Enhance partial merge to support multiple arguments

Summary:
* PartialMerge api now takes a list of operands instead of two operands.
* Add min_pertial_merge_operands to Options, indicating the minimum
  number of operands to trigger partial merge.
* This diff is based on Schalk's previous diff (D14601), but it also
  includes necessary changes such as updating the pure C api for
  partial merge.

Test Plan:
* make check all
* develop tests for cases where partial merge takes more than two
  operands.

TODOs (from Schalk):
* Add test with min_partial_merge_operands > 2.
* Perform benchmarks to measure the performance improvements (can probably
  use results of task #2837810.)
* Add description of problem to doc/index.html.
* Change wiki pages to reflect the interface changes.

Reviewers: haobo, igor, vamsi

Reviewed By: haobo

CC: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D16815

db/builder.cc

@@ -73,6 +73,7 @@ Status BuildTable(const std::string& dbname, Env* env, const Options& options,
 
     MergeHelper merge(internal_comparator.user_comparator(),
                       options.merge_operator.get(), options.info_log.get(),
+                      options.min_partial_merge_operands,
                       true /* internal key corruption is not ok */);
 
     if (purge) {

db/c.cc

@@ -159,12 +159,10 @@ struct rocksdb_mergeoperator_t : public MergeOperator {
       const char* const* operands_list, const size_t* operands_list_length,
       int num_operands,
       unsigned char* success, size_t* new_value_length);
-  char* (*partial_merge_)(
-      void*,
-      const char* key, size_t key_length,
-      const char* left_operand, size_t left_operand_length,
-      const char* right_operand, size_t right_operand_length,
-      unsigned char* success, size_t* new_value_length);
+  char* (*partial_merge_)(void*, const char* key, size_t key_length,
+                          const char* const* operands_list,
+                          const size_t* operands_list_length, int num_operands,
+                          unsigned char* success, size_t* new_value_length);
   void (*delete_value_)(
       void*,
       const char* value, size_t value_length);
@@ -219,21 +217,23 @@ struct rocksdb_mergeoperator_t : public MergeOperator {
     return success;
   }
 
-  virtual bool PartialMerge(
-      const Slice& key,
-      const Slice& left_operand,
-      const Slice& right_operand,
-      std::string* new_value,
-      Logger* logger) const {
+  virtual bool PartialMergeMulti(const Slice& key,
+                                 const std::deque<Slice>& operand_list,
+                                 std::string* new_value, Logger* logger) const {
+    size_t operand_count = operand_list.size();
+    std::vector<const char*> operand_pointers(operand_count);
+    std::vector<size_t> operand_sizes(operand_count);
+    for (size_t i = 0; i < operand_count; ++i) {
+      Slice operand(operand_list[i]);
+      operand_pointers[i] = operand.data();
+      operand_sizes[i] = operand.size();
+    }
 
     unsigned char success;
     size_t new_value_len;
     char* tmp_new_value = (*partial_merge_)(
-        state_,
-        key.data(), key.size(),
-        left_operand.data(), left_operand.size(),
-        right_operand.data(), right_operand.size(),
-        &success, &new_value_len);
+        state_, key.data(), key.size(), &operand_pointers[0], &operand_sizes[0],
+        operand_count, &success, &new_value_len);
     new_value->assign(tmp_new_value, new_value_len);
 
     if (delete_value_ != nullptr) {
@@ -1094,24 +1094,18 @@ rocksdb_filterpolicy_t* rocksdb_filterpolicy_create_bloom(int bits_per_key) {
 }
 
