rocksdb/db/file_indexer.h

//  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
//  This source code is licensed under the BSD-style license found in the
//  LICENSE file in the root directory of this source tree. An additional grant
//  of patent rights can be found in the PATENTS file in the same directory.
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#pragma once
#include <cstdint>
#include <functional>
#include <limits>
#include <vector>

namespace rocksdb {

class Comparator;
struct FileMetaData;

// The file tree structure in Version is prebuilt and the range of each file
// is known. On Version::Get(), it uses binary search to find a potential file
// and then check if a target key can be found in the file by comparing the key
// to each file's smallest and largest key. The results of these comparisions
// can be reused beyond checking if a key falls into a file's range.
// With some pre-calculated knowledge, each key comparision that has been done
// can serve as a hint to narrow down further searches: if a key compared to
// be smaller than a file's smallest or largest, that comparison can be used
// to find out the right bound of next binary search. Similarly, if a key
// compared to be larger than a file's smallest or largest, it can be utilized
// to find out the left bound of next binary search.
// With these hints: it can greatly reduce the range of binary search,
// especially for bottom levels, given that one file most likely overlaps with
// only N files from level below (where N is max_bytes_for_level_multiplier).
// So on level L, we will only look at ~N files instead of N^L files on the
// naive approach.
class FileIndexer {
 public:
  FileIndexer(const uint32_t num_levels, const Comparator* ucmp);

  uint32_t NumLevelIndex();

  uint32_t LevelIndexSize(uint32_t level);

  // Return a file index range in the next level to search for a key based on
  // smallest and largest key comparision for the current file specified by
  // level and file_index. When *left_index < *right_index, both index should
  // be valid and fit in the vector size.
  void GetNextLevelIndex(
    const uint32_t level, const uint32_t file_index, const int cmp_smallest,
    const int cmp_largest, int32_t* left_bound, int32_t* right_bound);

  void ClearIndex();

  void UpdateIndex(std::vector<FileMetaData*>* const files);

  enum {
    kLevelMaxIndex = std::numeric_limits<int32_t>::max()
  };

 private:
  const uint32_t num_levels_;
  const Comparator* ucmp_;

  struct IndexUnit {
    IndexUnit()
      : smallest_lb(0), largest_lb(0), smallest_rb(-1), largest_rb(-1) {}
    // During file search, a key is compared against smallest and largest
    // from a FileMetaData. It can have 3 possible outcomes:
    // (1) key is smaller than smallest, implying it is also smaller than
    //     larger. Precalculated index based on "smallest < smallest" can
    //     be used to provide right bound.
    // (2) key is in between smallest and largest.
    //     Precalculated index based on "smallest > greatest" can be used to
    //     provide left bound.
    //     Precalculated index based on "largest < smallest" can be used to
    //     provide right bound.
    // (3) key is larger than largest, implying it is also larger than smallest.
    //     Precalculated index based on "largest > largest" can be used to
    //     provide left bound.
    //
    // As a result, we will need to do:
    // Compare smallest (<=) and largest keys from upper level file with
    // smallest key from lower level to get a right bound.
    // Compare smallest (>=) and largest keys from upper level file with
    // largest key from lower level to get a left bound.
    //
    // Example:
    //    level 1:              [50 - 60]
    //    level 2:        [1 - 40], [45 - 55], [58 - 80]
    // A key 35, compared to be less than 50, 3rd file on level 2 can be
    // skipped according to rule (1). LB = 0, RB = 1.
    // A key 53, sits in the middle 50 and 60. 1st file on level 2 can be
    // skipped according to rule (2)-a, but the 3rd file cannot be skipped
    // because 60 is greater than 58. LB = 1, RB = 2.
    // A key 70, compared to be larger than 60. 1st and 2nd file can be skipped
    // according to rule (3). LB = 2, RB = 2.
    //
    // Point to a left most file in a lower level that may contain a key,
    // which compares greater than smallest of a FileMetaData (upper level)
    int32_t smallest_lb;
    // Point to a left most file in a lower level that may contain a key,
    // which compares greater than largest of a FileMetaData (upper level)
    int32_t largest_lb;
    // Point to a right most file in a lower level that may contain a key,
    // which compares smaller than smallest of a FileMetaData (upper level)
    int32_t smallest_rb;
    // Point to a right most file in a lower level that may contain a key,
    // which compares smaller than largest of a FileMetaData (upper level)
    int32_t largest_rb;
  };

  void CalculateLB(const std::vector<FileMetaData*>& upper_files,
    const std::vector<FileMetaData*>& lower_files,
    std::vector<IndexUnit>* index,
    std::function<int(const FileMetaData*, const FileMetaData*)> cmp_op,
    std::function<void(IndexUnit*, int32_t)> set_index);

  void CalculateRB(const std::vector<FileMetaData*>& upper_files,
    const std::vector<FileMetaData*>& lower_files,
    std::vector<IndexUnit>* index,
    std::function<int(const FileMetaData*, const FileMetaData*)> cmp_op,
    std::function<void(IndexUnit*, int32_t)> set_index);

  std::vector<std::vector<IndexUnit>> next_level_index_;
  std::vector<int32_t> level_rb_;
};

}  // namespace rocksdb