Refactor BlockIter (#4121)

Summary:
BlockIter is getting crowded including details that specific only to either index or data blocks. The patch moves down such details to DataBlockIter and IndexBlockIter, both inheriting from BlockIter.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4121

Differential Revision: D8816832

Pulled By: maysamyabandeh

fbshipit-source-id: d492e74155c11d8a0c1c85cd7ee33d24c7456197

table/block.cc

@@ -140,6 +140,31 @@ void BlockIter::Prev() {
 }
 
 void BlockIter::Seek(const Slice& target) {
+  Slice seek_key = target;
+  if (!key_includes_seq_) {
+    seek_key = ExtractUserKey(target);
+  }
+  PERF_TIMER_GUARD(block_seek_nanos);
+  if (data_ == nullptr) {  // Not init yet
+    return;
+  }
+  uint32_t index = 0;
+  bool ok = BinarySeek(seek_key, 0, num_restarts_ - 1, &index);
+
+  if (!ok) {
+    return;
+  }
+  SeekToRestartPoint(index);
+  // Linear search (within restart block) for first key >= target
+
+  while (true) {
+    if (!ParseNextKey() || Compare(key_, seek_key) >= 0) {
+      return;
+    }
+  }
+}
+
+void IndexBlockIter::Seek(const Slice& target) {
   Slice seek_key = target;
   if (!key_includes_seq_) {
     seek_key = ExtractUserKey(target);
@@ -329,7 +354,7 @@ bool BlockIter::BinarySeek(const Slice& target, uint32_t left, uint32_t right,
 
 // Compare target key and the block key of the block of `block_index`.
 // Return -1 if error.
-int BlockIter::CompareBlockKey(uint32_t block_index, const Slice& target) {
+int IndexBlockIter::CompareBlockKey(uint32_t block_index, const Slice& target) {
   uint32_t region_offset = GetRestartPoint(block_index);
   uint32_t shared, non_shared, value_length;
   const char* key_ptr = DecodeEntry(data_ + region_offset, data_ + restarts_,
@@ -344,9 +369,9 @@ int BlockIter::CompareBlockKey(uint32_t block_index, const Slice& target) {
 
 // Binary search in block_ids to find the first block
 // with a key >= target
-bool BlockIter::BinaryBlockIndexSeek(const Slice& target, uint32_t* block_ids,
-                                     uint32_t left, uint32_t right,
-                                     uint32_t* index) {
+bool IndexBlockIter::BinaryBlockIndexSeek(const Slice& target,
+                                          uint32_t* block_ids, uint32_t left,
+                                          uint32_t right, uint32_t* index) {
   assert(left <= right);
   uint32_t left_bound = left;
 
@@ -394,7 +419,7 @@ bool BlockIter::BinaryBlockIndexSeek(const Slice& target, uint32_t* block_ids,
   }
 }
 
-bool BlockIter::PrefixSeek(const Slice& target, uint32_t* index) {
+bool IndexBlockIter::PrefixSeek(const Slice& target, uint32_t* index) {
   assert(prefix_index_);
   Slice seek_key = target;
   if (!key_includes_seq_) {
@@ -445,9 +470,12 @@ Block::Block(BlockContents&& contents, SequenceNumber _global_seqno,
   }
 }
 
+template <>
 BlockIter* Block::NewIterator(const Comparator* cmp, const Comparator* ucmp,
-                              BlockIter* iter, bool total_order_seek,
-                              Statistics* stats, bool key_includes_seq) {
+                              BlockIter* iter, Statistics* /*stats*/,
+                              bool /*total_order_seek*/,
+                              bool /*key_includes_seq*/,
+                              BlockPrefixIndex* /*prefix_index*/) {
   BlockIter* ret_iter;
   if (iter != nullptr) {
     ret_iter = iter;
@@ -463,12 +491,37 @@ BlockIter* Block::NewIterator(const Comparator* cmp, const Comparator* ucmp,
     ret_iter->Invalidate(Status::OK());
     return ret_iter;
   } else {
-    BlockPrefixIndex* prefix_index_ptr =
-        total_order_seek ? nullptr : prefix_index_.get();
-    ret_iter->Initialize(cmp, ucmp, data_, restart_offset_, num_restarts_,
-                         prefix_index_ptr, global_seqno_,
-                         read_amp_bitmap_.get(), key_includes_seq, cachable());
+    const bool kKeyIncludesSeq = true;
+    ret_iter->InitializeBase(cmp, ucmp, data_, restart_offset_, num_restarts_,
+                             global_seqno_, kKeyIncludesSeq, cachable());
+  }
 
+  return ret_iter;
+}
+
+template <>
+DataBlockIter* Block::NewIterator(const Comparator* cmp, const Comparator* ucmp,
+                                  DataBlockIter* iter, Statistics* stats,
+                                  bool /*total_order_seek*/,
+                                  bool /*key_includes_seq*/,
+                                  BlockPrefixIndex* /*prefix_index*/) {
+  DataBlockIter* ret_iter;
+  if (iter != nullptr) {
+    ret_iter = iter;
+  } else {
+    ret_iter = new DataBlockIter;
+  }
+  if (size_ < 2 * sizeof(uint32_t)) {
+    ret_iter->Invalidate(Status::Corruption("bad block contents"));
+    return ret_iter;
+  }
+  if (num_restarts_ == 0) {
+    // Empty block.
+    ret_iter->Invalidate(Status::OK());
+    return ret_iter;
+  } else {
+    ret_iter->Initialize(cmp, ucmp, data_, restart_offset_, num_restarts_,
+                         global_seqno_, read_amp_bitmap_.get(), cachable());
     if (read_amp_bitmap_) {
       if (read_amp_bitmap_->GetStatistics() != stats) {
         // DB changed the Statistics pointer, we need to notify read_amp_bitmap_
@@ -480,8 +533,34 @@ BlockIter* Block::NewIterator(const Comparator* cmp, const Comparator* ucmp,
   return ret_iter;
 }
 
-void Block::SetBlockPrefixIndex(BlockPrefixIndex* prefix_index) {
-  prefix_index_.reset(prefix_index);
+template <>
+IndexBlockIter* Block::NewIterator(const Comparator* cmp,
+                                   const Comparator* ucmp, IndexBlockIter* iter,
+                                   Statistics* /*stats*/, bool total_order_seek,
+                                   bool key_includes_seq,
+                                   BlockPrefixIndex* prefix_index) {
+  IndexBlockIter* ret_iter;
+  if (iter != nullptr) {
+    ret_iter = iter;
+  } else {
+    ret_iter = new IndexBlockIter;
+  }
+  if (size_ < 2 * sizeof(uint32_t)) {
+    ret_iter->Invalidate(Status::Corruption("bad block contents"));
+    return ret_iter;
+  }
+  if (num_restarts_ == 0) {
+    // Empty block.
+    ret_iter->Invalidate(Status::OK());
+    return ret_iter;
+  } else {
+    BlockPrefixIndex* prefix_index_ptr =
+        total_order_seek ? nullptr : prefix_index;
+    ret_iter->Initialize(cmp, ucmp, data_, restart_offset_, num_restarts_,
+                         prefix_index_ptr, key_includes_seq, cachable());
+  }
+
+  return ret_iter;
 }
 
 size_t Block::ApproximateMemoryUsage() const {
@@ -491,9 +570,6 @@ size_t Block::ApproximateMemoryUsage() const {
 #else
   usage += sizeof(*this);
 #endif  // ROCKSDB_MALLOC_USABLE_SIZE
-  if (prefix_index_) {
-    usage += prefix_index_->ApproximateMemoryUsage();
-  }
   if (read_amp_bitmap_) {
     usage += read_amp_bitmap_->ApproximateMemoryUsage();
   }

table/block.h

@@ -36,6 +36,8 @@ namespace rocksdb {
 struct BlockContents;
 class Comparator;
 class BlockIter;
+class DataBlockIter;
+class IndexBlockIter;
 class BlockPrefixIndex;
 