 rocksdb_mergeoperator_t* rocksdb_mergeoperator_create(
-    void* state,
-    void (*destructor)(void*),
-    char* (*full_merge)(
-        void*,
-        const char* key, size_t key_length,
-        const char* existing_value, size_t existing_value_length,
-        const char* const* operands_list, const size_t* operands_list_length,
-        int num_operands,
-        unsigned char* success, size_t* new_value_length),
-    char* (*partial_merge)(
-        void*,
-        const char* key, size_t key_length,
-        const char* left_operand, size_t left_operand_length,
-        const char* right_operand, size_t right_operand_length,
-        unsigned char* success, size_t* new_value_length),
-    void (*delete_value)(
-        void*,
-        const char* value, size_t value_length),
+    void* state, void (*destructor)(void*),
+    char* (*full_merge)(void*, const char* key, size_t key_length,
+                        const char* existing_value,
+                        size_t existing_value_length,
+                        const char* const* operands_list,
+                        const size_t* operands_list_length, int num_operands,
+                        unsigned char* success, size_t* new_value_length),
+    char* (*partial_merge)(void*, const char* key, size_t key_length,
+                           const char* const* operands_list,
+                           const size_t* operands_list_length, int num_operands,
+                           unsigned char* success, size_t* new_value_length),
+    void (*delete_value)(void*, const char* value, size_t value_length),
     const char* (*name)(void*)) {
   rocksdb_mergeoperator_t* result = new rocksdb_mergeoperator_t;
   result->state_ = state;

db/c_test.c

@@ -175,8 +175,8 @@ static char* MergeOperatorFullMerge(
 static char* MergeOperatorPartialMerge(
     void* arg,
     const char* key, size_t key_length,
-    const char* left_operand, size_t left_operand_length,
-    const char* right_operand, size_t right_operand_length,
+    const char* const* operands_list, const size_t* operands_list_length,
+    int num_operands,
     unsigned char* success, size_t* new_value_length) {
   *new_value_length = 4;
   *success = 1;

db/db_impl.cc

@@ -2463,6 +2463,7 @@ Status DBImpl::DoCompactionWork(CompactionState* compact,
   std::vector<char> delete_key; // for compaction filter
   MergeHelper merge(user_comparator(), options_.merge_operator.get(),
                     options_.info_log.get(),
+                    options_.min_partial_merge_operands,
                     false /* internal key corruption is expected */);
   auto compaction_filter = options_.compaction_filter;
   std::unique_ptr<CompactionFilter> compaction_filter_from_factory = nullptr;

db/db_iter.cc

@@ -313,21 +313,6 @@ void DBIter::MergeValuesNewToOld() {
       // when complete, add result to operands and continue.
       const Slice& value = iter_->value();
       operands.push_front(value.ToString());
-      while(operands.size() >= 2) {
-        // Call user associative-merge until it returns false
-        if (user_merge_operator_->PartialMerge(ikey.user_key,
-                                               Slice(operands[0]),
-                                               Slice(operands[1]),
-                                               &merge_result,
-                                               logger_)) {
-          operands.pop_front();
-          swap(operands.front(), merge_result);
-        } else {
-          // Associative merge returns false ==> stack the operands
-          break;
-        }
-      }
-
     }
   }
 

db/memtable.cc

@@ -36,7 +36,7 @@ MemTable::MemTable(const InternalKeyComparator& cmp, const Options& options)
       kWriteBufferSize(options.write_buffer_size),
       arena_(options.arena_block_size),
       table_(options.memtable_factory->CreateMemTableRep(
-             comparator_, &arena_, options.prefix_extractor.get())),
+          comparator_, &arena_, options.prefix_extractor.get())),
       flush_in_progress_(false),
       flush_completed_(false),
       file_number_(0),
@@ -353,17 +353,6 @@ static bool SaveValue(void* arg, const char* entry) {
         Slice v = GetLengthPrefixedSlice(key_ptr + key_length);
         *(s->merge_in_progress) = true;
         merge_context->PushOperand(v);
-        while (merge_context->GetNumOperands() >= 2) {
-          // Attempt to associative merge. (Returns true if successful)
-          if (merge_operator->PartialMerge(
-                  s->key->user_key(), merge_context->GetOperand(0),
-                  merge_context->GetOperand(1), &merge_result, s->logger)) {
-            merge_context->PushPartialMergeResult(merge_result);
-          } else {
-            // Stack them because user can't associative merge
-            break;
-          }
-        }
         return true;
       }
       default:

db/merge_context.h

@@ -25,12 +25,12 @@ public:
       operand_list->clear();
     }
   }
-  // Replace the first two operands of merge_result, which are expected be the
-  // merge results of them.
+  // Replace all operands with merge_result, which are expected to be the
+  // merge result of them.
   void PushPartialMergeResult(std::string& merge_result) {
     assert (operand_list);
-    operand_list->pop_front();
-    swap(operand_list->front(), merge_result);
+    operand_list->clear();
+    operand_list->push_front(std::move(merge_result));
   }
   // Push a merge operand
   void PushOperand(const Slice& operand_slice) {

