Fix memtable-only iterator regression (#10705)

Summary:
when there is a single memtable without range tombstones and no SST files in the database, DBIter should wrap memtable iterator directly. Currently we create a merging iterator on top of the memtable iterator, and have DBIter wrap around it. This causes iterator regression and this PR fixes this issue.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10705

Test Plan:
- `make check`
- Performance:
  - Set up: `./db_bench -benchmarks=filluniquerandom -write_buffer_size=$((1 << 30)) -num=10000`
  - Benchmark: `./db_bench -benchmarks=seekrandom -use_existing_db=true -avoid_flush_during_recovery=true -write_buffer_size=$((1 << 30)) -num=10000 -threads=16 -duration=60 -seek_nexts=$seek_nexts`
```
seek_nexts    main op/sec    https://github.com/facebook/rocksdb/issues/10705      RocksDB v7.6
0             5746568        5749033     5786180
30            2411690        3006466     2837699
1000          102556         128902      124667
```

Reviewed By: ajkr

Differential Revision: D39644221

Pulled By: cbi42

fbshipit-source-id: 8063ff611ba31b0e5670041da3927c8c54b2097d

db/arena_wrapped_db_iter.cc

@@ -58,48 +58,60 @@ Status ArenaWrappedDBIter::Refresh() {
   uint64_t cur_sv_number = cfd_->GetSuperVersionNumber();
   TEST_SYNC_POINT("ArenaWrappedDBIter::Refresh:1");
   TEST_SYNC_POINT("ArenaWrappedDBIter::Refresh:2");
-  while (true) {
-    if (sv_number_ != cur_sv_number) {
-      Env* env = db_iter_->env();
-      db_iter_->~DBIter();
-      arena_.~Arena();
-      new (&arena_) Arena();
+  auto reinit_internal_iter = [&]() {
+    Env* env = db_iter_->env();
+    db_iter_->~DBIter();
+    arena_.~Arena();
+    new (&arena_) Arena();
 
-      SuperVersion* sv = cfd_->GetReferencedSuperVersion(db_impl_);
-      SequenceNumber latest_seq = db_impl_->GetLatestSequenceNumber();
-      if (read_callback_) {
-        read_callback_->Refresh(latest_seq);
-      }
-      Init(env, read_options_, *(cfd_->ioptions()), sv->mutable_cf_options,
-           sv->current, latest_seq,
-           sv->mutable_cf_options.max_sequential_skip_in_iterations,
-           cur_sv_number, read_callback_, db_impl_, cfd_, expose_blob_index_,
-           allow_refresh_);
+    SuperVersion* sv = cfd_->GetReferencedSuperVersion(db_impl_);
+    SequenceNumber latest_seq = db_impl_->GetLatestSequenceNumber();
+    if (read_callback_) {
+      read_callback_->Refresh(latest_seq);
+    }
+    Init(env, read_options_, *(cfd_->ioptions()), sv->mutable_cf_options,
+         sv->current, latest_seq,
+         sv->mutable_cf_options.max_sequential_skip_in_iterations,
+         cur_sv_number, read_callback_, db_impl_, cfd_, expose_blob_index_,
+         allow_refresh_);
 
