[RocksDB Performance Branch] A more customized index in PlainTableReader

Summary:
PlainTableReader to use a more customized hash table. This patch assumes the SST file is smaller than 2GB:
(1) Every bucket uses 32-bit integer
(2) no key is stored in bucket
(3) use the first bit of the bucket value to distinguish it points to the file offset or a second level index.
This index schema fits the use case that most of prefixes have very small number of keys

Test Plan: plain_table_db_test

Reviewers: haobo, kailiu, dhruba

Reviewed By: haobo

CC: nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D14343

include/rocksdb/plain_table_factory.h

@@ -23,7 +23,9 @@ class TableBuilder;
 
 // IndexedTable requires fixed length key, configured as a constructor
 // parameter of the factory class. Output file format:
-// +--------------------------------------------+  <= key1 offset
+// +-------------+
+// | version     |
+// +-------------+------------------------------+  <= key1 offset
 // | key1            | value_size (4 bytes) |   |
 // +----------------------------------------+   |
 // | value1                                     |

table/plain_table_builder.cc

@@ -25,6 +25,10 @@ PlainTableBuilder::PlainTableBuilder(const Options& options,
                                      int user_key_size, int key_prefix_len) :
     options_(options), file_(file), user_key_size_(user_key_size),
     key_prefix_len_(key_prefix_len) {
+  std::string version;
+  PutFixed32(&version, 1 | 0x80000000);
+  file_->Append(Slice(version));
+  offset_ = 4;
 }
 
 PlainTableBuilder::~PlainTableBuilder() {
@@ -43,11 +47,11 @@ void PlainTableBuilder::Add(const Slice& key, const Slice& value) {
 
   std::string size;
   int value_size = value.size();
-  PutFixed32(&size, value_size);
+  PutVarint32(&size, value_size);
   Slice sizeSlice(size);
   file_->Append(sizeSlice);
   file_->Append(value);
-  offset_ += value_size + 4;
+  offset_ += value_size + size.length();
 
   num_entries_++;
 }

table/plain_table_reader.cc

@@ -5,6 +5,7 @@
 #include "table/plain_table_reader.h"
 
 #include <unordered_map>
+#include <map>
 
 #include "db/dbformat.h"
 
@@ -23,21 +24,35 @@
 #include "util/coding.h"
 #include "util/hash.h"
 #include "util/histogram.h"
+#include "util/murmurhash.h"
 #include "util/perf_context_imp.h"
 #include "util/stop_watch.h"
 
+
 namespace std {
 template<>
 struct hash<rocksdb::Slice> {
 public:
   std::size_t operator()(rocksdb::Slice const& s) const {
-    return rocksdb::Hash(s.data(), s.size(), 397);
+    return MurmurHash(s.data(), s.size(), 397);
+  }
+};
+
+class slice_comparator {
+public:
+  bool operator()(rocksdb::Slice const& s1, rocksdb::Slice const& s2) {
+    return s1.compare(s2) < 0;
   }
 };
 }
 
 namespace rocksdb {
 
+static uint32_t getBucketId(Slice const& s, size_t prefix_len,
+                            uint32_t num_buckets) {
+  return MurmurHash(s.data(), prefix_len, 397) % num_buckets;
+}
+
 PlainTableReader::PlainTableReader(const EnvOptions& storage_options,
                                    uint64_t file_size, int user_key_size,
                                    int key_prefix_len, int bloom_bits_per_key,
@@ -51,6 +66,8 @@ PlainTableReader::PlainTableReader(const EnvOptions& storage_options,
     filter_policy_ = nullptr;
   }
   hash_table_ = nullptr;
+  data_start_offset_ = 0;
+  data_end_offset_ = file_size;
 }
 
 PlainTableReader::~PlainTableReader() {
@@ -73,6 +90,10 @@ Status PlainTableReader::Open(const Options& options,
                               double hash_table_ratio) {
   assert(options.allow_mmap_reads);
 
+  if (file_size > 2147483646) {
+    return Status::NotSupported("File is too large for PlainTableReader!");
+  }
+
   PlainTableReader* t = new PlainTableReader(soptions, file_size,
                                              user_key_size,
                                              key_prefix_len,
@@ -101,104 +122,111 @@ Iterator* PlainTableReader::NewIterator(const ReadOptions& options) {
 }
 
