diff --git a/HISTORY.md b/HISTORY.md
index c7a9055b8..0cf2efc06 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -5,16 +5,18 @@
 * Added single delete operation as a more efficient way to delete keys that have not been overwritten.
 * Added experimental AddFile() to DB interface that allow users to add files created by SstFileWriter into an empty Database, see include/rocksdb/sst_file_writer.h and DB::AddFile() for more info.
 * Added support for opening SST files with .ldb suffix which enables opening LevelDB databases.
+* CompactionFilter now supports filtering of merge operands and merge results.
 
 ### Public API Changes
 * Added SingleDelete() to the DB interface.
 * Added AddFile() to DB interface.
 * Added SstFileWriter class.
+* CompactionFilter has a new method FilterMergeOperand() that RocksDB applies to every merge operand during compaction to decide whether to filter the operand.
 
 ## 4.0.0 (9/9/2015)
 ### New Features
 * Added support for transactions.  See include/rocksdb/utilities/transaction.h for more info.
-* DB::GetProperty() now accept "rocksdb.aggregated-table-properties" and "rocksdb.aggregated-table-properties-at-levelN", in which case it returns aggregated table properties of the target column family, or the aggregated table properties of the specified level N if the "at-level" version is used.
+* DB::GetProperty() now accepts "rocksdb.aggregated-table-properties" and "rocksdb.aggregated-table-properties-at-levelN", in which case it returns aggregated table properties of the target column family, or the aggregated table properties of the specified level N if the "at-level" version is used.
 * Add compression option kZSTDNotFinalCompression for people to experiment ZSTD although its format is not finalized.
 * We removed the need for LATEST_BACKUP file in BackupEngine. We still keep writing it when we create new backups (because of backward compatibility), but we don't read it anymore.
 
diff --git a/db/builder.cc b/db/builder.cc
index 388c94126..3d07a0f30 100644
--- a/db/builder.cc
+++ b/db/builder.cc
@@ -85,10 +85,11 @@ Status BuildTable(
           file_writer.get(), compression, compression_opts);
     }
 
-    MergeHelper merge(internal_comparator.user_comparator(),
-                      ioptions.merge_operator, ioptions.info_log,
+    MergeHelper merge(env, internal_comparator.user_comparator(),
+                      ioptions.merge_operator, nullptr, ioptions.info_log,
                       ioptions.min_partial_merge_operands,
-                      true /* internal key corruption is not ok */);
+                      true /* internal key corruption is not ok */,
+                      snapshots.empty() ? 0 : snapshots.back());
 
     CompactionIterator c_iter(iter, internal_comparator.user_comparator(),
                               &merge, kMaxSequenceNumber, &snapshots, env,
diff --git a/db/compaction_iterator.cc b/db/compaction_iterator.cc
index e6f1bb60a..d242291dd 100644
--- a/db/compaction_iterator.cc
+++ b/db/compaction_iterator.cc
@@ -12,16 +12,14 @@ namespace rocksdb {
 CompactionIterator::CompactionIterator(
     Iterator* input, const Comparator* cmp, MergeHelper* merge_helper,
     SequenceNumber last_sequence, std::vector<SequenceNumber>* snapshots,
-    Env* env, bool expect_valid_internal_key, Statistics* stats,
-    Compaction* compaction, const CompactionFilter* compaction_filter,
-    LogBuffer* log_buffer)
+    Env* env, bool expect_valid_internal_key, Compaction* compaction,
+    const CompactionFilter* compaction_filter, LogBuffer* log_buffer)
     : input_(input),
       cmp_(cmp),
       merge_helper_(merge_helper),
       snapshots_(snapshots),
       env_(env),
       expect_valid_internal_key_(expect_valid_internal_key),
-      stats_(stats),
       compaction_(compaction),
       compaction_filter_(compaction_filter),
       log_buffer_(log_buffer),
@@ -277,24 +275,30 @@ void CompactionIterator::NextFromInput() {
       // have hit (A)
       // We encapsulate the merge related state machine in a different
       // object to minimize change to the existing flow.
-      merge_helper_->MergeUntil(input_, prev_snapshot, bottommost_level_,
-                                stats_, env_);
+      merge_helper_->MergeUntil(input_, prev_snapshot, bottommost_level_);
       merge_out_iter_.SeekToFirst();
 
-      // NOTE: key, value, and ikey_ refer to old entries.
-      //       These will be correctly set below.
-      key_ = merge_out_iter_.key();
-      value_ = merge_out_iter_.value();
-      bool valid_key __attribute__((__unused__)) =
-          ParseInternalKey(key_, &ikey_);
-      // MergeUntil stops when it encounters a corrupt key and does not
-      // include them in the result, so we expect the keys here to valid.
-      assert(valid_key);
-      // Keep current_key_ in sync.
-      current_key_.UpdateInternalKey(ikey_.sequence, ikey_.type);
-      key_ = current_key_.GetKey();
-      ikey_.user_key = current_key_.GetUserKey();
-      valid_ = true;
+      if (merge_out_iter_.Valid()) {
+        // NOTE: key, value, and ikey_ refer to old entries.
+        //       These will be correctly set below.
+        key_ = merge_out_iter_.key();
+        value_ = merge_out_iter_.value();
+        bool valid_key __attribute__((__unused__)) =
+            ParseInternalKey(key_, &ikey_);
+        // MergeUntil stops when it encounters a corrupt key and does not
+        // include them in the result, so we expect the keys here to valid.
+        assert(valid_key);
+        // Keep current_key_ in sync.
+        current_key_.UpdateInternalKey(ikey_.sequence, ikey_.type);
+        key_ = current_key_.GetKey();
+        ikey_.user_key = current_key_.GetUserKey();
+        valid_ = true;
+      } else {
+        // all merge operands were filtered out. reset the user key, since the
+        // batch consumed by the merge operator should not shadow any keys
+        // coming after the merges
+        has_current_user_key_ = false;
+      }
     } else {
       valid_ = true;
     }
diff --git a/db/compaction_iterator.h b/db/compaction_iterator.h
index 8229f2f00..da242f6aa 100644
--- a/db/compaction_iterator.h
+++ b/db/compaction_iterator.h
@@ -41,7 +41,6 @@ class CompactionIterator {
                      MergeHelper* merge_helper, SequenceNumber last_sequence,
                      std::vector<SequenceNumber>* snapshots, Env* env,
                      bool expect_valid_internal_key,
-                     Statistics* stats = nullptr,
                      Compaction* compaction = nullptr,
                      const CompactionFilter* compaction_filter = nullptr,
                      LogBuffer* log_buffer = nullptr);
@@ -91,7 +90,6 @@ class CompactionIterator {
   const std::vector<SequenceNumber>* snapshots_;
   Env* env_;
   bool expect_valid_internal_key_;
-  Statistics* stats_;
   Compaction* compaction_;
   const CompactionFilter* compaction_filter_;
   LogBuffer* log_buffer_;
diff --git a/db/compaction_iterator_test.cc b/db/compaction_iterator_test.cc
index 8b8a3c9a4..1148c2ac7 100644
--- a/db/compaction_iterator_test.cc
+++ b/db/compaction_iterator_test.cc
@@ -16,7 +16,8 @@ class CompactionIteratorTest : public testing::Test {
   void InitIterator(const std::vector<std::string>& ks,
                     const std::vector<std::string>& vs,
                     SequenceNumber last_sequence) {
-    merge_helper_.reset(new MergeHelper(cmp_, nullptr, nullptr, 0U, false));
+    merge_helper_.reset(new MergeHelper(Env::Default(), cmp_, nullptr, nullptr,
+                                        nullptr, 0U, false, 0));
     iter_.reset(new test::VectorIterator(ks, vs));
     iter_->SeekToFirst();
     c_iter_.reset(new CompactionIterator(iter_.get(), cmp_, merge_helper_.get(),
diff --git a/db/compaction_job.cc b/db/compaction_job.cc
index 691aa2f62..4d6656d4e 100644
--- a/db/compaction_job.cc
+++ b/db/compaction_job.cc
@@ -597,10 +597,6 @@ void CompactionJob::ProcessKeyValueCompaction(SubcompactionState* sub_compact) {
   }
 