-      InternalIterator* internal_iter = db_impl_->NewInternalIterator(
-          read_options_, cfd_, sv, &arena_, latest_seq,
-          /* allow_unprepared_value */ true, /* db_iter */ this);
-      SetIterUnderDBIter(internal_iter);
+    InternalIterator* internal_iter = db_impl_->NewInternalIterator(
+        read_options_, cfd_, sv, &arena_, latest_seq,
+        /* allow_unprepared_value */ true, /* db_iter */ this);
+    SetIterUnderDBIter(internal_iter);
+  };
+  while (true) {
+    if (sv_number_ != cur_sv_number) {
+      reinit_internal_iter();
       break;
     } else {
       SequenceNumber latest_seq = db_impl_->GetLatestSequenceNumber();
       // Refresh range-tombstones in MemTable
       if (!read_options_.ignore_range_deletions) {
-        assert(memtable_range_tombstone_iter_ != nullptr);
-        if (memtable_range_tombstone_iter_ != nullptr) {
-          SuperVersion* sv = cfd_->GetThreadLocalSuperVersion(db_impl_);
-          auto t = sv->mem->NewRangeTombstoneIterator(
-              read_options_, latest_seq, false /* immutable_memtable */);
-          delete *memtable_range_tombstone_iter_;
-          if (t == nullptr || t->empty()) {
+        SuperVersion* sv = cfd_->GetThreadLocalSuperVersion(db_impl_);
+        auto t = sv->mem->NewRangeTombstoneIterator(
+            read_options_, latest_seq, false /* immutable_memtable */);
+        if (!t || t->empty()) {
+          if (memtable_range_tombstone_iter_) {
             *memtable_range_tombstone_iter_ = nullptr;
+          }
+          delete t;
+        } else {  // current mutable memtable has range tombstones
+          if (!memtable_range_tombstone_iter_) {
+            delete t;
+            cfd_->ReturnThreadLocalSuperVersion(sv);
+            // The memtable under DBIter did not have range tombstone before
+            // refresh.
+            reinit_internal_iter();
+            break;
           } else {
+            delete *memtable_range_tombstone_iter_;
             *memtable_range_tombstone_iter_ = new TruncatedRangeDelIterator(
                 std::unique_ptr<FragmentedRangeTombstoneIterator>(t),
                 &cfd_->internal_comparator(), nullptr, nullptr);
           }
-          cfd_->ReturnThreadLocalSuperVersion(sv);
         }
+        cfd_->ReturnThreadLocalSuperVersion(sv);
       }
       // Refresh latest sequence number
       db_iter_->set_sequence(latest_seq);

db/arena_wrapped_db_iter.h

@@ -107,6 +107,8 @@ class ArenaWrappedDBIter : public Iterator {
   ReadCallback* read_callback_;
   bool expose_blob_index_ = false;
   bool allow_refresh_ = true;
+  // If this is nullptr, it means the mutable memtable does not contain range
+  // tombstone when added under this DBIter.
   TruncatedRangeDelIterator** memtable_range_tombstone_iter_ = nullptr;
 };
 

db/column_family.cc

@@ -1144,7 +1144,8 @@ Status ColumnFamilyData::RangesOverlapWithMemtables(
   MergeIteratorBuilder merge_iter_builder(&internal_comparator_, &arena);
   merge_iter_builder.AddIterator(
       super_version->mem->NewIterator(read_opts, &arena));
-  super_version->imm->AddIterators(read_opts, &merge_iter_builder);
+  super_version->imm->AddIterators(read_opts, &merge_iter_builder,
+                                   false /* add_range_tombstone_iter */);
   ScopedArenaIterator memtable_iter(merge_iter_builder.Finish());
 
   auto read_seq = super_version->current->version_set()->LastSequence();

db/db_impl/db_impl.cc

@@ -1803,31 +1803,31 @@ InternalIterator* DBImpl::NewInternalIterator(
       &cfd->internal_comparator(), arena,
       !read_options.total_order_seek &&
           super_version->mutable_cf_options.prefix_extractor != nullptr);
-  // Collect iterator for mutable mem
-  merge_iter_builder.AddIterator(
-      super_version->mem->NewIterator(read_options, arena));
+  // Collect iterator for mutable memtable
+  auto mem_iter = super_version->mem->NewIterator(read_options, arena);
   Status s;
   if (!read_options.ignore_range_deletions) {
+    TruncatedRangeDelIterator* mem_tombstone_iter = nullptr;
     auto range_del_iter = super_version->mem->NewRangeTombstoneIterator(
         read_options, sequence, false /* immutable_memtable */);
     if (range_del_iter == nullptr || range_del_iter->empty()) {
       delete range_del_iter;
-      merge_iter_builder.AddRangeTombstoneIterator(nullptr);
     } else {
-      merge_iter_builder.AddRangeTombstoneIterator(
-          new TruncatedRangeDelIterator(
-              std::unique_ptr<FragmentedRangeTombstoneIterator>(range_del_iter),
-              &cfd->ioptions()->internal_comparator, nullptr /* smallest */,
-              nullptr /* largest */));
+      mem_tombstone_iter = new TruncatedRangeDelIterator(
+          std::unique_ptr<FragmentedRangeTombstoneIterator>(range_del_iter),
+          &cfd->ioptions()->internal_comparator, nullptr /* smallest */,
+          nullptr /* largest */);
     }
+    merge_iter_builder.AddPointAndTombstoneIterator(mem_iter,
+                                                    mem_tombstone_iter);
+  } else {
+    merge_iter_builder.AddIterator(mem_iter);
   }
+
   // Collect all needed child iterators for immutable memtables
   if (s.ok()) {
-    super_version->imm->AddIterators(read_options, &merge_iter_builder);
-    if (!read_options.ignore_range_deletions) {
-      s = super_version->imm->AddRangeTombstoneIterators(read_options, arena,
-                                                         merge_iter_builder);
-    }
+    super_version->imm->AddIterators(read_options, &merge_iter_builder,
+                                     !read_options.ignore_range_deletions);
   }
   TEST_SYNC_POINT_CALLBACK("DBImpl::NewInternalIterator:StatusCallback", &s);
   if (s.ok()) {

