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rocksdb/table/merging_iterator.cc

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// 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/merging_iterator.h"
#include "db/arena_wrapped_db_iter.h"
#include "db/dbformat.h"
#include "db/pinned_iterators_manager.h"
#include "memory/arena.h"
#include "monitoring/perf_context_imp.h"
#include "rocksdb/comparator.h"
#include "rocksdb/iterator.h"
#include "rocksdb/options.h"
#include "table/internal_iterator.h"
#include "table/iter_heap.h"
#include "table/iterator_wrapper.h"
#include "test_util/sync_point.h"
#include "util/autovector.h"
#include "util/heap.h"
#include "util/stop_watch.h"
namespace ROCKSDB_NAMESPACE {
// For merging iterator to process range tombstones, we treat the start and end
// keys of a range tombstone as point keys and put them into the minHeap/maxHeap
// used in merging iterator. Take minHeap for example, we are able to keep track
// of currently "active" range tombstones (the ones whose start keys are popped
// but end keys are still in the heap) in `active_`. This `active_` set of range
// tombstones is then used to quickly determine whether the point key at heap
// top is deleted (by heap property, the point key at heap top must be within
// internal key range of active range tombstones).
//
// The HeapItem struct represents 3 types of elements in the minHeap/maxHeap:
// point key and the start and end keys of a range tombstone.
struct HeapItem {
HeapItem() = default;
enum Type { ITERATOR, DELETE_RANGE_START, DELETE_RANGE_END };
IteratorWrapper iter;
size_t level = 0;
std::string pinned_key;
// Will be overwritten before use, initialize here so compiler does not
// complain.
Type type = ITERATOR;
explicit HeapItem(size_t _level, InternalIteratorBase<Slice>* _iter)
: level(_level), type(Type::ITERATOR) {
iter.Set(_iter);
}
void SetTombstoneKey(ParsedInternalKey&& pik) {
pinned_key.clear();
// Range tombstone end key is exclusive. If a point internal key has the
// same user key and sequence number as the start or end key of a range
// tombstone, the order will be start < end key < internal key with the
// following op_type change. This is helpful to ensure keys popped from
// heap are in expected order since range tombstone start/end keys will
// be distinct from point internal keys. Strictly speaking, this is only
// needed for tombstone end points that are truncated in
// TruncatedRangeDelIterator since untruncated tombstone end points always
// have kMaxSequenceNumber and kTypeRangeDeletion (see
// TruncatedRangeDelIterator::start_key()/end_key()).
ParsedInternalKey p(pik.user_key, pik.sequence, kTypeMaxValid);
AppendInternalKey(&pinned_key, p);
}
Slice key() const {
if (type == Type::ITERATOR) {
return iter.key();
}
return pinned_key;
}
bool IsDeleteRangeSentinelKey() const {
if (type == Type::ITERATOR) {
return iter.IsDeleteRangeSentinelKey();
}
return false;
}
};
class MinHeapItemComparator {
public:
MinHeapItemComparator(const InternalKeyComparator* comparator)
: comparator_(comparator) {}
bool operator()(HeapItem* a, HeapItem* b) const {
return comparator_->Compare(a->key(), b->key()) > 0;
}
private:
const InternalKeyComparator* comparator_;
};
class MaxHeapItemComparator {
public:
MaxHeapItemComparator(const InternalKeyComparator* comparator)
: comparator_(comparator) {}
bool operator()(HeapItem* a, HeapItem* b) const {
return comparator_->Compare(a->key(), b->key()) < 0;
}
private:
const InternalKeyComparator* comparator_;
};
// Without anonymous namespace here, we fail the warning -Wmissing-prototypes
namespace {
using MergerMinIterHeap = BinaryHeap<HeapItem*, MinHeapItemComparator>;
using MergerMaxIterHeap = BinaryHeap<HeapItem*, MaxHeapItemComparator>;
} // namespace
class MergingIterator : public InternalIterator {
public:
MergingIterator(const InternalKeyComparator* comparator,
InternalIterator** children, int n, bool is_arena_mode,
bool prefix_seek_mode)
: is_arena_mode_(is_arena_mode),
prefix_seek_mode_(prefix_seek_mode),
direction_(kForward),
comparator_(comparator),
current_(nullptr),
minHeap_(comparator_),
pinned_iters_mgr_(nullptr) {
children_.resize(n);
for (int i = 0; i < n; i++) {
children_[i].level = i;
children_[i].iter.Set(children[i]);
}
}
void considerStatus(Status s) {
if (!s.ok() && status_.ok()) {
status_ = s;
}
}
virtual void AddIterator(InternalIterator* iter) {
children_.emplace_back(children_.size(), iter);
if (pinned_iters_mgr_) {
iter->SetPinnedItersMgr(pinned_iters_mgr_);
}
// Invalidate to ensure `Seek*()` is called to construct the heaps before
// use.
