+        start_seqnum_(read_options.iter_start_seqnum) {
+    RecordTick(statistics_, NO_ITERATOR_CREATED);
+    prefix_extractor_ = mutable_cf_options.prefix_extractor.get();
+    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_);
+    }
+  }
+  ~DBIter() override {
+    // Release pinned data if any
+    if (pinned_iters_mgr_.PinningEnabled()) {
+      pinned_iters_mgr_.ReleasePinnedData();
+    }
+    RecordTick(statistics_, NO_ITERATOR_DELETED);
+    ResetInternalKeysSkippedCounter();
+    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 ReadRangeDelAggregator* GetRangeDelAggregator() {
+    return &range_del_agg_;
+  }
+
+  bool Valid() const override { return valid_; }
+  Slice key() const override {
+    assert(valid_);
+    if(start_seqnum_ > 0) {
+      return saved_key_.GetInternalKey();
+    } else {
+      return saved_key_.GetUserKey();
+    }
+  }
+  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();
+    }
+  }
+  Status status() const override {
+    if (status_.ok()) {
+      return iter_->status();
+    } else {
+      assert(!valid_);
+      return status_;
+    }
+  }
+  bool IsBlob() const {
+    assert(valid_ && (allow_blob_ || !is_blob_));
+    return is_blob_;
+  }
+
+  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();
+    } else if (prop_name == "rocksdb.iterator.internal-key") {
+      *prop = saved_key_.GetUserKey().ToString();
+      return Status::OK();
+    }
+    return Status::InvalidArgument("Unidentified property.");
+  }
+
+  void Next() override;
+  void Prev() override;
+  void Seek(const Slice& target) override;
+  void SeekForPrev(const Slice& target) override;
+  void SeekToFirst() override;
+  void SeekToLast() override;
+  Env* env() { return env_; }
+  void set_sequence(uint64_t s) {
+    sequence_ = s;
+    if (read_callback_) {
+      read_callback_->Refresh(s);
+    }
+  }
+  void set_valid(bool v) { valid_ = v; }
+
+ private:
+  // For all methods in this block:
+  // PRE: iter_->Valid() && status_.ok()
+  // Return false if there was an error, and status() is non-ok, valid_ = false;
+  // in this case callers would usually stop what they were doing and return.
+  bool ReverseToForward();
+  bool ReverseToBackward();
+  bool FindValueForCurrentKey();
+  bool FindValueForCurrentKeyUsingSeek();
+  bool FindUserKeyBeforeSavedKey();
+  inline bool FindNextUserEntry(bool skipping, bool prefix_check);
+  bool FindNextUserEntryInternal(bool skipping, bool prefix_check);
+  bool ParseKey(ParsedInternalKey* key);
+  bool MergeValuesNewToOld();
+
+  void PrevInternal();
+  bool TooManyInternalKeysSkipped(bool increment = true);
+  inline bool IsVisible(SequenceNumber sequence);
+
+  // CanReseekToSkip() returns whether the iterator can use the optimization
+  // where it reseek by sequence number to get the next key when there are too
+  // many versions. This is disabled for write unprepared because seeking to
+  // sequence number does not guarantee that it is visible.
+  inline bool CanReseekToSkip();
+
+  // 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() {
+    local_stats_.skip_count_ += num_internal_keys_skipped_;
+    if (valid_) {
+      local_stats_.skip_count_--;
+    }
+    num_internal_keys_skipped_ = 0;
+  }
+
+  const SliceTransform* prefix_extractor_;
+  Env* const env_;
+  Logger* logger_;
+  UserComparatorWrapper user_comparator_;
+  const MergeOperator* const merge_operator_;
+  InternalIterator* iter_;
+  ReadCallback* read_callback_;
+  // Max visible sequence number. It is normally the snapshot seq unless we have
+  // uncommitted data in db as in WriteUnCommitted.