db/db_range_del_test.cc

@@ -2303,7 +2303,6 @@ TEST_F(DBRangeDelTest, TombstoneOnlyLevel) {
   InternalIterator* level_iter = sv->current->TEST_GetLevelIterator(
       read_options, &merge_iter_builder, 1 /* level */, true);
   // This is needed to make LevelIterator range tombstone aware
-  merge_iter_builder.AddIterator(level_iter);
   auto miter = merge_iter_builder.Finish();
   auto k = Key(3);
   IterKey target;

db/memtable_list.cc

@@ -210,30 +210,6 @@ Status MemTableListVersion::AddRangeTombstoneIterators(
   return Status::OK();
 }
 
-Status MemTableListVersion::AddRangeTombstoneIterators(
-    const ReadOptions& read_opts, Arena* /*arena*/,
-    MergeIteratorBuilder& builder) {
-  // Except for snapshot read, using kMaxSequenceNumber is OK because these
-  // are immutable memtables.
-  SequenceNumber read_seq = read_opts.snapshot != nullptr
-                                ? read_opts.snapshot->GetSequenceNumber()
-                                : kMaxSequenceNumber;
-  for (auto& m : memlist_) {
-    auto range_del_iter = m->NewRangeTombstoneIterator(
-        read_opts, read_seq, true /* immutale_memtable */);
-    if (range_del_iter == nullptr || range_del_iter->empty()) {
-      delete range_del_iter;
-      builder.AddRangeTombstoneIterator(nullptr);
-    } else {
-      builder.AddRangeTombstoneIterator(new TruncatedRangeDelIterator(
-          std::unique_ptr<FragmentedRangeTombstoneIterator>(range_del_iter),
-          &m->GetInternalKeyComparator(), nullptr /* smallest */,
-          nullptr /* largest */));
-    }
-  }
-  return Status::OK();
-}
-
 void MemTableListVersion::AddIterators(
     const ReadOptions& options, std::vector<InternalIterator*>* iterator_list,
     Arena* arena) {
@@ -242,11 +218,33 @@ void MemTableListVersion::AddIterators(
   }
 }
 
-void MemTableListVersion::AddIterators(
-    const ReadOptions& options, MergeIteratorBuilder* merge_iter_builder) {
+void MemTableListVersion::AddIterators(const ReadOptions& options,
+                                       MergeIteratorBuilder* merge_iter_builder,
+                                       bool add_range_tombstone_iter) {
   for (auto& m : memlist_) {
-    merge_iter_builder->AddIterator(
-        m->NewIterator(options, merge_iter_builder->GetArena()));
+    auto mem_iter = m->NewIterator(options, merge_iter_builder->GetArena());
+    if (!add_range_tombstone_iter || options.ignore_range_deletions) {
+      merge_iter_builder->AddIterator(mem_iter);
+    } else {
+      // Except for snapshot read, using kMaxSequenceNumber is OK because these
+      // are immutable memtables.
+      SequenceNumber read_seq = options.snapshot != nullptr
+                                    ? options.snapshot->GetSequenceNumber()
+                                    : kMaxSequenceNumber;
+      TruncatedRangeDelIterator* mem_tombstone_iter = nullptr;
+      auto range_del_iter = m->NewRangeTombstoneIterator(
+          options, read_seq, true /* immutale_memtable */);
+      if (range_del_iter == nullptr || range_del_iter->empty()) {
+        delete range_del_iter;
+      } else {
+        mem_tombstone_iter = new TruncatedRangeDelIterator(
+            std::unique_ptr<FragmentedRangeTombstoneIterator>(range_del_iter),
+            &m->GetInternalKeyComparator(), nullptr /* smallest */,
+            nullptr /* largest */);
+      }
+      merge_iter_builder->AddPointAndTombstoneIterator(mem_iter,
+                                                       mem_tombstone_iter);
+    }
   }
 }
 

db/memtable_list.h

@@ -111,15 +111,13 @@ class MemTableListVersion {
   Status AddRangeTombstoneIterators(const ReadOptions& read_opts, Arena* arena,
                                     RangeDelAggregator* range_del_agg);
 
