Use only "local" range tombstones during Get (#4449)

Summary:
Previously, range tombstones were accumulated from every level, which
was necessary if a range tombstone in a higher level covered a key in a lower
level. However, RangeDelAggregator::AddTombstones's complexity is based on
the number of tombstones that are currently stored in it, which is wasteful in
the Get case, where we only need to know the highest sequence number of range
tombstones that cover the key from higher levels, and compute the highest covering
sequence number at the current level. This change introduces this optimization, and
removes the use of RangeDelAggregator from the Get path.

In the benchmark results, the following command was used to initialize the database:
```
./db_bench -db=/dev/shm/5k-rts -use_existing_db=false -benchmarks=filluniquerandom -write_buffer_size=1048576 -compression_type=lz4 -target_file_size_base=1048576 -max_bytes_for_level_base=4194304 -value_size=112 -key_size=16 -block_size=4096 -level_compaction_dynamic_level_bytes=true -num=5000000 -max_background_jobs=12 -benchmark_write_rate_limit=20971520 -range_tombstone_width=100 -writes_per_range_tombstone=100 -max_num_range_tombstones=50000 -bloom_bits=8
```

...and the following command was used to measure read throughput:
```
./db_bench -db=/dev/shm/5k-rts/ -use_existing_db=true -benchmarks=readrandom -disable_auto_compactions=true -num=5000000 -reads=100000 -threads=32
```

The filluniquerandom command was only run once, and the resulting database was used
to measure read performance before and after the PR. Both binaries were compiled with
`DEBUG_LEVEL=0`.

Readrandom results before PR:
```
readrandom   :       4.544 micros/op 220090 ops/sec;   16.9 MB/s (63103 of 100000 found)
```

Readrandom results after PR:
```
readrandom   :      11.147 micros/op 89707 ops/sec;    6.9 MB/s (63103 of 100000 found)
```

So it's actually slower right now, but this PR paves the way for future optimizations (see #4493).

----
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4449

Differential Revision: D10370575

Pulled By: abhimadan

fbshipit-source-id: 9a2e152be1ef36969055c0e9eb4beb0d96c11f4d

CMakeLists.txt

@@ -503,6 +503,7 @@ set(SOURCES
         db/merge_helper.cc
         db/merge_operator.cc
         db/range_del_aggregator.cc
+        db/range_tombstone_fragmenter.cc
         db/repair.cc
         db/snapshot_impl.cc
         db/table_cache.cc
@@ -901,6 +902,8 @@ if(WITH_TESTS)
         db/perf_context_test.cc
         db/plain_table_db_test.cc
         db/prefix_test.cc
+        db/range_del_aggregator_test.cc
+        db/range_tombstone_fragmenter_test.cc
         db/repair_test.cc
         db/table_properties_collector_test.cc
         db/version_builder_test.cc

Makefile

@@ -530,6 +530,7 @@ TESTS = \
 	db_universal_compaction_test \
 	trace_analyzer_test \
 	repeatable_thread_test \
+	range_tombstone_fragmenter_test \
 
 PARALLEL_TEST = \
 	backupable_db_test \
@@ -1556,6 +1557,9 @@ blob_db_test: utilities/blob_db/blob_db_test.o $(LIBOBJECTS) $(TESTHARNESS)
 repeatable_thread_test: util/repeatable_thread_test.o $(LIBOBJECTS) $(TESTHARNESS)
 	$(AM_LINK)
 
+range_tombstone_fragmenter_test: db/range_tombstone_fragmenter_test.o db/db_test_util.o $(LIBOBJECTS) $(TESTHARNESS)
+	$(AM_LINK)
+
 #-------------------------------------------------
 # make install related stuff
 INSTALL_PATH ?= /usr/local

TARGETS

@@ -123,6 +123,7 @@ cpp_library(
         "db/merge_helper.cc",
         "db/merge_operator.cc",
         "db/range_del_aggregator.cc",
+        "db/range_tombstone_fragmenter.cc",
         "db/repair.cc",
         "db/snapshot_impl.cc",
         "db/table_cache.cc",

db/db_impl.cc

@@ -1240,7 +1240,7 @@ Status DBImpl::GetImpl(const ReadOptions& read_options,
 
   // Prepare to store a list of merge operations if merge occurs.
   MergeContext merge_context;
-  RangeDelAggregator range_del_agg(cfd->internal_comparator(), snapshot);
+  SequenceNumber max_covering_tombstone_seq = 0;
 
   Status s;
   // First look in the memtable, then in the immutable memtable (if any).
@@ -1254,13 +1254,14 @@ Status DBImpl::GetImpl(const ReadOptions& read_options,
   bool done = false;
   if (!skip_memtable) {
     if (sv->mem->Get(lkey, pinnable_val->GetSelf(), &s, &merge_context,
-                     &range_del_agg, read_options, callback, is_blob_index)) {
+                     &max_covering_tombstone_seq, read_options, callback,
+                     is_blob_index)) {
       done = true;
       pinnable_val->PinSelf();
       RecordTick(stats_, MEMTABLE_HIT);
     } else if ((s.ok() || s.IsMergeInProgress()) &&
                sv->imm->Get(lkey, pinnable_val->GetSelf(), &s, &merge_context,
-                            &range_del_agg, read_options, callback,
+                            &max_covering_tombstone_seq, read_options, callback,
                             is_blob_index)) {
       done = true;
       pinnable_val->PinSelf();
@@ -1274,8 +1275,8 @@ Status DBImpl::GetImpl(const ReadOptions& read_options,
   if (!done) {
     PERF_TIMER_GUARD(get_from_output_files_time);
     sv->current->Get(read_options, lkey, pinnable_val, &s, &merge_context,
-                     &range_del_agg, value_found, nullptr, nullptr, callback,
+                     &max_covering_tombstone_seq, value_found, nullptr, nullptr,
-                     is_blob_index);
+                     callback, is_blob_index);
     RecordTick(stats_, MEMTABLE_MISS);
   }
 
@@ -1357,8 +1358,7 @@ std::vector<Status> DBImpl::MultiGet(
 
     LookupKey lkey(keys[i], snapshot);
     auto cfh = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family[i]);
-    RangeDelAggregator range_del_agg(cfh->cfd()->internal_comparator(),
+    SequenceNumber max_covering_tombstone_seq = 0;
-                                     snapshot);
     auto mgd_iter = multiget_cf_data.find(cfh->cfd()->GetID());
     assert(mgd_iter != multiget_cf_data.end());
     auto mgd = mgd_iter->second;
@@ -1369,11 +1369,12 @@ std::vector<Status> DBImpl::MultiGet(
     bool done = false;
     if (!skip_memtable) {
       if (super_version->mem->Get(lkey, value, &s, &merge_context,
-                                  &range_del_agg, read_options)) {
+                                  &max_covering_tombstone_seq, read_options)) {
         done = true;
         RecordTick(stats_, MEMTABLE_HIT);
       } else if (super_version->imm->Get(lkey, value, &s, &merge_context,
-                                         &range_del_agg, read_options)) {
+                                         &max_covering_tombstone_seq,
+                                         read_options)) {
         done = true;
         RecordTick(stats_, MEMTABLE_HIT);
       }
@@ -1382,7 +1383,7 @@ std::vector<Status> DBImpl::MultiGet(
       PinnableSlice pinnable_val;
       PERF_TIMER_GUARD(get_from_output_files_time);
       super_version->current->Get(read_options, lkey, &pinnable_val, &s,
-                                  &merge_context, &range_del_agg);
+                                  &merge_context, &max_covering_tombstone_seq);
       value->assign(pinnable_val.data(), pinnable_val.size());
       RecordTick(stats_, MEMTABLE_MISS);
     }
@@ -2913,8 +2914,7 @@ Status DBImpl::GetLatestSequenceForKey(SuperVersion* sv, const Slice& key,
                                        bool* is_blob_index) {
   Status s;
   MergeContext merge_context;
-  RangeDelAggregator range_del_agg(sv->mem->GetInternalKeyComparator(),
+  SequenceNumber max_covering_tombstone_seq = 0;
-                                   kMaxSequenceNumber);
 
   ReadOptions read_options;
   SequenceNumber current_seq = versions_->LastSequence();
@@ -2924,8 +2924,8 @@ Status DBImpl::GetLatestSequenceForKey(SuperVersion* sv, const Slice& key,
   *found_record_for_key = false;
 
   // Check if there is a record for this key in the latest memtable
-  sv->mem->Get(lkey, nullptr, &s, &merge_context, &range_del_agg, seq,
+  sv->mem->Get(lkey, nullptr, &s, &merge_context, &max_covering_tombstone_seq,
-               read_options, nullptr /*read_callback*/, is_blob_index);
+               seq, read_options, nullptr /*read_callback*/, is_blob_index);
 
   if (!(s.ok() || s.IsNotFound() || s.IsMergeInProgress())) {
     // unexpected error reading memtable.
@@ -2943,8 +2943,8 @@ Status DBImpl::GetLatestSequenceForKey(SuperVersion* sv, const Slice& key,
   }
 