 Status PlainTableReader::PopulateIndex(uint64_t file_size) {
+  // Get mmapped memory to file_data_.
+  Status s = file_->Read(0, file_size_, &file_data_, nullptr);
+  if (!s.ok()) {
+    return s;
+  }
+  version_ = DecodeFixed32(file_data_.data());
+  version_ ^= 0x80000000;
+  assert(version_ == 1);
+  data_start_offset_ = 4;
+  data_end_offset_ = file_size;
+
   Slice key_slice;
   Slice key_prefix_slice;
   Slice key_suffix_slice;
   Slice value_slice;
-  Slice tmp_slice;
   Slice prev_key_prefix_slice;
-  uint64_t pos = 0;
-  uint64_t data_offset_for_cur_prefix = 0;
-  int count_prefix = 0;
+  uint32_t pos = data_start_offset_;
+  int key_index_within_prefix = 0;
   bool first = true;
   std::string prefix_sub_index;
   HistogramImpl keys_per_prefix_hist;
-  std::unordered_map<Slice, uint64_t> tmp_index;
+  // Need map to be ordered to make sure sub indexes generated
+  // are in order.
+  std::map<Slice, std::string, std::slice_comparator> prefix2map;
 
   while (pos < file_size) {
-    uint64_t key_offset = pos;
-    pos = Next(pos, &key_slice, &value_slice, &tmp_slice);
+    uint32_t key_offset = pos;
+    status_ = Next(pos, &key_slice, &value_slice, pos);
     key_prefix_slice = Slice(key_slice.data(), key_prefix_len_);
 
     if (first || prev_key_prefix_slice != key_prefix_slice) {
       if (!first) {
-        if (count_prefix < 8 || key_prefix_len_ == user_key_size_) {
-          tmp_index[prev_key_prefix_slice] = data_offset_for_cur_prefix;
-        } else {
-          tmp_index[prev_key_prefix_slice] = sub_index_.length()
-              | kSubIndexMask;
-          PutFixed32(&sub_index_, (count_prefix - 1) / 8 + 1);
-          sub_index_.append(prefix_sub_index);
-        }
-        prefix_sub_index.clear();
-        data_offset_for_cur_prefix = key_offset;
-        keys_per_prefix_hist.Add(count_prefix);
+        keys_per_prefix_hist.Add(key_index_within_prefix);
       }
+      key_index_within_prefix = 0;
       prev_key_prefix_slice = key_prefix_slice;
-      count_prefix = 1;
-    } else {
-      count_prefix++;
-    }
-    if (key_prefix_len_ < user_key_size_ && count_prefix % 8 == 1) {
-      prefix_sub_index.append(key_slice.data() + key_prefix_len_,
-                              user_key_size_ - key_prefix_len_);
-      PutFixed64(&prefix_sub_index, key_offset);
     }
 
-    first = false;
+    if (key_index_within_prefix++ % 8 == 0) {
+      // Add an index key for every 8 keys
+      std::string& prefix_index = prefix2map[key_prefix_slice];
+      PutFixed32(&prefix_index, key_offset);
     }
-  keys_per_prefix_hist.Add(count_prefix);
-  if (count_prefix <= 2 || key_prefix_len_ == user_key_size_) {
-    tmp_index[prev_key_prefix_slice] = data_offset_for_cur_prefix;
-  } else {
-    tmp_index[prev_key_prefix_slice] = sub_index_.length() | kSubIndexMask;
-    PutFixed32(&sub_index_, (count_prefix - 1) / 8 + 1);
-    sub_index_.append(prefix_sub_index);
+    first = false;
   }
-
+  keys_per_prefix_hist.Add(key_index_within_prefix);
   if (hash_table_ != nullptr) {
     delete[] hash_table_;
   }
-  // Make the hash table 3/5 full
   std::vector<Slice> filter_entries(0); // for creating bloom filter;
   if (filter_policy_ != nullptr) {
-    filter_entries.reserve(tmp_index.size());
+    filter_entries.reserve(prefix2map.size());
   }
   double hash_table_size_multipier =
       (hash_table_ratio_ > 1.0) ? 1.0 : 1.0 / hash_table_ratio_;
-  hash_table_size_ = tmp_index.size() * hash_table_size_multipier + 1;
-  hash_table_ = new char[GetHashTableRecordLen() * hash_table_size_];
-  for (int i = 0; i < hash_table_size_; i++) {
-    memcpy(GetHashTableBucketPtr(i) + key_prefix_len_, &file_size_,
-           kOffsetLen);
-  }
+  hash_table_size_ = prefix2map.size() * hash_table_size_multipier + 1;
+  hash_table_ = new uint32_t[hash_table_size_];
+  std::vector<std::string> hash2map(hash_table_size_);
 