-  Status AddRangeTombstoneIterators(const ReadOptions& read_opts, Arena* arena,
-                                    MergeIteratorBuilder& builder);
-
   void AddIterators(const ReadOptions& options,
                     std::vector<InternalIterator*>* iterator_list,
                     Arena* arena);
 
   void AddIterators(const ReadOptions& options,
-                    MergeIteratorBuilder* merge_iter_builder);
+                    MergeIteratorBuilder* merge_iter_builder,
+                    bool add_range_tombstone_iter);
 
   uint64_t GetTotalNumEntries() const;
 

db/version_set.cc

@@ -941,18 +941,18 @@ namespace {
 class LevelIterator final : public InternalIterator {
  public:
   // @param read_options Must outlive this iterator.
-  LevelIterator(TableCache* table_cache, const ReadOptions& read_options,
-                const FileOptions& file_options,
-                const InternalKeyComparator& icomparator,
-                const LevelFilesBrief* flevel,
-                const std::shared_ptr<const SliceTransform>& prefix_extractor,
-                bool should_sample, HistogramImpl* file_read_hist,
-                TableReaderCaller caller, bool skip_filters, int level,
-                RangeDelAggregator* range_del_agg,
-                const std::vector<AtomicCompactionUnitBoundary>*
-                    compaction_boundaries = nullptr,
-                bool allow_unprepared_value = false,
-                MergeIteratorBuilder* merge_iter_builder = nullptr)
+  LevelIterator(
+      TableCache* table_cache, const ReadOptions& read_options,
+      const FileOptions& file_options, const InternalKeyComparator& icomparator,
+      const LevelFilesBrief* flevel,
+      const std::shared_ptr<const SliceTransform>& prefix_extractor,
+      bool should_sample, HistogramImpl* file_read_hist,
+      TableReaderCaller caller, bool skip_filters, int level,
+      RangeDelAggregator* range_del_agg,
+      const std::vector<AtomicCompactionUnitBoundary>* compaction_boundaries =
+          nullptr,
+      bool allow_unprepared_value = false,
+      TruncatedRangeDelIterator**** range_tombstone_iter_ptr_ = nullptr)
       : table_cache_(table_cache),
         read_options_(read_options),
         file_options_(file_options),
@@ -975,10 +975,8 @@ class LevelIterator final : public InternalIterator {
         to_return_sentinel_(false) {
     // Empty level is not supported.
     assert(flevel_ != nullptr && flevel_->num_files > 0);
-    if (merge_iter_builder && !read_options.ignore_range_deletions) {
-      // lazily initialize range_tombstone_iter_ together with file_iter_
-      merge_iter_builder->AddRangeTombstoneIterator(nullptr,
-                                                    &range_tombstone_iter_);
+    if (range_tombstone_iter_ptr_) {
+      *range_tombstone_iter_ptr_ = &range_tombstone_iter_;
     }
   }
 
@@ -1840,14 +1838,22 @@ InternalIterator* Version::TEST_GetLevelIterator(
     int level, bool allow_unprepared_value) {
   auto* arena = merge_iter_builder->GetArena();
   auto* mem = arena->AllocateAligned(sizeof(LevelIterator));
-  return new (mem) LevelIterator(
+  TruncatedRangeDelIterator*** tombstone_iter_ptr = nullptr;
+  auto level_iter = new (mem) LevelIterator(
       cfd_->table_cache(), read_options, file_options_,
       cfd_->internal_comparator(), &storage_info_.LevelFilesBrief(level),
       mutable_cf_options_.prefix_extractor, should_sample_file_read(),
       cfd_->internal_stats()->GetFileReadHist(level),
       TableReaderCaller::kUserIterator, IsFilterSkipped(level), level,
       nullptr /* range_del_agg */, nullptr /* compaction_boundaries */,
-      allow_unprepared_value, merge_iter_builder);
+      allow_unprepared_value, &tombstone_iter_ptr);
+  if (read_options.ignore_range_deletions) {
+    merge_iter_builder->AddIterator(level_iter);
+  } else {
+    merge_iter_builder->AddPointAndTombstoneIterator(
+        level_iter, nullptr /* tombstone_iter */, tombstone_iter_ptr);
+  }
+  return level_iter;
 }
 