   // Check if there is a record for this key in the immutable memtables
-  sv->imm->Get(lkey, nullptr, &s, &merge_context, &range_del_agg, seq,
+  sv->imm->Get(lkey, nullptr, &s, &merge_context, &max_covering_tombstone_seq,
-               read_options, nullptr /*read_callback*/, is_blob_index);
+               seq, read_options, nullptr /*read_callback*/, is_blob_index);
 
   if (!(s.ok() || s.IsNotFound() || s.IsMergeInProgress())) {
     // unexpected error reading memtable.
@@ -2962,8 +2962,9 @@ Status DBImpl::GetLatestSequenceForKey(SuperVersion* sv, const Slice& key,
   }
 
   // Check if there is a record for this key in the immutable memtables
-  sv->imm->GetFromHistory(lkey, nullptr, &s, &merge_context, &range_del_agg,
+  sv->imm->GetFromHistory(lkey, nullptr, &s, &merge_context,
-                          seq, read_options, is_blob_index);
+                          &max_covering_tombstone_seq, seq, read_options,
+                          is_blob_index);
 
   if (!(s.ok() || s.IsNotFound() || s.IsMergeInProgress())) {
     // unexpected error reading memtable.
@@ -2986,7 +2987,7 @@ Status DBImpl::GetLatestSequenceForKey(SuperVersion* sv, const Slice& key,
   if (!cache_only) {
     // Check tables
     sv->current->Get(read_options, lkey, nullptr, &s, &merge_context,
-                     &range_del_agg, nullptr /* value_found */,
+                     &max_covering_tombstone_seq, nullptr /* value_found */,
                      found_record_for_key, seq, nullptr /*read_callback*/,
                      is_blob_index);
 

db/db_impl_readonly.cc

@@ -45,17 +45,17 @@ Status DBImplReadOnly::Get(const ReadOptions& read_options,
   }
   SuperVersion* super_version = cfd->GetSuperVersion();
   MergeContext merge_context;
-  RangeDelAggregator range_del_agg(cfd->internal_comparator(), snapshot);
+  SequenceNumber max_covering_tombstone_seq = 0;
   LookupKey lkey(key, snapshot);
   PERF_TIMER_STOP(get_snapshot_time);
   if (super_version->mem->Get(lkey, pinnable_val->GetSelf(), &s, &merge_context,
-                              &range_del_agg, read_options)) {
+                              &max_covering_tombstone_seq, read_options)) {
     pinnable_val->PinSelf();
     RecordTick(stats_, MEMTABLE_HIT);
   } else {
     PERF_TIMER_GUARD(get_from_output_files_time);
     super_version->current->Get(read_options, lkey, pinnable_val, &s,
-                                &merge_context, &range_del_agg);
+                                &merge_context, &max_covering_tombstone_seq);
     RecordTick(stats_, MEMTABLE_MISS);
   }
   RecordTick(stats_, NUMBER_KEYS_READ);

db/memtable.cc

@@ -17,6 +17,7 @@
 #include "db/merge_context.h"
 #include "db/merge_helper.h"
 #include "db/pinned_iterators_manager.h"
+#include "db/range_tombstone_fragmenter.h"
 #include "db/read_callback.h"
 #include "monitoring/perf_context_imp.h"
 #include "monitoring/statistics.h"
@@ -570,7 +571,7 @@ struct Saver {
   const MergeOperator* merge_operator;
   // the merge operations encountered;
   MergeContext* merge_context;
-  RangeDelAggregator* range_del_agg;
+  SequenceNumber max_covering_tombstone_seq;
   MemTable* mem;
   Logger* logger;
   Statistics* statistics;
@@ -592,10 +593,10 @@ static bool SaveValue(void* arg, const char* entry) {
   Saver* s = reinterpret_cast<Saver*>(arg);
   assert(s != nullptr);
   MergeContext* merge_context = s->merge_context;
-  RangeDelAggregator* range_del_agg = s->range_del_agg;
+  SequenceNumber max_covering_tombstone_seq = s->max_covering_tombstone_seq;
   const MergeOperator* merge_operator = s->merge_operator;
 
-  assert(merge_context != nullptr && range_del_agg != nullptr);
+  assert(merge_context != nullptr);
 
   // entry format is:
   //    klength  varint32
@@ -623,7 +624,7 @@ static bool SaveValue(void* arg, const char* entry) {
     s->seq = seq;
 
     if ((type == kTypeValue || type == kTypeMerge || type == kTypeBlobIndex) &&
-        range_del_agg->ShouldDelete(Slice(key_ptr, key_length))) {
+        max_covering_tombstone_seq > seq) {
       type = kTypeRangeDeletion;
     }
     switch (type) {
@@ -720,9 +721,9 @@ static bool SaveValue(void* arg, const char* entry) {
 
 bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
                    MergeContext* merge_context,
-                   RangeDelAggregator* range_del_agg, SequenceNumber* seq,
+                   SequenceNumber* max_covering_tombstone_seq,
-                   const ReadOptions& read_opts, ReadCallback* callback,
+                   SequenceNumber* seq, const ReadOptions& read_opts,
-                   bool* is_blob_index) {
+                   ReadCallback* callback, bool* is_blob_index) {
   // The sequence number is updated synchronously in version_set.h
   if (IsEmpty()) {
     // Avoiding recording stats for speed.
@@ -732,11 +733,13 @@ bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
 
   std::unique_ptr<InternalIterator> range_del_iter(
       NewRangeTombstoneIterator(read_opts));
-  Status status = range_del_agg->AddTombstones(std::move(range_del_iter));
+  SequenceNumber snapshot = GetInternalKeySeqno(key.internal_key());
-  if (!status.ok()) {
+  FragmentedRangeTombstoneIterator fragment_iter(
-    *s = status;
+      std::move(range_del_iter), comparator_.comparator, snapshot);
-    return false;
+  *max_covering_tombstone_seq = std::max(
-  }
+      *max_covering_tombstone_seq,
+      MaxCoveringTombstoneSeqnum(&fragment_iter, key.internal_key(),
+                                 comparator_.comparator.user_comparator()));
 
   Slice user_key = key.user_key();
   bool found_final_value = false;
@@ -762,7 +765,7 @@ bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
     saver.seq = kMaxSequenceNumber;
     saver.mem = this;
     saver.merge_context = merge_context;
-    saver.range_del_agg = range_del_agg;
+    saver.max_covering_tombstone_seq = *max_covering_tombstone_seq;
     saver.merge_operator = moptions_.merge_operator;
     saver.logger = moptions_.info_log;
     saver.inplace_update_support = moptions_.inplace_update_support;

db/memtable.h

@@ -187,17 +187,19 @@ class MemTable {
   // On success, *s may be set to OK, NotFound, or MergeInProgress.  Any other
   // status returned indicates a corruption or other unexpected error.
   bool Get(const LookupKey& key, std::string* value, Status* s,
-           MergeContext* merge_context, RangeDelAggregator* range_del_agg,
+           MergeContext* merge_context,
-           SequenceNumber* seq, const ReadOptions& read_opts,
+           SequenceNumber* max_covering_tombstone_seq, SequenceNumber* seq,
-           ReadCallback* callback = nullptr, bool* is_blob_index = nullptr);
+           const ReadOptions& read_opts, ReadCallback* callback = nullptr,
+           bool* is_blob_index = nullptr);
 
   bool Get(const LookupKey& key, std::string* value, Status* s,
-           MergeContext* merge_context, RangeDelAggregator* range_del_agg,
+           MergeContext* merge_context,
+           SequenceNumber* max_covering_tombstone_seq,
            const ReadOptions& read_opts, ReadCallback* callback = nullptr,
            bool* is_blob_index = nullptr) {
     SequenceNumber seq;
-    return Get(key, value, s, merge_context, range_del_agg, &seq, read_opts,
+    return Get(key, value, s, merge_context, max_covering_tombstone_seq, &seq,
-               callback, is_blob_index);
+               read_opts, callback, is_blob_index);
   }
 
   // Attempts to update the new_value inplace, else does normal Add

db/memtable_list.cc

@@ -104,34 +104,36 @@ int MemTableList::NumFlushed() const {
 // Operands stores the list of merge operations to apply, so far.
 bool MemTableListVersion::Get(const LookupKey& key, std::string* value,
                               Status* s, MergeContext* merge_context,
-                              RangeDelAggregator* range_del_agg,
+                              SequenceNumber* max_covering_tombstone_seq,
                               SequenceNumber* seq, const ReadOptions& read_opts,
                               ReadCallback* callback, bool* is_blob_index) {
-  return GetFromList(&memlist_, key, value, s, merge_context, range_del_agg,
+  return GetFromList(&memlist_, key, value, s, merge_context,
-                     seq, read_opts, callback, is_blob_index);
+                     max_covering_tombstone_seq, seq, read_opts, callback,
+                     is_blob_index);
 }
 
 bool MemTableListVersion::GetFromHistory(
     const LookupKey& key, std::string* value, Status* s,
-    MergeContext* merge_context, RangeDelAggregator* range_del_agg,
+    MergeContext* merge_context, SequenceNumber* max_covering_tombstone_seq,
     SequenceNumber* seq, const ReadOptions& read_opts, bool* is_blob_index) {
   return GetFromList(&memlist_history_, key, value, s, merge_context,
-                     range_del_agg, seq, read_opts, nullptr /*read_callback*/,
+                     max_covering_tombstone_seq, seq, read_opts,
-                     is_blob_index);
+                     nullptr /*read_callback*/, is_blob_index);
 }
 
 bool MemTableListVersion::GetFromList(
     std::list<MemTable*>* list, const LookupKey& key, std::string* value,
-    Status* s, MergeContext* merge_context, RangeDelAggregator* range_del_agg,
+    Status* s, MergeContext* merge_context,
-    SequenceNumber* seq, const ReadOptions& read_opts, ReadCallback* callback,
+    SequenceNumber* max_covering_tombstone_seq, SequenceNumber* seq,
-    bool* is_blob_index) {
+    const ReadOptions& read_opts, ReadCallback* callback, bool* is_blob_index) {
   *seq = kMaxSequenceNumber;
 
   for (auto& memtable : *list) {
     SequenceNumber current_seq = kMaxSequenceNumber;
 