   ColumnFamilyData* cfd = sub_compact->compaction->column_family_data();
-  MergeHelper merge(cfd->user_comparator(), cfd->ioptions()->merge_operator,
-                    db_options_.info_log.get(),
-                    cfd->ioptions()->min_partial_merge_operands,
-                    false /* internal key corruption is expected */);
   auto compaction_filter = cfd->ioptions()->compaction_filter;
   std::unique_ptr<CompactionFilter> compaction_filter_from_factory = nullptr;
   if (compaction_filter == nullptr) {
@@ -608,6 +604,13 @@ void CompactionJob::ProcessKeyValueCompaction(SubcompactionState* sub_compact) {
         sub_compact->compaction->CreateCompactionFilter();
     compaction_filter = compaction_filter_from_factory.get();
   }
+  MergeHelper merge(
+      env_, cfd->user_comparator(), cfd->ioptions()->merge_operator,
+      compaction_filter, db_options_.info_log.get(),
+      cfd->ioptions()->min_partial_merge_operands,
+      false /* internal key corruption is expected */,
+      existing_snapshots_.empty() ? 0 : existing_snapshots_.back(),
+      compact_->compaction->level(), db_options_.statistics.get());
 
   TEST_SYNC_POINT("CompactionJob::Run():Inprogress");
 
@@ -624,8 +627,8 @@ void CompactionJob::ProcessKeyValueCompaction(SubcompactionState* sub_compact) {
   Status status;
   sub_compact->c_iter.reset(new CompactionIterator(
       input.get(), cfd->user_comparator(), &merge, versions_->LastSequence(),
-      &existing_snapshots_, env_, false, db_options_.statistics.get(),
-      sub_compact->compaction, compaction_filter));
+      &existing_snapshots_, env_, false, sub_compact->compaction,
+      compaction_filter));
   auto c_iter = sub_compact->c_iter.get();
   c_iter->SeekToFirst();
   const auto& c_iter_stats = c_iter->iter_stats();
diff --git a/db/compaction_job_test.cc b/db/compaction_job_test.cc
index 795607afe..1e57c9c36 100644
--- a/db/compaction_job_test.cc
+++ b/db/compaction_job_test.cc
@@ -182,7 +182,7 @@ class CompactionJobTest : public testing::Test {
     return expected_results;
   }
 
-  void NewDB(std::shared_ptr<MergeOperator> merge_operator = nullptr) {
+  void NewDB() {
     VersionEdit new_db;
     new_db.SetLogNumber(0);
     new_db.SetNextFile(2);
@@ -207,7 +207,8 @@ class CompactionJobTest : public testing::Test {
 
     std::vector<ColumnFamilyDescriptor> column_families;
     cf_options_.table_factory = mock_table_factory_;
-    cf_options_.merge_operator = merge_operator;
+    cf_options_.merge_operator = merge_op_;
+    cf_options_.compaction_filter = compaction_filter_.get();
     column_families.emplace_back(kDefaultColumnFamilyName, cf_options_);
 
     EXPECT_OK(versions_->Recover(column_families, false));
@@ -258,10 +259,16 @@ class CompactionJobTest : public testing::Test {
                                      &mutex_));
     mutex_.Unlock();
 
-    ASSERT_GE(compaction_job_stats_.elapsed_micros, 0U);
-    ASSERT_EQ(compaction_job_stats_.num_input_files, num_input_files);
-    ASSERT_EQ(compaction_job_stats_.num_output_files, 1U);
-    mock_table_factory_->AssertLatestFile(expected_results);
+    if (expected_results.size() == 0) {
+      ASSERT_GE(compaction_job_stats_.elapsed_micros, 0U);
+      ASSERT_EQ(compaction_job_stats_.num_input_files, num_input_files);
+      ASSERT_EQ(compaction_job_stats_.num_output_files, 0U);
+    } else {
+      ASSERT_GE(compaction_job_stats_.elapsed_micros, 0U);
+      ASSERT_EQ(compaction_job_stats_.num_input_files, num_input_files);
+      ASSERT_EQ(compaction_job_stats_.num_output_files, 1U);
+      mock_table_factory_->AssertLatestFile(expected_results);
+    }
   }
 
   Env* env_;
@@ -279,6 +286,8 @@ class CompactionJobTest : public testing::Test {
   std::shared_ptr<mock::MockTableFactory> mock_table_factory_;
   CompactionJobStats compaction_job_stats_;
   ColumnFamilyData* cfd_;
+  std::unique_ptr<CompactionFilter> compaction_filter_;
+  std::shared_ptr<MergeOperator> merge_op_;
 };
 
 TEST_F(CompactionJobTest, Simple) {
@@ -297,7 +306,7 @@ TEST_F(CompactionJobTest, SimpleCorrupted) {
   auto expected_results = CreateTwoFiles(true);
   auto cfd = versions_->GetColumnFamilySet()->GetDefault();
   auto files = cfd->current()->storage_info()->LevelFiles(0);
-  RunCompaction({ files }, expected_results);
+  RunCompaction({files}, expected_results);
   ASSERT_EQ(compaction_job_stats_.num_corrupt_keys, 400U);
 }
 
@@ -317,7 +326,7 @@ TEST_F(CompactionJobTest, SimpleDeletion) {
 
   SetLastSequence(4U);
   auto files = cfd_->current()->storage_info()->LevelFiles(0);
-  RunCompaction({ files }, expected_results);
+  RunCompaction({files}, expected_results);
 }
 
 TEST_F(CompactionJobTest, SimpleOverwrite) {
@@ -339,7 +348,7 @@ TEST_F(CompactionJobTest, SimpleOverwrite) {
 
   SetLastSequence(4U);
   auto files = cfd_->current()->storage_info()->LevelFiles(0);
-  RunCompaction({ files }, expected_results);
+  RunCompaction({files}, expected_results);
 }
 
 TEST_F(CompactionJobTest, SimpleNonLastLevel) {
@@ -368,12 +377,12 @@ TEST_F(CompactionJobTest, SimpleNonLastLevel) {
   SetLastSequence(6U);
   auto lvl0_files = cfd_->current()->storage_info()->LevelFiles(0);
   auto lvl1_files = cfd_->current()->storage_info()->LevelFiles(1);
-  RunCompaction({ lvl0_files, lvl1_files }, expected_results);
+  RunCompaction({lvl0_files, lvl1_files}, expected_results);
 }
 
 TEST_F(CompactionJobTest, SimpleMerge) {
-  auto merge_op = MergeOperators::CreateStringAppendOperator();
-  NewDB(merge_op);
+  merge_op_ = MergeOperators::CreateStringAppendOperator();
+  NewDB();
 
   auto file1 = mock::MakeMockFile({
       {KeyStr("a", 5U, kTypeMerge), "5"},
@@ -392,12 +401,12 @@ TEST_F(CompactionJobTest, SimpleMerge) {
 
   SetLastSequence(5U);
   auto files = cfd_->current()->storage_info()->LevelFiles(0);
-  RunCompaction({ files }, expected_results);
+  RunCompaction({files}, expected_results);
 }
 
 TEST_F(CompactionJobTest, NonAssocMerge) {
-  auto merge_op = MergeOperators::CreateStringAppendTESTOperator();
-  NewDB(merge_op);
+  merge_op_ = MergeOperators::CreateStringAppendTESTOperator();
+  NewDB();
 
   auto file1 = mock::MakeMockFile({
       {KeyStr("a", 5U, kTypeMerge), "5"},
@@ -417,7 +426,104 @@ TEST_F(CompactionJobTest, NonAssocMerge) {
 