current_ = nullptr;
}
// Merging iterator can optionally process range tombstones: if a key is
// covered by a range tombstone, the merging iterator will not output it but
// skip it.
//
// Add the next range tombstone iterator to this merging iterator.
// There must be either no range tombstone iterator, or same number of
// range tombstone iterators as point iterators after all range tombstone
// iters are added. The i-th added range tombstone iterator and the i-th point
// iterator must point to the same sorted run.
// Merging iterator takes ownership of the range tombstone iterator and
// is responsible for freeing it. Note that during Iterator::Refresh()
// and when a level iterator moves to a different SST file, the range
// tombstone iterator could be updated. In that case, the merging iterator
// is only responsible to freeing the new range tombstone iterator
// that it has pointers to in range_tombstone_iters_.
void AddRangeTombstoneIterator(TruncatedRangeDelIterator* iter) {
range_tombstone_iters_.emplace_back(iter);
}
// Called by MergingIteratorBuilder when all point iterators and range
// tombstone iterators are added. Initializes HeapItems for range tombstone
// iterators so that no further allocation is needed for HeapItem.
void Finish() {
if (!range_tombstone_iters_.empty()) {
pinned_heap_item_.resize(range_tombstone_iters_.size());
for (size_t i = 0; i < range_tombstone_iters_.size(); ++i) {
pinned_heap_item_[i].level = i;
}
}
}
~MergingIterator() override {
for (auto child : range_tombstone_iters_) {
delete child;
}
for (auto& child : children_) {
child.iter.DeleteIter(is_arena_mode_);
}
status_.PermitUncheckedError();
}
bool Valid() const override { return current_ != nullptr && status_.ok(); }
Status status() const override { return status_; }
// Add range_tombstone_iters_[level] into min heap.
// Updates active_ if the end key of a range tombstone is inserted.
// @param start_key specifies which end point of the range tombstone to add.
void InsertRangeTombstoneToMinHeap(size_t level, bool start_key = true,
bool replace_top = false) {
assert(!range_tombstone_iters_.empty() &&
range_tombstone_iters_[level]->Valid());
if (start_key) {
pinned_heap_item_[level].SetTombstoneKey(
range_tombstone_iters_[level]->start_key());
pinned_heap_item_[level].type = HeapItem::DELETE_RANGE_START;
assert(active_.count(level) == 0);
} else {
pinned_heap_item_[level].SetTombstoneKey(
range_tombstone_iters_[level]->end_key());
pinned_heap_item_[level].type = HeapItem::DELETE_RANGE_END;
active_.insert(level);
}
if (replace_top) {
minHeap_.replace_top(&pinned_heap_item_[level]);
} else {
minHeap_.push(&pinned_heap_item_[level]);
}
}
// Add range_tombstone_iters_[level] into max heap.
// Updates active_ if the start key of a range tombstone is inserted.
