Add Prev() for merge operator

Summary: Implement Prev() with merge operator for DBIterator. Request from mongoDB. Task 4673663.

Test Plan: make all check

Reviewers: sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19743

db/db_iter.cc

@@ -10,6 +10,7 @@
 #include "db/db_iter.h"
 #include <stdexcept>
 #include <deque>
+#include <string>
 
 #include "db/filename.h"
 #include "db/dbformat.h"
@@ -109,9 +110,14 @@ class DBIter: public Iterator {
   virtual void SeekToLast();
 
  private:
+  void PrevInternal();
+  void FindParseableKey(ParsedInternalKey* ikey, Direction direction);
+  bool FindValueForCurrentKey();
+  bool FindValueForCurrentKeyUsingSeek();
+  void FindPrevUserKey();
+  void FindNextUserKey();
   inline void FindNextUserEntry(bool skipping);
   void FindNextUserEntryInternal(bool skipping);
-  void FindPrevUserEntry();
   bool ParseKey(ParsedInternalKey* key);
   void MergeValuesNewToOld();
 
@@ -133,8 +139,8 @@ class DBIter: public Iterator {
   SequenceNumber const sequence_;
 
   Status status_;
-  IterKey saved_key_;   // == current key when direction_==kReverse
+  IterKey saved_key_;
-  std::string saved_value_;   // == current raw value when direction_==kReverse
+  std::string saved_value_;
   Direction direction_;
   bool valid_;
   bool current_entry_is_merged_;
@@ -160,20 +166,11 @@ inline bool DBIter::ParseKey(ParsedInternalKey* ikey) {
 void DBIter::Next() {
   assert(valid_);
 
-  if (direction_ == kReverse) {  // Switch directions?
+  if (direction_ == kReverse) {
+    FindNextUserKey();
     direction_ = kForward;
-    // iter_ is pointing just before the entries for this->key(),
-    // so advance into the range of entries for this->key() and then
-    // use the normal skipping code below.
     if (!iter_->Valid()) {
       iter_->SeekToFirst();
-    } else {
-      iter_->Next();
-    }
-    if (!iter_->Valid()) {
-      valid_ = false;
-      saved_key_.Clear();
-      return;
     }
   }
 
@@ -185,7 +182,6 @@ void DBIter::Next() {
   FindNextUserEntry(true /* skipping the current user key */);
 }
 
-
 // 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
@@ -327,100 +323,228 @@ void DBIter::MergeValuesNewToOld() {
 
 void DBIter::Prev() {
   assert(valid_);
-
+  if (direction_ == kForward) {
-  // Throw an exception now if merge_operator is provided
+    FindPrevUserKey();
-  // TODO: support backward iteration
-  if (user_merge_operator_) {
-    Log(logger_, "Prev not supported yet if merge_operator is provided");
-    throw std::logic_error("DBIter::Prev backward iteration not supported"
-                           " if merge_operator is provided");
-  }
-
-  if (direction_ == kForward) {  // Switch directions?
-    // iter_ is pointing at the current entry.  Scan backwards until
-    // the key changes so we can use the normal reverse scanning code.
-    assert(iter_->Valid());  // Otherwise valid_ would have been false
-    saved_key_.SetKey(ExtractUserKey(iter_->key()));
-    while (true) {
-      iter_->Prev();
-      if (!iter_->Valid()) {
-        valid_ = false;
-        saved_key_.Clear();
-        ClearSavedValue();
-        return;
-      }
-      if (user_comparator_->Compare(ExtractUserKey(iter_->key()),
-                                    saved_key_.GetKey()) < 0) {
-        break;
-      }
-    }
     direction_ = kReverse;
   }
-
+  PrevInternal();
-  FindPrevUserEntry();
 }
 
