db_iter.h

//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root directory).
//
// 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 <stdint.h>
#include <string>
#include "db/db_impl/db_impl.h"
#include "db/dbformat.h"
#include "db/range_del_aggregator.h"
#include "memory/arena.h"
#include "options/cf_options.h"
#include "rocksdb/db.h"
#include "rocksdb/iterator.h"
#include "table/iterator_wrapper.h"
#include "util/autovector.h"

namespace rocksdb {

// This file declares the factory functions of DBIter, in its original form
// or a wrapped form with class ArenaWrappedDBIter, which is defined here.
// Class DBIter, which is declared and implemented inside db_iter.cc, is
// a iterator that converts internal keys (yielded by an InternalIterator)
// that were live at the specified sequence number into appropriate user
// keys.
// Each internal key is consist of a user key, a sequence number, and a value
// type. DBIter deals with multiple key versions, tombstones, merge operands,
// etc, and exposes an Iterator.
// For example, DBIter may wrap following InternalIterator:
//    user key: AAA  value: v3   seqno: 100    type: Put
//    user key: AAA  value: v2   seqno: 97     type: Put
//    user key: AAA  value: v1   seqno: 95     type: Put
//    user key: BBB  value: v1   seqno: 90     type: Put
//    user key: BBC  value: N/A  seqno: 98     type: Delete
//    user key: BBC  value: v1   seqno: 95     type: Put
// If the snapshot passed in is 102, then the DBIter is expected to
// expose the following iterator:
//    key: AAA  value: v3
//    key: BBB  value: v1
// If the snapshot passed in is 96, then it should expose:
//    key: AAA  value: v1
//    key: BBB  value: v1
//    key: BBC  value: v1
//

// Memtables and sstables that make the DB representation contain
// (userkey,seq,type) => uservalue entries.  DBIter
// combines multiple entries for the same userkey found in the DB
// representation into a single entry while accounting for sequence
// numbers, deletion markers, overwrites, etc.
class DBIter final: public Iterator {
 public:
  // The following is grossly complicated. TODO: clean it up
  // Which direction is the iterator currently moving?
  // (1) When moving forward:
  //   (1a) if current_entry_is_merged_ = false, the internal iterator is
  //        positioned at the exact entry that yields this->key(), this->value()
  //   (1b) if current_entry_is_merged_ = true, the internal iterator is
  //        positioned immediately after the last entry that contributed to the
  //        current this->value(). That entry may or may not have key equal to
  //        this->key().
  // (2) When moving backwards, the internal iterator is positioned
  //     just before all entries whose user key == this->key().
  enum Direction {
    kForward,
    kReverse
  };

  // LocalStatistics contain Statistics counters that will be aggregated per
  // each iterator instance and then will be sent to the global statistics when
  // the iterator is destroyed.
  //
  // The purpose of this approach is to avoid perf regression happening
  // when multiple threads bump the atomic counters from a DBIter::Next().
  struct LocalStatistics {
    explicit LocalStatistics() { ResetCounters(); }

    void ResetCounters() {
      next_count_ = 0;
      next_found_count_ = 0;
      prev_count_ = 0;
      prev_found_count_ = 0;
      bytes_read_ = 0;
      skip_count_ = 0;
    }

    void BumpGlobalStatistics(Statistics* global_statistics) {
      RecordTick(global_statistics, NUMBER_DB_NEXT, next_count_);
      RecordTick(global_statistics, NUMBER_DB_NEXT_FOUND, next_found_count_);
      RecordTick(global_statistics, NUMBER_DB_PREV, prev_count_);
      RecordTick(global_statistics, NUMBER_DB_PREV_FOUND, prev_found_count_);
      RecordTick(global_statistics, ITER_BYTES_READ, bytes_read_);
      RecordTick(global_statistics, NUMBER_ITER_SKIP, skip_count_);
      PERF_COUNTER_ADD(iter_read_bytes, bytes_read_);
      ResetCounters();
    }

    // Map to Tickers::NUMBER_DB_NEXT
    uint64_t next_count_;
    // Map to Tickers::NUMBER_DB_NEXT_FOUND
    uint64_t next_found_count_;
    // Map to Tickers::NUMBER_DB_PREV
    uint64_t prev_count_;
    // Map to Tickers::NUMBER_DB_PREV_FOUND
    uint64_t prev_found_count_;
    // Map to Tickers::ITER_BYTES_READ
    uint64_t bytes_read_;
    // Map to Tickers::NUMBER_ITER_SKIP
    uint64_t skip_count_;
  };

