table/block_based/block_based_table_iterator.cc

//  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.
#include "table/block_based/block_based_table_iterator.h"

namespace ROCKSDB_NAMESPACE {
void BlockBasedTableIterator::Seek(const Slice& target) { SeekImpl(&target); }

void BlockBasedTableIterator::SeekToFirst() { SeekImpl(nullptr); }

void BlockBasedTableIterator::SeekImpl(const Slice* target) {
  is_out_of_bound_ = false;
  is_at_first_key_from_index_ = false;
  if (target && !CheckPrefixMayMatch(*target, IterDirection::kForward)) {
    ResetDataIter();
    return;
  }

  bool need_seek_index = true;
  if (block_iter_points_to_real_block_ && block_iter_.Valid()) {
    // Reseek.
    prev_block_offset_ = index_iter_->value().handle.offset();

    if (target) {
      // We can avoid an index seek if:
      // 1. The new seek key is larger than the current key
      // 2. The new seek key is within the upper bound of the block
      // Since we don't necessarily know the internal key for either
      // the current key or the upper bound, we check user keys and
      // exclude the equality case. Considering internal keys can
      // improve for the boundary cases, but it would complicate the
      // code.
      if (user_comparator_.Compare(ExtractUserKey(*target),
                                   block_iter_.user_key()) > 0 &&
          user_comparator_.Compare(ExtractUserKey(*target),
                                   index_iter_->user_key()) < 0) {
        need_seek_index = false;
      }
    }
  }

  if (need_seek_index) {
    if (target) {
      index_iter_->Seek(*target);
    } else {
      index_iter_->SeekToFirst();
    }

    if (!index_iter_->Valid()) {
      ResetDataIter();
      return;
    }
  }

  IndexValue v = index_iter_->value();
  const bool same_block = block_iter_points_to_real_block_ &&
                          v.handle.offset() == prev_block_offset_;

  // TODO(kolmike): Remove the != kBlockCacheTier condition.
  if (!v.first_internal_key.empty() && !same_block &&
      (!target || icomp_.Compare(*target, v.first_internal_key) <= 0) &&
      read_options_.read_tier != kBlockCacheTier) {
    // Index contains the first key of the block, and it's >= target.
    // We can defer reading the block.
    is_at_first_key_from_index_ = true;
    // ResetDataIter() will invalidate block_iter_. Thus, there is no need to
    // call CheckDataBlockWithinUpperBound() to check for iterate_upper_bound
    // as that will be done later when the data block is actually read.
    ResetDataIter();
  } else {
    // Need to use the data block.
    if (!same_block) {
      InitDataBlock();
    } else {
      // When the user does a reseek, the iterate_upper_bound might have
      // changed. CheckDataBlockWithinUpperBound() needs to be called
      // explicitly if the reseek ends up in the same data block.
      // If the reseek ends up in a different block, InitDataBlock() will do
      // the iterator upper bound check.
      CheckDataBlockWithinUpperBound();
    }

    if (target) {
      block_iter_.Seek(*target);
    } else {
      block_iter_.SeekToFirst();
    }
    FindKeyForward();
  }

  CheckOutOfBound();

  if (target) {
    assert(!Valid() || icomp_.Compare(*target, key()) <= 0);
  }
}

void BlockBasedTableIterator::SeekForPrev(const Slice& target) {
  is_out_of_bound_ = false;
  is_at_first_key_from_index_ = false;
  // For now totally disable prefix seek in auto prefix mode because we don't
  // have logic
  if (!CheckPrefixMayMatch(target, IterDirection::kBackward)) {
    ResetDataIter();
    return;
  }

  SavePrevIndexValue();

  // Call Seek() rather than SeekForPrev() in the index block, because the
  // target data block will likely to contain the position for `target`, the
  // same as Seek(), rather than than before.
  // For example, if we have three data blocks, each containing two keys:
  //   [2, 4]  [6, 8] [10, 12]
  //  (the keys in the index block would be [4, 8, 12])
  // and the user calls SeekForPrev(7), we need to go to the second block,
  // just like if they call Seek(7).
  // The only case where the block is difference is when they seek to a position
  // in the boundary. For example, if they SeekForPrev(5), we should go to the
  // first block, rather than the second. However, we don't have the information
  // to distinguish the two unless we read the second block. In this case, we'll
  // end up with reading two blocks.
  index_iter_->Seek(target);

  if (!index_iter_->Valid()) {
    auto seek_status = index_iter_->status();
    // Check for IO error
    if (!seek_status.IsNotFound() && !seek_status.ok()) {
      ResetDataIter();
      return;
    }