-void DBIter::FindPrevUserEntry() {
+void DBIter::PrevInternal() {
-  assert(direction_ == kReverse);
+  if (!iter_->Valid()) {
-  uint64_t num_skipped = 0;
+    valid_ = false;
-
+    return;
-  ValueType value_type = kTypeDeletion;
-  bool saved_key_valid = true;
-  if (iter_->Valid()) {
-    do {
-      ParsedInternalKey ikey;
-      if (ParseKey(&ikey) && ikey.sequence <= sequence_) {
-        if ((value_type != kTypeDeletion) &&
-            user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) < 0) {
-          // We encountered a non-deleted value in entries for previous keys,
-          break;
-        }
-        value_type = ikey.type;
-        if (value_type == kTypeDeletion) {
-          saved_key_.Clear();
-          ClearSavedValue();
-          saved_key_valid = false;
-        } else {
-          Slice raw_value = iter_->value();
-          if (saved_value_.capacity() > raw_value.size() + 1048576) {
-            std::string empty;
-            swap(empty, saved_value_);
-          }
-          saved_key_.SetKey(ExtractUserKey(iter_->key()));
-          saved_value_.assign(raw_value.data(), raw_value.size());
-        }
-      } else {
-        // In the case of ikey.sequence > sequence_, we might have already
-        // iterated to a different user key.
-        saved_key_valid = false;
-      }
-      num_skipped++;
-      // If we have sequentially iterated via numerous keys and still not
-      // found the prev user-key, then it is better to seek so that we can
-      // avoid too many key comparisons. We seek to the first occurence of
-      // our current key by looking for max sequence number.
-      if (saved_key_valid && num_skipped > max_skip_) {
-        num_skipped = 0;
-        std::string last_key;
-        AppendInternalKey(&last_key, ParsedInternalKey(saved_key_.GetKey(),
-                                                       kMaxSequenceNumber,
-                                                       kValueTypeForSeek));
-        iter_->Seek(last_key);
-        RecordTick(statistics_, NUMBER_OF_RESEEKS_IN_ITERATION);
-      } else {
-        iter_->Prev();
-      }
-    } while (iter_->Valid());
   }
 
-  if (value_type == kTypeDeletion) {
+  ParsedInternalKey ikey;
-    // End
+
+  while (iter_->Valid()) {
+    saved_key_.SetKey(ExtractUserKey(iter_->key()));
+    if (FindValueForCurrentKey()) {
+      valid_ = true;
+      if (!iter_->Valid()) {
+        return;
+      }
+      FindParseableKey(&ikey, kReverse);
+      if (user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) == 0) {
+        FindPrevUserKey();
+      }
+      return;
+    }
+    if (!iter_->Valid()) {
+      break;
+    }
+    FindParseableKey(&ikey, kReverse);
+    if (user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) == 0) {
+
+      FindPrevUserKey();
+    }
+  }
+  // We haven't found any key - iterator is not valid
+  assert(!iter_->Valid());
+  valid_ = false;
+}
+
+// This function checks, if the entry with biggest sequence_number <= sequence_
+// is non kTypeDeletion. If it's not, we save value in saved_value_
+bool DBIter::FindValueForCurrentKey() {
+  assert(iter_->Valid());
+  // Contains operands for merge operator.
+  std::deque<std::string> operands;
+  // last entry before merge (could be kTypeDeletion or kTypeValue)
+  ValueType last_not_merge_type = kTypeDeletion;
+  ValueType last_key_entry_type = kTypeDeletion;
+
+  ParsedInternalKey ikey;
+  FindParseableKey(&ikey, kReverse);
+
+  size_t num_skipped = 0;
+  while (iter_->Valid() && ikey.sequence <= sequence_ &&
+         (user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) == 0)) {
+    // 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:
+        operands.clear();
+        saved_value_ = iter_->value().ToString();
+        last_not_merge_type = kTypeValue;
+        break;
+      case kTypeDeletion:
+        operands.clear();
+        last_not_merge_type = kTypeDeletion;
+        break;
+      case kTypeMerge:
+        assert(user_merge_operator_ != nullptr);
+        operands.push_back(iter_->value().ToString());
+        break;
+      default:
+        assert(false);
+    }
+
+    assert(user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) == 0);
+    iter_->Prev();
+    ++num_skipped;
+    FindParseableKey(&ikey, kReverse);
+  }
+
+  switch (last_key_entry_type) {
+    case kTypeDeletion:
+      valid_ = false;
+      return false;
+    case kTypeMerge:
+      if (last_not_merge_type == kTypeDeletion) {
+        user_merge_operator_->FullMerge(saved_key_.GetKey(), nullptr, operands,
+                                        &saved_value_, logger_);
+      } else {
+        assert(last_not_merge_type == kTypeValue);
+        std::string last_put_value = saved_value_;
+        Slice temp_slice(last_put_value);
+        user_merge_operator_->FullMerge(saved_key_.GetKey(), &temp_slice,
+                                        operands, &saved_value_, logger_);
+      }
+      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() {
+  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) {
+    if (ikey.type == kTypeValue) {
+      saved_value_ = iter_->value().ToString();
+      valid_ = true;
+      return true;
+    }
     valid_ = false;
-    saved_key_.Clear();
+    return false;
-    ClearSavedValue();
+  }
-    direction_ = kForward;
+
-  } else {
+  // kTypeMerge. We need to collect all kTypeMerge values and save them
+  // in operands
+  std::deque<std::string> operands;
+  while (iter_->Valid() &&
+         (user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) == 0) &&
+         ikey.type == kTypeMerge) {
+    operands.push_front(iter_->value().ToString());
+    iter_->Next();
+    FindParseableKey(&ikey, kForward);
+  }
+
+  if (!iter_->Valid() ||
+      (user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) != 0) ||
+      ikey.type == kTypeDeletion) {
+    user_merge_operator_->FullMerge(saved_key_.GetKey(), nullptr, operands,
+                                    &saved_value_, logger_);
+
+    // Make iter_ valid and point to saved_key_
+    if (!iter_->Valid() ||
+        (user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) != 0)) {
+      iter_->Seek(last_key);
+      RecordTick(statistics_, NUMBER_OF_RESEEKS_IN_ITERATION);
+    }
     valid_ = true;
+    return true;
+  }
+
+  const Slice& value = iter_->value();
+  user_merge_operator_->FullMerge(saved_key_.GetKey(), &value, operands,
+                                  &saved_value_, logger_);
+  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_->Compare(ikey.user_key, saved_key_.GetKey()) != 0) {
+    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);
+  while (iter_->Valid() &&
+         user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) == 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);
+    }
+
+    iter_->Prev();
+    ++num_skipped;
+    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();
+    }
   }
 }
 