 // BlockReadAmpBitmap is a bitmap that map the rocksdb::Block data bytes to
@@ -156,29 +158,30 @@ class Block {
     return contents_.compression_type;
   }
 
-  // If hash index lookup is enabled and `use_hash_index` is true. This block
-  // will do hash lookup for the key prefix.
-  //
-  // NOTE: for the hash based lookup, if a key prefix doesn't match any key,
-  // the iterator will simply be set as "invalid", rather than returning
-  // the key that is just pass the target key.
-  //
   // If comparator is InternalKeyComparator, user_comparator is its user
   // comparator; they are equal otherwise.
   //
   // If iter is null, return new Iterator
   // If iter is not null, update this one and return it as Iterator*
   //
-  // If total_order_seek is true, hash_index_ and prefix_index_ are ignored.
-  // This option only applies for index block. For data block, hash_index_
-  // and prefix_index_ are null, so this option does not matter.
-  BlockIter* NewIterator(const Comparator* comparator,
-                         const Comparator* user_comparator,
-                         BlockIter* iter = nullptr,
-                         bool total_order_seek = true,
-                         Statistics* stats = nullptr,
-                         bool key_includes_seq = true);
-  void SetBlockPrefixIndex(BlockPrefixIndex* prefix_index);
+  // NewIterator<DataBlockIter>
+  // Same as above but also updates read_amp_bitmap_ if it is not nullptr.
+  //
+  // NewIterator<IndexBlockIter>
+  // If `prefix_index` is not nullptr this block will do hash lookup for the key
+  // prefix. If total_order_seek is true, prefix_index_ is ignored.
+  //
+  // NOTE: for the hash based lookup, if a key prefix doesn't match any key,
+  // the iterator will simply be set as "invalid", rather than returning
+  // the key that is just pass the target key.
+  template <typename TBlockIter>
+  TBlockIter* NewIterator(const Comparator* comparator,
+                          const Comparator* user_comparator,
+                          TBlockIter* iter = nullptr,
+                          Statistics* stats = nullptr,
+                          bool total_order_seek = true,
+                          bool key_includes_seq = true,
+                          BlockPrefixIndex* prefix_index = nullptr);
 
   // Report an approximation of how much memory has been used.
   size_t ApproximateMemoryUsage() const;
@@ -191,7 +194,6 @@ class Block {
   size_t size_;                 // contents_.data.size()
   uint32_t restart_offset_;     // Offset in data_ of restart array
   uint32_t num_restarts_;
-  std::unique_ptr<BlockPrefixIndex> prefix_index_;
   std::unique_ptr<BlockReadAmpBitmap> read_amp_bitmap_;
   // All keys in the block will have seqno = global_seqno_, regardless of
   // the encoded value (kDisableGlobalSequenceNumber means disabled)
@@ -202,7 +204,7 @@ class Block {
   void operator=(const Block&) = delete;
 };
 
-class BlockIter final : public InternalIterator {
+class BlockIter : public InternalIterator {
  public:
   // Object created using this constructor will behave like an iterator
   // against an empty block. The state after the creation: Valid()=false
@@ -211,36 +213,30 @@ class BlockIter final : public InternalIterator {
       : comparator_(nullptr),
         user_comparator_(nullptr),
         data_(nullptr),
-        restarts_(0),
         num_restarts_(0),
-        current_(0),
         restart_index_(0),
+        restarts_(0),
+        current_(0),
         status_(Status::OK()),
-        prefix_index_(nullptr),
         key_pinned_(false),
+        block_contents_pinned_(false),
         key_includes_seq_(true),
-        global_seqno_(kDisableGlobalSequenceNumber),
-        read_amp_bitmap_(nullptr),
-        last_bitmap_offset_(0),
-        block_contents_pinned_(false) {}
+        global_seqno_(kDisableGlobalSequenceNumber) {}
 
   BlockIter(const Comparator* comparator, const Comparator* user_comparator,
             const char* data, uint32_t restarts, uint32_t num_restarts,
-            BlockPrefixIndex* prefix_index, SequenceNumber global_seqno,
-            BlockReadAmpBitmap* read_amp_bitmap, bool key_includes_seq,
+            SequenceNumber global_seqno, bool key_includes_seq,
             bool block_contents_pinned)
       : BlockIter() {
-    Initialize(comparator, user_comparator, data, restarts, num_restarts,
-               prefix_index, global_seqno, read_amp_bitmap, key_includes_seq,
-               block_contents_pinned);
+    InitializeBase(comparator, user_comparator, data, restarts, num_restarts,
+                   global_seqno, key_includes_seq, block_contents_pinned);
   }
 
-  void Initialize(const Comparator* comparator,
-                  const Comparator* user_comparator, const char* data,
-                  uint32_t restarts, uint32_t num_restarts,
-                  BlockPrefixIndex* prefix_index, SequenceNumber global_seqno,
-                  BlockReadAmpBitmap* read_amp_bitmap, bool key_includes_seq,
-                  bool block_contents_pinned) {
+  void InitializeBase(const Comparator* comparator,
+                      const Comparator* user_comparator, const char* data,
+                      uint32_t restarts, uint32_t num_restarts,
+                      SequenceNumber global_seqno, bool key_includes_seq,
+                      bool block_contents_pinned) {
     assert(data_ == nullptr);           // Ensure it is called only once
     assert(num_restarts > 0);           // Ensure the param is valid
 
@@ -251,10 +247,7 @@ class BlockIter final : public InternalIterator {
     num_restarts_ = num_restarts;
     current_ = restarts_;
     restart_index_ = num_restarts_;
-    prefix_index_ = prefix_index;
     global_seqno_ = global_seqno;
-    read_amp_bitmap_ = read_amp_bitmap;
-    last_bitmap_offset_ = current_ + 1;
     key_includes_seq_ = key_includes_seq;
     block_contents_pinned_ = block_contents_pinned;
   }
@@ -283,16 +276,10 @@ class BlockIter final : public InternalIterator {
   virtual Status status() const override { return status_; }
   virtual Slice key() const override {
     assert(Valid());
-    return key_includes_seq_ ? key_.GetInternalKey() : key_.GetUserKey();
+    return key_.GetInternalKey();
   }
   virtual Slice value() const override {
     assert(Valid());
-    if (read_amp_bitmap_ && current_ < restarts_ &&
-        current_ != last_bitmap_offset_) {
-      read_amp_bitmap_->Mark(current_ /* current entry offset */,
-                             NextEntryOffset() - 1);
-      last_bitmap_offset_ = current_;
-    }
     return value_;
   }
 
@@ -309,7 +296,7 @@ class BlockIter final : public InternalIterator {
   virtual void SeekToLast() override;
 
 #ifndef NDEBUG
-  ~BlockIter() {
+  virtual ~BlockIter() {
     // Assert that the BlockIter is never deleted while Pinning is Enabled.
     assert(!pinned_iters_mgr_ ||
            (pinned_iters_mgr_ && !pinned_iters_mgr_->PinningEnabled()));
@@ -333,36 +320,29 @@ class BlockIter final : public InternalIterator {
     return static_cast<uint32_t>(value_.data() - data_);
   }
 