db/merge_helper.cc

@@ -129,28 +129,10 @@ void MergeHelper::MergeUntil(Iterator* iter, SequenceNumber stop_before,
       //   => 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
+      // request or later did a partial merge.
       keys_.push_front(iter->key().ToString());
       operands_.push_front(iter->value().ToString());
-      while (operands_.size() >= 2) {
-        // Returns false when the merge_operator can no longer process it
-        if (user_merge_operator_->PartialMerge(ikey.user_key,
-                                               Slice(operands_[0]),
-                                               Slice(operands_[1]),
-                                               &merge_result,
-                                               logger_)) {
-          // Merging of operands (associative merge) was successful.
-          // Replace these frontmost two operands with the merge result
-          keys_.pop_front();
-          operands_.pop_front();
-          swap(operands_.front(), merge_result);
-        } else {
-          // Merging of operands (associative merge) returned false.
-          // The user merge_operator does not know how to merge these operands.
-          // So we just stack them up until we find a Put/Delete or end of key.
-          break;
-        }
-      }
-      continue;
     }
   }
 
@@ -192,6 +174,23 @@ void MergeHelper::MergeUntil(Iterator* iter, SequenceNumber stop_before,
       RecordTick(stats, NUMBER_MERGE_FAILURES);
       // Do nothing if not success_. Leave keys() and operands() as they are.
     }
+  } else {
+    // We haven't seen the beginning of the key nor a Put/Delete.
+    // Attempt to use the user's associative merge function to
+    // merge the stacked merge operands into a single operand.
+
+    if (operands_.size() >= 2 &&
+        operands_.size() >= min_partial_merge_operands_ &&
+        user_merge_operator_->PartialMergeMulti(
+            ikey.user_key,
+            std::deque<Slice>(operands_.begin(), operands_.end()),
+            &merge_result, logger_)) {
+      // Merging of operands (associative merge) was successful.
+      // Replace operands with the merge result
+      operands_.clear();
+      operands_.push_front(std::move(merge_result));
+      keys_.erase(keys_.begin(), keys_.end() - 1);
+    }
   }
 }
 

db/merge_helper.h

@@ -22,12 +22,13 @@ class Statistics;
 class MergeHelper {
  public:
   MergeHelper(const Comparator* user_comparator,
-              const MergeOperator* user_merge_operator,
-              Logger* logger,
+              const MergeOperator* user_merge_operator, Logger* logger,
+              unsigned min_partial_merge_operands,
               bool assert_valid_internal_key)
       : user_comparator_(user_comparator),
         user_merge_operator_(user_merge_operator),
         logger_(logger),
+        min_partial_merge_operands_(min_partial_merge_operands),
         assert_valid_internal_key_(assert_valid_internal_key),
         keys_(),
         operands_(),
@@ -88,6 +89,7 @@ class MergeHelper {
   const Comparator* user_comparator_;
   const MergeOperator* user_merge_operator_;
   Logger* logger_;
+  unsigned min_partial_merge_operands_;
   bool assert_valid_internal_key_; // enforce no internal key corruption?
 
   // the scratch area that holds the result of MergeUntil

db/merge_operator.cc

@@ -11,6 +11,28 @@
 
 namespace rocksdb {
 
+// The default implementation of PartialMergeMulti, which invokes
+// PartialMerge multiple times internally and merges two operands at
+// a time.
+bool MergeOperator::PartialMergeMulti(const Slice& key,
+                                      const std::deque<Slice>& operand_list,
+                                      std::string* new_value,
+                                      Logger* logger) const {
+  // Simply loop through the operands
+  std::string temp_value;
+  Slice temp_slice;
+  for (const auto& operand : operand_list) {
+    if (!PartialMerge(key, temp_slice, operand, &temp_value, logger)) {
+      return false;
+    }
+    swap(temp_value, *new_value);
+    temp_slice = Slice(*new_value);
+  }
+
+  // The result will be in *new_value. All merges succeeded.
+  return true;
+}
+
 // Given a "real" merge from the library, call the user's
 // associative merge function one-by-one on each of the operands.
 // NOTE: It is assumed that the client's merge-operator will handle any errors.
@@ -46,7 +68,6 @@ bool AssociativeMergeOperator::PartialMerge(
     const Slice& right_operand,
     std::string* new_value,
     Logger* logger) const {
-
   return Merge(key, &left_operand, right_operand, new_value, logger);
 }
 