   SetLastSequence(5U);
   auto files = cfd_->current()->storage_info()->LevelFiles(0);
-  RunCompaction({ files }, expected_results);
+  RunCompaction({files}, expected_results);
+}
+
+// Filters merge operands with value 10.
+TEST_F(CompactionJobTest, MergeOperandFilter) {
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
+  compaction_filter_.reset(new test::FilterNumber(10U));
+  NewDB();
+
+  auto file1 = mock::MakeMockFile(
+      {{KeyStr("a", 5U, kTypeMerge), test::EncodeInt(5U)},
+       {KeyStr("a", 4U, kTypeMerge), test::EncodeInt(10U)},  // Filtered
+       {KeyStr("a", 3U, kTypeMerge), test::EncodeInt(3U)}});
+  AddMockFile(file1);
+
+  auto file2 = mock::MakeMockFile({
+      {KeyStr("b", 2U, kTypeMerge), test::EncodeInt(2U)},
+      {KeyStr("b", 1U, kTypeMerge), test::EncodeInt(10U)}  // Filtered
+  });
+  AddMockFile(file2);
+
+  auto expected_results =
+      mock::MakeMockFile({{KeyStr("a", 0U, kTypeValue), test::EncodeInt(8U)},
+                          {KeyStr("b", 2U, kTypeMerge), test::EncodeInt(2U)}});
+
+  SetLastSequence(5U);
+  auto files = cfd_->current()->storage_info()->LevelFiles(0);
+  RunCompaction({files}, expected_results);
+}
+
+TEST_F(CompactionJobTest, FilterSomeMergeOperands) {
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
+  compaction_filter_.reset(new test::FilterNumber(10U));
+  NewDB();
+
+  auto file1 = mock::MakeMockFile(
+      {{KeyStr("a", 5U, kTypeMerge), test::EncodeInt(5U)},
+       {KeyStr("a", 4U, kTypeMerge), test::EncodeInt(10U)},  // Filtered
+       {KeyStr("a", 3U, kTypeValue), test::EncodeInt(5U)},
+       {KeyStr("d", 8U, kTypeMerge), test::EncodeInt(10U)}});
+  AddMockFile(file1);
+
+  auto file2 =
+      mock::MakeMockFile({{KeyStr("b", 2U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("b", 1U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("c", 2U, kTypeMerge), test::EncodeInt(3U)},
+                          {KeyStr("c", 1U, kTypeValue), test::EncodeInt(7U)},
+                          {KeyStr("d", 1U, kTypeValue), test::EncodeInt(6U)}});
+  AddMockFile(file2);
+
+  auto file3 =
+      mock::MakeMockFile({{KeyStr("a", 1U, kTypeMerge), test::EncodeInt(3U)}});
+  AddMockFile(file3, 2);
+
+  auto expected_results = mock::MakeMockFile({
+      {KeyStr("a", 5U, kTypeValue), test::EncodeInt(10U)},
+      {KeyStr("c", 2U, kTypeValue), test::EncodeInt(10U)},
+      {KeyStr("d", 1U, kTypeValue), test::EncodeInt(6U)}
+      // b does not appear because the operands are filtered
+  });
+
+  SetLastSequence(5U);
+  auto files = cfd_->current()->storage_info()->LevelFiles(0);
+  RunCompaction({files}, expected_results);
+}
+
+// Test where all operands/merge results are filtered out.
+TEST_F(CompactionJobTest, FilterAllMergeOperands) {
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
+  compaction_filter_.reset(new test::FilterNumber(10U));
+  NewDB();
+
+  auto file1 =
+      mock::MakeMockFile({{KeyStr("a", 11U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("a", 10U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("a", 9U, kTypeMerge), test::EncodeInt(10U)}});
+  AddMockFile(file1);
+
+  auto file2 =
+      mock::MakeMockFile({{KeyStr("b", 8U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("b", 7U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("b", 6U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("b", 5U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("b", 4U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("b", 3U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("b", 2U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("c", 2U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("c", 1U, kTypeMerge), test::EncodeInt(10U)}});
+  AddMockFile(file2);
+
+  auto file3 =
+      mock::MakeMockFile({{KeyStr("a", 2U, kTypeMerge), test::EncodeInt(10U)},
+                          {KeyStr("b", 1U, kTypeMerge), test::EncodeInt(10U)}});
+  AddMockFile(file3, 2);
+
+  SetLastSequence(11U);
+  auto files = cfd_->current()->storage_info()->LevelFiles(0);
+  RunCompaction({files}, {});
 }
 
 TEST_F(CompactionJobTest, SimpleSingleDelete) {
diff --git a/db/merge_helper.cc b/db/merge_helper.cc
index 325fc33fa..f9cb67e9c 100644
--- a/db/merge_helper.cc
+++ b/db/merge_helper.cc
@@ -14,7 +14,6 @@
 #include "rocksdb/merge_operator.h"
 #include "util/perf_context_imp.h"
 #include "util/statistics.h"
-#include "util/stop_watch.h"
 
 namespace rocksdb {
 
@@ -41,8 +40,7 @@ Status MergeHelper::TimedFullMerge(const Slice& key, const Slice* value,
   bool success =
       merge_operator->FullMerge(key, value, operands, result, logger);
 
-  RecordTick(statistics, MERGE_OPERATION_TOTAL_TIME,
-             env != nullptr ? timer.ElapsedNanos() : 0);
+  RecordTick(statistics, MERGE_OPERATION_TOTAL_TIME, timer.ElapsedNanosSafe());
 
   if (!success) {
     RecordTick(statistics, NUMBER_MERGE_FAILURES);
@@ -59,30 +57,33 @@ Status MergeHelper::TimedFullMerge(const Slice& key, const Slice* value,
 //       operands_ stores the list of merge operands encountered while merging.
 //       keys_[i] corresponds to operands_[i] for each i.
 Status MergeHelper::MergeUntil(Iterator* iter, const SequenceNumber stop_before,
-                               const bool at_bottom, Statistics* stats,
-                               Env* env_) {
+                               const bool at_bottom) {
   // Get a copy of the internal key, before it's invalidated by iter->Next()
   // Also maintain the list of merge operands seen.
   assert(HasOperator());
   keys_.clear();
   operands_.clear();
-  keys_.push_front(iter->key().ToString());
-  operands_.push_front(iter->value().ToString());
   assert(user_merge_operator_);
+  bool first_key = true;
 
   // We need to parse the internal key again as the parsed key is
   // backed by the internal key!
   // Assume no internal key corruption as it has been successfully parsed
   // by the caller.
-  // Invariant: keys_.back() will not change. Hence, orig_ikey is always valid.
+  // original_key_is_iter variable is just caching the information:
+  // original_key_is_iter == (iter->key().ToString() == original_key)
+  bool original_key_is_iter = true;
+  std::string original_key = iter->key().ToString();
+  // Important:
+  // orig_ikey is backed by original_key if keys_.empty()
+  // orig_ikey is backed by keys_.back() if !keys_.empty()
   ParsedInternalKey orig_ikey;
-  ParseInternalKey(keys_.back(), &orig_ikey);
+  ParseInternalKey(original_key, &orig_ikey);
 
   Status s;
   bool hit_the_next_user_key = false;
-  for (iter->Next(); iter->Valid(); iter->Next()) {
+  for (; iter->Valid(); iter->Next(), original_key_is_iter = false) {
     ParsedInternalKey ikey;
-    assert(operands_.size() >= 1);        // Should be invariants!
     assert(keys_.size() == operands_.size());
 
     if (!ParseInternalKey(iter->key(), &ikey)) {
@@ -92,6 +93,9 @@ Status MergeHelper::MergeUntil(Iterator* iter, const SequenceNumber stop_before,
         return Status::Corruption("Corrupted internal key not expected.");
       }
       break;
+    } else if (first_key) {
+      assert(user_comparator_->Equal(ikey.user_key, orig_ikey.user_key));
+      first_key = false;
     } else if (!user_comparator_->Equal(ikey.user_key, orig_ikey.user_key)) {
       // hit a different user key, stop right here
       hit_the_next_user_key = true;
@@ -105,16 +109,29 @@ Status MergeHelper::MergeUntil(Iterator* iter, const SequenceNumber stop_before,
 
     assert(IsValueType(ikey.type));
     if (ikey.type != kTypeMerge) {
-      // Merges operands can only be used with puts and deletions, single
-      // deletions are not supported.
-      assert(ikey.type == kTypeValue || ikey.type == kTypeDeletion);
+      if (ikey.type != kTypeValue && ikey.type != kTypeDeletion) {
+        // Merges operands can only be used with puts and deletions, single
+        // deletions are not supported.
+        assert(false);
+        // release build doesn't have asserts, so we return error status
+        return Status::InvalidArgument(
+            " Merges operands can only be used with puts and deletions, single "
+            "deletions are not supported.");
+      }
 