-    bool done = memtable->Get(key, value, s, merge_context, range_del_agg,
+    bool done =
-                              &current_seq, read_opts, callback, is_blob_index);
+        memtable->Get(key, value, s, merge_context, max_covering_tombstone_seq,
+                      &current_seq, read_opts, callback, is_blob_index);
     if (*seq == kMaxSequenceNumber) {
       // Store the most recent sequence number of any operation on this key.
       // Since we only care about the most recent change, we only need to

db/memtable_list.h

@@ -55,17 +55,19 @@ class MemTableListVersion {
   // will be stored in *seq on success (regardless of whether true/false is
   // returned).  Otherwise, *seq will be set to kMaxSequenceNumber.
   bool Get(const LookupKey& key, std::string* value, Status* s,
-           MergeContext* merge_context, RangeDelAggregator* range_del_agg,
+           MergeContext* merge_context,
-           SequenceNumber* seq, const ReadOptions& read_opts,
+           SequenceNumber* max_covering_tombstone_seq, SequenceNumber* seq,
-           ReadCallback* callback = nullptr, bool* is_blob_index = nullptr);
+           const ReadOptions& read_opts, ReadCallback* callback = nullptr,
+           bool* is_blob_index = nullptr);
 
   bool Get(const LookupKey& key, std::string* value, Status* s,
-           MergeContext* merge_context, RangeDelAggregator* range_del_agg,
+           MergeContext* merge_context,
+           SequenceNumber* max_covering_tombstone_seq,
            const ReadOptions& read_opts, ReadCallback* callback = nullptr,
            bool* is_blob_index = nullptr) {
     SequenceNumber seq;
-    return Get(key, value, s, merge_context, range_del_agg, &seq, read_opts,
+    return Get(key, value, s, merge_context, max_covering_tombstone_seq, &seq,
-               callback, is_blob_index);
+               read_opts, callback, is_blob_index);
   }
 
   // Similar to Get(), but searches the Memtable history of memtables that
@@ -74,17 +76,18 @@ class MemTableListVersion {
   // writes that are also present in the SST files.
   bool GetFromHistory(const LookupKey& key, std::string* value, Status* s,
                       MergeContext* merge_context,
-                      RangeDelAggregator* range_del_agg, SequenceNumber* seq,
+                      SequenceNumber* max_covering_tombstone_seq,
-                      const ReadOptions& read_opts,
+                      SequenceNumber* seq, const ReadOptions& read_opts,
                       bool* is_blob_index = nullptr);
   bool GetFromHistory(const LookupKey& key, std::string* value, Status* s,
                       MergeContext* merge_context,
-                      RangeDelAggregator* range_del_agg,
+                      SequenceNumber* max_covering_tombstone_seq,
                       const ReadOptions& read_opts,
                       bool* is_blob_index = nullptr) {
     SequenceNumber seq;
-    return GetFromHistory(key, value, s, merge_context, range_del_agg, &seq,
+    return GetFromHistory(key, value, s, merge_context,
-                          read_opts, is_blob_index);
+                          max_covering_tombstone_seq, &seq, read_opts,
+                          is_blob_index);
   }
 
   Status AddRangeTombstoneIterators(const ReadOptions& read_opts, Arena* arena,
@@ -123,8 +126,8 @@ class MemTableListVersion {
 
   bool GetFromList(std::list<MemTable*>* list, const LookupKey& key,
                    std::string* value, Status* s, MergeContext* merge_context,
-                   RangeDelAggregator* range_del_agg, SequenceNumber* seq,
+                   SequenceNumber* max_covering_tombstone_seq,
-                   const ReadOptions& read_opts,
+                   SequenceNumber* seq, const ReadOptions& read_opts,
                    ReadCallback* callback = nullptr,
                    bool* is_blob_index = nullptr);
 

db/memtable_list_test.cc

@@ -198,12 +198,12 @@ TEST_F(MemTableListTest, GetTest) {
   Status s;
   MergeContext merge_context;
   InternalKeyComparator ikey_cmp(options.comparator);
-  RangeDelAggregator range_del_agg(ikey_cmp, {} /* snapshots */);
+  SequenceNumber max_covering_tombstone_seq = 0;
   autovector<MemTable*> to_delete;
 
   LookupKey lkey("key1", seq);
   bool found = list.current()->Get(lkey, &value, &s, &merge_context,
-                                   &range_del_agg, ReadOptions());
+                                   &max_covering_tombstone_seq, ReadOptions());
   ASSERT_FALSE(found);
 
   // Create a MemTable
@@ -226,19 +226,19 @@ TEST_F(MemTableListTest, GetTest) {
   // Fetch the newly written keys
   merge_context.Clear();
   found = mem->Get(LookupKey("key1", seq), &value, &s, &merge_context,
-                   &range_del_agg, ReadOptions());
+                   &max_covering_tombstone_seq, ReadOptions());
   ASSERT_TRUE(s.ok() && found);
   ASSERT_EQ(value, "value1");
 
   merge_context.Clear();
   found = mem->Get(LookupKey("key1", 2), &value, &s, &merge_context,
-                   &range_del_agg, ReadOptions());
+                   &max_covering_tombstone_seq, ReadOptions());
   // MemTable found out that this key is *not* found (at this sequence#)
   ASSERT_TRUE(found && s.IsNotFound());
 
   merge_context.Clear();
   found = mem->Get(LookupKey("key2", seq), &value, &s, &merge_context,
-                   &range_del_agg, ReadOptions());
+                   &max_covering_tombstone_seq, ReadOptions());
   ASSERT_TRUE(s.ok() && found);
   ASSERT_EQ(value, "value2.2");
 
@@ -264,25 +264,28 @@ TEST_F(MemTableListTest, GetTest) {
 
   // Fetch keys via MemTableList
   merge_context.Clear();
-  found = list.current()->Get(LookupKey("key1", seq), &value, &s,
+  found =
-                              &merge_context, &range_del_agg, ReadOptions());
+      list.current()->Get(LookupKey("key1", seq), &value, &s, &merge_context,
+                          &max_covering_tombstone_seq, ReadOptions());
   ASSERT_TRUE(found && s.IsNotFound());
 
   merge_context.Clear();
   found = list.current()->Get(LookupKey("key1", saved_seq), &value, &s,
-                              &merge_context, &range_del_agg, ReadOptions());
+                              &merge_context, &max_covering_tombstone_seq,
+                              ReadOptions());
   ASSERT_TRUE(s.ok() && found);
   ASSERT_EQ("value1", value);
 
   merge_context.Clear();
-  found = list.current()->Get(LookupKey("key2", seq), &value, &s,
+  found =
-                              &merge_context, &range_del_agg, ReadOptions());
+      list.current()->Get(LookupKey("key2", seq), &value, &s, &merge_context,
+                          &max_covering_tombstone_seq, ReadOptions());
   ASSERT_TRUE(s.ok() && found);
   ASSERT_EQ(value, "value2.3");
 
   merge_context.Clear();
   found = list.current()->Get(LookupKey("key2", 1), &value, &s, &merge_context,
-                              &range_del_agg, ReadOptions());
+                              &max_covering_tombstone_seq, ReadOptions());
   ASSERT_FALSE(found);
 
   ASSERT_EQ(2, list.NumNotFlushed());
@@ -305,12 +308,12 @@ TEST_F(MemTableListTest, GetFromHistoryTest) {
   Status s;
   MergeContext merge_context;
   InternalKeyComparator ikey_cmp(options.comparator);
-  RangeDelAggregator range_del_agg(ikey_cmp, {} /* snapshots */);
+  SequenceNumber max_covering_tombstone_seq = 0;
   autovector<MemTable*> to_delete;
 
   LookupKey lkey("key1", seq);
   bool found = list.current()->Get(lkey, &value, &s, &merge_context,
-                                   &range_del_agg, ReadOptions());
+                                   &max_covering_tombstone_seq, ReadOptions());
   ASSERT_FALSE(found);
 