db/merge_test.cc

@@ -24,10 +24,14 @@ using namespace rocksdb;
 
 namespace {
   int numMergeOperatorCalls;
-
   void resetNumMergeOperatorCalls() {
     numMergeOperatorCalls = 0;
   }
+
+  int num_partial_merge_calls;
+  void resetNumPartialMergeCalls() {
+    num_partial_merge_calls = 0;
+  }
 }
 
 class CountMergeOperator : public AssociativeMergeOperator {
@@ -42,6 +46,11 @@ class CountMergeOperator : public AssociativeMergeOperator {
                      std::string* new_value,
                      Logger* logger) const override {
     ++numMergeOperatorCalls;
+    if (existing_value == nullptr) {
+      new_value->assign(value.data(), value.size());
+      return true;
+    }
+
     return mergeOperator_->PartialMerge(
         key,
         *existing_value,
@@ -50,6 +59,14 @@ class CountMergeOperator : public AssociativeMergeOperator {
         logger);
   }
 
+  virtual bool PartialMergeMulti(const Slice& key,
+                                 const std::deque<Slice>& operand_list,
+                                 std::string* new_value, Logger* logger) const {
+    ++num_partial_merge_calls;
+    return mergeOperator_->PartialMergeMulti(key, operand_list, new_value,
+                                             logger);
+  }
+
   virtual const char* Name() const override {
     return "UInt64AddOperator";
   }
@@ -58,16 +75,16 @@ class CountMergeOperator : public AssociativeMergeOperator {
   std::shared_ptr<MergeOperator> mergeOperator_;
 };
 
-std::shared_ptr<DB> OpenDb(
-    const string& dbname,
-    const bool ttl = false,
-    const unsigned max_successive_merges = 0) {
+std::shared_ptr<DB> OpenDb(const string& dbname, const bool ttl = false,
+                           const size_t max_successive_merges = 0,
+                           const uint32_t min_partial_merge_operands = 2) {
   DB* db;
   StackableDB* sdb;
   Options options;
   options.create_if_missing = true;
   options.merge_operator = std::make_shared<CountMergeOperator>();
   options.max_successive_merges = max_successive_merges;
+  options.min_partial_merge_operands = min_partial_merge_operands;
   Status s;
   DestroyDB(dbname, Options());
   if (ttl) {
@@ -306,6 +323,44 @@ void testSuccessiveMerge(
   }
 }
 
+void testPartialMerge(Counters* counters, DB* db, int max_merge, int min_merge,
+                      int count) {
+  FlushOptions o;
+  o.wait = true;
+
+  // Test case 1: partial merge should be called when the number of merge
+  //              operands exceeds the threshold.
+  uint64_t tmp_sum = 0;
+  resetNumPartialMergeCalls();
+  for (int i = 1; i <= count; i++) {
+    counters->assert_add("b", i);
+    tmp_sum += i;
+  }
+  db->Flush(o);
+  db->CompactRange(nullptr, nullptr);
+  ASSERT_EQ(tmp_sum, counters->assert_get("b"));
+  if (count > max_merge) {
+    // in this case, FullMerge should be called instead.
+    ASSERT_EQ(num_partial_merge_calls, 0);
+  } else {
+    // if count >= min_merge, then partial merge should be called once.
+    ASSERT_EQ((count >= min_merge), (num_partial_merge_calls == 1));
+  }
+
+  // Test case 2: partial merge should not be called when a put is found.
+  resetNumPartialMergeCalls();
+  tmp_sum = 0;
+  db->Put(rocksdb::WriteOptions(), "c", "10");
+  for (int i = 1; i <= count; i++) {
+    counters->assert_add("c", i);
+    tmp_sum += i;
+  }
+  db->Flush(o);
+  db->CompactRange(nullptr, nullptr);
+  ASSERT_EQ(tmp_sum, counters->assert_get("c"));
+  ASSERT_EQ(num_partial_merge_calls, 0);
+}
+
 void testSingleBatchSuccessiveMerge(
     DB* db,
     int max_num_merges,
@@ -370,20 +425,40 @@ void runTest(int argc, const string& dbname, const bool use_ttl = false) {
 