@@ -454,20 +578,13 @@ void DBIter::SeekToFirst() {
 }
 
 void DBIter::SeekToLast() {
-  // Throw an exception for now if merge_operator is provided
-  // TODO: support backward iteration
-  if (user_merge_operator_) {
-    Log(logger_, "SeekToLast not supported yet if merge_operator is provided");
-    throw std::logic_error("DBIter::SeekToLast: backward iteration not"
-                           " supported if merge_operator is provided");
-  }
-
   direction_ = kReverse;
   ClearSavedValue();
   PERF_TIMER_AUTO(seek_internal_seek_time);
   iter_->SeekToLast();
   PERF_TIMER_STOP(seek_internal_seek_time);
-  FindPrevUserEntry();
+
+  PrevInternal();
 }
 
 Iterator* NewDBIterator(Env* env, const Options& options,

db/db_test.cc

@@ -12,8 +12,10 @@
 #include <set>
 #include <unistd.h>
 #include <unordered_set>
+#include <utility>
 
 #include "db/dbformat.h"
+#include "db/db_iter.h"
 #include "db/db_impl.h"
 #include "db/filename.h"
 #include "db/version_set.h"
@@ -1162,6 +1164,952 @@ TEST(DBTest, ReadOnlyDB) {
   delete iter;
 }
 