   // Create a MemTable
@@ -332,13 +335,13 @@ TEST_F(MemTableListTest, GetFromHistoryTest) {
   // Fetch the newly written keys
   merge_context.Clear();
   found = mem->Get(LookupKey("key1", seq), &value, &s, &merge_context,
-                   &range_del_agg, ReadOptions());
+                   &max_covering_tombstone_seq, ReadOptions());
   // MemTable found out that this key is *not* found (at this sequence#)
   ASSERT_TRUE(found && s.IsNotFound());
 
   merge_context.Clear();
   found = mem->Get(LookupKey("key2", seq), &value, &s, &merge_context,
-                   &range_del_agg, ReadOptions());
+                   &max_covering_tombstone_seq, ReadOptions());
   ASSERT_TRUE(s.ok() && found);
   ASSERT_EQ(value, "value2.2");
 
@@ -348,13 +351,15 @@ TEST_F(MemTableListTest, GetFromHistoryTest) {
 
   // Fetch keys via MemTableList
   merge_context.Clear();
-  found = list.current()->Get(LookupKey("key1", seq), &value, &s,
+  found =
-                              &merge_context, &range_del_agg, ReadOptions());
+      list.current()->Get(LookupKey("key1", seq), &value, &s, &merge_context,
+                          &max_covering_tombstone_seq, ReadOptions());
   ASSERT_TRUE(found && s.IsNotFound());
 
   merge_context.Clear();
-  found = list.current()->Get(LookupKey("key2", seq), &value, &s,
+  found =
-                              &merge_context, &range_del_agg, ReadOptions());
+      list.current()->Get(LookupKey("key2", seq), &value, &s, &merge_context,
+                          &max_covering_tombstone_seq, ReadOptions());
   ASSERT_TRUE(s.ok() && found);
   ASSERT_EQ("value2.2", value);
 
@@ -374,26 +379,28 @@ TEST_F(MemTableListTest, GetFromHistoryTest) {
 
   // Verify keys are no longer in MemTableList
   merge_context.Clear();
-  found = list.current()->Get(LookupKey("key1", seq), &value, &s,
+  found =
-                              &merge_context, &range_del_agg, ReadOptions());
+      list.current()->Get(LookupKey("key1", seq), &value, &s, &merge_context,
+                          &max_covering_tombstone_seq, ReadOptions());
   ASSERT_FALSE(found);
 
   merge_context.Clear();
-  found = list.current()->Get(LookupKey("key2", seq), &value, &s,
+  found =
-                              &merge_context, &range_del_agg, ReadOptions());
+      list.current()->Get(LookupKey("key2", seq), &value, &s, &merge_context,
+                          &max_covering_tombstone_seq, ReadOptions());
   ASSERT_FALSE(found);
 
   // Verify keys are present in history
   merge_context.Clear();
-  found = list.current()->GetFromHistory(LookupKey("key1", seq), &value, &s,
+  found = list.current()->GetFromHistory(
-                                         &merge_context, &range_del_agg,
+      LookupKey("key1", seq), &value, &s, &merge_context,
-                                         ReadOptions());
+      &max_covering_tombstone_seq, ReadOptions());
   ASSERT_TRUE(found && s.IsNotFound());
 
   merge_context.Clear();
-  found = list.current()->GetFromHistory(LookupKey("key2", seq), &value, &s,
+  found = list.current()->GetFromHistory(
-                                         &merge_context, &range_del_agg,
+      LookupKey("key2", seq), &value, &s, &merge_context,
-                                         ReadOptions());
+      &max_covering_tombstone_seq, ReadOptions());
   ASSERT_TRUE(found);
   ASSERT_EQ("value2.2", value);
 
@@ -434,38 +441,42 @@ TEST_F(MemTableListTest, GetFromHistoryTest) {
 
   // Verify keys are no longer in MemTableList
   merge_context.Clear();
-  found = list.current()->Get(LookupKey("key1", seq), &value, &s,
+  found =
-                              &merge_context, &range_del_agg, ReadOptions());
+      list.current()->Get(LookupKey("key1", seq), &value, &s, &merge_context,
+                          &max_covering_tombstone_seq, ReadOptions());
   ASSERT_FALSE(found);
 
   merge_context.Clear();
-  found = list.current()->Get(LookupKey("key2", seq), &value, &s,
+  found =
-                              &merge_context, &range_del_agg, ReadOptions());
+      list.current()->Get(LookupKey("key2", seq), &value, &s, &merge_context,
+                          &max_covering_tombstone_seq, ReadOptions());
   ASSERT_FALSE(found);
 
   merge_context.Clear();
-  found = list.current()->Get(LookupKey("key3", seq), &value, &s,
+  found =
-                              &merge_context, &range_del_agg, ReadOptions());
+      list.current()->Get(LookupKey("key3", seq), &value, &s, &merge_context,
+                          &max_covering_tombstone_seq, ReadOptions());
   ASSERT_FALSE(found);
 
   // Verify that the second memtable's keys are in the history
   merge_context.Clear();
-  found = list.current()->GetFromHistory(LookupKey("key1", seq), &value, &s,
+  found = list.current()->GetFromHistory(
-                                         &merge_context, &range_del_agg,
+      LookupKey("key1", seq), &value, &s, &merge_context,
-                                         ReadOptions());
+      &max_covering_tombstone_seq, ReadOptions());
   ASSERT_TRUE(found && s.IsNotFound());
 
   merge_context.Clear();
-  found = list.current()->GetFromHistory(LookupKey("key3", seq), &value, &s,
+  found = list.current()->GetFromHistory(
-                                         &merge_context, &range_del_agg,
+      LookupKey("key3", seq), &value, &s, &merge_context,
-                                         ReadOptions());
+      &max_covering_tombstone_seq, ReadOptions());
   ASSERT_TRUE(found);
   ASSERT_EQ("value3", value);
 
   // Verify that key2 from the first memtable is no longer in the history
   merge_context.Clear();
-  found = list.current()->Get(LookupKey("key2", seq), &value, &s,
+  found =
-                              &merge_context, &range_del_agg, ReadOptions());
+      list.current()->Get(LookupKey("key2", seq), &value, &s, &merge_context,
+                          &max_covering_tombstone_seq, ReadOptions());
   ASSERT_FALSE(found);
 