   {
     cout << "Test merge in memtable... \n";
-    unsigned maxMerge = 5;
-    auto db = OpenDb(dbname, use_ttl, maxMerge);
+    size_t max_merge = 5;
+    auto db = OpenDb(dbname, use_ttl, max_merge);
     MergeBasedCounters counters(db, 0);
     testCounters(counters, db.get(), compact);
-    testSuccessiveMerge(counters, maxMerge, maxMerge * 2);
+    testSuccessiveMerge(counters, max_merge, max_merge * 2);
     testSingleBatchSuccessiveMerge(db.get(), 5, 7);
     DestroyDB(dbname, Options());
   }
 
+  {
+    cout << "Test Partial-Merge\n";
+    size_t max_merge = 100;
+    for (uint32_t min_merge = 5; min_merge < 25; min_merge += 5) {
+      for (uint32_t count = min_merge - 1; count <= min_merge + 1; count++) {
+        auto db = OpenDb(dbname, use_ttl, max_merge, min_merge);
+        MergeBasedCounters counters(db, 0);
+        testPartialMerge(&counters, db.get(), max_merge, min_merge, count);
+        DestroyDB(dbname, Options());
+      }
+      {
+        auto db = OpenDb(dbname, use_ttl, max_merge, min_merge);
+        MergeBasedCounters counters(db, 0);
+        testPartialMerge(&counters, db.get(), max_merge, min_merge,
+                         min_merge * 10);
+        DestroyDB(dbname, Options());
+      }
+    }
+  }
 }
 
 int main(int argc, char *argv[]) {
   //TODO: Make this test like a general rocksdb unit-test
   runTest(argc, test::TmpDir() + "/merge_testdb");
   runTest(argc, test::TmpDir() + "/merge_testdbttl", true); // Run test on TTL database
+  printf("Passed all tests!\n");
   return 0;
 }

db/version_set.cc

@@ -419,18 +419,7 @@ static bool SaveValue(void* arg, const ParsedInternalKey& parsed_key,
         assert(s->state == kNotFound || s->state == kMerge);
         s->state = kMerge;
         merge_contex->PushOperand(v);
-        while (merge_contex->GetNumOperands() >= 2) {
-          // Attempt to merge operands together via user associateive merge
-          if (s->merge_operator->PartialMerge(
-                  s->user_key, merge_contex->GetOperand(0),
-                  merge_contex->GetOperand(1), &merge_result, s->logger)) {
-            merge_contex->PushPartialMergeResult(merge_result);
-          } else {
-            // Associative merge returns false ==> stack the operands
-          break;
-          }
-      }
-      return true;
+        return true;
 
       default:
         assert(false);

include/rocksdb/c.h

@@ -425,8 +425,8 @@ extern rocksdb_mergeoperator_t* rocksdb_mergeoperator_create(
     char* (*partial_merge)(
         void*,
         const char* key, size_t key_length,
-        const char* left_operand, size_t left_operand_length,
-        const char* right_operand, size_t right_operand_length,
+        const char* const* operands_list, const size_t* operands_list_length,
+        int num_operands,
         unsigned char* success, size_t* new_value_length),
     void (*delete_value)(
         void*,