    // With prefix index, Seek() returns NotFound if the prefix doesn't exist
    if (seek_status.IsNotFound()) {
      // Any key less than the target is fine for prefix seek
      ResetDataIter();
      return;
    } else {
      index_iter_->SeekToLast();
    }
    // Check for IO error
    if (!index_iter_->Valid()) {
      ResetDataIter();
      return;
    }
  }

  InitDataBlock();

  block_iter_.SeekForPrev(target);

  FindKeyBackward();
  CheckDataBlockWithinUpperBound();
  assert(!block_iter_.Valid() ||
         icomp_.Compare(target, block_iter_.key()) >= 0);
}

void BlockBasedTableIterator::SeekToLast() {
  is_out_of_bound_ = false;
  is_at_first_key_from_index_ = false;
  SavePrevIndexValue();
  index_iter_->SeekToLast();
  if (!index_iter_->Valid()) {
    ResetDataIter();
    return;
  }
  InitDataBlock();
  block_iter_.SeekToLast();
  FindKeyBackward();
  CheckDataBlockWithinUpperBound();
}

void BlockBasedTableIterator::Next() {
  if (is_at_first_key_from_index_ && !MaterializeCurrentBlock()) {
    return;
  }
  assert(block_iter_points_to_real_block_);
  block_iter_.Next();
  FindKeyForward();
  CheckOutOfBound();
}

bool BlockBasedTableIterator::NextAndGetResult(IterateResult* result) {
  Next();
  bool is_valid = Valid();
  if (is_valid) {
    result->key = key();
    result->may_be_out_of_upper_bound = MayBeOutOfUpperBound();
  }
  return is_valid;
}

void BlockBasedTableIterator::Prev() {
  if (is_at_first_key_from_index_) {
    is_at_first_key_from_index_ = false;

    index_iter_->Prev();
    if (!index_iter_->Valid()) {
      return;
    }

    InitDataBlock();
    block_iter_.SeekToLast();
  } else {
    assert(block_iter_points_to_real_block_);
    block_iter_.Prev();
  }

  FindKeyBackward();
}

void BlockBasedTableIterator::InitDataBlock() {
  BlockHandle data_block_handle = index_iter_->value().handle;
  if (!block_iter_points_to_real_block_ ||
      data_block_handle.offset() != prev_block_offset_ ||
      // if previous attempt of reading the block missed cache, try again
      block_iter_.status().IsIncomplete()) {
    if (block_iter_points_to_real_block_) {
      ResetDataIter();
    }
    auto* rep = table_->get_rep();

    bool is_for_compaction =
        lookup_context_.caller == TableReaderCaller::kCompaction;
    // Prefetch additional data for range scans (iterators).
    // Implicit auto readahead:
    //   Enabled after 2 sequential IOs when ReadOptions.readahead_size == 0.
    // Explicit user requested readahead:
    //   Enabled from the very first IO when ReadOptions.readahead_size is set.
    block_prefetcher_.PrefetchIfNeeded(rep, data_block_handle,
                                       read_options_.readahead_size,
                                       is_for_compaction);