+  SequenceNumber sequence_;
+
+  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_;
+  // 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_lower_bound_;
+  const Slice* iterate_upper_bound_;
+
+  IterKey prefix_start_buf_;
+
+  Status status_;
+  Slice prefix_start_key_;
+  Direction direction_;
+  bool valid_;
+  bool current_entry_is_merged_;
+  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_;
+  bool allow_blob_;
+  bool is_blob_;
+  bool arena_mode_;
+  // List of operands for merge operator.
+  MergeContext merge_context_;
+  ReadRangeDelAggregator range_del_agg_;
+  LocalStatistics local_stats_;
+  PinnedIteratorsManager pinned_iters_mgr_;
+  DBImpl* db_impl_;
+  ColumnFamilyData* cfd_;
+  // for diff snapshots we want the lower bound on the seqnum;
+  // if this value > 0 iterator will return internal keys
+  SequenceNumber start_seqnum_;
+
+  // 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");
+    valid_ = false;
+    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_);
+  assert(status_.ok());
+
+  PERF_CPU_TIMER_GUARD(iter_next_cpu_nanos, env_);
+  // Release temporarily pinned blocks from last operation
+  ReleaseTempPinnedData();
+  ResetInternalKeysSkippedCounter();
+  bool ok = true;
+  if (direction_ == kReverse) {
+    if (!ReverseToForward()) {
+      ok = false;
+    }
+  } 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_++;
+  }
+  if (ok && iter_->Valid()) {
+    FindNextUserEntry(true /* skipping the current user key */,
+                      prefix_same_as_start_);
+  } else {
+    valid_ = false;
+  }
+  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 bool DBIter::FindNextUserEntry(bool skipping, bool prefix_check) {
+  PERF_TIMER_GUARD(find_next_user_entry_time);
+  return FindNextUserEntryInternal(skipping, prefix_check);
+}
+
+// Actual implementation of DBIter::FindNextUserEntry()
+bool DBIter::FindNextUserEntryInternal(bool skipping, bool prefix_check) {
+  // Loop until we hit an acceptable entry to yield
+  assert(iter_->Valid());
+  assert(status_.ok());
+  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_)) {
+      return false;
+    }
+
+    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 false;
+    }
+
+    if (IsVisible(ikey_.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.
+            // if iterartor specified start_seqnum we
+            // 1) return internal key, including the type
+            // 2) return ikey only if ikey.seqnum >= start_seqnum_
+            // note that if deletion seqnum is < start_seqnum_ we
+            // just skip it like in normal iterator.
+            if (start_seqnum_ > 0 && ikey_.sequence >= start_seqnum_)  {
+              saved_key_.SetInternalKey(ikey_);
+              valid_ = true;
+              return true;
+            } else {
+              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:
+            if (start_seqnum_ > 0) {
+              // we are taking incremental snapshot here
+              // incremental snapshots aren't supported on DB with range deletes
+              assert(!(
+                (ikey_.type == kTypeBlobIndex) && (start_seqnum_ > 0)
+              ));
+              if (ikey_.sequence >= start_seqnum_) {
+                saved_key_.SetInternalKey(ikey_);
+                valid_ = true;
+                return true;
+              } else {
+                // this key and all previous versions shouldn't be included,
+                // skipping
+                saved_key_.SetUserKey(ikey_.user_key,
+                  !pin_thru_lifetime_ || !iter_->IsKeyPinned() /* copy */);
+                skipping = true;
+              }
+            } else {
+              saved_key_.SetUserKey(
+                  ikey_.user_key,
+                  !pin_thru_lifetime_ || !iter_->IsKeyPinned() /* copy */);
+              if (range_del_agg_.ShouldDelete(
+                      ikey_, RangeDelPositioningMode::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;
+                  return false;
+                }
+
+                is_blob_ = true;
+                valid_ = true;
+                return true;
+              } else {
+                valid_ = true;
+                return true;
+              }
+            }
+            break;
+          case kTypeMerge:
+            saved_key_.SetUserKey(
+                ikey_.user_key,
+                !pin_thru_lifetime_ || !iter_->IsKeyPinned() /* copy */);
+            if (range_del_agg_.ShouldDelete(
+                    ikey_, RangeDelPositioningMode::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;
+              return MergeValuesNewToOld();  // Go to a different state machine
+            }
+            break;
+          default:
+            assert(false);
+            break;
+        }
+      }
+    } else {
+      PERF_COUNTER_ADD(internal_recent_skipped_count, 1);
+
+      // This key was inserted after our snapshot was taken.