   // Cleanup

db/range_tombstone_fragmenter.cc

@@ -0,0 +1,277 @@
+//  Copyright (c) 2018-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+
+#include "db/range_tombstone_fragmenter.h"
+
+#include <algorithm>
+#include <functional>
+#include <set>
+
+#include <inttypes.h>
+#include <stdio.h>
+
+#include "util/kv_map.h"
+#include "util/vector_iterator.h"
+
+namespace rocksdb {
+
+namespace {
+
+struct ParsedInternalKeyComparator {
+  explicit ParsedInternalKeyComparator(const InternalKeyComparator* c)
+      : cmp(c) {}
+
+  bool operator()(const ParsedInternalKey& a,
+                  const ParsedInternalKey& b) const {
+    return cmp->Compare(a, b) < 0;
+  }
+
+  const InternalKeyComparator* cmp;
+};
+
+}  // anonymous namespace
+
+FragmentedRangeTombstoneIterator::FragmentedRangeTombstoneIterator(
+    std::unique_ptr<InternalIterator> unfragmented_tombstones,
+    const InternalKeyComparator& icmp, SequenceNumber snapshot)
+    : tombstone_cmp_(icmp.user_comparator()),
+      icmp_(&icmp),
+      ucmp_(icmp.user_comparator()) {
+  if (unfragmented_tombstones == nullptr) {
+    pos_ = tombstones_.end();
+    return;
+  }
+  bool is_sorted = true;
+  int num_tombstones = 0;
+  InternalKey pinned_last_start_key;
+  Slice last_start_key;
+  for (unfragmented_tombstones->SeekToFirst(); unfragmented_tombstones->Valid();
+       unfragmented_tombstones->Next(), num_tombstones++) {
+    if (num_tombstones > 0 &&
+        icmp_->Compare(last_start_key, unfragmented_tombstones->key()) > 0) {
+      is_sorted = false;
+      break;
+    }
+    if (unfragmented_tombstones->IsKeyPinned()) {
+      last_start_key = unfragmented_tombstones->key();
+    } else {
+      pinned_last_start_key.DecodeFrom(unfragmented_tombstones->key());
+      last_start_key = pinned_last_start_key.Encode();
+    }
+  }
+  if (is_sorted) {
+    FragmentTombstones(std::move(unfragmented_tombstones), snapshot);
+    return;
+  }
+
+  // Sort the tombstones before fragmenting them.
+  std::vector<std::string> keys, values;
+  keys.reserve(num_tombstones);
+  values.reserve(num_tombstones);
+  for (unfragmented_tombstones->SeekToFirst(); unfragmented_tombstones->Valid();
+       unfragmented_tombstones->Next()) {
+    keys.emplace_back(unfragmented_tombstones->key().data(),
+                      unfragmented_tombstones->key().size());
+    values.emplace_back(unfragmented_tombstones->value().data(),
+                        unfragmented_tombstones->value().size());
+  }
+  // VectorIterator implicitly sorts by key during construction.
+  auto iter = std::unique_ptr<VectorIterator>(
+      new VectorIterator(std::move(keys), std::move(values), icmp_));
+  FragmentTombstones(std::move(iter), snapshot);
+}
+
+void FragmentedRangeTombstoneIterator::FragmentTombstones(
+    std::unique_ptr<InternalIterator> unfragmented_tombstones,
+    SequenceNumber snapshot) {
+  Slice cur_start_key(nullptr, 0);
+  auto cmp = ParsedInternalKeyComparator(icmp_);
+
+  // Stores the end keys and sequence numbers of range tombstones with a start
+  // key less than or equal to cur_start_key. Provides an ordering by end key
+  // for use in flush_current_tombstones.
+  std::set<ParsedInternalKey, ParsedInternalKeyComparator> cur_end_keys(cmp);
+
+  // Given the next start key in unfragmented_tombstones,
+  // flush_current_tombstones writes every tombstone fragment that starts
+  // and ends with a key before next_start_key, and starts with a key greater
+  // than or equal to cur_start_key.
+  auto flush_current_tombstones = [&](const Slice& next_start_key) {
+    auto it = cur_end_keys.begin();
+    bool reached_next_start_key = false;
+    for (; it != cur_end_keys.end() && !reached_next_start_key; ++it) {
+      Slice cur_end_key = it->user_key;
+      if (icmp_->user_comparator()->Compare(cur_start_key, cur_end_key) == 0) {
+        // Empty tombstone.
+        continue;
+      }
+      if (icmp_->user_comparator()->Compare(next_start_key, cur_end_key) <= 0) {
+        // All of the end keys in [it, cur_end_keys.end()) are after
+        // next_start_key, so the tombstones they represent can be used in
+        // fragments that start with keys greater than or equal to
+        // next_start_key. However, the end keys we already passed will not be
+        // used in any more tombstone fragments.
+        //
+        // Remove the fully fragmented tombstones and stop iteration after a
+        // final round of flushing to preserve the tombstones we can create more
+        // fragments from.
+        reached_next_start_key = true;
+        cur_end_keys.erase(cur_end_keys.begin(), it);
+        cur_end_key = next_start_key;
+      }
+
+      // Flush a range tombstone fragment [cur_start_key, cur_end_key), which
+      // should not overlap with the last-flushed tombstone fragment.
+      assert(tombstones_.empty() ||
+             icmp_->user_comparator()->Compare(tombstones_.back().end_key_,
+                                               cur_start_key) <= 0);
+
+      SequenceNumber max_seqnum = 0;
+      for (auto flush_it = it; flush_it != cur_end_keys.end(); ++flush_it) {
+        max_seqnum = std::max(max_seqnum, flush_it->sequence);
+      }
+      // Flush only the tombstone fragment with the highest sequence
+      // number.
+      tombstones_.push_back(
+          RangeTombstone(cur_start_key, cur_end_key, max_seqnum));
+      cur_start_key = cur_end_key;
+    }
+    if (!reached_next_start_key) {
+      // There is a gap between the last flushed tombstone fragment and
+      // the next tombstone's start key. Remove all the end keys in
+      // the working set, since we have fully fragmented their corresponding
+      // tombstones.
+      cur_end_keys.clear();
+    }
+    cur_start_key = next_start_key;
+  };
+
+  pinned_iters_mgr_.StartPinning();
+
+  bool no_tombstones = true;
+  for (unfragmented_tombstones->SeekToFirst(); unfragmented_tombstones->Valid();
+       unfragmented_tombstones->Next()) {
+    const Slice& ikey = unfragmented_tombstones->key();
+    Slice tombstone_start_key = ExtractUserKey(ikey);
+    SequenceNumber tombstone_seq = GetInternalKeySeqno(ikey);
+    if (tombstone_seq > snapshot) {
+      // The tombstone is not visible by this snapshot.
+      continue;
+    }
+    no_tombstones = false;
+
+    Slice tombstone_end_key = unfragmented_tombstones->value();
+    if (!unfragmented_tombstones->IsValuePinned()) {
+      pinned_slices_.emplace_back(tombstone_end_key.data(),
+                                  tombstone_end_key.size());
+      tombstone_end_key = pinned_slices_.back();
+    }
+    if (!cur_end_keys.empty() && icmp_->user_comparator()->Compare(
+                                     cur_start_key, tombstone_start_key) != 0) {
+      // The start key has changed. Flush all tombstones that start before
+      // this new start key.
+      flush_current_tombstones(tombstone_start_key);
+    }
+    if (unfragmented_tombstones->IsKeyPinned()) {
+      cur_start_key = tombstone_start_key;
+    } else {
+      pinned_slices_.emplace_back(tombstone_start_key.data(),
+                                  tombstone_start_key.size());
+      cur_start_key = pinned_slices_.back();
+    }
+
+    cur_end_keys.emplace(tombstone_end_key, tombstone_seq, kTypeRangeDeletion);
+  }
+  if (!cur_end_keys.empty()) {
+    ParsedInternalKey last_end_key = *std::prev(cur_end_keys.end());
+    flush_current_tombstones(last_end_key.user_key);
+  }
+
+  if (!no_tombstones) {
+    pinned_iters_mgr_.PinIterator(unfragmented_tombstones.release(),
+                                  false /* arena */);
+  }
+
+  // With this, the caller must Seek before the iterator is valid.
+  pos_ = tombstones_.end();
+  pinned_pos_ = tombstones_.end();
+}
+
+void FragmentedRangeTombstoneIterator::SeekToFirst() {
+  pos_ = tombstones_.begin();
+}
+
+void FragmentedRangeTombstoneIterator::SeekToLast() {
+  pos_ = tombstones_.end();
+  Prev();
+}
+
+void FragmentedRangeTombstoneIterator::Seek(const Slice& target) {
+  if (tombstones_.empty()) {
+    pos_ = tombstones_.end();
+    return;
+  }
+  RangeTombstone search(ExtractUserKey(target), ExtractUserKey(target),
+                        GetInternalKeySeqno(target));
+  pos_ = std::lower_bound(tombstones_.begin(), tombstones_.end(), search,
+                          tombstone_cmp_);
+}
+
+void FragmentedRangeTombstoneIterator::SeekForPrev(const Slice& target) {
+  Seek(target);
+  if (!Valid()) {
+    SeekToLast();
+  }
+  ParsedInternalKey parsed_target;
+  if (!ParseInternalKey(target, &parsed_target)) {
+    assert(false);
+  }
+  ParsedInternalKey parsed_start_key;
+  ParseKey(&parsed_start_key);
+  while (Valid() && icmp_->Compare(parsed_target, parsed_start_key) < 0) {
+    Prev();
+    ParseKey(&parsed_start_key);
+  }
+}
+
+void FragmentedRangeTombstoneIterator::Next() { ++pos_; }
+
+void FragmentedRangeTombstoneIterator::Prev() {
+  if (pos_ == tombstones_.begin()) {
+    pos_ = tombstones_.end();
+    return;
+  }
+  --pos_;
+}
+
+bool FragmentedRangeTombstoneIterator::Valid() const {
+  return pos_ != tombstones_.end();
+}
+
+SequenceNumber MaxCoveringTombstoneSeqnum(
+    FragmentedRangeTombstoneIterator* tombstone_iter, const Slice& lookup_key,
+    const Comparator* ucmp) {
+  SequenceNumber snapshot = GetInternalKeySeqno(lookup_key);
+  Slice user_key = ExtractUserKey(lookup_key);
+
+  tombstone_iter->Seek(lookup_key);
+  SequenceNumber highest_covering_seqnum = 0;
+  if (!tombstone_iter->Valid()) {
+    // Seeked past the last tombstone
+    tombstone_iter->Prev();
+  }
+  while (tombstone_iter->Valid() &&
+         ucmp->Compare(user_key, tombstone_iter->value()) < 0) {
+    if (tombstone_iter->seq() <= snapshot &&
+        ucmp->Compare(tombstone_iter->user_key(), user_key) <= 0) {
+      highest_covering_seqnum =
+          std::max(highest_covering_seqnum, tombstone_iter->seq());
+    }
+    tombstone_iter->Prev();
+  }
+  return highest_covering_seqnum;
+}
+
+}  // namespace rocksdb