- private:
+ protected:
   // Note: The type could be changed to InternalKeyComparator but we see a weird
   // performance drop by that.
   const Comparator* comparator_;
   // Same as comparator_ if comparator_ is not InernalKeyComparator
   const Comparator* user_comparator_;
   const char* data_;       // underlying block contents
-  uint32_t restarts_;      // Offset of restart array (list of fixed32)
   uint32_t num_restarts_;  // Number of uint32_t entries in restart array
 
+  uint32_t restart_index_;  // Index of restart block in which current_ falls
+  uint32_t restarts_;       // Offset of restart array (list of fixed32)
   // current_ is offset in data_ of current entry.  >= restarts_ if !Valid
   uint32_t current_;
-  uint32_t restart_index_;  // Index of restart block in which current_ falls
   IterKey key_;
   Slice value_;
   Status status_;
-  BlockPrefixIndex* prefix_index_;
   bool key_pinned_;
+  // whether the block data is guaranteed to outlive this iterator
+  bool block_contents_pinned_;
   // Key is in InternalKey format
   bool key_includes_seq_;
   SequenceNumber global_seqno_;
 
- public:
-  // read-amp bitmap
-  BlockReadAmpBitmap* read_amp_bitmap_;
-  // last `current_` value we report to read-amp bitmp
-  mutable uint32_t last_bitmap_offset_;
-  // whether the block data is guaranteed to outlive this iterator
-  bool block_contents_pinned_;
-
   struct CachedPrevEntry {
     explicit CachedPrevEntry(uint32_t _offset, const char* _key_ptr,
                              size_t _key_offset, size_t _key_size, Slice _value)
@@ -387,6 +367,7 @@ class BlockIter final : public InternalIterator {
   std::vector<CachedPrevEntry> prev_entries_;
   int32_t prev_entries_idx_ = -1;
 
+ public:
   inline int Compare(const Slice& a, const Slice& b) const {
     if (key_includes_seq_) {
       return comparator_->Compare(a, b);
@@ -430,15 +411,90 @@ class BlockIter final : public InternalIterator {
 
   bool BinarySeek(const Slice& target, uint32_t left, uint32_t right,
                   uint32_t* index);
+};
 
-  int CompareBlockKey(uint32_t block_index, const Slice& target);
+class DataBlockIter final : public BlockIter {
+ public:
+  DataBlockIter()
+      : BlockIter(), read_amp_bitmap_(nullptr), last_bitmap_offset_(0) {}
+  DataBlockIter(const Comparator* comparator, const Comparator* user_comparator,
+                const char* data, uint32_t restarts, uint32_t num_restarts,
+                SequenceNumber global_seqno,
+                BlockReadAmpBitmap* read_amp_bitmap, bool block_contents_pinned)
+      : DataBlockIter() {
+    Initialize(comparator, user_comparator, data, restarts, num_restarts,
+               global_seqno, read_amp_bitmap, block_contents_pinned);
+  }
+  void Initialize(const Comparator* comparator,
+                  const Comparator* user_comparator, const char* data,
+                  uint32_t restarts, uint32_t num_restarts,
+                  SequenceNumber global_seqno,
+                  BlockReadAmpBitmap* read_amp_bitmap,
+                  bool block_contents_pinned) {
+    const bool kKeyIncludesSeq = true;
+    InitializeBase(comparator, user_comparator, data, restarts, num_restarts,
+                   global_seqno, kKeyIncludesSeq, block_contents_pinned);
+    read_amp_bitmap_ = read_amp_bitmap;
+    last_bitmap_offset_ = current_ + 1;
+  }
+
+  virtual Slice value() const override {
+    assert(Valid());
+    if (read_amp_bitmap_ && current_ < restarts_ &&
+        current_ != last_bitmap_offset_) {
+      read_amp_bitmap_->Mark(current_ /* current entry offset */,
+                             NextEntryOffset() - 1);
+      last_bitmap_offset_ = current_;
+    }
+    return value_;
+  }
+
+ private:
+  // read-amp bitmap
+  BlockReadAmpBitmap* read_amp_bitmap_;
+  // last `current_` value we report to read-amp bitmp
+  mutable uint32_t last_bitmap_offset_;
+};
 
+class IndexBlockIter final : public BlockIter {
+ public:
+  IndexBlockIter() : BlockIter(), prefix_index_(nullptr) {}
+
+  virtual Slice key() const override {
+    assert(Valid());
+    return key_includes_seq_ ? key_.GetInternalKey() : key_.GetUserKey();
+  }
+  IndexBlockIter(const Comparator* comparator,
+                 const Comparator* user_comparator, const char* data,
+                 uint32_t restarts, uint32_t num_restarts,
+                 BlockPrefixIndex* prefix_index, bool key_includes_seq,
+                 bool block_contents_pinned)
+      : IndexBlockIter() {
+    Initialize(comparator, user_comparator, data, restarts, num_restarts,
+               prefix_index, key_includes_seq, block_contents_pinned);
+  }
+
+  void Initialize(const Comparator* comparator,
+                  const Comparator* user_comparator, const char* data,
+                  uint32_t restarts, uint32_t num_restarts,
+                  BlockPrefixIndex* prefix_index, bool key_includes_seq,
+                  bool block_contents_pinned) {
+    InitializeBase(comparator, user_comparator, data, restarts, num_restarts,
+                   kDisableGlobalSequenceNumber, key_includes_seq,
+                   block_contents_pinned);
+    prefix_index_ = prefix_index;
+  }
+
+  virtual void Seek(const Slice& target) override;
+
+ private:
+  bool PrefixSeek(const Slice& target, uint32_t* index);
   bool BinaryBlockIndexSeek(const Slice& target, uint32_t* block_ids,
                             uint32_t left, uint32_t right,
                             uint32_t* index);
+  int CompareBlockKey(uint32_t block_index, const Slice& target);
 
-  bool PrefixSeek(const Slice& target, uint32_t* index);
-
+  BlockPrefixIndex* prefix_index_;
 };
 
 }  // namespace rocksdb

table/block_based_table_reader.cc

@@ -229,26 +229,27 @@ class PartitionIndexReader : public IndexReader, public Cleanable {
   }
 