    Status s;
    table_->NewDataBlockIterator<DataBlockIter>(
        read_options_, data_block_handle, &block_iter_, BlockType::kData,
        /*get_context=*/nullptr, &lookup_context_, s,
        block_prefetcher_.prefetch_buffer(),
        /*for_compaction=*/is_for_compaction);
    block_iter_points_to_real_block_ = true;
    CheckDataBlockWithinUpperBound();
  }
}

bool BlockBasedTableIterator::MaterializeCurrentBlock() {
  assert(is_at_first_key_from_index_);
  assert(!block_iter_points_to_real_block_);
  assert(index_iter_->Valid());

  is_at_first_key_from_index_ = false;
  InitDataBlock();
  assert(block_iter_points_to_real_block_);
  block_iter_.SeekToFirst();

  if (!block_iter_.Valid() ||
      icomp_.Compare(block_iter_.key(),
                     index_iter_->value().first_internal_key) != 0) {
    // Uh oh.
    block_iter_.Invalidate(Status::Corruption(
        "first key in index doesn't match first key in block"));
    return false;
  }

  return true;
}

void BlockBasedTableIterator::FindKeyForward() {
  // This method's code is kept short to make it likely to be inlined.

  assert(!is_out_of_bound_);
  assert(block_iter_points_to_real_block_);

  if (!block_iter_.Valid()) {
    // This is the only call site of FindBlockForward(), but it's extracted into
    // a separate method to keep FindKeyForward() short and likely to be
    // inlined. When transitioning to a different block, we call
    // FindBlockForward(), which is much longer and is probably not inlined.
    FindBlockForward();
  } else {
    // This is the fast path that avoids a function call.
  }
}

void BlockBasedTableIterator::FindBlockForward() {
  // 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".
  do {
    if (!block_iter_.status().ok()) {
      return;
    }
    // Whether next data block is out of upper bound, if there is one.
    const bool next_block_is_out_of_bound =
        read_options_.iterate_upper_bound != nullptr &&
        block_iter_points_to_real_block_ && !data_block_within_upper_bound_;
    assert(!next_block_is_out_of_bound ||
           user_comparator_.CompareWithoutTimestamp(
               *read_options_.iterate_upper_bound, /*a_has_ts=*/false,
               index_iter_->user_key(), /*b_has_ts=*/true) <= 0);
    ResetDataIter();
    index_iter_->Next();
    if (next_block_is_out_of_bound) {
      // The next block is out of bound. No need to read it.
      TEST_SYNC_POINT_CALLBACK("BlockBasedTableIterator:out_of_bound", nullptr);
      // We need to make sure this is not the last data block before setting
      // is_out_of_bound_, since the index key for the last data block can be
      // larger than smallest key of the next file on the same level.
      if (index_iter_->Valid()) {
        is_out_of_bound_ = true;
      }
      return;
    }

    if (!index_iter_->Valid()) {
      return;
    }

    IndexValue v = index_iter_->value();

    // TODO(kolmike): Remove the != kBlockCacheTier condition.
    if (!v.first_internal_key.empty() &&
        read_options_.read_tier != kBlockCacheTier) {
      // Index contains the first key of the block. Defer reading the block.
      is_at_first_key_from_index_ = true;
      return;
    }

    InitDataBlock();
    block_iter_.SeekToFirst();
  } while (!block_iter_.Valid());
}

void BlockBasedTableIterator::FindKeyBackward() {
  while (!block_iter_.Valid()) {
    if (!block_iter_.status().ok()) {
      return;
    }

    ResetDataIter();
    index_iter_->Prev();

    if (index_iter_->Valid()) {
      InitDataBlock();
      block_iter_.SeekToLast();
    } else {
      return;
    }
  }

  // We could have check lower bound here too, but we opt not to do it for
  // code simplicity.
}

void BlockBasedTableIterator::CheckOutOfBound() {
  if (read_options_.iterate_upper_bound != nullptr && Valid()) {
    is_out_of_bound_ =
        user_comparator_.CompareWithoutTimestamp(
            *read_options_.iterate_upper_bound, /*a_has_ts=*/false, user_key(),
            /*b_has_ts=*/true) <= 0;
  }
}

void BlockBasedTableIterator::CheckDataBlockWithinUpperBound() {
  if (read_options_.iterate_upper_bound != nullptr &&
      block_iter_points_to_real_block_) {
    data_block_within_upper_bound_ =
        (user_comparator_.CompareWithoutTimestamp(
             *read_options_.iterate_upper_bound, /*a_has_ts=*/false,
             index_iter_->user_key(),
             /*b_has_ts=*/true) > 0);
  }
}
}  // namespace ROCKSDB_NAMESPACE