include/rocksdb/merge_operator.h

@@ -32,9 +32,9 @@ class Logger;
 //
 //  b) MergeOperator - the generic class for all the more abstract / complex
 //    operations; one method (FullMerge) to merge a Put/Delete value with a
-//    merge operand; and another method (PartialMerge) that merges two
-//    operands together. this is especially useful if your key values have a
-//    complex structure but you would still like to support client-specific
+//    merge operand; and another method (PartialMerge) that merges multiple
+//    operands together. this is especially useful if your key values have
+//    complex structures but you would still like to support client-specific
 //    incremental updates.
 //
 // AssociativeMergeOperator is simpler to implement. MergeOperator is simply
@@ -80,6 +80,13 @@ class MergeOperator {
   // DB::Merge(key, *new_value) would yield the same result as a call
   // to DB::Merge(key, left_op) followed by DB::Merge(key, right_op).
   //
+  // The default implementation of PartialMergeMulti will use this function
+  // as a helper, for backward compatibility.  Any successor class of
+  // MergeOperator should either implement PartialMerge or PartialMergeMulti,
+  // although implementing PartialMergeMulti is suggested as it is in general
+  // more effective to merge multiple operands at a time instead of two
+  // operands at a time.
+  //
   // If it is impossible or infeasible to combine the two operations,
   // leave new_value unchanged and return false. The library will
   // internally keep track of the operations, and apply them in the
@@ -89,12 +96,38 @@ class MergeOperator {
   // and simply "return false". For now, the client should simply return
   // false in any case it cannot perform partial-merge, regardless of reason.
   // If there is corruption in the data, handle it in the FullMerge() function,
-  // and return false there.
-  virtual bool PartialMerge(const Slice& key,
-                            const Slice& left_operand,
-                            const Slice& right_operand,
-                            std::string* new_value,
-                            Logger* logger) const = 0;
+  // and return false there.  The default implementation of PartialMerge will
+  // always return false.
+  virtual bool PartialMerge(const Slice& key, const Slice& left_operand,
+                            const Slice& right_operand, std::string* new_value,
+                            Logger* logger) const {
+    return false;
+  }
+
+  // This function performs merge when all the operands are themselves merge
+  // operation types that you would have passed to a DB::Merge() call in the
+  // same order (front() first)
+  // (i.e. DB::Merge(key, operand_list[0]), followed by
+  //  DB::Merge(key, operand_list[1]), ...)
+  //
+  // PartialMergeMulti should combine them into a single merge operation that is
+  // saved into *new_value, and then it should return true.  *new_value should
+  // be constructed such that a call to DB::Merge(key, *new_value) would yield
+  // the same result as subquential individual calls to DB::Merge(key, operand)
+  // for each operand in operand_list from front() to back().
+  //
+  // The PartialMergeMulti function will be called only when the list of
+  // operands are long enough. The minimum amount of operands that will be
+  // passed to the function are specified by the "min_partial_merge_operands"
+  // option.
+  //
+  // In the default implementation, PartialMergeMulti will invoke PartialMerge
+  // multiple times, where each time it only merges two operands.  Developers
+  // should either implement PartialMergeMulti, or implement PartialMerge which
+  // is served as the helper function of the default PartialMergeMulti.
+  virtual bool PartialMergeMulti(const Slice& key,
+                                 const std::deque<Slice>& operand_list,
+                                 std::string* new_value, Logger* logger) const;
 
   // The name of the MergeOperator. Used to check for MergeOperator
   // mismatches (i.e., a DB created with one MergeOperator is

include/rocksdb/options.h

@@ -723,6 +723,15 @@ struct Options {
   // Default: 0 (disabled)
   size_t max_successive_merges;
 
+  // The number of partial merge operands to accumulate before partial
+  // merge will be performed. Partial merge will not be called
+  // if the list of values to merge is less than min_partial_merge_operands.
+  //
+  // If min_partial_merge_operands < 2, then it will be treated as 2.
+  //
+  // Default: 2
+  uint32_t min_partial_merge_operands;
+
   // Allow RocksDB to use thread local storage to optimize performance.
   // Default: true
   bool allow_thread_local;