   // return a two-level iterator: first level is on the partition index
-  virtual InternalIterator* NewIterator(BlockIter* /*iter*/ = nullptr,
+  virtual InternalIterator* NewIterator(IndexBlockIter* /*iter*/ = nullptr,
                                         bool /*dont_care*/ = true,
                                         bool fill_cache = true) override {
+    Statistics* kNullStats = nullptr;
     // Filters are already checked before seeking the index
     if (!partition_map_.empty()) {
       return NewTwoLevelIterator(
           new BlockBasedTable::PartitionedIndexIteratorState(
               table_, &partition_map_, index_key_includes_seq_),
-          index_block_->NewIterator(icomparator_,
-                                    icomparator_->user_comparator(), nullptr,
-                                    true, nullptr, index_key_includes_seq_));
+          index_block_->NewIterator<IndexBlockIter>(
+              icomparator_, icomparator_->user_comparator(), nullptr,
+              kNullStats, true, index_key_includes_seq_));
     } else {
       auto ro = ReadOptions();
       ro.fill_cache = fill_cache;
       bool kIsIndex = true;
-      return new BlockBasedTableIterator(
+      return new BlockBasedTableIterator<IndexBlockIter>(
           table_, ro, *icomparator_,
-          index_block_->NewIterator(icomparator_,
-                                    icomparator_->user_comparator(), nullptr,
-                                    true, nullptr, index_key_includes_seq_),
+          index_block_->NewIterator<IndexBlockIter>(
+              icomparator_, icomparator_->user_comparator(), nullptr,
+              kNullStats, true, index_key_includes_seq_),
           false, true, /* prefix_extractor */ nullptr, kIsIndex,
           index_key_includes_seq_);
     }
@@ -261,10 +262,12 @@ class PartitionIndexReader : public IndexReader, public Cleanable {
   virtual void CacheDependencies(bool pin) override {
     // Before read partitions, prefetch them to avoid lots of IOs
     auto rep = table_->rep_;
-    BlockIter biter;
+    IndexBlockIter biter;
     BlockHandle handle;
-    index_block_->NewIterator(icomparator_, icomparator_->user_comparator(),
-                              &biter, true, nullptr, index_key_includes_seq_);
+    Statistics* kNullStats = nullptr;
+    index_block_->NewIterator<IndexBlockIter>(
+        icomparator_, icomparator_->user_comparator(), &biter, kNullStats, true,
+        index_key_includes_seq_);
     // Index partitions are assumed to be consecuitive. Prefetch them all.
     // Read the first block offset
     biter.SeekToFirst();
@@ -415,12 +418,13 @@ class BinarySearchIndexReader : public IndexReader {
     return s;
   }
 
-  virtual InternalIterator* NewIterator(BlockIter* iter = nullptr,
+  virtual InternalIterator* NewIterator(IndexBlockIter* iter = nullptr,
                                         bool /*dont_care*/ = true,
                                         bool /*dont_care*/ = true) override {
-    return index_block_->NewIterator(icomparator_,
-                                     icomparator_->user_comparator(), iter,
-                                     true, nullptr, index_key_includes_seq_);
+    Statistics* kNullStats = nullptr;
+    return index_block_->NewIterator<IndexBlockIter>(
+        icomparator_, icomparator_->user_comparator(), iter, kNullStats, true,
+        index_key_includes_seq_);
   }
 
   virtual size_t size() const override { return index_block_->size(); }
@@ -532,18 +536,19 @@ class HashIndexReader : public IndexReader {
                                  prefixes_meta_contents.data, &prefix_index);
     // TODO: log error
     if (s.ok()) {
-      new_index_reader->index_block_->SetBlockPrefixIndex(prefix_index);
+      new_index_reader->prefix_index_.reset(prefix_index);
     }
 
     return Status::OK();
   }
 
-  virtual InternalIterator* NewIterator(BlockIter* iter = nullptr,
+  virtual InternalIterator* NewIterator(IndexBlockIter* iter = nullptr,
                                         bool total_order_seek = true,
                                         bool /*dont_care*/ = true) override {
-    return index_block_->NewIterator(
-        icomparator_, icomparator_->user_comparator(), iter, total_order_seek,
-        nullptr, index_key_includes_seq_);
+    Statistics* kNullStats = nullptr;
+    return index_block_->NewIterator<IndexBlockIter>(
+        icomparator_, icomparator_->user_comparator(), iter, kNullStats,
+        total_order_seek, index_key_includes_seq_, prefix_index_.get());
   }
 
   virtual size_t size() const override { return index_block_->size(); }
@@ -558,6 +563,9 @@ class HashIndexReader : public IndexReader {
 #ifdef ROCKSDB_MALLOC_USABLE_SIZE
     usage += malloc_usable_size((void*)this);
 #else
+    if (prefix_index_) {
+      usage += prefix_index_->ApproximateMemoryUsage();
+    }
     usage += sizeof(*this);
 #endif  // ROCKSDB_MALLOC_USABLE_SIZE
     return usage;
@@ -577,6 +585,7 @@ class HashIndexReader : public IndexReader {
   }
 
   std::unique_ptr<Block> index_block_;
+  std::unique_ptr<BlockPrefixIndex> prefix_index_;
   BlockContents prefixes_contents_;
   const bool index_key_includes_seq_;
 };
@@ -1110,8 +1119,8 @@ Status BlockBasedTable::ReadMetaBlock(Rep* rep,
 
   *meta_block = std::move(meta);
   // meta block uses bytewise comparator.
-  iter->reset(meta_block->get()->NewIterator(BytewiseComparator(),
-                                             BytewiseComparator()));
+  iter->reset(meta_block->get()->NewIterator<BlockIter>(BytewiseComparator(),
+                                                        BytewiseComparator()));
   return Status::OK();
 }
 
@@ -1477,7 +1486,7 @@ BlockBasedTable::CachableEntry<FilterBlockReader> BlockBasedTable::GetFilter(
 // differs from the one in mutable_cf_options and index type is HashBasedIndex
 InternalIterator* BlockBasedTable::NewIndexIterator(
     const ReadOptions& read_options, bool disable_prefix_seek,
-    BlockIter* input_iter, CachableEntry<IndexReader>* index_entry,
+    IndexBlockIter* input_iter, CachableEntry<IndexReader>* index_entry,
     GetContext* get_context) {
   // index reader has already been pre-populated.
   if (rep_->index_reader) {
@@ -1578,28 +1587,29 @@ InternalIterator* BlockBasedTable::NewIndexIterator(
   return iter;
 }
 
-BlockIter* BlockBasedTable::NewDataBlockIterator(
+template <typename TBlockIter>
+TBlockIter* BlockBasedTable::NewDataBlockIterator(
     Rep* rep, const ReadOptions& ro, const Slice& index_value,
-    BlockIter* input_iter, bool is_index, bool key_includes_seq,
-    GetContext* get_context,
-    FilePrefetchBuffer* prefetch_buffer) {
+    TBlockIter* input_iter, bool is_index, bool key_includes_seq,
+    GetContext* get_context, FilePrefetchBuffer* prefetch_buffer) {
   BlockHandle handle;
   Slice input = index_value;
   // We intentionally allow extra stuff in index_value so that we
   // can add more features in the future.
   Status s = handle.DecodeFrom(&input);
-  return NewDataBlockIterator(rep, ro, handle, input_iter, is_index,
-                              key_includes_seq, get_context, s,
-                              prefetch_buffer);
+  return NewDataBlockIterator<TBlockIter>(rep, ro, handle, input_iter, is_index,
+                                          key_includes_seq, get_context, s,
+                                          prefetch_buffer);
 }
 
 // Convert an index iterator value (i.e., an encoded BlockHandle)
 // into an iterator over the contents of the corresponding block.
 // If input_iter is null, new a iterator
 // If input_iter is not null, update this iter and return it
-BlockIter* BlockBasedTable::NewDataBlockIterator(
+template <typename TBlockIter>
+TBlockIter* BlockBasedTable::NewDataBlockIterator(
     Rep* rep, const ReadOptions& ro, const BlockHandle& handle,
-    BlockIter* input_iter, bool is_index, bool key_includes_seq,
+    TBlockIter* input_iter, bool is_index, bool key_includes_seq,
     GetContext* get_context, Status s, FilePrefetchBuffer* prefetch_buffer) {
   PERF_TIMER_GUARD(new_table_block_iter_nanos);
 
@@ -1616,11 +1626,11 @@ BlockIter* BlockBasedTable::NewDataBlockIterator(
                                   get_context);
   }
 