  DBIter(Env* _env, const ReadOptions& read_options,
         const ImmutableCFOptions& cf_options,
         const MutableCFOptions& mutable_cf_options, const Comparator* cmp,
         InternalIterator* iter, SequenceNumber s, bool arena_mode,
         uint64_t max_sequential_skip_in_iterations,
         ReadCallback* read_callback, DBImpl* db_impl, ColumnFamilyData* cfd,
         bool allow_blob);

  // No copying allowed
  DBIter(const DBIter&) = delete;
  void operator=(const DBIter&) = delete;

  ~DBIter() override {
    // Release pinned data if any
    if (pinned_iters_mgr_.PinningEnabled()) {
      pinned_iters_mgr_.ReleasePinnedData();
    }
    RecordTick(statistics_, NO_ITERATOR_DELETED);
    ResetInternalKeysSkippedCounter();
    local_stats_.BumpGlobalStatistics(statistics_);
    iter_.DeleteIter(arena_mode_);
  }
  virtual void SetIter(InternalIterator* iter) {
    assert(iter_.iter() == nullptr);
    iter_.Set(iter);
    iter_.iter()->SetPinnedItersMgr(&pinned_iters_mgr_);
  }
  virtual ReadRangeDelAggregator* GetRangeDelAggregator() {
    return &range_del_agg_;
  }

  bool Valid() const override { return valid_; }
  Slice key() const override {
    assert(valid_);
    if(start_seqnum_ > 0) {
      return saved_key_.GetInternalKey();
    } else {
      return saved_key_.GetUserKey();
    }
  }
  Slice value() const override {
    assert(valid_);
    if (current_entry_is_merged_) {
      // If pinned_value_ is set then the result of merge operator is one of
      // the merge operands and we should return it.
      return pinned_value_.data() ? pinned_value_ : saved_value_;
    } else if (direction_ == kReverse) {
      return pinned_value_;
    } else {
      return iter_.value();
    }
  }
  Status status() const override {
    if (status_.ok()) {
      return iter_.status();
    } else {
      assert(!valid_);
      return status_;
    }
  }
  bool IsBlob() const {
    assert(valid_ && (allow_blob_ || !is_blob_));
    return is_blob_;
  }

  Status GetProperty(std::string prop_name, std::string* prop) override;

  void Next() final override;
  void Prev() final override;
  void Seek(const Slice& target) final override;
  void SeekForPrev(const Slice& target) final override;
  void SeekToFirst() final override;
  void SeekToLast() final override;
  Env* env() { return env_; }
  void set_sequence(uint64_t s) {
    sequence_ = s;
    if (read_callback_) {
      read_callback_->Refresh(s);
    }
  }
  void set_valid(bool v) { valid_ = v; }

 private:
  // For all methods in this block:
  // PRE: iter_->Valid() && status_.ok()
  // Return false if there was an error, and status() is non-ok, valid_ = false;
  // in this case callers would usually stop what they were doing and return.
  bool ReverseToForward();
  bool ReverseToBackward();
  // Set saved_key_ to the seek key to target, with proper sequence number set.
  // It might get adjusted if the seek key is smaller than iterator lower bound.
  void SetSavedKeyToSeekTarget(const Slice& /*target*/);
  // Set saved_key_ to the seek key to target, with proper sequence number set.
  // It might get adjusted if the seek key is larger than iterator upper bound.
  void SetSavedKeyToSeekForPrevTarget(const Slice& /*target*/);
  bool FindValueForCurrentKey();
  bool FindValueForCurrentKeyUsingSeek();
  bool FindUserKeyBeforeSavedKey();
  // If `skipping_saved_key` is true, the function will keep iterating until it
  // finds a user key that is larger than `saved_key_`.
  // If `prefix` is not null, the iterator needs to stop when all keys for the
  // prefix are exhausted and the interator is set to invalid.
  bool FindNextUserEntry(bool skipping_saved_key, const Slice* prefix);
  // Internal implementation of FindNextUserEntry().
  bool FindNextUserEntryInternal(bool skipping_saved_key, const Slice* prefix);
  bool ParseKey(ParsedInternalKey* key);
  bool MergeValuesNewToOld();