       // hit a put/delete
       //   => merge the put value or a nullptr with operands_
       //   => store result in operands_.back() (and update keys_.back())
       //   => change the entry type to kTypeValue for keys_.back()
       // We are done! Success!
-      //
+
+      // If there are no operands, just return the Status::OK(). That will cause
+      // the compaction iterator to write out the key we're currently at, which
+      // is the put/delete we just encountered.
+      if (keys_.empty()) {
+        return Status::OK();
+      }
+
       // TODO(noetzli) If the merge operator returns false, we are currently
       // (almost) silently dropping the put/delete. That's probably not what we
       // want.
@@ -122,14 +139,14 @@ Status MergeHelper::MergeUntil(Iterator* iter, const SequenceNumber stop_before,
       const Slice* val_ptr = (kTypeValue == ikey.type) ? &val : nullptr;
       std::string merge_result;
       s = TimedFullMerge(ikey.user_key, val_ptr, operands_,
-                         user_merge_operator_, stats, env_, logger_,
+                         user_merge_operator_, stats_, env_, logger_,
                          &merge_result);
 
       // We store the result in keys_.back() and operands_.back()
       // if nothing went wrong (i.e.: no operand corruption on disk)
       if (s.ok()) {
         // The original key encountered
-        std::string original_key = std::move(keys_.back());
+        original_key = std::move(keys_.back());
         orig_ikey.type = kTypeValue;
         UpdateInternalKey(&original_key, orig_ikey.sequence, orig_ikey.type);
         keys_.clear();
@@ -143,18 +160,42 @@ Status MergeHelper::MergeUntil(Iterator* iter, const SequenceNumber stop_before,
       return s;
     } else {
       // hit a merge
+      //   => if there is a compaction filter, apply it.
       //   => merge the operand into the front of the operands_ list
-      //   => use the user's associative merge function to determine how.
+      //      if not filtered
       //   => then continue because we haven't yet seen a Put/Delete.
-      assert(!operands_.empty()); // Should have at least one element in it
-
-      // keep queuing keys and operands until we either meet a put / delete
+      //
+      // Keep queuing keys and operands until we either meet a put / delete
       // request or later did a partial merge.
-      keys_.push_front(iter->key().ToString());
-      operands_.push_front(iter->value().ToString());
+
+      Slice value_slice = iter->value();
+      // add an operand to the list if:
+      // 1) it's included in one of the snapshots. in that case we *must* write
+      // it out, no matter what compaction filter says
+      // 2) it's not filtered by a compaction filter
+      if (ikey.sequence <= latest_snapshot_ ||
+          !FilterMerge(orig_ikey.user_key, value_slice)) {
+        if (original_key_is_iter) {
+          // this is just an optimization that saves us one memcpy
+          keys_.push_front(std::move(original_key));
+        } else {
+          keys_.push_front(iter->key().ToString());
+        }
+        if (keys_.size() == 1) {
+          // we need to re-anchor the orig_ikey because it was anchored by
+          // original_key before
+          ParseInternalKey(keys_.back(), &orig_ikey);
+        }
+        operands_.push_front(value_slice.ToString());
+      }
     }
   }
 
+  if (operands_.size() == 0) {
+    // we filtered out all the merge operands
+    return Status::OK();
+  }
+
   // We are sure we have seen this key's entire history if we are at the
   // last level and exhausted all internal keys of this user key.
   // NOTE: !iter->Valid() does not necessarily mean we hit the
@@ -179,11 +220,13 @@ Status MergeHelper::MergeUntil(Iterator* iter, const SequenceNumber stop_before,
     assert(operands_.size() == keys_.size());
     std::string merge_result;
     s = TimedFullMerge(orig_ikey.user_key, nullptr, operands_,
-                       user_merge_operator_, stats, env_, logger_,
+                       user_merge_operator_, stats_, env_, logger_,
                        &merge_result);
     if (s.ok()) {
       // The original key encountered
-      std::string original_key = std::move(keys_.back());
+      // We are certain that keys_ is not empty here (see assertions couple of
+      // lines before).
+      original_key = std::move(keys_.back());
       orig_ikey.type = kTypeValue;
       UpdateInternalKey(&original_key, orig_ikey.sequence, orig_ikey.type);
       keys_.clear();
@@ -205,14 +248,14 @@ Status MergeHelper::MergeUntil(Iterator* iter, const SequenceNumber stop_before,
       bool merge_success = false;
       std::string merge_result;
       {
-        StopWatchNano timer(env_, stats != nullptr);
+        StopWatchNano timer(env_, stats_ != nullptr);
         PERF_TIMER_GUARD(merge_operator_time_nanos);
         merge_success = user_merge_operator_->PartialMergeMulti(
             orig_ikey.user_key,
             std::deque<Slice>(operands_.begin(), operands_.end()),
             &merge_result, logger_);
-        RecordTick(stats, MERGE_OPERATION_TOTAL_TIME,
-                   env_ != nullptr ? timer.ElapsedNanos() : 0);
+        RecordTick(stats_, MERGE_OPERATION_TOTAL_TIME,
+                   timer.ElapsedNanosSafe());
       }
       if (merge_success) {
         // Merging of operands (associative merge) was successful.
@@ -236,7 +279,6 @@ MergeOutputIterator::MergeOutputIterator(const MergeHelper* merge_helper)
 void MergeOutputIterator::SeekToFirst() {
   const auto& keys = merge_helper_->keys();
   const auto& values = merge_helper_->values();
-  assert(keys.size() > 0);
   assert(keys.size() == values.size());
   it_keys_ = keys.rbegin();
   it_values_ = values.rbegin();
@@ -247,4 +289,17 @@ void MergeOutputIterator::Next() {
   ++it_values_;
 }
 
+bool MergeHelper::FilterMerge(const Slice& user_key, const Slice& value_slice) {
+  if (compaction_filter_ == nullptr) {
+    return false;
+  }
+  if (stats_ != nullptr) {
+    filter_timer_.Start();
+  }
+  bool to_delete =
+      compaction_filter_->FilterMergeOperand(level_, user_key, value_slice);
+  total_filter_time_ += filter_timer_.ElapsedNanosSafe();
+  return to_delete;
+}
+
 } // namespace rocksdb
diff --git a/db/merge_helper.h b/db/merge_helper.h
index 39c7126de..ade3d71a6 100644
--- a/db/merge_helper.h
+++ b/db/merge_helper.h
@@ -10,8 +10,10 @@
 #include <string>
 
 #include "db/dbformat.h"
+#include "rocksdb/compaction_filter.h"
 #include "rocksdb/env.h"
 #include "rocksdb/slice.h"
+#include "util/stop_watch.h"
 
 namespace rocksdb {
 
@@ -23,17 +25,26 @@ class Statistics;
 
 class MergeHelper {
  public:
-  MergeHelper(const Comparator* user_comparator,
-              const MergeOperator* user_merge_operator, Logger* logger,
+  MergeHelper(Env* env, const Comparator* user_comparator,
+              const MergeOperator* user_merge_operator,
+              const CompactionFilter* compaction_filter, Logger* logger,
               unsigned min_partial_merge_operands,
-              bool assert_valid_internal_key)
-      : user_comparator_(user_comparator),
+              bool assert_valid_internal_key, SequenceNumber latest_snapshot,
+              int level = 0, Statistics* stats = nullptr)
+      : env_(env),
+        user_comparator_(user_comparator),
         user_merge_operator_(user_merge_operator),
+        compaction_filter_(compaction_filter),
         logger_(logger),
         min_partial_merge_operands_(min_partial_merge_operands),
         assert_valid_internal_key_(assert_valid_internal_key),
+        latest_snapshot_(latest_snapshot),
+        level_(level),
         keys_(),
-        operands_() {
+        operands_(),
+        filter_timer_(env_),
+        total_filter_time_(0U),
+        stats_(stats) {
     assert(user_comparator_ != nullptr);
   }
 