 uint64_t VersionStorageInfo::GetEstimatedActiveKeys() const {
@@ -1927,10 +1933,10 @@ void Version::AddIteratorsForLevel(const ReadOptions& read_options,
   auto* arena = merge_iter_builder->GetArena();
   if (level == 0) {
     // Merge all level zero files together since they may overlap
-    TruncatedRangeDelIterator* iter = nullptr;
+    TruncatedRangeDelIterator* tombstone_iter = nullptr;
     for (size_t i = 0; i < storage_info_.LevelFilesBrief(0).num_files; i++) {
       const auto& file = storage_info_.LevelFilesBrief(0).files[i];
-      merge_iter_builder->AddIterator(cfd_->table_cache()->NewIterator(
+      auto table_iter = cfd_->table_cache()->NewIterator(
           read_options, soptions, cfd_->internal_comparator(),
           *file.file_metadata, /*range_del_agg=*/nullptr,
           mutable_cf_options_.prefix_extractor, nullptr,
@@ -1938,9 +1944,13 @@ void Version::AddIteratorsForLevel(const ReadOptions& read_options,
           TableReaderCaller::kUserIterator, arena,
           /*skip_filters=*/false, /*level=*/0, max_file_size_for_l0_meta_pin_,
           /*smallest_compaction_key=*/nullptr,
-          /*largest_compaction_key=*/nullptr, allow_unprepared_value, &iter));
-      if (!read_options.ignore_range_deletions) {
-        merge_iter_builder->AddRangeTombstoneIterator(iter);
+          /*largest_compaction_key=*/nullptr, allow_unprepared_value,
+          &tombstone_iter);
+      if (read_options.ignore_range_deletions) {
+        merge_iter_builder->AddIterator(table_iter);
+      } else {
+        merge_iter_builder->AddPointAndTombstoneIterator(table_iter,
+                                                         tombstone_iter);
       }
     }
     if (should_sample) {
@@ -1957,14 +1967,21 @@ void Version::AddIteratorsForLevel(const ReadOptions& read_options,
     // walks through the non-overlapping files in the level, opening them
     // lazily.
     auto* mem = arena->AllocateAligned(sizeof(LevelIterator));
-    merge_iter_builder->AddIterator(new (mem) LevelIterator(
+    TruncatedRangeDelIterator*** tombstone_iter_ptr = nullptr;
+    auto level_iter = new (mem) LevelIterator(
         cfd_->table_cache(), read_options, soptions,
         cfd_->internal_comparator(), &storage_info_.LevelFilesBrief(level),
         mutable_cf_options_.prefix_extractor, should_sample_file_read(),
         cfd_->internal_stats()->GetFileReadHist(level),
         TableReaderCaller::kUserIterator, IsFilterSkipped(level), level,
         /*range_del_agg=*/nullptr, /*compaction_boundaries=*/nullptr,
-        allow_unprepared_value, merge_iter_builder));
+        allow_unprepared_value, &tombstone_iter_ptr);
+    if (read_options.ignore_range_deletions) {
+      merge_iter_builder->AddIterator(level_iter);
+    } else {
+      merge_iter_builder->AddPointAndTombstoneIterator(
+          level_iter, nullptr /* tombstone_iter */, tombstone_iter_ptr);
+    }
   }
 }
 

table/merging_iterator.cc

@@ -1295,22 +1295,40 @@ void MergeIteratorBuilder::AddIterator(InternalIterator* iter) {
   }
 }
 