db/range_tombstone_fragmenter.h

@@ -0,0 +1,103 @@
+//  Copyright (c) 2018-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+
+#pragma once
+
+#include <list>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "db/dbformat.h"
+#include "db/pinned_iterators_manager.h"
+#include "rocksdb/status.h"
+#include "table/internal_iterator.h"
+
+namespace rocksdb {
+
+// FragmentedRangeTombstoneIterator converts an InternalIterator of a range-del
+// meta block into an iterator over non-overlapping tombstone fragments. The
+// tombstone fragmentation process should be more efficient than the range
+// tombstone collapsing algorithm in RangeDelAggregator because this leverages
+// the internal key ordering already provided by the input iterator, if
+// applicable (when the iterator is unsorted, a new sorted iterator is created
+// before proceeding). If there are few overlaps, creating a
+// FragmentedRangeTombstoneIterator should be O(n), while the RangeDelAggregator
+// tombstone collapsing is always O(n log n).
+class FragmentedRangeTombstoneIterator : public InternalIterator {
+ public:
+  FragmentedRangeTombstoneIterator(
+      std::unique_ptr<InternalIterator> unfragmented_tombstones,
+      const InternalKeyComparator& icmp, SequenceNumber snapshot);
+  void SeekToFirst() override;
+  void SeekToLast() override;
+  void Seek(const Slice& target) override;
+  void SeekForPrev(const Slice& target) override;
+  void Next() override;
+  void Prev() override;
+  bool Valid() const override;
+  Slice key() const override {
+    MaybePinKey();
+    return current_start_key_.Encode();
+  }
+  Slice value() const override { return pos_->end_key_; }
+  bool IsKeyPinned() const override { return false; }
+  bool IsValuePinned() const override { return true; }
+  Status status() const override { return Status::OK(); }
+
+  Slice user_key() const { return pos_->start_key_; }
+  SequenceNumber seq() const { return pos_->seq_; }
+
+ private:
+  struct FragmentedRangeTombstoneComparator {
+    explicit FragmentedRangeTombstoneComparator(const Comparator* c) : cmp(c) {}
+
+    bool operator()(const RangeTombstone& a, const RangeTombstone& b) const {
+      int user_key_cmp = cmp->Compare(a.start_key_, b.start_key_);
+      if (user_key_cmp != 0) {
+        return user_key_cmp < 0;
+      }
+      return a.seq_ > b.seq_;
+    }
+
+    const Comparator* cmp;
+  };
+
+  void MaybePinKey() const {
+    if (pos_ != tombstones_.end() && pinned_pos_ != pos_) {
+      current_start_key_.Set(pos_->start_key_, pos_->seq_, kTypeRangeDeletion);
+      pinned_pos_ = pos_;
+    }
+  }
+
+  void ParseKey(ParsedInternalKey* parsed) const {
+    parsed->user_key = pos_->start_key_;
+    parsed->sequence = pos_->seq_;
+    parsed->type = kTypeRangeDeletion;
+  }
+
+  // Given an ordered range tombstone iterator unfragmented_tombstones,
+  // "fragment" the tombstones into non-overlapping pieces, and store them in
+  // tombstones_.
+  void FragmentTombstones(
+      std::unique_ptr<InternalIterator> unfragmented_tombstones,
+      SequenceNumber snapshot);
+
+  const FragmentedRangeTombstoneComparator tombstone_cmp_;
+  const InternalKeyComparator* icmp_;
+  const Comparator* ucmp_;
+  std::vector<RangeTombstone> tombstones_;
+  std::list<std::string> pinned_slices_;
+  std::vector<RangeTombstone>::const_iterator pos_;
+  mutable std::vector<RangeTombstone>::const_iterator pinned_pos_;
+  mutable InternalKey current_start_key_;
+  PinnedIteratorsManager pinned_iters_mgr_;
+};
+
+SequenceNumber MaxCoveringTombstoneSeqnum(
+    FragmentedRangeTombstoneIterator* tombstone_iter, const Slice& key,
+    const Comparator* ucmp);
+
+}  // namespace rocksdb

db/range_tombstone_fragmenter_test.cc

@@ -0,0 +1,275 @@
+//  Copyright (c) 2018-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+
+#include "db/range_tombstone_fragmenter.h"
+
+#include "db/db_test_util.h"
+#include "rocksdb/comparator.h"
+#include "util/testutil.h"
+
+namespace rocksdb {
+
+class RangeTombstoneFragmenterTest : public testing::Test {};
+
+namespace {
+
+static auto bytewise_icmp = InternalKeyComparator(BytewiseComparator());
+
+std::unique_ptr<InternalIterator> MakeRangeDelIter(
+    const std::vector<RangeTombstone>& range_dels) {
+  std::vector<std::string> keys, values;
+  for (const auto& range_del : range_dels) {
+    auto key_and_value = range_del.Serialize();
+    keys.push_back(key_and_value.first.Encode().ToString());
+    values.push_back(key_and_value.second.ToString());
+  }
+  return std::unique_ptr<test::VectorIterator>(
+      new test::VectorIterator(keys, values));
+}
+
+void VerifyFragmentedRangeDels(
+    FragmentedRangeTombstoneIterator* iter,
+    const std::vector<RangeTombstone>& expected_tombstones) {
+  iter->SeekToFirst();
+  for (size_t i = 0; i < expected_tombstones.size() && iter->Valid();
+       i++, iter->Next()) {
+    EXPECT_EQ(ExtractUserKey(iter->key()), expected_tombstones[i].start_key_);
+    EXPECT_EQ(iter->value(), expected_tombstones[i].end_key_);
+    EXPECT_EQ(GetInternalKeySeqno(iter->key()), expected_tombstones[i].seq_);
+  }
+  EXPECT_FALSE(iter->Valid());
+}
+
+struct SeekForPrevTestCase {
+  Slice seek_target;
+  RangeTombstone expected_position;
+  bool out_of_range;
+};
+
+void VerifySeekForPrev(FragmentedRangeTombstoneIterator* iter,
+                       const std::vector<SeekForPrevTestCase>& cases) {
+  for (const auto& testcase : cases) {
+    InternalKey ikey_seek_target(testcase.seek_target, 0, kTypeRangeDeletion);
+    iter->SeekForPrev(ikey_seek_target.Encode());
+    if (testcase.out_of_range) {
+      ASSERT_FALSE(iter->Valid());
+    } else {
+      ASSERT_TRUE(iter->Valid());
+      EXPECT_EQ(ExtractUserKey(iter->key()),
+                testcase.expected_position.start_key_);
+      EXPECT_EQ(iter->value(), testcase.expected_position.end_key_);
+      EXPECT_EQ(GetInternalKeySeqno(iter->key()),
+                testcase.expected_position.seq_);
+    }
+  }
+}
+
+struct MaxCoveringTombstoneSeqnumTestCase {
+  Slice user_key;
+  int result;
+};
+
+void VerifyMaxCoveringTombstoneSeqnum(
+    FragmentedRangeTombstoneIterator* iter, const Comparator* ucmp,
+    const std::vector<MaxCoveringTombstoneSeqnumTestCase>& cases) {
+  for (const auto& testcase : cases) {
+    InternalKey key_and_snapshot(testcase.user_key, kMaxSequenceNumber,
+                                 kTypeValue);
+    EXPECT_EQ(testcase.result, MaxCoveringTombstoneSeqnum(
+                                   iter, key_and_snapshot.Encode(), ucmp));
+  }
+}
+
+}  // anonymous namespace
+
+TEST_F(RangeTombstoneFragmenterTest, NonOverlappingTombstones) {
+  auto range_del_iter = MakeRangeDelIter({{"a", "b", 10}, {"c", "d", 5}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, kMaxSequenceNumber);
+  VerifyFragmentedRangeDels(&iter, {{"a", "b", 10}, {"c", "d", 5}});
+  VerifyMaxCoveringTombstoneSeqnum(&iter, bytewise_icmp.user_comparator(),
+                                   {{"", 0}, {"a", 10}, {"b", 0}, {"c", 5}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, OverlappingTombstones) {
+  auto range_del_iter = MakeRangeDelIter({{"a", "e", 10}, {"c", "g", 15}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, kMaxSequenceNumber);
+  VerifyFragmentedRangeDels(&iter,
+                            {{"a", "c", 10}, {"c", "e", 15}, {"e", "g", 15}});
+  VerifyMaxCoveringTombstoneSeqnum(&iter, bytewise_icmp.user_comparator(),
+                                   {{"a", 10}, {"c", 15}, {"e", 15}, {"g", 0}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, ContiguousTombstones) {
+  auto range_del_iter = MakeRangeDelIter(
+      {{"a", "c", 10}, {"c", "e", 20}, {"c", "e", 5}, {"e", "g", 15}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, kMaxSequenceNumber);
+  VerifyFragmentedRangeDels(&iter,
+                            {{"a", "c", 10}, {"c", "e", 20}, {"e", "g", 15}});
+  VerifyMaxCoveringTombstoneSeqnum(&iter, bytewise_icmp.user_comparator(),
+                                   {{"a", 10}, {"c", 20}, {"e", 15}, {"g", 0}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, RepeatedStartAndEndKey) {
+  auto range_del_iter =
+      MakeRangeDelIter({{"a", "c", 10}, {"a", "c", 7}, {"a", "c", 3}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, kMaxSequenceNumber);
+  VerifyFragmentedRangeDels(&iter, {{"a", "c", 10}});
+  VerifyMaxCoveringTombstoneSeqnum(&iter, bytewise_icmp.user_comparator(),
+                                   {{"a", 10}, {"b", 10}, {"c", 0}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, RepeatedStartKeyDifferentEndKeys) {
+  auto range_del_iter =
+      MakeRangeDelIter({{"a", "e", 10}, {"a", "g", 7}, {"a", "c", 3}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, kMaxSequenceNumber);
+  VerifyFragmentedRangeDels(&iter,
+                            {{"a", "c", 10}, {"c", "e", 10}, {"e", "g", 7}});
+  VerifyMaxCoveringTombstoneSeqnum(&iter, bytewise_icmp.user_comparator(),
+                                   {{"a", 10}, {"c", 10}, {"e", 7}, {"g", 0}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, RepeatedStartKeyMixedEndKeys) {
+  auto range_del_iter = MakeRangeDelIter({{"a", "c", 30},
+                                          {"a", "g", 20},
+                                          {"a", "e", 10},
+                                          {"a", "g", 7},
+                                          {"a", "c", 3}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, kMaxSequenceNumber);
+  VerifyFragmentedRangeDels(&iter,
+                            {{"a", "c", 30}, {"c", "e", 20}, {"e", "g", 20}});
+  VerifyMaxCoveringTombstoneSeqnum(&iter, bytewise_icmp.user_comparator(),
+                                   {{"a", 30}, {"c", 20}, {"e", 20}, {"g", 0}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, OverlapAndRepeatedStartKey) {
+  auto range_del_iter = MakeRangeDelIter({{"a", "e", 10},
+                                          {"c", "g", 8},
+                                          {"c", "i", 6},
+                                          {"j", "n", 4},
+                                          {"j", "l", 2}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, kMaxSequenceNumber);
+  VerifyFragmentedRangeDels(&iter, {{"a", "c", 10},
+                                    {"c", "e", 10},
+                                    {"e", "g", 8},
+                                    {"g", "i", 6},
+                                    {"j", "l", 4},
+                                    {"l", "n", 4}});
+  VerifyMaxCoveringTombstoneSeqnum(
+      &iter, bytewise_icmp.user_comparator(),
+      {{"a", 10}, {"c", 10}, {"e", 8}, {"i", 0}, {"j", 4}, {"m", 4}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, OverlapAndRepeatedStartKeyWithSnapshot) {
+  auto range_del_iter = MakeRangeDelIter({{"a", "e", 10},
+                                          {"c", "g", 8},
+                                          {"c", "i", 6},
+                                          {"j", "n", 4},
+                                          {"j", "l", 2}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, 9);
+  VerifyFragmentedRangeDels(
+      &iter, {{"c", "g", 8}, {"g", "i", 6}, {"j", "l", 4}, {"l", "n", 4}});
+  VerifyMaxCoveringTombstoneSeqnum(
+      &iter, bytewise_icmp.user_comparator(),
+      {{"a", 0}, {"c", 8}, {"e", 8}, {"i", 0}, {"j", 4}, {"m", 4}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, OverlapAndRepeatedStartKeyUnordered) {
+  auto range_del_iter = MakeRangeDelIter({{"a", "e", 10},
+                                          {"j", "n", 4},
+                                          {"c", "i", 6},
+                                          {"c", "g", 8},
+                                          {"j", "l", 2}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, 9);
+  VerifyFragmentedRangeDels(
+      &iter, {{"c", "g", 8}, {"g", "i", 6}, {"j", "l", 4}, {"l", "n", 4}});
+  VerifyMaxCoveringTombstoneSeqnum(
+      &iter, bytewise_icmp.user_comparator(),
+      {{"a", 0}, {"c", 8}, {"e", 8}, {"i", 0}, {"j", 4}, {"m", 4}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, SeekForPrevStartKey) {
+  // Same tombstones as OverlapAndRepeatedStartKey.
+  auto range_del_iter = MakeRangeDelIter({{"a", "e", 10},
+                                          {"c", "g", 8},
+                                          {"c", "i", 6},
+                                          {"j", "n", 4},
+                                          {"j", "l", 2}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, kMaxSequenceNumber);
+  VerifySeekForPrev(
+      &iter,
+      {{"a", {"a", "c", 10}}, {"e", {"e", "g", 8}}, {"l", {"l", "n", 4}}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, SeekForPrevCovered) {
+  // Same tombstones as OverlapAndRepeatedStartKey.
+  auto range_del_iter = MakeRangeDelIter({{"a", "e", 10},
+                                          {"c", "g", 8},
+                                          {"c", "i", 6},
+                                          {"j", "n", 4},
+                                          {"j", "l", 2}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, kMaxSequenceNumber);
+  VerifySeekForPrev(
+      &iter,
+      {{"b", {"a", "c", 10}}, {"f", {"e", "g", 8}}, {"m", {"l", "n", 4}}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, SeekForPrevEndKey) {
+  // Same tombstones as OverlapAndRepeatedStartKey.
+  auto range_del_iter = MakeRangeDelIter({{"a", "e", 10},
+                                          {"c", "g", 8},
+                                          {"c", "i", 6},
+                                          {"j", "n", 4},
+                                          {"j", "l", 2}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, kMaxSequenceNumber);
+  VerifySeekForPrev(&iter, {{"c", {"c", "e", 10}},
+                            {"g", {"g", "i", 6}},
+                            {"i", {"g", "i", 6}},
+                            {"n", {"l", "n", 4}}});
+}
+
+TEST_F(RangeTombstoneFragmenterTest, SeekForPrevOutOfBounds) {
+  // Same tombstones as OverlapAndRepeatedStartKey.
+  auto range_del_iter = MakeRangeDelIter({{"a", "e", 10},
+                                          {"c", "g", 8},
+                                          {"c", "i", 6},
+                                          {"j", "n", 4},
+                                          {"j", "l", 2}});
+
+  FragmentedRangeTombstoneIterator iter(std::move(range_del_iter),
+                                        bytewise_icmp, kMaxSequenceNumber);
+  VerifySeekForPrev(&iter,
+                    {{"", {}, true /* out of range */}, {"z", {"l", "n", 4}}});
+}
+
+}  // namespace rocksdb
+
+int main(int argc, char** argv) {
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}