  // If prefix is not null, we need to set the iterator to invalid if no more
  // entry can be found within the prefix.
  void PrevInternal(const Slice* /*prefix*/);
  bool TooManyInternalKeysSkipped(bool increment = true);
  bool IsVisible(SequenceNumber sequence);

  // Temporarily pin the blocks that we encounter until ReleaseTempPinnedData()
  // is called
  void TempPinData() {
    if (!pin_thru_lifetime_) {
      pinned_iters_mgr_.StartPinning();
    }
  }

  // Release blocks pinned by TempPinData()
  void ReleaseTempPinnedData() {
    if (!pin_thru_lifetime_ && pinned_iters_mgr_.PinningEnabled()) {
      pinned_iters_mgr_.ReleasePinnedData();
    }
  }

  inline void ClearSavedValue() {
    if (saved_value_.capacity() > 1048576) {
      std::string empty;
      swap(empty, saved_value_);
    } else {
      saved_value_.clear();
    }
  }

  inline void ResetInternalKeysSkippedCounter() {
    local_stats_.skip_count_ += num_internal_keys_skipped_;
    if (valid_) {
      local_stats_.skip_count_--;
    }
    num_internal_keys_skipped_ = 0;
  }

  const SliceTransform* prefix_extractor_;
  Env* const env_;
  Logger* logger_;
  UserComparatorWrapper user_comparator_;
  const MergeOperator* const merge_operator_;
  IteratorWrapper iter_;
  ReadCallback* read_callback_;
  // Max visible sequence number. It is normally the snapshot seq unless we have
  // uncommitted data in db as in WriteUnCommitted.
  SequenceNumber sequence_;

  IterKey saved_key_;
  // Reusable internal key data structure. This is only used inside one function
  // and should not be used across functions. Reusing this object can reduce
  // overhead of calling construction of the function if creating it each time.
  ParsedInternalKey ikey_;
  std::string saved_value_;
  Slice pinned_value_;
  // for prefix seek mode to support prev()
  Statistics* statistics_;
  uint64_t max_skip_;
  uint64_t max_skippable_internal_keys_;
  uint64_t num_internal_keys_skipped_;
  const Slice* iterate_lower_bound_;
  const Slice* iterate_upper_bound_;

  // The prefix of the seek key. It is only used when prefix_same_as_start_
  // is true and prefix extractor is not null. In Next() or Prev(), current keys
  // will be checked against this prefix, so that the iterator can be
  // invalidated if the keys in this prefix has been exhausted. Set it using
  // SetUserKey() and use it using GetUserKey().
  IterKey prefix_;

  Status status_;
  Direction direction_;
  bool valid_;
  bool current_entry_is_merged_;
  // True if we know that the current entry's seqnum is 0.
  // This information is used as that the next entry will be for another
  // user key.
  bool is_key_seqnum_zero_;
  const bool prefix_same_as_start_;
  // Means that we will pin all data blocks we read as long the Iterator
  // is not deleted, will be true if ReadOptions::pin_data is true
  const bool pin_thru_lifetime_;
  const bool total_order_seek_;
  bool allow_blob_;
  bool is_blob_;
  bool arena_mode_;
  // List of operands for merge operator.
  MergeContext merge_context_;
  ReadRangeDelAggregator range_del_agg_;
  LocalStatistics local_stats_;
  PinnedIteratorsManager pinned_iters_mgr_;
#ifdef ROCKSDB_LITE
  ROCKSDB_FIELD_UNUSED
#endif
  DBImpl* db_impl_;
#ifdef ROCKSDB_LITE
  ROCKSDB_FIELD_UNUSED
#endif
  ColumnFamilyData* cfd_;
  // for diff snapshots we want the lower bound on the seqnum;
  // if this value > 0 iterator will return internal keys
  SequenceNumber start_seqnum_;
};
// Return a new iterator that converts internal keys (yielded by
// "*internal_iter") that were live at the specified `sequence` number
// into appropriate user keys.
extern Iterator* NewDBIterator(
    Env* env, const ReadOptions& read_options,
    const ImmutableCFOptions& cf_options,
    const MutableCFOptions& mutable_cf_options,
    const Comparator* user_key_comparator, InternalIterator* internal_iter,
    const SequenceNumber& sequence, uint64_t max_sequential_skip_in_iterations,
    ReadCallback* read_callback, DBImpl* db_impl = nullptr,
    ColumnFamilyData* cfd = nullptr, bool allow_blob = false);

}  // namespace rocksdb