+class TestIterator : public Iterator {
+ public:
+  explicit TestIterator(const Comparator* comparator)
+      : initialized_(false),
+        valid_(false),
+        sequence_number_(0),
+        iter_(0),
+        cmp(comparator) {}
+
+  void AddMerge(std::string key, std::string value) {
+    Add(key, kTypeMerge, value);
+  }
+
+  void AddDeletion(std::string key) { Add(key, kTypeDeletion, std::string()); }
+
+  void AddPut(std::string key, std::string value) {
+    Add(key, kTypeValue, value);
+  }
+
+  void Add(std::string key, ValueType type, std::string value) {
+    valid_ = true;
+    ParsedInternalKey internal_key(key, sequence_number_++, type);
+    data_.push_back(std::pair<std::string, std::string>(std::string(), value));
+    AppendInternalKey(&data_.back().first, internal_key);
+  }
+
+  // should be called before operations with iterator
+  void Finish() {
+    initialized_ = true;
+    std::sort(data_.begin(), data_.end(),
+              [this](std::pair<std::string, std::string> a,
+                     std::pair<std::string, std::string> b) {
+      return (cmp.Compare(a.first, b.first) < 0);
+    });
+  }
+
+  virtual bool Valid() const override {
+    assert(initialized_);
+    return valid_;
+  }
+
+  virtual void SeekToFirst() override {
+    assert(initialized_);
+    valid_ = (data_.size() > 0);
+    iter_ = 0;
+  }
+
+  virtual void SeekToLast() override {
+    assert(initialized_);
+    valid_ = (data_.size() > 0);
+    iter_ = data_.size() - 1;
+  }
+
+  virtual void Seek(const Slice& target) override {
+    assert(initialized_);
+    SeekToFirst();
+    if (!valid_) {
+      return;
+    }
+    while (iter_ < data_.size() &&
+           (cmp.Compare(data_[iter_].first, target) < 0)) {
+      ++iter_;
+    }
+
+    if (iter_ == data_.size()) {
+      valid_ = false;
+    }
+  }
+
+  virtual void Next() override {
+    assert(initialized_);
+    if (data_.empty() || (iter_ == data_.size() - 1)) {
+      valid_ = false;
+    } else {
+      ++iter_;
+    }
+  }
+
+  virtual void Prev() override {
+    assert(initialized_);
+    if (iter_ == 0) {
+      valid_ = false;
+    } else {
+      --iter_;
+    }
+  }
+
+  virtual Slice key() const override {
+    assert(initialized_);
+    return data_[iter_].first;
+  }
+
+  virtual Slice value() const override {
+    assert(initialized_);
+    return data_[iter_].second;
+  }
+
+  virtual Status status() const override {
+    assert(initialized_);
+    return Status::OK();
+  }
+
+ private:
+  bool initialized_;
+  bool valid_;
+  size_t sequence_number_;
+  size_t iter_;
+
+  InternalKeyComparator cmp;
+  std::vector<std::pair<std::string, std::string>> data_;
+};
+
+TEST(DBTest, DBIteratorPrevNext) {
+  Options options;
+  TestIterator internal_iter(BytewiseComparator());
+  internal_iter.AddDeletion("a");
+  internal_iter.AddDeletion("a");
+  internal_iter.AddDeletion("a");
+  internal_iter.AddDeletion("a");
+  internal_iter.AddPut("a", "val_a");
+
+  internal_iter.AddPut("b", "val_b");
+  internal_iter.Finish();
+
+  {
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 10);
+    db_iter->SeekToLast();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "b");
+    ASSERT_EQ(db_iter->value().ToString(), "val_b");
+
+    db_iter->Prev();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "a");
+    ASSERT_EQ(db_iter->value().ToString(), "val_a");
+
+    db_iter->Next();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "b");
+    ASSERT_EQ(db_iter->value().ToString(), "val_b");
+
+    db_iter->Next();
+    ASSERT_TRUE(!db_iter->Valid());
+  }
+
+  {
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 10);
+    db_iter->SeekToFirst();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "a");
+    ASSERT_EQ(db_iter->value().ToString(), "val_a");
+
+    db_iter->Next();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "b");
+    ASSERT_EQ(db_iter->value().ToString(), "val_b");
+
+    db_iter->Prev();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "a");
+    ASSERT_EQ(db_iter->value().ToString(), "val_a");
+
+    db_iter->Prev();
+    ASSERT_TRUE(!db_iter->Valid());
+  }
+
+  {
+    Options options;
+    TestIterator internal_iter(BytewiseComparator());
+    internal_iter.AddPut("a", "val_a");
+    internal_iter.AddPut("b", "val_b");
+
+    internal_iter.AddPut("a", "val_a");
+    internal_iter.AddPut("b", "val_b");
+
+    internal_iter.AddPut("a", "val_a");
+    internal_iter.AddPut("b", "val_b");
+
+    internal_iter.AddPut("a", "val_a");
+    internal_iter.AddPut("b", "val_b");
+
+    internal_iter.AddPut("a", "val_a");
+    internal_iter.AddPut("b", "val_b");
+    internal_iter.Finish();
+
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 2);
+    db_iter->SeekToLast();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "b");
+    ASSERT_EQ(db_iter->value().ToString(), "val_b");
+
+    db_iter->Next();
+    ASSERT_TRUE(!db_iter->Valid());
+
+    db_iter->SeekToLast();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "b");
+    ASSERT_EQ(db_iter->value().ToString(), "val_b");
+  }
+
+  {