@@ -62,6 +73,7 @@ class MergeHelper {
   //                   0 means no restriction
   // at_bottom:   (IN) true if the iterator covers the bottem level, which means
   //                   we could reach the start of the history of this user key.
+  //
   // Returns one of the following statuses:
   // - OK: Entries were successfully merged.
   // - MergeInProgress: Put/Delete not encountered and unable to merge operands.
@@ -71,8 +83,11 @@ class MergeHelper {
   //
   // REQUIRED: The first key in the input is not corrupted.
   Status MergeUntil(Iterator* iter, const SequenceNumber stop_before = 0,
-                    const bool at_bottom = false, Statistics* stats = nullptr,
-                    Env* env_ = nullptr);
+                    const bool at_bottom = false);
+
+  // Filters a merge operand using the compaction filter specified
+  // in the constructor. Returns true if the operand should be filtered out.
+  bool FilterMerge(const Slice& user_key, const Slice& value_slice);
 
   // Query the merge result
   // These are valid until the next MergeUntil call
@@ -101,19 +116,28 @@ class MergeHelper {
   // TODO: Re-style this comment to be like the first one
   const std::deque<std::string>& keys() const { return keys_; }
   const std::deque<std::string>& values() const { return operands_; }
+  uint64_t TotalFilterTime() const { return total_filter_time_; }
   bool HasOperator() const { return user_merge_operator_ != nullptr; }
 
  private:
+  Env* env_;
   const Comparator* user_comparator_;
   const MergeOperator* user_merge_operator_;
+  const CompactionFilter* compaction_filter_;
   Logger* logger_;
   unsigned min_partial_merge_operands_;
   bool assert_valid_internal_key_; // enforce no internal key corruption?
+  SequenceNumber latest_snapshot_;
+  int level_;
 
   // the scratch area that holds the result of MergeUntil
   // valid up to the next MergeUntil call
   std::deque<std::string> keys_;    // Keeps track of the sequence of keys seen
   std::deque<std::string> operands_;  // Parallel with keys_; stores the values
+
+  StopWatchNano filter_timer_;
+  uint64_t total_filter_time_;
+  Statistics* stats_;
 };
 
 // MergeOutputIterator can be used to iterate over the result of a merge.
diff --git a/db/merge_helper_test.cc b/db/merge_helper_test.cc
index 209ede01f..2ef0d39e4 100644
--- a/db/merge_helper_test.cc
+++ b/db/merge_helper_test.cc
@@ -18,26 +18,18 @@ namespace rocksdb {
 
 class MergeHelperTest : public testing::Test {
  public:
-  MergeHelperTest() = default;
-  ~MergeHelperTest() = default;
+  MergeHelperTest() { env_ = Env::Default(); }
 
-  Status RunUInt64MergeHelper(SequenceNumber stop_before, bool at_bottom) {
-    InitIterator();
-    merge_op_ = MergeOperators::CreateUInt64AddOperator();
-    merge_helper_.reset(new MergeHelper(BytewiseComparator(), merge_op_.get(),
-                                        nullptr, 2U, false));
-    return merge_helper_->MergeUntil(iter_.get(), stop_before, at_bottom,
-                                     nullptr, Env::Default());
-  }
+  ~MergeHelperTest() = default;
 
-  Status RunStringAppendMergeHelper(SequenceNumber stop_before,
-                                    bool at_bottom) {
-    InitIterator();
-    merge_op_ = MergeOperators::CreateStringAppendTESTOperator();
-    merge_helper_.reset(new MergeHelper(BytewiseComparator(), merge_op_.get(),
-                                        nullptr, 2U, false));
-    return merge_helper_->MergeUntil(iter_.get(), stop_before, at_bottom,
-                                     nullptr, Env::Default());
+  Status Run(SequenceNumber stop_before, bool at_bottom,
+             SequenceNumber latest_snapshot = 0) {
+    iter_.reset(new test::VectorIterator(ks_, vs_));
+    iter_->SeekToFirst();
+    merge_helper_.reset(new MergeHelper(env_, BytewiseComparator(),
+                                        merge_op_.get(), filter_.get(), nullptr,
+                                        2U, false, latest_snapshot));
+    return merge_helper_->MergeUntil(iter_.get(), stop_before, at_bottom);
   }
 
   void AddKeyVal(const std::string& user_key, const SequenceNumber& seq,
@@ -51,66 +43,63 @@ class MergeHelperTest : public testing::Test {
     vs_.push_back(val);
   }
 
-  void InitIterator() {
-    iter_.reset(new test::VectorIterator(ks_, vs_));
-    iter_->SeekToFirst();
-  }
-
-  std::string EncodeInt(uint64_t x) {
-    std::string result;
-    PutFixed64(&result, x);
-    return result;
-  }
-
+  Env* env_;
   std::unique_ptr<test::VectorIterator> iter_;
   std::shared_ptr<MergeOperator> merge_op_;
   std::unique_ptr<MergeHelper> merge_helper_;
   std::vector<std::string> ks_;
   std::vector<std::string> vs_;
+  std::unique_ptr<test::FilterNumber> filter_;
 };
 
 // If MergeHelper encounters a new key on the last level, we know that
 // the key has no more history and it can merge keys.
 TEST_F(MergeHelperTest, MergeAtBottomSuccess) {
-  AddKeyVal("a", 20, kTypeMerge, EncodeInt(1U));
-  AddKeyVal("a", 10, kTypeMerge, EncodeInt(3U));
-  AddKeyVal("b", 10, kTypeMerge, EncodeInt(4U));  // <- Iterator after merge
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
+
+  AddKeyVal("a", 20, kTypeMerge, test::EncodeInt(1U));
+  AddKeyVal("a", 10, kTypeMerge, test::EncodeInt(3U));
+  AddKeyVal("b", 10, kTypeMerge, test::EncodeInt(4U));  // <- iter_ after merge
 
-  ASSERT_TRUE(RunUInt64MergeHelper(0, true).ok());
+  ASSERT_TRUE(Run(0, true).ok());
   ASSERT_EQ(ks_[2], iter_->key());
   ASSERT_EQ(test::KeyStr("a", 20, kTypeValue), merge_helper_->keys()[0]);
-  ASSERT_EQ(EncodeInt(4U), merge_helper_->values()[0]);
+  ASSERT_EQ(test::EncodeInt(4U), merge_helper_->values()[0]);
   ASSERT_EQ(1U, merge_helper_->keys().size());
   ASSERT_EQ(1U, merge_helper_->values().size());
 }
 
 // Merging with a value results in a successful merge.
 TEST_F(MergeHelperTest, MergeValue) {
-  AddKeyVal("a", 40, kTypeMerge, EncodeInt(1U));
-  AddKeyVal("a", 30, kTypeMerge, EncodeInt(3U));
-  AddKeyVal("a", 20, kTypeValue, EncodeInt(4U));  // <- Iterator after merge
-  AddKeyVal("a", 10, kTypeMerge, EncodeInt(1U));
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
+
+  AddKeyVal("a", 40, kTypeMerge, test::EncodeInt(1U));
+  AddKeyVal("a", 30, kTypeMerge, test::EncodeInt(3U));
+  AddKeyVal("a", 20, kTypeValue, test::EncodeInt(4U));  // <- iter_ after merge
+  AddKeyVal("a", 10, kTypeMerge, test::EncodeInt(1U));
 
-  ASSERT_TRUE(RunUInt64MergeHelper(0, false).ok());
+  ASSERT_TRUE(Run(0, false).ok());
   ASSERT_EQ(ks_[3], iter_->key());
   ASSERT_EQ(test::KeyStr("a", 40, kTypeValue), merge_helper_->keys()[0]);
-  ASSERT_EQ(EncodeInt(8U), merge_helper_->values()[0]);
+  ASSERT_EQ(test::EncodeInt(8U), merge_helper_->values()[0]);
   ASSERT_EQ(1U, merge_helper_->keys().size());
   ASSERT_EQ(1U, merge_helper_->values().size());
 }
 