util/options.cc

@@ -30,16 +30,15 @@ Options::Options()
     : comparator(BytewiseComparator()),
       merge_operator(nullptr),
       compaction_filter(nullptr),
-      compaction_filter_factory(
-          std::shared_ptr<CompactionFilterFactory>(
-            new DefaultCompactionFilterFactory())),
+      compaction_filter_factory(std::shared_ptr<CompactionFilterFactory>(
+          new DefaultCompactionFilterFactory())),
       create_if_missing(false),
       error_if_exists(false),
       paranoid_checks(false),
       env(Env::Default()),
       info_log(nullptr),
       info_log_level(INFO),
-      write_buffer_size(4<<20),
+      write_buffer_size(4 << 20),
       max_write_buffer_number(2),
       min_write_buffer_number_to_merge(1),
       max_open_files(1000),
@@ -95,7 +94,7 @@ Options::Options()
       is_fd_close_on_exec(true),
       skip_log_error_on_recovery(false),
       stats_dump_period_sec(3600),
-      block_size_deviation (10),
+      block_size_deviation(10),
       advise_random_on_open(true),
       access_hint_on_compaction_start(NORMAL),
       use_adaptive_mutex(false),
@@ -106,13 +105,14 @@ Options::Options()
       max_sequential_skip_in_iterations(8),
       memtable_factory(std::shared_ptr<SkipListFactory>(new SkipListFactory)),
       table_factory(
-        std::shared_ptr<TableFactory>(new BlockBasedTableFactory())),
+          std::shared_ptr<TableFactory>(new BlockBasedTableFactory())),
       inplace_update_support(false),
       inplace_update_num_locks(10000),
       inplace_callback(nullptr),
       memtable_prefix_bloom_bits(0),
       memtable_prefix_bloom_probes(6),
       max_successive_merges(0),
+      min_partial_merge_operands(2),
       allow_thread_local(true) {
   assert(memtable_factory.get() != nullptr);
 }
@@ -306,6 +306,8 @@ Options::Dump(Logger* log) const
         inplace_update_support);
     Log(log, "                Options.inplace_update_num_locks: %zd",
         inplace_update_num_locks);
+    Log(log, "              Options.min_partial_merge_operands: %u",
+        min_partial_merge_operands);
     // TODO: easier config for bloom (maybe based on avg key/value size)
     Log(log, "              Options.memtable_prefix_bloom_bits: %d",
         memtable_prefix_bloom_bits);

utilities/merge_operators/put.cc

@@ -1,3 +1,8 @@
+//  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 <memory>
 #include "rocksdb/slice.h"
 #include "rocksdb/merge_operator.h"
@@ -38,6 +43,15 @@ class PutOperator : public MergeOperator {
     return true;
   }
 
+  using MergeOperator::PartialMergeMulti;
+  virtual bool PartialMergeMulti(const Slice& key,
+                                 const std::deque<Slice>& operand_list,
+                                 std::string* new_value, Logger* logger) const
+      override {
+    new_value->assign(operand_list.back().data(), operand_list.back().size());
+    return true;
+  }
+
   virtual const char* Name() const override {
     return "PutOperator";
   }

utilities/merge_operators/string_append/stringappend2.cc

@@ -6,6 +6,7 @@
 #include "stringappend2.h"
 
 #include <memory>
+#include <string>
 #include <assert.h>
 
 #include "rocksdb/slice.h"
@@ -61,31 +62,39 @@ bool StringAppendTESTOperator::FullMerge(
   return true;
 }
 
-bool StringAppendTESTOperator::PartialMerge(const Slice& key,
-                                            const Slice& left_operand,
-                                            const Slice& right_operand,
-                                            std::string* new_value,
-                                            Logger* logger) const {
+bool StringAppendTESTOperator::PartialMergeMulti(
+    const Slice& key, const std::deque<Slice>& operand_list,
+    std::string* new_value, Logger* logger) const {
   return false;
 }
 
 // A version of PartialMerge that actually performs "partial merging".
 // Use this to simulate the exact behaviour of the StringAppendOperator.
-bool StringAppendTESTOperator::_AssocPartialMerge(const Slice& key,
-                                            const Slice& left_operand,
-                                            const Slice& right_operand,
-                                            std::string* new_value,
-                                            Logger* logger) const {
-  // Clear the *new_value for writing.
+bool StringAppendTESTOperator::_AssocPartialMergeMulti(
+    const Slice& key, const std::deque<Slice>& operand_list,
+    std::string* new_value, Logger* logger) const {
+  // Clear the *new_value for writing
   assert(new_value);
   new_value->clear();
+  assert(operand_list.size() >= 2);
 