-void MergeIteratorBuilder::AddRangeTombstoneIterator(
-    TruncatedRangeDelIterator* iter, TruncatedRangeDelIterator*** iter_ptr) {
-  if (!use_merging_iter) {
+void MergeIteratorBuilder::AddPointAndTombstoneIterator(
+    InternalIterator* point_iter, TruncatedRangeDelIterator* tombstone_iter,
+    TruncatedRangeDelIterator*** tombstone_iter_ptr) {
+  // tombstone_iter_ptr != nullptr means point_iter is a LevelIterator.
+  bool add_range_tombstone = tombstone_iter ||
+                             !merge_iter->range_tombstone_iters_.empty() ||
+                             tombstone_iter_ptr;
+  if (!use_merging_iter && (add_range_tombstone || first_iter)) {
     use_merging_iter = true;
     if (first_iter) {
       merge_iter->AddIterator(first_iter);
       first_iter = nullptr;
     }
   }
-  merge_iter->AddRangeTombstoneIterator(iter);
-  if (iter_ptr) {
-    // This is needed instead of setting to &range_tombstone_iters_[i] directly
-    // here since the memory address of range_tombstone_iters_[i] might change
-    // during vector resizing.
-    range_del_iter_ptrs_.emplace_back(
-        merge_iter->range_tombstone_iters_.size() - 1, iter_ptr);
+  if (use_merging_iter) {
+    merge_iter->AddIterator(point_iter);
+    if (add_range_tombstone) {
+      // If there was a gap, fill in nullptr as empty range tombstone iterators.
+      while (merge_iter->range_tombstone_iters_.size() <
+             merge_iter->children_.size() - 1) {
+        merge_iter->AddRangeTombstoneIterator(nullptr);
+      }
+      merge_iter->AddRangeTombstoneIterator(tombstone_iter);
+    }
+
+    if (tombstone_iter_ptr) {
+      // This is needed instead of setting to &range_tombstone_iters_[i]
+      // directly here since the memory address of range_tombstone_iters_[i]
+      // might change during vector resizing.
+      range_del_iter_ptrs_.emplace_back(
+          merge_iter->range_tombstone_iters_.size() - 1, tombstone_iter_ptr);
+    }
+  } else {
+    first_iter = point_iter;
   }
 }
 
@@ -1323,8 +1341,7 @@ InternalIterator* MergeIteratorBuilder::Finish(ArenaWrappedDBIter* db_iter) {
     for (auto& p : range_del_iter_ptrs_) {
       *(p.second) = &(merge_iter->range_tombstone_iters_[p.first]);
     }
-    if (db_iter) {
-      assert(!merge_iter->range_tombstone_iters_.empty());
+    if (db_iter && !merge_iter->range_tombstone_iters_.empty()) {
       // memtable is always the first level
       db_iter->SetMemtableRangetombstoneIter(
           &merge_iter->range_tombstone_iters_.front());

table/merging_iterator.h

@@ -38,6 +38,8 @@ extern InternalIterator* NewMergingIterator(
 class MergingIterator;
 
 // A builder class to build a merging iterator by adding iterators one by one.
+// User should call only one of AddIterator() or AddPointAndTombstoneIterator()
+// exclusively for the same builder.
 class MergeIteratorBuilder {
  public:
   // comparator: the comparator used in merging comparator
@@ -49,16 +51,20 @@ class MergeIteratorBuilder {
   // Add iter to the merging iterator.
   void AddIterator(InternalIterator* iter);
 
-  // Add a range tombstone iterator to underlying merge iterator.
-  // See MergingIterator::AddRangeTombstoneIterator() for more detail.
-  //
-  // If `iter_ptr` is not nullptr, *iter_ptr will be set to where the merging
-  // iterator stores `iter` when MergeIteratorBuilder::Finish() is called. This
-  // is used by level iterator to update range tombstone iters when switching to
-  // a different SST file.
-  void AddRangeTombstoneIterator(
-      TruncatedRangeDelIterator* iter,
-      TruncatedRangeDelIterator*** iter_ptr = nullptr);
+  // Add a point key iterator and a range tombstone iterator.
+  // `tombstone_iter_ptr` should and only be set by LevelIterator.
+  // *tombstone_iter_ptr will be set to where the merging iterator stores
+  // `tombstone_iter` when MergeIteratorBuilder::Finish() is called. This is
+  // used by LevelIterator to update range tombstone iters when switching to a
+  // different SST file. If a single point iterator with a nullptr range
+  // tombstone iterator is provided, and the point iterator is not a level
+  // iterator, then this builder will return the point iterator directly,
+  // instead of creating a merging iterator on top of it. Internally, if all
+  // point iterators are not LevelIterator, then range tombstone iterator is
+  // only added to the merging iter if there is a non-null `tombstone_iter`.
+  void AddPointAndTombstoneIterator(
+      InternalIterator* point_iter, TruncatedRangeDelIterator* tombstone_iter,
+      TruncatedRangeDelIterator*** tombstone_iter_ptr = nullptr);
 
   // Get arena used to build the merging iterator. It is called one a child
   // iterator needs to be allocated.