db/table_cache.cc

@@ -10,6 +10,7 @@
 #include "db/table_cache.h"
 
 #include "db/dbformat.h"
+#include "db/range_tombstone_fragmenter.h"
 #include "db/version_edit.h"
 #include "util/filename.h"
 
@@ -372,19 +373,19 @@ Status TableCache::Get(const ReadOptions& options,
         t = GetTableReaderFromHandle(handle);
       }
     }
-    if (s.ok() && get_context->range_del_agg() != nullptr &&
+    SequenceNumber* max_covering_tombstone_seq =
+        get_context->max_covering_tombstone_seq();
+    if (s.ok() && max_covering_tombstone_seq != nullptr &&
         !options.ignore_range_deletions) {
       std::unique_ptr<InternalIterator> range_del_iter(
           t->NewRangeTombstoneIterator(options));
-      if (range_del_iter != nullptr) {
+      FragmentedRangeTombstoneIterator fragment_iter(std::move(range_del_iter),
-        s = range_del_iter->status();
+                                                     internal_comparator,
-      }
+                                                     GetInternalKeySeqno(k));
-      if (s.ok()) {
+      *max_covering_tombstone_seq = std::max(
-        s = get_context->range_del_agg()->AddTombstones(
+          *max_covering_tombstone_seq,
-            std::move(range_del_iter),
+          MaxCoveringTombstoneSeqnum(&fragment_iter, k,
-            &file_meta.smallest,
+                                     internal_comparator.user_comparator()));
-            &file_meta.largest);
-      }
     }
     if (s.ok()) {
       get_context->SetReplayLog(row_cache_entry);  // nullptr if no cache.

db/version_set.cc

@@ -1177,7 +1177,7 @@ Version::Version(ColumnFamilyData* column_family_data, VersionSet* vset,
 void Version::Get(const ReadOptions& read_options, const LookupKey& k,
                   PinnableSlice* value, Status* status,
                   MergeContext* merge_context,
-                  RangeDelAggregator* range_del_agg, bool* value_found,
+                  SequenceNumber* max_covering_tombstone_seq, bool* value_found,
                   bool* key_exists, SequenceNumber* seq, ReadCallback* callback,
                   bool* is_blob) {
   Slice ikey = k.internal_key();
@@ -1194,8 +1194,8 @@ void Version::Get(const ReadOptions& read_options, const LookupKey& k,
   GetContext get_context(
       user_comparator(), merge_operator_, info_log_, db_statistics_,
       status->ok() ? GetContext::kNotFound : GetContext::kMerge, user_key,
-      value, value_found, merge_context, range_del_agg, this->env_, seq,
+      value, value_found, merge_context, max_covering_tombstone_seq, this->env_,
-      merge_operator_ ? &pinned_iters_mgr : nullptr, callback, is_blob);
+      seq, merge_operator_ ? &pinned_iters_mgr : nullptr, callback, is_blob);
 
   // Pin blocks that we read to hold merge operands
   if (merge_operator_) {

db/version_set.h

@@ -567,9 +567,10 @@ class Version {
   // REQUIRES: lock is not held
   void Get(const ReadOptions&, const LookupKey& key, PinnableSlice* value,
            Status* status, MergeContext* merge_context,
-           RangeDelAggregator* range_del_agg, bool* value_found = nullptr,
+           SequenceNumber* max_covering_tombstone_seq,
-           bool* key_exists = nullptr, SequenceNumber* seq = nullptr,
+           bool* value_found = nullptr, bool* key_exists = nullptr,
-           ReadCallback* callback = nullptr, bool* is_blob = nullptr);
+           SequenceNumber* seq = nullptr, ReadCallback* callback = nullptr,
+           bool* is_blob = nullptr);
 