-  BlockIter* iter;
+  TBlockIter* iter;
   if (input_iter != nullptr) {
     iter = input_iter;
   } else {
-    iter = new BlockIter;
+    iter = new TBlockIter;
   }
   // Didn't get any data from block caches.
   if (s.ok() && block.value == nullptr) {
@@ -1647,9 +1657,10 @@ BlockIter* BlockBasedTable::NewDataBlockIterator(
 
   if (s.ok()) {
     assert(block.value != nullptr);
-    iter = block.value->NewIterator(
+    const bool kTotalOrderSeek = true;
+    iter = block.value->NewIterator<TBlockIter>(
         &rep->internal_comparator, rep->internal_comparator.user_comparator(),
-        iter, true, rep->ioptions.statistics, key_includes_seq);
+        iter, rep->ioptions.statistics, kTotalOrderSeek, key_includes_seq);
     if (block.cache_handle != nullptr) {
       iter->RegisterCleanup(&ReleaseCachedEntry, block_cache,
                             block.cache_handle);
@@ -1769,7 +1780,9 @@ BlockBasedTable::PartitionedIndexIteratorState::PartitionedIndexIteratorState(
       block_map_(block_map),
       index_key_includes_seq_(index_key_includes_seq) {}
 
-const size_t BlockBasedTableIterator::kMaxReadaheadSize = 256 * 1024;
+template <class TBlockIter>
+const size_t BlockBasedTableIterator<TBlockIter>::kMaxReadaheadSize =
+    256 * 1024;
 
 InternalIterator*
 BlockBasedTable::PartitionedIndexIteratorState::NewSecondaryIterator(
@@ -1790,9 +1803,10 @@ BlockBasedTable::PartitionedIndexIteratorState::NewSecondaryIterator(
     assert(block_cache);
     RecordTick(rep->ioptions.statistics, BLOCK_CACHE_BYTES_READ,
                block_cache->GetUsage(block->second.cache_handle));
-    return block->second.value->NewIterator(
+    Statistics* kNullStats = nullptr;
+    return block->second.value->NewIterator<IndexBlockIter>(
         &rep->internal_comparator, rep->internal_comparator.user_comparator(),
-        nullptr, true, rep->ioptions.statistics, index_key_includes_seq_);
+        nullptr, kNullStats, true, index_key_includes_seq_);
   }
   // Create an empty iterator
   return new BlockIter();
@@ -1928,7 +1942,8 @@ bool BlockBasedTable::PrefixMayMatch(
   return may_match;
 }
 
-void BlockBasedTableIterator::Seek(const Slice& target) {
+template <class TBlockIter>
+void BlockBasedTableIterator<TBlockIter>::Seek(const Slice& target) {
   if (!CheckPrefixMayMatch(target)) {
     ResetDataIter();
     return;
@@ -1945,18 +1960,19 @@ void BlockBasedTableIterator::Seek(const Slice& target) {
 
   InitDataBlock();
 
-  data_block_iter_.Seek(target);
+  block_iter_.Seek(target);
 
   FindKeyForward();
-  assert(!data_block_iter_.Valid() ||
-         (key_includes_seq_ &&
-          icomp_.Compare(target, data_block_iter_.key()) <= 0) ||
-         (!key_includes_seq_ &&
-          icomp_.user_comparator()->Compare(ExtractUserKey(target),
-                                            data_block_iter_.key()) <= 0));
+  assert(
+      !block_iter_.Valid() ||
+      (key_includes_seq_ && icomp_.Compare(target, block_iter_.key()) <= 0) ||
+      (!key_includes_seq_ &&
+       icomp_.user_comparator()->Compare(ExtractUserKey(target),
+                                         block_iter_.key()) <= 0));
 }
 
-void BlockBasedTableIterator::SeekForPrev(const Slice& target) {
+template <class TBlockIter>
+void BlockBasedTableIterator<TBlockIter>::SeekForPrev(const Slice& target) {
   if (!CheckPrefixMayMatch(target)) {
     ResetDataIter();
     return;
@@ -1990,14 +2006,15 @@ void BlockBasedTableIterator::SeekForPrev(const Slice& target) {
 
   InitDataBlock();
 
-  data_block_iter_.SeekForPrev(target);
+  block_iter_.SeekForPrev(target);
 
   FindKeyBackward();
-  assert(!data_block_iter_.Valid() ||
-         icomp_.Compare(target, data_block_iter_.key()) >= 0);
+  assert(!block_iter_.Valid() ||
+         icomp_.Compare(target, block_iter_.key()) >= 0);
 }
 
-void BlockBasedTableIterator::SeekToFirst() {
+template <class TBlockIter>
+void BlockBasedTableIterator<TBlockIter>::SeekToFirst() {
   SavePrevIndexValue();
   index_iter_->SeekToFirst();
   if (!index_iter_->Valid()) {
@@ -2005,11 +2022,12 @@ void BlockBasedTableIterator::SeekToFirst() {
     return;
   }
   InitDataBlock();
-  data_block_iter_.SeekToFirst();
+  block_iter_.SeekToFirst();
   FindKeyForward();
 }
 
-void BlockBasedTableIterator::SeekToLast() {
+template <class TBlockIter>
+void BlockBasedTableIterator<TBlockIter>::SeekToLast() {
   SavePrevIndexValue();
   index_iter_->SeekToLast();
   if (!index_iter_->Valid()) {
@@ -2017,29 +2035,32 @@ void BlockBasedTableIterator::SeekToLast() {
     return;
   }
   InitDataBlock();
-  data_block_iter_.SeekToLast();
+  block_iter_.SeekToLast();
   FindKeyBackward();
 }
 
-void BlockBasedTableIterator::Next() {
+template <class TBlockIter>
+void BlockBasedTableIterator<TBlockIter>::Next() {
   assert(block_iter_points_to_real_block_);
-  data_block_iter_.Next();
+  block_iter_.Next();
   FindKeyForward();
 }
 
-void BlockBasedTableIterator::Prev() {
+template <class TBlockIter>
+void BlockBasedTableIterator<TBlockIter>::Prev() {
   assert(block_iter_points_to_real_block_);
-  data_block_iter_.Prev();
+  block_iter_.Prev();
   FindKeyBackward();
 }
 
-void BlockBasedTableIterator::InitDataBlock() {
+template <class TBlockIter>
+void BlockBasedTableIterator<TBlockIter>::InitDataBlock() {
   BlockHandle data_block_handle;
   Slice handle_slice = index_iter_->value();
   if (!block_iter_points_to_real_block_ ||
       handle_slice.compare(prev_index_value_) != 0 ||
       // if previous attempt of reading the block missed cache, try again
-      data_block_iter_.status().IsIncomplete()) {
+      block_iter_.status().IsIncomplete()) {
     if (block_iter_points_to_real_block_) {
       ResetDataIter();
     }
@@ -2075,21 +2096,21 @@ void BlockBasedTableIterator::InitDataBlock() {
       }
     }
 
-    BlockBasedTable::NewDataBlockIterator(rep, read_options_, data_block_handle,
-                                          &data_block_iter_, is_index_,
-                                          key_includes_seq_,
-                                          /* get_context */ nullptr, s,
-                                          prefetch_buffer_.get());
+    BlockBasedTable::NewDataBlockIterator<TBlockIter>(
+        rep, read_options_, data_block_handle, &block_iter_, is_index_,
+        key_includes_seq_,
+        /* get_context */ nullptr, s, prefetch_buffer_.get());
     block_iter_points_to_real_block_ = true;
   }
 }
 