// @param end_key specifies which end point of the range tombstone to add.
void InsertRangeTombstoneToMaxHeap(size_t level, bool end_key = true,
bool replace_top = false) {
assert(!range_tombstone_iters_.empty() &&
range_tombstone_iters_[level]->Valid());
if (end_key) {
pinned_heap_item_[level].SetTombstoneKey(
range_tombstone_iters_[level]->end_key());
pinned_heap_item_[level].type = HeapItem::DELETE_RANGE_END;
assert(active_.count(level) == 0);
} else {
pinned_heap_item_[level].SetTombstoneKey(
range_tombstone_iters_[level]->start_key());
pinned_heap_item_[level].type = HeapItem::DELETE_RANGE_START;
active_.insert(level);
}
if (replace_top) {
maxHeap_->replace_top(&pinned_heap_item_[level]);
} else {
maxHeap_->push(&pinned_heap_item_[level]);
}
}
// Remove HeapItems from top of minHeap_ that are of type DELETE_RANGE_START
// until minHeap_ is empty or the top of the minHeap_ is not of type
// DELETE_RANGE_START. Each such item means a range tombstone becomes active,
// so `active_` is updated accordingly.
void PopDeleteRangeStart() {
while (!minHeap_.empty() &&
minHeap_.top()->type == HeapItem::DELETE_RANGE_START) {
// insert end key of this range tombstone and updates active_
InsertRangeTombstoneToMinHeap(
minHeap_.top()->level, false /* start_key */, true /* replace_top */);
}
}
// Remove HeapItems from top of maxHeap_ that are of type DELETE_RANGE_END
// until maxHeap_ is empty or the top of the maxHeap_ is not of type
// DELETE_RANGE_END. Each such item means a range tombstone becomes active,
// so `active_` is updated accordingly.
void PopDeleteRangeEnd() {
while (!maxHeap_->empty() &&
maxHeap_->top()->type == HeapItem::DELETE_RANGE_END) {
// insert start key of this range tombstone and updates active_
InsertRangeTombstoneToMaxHeap(maxHeap_->top()->level, false /* end_key */,
true /* replace_top */);
}
}
void SeekToFirst() override {
ClearHeaps();
status_ = Status::OK();
for (auto& child : children_) {
child.iter.SeekToFirst();
AddToMinHeapOrCheckStatus(&child);
}
for (size_t i = 0; i < range_tombstone_iters_.size(); ++i) {
if (range_tombstone_iters_[i]) {
range_tombstone_iters_[i]->SeekToFirst();
if (range_tombstone_iters_[i]->Valid()) {
// It is possible to be invalid due to snapshots.
InsertRangeTombstoneToMinHeap(i);
}
}
}
FindNextVisibleKey();
direction_ = kForward;
current_ = CurrentForward();
}
void SeekToLast() override {
ClearHeaps();
InitMaxHeap();
status_ = Status::OK();
for (auto& child : children_) {
child.iter.SeekToLast();
AddToMaxHeapOrCheckStatus(&child);
}
for (size_t i = 0; i < range_tombstone_iters_.size(); ++i) {
if (range_tombstone_iters_[i]) {
range_tombstone_iters_[i]->SeekToLast();
if (range_tombstone_iters_[i]->Valid()) {
// It is possible to be invalid due to snapshots.
InsertRangeTombstoneToMaxHeap(i);
}
}
}
FindPrevVisibleKey();
direction_ = kReverse;
current_ = CurrentReverse();
}
// Position this merging iterator at the first key >= target (internal key).
// If range tombstones are present, keys covered by range tombstones are
// skipped, and this merging iter points to the first non-range-deleted key >=
// target after Seek(). If !Valid() and status().ok() then end of the iterator
// is reached.
//
// Internally, this involves positioning all child iterators at the first key
// >= target. If range tombstones are present, we apply a similar
// optimization, cascading seek, as in Pebble
// (https://github.com/cockroachdb/pebble). Specifically, if there is a range
// tombstone [start, end) that covers the target user key at level L, then
// this range tombstone must cover the range [target key, end) in all levels >
// L. So for all levels > L, we can pretend the target key is `end`. This
// optimization is applied at each level and hence the name "cascading seek".
// After a round of (cascading) seeks, the top of the heap is checked to see
// if it is covered by a range tombstone (see FindNextVisibleKey() for more
// detail), and advanced if so. The process is repeated until a
// non-range-deleted key is at the top of the heap, or heap becomes empty.