+      // If this happens too many times in a row for the same user key, we want
+      // to seek to the target sequence number.
+      int cmp =
+          user_comparator_.Compare(ikey_.user_key, saved_key_.GetUserKey());
+      if (cmp == 0 || (skipping && cmp <= 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_ && CanReseekToSkip()) {
+      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;
+  return iter_->status().ok();
+}
+
+// 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)
+bool 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 false;
+  }
+
+  // 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 */);
+  TEST_SYNC_POINT("DBIter::MergeValuesNewToOld:PushedFirstOperand");
+
+  ParsedInternalKey ikey;
+  Status s;
+  for (iter_->Next(); iter_->Valid(); iter_->Next()) {
+    TEST_SYNC_POINT("DBIter::MergeValuesNewToOld:SteppedToNextOperand");
+    if (!ParseKey(&ikey)) {
+      return false;
+    }
+
+    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, RangeDelPositioningMode::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!
+      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()) {
+        valid_ = false;
+        status_ = s;
+        return false;
+      }
+      // iter_ is positioned after put
+      iter_->Next();
+      if (!iter_->status().ok()) {
+        valid_ = false;
+        return false;
+      }
+      return true;
+    } 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 false;
+    } else {
+      assert(false);
+    }
+  }
+
+  if (!iter_->status().ok()) {
+    valid_ = false;
+    return 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()) {
+    valid_ = false;
+    status_ = s;
+    return false;
+  }
+
+  assert(status_.ok());
+  return true;
+}
+
+void DBIter::Prev() {
+  assert(valid_);
+  assert(status_.ok());
+
+  PERF_CPU_TIMER_GUARD(iter_prev_cpu_nanos, env_);
+  ReleaseTempPinnedData();
+  ResetInternalKeysSkippedCounter();
+  bool ok = true;
+  if (direction_ == kForward) {
+    if (!ReverseToBackward()) {
+      ok = false;
+    }
+  }
+  if (ok) {
+    PrevInternal();
+  }
+  if (statistics_ != nullptr) {
+    local_stats_.prev_count_++;
+    if (valid_) {
+      local_stats_.prev_found_count_++;
+      local_stats_.bytes_read_ += (key().size() + value().size());
+    }
+  }
+}
+
+bool DBIter::ReverseToForward() {
+  assert(iter_->status().ok());
+
+  // When moving backwards, iter_ is positioned on _previous_ key, which may
+  // not exist or may have different prefix than the current key().
+  // If that's the case, seek iter_ to current key.
+  if ((prefix_extractor_ != nullptr && !total_order_seek_) || !iter_->Valid()) {
+    IterKey last_key;
+    last_key.SetInternalKey(ParsedInternalKey(
+        saved_key_.GetUserKey(), kMaxSequenceNumber, kValueTypeForSeek));
+    iter_->Seek(last_key.GetInternalKey());
+  }
+
+  direction_ = kForward;
+  // Skip keys less than the current key() (a.k.a. saved_key_).
+  while (iter_->Valid()) {
+    ParsedInternalKey ikey;
+    if (!ParseKey(&ikey)) {
+      return false;
+    }
+    if (user_comparator_.Compare(ikey.user_key, saved_key_.GetUserKey()) >= 0) {
+      return true;
+    }
+    iter_->Next();
+  }
+
+  if (!iter_->status().ok()) {
+    valid_ = false;
+    return false;
+  }
+
+  return true;
+}
+
+// Move iter_ to the key before saved_key_.
+bool DBIter::ReverseToBackward() {
+  assert(iter_->status().ok());
+
+  // When current_entry_is_merged_ is true, iter_ may be positioned on the next
+  // key, which may not exist or may have prefix different from current.