-  for (auto it = tmp_index.begin(); it != tmp_index.end(); ++it) {
+  size_t sub_index_size_needed = 0;
+  for (auto& p: prefix2map) {
+    auto& sub_index = hash2map[getBucketId(p.first, key_prefix_len_,
+                                           hash_table_size_)];
+    if (sub_index.length() > 0 || p.second.length() > kOffsetLen) {
+      if (sub_index.length() <= kOffsetLen) {
+        sub_index_size_needed += sub_index.length() + 4;
+      }
+      sub_index_size_needed += p.second.length();
+    }
+    sub_index.append(p.second);
     if (filter_policy_ != nullptr) {
-      filter_entries.push_back(it->first);
+      filter_entries.push_back(p.first);
+    }
   }
 
-    int bucket = GetHashTableBucket(it->first);
-    uint64_t* hash_value;
-    while (true) {
-      GetHashValue(bucket, &hash_value);
-      if (*hash_value == file_size_) {
+  sub_index_.clear();
+  Log(options_.info_log, "Reserving %zu bytes for sub index",
+      sub_index_size_needed);
+  sub_index_.reserve(sub_index_size_needed);
+  for (int i = 0; i < hash_table_size_; i++) {
+    uint32_t num_keys_for_bucket = hash2map[i].length() / kOffsetLen;
+    switch (num_keys_for_bucket) {
+    case 0:
+      // No key for bucket
+      hash_table_[i] = data_end_offset_;
       break;
+    case 1:
+      // point directly to the file offset
+      hash_table_[i] = DecodeFixed32(hash2map[i].data());
+      break;
+    default:
+      // point to index block
+      hash_table_[i] = sub_index_.length() | kSubIndexMask;
+      PutFixed32(&sub_index_, num_keys_for_bucket);
+      sub_index_.append(hash2map[i]);
     }
-      bucket = (bucket + 1) % hash_table_size_;
-    }
-
-    char* bucket_ptr = GetHashTableBucketPtr(bucket);
-    memcpy(bucket_ptr, it->first.data(), key_prefix_len_);
-    memcpy(bucket_ptr + key_prefix_len_, &it->second, kOffsetLen);
   }
   if (filter_policy_ != nullptr) {
+    filter_str_.clear();
     filter_policy_->CreateFilter(&filter_entries[0], filter_entries.size(),
                                  &filter_str_);
     filter_slice_ = Slice(filter_str_.data(), filter_str_.size());
   }
 
-  Log(options_.info_log, "Number of prefixes: %d, suffix_map length %ld",
+  Log(options_.info_log, "hash table size: %d, suffix_map length %zu",
       hash_table_size_, sub_index_.length());
   Log(options_.info_log, "Number of Keys per prefix Histogram: %s",
       keys_per_prefix_hist.ToString().c_str());
@@ -206,51 +234,33 @@ Status PlainTableReader::PopulateIndex(uint64_t file_size) {
   return Status::OK();
 }
 
-inline int PlainTableReader::GetHashTableBucket(Slice key) {
-  return rocksdb::Hash(key.data(), key_prefix_len_, 397) % hash_table_size_;
-}
-
-inline void PlainTableReader::GetHashValue(int bucket, uint64_t** ret_value) {
-  *ret_value = (uint64_t*) (GetHashTableBucketPtr(bucket) + key_prefix_len_);
-}
-
-Status PlainTableReader::GetOffset(const Slice& target, uint64_t* offset) {
-  Status s;
-  int bucket = GetHashTableBucket(target);
-  uint64_t* found_value;
-  Slice hash_key;
-  while (true) {
-    GetHashValue(bucket, &found_value);
-    if (*found_value == file_size_) {
-      break;
-    }
-    GetHashKey(bucket, &hash_key);
-    if (target.starts_with(hash_key)) {
-      break;
-    }
-    bucket = (bucket + 1) % hash_table_size_;
-  }
-
-  if (*found_value == file_size_ || (*found_value & kSubIndexMask) == 0) {
-    *offset = *found_value;
-    return Status::OK();
+uint32_t PlainTableReader::GetOffset(const Slice& target,
+                                     bool& prefix_matched) {
+  prefix_matched = false;
+  int bucket = getBucketId(target, key_prefix_len_, hash_table_size_);
+  uint32_t bucket_value = hash_table_[bucket];
+  if (bucket_value == data_end_offset_) {
+    return data_end_offset_;
+  } else if ((bucket_value & kSubIndexMask) == 0) {
+    // point directly to the file
+    return bucket_value;
   }
+  // point to sub-index, need to do a binary search
 