//
// As mentioned in comments above HeapItem, to make the checking of whether
// top of the heap is covered by some range tombstone efficient, we treat each
// range deletion [start, end) as two point keys and insert them into the same
// min/maxHeap_ where point iterators are. The set `active_` tracks the levels
// that have active range tombstones. If level L is in `active_`, and the
// point key at top of the heap is from level >= L, then the point key is
// within the internal key range of the range tombstone that
// range_tombstone_iters_[L] currently points to. For correctness reasoning,
// one invariant that Seek() (and every other public APIs Seek*(),
// Next/Prev()) guarantees is as follows. After Seek(), suppose `k` is the
// current key of level L's point iterator. Then for each range tombstone
// iterator at level <= L, it is at or before the first range tombstone with
// end key > `k`. This ensures that when level L's point iterator reaches top
// of the heap, `active_` is calculated correctly (it contains the covering
// range tombstone's level if there is one), since no range tombstone iterator
// was skipped beyond that point iterator's current key during Seek().
// Next()/Prev() maintains a stronger version of this invariant where all
// range tombstone iterators from level <= L are *at* the first range
// tombstone with end key > `k`.
void Seek(const Slice& target) override {
assert(range_tombstone_iters_.empty() ||
range_tombstone_iters_.size() == children_.size());
SeekImpl(target);
FindNextVisibleKey();
direction_ = kForward;
{
PERF_TIMER_GUARD(seek_min_heap_time);
current_ = CurrentForward();
}
}
void SeekForPrev(const Slice& target) override {
assert(range_tombstone_iters_.empty() ||
range_tombstone_iters_.size() == children_.size());
SeekForPrevImpl(target);
FindPrevVisibleKey();
direction_ = kReverse;
{
PERF_TIMER_GUARD(seek_max_heap_time);
current_ = CurrentReverse();
}
}
void Next() override {
assert(Valid());
// Ensure that all children are positioned after key().
// If we are moving in the forward direction, it is already
// true for all of the non-current children since current_ is
// the smallest child and key() == current_->key().
if (direction_ != kForward) {
// The loop advanced all non-current children to be > key() so current_
// should still be strictly the smallest key.
SwitchToForward();
}
// For the heap modifications below to be correct, current_ must be the
// current top of the heap.
assert(current_ == CurrentForward());
// as the current points to the current record. move the iterator forward.
current_->Next();
if (current_->Valid()) {
// current is still valid after the Next() call above. Call
// replace_top() to restore the heap property. When the same child
// iterator yields a sequence of keys, this is cheap.
assert(current_->status().ok());
minHeap_.replace_top(minHeap_.top());
} else {
// current stopped being valid, remove it from the heap.
considerStatus(current_->status());
minHeap_.pop();
}
FindNextVisibleKey();
current_ = CurrentForward();
}
bool NextAndGetResult(IterateResult* result) override {
Next();
bool is_valid = Valid();
if (is_valid) {
result->key = key();
result->bound_check_result = UpperBoundCheckResult();
result->value_prepared = current_->IsValuePrepared();
}
return is_valid;
}
void Prev() override {
assert(Valid());
// Ensure that all children are positioned before key().
// If we are moving in the reverse direction, it is already
// true for all of the non-current children since current_ is
// the largest child and key() == current_->key().
if (direction_ != kReverse) {
// Otherwise, retreat the non-current children. We retreat current_
// just after the if-block.
SwitchToBackward();
}
// For the heap modifications below to be correct, current_ must be the
// current top of the heap.
assert(current_ == CurrentReverse());
current_->Prev();
if (current_->Valid()) {
// current is still valid after the Prev() call above. Call
// replace_top() to restore the heap property. When the same child
// iterator yields a sequence of keys, this is cheap.
assert(current_->status().ok());
maxHeap_->replace_top(maxHeap_->top());
} else {
// current stopped being valid, remove it from the heap.