+    Options options;
+    TestIterator internal_iter(BytewiseComparator());
+    internal_iter.AddPut("a", "val_a");
+    internal_iter.AddPut("a", "val_a");
+    internal_iter.AddPut("a", "val_a");
+    internal_iter.AddPut("a", "val_a");
+    internal_iter.AddPut("a", "val_a");
+
+    internal_iter.AddPut("b", "val_b");
+
+    internal_iter.AddPut("c", "val_c");
+    internal_iter.Finish();
+
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 10);
+    db_iter->SeekToLast();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "c");
+    ASSERT_EQ(db_iter->value().ToString(), "val_c");
+
+    db_iter->Prev();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "b");
+    ASSERT_EQ(db_iter->value().ToString(), "val_b");
+
+    db_iter->Next();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "c");
+    ASSERT_EQ(db_iter->value().ToString(), "val_c");
+  }
+}
+
+TEST(DBTest, DBIteratorEmpty) {
+  Options options = CurrentOptions();
+  TestIterator internal_iter(BytewiseComparator());
+  internal_iter.Finish();
+
+  {
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 0);
+    db_iter->SeekToLast();
+    ASSERT_TRUE(!db_iter->Valid());
+  }
+
+  {
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 0);
+    db_iter->SeekToFirst();
+    ASSERT_TRUE(!db_iter->Valid());
+  }
+}
+
+TEST(DBTest, DBIteratorUseSkipCountSkips) {
+  Options options = CurrentOptions();
+  options.statistics = rocksdb::CreateDBStatistics();
+  options.merge_operator = MergeOperators::CreateFromStringId("stringappend");
+
+  TestIterator internal_iter(BytewiseComparator());
+  for (size_t i = 0; i < 200; ++i) {
+    internal_iter.AddPut("a", "a");
+    internal_iter.AddPut("b", "b");
+    internal_iter.AddPut("c", "c");
+  }
+  internal_iter.Finish();
+
+  auto db_iter =
+      NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 2);
+  db_iter->SeekToLast();
+  ASSERT_TRUE(db_iter->Valid());
+  ASSERT_EQ(db_iter->key().ToString(), "c");
+  ASSERT_EQ(db_iter->value().ToString(), "c");
+  ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 1);
+
+  db_iter->Prev();
+  ASSERT_TRUE(db_iter->Valid());
+  ASSERT_EQ(db_iter->key().ToString(), "b");
+  ASSERT_EQ(db_iter->value().ToString(), "b");
+  ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 2);
+
+  db_iter->Prev();
+  ASSERT_TRUE(db_iter->Valid());
+  ASSERT_EQ(db_iter->key().ToString(), "a");
+  ASSERT_EQ(db_iter->value().ToString(), "a");
+  ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 3);
+
+  db_iter->Prev();
+  ASSERT_TRUE(!db_iter->Valid());
+  ASSERT_EQ(TestGetTickerCount(options, NUMBER_OF_RESEEKS_IN_ITERATION), 3);
+}
+
+TEST(DBTest, DBIteratorUseSkip) {
+  Options options = CurrentOptions();
+  options.merge_operator = MergeOperators::CreateFromStringId("stringappend");
+  {
+    TestIterator internal_iter(BytewiseComparator());
+    internal_iter.AddMerge("b", "merge_1");
+    internal_iter.AddMerge("a", "merge_2");
+    for (size_t i = 0; i < 200; ++i) {
+      internal_iter.AddPut("c", std::to_string(i));
+    }
+    internal_iter.Finish();
+
+    for (size_t i = 0; i < 200; ++i) {
+      options.statistics = rocksdb::CreateDBStatistics();
+      auto db_iter = NewDBIterator(env_, options, BytewiseComparator(),
+                                   &internal_iter, i + 2);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "c");
+      ASSERT_EQ(db_iter->value().ToString(), std::to_string(i));
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_2");
+      db_iter->Prev();
+
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+  }
+
+  {
+    TestIterator internal_iter(BytewiseComparator());
+    internal_iter.AddMerge("b", "merge_1");
+    internal_iter.AddMerge("a", "merge_2");
+    for (size_t i = 0; i < 200; ++i) {
+      internal_iter.AddDeletion("c");
+    }
+    internal_iter.AddPut("c", "200");
+    internal_iter.Finish();
+
+    for (size_t i = 0; i < 200; ++i) {
+      auto db_iter = NewDBIterator(env_, options, BytewiseComparator(),
+                                   &internal_iter, i + 2);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_2");
+      db_iter->Prev();
+
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter = NewDBIterator(env_, options, BytewiseComparator(),
+                                   &internal_iter, 202);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "c");
+      ASSERT_EQ(db_iter->value().ToString(), "200");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_2");
+      db_iter->Prev();
+
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+  }
+
+  {
+    TestIterator internal_iter(BytewiseComparator());
+    for (size_t i = 0; i < 200; ++i) {
+      internal_iter.AddDeletion("c");
+    }
+    internal_iter.AddPut("c", "200");
+    internal_iter.Finish();
+
+    for (size_t i = 0; i < 200; ++i) {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, i);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(!db_iter->Valid());
+
+      db_iter->SeekToFirst();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 200);
+    db_iter->SeekToLast();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "c");