-void BlockBasedTableIterator::FindKeyForward() {
+template <class TBlockIter>
+void BlockBasedTableIterator<TBlockIter>::FindKeyForward() {
   is_out_of_bound_ = false;
   // TODO the while loop inherits from two-level-iterator. We don't know
   // whether a block can be empty so it can be replaced by an "if".
-  while (!data_block_iter_.Valid()) {
-    if (!data_block_iter_.status().ok()) {
+  while (!block_iter_.Valid()) {
+    if (!block_iter_.status().ok()) {
       return;
     }
     ResetDataIter();
@@ -2101,7 +2122,7 @@ void BlockBasedTableIterator::FindKeyForward() {
 
     if (index_iter_->Valid()) {
       InitDataBlock();
-      data_block_iter_.SeekToFirst();
+      block_iter_.SeekToFirst();
     } else {
       return;
     }
@@ -2110,8 +2131,8 @@ void BlockBasedTableIterator::FindKeyForward() {
   // Check upper bound on the current key
   bool reached_upper_bound =
       (read_options_.iterate_upper_bound != nullptr &&
-       block_iter_points_to_real_block_ && data_block_iter_.Valid() &&
-       icomp_.user_comparator()->Compare(ExtractUserKey(data_block_iter_.key()),
+       block_iter_points_to_real_block_ && block_iter_.Valid() &&
+       icomp_.user_comparator()->Compare(ExtractUserKey(block_iter_.key()),
                                          *read_options_.iterate_upper_bound) >=
            0);
   TEST_SYNC_POINT_CALLBACK(
@@ -2123,9 +2144,10 @@ void BlockBasedTableIterator::FindKeyForward() {
   }
 }
 
-void BlockBasedTableIterator::FindKeyBackward() {
-  while (!data_block_iter_.Valid()) {
-    if (!data_block_iter_.status().ok()) {
+template <class TBlockIter>
+void BlockBasedTableIterator<TBlockIter>::FindKeyBackward() {
+  while (!block_iter_.Valid()) {
+    if (!block_iter_.status().ok()) {
       return;
     }
 
@@ -2134,7 +2156,7 @@ void BlockBasedTableIterator::FindKeyBackward() {
 
     if (index_iter_->Valid()) {
       InitDataBlock();
-      data_block_iter_.SeekToLast();
+      block_iter_.SeekToLast();
     } else {
       return;
     }
@@ -2151,7 +2173,7 @@ InternalIterator* BlockBasedTable::NewIterator(
       PrefixExtractorChanged(rep_->table_properties.get(), prefix_extractor);
   const bool kIsNotIndex = false;
   if (arena == nullptr) {
-    return new BlockBasedTableIterator(
+    return new BlockBasedTableIterator<DataBlockIter>(
         this, read_options, rep_->internal_comparator,
         NewIndexIterator(
             read_options,
@@ -2162,8 +2184,9 @@ InternalIterator* BlockBasedTable::NewIterator(
         need_upper_bound_check, prefix_extractor, kIsNotIndex,
         true /*key_includes_seq*/, for_compaction);
   } else {
-    auto* mem = arena->AllocateAligned(sizeof(BlockBasedTableIterator));
-    return new (mem) BlockBasedTableIterator(
+    auto* mem =
+        arena->AllocateAligned(sizeof(BlockBasedTableIterator<DataBlockIter>));
+    return new (mem) BlockBasedTableIterator<DataBlockIter>(
         this, read_options, rep_->internal_comparator,
         NewIndexIterator(read_options, need_upper_bound_check),
         !skip_filters && !read_options.total_order_seek &&
@@ -2188,10 +2211,9 @@ InternalIterator* BlockBasedTable::NewRangeTombstoneIterator(
     Cache* block_cache = rep_->table_options.block_cache.get();
     assert(block_cache != nullptr);
     if (block_cache->Ref(rep_->range_del_entry.cache_handle)) {
-      auto iter = rep_->range_del_entry.value->NewIterator(
+      auto iter = rep_->range_del_entry.value->NewIterator<BlockIter>(
           &rep_->internal_comparator,
-          rep_->internal_comparator.user_comparator(), nullptr /* iter */,
-          true /* total_order_seek */, rep_->ioptions.statistics);
+          rep_->internal_comparator.user_comparator());
       iter->RegisterCleanup(&ReleaseCachedEntry, block_cache,
                             rep_->range_del_entry.cache_handle);
       return iter;
@@ -2201,7 +2223,7 @@ InternalIterator* BlockBasedTable::NewRangeTombstoneIterator(
   rep_->range_del_handle.EncodeTo(&str);
   // The meta-block exists but isn't in uncompressed block cache (maybe
   // because it is disabled), so go through the full lookup process.
-  return NewDataBlockIterator(rep_, read_options, Slice(str));
+  return NewDataBlockIterator<BlockIter>(rep_, read_options, Slice(str));
 }
 
 bool BlockBasedTable::FullFilterKeyMayMatch(
@@ -2253,7 +2275,7 @@ Status BlockBasedTable::Get(const ReadOptions& read_options, const Slice& key,
                              prefix_extractor)) {
     RecordTick(rep_->ioptions.statistics, BLOOM_FILTER_USEFUL);
   } else {
-    BlockIter iiter_on_stack;
+    IndexBlockIter iiter_on_stack;
     // if prefix_extractor found in block differs from options, disable
     // BlockPrefixIndex. Only do this check when index_type is kHashSearch.
     bool need_upper_bound_check = false;
@@ -2288,9 +2310,10 @@ Status BlockBasedTable::Get(const ReadOptions& read_options, const Slice& key,
         RecordTick(rep_->ioptions.statistics, BLOOM_FILTER_USEFUL);
         break;
       } else {
-        BlockIter biter;
-        NewDataBlockIterator(rep_, read_options, iiter->value(), &biter, false,
-                             true /* key_includes_seq */, get_context);
+        DataBlockIter biter;
+        NewDataBlockIterator<DataBlockIter>(
+            rep_, read_options, iiter->value(), &biter, false,
+            true /* key_includes_seq */, get_context);
 
         if (read_options.read_tier == kBlockCacheTier &&
             biter.status().IsIncomplete()) {
@@ -2352,7 +2375,7 @@ Status BlockBasedTable::Prefetch(const Slice* const begin,
     return Status::InvalidArgument(*begin, *end);
   }
 
-  BlockIter iiter_on_stack;
+  IndexBlockIter iiter_on_stack;
   auto iiter = NewIndexIterator(ReadOptions(), false, &iiter_on_stack);
   std::unique_ptr<InternalIterator> iiter_unique_ptr;
   if (iiter != &iiter_on_stack) {
@@ -2386,8 +2409,9 @@ Status BlockBasedTable::Prefetch(const Slice* const begin,
     }
 