 // Merging stops before a snapshot.
 TEST_F(MergeHelperTest, SnapshotBeforeValue) {
-  AddKeyVal("a", 50, kTypeMerge, EncodeInt(1U));
-  AddKeyVal("a", 40, kTypeMerge, EncodeInt(3U));  // <- Iterator after merge
-  AddKeyVal("a", 30, kTypeMerge, EncodeInt(1U));
-  AddKeyVal("a", 20, kTypeValue, EncodeInt(4U));
-  AddKeyVal("a", 10, kTypeMerge, EncodeInt(1U));
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
 
-  ASSERT_TRUE(RunUInt64MergeHelper(31, true).IsMergeInProgress());
+  AddKeyVal("a", 50, kTypeMerge, test::EncodeInt(1U));
+  AddKeyVal("a", 40, kTypeMerge, test::EncodeInt(3U));  // <- iter_ after merge
+  AddKeyVal("a", 30, kTypeMerge, test::EncodeInt(1U));
+  AddKeyVal("a", 20, kTypeValue, test::EncodeInt(4U));
+  AddKeyVal("a", 10, kTypeMerge, test::EncodeInt(1U));
+
+  ASSERT_TRUE(Run(31, true).IsMergeInProgress());
   ASSERT_EQ(ks_[2], iter_->key());
   ASSERT_EQ(test::KeyStr("a", 50, kTypeMerge), merge_helper_->keys()[0]);
-  ASSERT_EQ(EncodeInt(4U), merge_helper_->values()[0]);
+  ASSERT_EQ(test::EncodeInt(4U), merge_helper_->values()[0]);
   ASSERT_EQ(1U, merge_helper_->keys().size());
   ASSERT_EQ(1U, merge_helper_->values().size());
 }
@@ -118,11 +107,13 @@ TEST_F(MergeHelperTest, SnapshotBeforeValue) {
 // MergeHelper preserves the operand stack for merge operators that
 // cannot do a partial merge.
 TEST_F(MergeHelperTest, NoPartialMerge) {
+  merge_op_ = MergeOperators::CreateStringAppendTESTOperator();
+
   AddKeyVal("a", 50, kTypeMerge, "v2");
-  AddKeyVal("a", 40, kTypeMerge, "v");  // <- Iterator after merge
+  AddKeyVal("a", 40, kTypeMerge, "v");  // <- iter_ after merge
   AddKeyVal("a", 30, kTypeMerge, "v");
 
-  ASSERT_TRUE(RunStringAppendMergeHelper(31, true).IsMergeInProgress());
+  ASSERT_TRUE(Run(31, true).IsMergeInProgress());
   ASSERT_EQ(ks_[2], iter_->key());
   ASSERT_EQ(test::KeyStr("a", 40, kTypeMerge), merge_helper_->keys()[0]);
   ASSERT_EQ("v", merge_helper_->values()[0]);
@@ -134,44 +125,162 @@ TEST_F(MergeHelperTest, NoPartialMerge) {
 
 // A single operand can not be merged.
 TEST_F(MergeHelperTest, SingleOperand) {
-  AddKeyVal("a", 50, kTypeMerge, EncodeInt(1U));
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
 
-  ASSERT_TRUE(RunUInt64MergeHelper(31, true).IsMergeInProgress());
+  AddKeyVal("a", 50, kTypeMerge, test::EncodeInt(1U));
+
+  ASSERT_TRUE(Run(31, true).IsMergeInProgress());
   ASSERT_FALSE(iter_->Valid());
   ASSERT_EQ(test::KeyStr("a", 50, kTypeMerge), merge_helper_->keys()[0]);
-  ASSERT_EQ(EncodeInt(1U), merge_helper_->values()[0]);
+  ASSERT_EQ(test::EncodeInt(1U), merge_helper_->values()[0]);
   ASSERT_EQ(1U, merge_helper_->keys().size());
   ASSERT_EQ(1U, merge_helper_->values().size());
 }
 
 // Merging with a deletion turns the deletion into a value
 TEST_F(MergeHelperTest, MergeDeletion) {
-  AddKeyVal("a", 30, kTypeMerge, EncodeInt(3U));
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
+
+  AddKeyVal("a", 30, kTypeMerge, test::EncodeInt(3U));
   AddKeyVal("a", 20, kTypeDeletion, "");
 
-  ASSERT_TRUE(RunUInt64MergeHelper(15, false).ok());
+  ASSERT_TRUE(Run(15, false).ok());
   ASSERT_FALSE(iter_->Valid());
   ASSERT_EQ(test::KeyStr("a", 30, kTypeValue), merge_helper_->keys()[0]);
-  ASSERT_EQ(EncodeInt(3U), merge_helper_->values()[0]);
+  ASSERT_EQ(test::EncodeInt(3U), merge_helper_->values()[0]);
   ASSERT_EQ(1U, merge_helper_->keys().size());
   ASSERT_EQ(1U, merge_helper_->values().size());
 }
 
 // The merge helper stops upon encountering a corrupt key
 TEST_F(MergeHelperTest, CorruptKey) {
-  AddKeyVal("a", 30, kTypeMerge, EncodeInt(3U));
-  AddKeyVal("a", 25, kTypeMerge, EncodeInt(1U));
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
+
+  AddKeyVal("a", 30, kTypeMerge, test::EncodeInt(3U));
+  AddKeyVal("a", 25, kTypeMerge, test::EncodeInt(1U));
   // Corrupt key
-  AddKeyVal("a", 20, kTypeDeletion, "", true);  // <- Iterator after merge
+  AddKeyVal("a", 20, kTypeDeletion, "", true);  // <- iter_ after merge
 
-  ASSERT_TRUE(RunUInt64MergeHelper(15, false).IsMergeInProgress());
+  ASSERT_TRUE(Run(15, false).IsMergeInProgress());
   ASSERT_EQ(ks_[2], iter_->key());
   ASSERT_EQ(test::KeyStr("a", 30, kTypeMerge), merge_helper_->keys()[0]);
-  ASSERT_EQ(EncodeInt(4U), merge_helper_->values()[0]);
+  ASSERT_EQ(test::EncodeInt(4U), merge_helper_->values()[0]);
   ASSERT_EQ(1U, merge_helper_->keys().size());
   ASSERT_EQ(1U, merge_helper_->values().size());
 }
 