   uint32_t low = 0;
-  uint64_t prefix_index_offset = *found_value ^ kSubIndexMask;
-  uint32_t high = DecodeFixed32(sub_index_.data() + prefix_index_offset);
+  uint64_t prefix_index_offset = bucket_value ^ kSubIndexMask;
+  uint32_t upper_bound = DecodeFixed32(sub_index_.data() + prefix_index_offset);
+  uint32_t high = upper_bound;
   uint64_t base_offset = prefix_index_offset + 4;
-  char* mid_key_str = new char[target.size()];
-  memcpy(mid_key_str, target.data(), target.size());
-  Slice mid_key = Slice(mid_key_str, target.size());
+  Slice mid_key;
 
-  // The key is between (low, high). Do a binary search between it.
+  // The key is between [low, high). Do a binary search between it.
   while (high - low > 1) {
     uint32_t mid = (high + low) / 2;
-    const char* base = sub_index_.data() + base_offset
-        + (user_key_size_ - key_prefix_len_ + kOffsetLen) * mid;
-    memcpy(mid_key_str + key_prefix_len_, base,
-           user_key_size_ - key_prefix_len_);
+    const char* index_offset = sub_index_.data() + base_offset
+        + kOffsetLen * mid;
+    uint32_t file_offset = DecodeFixed32(index_offset);
+    mid_key = Slice(file_data_.data() + file_offset, user_key_size_);
 
     int cmp_result = options_.comparator->Compare(target, mid_key);
     if (cmp_result > 0) {
@@ -259,21 +269,32 @@ Status PlainTableReader::GetOffset(const Slice& target, uint64_t* offset) {
       if (cmp_result == 0) {
         // Happen to have found the exact key or target is smaller than the
         // first key after base_offset.
-        *offset = DecodeFixed64(base + user_key_size_ - key_prefix_len_);
-        delete[] mid_key_str;
-        return s;
+        prefix_matched = true;
+        return file_offset;
       } else {
         high = mid;
       }
     }
   }
 
-  const char* base = sub_index_.data() + base_offset
-      + (user_key_size_ - key_prefix_len_ + kOffsetLen) * low;
-  *offset = DecodeFixed64(base + user_key_size_ - key_prefix_len_);
-
-  delete[] mid_key_str;
-  return s;
+  // The key is between low and low+1 (if exists). Both of them can have the
+  // correct prefix. Need to rule out at least one, to avoid to miss the
+  // correct one.
+  uint32_t low_key_offset = DecodeFixed32(
+      sub_index_.data() + base_offset + kOffsetLen * low);
+  if (low + 1 < upper_bound) {
+    if (Slice(file_data_.data() + low_key_offset, key_prefix_len_)
+        == Slice(target.data(), key_prefix_len_)) {
+      prefix_matched = true;
+    } else {
+      prefix_matched = false;
+      return DecodeFixed32(
+          sub_index_.data() + base_offset + kOffsetLen * (low + 1));
+    }
+  } else {
+    prefix_matched = false;
+  }
+  return low_key_offset;
 }
 
 bool PlainTableReader::MayHavePrefix(const Slice& target_prefix) {
@@ -282,46 +303,74 @@ bool PlainTableReader::MayHavePrefix(const Slice& target_prefix) {
 }
 
 
-uint64_t PlainTableReader::Next(uint64_t offset, Slice* key, Slice* value,
-                                Slice* tmp_slice) {
-  if (offset >= file_size_) {
-    return file_size_;
+Status PlainTableReader::Next(uint32_t offset, Slice* key, Slice* value,
+                              uint32_t& next_offset) {
+  if (offset == data_end_offset_) {
+    next_offset = data_end_offset_;
+    return Status::OK();
   }
-  int internal_key_size = GetInternalKeyLength();
 