+  // If that's the case, seek to saved_key_.
+  if (current_entry_is_merged_ &&
+      ((prefix_extractor_ != nullptr && !total_order_seek_) ||
+       !iter_->Valid())) {
+    IterKey last_key;
+    // Using kMaxSequenceNumber and kValueTypeForSeek
+    // (not kValueTypeForSeekForPrev) to seek to a key strictly smaller
+    // than saved_key_.
+    last_key.SetInternalKey(ParsedInternalKey(
+        saved_key_.GetUserKey(), kMaxSequenceNumber, kValueTypeForSeek));
+    if (prefix_extractor_ != nullptr && !total_order_seek_) {
+      iter_->SeekForPrev(last_key.GetInternalKey());
+    } else {
+      // Some iterators may not support SeekForPrev(), so we avoid using it
+      // when prefix seek mode is disabled. This is somewhat expensive
+      // (an extra Prev(), as well as an extra change of direction of iter_),
+      // so we may need to reconsider it later.
+      iter_->Seek(last_key.GetInternalKey());
+      if (!iter_->Valid() && iter_->status().ok()) {
+        iter_->SeekToLast();
+      }
+    }
+  }
+
+  direction_ = kReverse;
+  return FindUserKeyBeforeSavedKey();
+}
+
+void DBIter::PrevInternal() {
+  while (iter_->Valid()) {
+    saved_key_.SetUserKey(
+        ExtractUserKey(iter_->key()),
+        !iter_->IsKeyPinned() || !pin_thru_lifetime_ /* copy */);
+
+    if (prefix_extractor_ && prefix_same_as_start_ &&
+        prefix_extractor_->Transform(saved_key_.GetUserKey())
+                .compare(prefix_start_key_) != 0) {
+      // Current key does not have the same prefix as start
+      valid_ = false;
+      return;
+    }
+
+    if (iterate_lower_bound_ != nullptr &&
+        user_comparator_.Compare(saved_key_.GetUserKey(),
+                                 *iterate_lower_bound_) < 0) {
+      // We've iterated earlier than the user-specified lower bound.
+      valid_ = false;
+      return;
+    }
+
+    if (!FindValueForCurrentKey()) {  // assigns valid_
+      return;
+    }
+
+    // Whether or not we found a value for current key, we need iter_ to end up
+    // on a smaller key.
+    if (!FindUserKeyBeforeSavedKey()) {
+      return;
+    }
+
+    if (valid_) {
+      // Found the value.
+      return;
+    }
+
+    if (TooManyInternalKeysSkipped(false)) {
+      return;
+    }
+  }
+
+  // We haven't found any key - iterator is not valid
+  valid_ = false;
+}
+
+// Used for backwards iteration.
+// Looks at the entries with user key saved_key_ and finds the most up-to-date
+// value for it, or executes a merge, or determines that the value was deleted.
+// Sets valid_ to true if the value is found and is ready to be presented to
+// the user through value().
+// Sets valid_ to false if the value was deleted, and we should try another key.
+// Returns false if an error occurred, and !status().ok() and !valid_.
+//
+// PRE: iter_ is positioned on the last entry with user key equal to saved_key_.
+// POST: iter_ is positioned on one of the entries equal to saved_key_, or on
+//       the entry just before them, or on the entry just after them.
+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;
+
+  // Temporarily pin blocks that hold (merge operands / the value)
+  ReleaseTempPinnedData();
+  TempPinData();
+  size_t num_skipped = 0;
+  while (iter_->Valid()) {
+    ParsedInternalKey ikey;
+    if (!ParseKey(&ikey)) {
+      return false;
+    }
+
+    if (!IsVisible(ikey.sequence) ||
+        !user_comparator_.Equal(ikey.user_key, saved_key_.GetUserKey())) {
+      break;
+    }
+    if (TooManyInternalKeysSkipped()) {
+      return false;
+    }
+
+    // This user key has lots of entries.
+    // We're going from old to new, and it's taking too long. Let's do a Seek()
+    // and go from new to old. This helps when a key was overwritten many times.