+// The compaction filter is called on every merge operand
+TEST_F(MergeHelperTest, FilterMergeOperands) {
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
+  filter_.reset(new test::FilterNumber(5U));
+
+  AddKeyVal("a", 30, kTypeMerge, test::EncodeInt(3U));
+  AddKeyVal("a", 29, kTypeMerge, test::EncodeInt(5U));  // Filtered
+  AddKeyVal("a", 28, kTypeMerge, test::EncodeInt(3U));
+  AddKeyVal("a", 27, kTypeMerge, test::EncodeInt(1U));
+  AddKeyVal("a", 26, kTypeMerge, test::EncodeInt(5U));  // Filtered
+  AddKeyVal("a", 25, kTypeValue, test::EncodeInt(1U));
+
+  ASSERT_TRUE(Run(15, false).ok());
+  ASSERT_FALSE(iter_->Valid());
+  MergeOutputIterator merge_output_iter(merge_helper_.get());
+  merge_output_iter.SeekToFirst();
+  ASSERT_EQ(test::KeyStr("a", 30, kTypeValue),
+            merge_output_iter.key().ToString());
+  ASSERT_EQ(test::EncodeInt(8U), merge_output_iter.value().ToString());
+  merge_output_iter.Next();
+  ASSERT_FALSE(merge_output_iter.Valid());
+}
+
+TEST_F(MergeHelperTest, FilterAllMergeOperands) {
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
+  filter_.reset(new test::FilterNumber(5U));
+
+  AddKeyVal("a", 30, kTypeMerge, test::EncodeInt(5U));
+  AddKeyVal("a", 29, kTypeMerge, test::EncodeInt(5U));
+  AddKeyVal("a", 28, kTypeMerge, test::EncodeInt(5U));
+  AddKeyVal("a", 27, kTypeMerge, test::EncodeInt(5U));
+  AddKeyVal("a", 26, kTypeMerge, test::EncodeInt(5U));
+  AddKeyVal("a", 25, kTypeMerge, test::EncodeInt(5U));
+
+  // filtered out all
+  ASSERT_TRUE(Run(15, false).ok());
+  ASSERT_FALSE(iter_->Valid());
+  MergeOutputIterator merge_output_iter(merge_helper_.get());
+  merge_output_iter.SeekToFirst();
+  ASSERT_FALSE(merge_output_iter.Valid());
+
+  // we have one operand that will survive because it's a delete
+  AddKeyVal("a", 24, kTypeDeletion, test::EncodeInt(5U));
+  AddKeyVal("b", 23, kTypeValue, test::EncodeInt(5U));
+  ASSERT_TRUE(Run(15, true).ok());
+  merge_output_iter = MergeOutputIterator(merge_helper_.get());
+  ASSERT_TRUE(iter_->Valid());
+  merge_output_iter.SeekToFirst();
+  ASSERT_FALSE(merge_output_iter.Valid());
+
+  // when all merge operands are filtered out, we leave the iterator pointing to
+  // the Put/Delete that survived
+  ASSERT_EQ(test::KeyStr("a", 24, kTypeDeletion), iter_->key().ToString());
+  ASSERT_EQ(test::EncodeInt(5U), iter_->value().ToString());
+}
+
+// Make sure that merge operands are filtered at the beginning
+TEST_F(MergeHelperTest, FilterFirstMergeOperand) {
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
+  filter_.reset(new test::FilterNumber(5U));
+
+  AddKeyVal("a", 31, kTypeMerge, test::EncodeInt(5U));  // Filtered
+  AddKeyVal("a", 30, kTypeMerge, test::EncodeInt(5U));  // Filtered
+  AddKeyVal("a", 29, kTypeMerge, test::EncodeInt(2U));
+  AddKeyVal("a", 28, kTypeMerge, test::EncodeInt(1U));
+  AddKeyVal("a", 27, kTypeMerge, test::EncodeInt(3U));
+  AddKeyVal("a", 26, kTypeMerge, test::EncodeInt(5U));  // Filtered
+  AddKeyVal("a", 25, kTypeMerge, test::EncodeInt(5U));  // Filtered
+  AddKeyVal("b", 24, kTypeValue, test::EncodeInt(5U));  // next user key
+
+  ASSERT_OK(Run(15, true));
+  ASSERT_TRUE(iter_->Valid());
+  MergeOutputIterator merge_output_iter(merge_helper_.get());
+  merge_output_iter.SeekToFirst();
+  // sequence number is 29 here, because the first merge operand got filtered
+  // out
+  ASSERT_EQ(test::KeyStr("a", 29, kTypeValue),
+            merge_output_iter.key().ToString());
+  ASSERT_EQ(test::EncodeInt(6U), merge_output_iter.value().ToString());
+  merge_output_iter.Next();
+  ASSERT_FALSE(merge_output_iter.Valid());
+
+  // make sure that we're passing user keys into the filter
+  ASSERT_EQ("a", filter_->last_merge_operand_key());
+}
+
+// Make sure that merge operands are not filtered out if there's a snapshot
+// pointing at them
+TEST_F(MergeHelperTest, DontFilterMergeOperandsBeforeSnapshotTest) {
+  merge_op_ = MergeOperators::CreateUInt64AddOperator();
+  filter_.reset(new test::FilterNumber(5U));
+
+  AddKeyVal("a", 31, kTypeMerge, test::EncodeInt(5U));
+  AddKeyVal("a", 30, kTypeMerge, test::EncodeInt(5U));
+  AddKeyVal("a", 29, kTypeMerge, test::EncodeInt(2U));
+  AddKeyVal("a", 28, kTypeMerge, test::EncodeInt(1U));
+  AddKeyVal("a", 27, kTypeMerge, test::EncodeInt(3U));
+  AddKeyVal("a", 26, kTypeMerge, test::EncodeInt(5U));
+  AddKeyVal("a", 25, kTypeMerge, test::EncodeInt(5U));
+  AddKeyVal("b", 24, kTypeValue, test::EncodeInt(5U));
+
+  ASSERT_OK(Run(15, true, 32));
+  ASSERT_TRUE(iter_->Valid());
+  MergeOutputIterator merge_output_iter(merge_helper_.get());
+  merge_output_iter.SeekToFirst();
+  ASSERT_EQ(test::KeyStr("a", 31, kTypeValue),
+            merge_output_iter.key().ToString());
+  ASSERT_EQ(test::EncodeInt(26U), merge_output_iter.value().ToString());
+  merge_output_iter.Next();
+  ASSERT_FALSE(merge_output_iter.Valid());
+}
+
 }  // namespace rocksdb
 
 int main(int argc, char** argv) {
diff --git a/examples/.gitignore b/examples/.gitignore
index 1c63561a6..8c06e7972 100644
--- a/examples/.gitignore
+++ b/examples/.gitignore
@@ -1,6 +1,7 @@
-column_families_example
-simple_example
 c_simple_example
+column_families_example
 compact_files_example
-transaction_example
+compaction_filter_example
 optimistic_transaction_example
+simple_example
+transaction_example
diff --git a/examples/Makefile b/examples/Makefile
index 0882f83e6..fe82d11cd 100644
--- a/examples/Makefile
+++ b/examples/Makefile
@@ -10,6 +10,9 @@ simple_example: librocksdb simple_example.cc
 column_families_example: librocksdb column_families_example.cc
 	$(CXX) $(CXXFLAGS) $@.cc -o$@ ../librocksdb.a -I../include -O2 -std=c++11 $(PLATFORM_LDFLAGS) $(PLATFORM_CXXFLAGS) $(EXEC_LDFLAGS)
 
+compaction_filter_example: librocksdb compaction_filter_example.cc
+	$(CXX) $(CXXFLAGS) $@.cc -o$@ ../librocksdb.a -I../include -O2 -std=c++11 $(PLATFORM_LDFLAGS) $(PLATFORM_CXXFLAGS) $(EXEC_LDFLAGS)
+
 compact_files_example: librocksdb compact_files_example.cc
 	$(CXX) $(CXXFLAGS) $@.cc -o$@ ../librocksdb.a -I../include -O2 -std=c++11 $(PLATFORM_LDFLAGS) $(PLATFORM_CXXFLAGS) $(EXEC_LDFLAGS)
 
@@ -26,7 +29,7 @@ transaction_example: librocksdb transaction_example.cc
 	$(CXX) $(CXXFLAGS) $@.cc -o$@ ../librocksdb.a -I../include -O2 -std=c++11 $(PLATFORM_LDFLAGS) $(PLATFORM_CXXFLAGS) $(EXEC_LDFLAGS)
 
 clean:
-	rm -rf ./simple_example ./column_families_example ./compact_files_example ./c_simple_example c_simple_example.o ./optimistic_transaction_example ./transaction_example
+	rm -rf ./simple_example ./column_families_example ./compact_files_example ./compaction_filter_example ./c_simple_example c_simple_example.o ./optimistic_transaction_example ./transaction_example
 