-  Status s = file_->Read(offset, internal_key_size, key, nullptr);
-  offset += internal_key_size;
+  if (offset > data_end_offset_) {
+    return Status::Corruption("Offset is out of file size");
+  }
 
-  s = file_->Read(offset, 4, tmp_slice, nullptr);
-  offset += 4;
-  uint32_t value_size = DecodeFixed32(tmp_slice->data());
+  int internal_key_size = GetInternalKeyLength();
+  if (offset + internal_key_size >= data_end_offset_) {
+    return Status::Corruption("Un able to read the next key");
+  }
 
-  s = file_->Read(offset, value_size, value, nullptr);
-  offset += value_size;
+  const char* key_ptr =  file_data_.data() + offset;
+  *key = Slice(key_ptr, internal_key_size);
 
-  return offset;
+  uint32_t value_size;
+  const char* value_ptr = GetVarint32Ptr(key_ptr + internal_key_size,
+                                         file_data_.data() + data_end_offset_,
+                                         &value_size);
+  if (value_ptr == nullptr) {
+    return Status::Corruption("Error reading value length.");
+  }
+  next_offset = offset + (value_ptr - key_ptr) + value_size;
+  if (next_offset > data_end_offset_) {
+    return Status::Corruption("Reach end of file when reading value");
+  }
+  *value = Slice(value_ptr, value_size);
+
+  return Status::OK();
 }
 
 Status PlainTableReader::Get(
     const ReadOptions& ro, const Slice& target, void* arg,
     bool (*saver)(void*, const Slice&, const Slice&, bool),
     void (*mark_key_may_exist)(void*)) {
-  uint64_t offset;
-  Status s = GetOffset(target, &offset);
-  if (!s.ok()) {
-    return s;
+  // Check bloom filter first.
+  if (!MayHavePrefix(Slice(target.data(), key_prefix_len_))) {
+    return Status::OK();
   }
+
+  uint32_t offset;
+  bool prefix_match;
+  offset = GetOffset(target, prefix_match);
   Slice found_key;
   Slice found_value;
-  Slice tmp_slice;
-  while (offset < file_size_) {
-    offset = Next(offset, &found_key, &found_value, &tmp_slice);
+  while (offset < data_end_offset_) {
+    Status s = Next(offset, &found_key, &found_value, offset);
+    if (!s.ok()) {
+      return s;
+    }
+    if (!prefix_match) {
+      // Need to verify prefix for the first key found if it is not yet
+      // checked.
+      if (!target.starts_with(Slice(found_key.data(), key_prefix_len_))) {
+        break;
+      }
+      prefix_match = true;
+    }
     if (options_.comparator->Compare(found_key, target) >= 0
         && !(*saver)(arg, found_key, found_value, true)) {
       break;
     }
   }
-  return s;
+  return Status::OK();
 }
 
 bool PlainTableReader::TEST_KeyInCache(const ReadOptions& options,
@@ -342,11 +391,12 @@ PlainTableIterator::~PlainTableIterator() {
 }
 
 bool PlainTableIterator::Valid() const {
-  return offset_ < table_->file_size_ && offset_ >= 0;
+  return offset_ < table_->data_end_offset_
+      && offset_ >= table_->data_start_offset_;
 }
 
 void PlainTableIterator::SeekToFirst() {
-  next_offset_ = 0;
+  next_offset_ = table_->data_start_offset_;
   Next();
 }
 
@@ -356,26 +406,35 @@ void PlainTableIterator::SeekToLast() {
 
 void PlainTableIterator::Seek(const Slice& target) {
   if (!table_->MayHavePrefix(Slice(target.data(), table_->key_prefix_len_))) {
-    offset_ = next_offset_ = table_->file_size_;
+    offset_ = next_offset_ = table_->data_end_offset_;
     return;
   }
+  bool prefix_match;
+  next_offset_ = table_->GetOffset(target, prefix_match);
 