+    if (num_skipped >= max_skip_ && CanReseekToSkip()) {
+      return FindValueForCurrentKeyUsingSeek();
+    }
+
+    last_key_entry_type = ikey.type;
+    switch (last_key_entry_type) {
+      case kTypeValue:
+      case kTypeBlobIndex:
+        if (range_del_agg_.ShouldDelete(
+                ikey, RangeDelPositioningMode::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, RangeDelPositioningMode::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);
+    iter_->Prev();
+    ++num_skipped;
+  }
+
+  if (!iter_->status().ok()) {
+    valid_ = false;
+    return false;
+  }
+
+  Status s;
+  is_blob_ = false;
+  switch (last_key_entry_type) {
+    case kTypeDeletion:
+    case kTypeSingleDeletion:
+    case kTypeRangeDeletion:
+      valid_ = false;
+      return true;
+    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 false;
+      } 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 pinned_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 false;
+      }
+      is_blob_ = true;
+      break;
+    default:
+      assert(false);
+      break;
+  }
+  if (!s.ok()) {
+    valid_ = false;
+    status_ = s;
+    return false;
+  }
+  valid_ = true;
+  return true;
+}
+
+// This function is used in FindValueForCurrentKey.
+// We use Seek() function instead of Prev() to find necessary value
+// TODO: This is very similar to FindNextUserEntry() and MergeValuesNewToOld().
+//       Would be nice to reuse some code.
+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);
+
+  // In case read_callback presents, the value we seek to may not be visible.
+  // Find the next value that's visible.
+  ParsedInternalKey ikey;
+  while (true) {
+    if (!iter_->Valid()) {
+      valid_ = false;
+      return iter_->status().ok();
+    }
+
+    if (!ParseKey(&ikey)) {
+      return false;
+    }
+    if (!user_comparator_.Equal(ikey.user_key, saved_key_.GetUserKey())) {
+      // No visible values for this key, even though FindValueForCurrentKey()
+      // has seen some. This is possible if we're using a tailing iterator, and
+      // the entries were discarded in a compaction.
+      valid_ = false;
+      return true;
+    }
+
+    if (IsVisible(ikey.sequence)) {
+      break;
+    }
+
+    iter_->Next();
+  }
+
+  if (ikey.type == kTypeDeletion || ikey.type == kTypeSingleDeletion ||
+      range_del_agg_.ShouldDelete(
+          ikey, RangeDelPositioningMode::kBackwardTraversal)) {
+    valid_ = false;
+    return true;
+  }
+  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 false;
+  }
+  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
+  assert(ikey.type == kTypeMerge);
+  current_entry_is_merged_ = true;
+  merge_context_.Clear();
+  merge_context_.PushOperand(iter_->value(),
+                             iter_->IsValuePinned() /* operand_pinned */);
+  while (true) {
+    iter_->Next();
+
+    if (!iter_->Valid()) {
+      if (!iter_->status().ok()) {
+        valid_ = false;
+        return false;
+      }
+      break;
+    }
+    if (!ParseKey(&ikey)) {
+      return false;
+    }
+    if (!user_comparator_.Equal(ikey.user_key, saved_key_.GetUserKey())) {
+      break;
+    }
+
+    if (ikey.type == kTypeDeletion || ikey.type == kTypeSingleDeletion ||
+        range_del_agg_.ShouldDelete(
+            ikey, RangeDelPositioningMode::kForwardTraversal)) {
+      break;
+    } else if (ikey.type == kTypeValue) {
+      const Slice val = iter_->value();
+      Status s = MergeHelper::TimedFullMerge(
+          merge_operator_, saved_key_.GetUserKey(), &val,
+          merge_context_.GetOperands(), &saved_value_, logger_, statistics_,
+          env_, &pinned_value_, true);
+      if (!s.ok()) {
+        valid_ = false;
+        status_ = s;
+        return false;
+      }
+      valid_ = true;
+      return true;
+    } else if (ikey.type == kTypeMerge) {
+      merge_context_.PushOperand(iter_->value(),
+                                 iter_->IsValuePinned() /* operand_pinned */);
+      PERF_COUNTER_ADD(internal_merge_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.");
+      } else {
+        status_ =
+            Status::NotSupported("Blob DB does not support merge operator.");
+      }
+      valid_ = false;
+      return false;
+    } else {
+      assert(false);
+    }
+  }
+
+  Status s = MergeHelper::TimedFullMerge(
+      merge_operator_, saved_key_.GetUserKey(), nullptr,
+      merge_context_.GetOperands(), &saved_value_, logger_, statistics_, env_,
+      &pinned_value_, true);
+  if (!s.ok()) {
+    valid_ = false;
+    status_ = s;
+    return false;
+  }
+
+  // Make sure we leave iter_ in a good state. If it's valid and we don't care
+  // about prefixes, that's already good enough. Otherwise it needs to be
+  // seeked to the current key.