+    ASSERT_EQ(db_iter->value().ToString(), "200");
+
+    db_iter->Prev();
+    ASSERT_TRUE(!db_iter->Valid());
+
+    db_iter->SeekToFirst();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "c");
+    ASSERT_EQ(db_iter->value().ToString(), "200");
+
+    db_iter->Next();
+    ASSERT_TRUE(!db_iter->Valid());
+  }
+
+  {
+    TestIterator internal_iter(BytewiseComparator());
+    internal_iter.AddMerge("b", "merge_1");
+    internal_iter.AddMerge("a", "merge_2");
+    for (size_t i = 0; i < 200; ++i) {
+      internal_iter.AddPut("d", std::to_string(i));
+    }
+
+    for (size_t i = 0; i < 200; ++i) {
+      internal_iter.AddPut("c", std::to_string(i));
+    }
+    internal_iter.Finish();
+
+    for (size_t i = 0; i < 200; ++i) {
+      auto db_iter = NewDBIterator(env_, options, BytewiseComparator(),
+                                   &internal_iter, i + 2);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "d");
+      ASSERT_EQ(db_iter->value().ToString(), std::to_string(i));
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_2");
+      db_iter->Prev();
+
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+  }
+
+  {
+    TestIterator internal_iter(BytewiseComparator());
+    internal_iter.AddMerge("b", "b");
+    internal_iter.AddMerge("a", "a");
+    for (size_t i = 0; i < 200; ++i) {
+      internal_iter.AddMerge("c", std::to_string(i));
+    }
+    internal_iter.Finish();
+
+    for (size_t i = 0; i < 200; ++i) {
+      auto db_iter = NewDBIterator(env_, options, BytewiseComparator(),
+                                   &internal_iter, i + 2);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "c");
+      std::string merge_result = "0";
+      for (size_t j = 1; j <= i; ++j) {
+        merge_result += "," + std::to_string(j);
+      }
+      ASSERT_EQ(db_iter->value().ToString(), merge_result);
+
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(), "b");
+
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "a");
+
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+  }
+}
+
+TEST(DBTest, DBIterator) {
+  Options options = CurrentOptions();
+  options.merge_operator = MergeOperators::CreateFromStringId("stringappend");
+  TestIterator internal_iter(BytewiseComparator());
+  internal_iter.AddPut("a", "0");
+  internal_iter.AddPut("b", "0");
+  internal_iter.AddDeletion("b");
+  internal_iter.AddMerge("a", "1");
+  internal_iter.AddMerge("b", "2");
+  internal_iter.Finish();
+  {
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 1);
+    db_iter->SeekToFirst();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "a");
+    ASSERT_EQ(db_iter->value().ToString(), "0");
+    db_iter->Next();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "b");
+  }
+
+  {
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 0);
+    db_iter->SeekToFirst();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "a");
+    ASSERT_EQ(db_iter->value().ToString(), "0");
+    db_iter->Next();
+    ASSERT_TRUE(!db_iter->Valid());
+  }
+
+  {
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 2);
+    db_iter->SeekToFirst();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "a");
+    ASSERT_EQ(db_iter->value().ToString(), "0");
+    db_iter->Next();
+    ASSERT_TRUE(!db_iter->Valid());
+  }
+
+  {
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 4);
+    db_iter->SeekToFirst();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "a");
+    ASSERT_EQ(db_iter->value().ToString(), "0,1");
+    db_iter->Next();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "b");
+    ASSERT_EQ(db_iter->value().ToString(), "2");
+    db_iter->Next();
+    ASSERT_TRUE(!db_iter->Valid());
+  }
+
+  {
+    TestIterator internal_iter(BytewiseComparator());
+    internal_iter.AddMerge("a", "merge_1");
+    internal_iter.AddMerge("a", "merge_2");
+    internal_iter.AddMerge("a", "merge_3");
+    internal_iter.AddPut("a", "put_1");
+    internal_iter.AddMerge("a", "merge_4");
+    internal_iter.AddMerge("a", "merge_5");
+    internal_iter.AddMerge("a", "merge_6");
+    internal_iter.Finish();
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 0);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 1);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1,merge_2");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 2);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1,merge_2,merge_3");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 3);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "put_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 4);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "put_1,merge_4");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 5);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "put_1,merge_4,merge_5");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 6);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "put_1,merge_4,merge_5,merge_6");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+  }
+
+  {
+    TestIterator internal_iter(BytewiseComparator());
+    internal_iter.AddMerge("a", "merge_1");