-  Status s = table_->GetOffset(target, &next_offset_);
-  if (!s.ok()) {
-    status_ = s;
+  if (next_offset_ < table_-> data_end_offset_) {
+    for (Next(); status_.ok() && Valid(); Next()) {
+      if (!prefix_match) {
+        // Need to verify the first key's prefix
+        if (!target.starts_with(Slice(key().data(), table_->key_prefix_len_))) {
+          offset_ = next_offset_ = table_->data_end_offset_;
+          break;
         }
-  if (next_offset_ < table_->file_size_) {
-    for (Next();
-        Valid() && table_->options_.comparator->Compare(key(), target) < 0;
-        Next()) {
+        prefix_match = true;
       }
+      if (table_->options_.comparator->Compare(key(), target) >= 0) {
+        break;
+      }
+    }
+  } else {
+    offset_ = table_->data_end_offset_;
   }
 }
 
 void PlainTableIterator::Next() {
   offset_ = next_offset_;
   Slice tmp_slice;
-  next_offset_ = table_->Next(next_offset_, &key_, &value_, &tmp_slice);
+  status_ = table_->Next(next_offset_, &key_, &value_, next_offset_);
 }
 
 void PlainTableIterator::Prev() {

table/plain_table_reader.h

@@ -25,7 +25,9 @@ using std::unique_ptr;
 using std::unordered_map;
 
 // Based on following output file format:
-// +--------------------------------------------+  <= key1_data_offset
+// +-------------+
+// | version     |
+// +-------------+------------------------------+  <= key1_data_offset
 // | key1            | value_size (4 bytes) |   |
 // +----------------------------------------+   |
 // | value1                                     |
@@ -85,7 +87,7 @@ public:
   ~PlainTableReader();
 
 private:
-  char* hash_table_;
+  uint32_t* hash_table_;
   int hash_table_size_;
   std::string sub_index_;
 
@@ -94,7 +96,11 @@ private:
   Status status_;
   unique_ptr<RandomAccessFile> file_;
 
-  uint64_t file_size_;
+  Slice file_data_;
+  uint32_t version_;
+  uint32_t file_size_;
+  uint32_t data_start_offset_;
+  uint32_t data_end_offset_;
   const size_t user_key_size_;
   const size_t key_prefix_len_;
   const double hash_table_ratio_;
@@ -105,32 +111,34 @@ private:
   TableProperties tbl_props;
 
   static const size_t kNumInternalBytes = 8;
-  static const uint64_t kSubIndexMask = 0x8000000000000000;
-  static const size_t kOffsetLen = sizeof(uint64_t);
+  static const uint32_t kSubIndexMask = 0x80000000;
+  static const size_t kOffsetLen = sizeof(uint32_t);
 
-  inline int GetHashTableBucket(Slice key);
   inline size_t GetInternalKeyLength() {
     return user_key_size_ + kNumInternalBytes;
   }
-  inline size_t GetHashTableRecordLen() {
-    return key_prefix_len_ + kOffsetLen;
-  }
-  inline char* GetHashTableBucketPtr(int bucket) {
-    return hash_table_ + GetHashTableRecordLen() * bucket;
-  }
-  inline void GetHashKey(int bucket, Slice* slice) {
-    *slice = Slice(GetHashTableBucketPtr(bucket), key_prefix_len_);
-  }
-  inline void GetHashValue(int bucket, uint64_t** ret_value);
 
   friend class TableCache;
   friend class PlainTableIterator;
 
+  // Populate the internal indexes. It must be called before
+  // any query to the table.
+  // This query will populate the hash table hash_table_, the second
+  // level of indexes sub_index_ and bloom filter filter_slice_ if enabled.
   Status PopulateIndex(uint64_t file_size);
-  uint64_t Next(uint64_t offset, Slice* key, Slice* value, Slice* tmp_slice);
-  Status GetOffset(const Slice& target, uint64_t* offset);
+
+  // Check bloom filter to see whether it might contain this prefix
   bool MayHavePrefix(const Slice& target_prefix);
 
+  // Read the key and value at offset to key and value.
+  // tmp_slice is a tmp slice.
+  // return next_offset as the offset for the next key.
+  Status Next(uint32_t offset, Slice* key, Slice* value, uint32_t& next_offset);
+  // Get file offset for key target.
+  // return value prefix_matched is set to true if the offset is confirmed
+  // for a key with the same prefix as target.
+  uint32_t GetOffset(const Slice& target, bool& prefix_matched);
+
   // No copying allowed
   explicit PlainTableReader(const TableReader&) = delete;
   void operator=(const TableReader&) = delete;
@@ -162,8 +170,8 @@ public:
 
 private:
   PlainTableReader* table_;
-  uint64_t offset_;
-  uint64_t next_offset_;
+  uint32_t offset_;
+  uint32_t next_offset_;
   Slice key_;
   Slice value_;
   Status status_;