+  if ((prefix_extractor_ != nullptr && !total_order_seek_) || !iter_->Valid()) {
+    if (prefix_extractor_ != nullptr && !total_order_seek_) {
+      iter_->SeekForPrev(last_key);
+    } else {
+      iter_->Seek(last_key);
+      if (!iter_->Valid() && iter_->status().ok()) {
+        iter_->SeekToLast();
+      }
+    }
+    RecordTick(statistics_, NUMBER_OF_RESEEKS_IN_ITERATION);
+  }
+
+  valid_ = true;
+  return true;
+}
+
+// Move backwards until the key smaller than saved_key_.
+// Changes valid_ only if return value is false.
+bool DBIter::FindUserKeyBeforeSavedKey() {
+  assert(status_.ok());
+  size_t num_skipped = 0;
+  while (iter_->Valid()) {
+    ParsedInternalKey ikey;
+    if (!ParseKey(&ikey)) {
+      return false;
+    }
+
+    if (user_comparator_.Compare(ikey.user_key, saved_key_.GetUserKey()) < 0) {
+      return true;
+    }
+
+    if (TooManyInternalKeysSkipped()) {
+      return false;
+    }
+
+    assert(ikey.sequence != kMaxSequenceNumber);
+    if (!IsVisible(ikey.sequence)) {
+      PERF_COUNTER_ADD(internal_recent_skipped_count, 1);
+    } else {
+      PERF_COUNTER_ADD(internal_key_skipped_count, 1);
+    }
+
+    if (num_skipped >= max_skip_ && CanReseekToSkip()) {
+      num_skipped = 0;
+      IterKey last_key;
+      last_key.SetInternalKey(ParsedInternalKey(
+          saved_key_.GetUserKey(), kMaxSequenceNumber, kValueTypeForSeek));
+      // It would be more efficient to use SeekForPrev() here, but some
+      // iterators may not support it.