   // Generic append
-  // Reserve correct size for *new_value, and apply concatenation.
-  new_value->reserve(left_operand.size() + 1 + right_operand.size());
-  new_value->assign(left_operand.data(), left_operand.size());
-  new_value->append(1,delim_);
-  new_value->append(right_operand.data(), right_operand.size());
+  // Determine and reserve correct size for *new_value.
+  size_t size = 0;
+  for (const auto& operand : operand_list) {
+    size += operand.size();
+  }
+  size += operand_list.size() - 1;  // Delimiters
+  new_value->reserve(size);
+
+  // Apply concatenation
+  new_value->assign(operand_list.front().data(), operand_list.front().size());
+
+  for (std::deque<Slice>::const_iterator it = operand_list.begin() + 1;
+       it != operand_list.end(); ++it) {
+    new_value->append(1, delim_);
+    new_value->append(it->data(), it->size());
+  }
 
   return true;
 }

utilities/merge_operators/string_append/stringappend2.h

@@ -11,6 +11,9 @@
  */
 
 #pragma once
+#include <deque>
+#include <string>
+
 #include "rocksdb/merge_operator.h"
 #include "rocksdb/slice.h"
 
@@ -18,8 +21,8 @@ namespace rocksdb {
 
 class StringAppendTESTOperator : public MergeOperator {
  public:
-
-  StringAppendTESTOperator(char delim_char);    /// Constructor with delimiter
+  // Constructor with delimiter
+  explicit StringAppendTESTOperator(char delim_char);
 
   virtual bool FullMerge(const Slice& key,
                          const Slice* existing_value,
@@ -27,22 +30,19 @@ class StringAppendTESTOperator : public MergeOperator {
                          std::string* new_value,
                          Logger* logger) const override;
 
-  virtual bool PartialMerge(const Slice& key,
-                            const Slice& left_operand,
-                            const Slice& right_operand,
-                            std::string* new_value,
-                            Logger* logger) const override;
+  virtual bool PartialMergeMulti(const Slice& key,
+                                 const std::deque<Slice>& operand_list,
+                                 std::string* new_value, Logger* logger) const
+      override;
 
   virtual const char* Name() const override;
 
  private:
   // A version of PartialMerge that actually performs "partial merging".
   // Use this to simulate the exact behaviour of the StringAppendOperator.
-  bool _AssocPartialMerge(const Slice& key,
-                          const Slice& left_operand,
-                          const Slice& right_operand,
-                          std::string* new_value,
-                          Logger* logger) const;
+  bool _AssocPartialMergeMulti(const Slice& key,
+                               const std::deque<Slice>& operand_list,
+                               std::string* new_value, Logger* logger) const;
 
   char delim_;         // The delimiter is inserted between elements
 

utilities/ttl/db_ttl.h

@@ -3,6 +3,9 @@
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
 
 #pragma once
+#include <deque>
+#include <string>
+
 #include "rocksdb/db.h"
 #include "rocksdb/env.h"
 #include "rocksdb/compaction_filter.h"
@@ -268,24 +271,27 @@ class TtlMergeOperator : public MergeOperator {
     }
   }
 
-  virtual bool PartialMerge(const Slice& key,
-                            const Slice& left_operand,
-                            const Slice& right_operand,
-                            std::string* new_value,
-                            Logger* logger) const override {
+  virtual bool PartialMergeMulti(const Slice& key,
+                                 const std::deque<Slice>& operand_list,
+                                 std::string* new_value, Logger* logger) const
+      override {
     const uint32_t ts_len = DBWithTTL::kTSLength;
+    std::deque<Slice> operands_without_ts;
 
-    if (left_operand.size() < ts_len || right_operand.size() < ts_len) {
-      Log(logger, "Error: Could not remove timestamp from value.");
-      return false;
+    for (const auto& operand : operand_list) {
+      if (operand.size() < ts_len) {
+        Log(logger, "Error: Could not remove timestamp from value.");
+        return false;
+      }
+
+      operands_without_ts.push_back(
+          Slice(operand.data(), operand.size() - ts_len));
     }
 
     // Apply the user partial-merge operator (store result in *new_value)
     assert(new_value);
-    Slice left_without_ts(left_operand.data(), left_operand.size() - ts_len);
-    Slice right_without_ts(right_operand.data(), right_operand.size() - ts_len);
-    if (!user_merge_op_->PartialMerge(key, left_without_ts, right_without_ts,
-                                      new_value, logger)) {
+    if (!user_merge_op_->PartialMergeMulti(key, operands_without_ts, new_value,
+                                           logger)) {
       return false;
     }