+    internal_iter.AddMerge("a", "merge_2");
+    internal_iter.AddMerge("a", "merge_3");
+    internal_iter.AddDeletion("a");
+    internal_iter.AddMerge("a", "merge_4");
+    internal_iter.AddMerge("a", "merge_5");
+    internal_iter.AddMerge("a", "merge_6");
+    internal_iter.Finish();
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 0);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 1);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1,merge_2");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 2);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1,merge_2,merge_3");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 3);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 4);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_4");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 5);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_4,merge_5");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 6);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_4,merge_5,merge_6");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+  }
+
+  {
+    TestIterator internal_iter(BytewiseComparator());
+    internal_iter.AddMerge("a", "merge_1");
+    internal_iter.AddPut("b", "val");
+    internal_iter.AddMerge("b", "merge_2");
+
+    internal_iter.AddDeletion("b");
+    internal_iter.AddMerge("b", "merge_3");
+
+    internal_iter.AddMerge("c", "merge_4");
+    internal_iter.AddMerge("c", "merge_5");
+
+    internal_iter.AddDeletion("b");
+    internal_iter.AddMerge("b", "merge_6");
+    internal_iter.AddMerge("b", "merge_7");
+    internal_iter.AddMerge("b", "merge_8");
+    internal_iter.AddMerge("b", "merge_9");
+    internal_iter.AddMerge("b", "merge_10");
+    internal_iter.AddMerge("b", "merge_11");
+
+    internal_iter.AddDeletion("c");
+    internal_iter.Finish();
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 0);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 2);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(), "val,merge_2");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 4);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_3");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 5);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "c");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_4");
+      db_iter->Prev();
+
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_3");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 6);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "c");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_4,merge_5");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_3");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 7);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "c");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_4,merge_5");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter =
+          NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 9);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "c");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_4,merge_5");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_6,merge_7");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter = NewDBIterator(env_, options, BytewiseComparator(),
+                                   &internal_iter, 13);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "c");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_4,merge_5");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_TRUE(db_iter->Valid());
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(),
+                "merge_6,merge_7,merge_8,merge_9,merge_10,merge_11");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+
+    {
+      auto db_iter = NewDBIterator(env_, options, BytewiseComparator(),
+                                   &internal_iter, 14);
+      db_iter->SeekToLast();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "b");
+      ASSERT_EQ(db_iter->value().ToString(),
+                "merge_6,merge_7,merge_8,merge_9,merge_10,merge_11");
+      db_iter->Prev();
+      ASSERT_TRUE(db_iter->Valid());
+
+      ASSERT_EQ(db_iter->key().ToString(), "a");
+      ASSERT_EQ(db_iter->value().ToString(), "merge_1");
+      db_iter->Prev();
+      ASSERT_TRUE(!db_iter->Valid());
+    }
+  }
+
+  {
+    Options options = CurrentOptions();
+    TestIterator internal_iter(BytewiseComparator());
+    internal_iter.AddDeletion("a");
+    internal_iter.AddPut("a", "0");
+    internal_iter.AddPut("b", "0");
+    internal_iter.Finish();
+
+    auto db_iter =
+        NewDBIterator(env_, options, BytewiseComparator(), &internal_iter, 10);
+    db_iter->SeekToLast();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "b");
+    ASSERT_EQ(db_iter->value().ToString(), "0");
+
+    db_iter->Prev();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "a");
+    ASSERT_EQ(db_iter->value().ToString(), "0");
+  }
+}
+
 // Make sure that when options.block_cache is set, after a new table is
 // created its index/filter blocks are added to block cache.
 TEST(DBTest, IndexAndFilterBlocksOfNewTableAddedToCache) {