+      iter_->Seek(last_key.GetInternalKey());
+      RecordTick(statistics_, NUMBER_OF_RESEEKS_IN_ITERATION);
+      if (!iter_->Valid()) {
+        break;
+      }
+    } else {
+      ++num_skipped;
+    }
+
+    iter_->Prev();
+  }
+
+  if (!iter_->status().ok()) {
+    valid_ = false;
+    return false;
+  }
+
+  return true;
+}
+
+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;
+}
+
+bool DBIter::IsVisible(SequenceNumber sequence) {
+  if (read_callback_ == nullptr) {
+    return sequence <= sequence_;
+  } else {
+    return read_callback_->IsVisible(sequence);
+  }
+}
+
+bool DBIter::CanReseekToSkip() {
+  return read_callback_ == nullptr || read_callback_->CanReseekToSkip();
+}
+
+void DBIter::Seek(const Slice& target) {
+  PERF_CPU_TIMER_GUARD(iter_seek_cpu_nanos, env_);
+  StopWatch sw(env_, statistics_, DB_SEEK);
+  status_ = Status::OK();
+  ReleaseTempPinnedData();
+  ResetInternalKeysSkippedCounter();
+
+  SequenceNumber seq = sequence_;
+  saved_key_.Clear();
+  saved_key_.SetInternalKey(target, seq);
+
+#ifndef ROCKSDB_LITE
+  if (db_impl_ != nullptr && cfd_ != nullptr) {
+    db_impl_->TraceIteratorSeek(cfd_->GetID(), target);
+  }
+#endif  // ROCKSDB_LITE
+
+  if (iterate_lower_bound_ != nullptr &&
+      user_comparator_.Compare(saved_key_.GetUserKey(), *iterate_lower_bound_) <
+          0) {
+    saved_key_.Clear();
+    saved_key_.SetInternalKey(*iterate_lower_bound_, seq);
+  }
+
+  {
+    PERF_TIMER_GUARD(seek_internal_seek_time);
+    iter_->Seek(saved_key_.GetInternalKey());
+    range_del_agg_.InvalidateRangeDelMapPositions();
+  }
+  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_) {
+        // Decrement since we don't want to count this key as skipped
+        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) {
+  PERF_CPU_TIMER_GUARD(iter_seek_cpu_nanos, env_);
+  StopWatch sw(env_, statistics_, DB_SEEK);
+  status_ = Status::OK();
+  ReleaseTempPinnedData();
+  ResetInternalKeysSkippedCounter();
+  saved_key_.Clear();
+  // now saved_key is used to store internal key.
+  saved_key_.SetInternalKey(target, 0 /* sequence_number */,
+                            kValueTypeForSeekForPrev);
+
+  if (iterate_upper_bound_ != nullptr &&
+      user_comparator_.Compare(saved_key_.GetUserKey(),
+                               *iterate_upper_bound_) >= 0) {
+    saved_key_.Clear();
+    saved_key_.SetInternalKey(*iterate_upper_bound_, kMaxSequenceNumber);
+  }
+
+  {
+    PERF_TIMER_GUARD(seek_internal_seek_time);
+    iter_->SeekForPrev(saved_key_.GetInternalKey());
+    range_del_agg_.InvalidateRangeDelMapPositions();
+  }
+
+#ifndef ROCKSDB_LITE
+  if (db_impl_ != nullptr && cfd_ != nullptr) {
+    db_impl_->TraceIteratorSeekForPrev(cfd_->GetID(), target);
+  }
+#endif  // ROCKSDB_LITE
+
+  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() {
+  if (iterate_lower_bound_ != nullptr) {
+    Seek(*iterate_lower_bound_);
+    return;
+  }
+  PERF_CPU_TIMER_GUARD(iter_seek_cpu_nanos, env_);
+  // Don't use iter_::Seek() if we set a prefix extractor
+  // because prefix seek will be used.
+  if (prefix_extractor_ != nullptr && !total_order_seek_) {
+    max_skip_ = std::numeric_limits<uint64_t>::max();
+  }
+  status_ = Status::OK();
+  direction_ = kForward;
+  ReleaseTempPinnedData();
+  ResetInternalKeysSkippedCounter();
+  ClearSavedValue();
+
+  {
+    PERF_TIMER_GUARD(seek_internal_seek_time);
+    iter_->SeekToFirst();
+    range_del_agg_.InvalidateRangeDelMapPositions();
+  }
+
+  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() {
+  if (iterate_upper_bound_ != nullptr) {
+    // Seek to last key strictly less than ReadOptions.iterate_upper_bound.
+    SeekForPrev(*iterate_upper_bound_);
+    if (Valid() && user_comparator_.Equal(*iterate_upper_bound_, key())) {
+      ReleaseTempPinnedData();
+      PrevInternal();
+    }
+    return;
+  }
+
+  PERF_CPU_TIMER_GUARD(iter_seek_cpu_nanos, env_);
+  // Don't use iter_::Seek() if we set a prefix extractor
+  // because prefix seek will be used.