 librocksdb:
 	cd .. && $(MAKE) librocksdb.a
diff --git a/examples/compaction_filter_example.cc b/examples/compaction_filter_example.cc
new file mode 100644
index 000000000..050f4611a
--- /dev/null
+++ b/examples/compaction_filter_example.cc
@@ -0,0 +1,84 @@
+// Copyright (c) 2013, Facebook, Inc.  All rights reserved.
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree. An additional grant
+// of patent rights can be found in the PATENTS file in the same directory.
+
+#include <rocksdb/compaction_filter.h>
+#include <rocksdb/db.h>
+#include <rocksdb/merge_operator.h>
+#include <rocksdb/options.h>
+
+class MyMerge : public rocksdb::MergeOperator {
+ public:
+  bool FullMerge(const rocksdb::Slice& key,
+                 const rocksdb::Slice* existing_value,
+                 const std::deque<std::string>& operand_list,
+                 std::string* new_value,
+                 rocksdb::Logger* logger) const override {
+    new_value->clear();
+    if (existing_value != nullptr) {
+      new_value->assign(existing_value->data(), existing_value->size());
+    }
+    for (const std::string& m : operand_list) {
+      fprintf(stderr, "Merge(%s)\n", m.c_str());
+      assert(m != "bad");  // the compaction filter filters out bad values
+      new_value->assign(m);
+    }
+    return true;
+  }
+
+  const char* Name() const override { return "MyMerge"; }
+};
+
+class MyFilter : public rocksdb::CompactionFilter {
+ public:
+  bool Filter(int level, const rocksdb::Slice& key,
+              const rocksdb::Slice& existing_value, std::string* new_value,
+              bool* value_changed) const override {
+    fprintf(stderr, "Filter(%s)\n", key.ToString().c_str());
+    ++count_;
+    assert(*value_changed == false);
+    return false;
+  }
+
+  bool FilterMergeOperand(int level, const rocksdb::Slice& key,
+                          const rocksdb::Slice& existing_value) const override {
+    fprintf(stderr, "FilterMerge(%s)\n", key.ToString().c_str());
+    ++merge_count_;
+    return existing_value == "bad";
+  }
+
+  const char* Name() const override { return "MyFilter"; }
+
+  mutable int count_ = 0;
+  mutable int merge_count_ = 0;
+};
+
+int main() {
+  rocksdb::DB* raw_db;
+  rocksdb::Status status;
+
+  MyFilter filter;
+
+  system("rm -rf /tmp/rocksmergetest");
+  rocksdb::Options options;
+  options.create_if_missing = true;
+  options.merge_operator.reset(new MyMerge);
+  options.compaction_filter = &filter;
+  status = rocksdb::DB::Open(options, "/tmp/rocksmergetest", &raw_db);
+  assert(status.ok());
+  std::unique_ptr<rocksdb::DB> db(raw_db);
+
+  rocksdb::WriteOptions wopts;
+  db->Merge(wopts, "0", "bad");  // This is filtered out
+  db->Merge(wopts, "1", "data1");
+  db->Merge(wopts, "1", "bad");
+  db->Merge(wopts, "1", "data2");
+  db->Merge(wopts, "1", "bad");
+  db->Merge(wopts, "3", "data3");
+  db->CompactRange(rocksdb::CompactRangeOptions(), nullptr, nullptr);
+  fprintf(stderr, "filter.count_ = %d\n", filter.count_);
+  assert(filter.count_ == 1);
+  fprintf(stderr, "filter.merge_count_ = %d\n", filter.merge_count_);
+  assert(filter.merge_count_ == 5);
+}
diff --git a/include/rocksdb/compaction_filter.h b/include/rocksdb/compaction_filter.h
index dce69d2d7..da809f544 100644
--- a/include/rocksdb/compaction_filter.h
+++ b/include/rocksdb/compaction_filter.h
@@ -51,10 +51,24 @@ class CompactionFilter {
   // output of the compaction.  The application can inspect
   // the existing value of the key and make decision based on it.
   //
+  // Key-Values that are results of merge operation during compaction are not
+  // passed into this function. Currently, when you have a mix of Put()s and
+  // Merge()s on a same key, we only guarantee to process the merge operands
+  // through the compaction filters. Put()s might be processed, or might not.
+  //
   // When the value is to be preserved, the application has the option
   // to modify the existing_value and pass it back through new_value.
   // value_changed needs to be set to true in this case.
   //
+  // If you use snapshot feature of RocksDB (i.e. call GetSnapshot() API on a
+  // DB* object), CompactionFilter might not be very useful for you. Due to
+  // guarantees we need to maintain, compaction process will not call Filter()
+  // on any keys that were written before the latest snapshot. In other words,
+  // compaction will only call Filter() on keys written after your most recent
+  // call to GetSnapshot(). In most cases, Filter() will not be called very
+  // often. This is something we're fixing. See the discussion at:
+  // https://www.facebook.com/groups/mysqlonrocksdb/permalink/999723240091865/
+  //
   // If multithreaded compaction is being used *and* a single CompactionFilter
   // instance was supplied via Options::compaction_filter, this method may be
   // called from different threads concurrently.  The application must ensure
@@ -70,6 +84,14 @@ class CompactionFilter {
                       std::string* new_value,
                       bool* value_changed) const = 0;
 
+  // The compaction process invokes this method on every merge operand. If this
+  // method returns true, the merge operand will be ignored and not written out
+  // in the compaction output
+  virtual bool FilterMergeOperand(int level, const Slice& key,
+                                  const Slice& operand) const {
+    return false;
+  }
+
   // Returns a name that identifies this compaction filter.
   // The name will be printed to LOG file on start up for diagnosis.
   virtual const char* Name() const = 0;
diff --git a/table/get_context.cc b/table/get_context.cc
index bfa53d7a4..609ca3083 100644
--- a/table/get_context.cc
+++ b/table/get_context.cc
@@ -92,7 +92,7 @@ bool GetContext::SaveValue(const ParsedInternalKey& parsed_key,
                 user_key_, &value, merge_context_->GetOperands(), value_,
                 logger_);
             RecordTick(statistics_, MERGE_OPERATION_TOTAL_TIME,
-                       env_ != nullptr ? timer.ElapsedNanos() : 0);
+                       timer.ElapsedNanosSafe());
           }
           if (!merge_success) {
             RecordTick(statistics_, NUMBER_MERGE_FAILURES);
@@ -118,7 +118,7 @@ bool GetContext::SaveValue(const ParsedInternalKey& parsed_key,
                 user_key_, nullptr, merge_context_->GetOperands(), value_,
                 logger_);
             RecordTick(statistics_, MERGE_OPERATION_TOTAL_TIME,
-                       env_ != nullptr ? timer.ElapsedNanos() : 0);
+                       timer.ElapsedNanosSafe());
           }
           if (!merge_success) {
             RecordTick(statistics_, NUMBER_MERGE_FAILURES);
diff --git a/util/stop_watch.h b/util/stop_watch.h
index 3637533bb..86cb2653c 100644
--- a/util/stop_watch.h
+++ b/util/stop_watch.h
@@ -67,6 +67,10 @@ class StopWatchNano {
     return elapsed;
   }
 
+  uint64_t ElapsedNanosSafe(bool reset = false) {
+    return (env_ != nullptr) ? ElapsedNanos(reset) : 0U;
+  }
+
  private:
   Env* const env_;
   uint64_t start_;
diff --git a/util/testutil.h b/util/testutil.h
index 1729b3ee0..29806285e 100644
--- a/util/testutil.h
+++ b/util/testutil.h
@@ -13,6 +13,7 @@
 #include <vector>
 
 #include "db/dbformat.h"
+#include "rocksdb/compaction_filter.h"
 #include "rocksdb/env.h"
 #include "rocksdb/iterator.h"
 #include "rocksdb/slice.h"
@@ -337,5 +338,42 @@ class SleepingBackgroundTask {
   bool sleeping_;
 };
 
+// Filters merge operands and values that are equal to `num`.
+class FilterNumber : public CompactionFilter {
+ public:
+  explicit FilterNumber(uint64_t num) : num_(num) {}
+
+  std::string last_merge_operand_key() { return last_merge_operand_key_; }
+
+  bool Filter(int level, const rocksdb::Slice& key, const rocksdb::Slice& value,
+              std::string* new_value, bool* value_changed) const override {
+    if (value.size() == sizeof(uint64_t)) {
+      return num_ == DecodeFixed64(value.data());
+    }
+    return true;
+  }
+
+  bool FilterMergeOperand(int level, const rocksdb::Slice& key,
+                          const rocksdb::Slice& value) const override {
+    last_merge_operand_key_ = key.ToString();
+    if (value.size() == sizeof(uint64_t)) {
+      return num_ == DecodeFixed64(value.data());
+    }
+    return true;
+  }
+
+  const char* Name() const override { return "FilterBadMergeOperand"; }
+
+ private:
+  mutable std::string last_merge_operand_key_;
+  uint64_t num_;
+};
+
+inline std::string EncodeInt(uint64_t x) {
+  std::string result;
+  PutFixed64(&result, x);
+  return result;
+}
+
 }  // namespace test
 }  // namespace rocksdb