     // Load the block specified by the block_handle into the block cache
-    BlockIter biter;
-    NewDataBlockIterator(rep_, ReadOptions(), block_handle, &biter);
+    DataBlockIter biter;
+    NewDataBlockIterator<DataBlockIter>(rep_, ReadOptions(), block_handle,
+                                        &biter);
 
     if (!biter.status().ok()) {
       // there was an unexpected error while pre-fetching
@@ -2413,7 +2437,7 @@ Status BlockBasedTable::VerifyChecksum() {
     return s;
   }
   // Check Data blocks
-  BlockIter iiter_on_stack;
+  IndexBlockIter iiter_on_stack;
   InternalIterator* iiter =
       NewIndexIterator(ReadOptions(), false, &iiter_on_stack);
   std::unique_ptr<InternalIterator> iiter_unique_ptr;
@@ -2645,8 +2669,8 @@ Status BlockBasedTable::GetKVPairsFromDataBlocks(
     }
 
     std::unique_ptr<InternalIterator> datablock_iter;
-    datablock_iter.reset(
-        NewDataBlockIterator(rep_, ReadOptions(), blockhandles_iter->value()));
+    datablock_iter.reset(NewDataBlockIterator<DataBlockIter>(
+        rep_, ReadOptions(), blockhandles_iter->value()));
     s = datablock_iter->status();
 
     if (!s.ok()) {
@@ -2927,8 +2951,8 @@ Status BlockBasedTable::DumpDataBlocks(WritableFile* out_file) {
     out_file->Append("--------------------------------------\n");
 
     std::unique_ptr<InternalIterator> datablock_iter;
-    datablock_iter.reset(
-        NewDataBlockIterator(rep_, ReadOptions(), blockhandles_iter->value()));
+    datablock_iter.reset(NewDataBlockIterator<DataBlockIter>(
+        rep_, ReadOptions(), blockhandles_iter->value()));
     s = datablock_iter->status();
 
     if (!s.ok()) {

table/block_based_table_reader.h

@@ -175,7 +175,7 @@ class BlockBasedTable : public TableReader {
     // to
     // a different object then iter and the callee has the ownership of the
     // returned object.
-    virtual InternalIterator* NewIterator(BlockIter* iter = nullptr,
+    virtual InternalIterator* NewIterator(IndexBlockIter* iter = nullptr,
                                           bool total_order_seek = true,
                                           bool fill_cache = true) = 0;
 
@@ -217,14 +217,16 @@ class BlockBasedTable : public TableReader {
   Rep* get_rep() { return rep_; }
 
   // input_iter: if it is not null, update this one and return it as Iterator
-  static BlockIter* NewDataBlockIterator(
+  template <typename TBlockIter>
+  static TBlockIter* NewDataBlockIterator(
       Rep* rep, const ReadOptions& ro, const Slice& index_value,
-      BlockIter* input_iter = nullptr, bool is_index = false,
+      TBlockIter* input_iter = nullptr, bool is_index = false,
       bool key_includes_seq = true, GetContext* get_context = nullptr,
       FilePrefetchBuffer* prefetch_buffer = nullptr);
-  static BlockIter* NewDataBlockIterator(
+  template <typename TBlockIter>
+  static TBlockIter* NewDataBlockIterator(
       Rep* rep, const ReadOptions& ro, const BlockHandle& block_hanlde,
-      BlockIter* input_iter = nullptr, bool is_index = false,
+      TBlockIter* input_iter = nullptr, bool is_index = false,
       bool key_includes_seq = true, GetContext* get_context = nullptr,
       Status s = Status(), FilePrefetchBuffer* prefetch_buffer = nullptr);
 
@@ -281,7 +283,7 @@ class BlockBasedTable : public TableReader {
   //     kBlockCacheTier
   InternalIterator* NewIndexIterator(
       const ReadOptions& read_options, bool need_upper_bound_check = false,
-      BlockIter* input_iter = nullptr,
+      IndexBlockIter* input_iter = nullptr,
       CachableEntry<IndexReader>* index_entry = nullptr,
       GetContext* get_context = nullptr);
 
@@ -516,6 +518,7 @@ struct BlockBasedTable::Rep {
   const bool immortal_table;
 };
 
+template <class TBlockIter>
 class BlockBasedTableIterator : public InternalIterator {
  public:
   BlockBasedTableIterator(BlockBasedTable* table,
@@ -549,21 +552,21 @@ class BlockBasedTableIterator : public InternalIterator {
   void Prev() override;
   bool Valid() const override {
     return !is_out_of_bound_ && block_iter_points_to_real_block_ &&
-           data_block_iter_.Valid();
+           block_iter_.Valid();
   }
   Slice key() const override {
     assert(Valid());
-    return data_block_iter_.key();
+    return block_iter_.key();
   }
   Slice value() const override {
     assert(Valid());
-    return data_block_iter_.value();
+    return block_iter_.value();
   }
   Status status() const override {
     if (!index_iter_->status().ok()) {
       return index_iter_->status();
     } else if (block_iter_points_to_real_block_) {
-      return data_block_iter_.status();
+      return block_iter_.status();
     } else {
       return Status::OK();
     }
@@ -576,7 +579,7 @@ class BlockBasedTableIterator : public InternalIterator {
   }
   bool IsKeyPinned() const override {
     return pinned_iters_mgr_ && pinned_iters_mgr_->PinningEnabled() &&
-           block_iter_points_to_real_block_ && data_block_iter_.IsKeyPinned();
+           block_iter_points_to_real_block_ && block_iter_.IsKeyPinned();
   }
   bool IsValuePinned() const override {
     // BlockIter::IsValuePinned() is always true. No need to check
@@ -601,9 +604,9 @@ class BlockBasedTableIterator : public InternalIterator {
   void ResetDataIter() {
     if (block_iter_points_to_real_block_) {
       if (pinned_iters_mgr_ != nullptr && pinned_iters_mgr_->PinningEnabled()) {
-        data_block_iter_.DelegateCleanupsTo(pinned_iters_mgr_);
+        block_iter_.DelegateCleanupsTo(pinned_iters_mgr_);
       }
-      data_block_iter_.Invalidate(Status::OK());
+      block_iter_.Invalidate(Status::OK());
       block_iter_points_to_real_block_ = false;
     }
   }
@@ -627,7 +630,7 @@ class BlockBasedTableIterator : public InternalIterator {
   const InternalKeyComparator& icomp_;
   InternalIterator* index_iter_;
   PinnedIteratorsManager* pinned_iters_mgr_;
-  BlockIter data_block_iter_;
+  TBlockIter block_iter_;
   bool block_iter_points_to_real_block_;
   bool is_out_of_bound_ = false;
   bool check_filter_;

table/block_test.cc

@@ -100,7 +100,7 @@ TEST_F(BlockTest, SimpleTest) {
   // read contents of block sequentially
   int count = 0;
   InternalIterator *iter =
-      reader.NewIterator(options.comparator, options.comparator);
+      reader.NewIterator<BlockIter>(options.comparator, options.comparator);
   for (iter->SeekToFirst();iter->Valid(); count++, iter->Next()) {
 
     // read kv from block
@@ -114,7 +114,7 @@ TEST_F(BlockTest, SimpleTest) {
   delete iter;
 
   // read block contents randomly
-  iter = reader.NewIterator(options.comparator, options.comparator);
+  iter = reader.NewIterator<BlockIter>(options.comparator, options.comparator);
   for (int i = 0; i < num_records; i++) {
 
     // find a random key in the lookaside array
@@ -163,8 +163,8 @@ void CheckBlockContents(BlockContents contents, const int max_key,
   std::unique_ptr<const SliceTransform> prefix_extractor(
       NewFixedPrefixTransform(prefix_size));
 
-  std::unique_ptr<InternalIterator> regular_iter(
-      reader2.NewIterator(BytewiseComparator(), BytewiseComparator()));
+  std::unique_ptr<InternalIterator> regular_iter(reader2.NewIterator<BlockIter>(
+      BytewiseComparator(), BytewiseComparator()));
 
   // Seek existent keys
   for (size_t i = 0; i < keys.size(); i++) {
@@ -389,8 +389,9 @@ TEST_F(BlockTest, BlockWithReadAmpBitmap) {
 