+  if (prefix_extractor_ != nullptr && !total_order_seek_) {
+    max_skip_ = std::numeric_limits<uint64_t>::max();
+  }
+  status_ = Status::OK();
+  direction_ = kReverse;
+  ReleaseTempPinnedData();
+  ResetInternalKeysSkippedCounter();
+  ClearSavedValue();
+
+  {
+    PERF_TIMER_GUARD(seek_internal_seek_time);
+    iter_->SeekToLast();
+    range_del_agg_.InvalidateRangeDelMapPositions();
+  }
+  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 MutableCFOptions& mutable_cf_options,
+                        const Comparator* user_key_comparator,
+                        InternalIterator* internal_iter,
+                        const SequenceNumber& sequence,
+                        uint64_t max_sequential_skip_in_iterations,
+                        ReadCallback* read_callback, DBImpl* db_impl,
+                        ColumnFamilyData* cfd, bool allow_blob) {
+  DBIter* db_iter = new DBIter(
+      env, read_options, cf_options, mutable_cf_options, user_key_comparator,
+      internal_iter, sequence, false, max_sequential_skip_in_iterations,
+      read_callback, db_impl, cfd, allow_blob);
+  return db_iter;
+}
+
+ArenaWrappedDBIter::~ArenaWrappedDBIter() { db_iter_->~DBIter(); }
+
+ReadRangeDelAggregator* 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 MutableCFOptions& mutable_cf_options,
+                              const SequenceNumber& sequence,
+                              uint64_t max_sequential_skip_in_iteration,
+                              uint64_t version_number,
+                              ReadCallback* read_callback, DBImpl* db_impl,
+                              ColumnFamilyData* cfd, bool allow_blob,
+                              bool allow_refresh) {
+  auto mem = arena_.AllocateAligned(sizeof(DBIter));
+  db_iter_ = new (mem) DBIter(env, read_options, cf_options, mutable_cf_options,
+                              cf_options.user_comparator, nullptr, sequence,
+                              true, max_sequential_skip_in_iteration,
+                              read_callback, db_impl, cfd, allow_blob);
+  sv_number_ = version_number;
+  allow_refresh_ = allow_refresh;
+}
+
+Status ArenaWrappedDBIter::Refresh() {
+  if (cfd_ == nullptr || db_impl_ == nullptr || !allow_refresh_) {
+    return Status::NotSupported("Creating renew iterator is not allowed.");
+  }
+  assert(db_iter_ != nullptr);
+  // TODO(yiwu): For last_seq_same_as_publish_seq_==false, this is not the
+  // correct behavior. Will be corrected automatically when we take a snapshot
+  // here for the case of WritePreparedTxnDB.
+  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());
+    if (read_callback_) {
+      read_callback_->Refresh(latest_seq);
+    }
+    Init(env, read_options_, *(cfd_->ioptions()), sv->mutable_cf_options,
+         latest_seq, sv->mutable_cf_options.max_sequential_skip_in_iterations,
+         cur_sv_number, read_callback_, db_impl_, cfd_, allow_blob_,
+         allow_refresh_);
+
+    InternalIterator* internal_iter = db_impl_->NewInternalIterator(
+        read_options_, cfd_, sv, &arena_, db_iter_->GetRangeDelAggregator(),
+        latest_seq);
+    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 MutableCFOptions& mutable_cf_options, const SequenceNumber& sequence,
+    uint64_t max_sequential_skip_in_iterations, uint64_t version_number,
+    ReadCallback* read_callback, DBImpl* db_impl, ColumnFamilyData* cfd,
+    bool allow_blob, bool allow_refresh) {
+  ArenaWrappedDBIter* iter = new ArenaWrappedDBIter();
+  iter->Init(env, read_options, cf_options, mutable_cf_options, sequence,
+             max_sequential_skip_in_iterations, version_number, read_callback,
+             db_impl, cfd, allow_blob, allow_refresh);
+  if (db_impl != nullptr && cfd != nullptr && allow_refresh) {
+    iter->StoreRefreshInfo(read_options, db_impl, cfd, read_callback,
+                           allow_blob);
+  }
+
+  return iter;
+}
+
+}  // namespace rocksdb