   // Loads some stats information from files. Call without mutex held. It needs
   // to be called before applying the version to the version set.

src.mk

@@ -43,6 +43,7 @@ LIB_SOURCES =                                                   \
   db/merge_helper.cc                                            \
   db/merge_operator.cc                                          \
   db/range_del_aggregator.cc                                    \
+  db/range_tombstone_fragmenter.cc                              \
   db/repair.cc                                                  \
   db/snapshot_impl.cc                                           \
   db/table_cache.cc                                             \
@@ -329,6 +330,7 @@ MAIN_SOURCES =                                                          \
   db/repair_test.cc                                                     \
   db/range_del_aggregator_test.cc                                       \
   db/range_del_aggregator_bench.cc                                      \
+  db/range_tombstone_fragmenter_test.cc                                 \
   db/table_properties_collector_test.cc                                 \
   db/util_merge_operators_test.cc                                       \
   db/version_builder_test.cc                                            \

table/data_block_hash_index_test.cc

@@ -7,12 +7,14 @@
 #include <string>
 #include <unordered_map>
 
+#include "db/table_properties_collector.h"
 #include "rocksdb/slice.h"
 #include "table/block.h"
 #include "table/block_based_table_reader.h"
 #include "table/block_builder.h"
 #include "table/data_block_hash_index.h"
 #include "table/get_context.h"
+#include "table/table_builder.h"
 #include "util/testharness.h"
 #include "util/testutil.h"
 

table/get_context.cc

@@ -43,7 +43,7 @@ GetContext::GetContext(const Comparator* ucmp,
                        Statistics* statistics, GetState init_state,
                        const Slice& user_key, PinnableSlice* pinnable_val,
                        bool* value_found, MergeContext* merge_context,
-                       RangeDelAggregator* _range_del_agg, Env* env,
+                       SequenceNumber* _max_covering_tombstone_seq, Env* env,
                        SequenceNumber* seq,
                        PinnedIteratorsManager* _pinned_iters_mgr,
                        ReadCallback* callback, bool* is_blob_index)
@@ -56,7 +56,7 @@ GetContext::GetContext(const Comparator* ucmp,
       pinnable_val_(pinnable_val),
       value_found_(value_found),
       merge_context_(merge_context),
-      range_del_agg_(_range_del_agg),
+      max_covering_tombstone_seq_(_max_covering_tombstone_seq),
       env_(env),
       seq_(seq),
       replay_log_(nullptr),
@@ -185,7 +185,8 @@ bool GetContext::SaveValue(const ParsedInternalKey& parsed_key,
     auto type = parsed_key.type;
     // Key matches. Process it
     if ((type == kTypeValue || type == kTypeMerge || type == kTypeBlobIndex) &&
-        range_del_agg_ != nullptr && range_del_agg_->ShouldDelete(parsed_key)) {
+        max_covering_tombstone_seq_ != nullptr &&
+        *max_covering_tombstone_seq_ > parsed_key.sequence) {
       type = kTypeRangeDeletion;
     }
     switch (type) {

table/get_context.h

@@ -6,7 +6,6 @@
 #pragma once
 #include <string>
 #include "db/merge_context.h"
-#include "db/range_del_aggregator.h"
 #include "db/read_callback.h"
 #include "rocksdb/env.h"
 #include "rocksdb/statistics.h"
@@ -52,8 +51,9 @@ class GetContext {
   GetContext(const Comparator* ucmp, const MergeOperator* merge_operator,
              Logger* logger, Statistics* statistics, GetState init_state,
              const Slice& user_key, PinnableSlice* value, bool* value_found,
-             MergeContext* merge_context, RangeDelAggregator* range_del_agg,
+             MergeContext* merge_context,
-             Env* env, SequenceNumber* seq = nullptr,
+             SequenceNumber* max_covering_tombstone_seq, Env* env,
+             SequenceNumber* seq = nullptr,
              PinnedIteratorsManager* _pinned_iters_mgr = nullptr,
              ReadCallback* callback = nullptr, bool* is_blob_index = nullptr);
 
@@ -76,7 +76,9 @@ class GetContext {
 
   GetState State() const { return state_; }
 
-  RangeDelAggregator* range_del_agg() { return range_del_agg_; }
+  SequenceNumber* max_covering_tombstone_seq() {
+    return max_covering_tombstone_seq_;
+  }
 
   PinnedIteratorsManager* pinned_iters_mgr() { return pinned_iters_mgr_; }
 
@@ -111,7 +113,7 @@ class GetContext {
   PinnableSlice* pinnable_val_;
   bool* value_found_;  // Is value set correctly? Used by KeyMayExist
   MergeContext* merge_context_;
-  RangeDelAggregator* range_del_agg_;
+  SequenceNumber* max_covering_tombstone_seq_;
   Env* env_;
   // If a key is found, seq_ will be set to the SequenceNumber of most recent
   // write to the key or kMaxSequenceNumber if unknown

table/table_reader_bench.cc

@@ -170,12 +170,12 @@ void TableReaderBenchmark(Options& opts, EnvOptions& env_options,
           if (!through_db) {
             PinnableSlice value;
             MergeContext merge_context;
-            RangeDelAggregator range_del_agg(ikc, {} /* snapshots */);
+            SequenceNumber max_covering_tombstone_seq = 0;
             GetContext get_context(ioptions.user_comparator,
                                    ioptions.merge_operator, ioptions.info_log,
                                    ioptions.statistics, GetContext::kNotFound,
                                    Slice(key), &value, nullptr, &merge_context,
-                                   &range_del_agg, env);
+                                   &max_covering_tombstone_seq, env);
             s = table_reader->Get(read_options, key, &get_context, nullptr);
           } else {
             s = db->Get(read_options, key, &result);

util/vector_iterator.h

@@ -0,0 +1,100 @@
+#pragma once
+
+#include <algorithm>
+#include <string>
+#include <vector>
+
+#include "db/dbformat.h"
+#include "rocksdb/iterator.h"
+#include "rocksdb/slice.h"
+#include "table/internal_iterator.h"
+
+namespace rocksdb {
+
+// Iterator over a vector of keys/values
+class VectorIterator : public InternalIterator {
+ public:
+  VectorIterator(std::vector<std::string> keys, std::vector<std::string> values,
+                 const InternalKeyComparator* icmp)
+      : keys_(std::move(keys)),
+        values_(std::move(values)),
+        indexed_cmp_(icmp, &keys_),
+        current_(keys.size()) {
+    assert(keys_.size() == values_.size());
+
+    indices_.reserve(keys_.size());
+    for (size_t i = 0; i < keys_.size(); i++) {
+      indices_.push_back(i);
+    }
+    std::sort(indices_.begin(), indices_.end(), indexed_cmp_);
+  }
+
+  virtual bool Valid() const override {
+    return !indices_.empty() && current_ < indices_.size();
+  }
+
+  virtual void SeekToFirst() override { current_ = 0; }
+  virtual void SeekToLast() override { current_ = indices_.size() - 1; }
+
+  virtual void Seek(const Slice& target) override {
+    current_ = std::lower_bound(indices_.begin(), indices_.end(), target,
+                                indexed_cmp_) -
+               indices_.begin();
+  }
+
+  virtual void SeekForPrev(const Slice& target) override {
+    current_ = std::lower_bound(indices_.begin(), indices_.end(), target,
+                                indexed_cmp_) -
+               indices_.begin();
+    if (!Valid()) {
+      SeekToLast();
+    } else {
+      Prev();
+    }
+  }
+
+  virtual void Next() override { current_++; }
+  virtual void Prev() override { current_--; }
+
+  virtual Slice key() const override {
+    return Slice(keys_[indices_[current_]]);
+  }
+  virtual Slice value() const override {
+    return Slice(values_[indices_[current_]]);
+  }
+
+  virtual Status status() const override { return Status::OK(); }
+
+  virtual bool IsKeyPinned() const override { return true; }
+  virtual bool IsValuePinned() const override { return true; }
+
+ private:
+  struct IndexedKeyComparator {
+    IndexedKeyComparator(const InternalKeyComparator* c,
+                         const std::vector<std::string>* ks)
+        : cmp(c), keys(ks) {}
+
+    bool operator()(size_t a, size_t b) const {
+      return cmp->Compare((*keys)[a], (*keys)[b]) < 0;
+    }
+
+    bool operator()(size_t a, const Slice& b) const {
+      return cmp->Compare((*keys)[a], b) < 0;
+    }
+
+    bool operator()(const Slice& a, size_t b) const {
+      return cmp->Compare(a, (*keys)[b]) < 0;
+    }
+
+    const InternalKeyComparator* cmp;
+    const std::vector<std::string>* keys;
+  };
+
+  std::vector<std::string> keys_;
+  std::vector<std::string> values_;
+  IndexedKeyComparator indexed_cmp_;
+  std::vector<size_t> indices_;
+  size_t current_;
+};
+
+}  // namespace rocksdb