     // read contents of block sequentially
     size_t read_bytes = 0;
-    BlockIter *iter = static_cast<BlockIter *>(reader.NewIterator(
-        options.comparator, options.comparator, nullptr, true, stats.get()));
+    DataBlockIter *iter =
+        static_cast<DataBlockIter *>(reader.NewIterator<DataBlockIter>(
+            options.comparator, options.comparator, nullptr, stats.get()));
     for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {
       iter->value();
       read_bytes += iter->TEST_CurrentEntrySize();
@@ -422,8 +423,9 @@ TEST_F(BlockTest, BlockWithReadAmpBitmap) {
                  kBytesPerBit, stats.get());
 
     size_t read_bytes = 0;
-    BlockIter *iter = static_cast<BlockIter *>(reader.NewIterator(
-        options.comparator, options.comparator, nullptr, true, stats.get()));
+    DataBlockIter *iter =
+        static_cast<DataBlockIter *>(reader.NewIterator<DataBlockIter>(
+            options.comparator, options.comparator, nullptr, stats.get()));
     for (int i = 0; i < num_records; i++) {
       Slice k(keys[i]);
 
@@ -458,8 +460,9 @@ TEST_F(BlockTest, BlockWithReadAmpBitmap) {
                  kBytesPerBit, stats.get());
 
     size_t read_bytes = 0;
-    BlockIter *iter = static_cast<BlockIter *>(reader.NewIterator(
-        options.comparator, options.comparator, nullptr, true, stats.get()));
+    DataBlockIter *iter =
+        static_cast<DataBlockIter *>(reader.NewIterator<DataBlockIter>(
+            options.comparator, options.comparator, nullptr, stats.get()));
     std::unordered_set<int> read_keys;
     for (int i = 0; i < num_records; i++) {
       int index = rnd.Uniform(num_records);

table/meta_blocks.cc

@@ -204,8 +204,8 @@ Status ReadProperties(const Slice& handle_value, RandomAccessFileReader* file,
   Block properties_block(std::move(block_contents),
                          kDisableGlobalSequenceNumber);
   BlockIter iter;
-  properties_block.NewIterator(BytewiseComparator(), BytewiseComparator(),
-                               &iter);
+  properties_block.NewIterator<BlockIter>(BytewiseComparator(),
+                                          BytewiseComparator(), &iter);
 
   auto new_table_properties = new TableProperties();
   // All pre-defined properties of type uint64_t
@@ -335,7 +335,8 @@ Status ReadTableProperties(RandomAccessFileReader* file, uint64_t file_size,
   Block metaindex_block(std::move(metaindex_contents),
                         kDisableGlobalSequenceNumber);
   std::unique_ptr<InternalIterator> meta_iter(
-      metaindex_block.NewIterator(BytewiseComparator(), BytewiseComparator()));
+      metaindex_block.NewIterator<BlockIter>(BytewiseComparator(),
+                                             BytewiseComparator()));
 
   // -- Read property block
   bool found_properties_block = true;
@@ -404,8 +405,8 @@ Status FindMetaBlock(RandomAccessFileReader* file, uint64_t file_size,
                         kDisableGlobalSequenceNumber);
 
   std::unique_ptr<InternalIterator> meta_iter;
-  meta_iter.reset(
-      metaindex_block.NewIterator(BytewiseComparator(), BytewiseComparator()));
+  meta_iter.reset(metaindex_block.NewIterator<BlockIter>(BytewiseComparator(),
+                                                         BytewiseComparator()));
 
   return FindMetaBlock(meta_iter.get(), meta_block_name, block_handle);
 }
@@ -451,8 +452,8 @@ Status ReadMetaBlock(RandomAccessFileReader* file,
                         kDisableGlobalSequenceNumber);
 
   std::unique_ptr<InternalIterator> meta_iter;
-  meta_iter.reset(
-      metaindex_block.NewIterator(BytewiseComparator(), BytewiseComparator()));
+  meta_iter.reset(metaindex_block.NewIterator<BlockIter>(BytewiseComparator(),
+                                                         BytewiseComparator()));
 
   BlockHandle block_handle;
   status = FindMetaBlock(meta_iter.get(), meta_block_name, &block_handle);

table/partitioned_filter_block.cc

@@ -129,10 +129,12 @@ PartitionedFilterBlockReader::~PartitionedFilterBlockReader() {
     return;
   }
   char cache_key[BlockBasedTable::kMaxCacheKeyPrefixSize + kMaxVarint64Length];
-  BlockIter biter;
+  IndexBlockIter biter;
   BlockHandle handle;
-  idx_on_fltr_blk_->NewIterator(&comparator_, comparator_.user_comparator(),
-                                &biter, true, nullptr, index_key_includes_seq_);
+  Statistics* kNullStats = nullptr;
+  idx_on_fltr_blk_->NewIterator<IndexBlockIter>(
+      &comparator_, comparator_.user_comparator(), &biter, kNullStats, true,
+      index_key_includes_seq_);
   biter.SeekToFirst();
   for (; biter.Valid(); biter.Next()) {
     auto input = biter.value();
@@ -225,9 +227,11 @@ bool PartitionedFilterBlockReader::PrefixMayMatch(
 
 Slice PartitionedFilterBlockReader::GetFilterPartitionHandle(
     const Slice& entry) {
-  BlockIter iter;
-  idx_on_fltr_blk_->NewIterator(&comparator_, comparator_.user_comparator(),
-                                &iter, true, nullptr, index_key_includes_seq_);
+  IndexBlockIter iter;
+  Statistics* kNullStats = nullptr;
+  idx_on_fltr_blk_->NewIterator<IndexBlockIter>(
+      &comparator_, comparator_.user_comparator(), &iter, kNullStats, true,
+      index_key_includes_seq_);
   iter.Seek(entry);
   if (UNLIKELY(!iter.Valid())) {
     return Slice();
@@ -294,10 +298,12 @@ void PartitionedFilterBlockReader::CacheDependencies(
     bool pin, const SliceTransform* prefix_extractor) {
   // Before read partitions, prefetch them to avoid lots of IOs
   auto rep = table_->rep_;
-  BlockIter biter;
+  IndexBlockIter biter;
   BlockHandle handle;
-  idx_on_fltr_blk_->NewIterator(&comparator_, comparator_.user_comparator(),
-                                &biter, true, nullptr, index_key_includes_seq_);
+  Statistics* kNullStats = nullptr;
+  idx_on_fltr_blk_->NewIterator<IndexBlockIter>(
+      &comparator_, comparator_.user_comparator(), &biter, kNullStats, true,
+      index_key_includes_seq_);
   // Index partitions are assumed to be consecuitive. Prefetch them all.
   // Read the first block offset
   biter.SeekToFirst();

table/table_test.cc

@@ -238,7 +238,7 @@ class BlockConstructor: public Constructor {
   }
   virtual InternalIterator* NewIterator(
       const SliceTransform* /*prefix_extractor*/) const override {
-    return block_->NewIterator(comparator_, comparator_);
+    return block_->NewIterator<BlockIter>(comparator_, comparator_);
   }
 
  private:
@@ -3473,8 +3473,9 @@ TEST_P(BlockBasedTableTest, PropertiesBlockRestartPointTest) {
     Block metaindex_block(std::move(metaindex_contents),
                           kDisableGlobalSequenceNumber);
 
-    std::unique_ptr<InternalIterator> meta_iter(metaindex_block.NewIterator(
-        BytewiseComparator(), BytewiseComparator()));
+    std::unique_ptr<InternalIterator> meta_iter(
+        metaindex_block.NewIterator<BlockIter>(BytewiseComparator(),
+                                               BytewiseComparator()));
     bool found_properties_block = true;
     ASSERT_OK(SeekToPropertiesBlock(meta_iter.get(), &found_properties_block));
     ASSERT_TRUE(found_properties_block);