considerStatus(current_->status());
maxHeap_->pop();
}
FindPrevVisibleKey();
current_ = CurrentReverse();
}
Slice key() const override {
assert(Valid());
return current_->key();
}
Slice value() const override {
assert(Valid());
return current_->value();
}
bool PrepareValue() override {
assert(Valid());
if (current_->PrepareValue()) {
return true;
}
considerStatus(current_->status());
assert(!status_.ok());
return false;
}
// Here we simply relay MayBeOutOfLowerBound/MayBeOutOfUpperBound result
// from current child iterator. Potentially as long as one of child iterator
// report out of bound is not possible, we know current key is within bound.
bool MayBeOutOfLowerBound() override {
assert(Valid());
return current_->MayBeOutOfLowerBound();
}
IterBoundCheck UpperBoundCheckResult() override {
assert(Valid());
return current_->UpperBoundCheckResult();
}
void SetPinnedItersMgr(PinnedIteratorsManager* pinned_iters_mgr) override {
pinned_iters_mgr_ = pinned_iters_mgr;
for (auto& child : children_) {
child.iter.SetPinnedItersMgr(pinned_iters_mgr);
}
}
bool IsKeyPinned() const override {
assert(Valid());
return pinned_iters_mgr_ && pinned_iters_mgr_->PinningEnabled() &&
current_->IsKeyPinned();
}
bool IsValuePinned() const override {
assert(Valid());
return pinned_iters_mgr_ && pinned_iters_mgr_->PinningEnabled() &&
current_->IsValuePinned();
}
private:
friend class MergeIteratorBuilder;
// Clears heaps for both directions, used when changing direction or seeking
void ClearHeaps(bool clear_active = true);
// Ensures that maxHeap_ is initialized when starting to go in the reverse
// direction
void InitMaxHeap();
// Advance this merging iterator until the current key (top of min heap) is
// not covered by any range tombstone or that there is no more keys (heap is
// empty). After this call, if Valid(), current_ points to the next key that
// is not covered by any range tombstone.
void FindNextVisibleKey();
void FindPrevVisibleKey();
void SeekImpl(const Slice& target, size_t starting_level = 0,
bool range_tombstone_reseek = false);
// Seek to fist key <= target key (internal key) for
// children_[starting_level:].
void SeekForPrevImpl(const Slice& target, size_t starting_level = 0,
bool range_tombstone_reseek = false);
bool is_arena_mode_;
bool prefix_seek_mode_;
// Which direction is the iterator moving?
enum Direction : uint8_t { kForward, kReverse };
Direction direction_;
const InternalKeyComparator* comparator_;
// We could also use an autovector with a larger reserved size.
// HeapItem for all child point iterators.
std::vector<HeapItem> children_;
// HeapItem for range tombstone start and end keys. Each range tombstone
// iterator will have at most one side (start key or end key) in a heap
// at the same time, so this vector will be of size children_.size();
// pinned_heap_item_[i] corresponds to the start key and end key HeapItem
// for range_tombstone_iters_[i].
std::vector<HeapItem> pinned_heap_item_;
// range_tombstone_iters_[i] contains range tombstones in the sorted run that
// corresponds to children_[i]. range_tombstone_iters_.empty() means not
// handling range tombstones in merging iterator. range_tombstone_iters_[i] ==
// nullptr means the sorted run of children_[i] does not have range
// tombstones.
std::vector<TruncatedRangeDelIterator*> range_tombstone_iters_;
// Levels (indices into range_tombstone_iters_/children_ ) that currently have
// "active" range tombstones. See comments above Seek() for meaning of
// "active".
std::set<size_t> active_;
bool SkipNextDeleted();
bool SkipPrevDeleted();
// Cached pointer to child iterator with the current key, or nullptr if no
// child iterators are valid. This is the top of minHeap_ or maxHeap_
// depending on the direction.
IteratorWrapper* current_;
// If any of the children have non-ok status, this is one of them.
Status status_;
MergerMinIterHeap minHeap_;
// Max heap is used for reverse iteration, which is way less common than
// forward. Lazily initialize it to save memory.
std::unique_ptr<MergerMaxIterHeap> maxHeap_;
PinnedIteratorsManager* pinned_iters_mgr_;
// In forward direction, process a child that is not in the min heap.
// If valid, add to the min heap. Otherwise, check status.
void AddToMinHeapOrCheckStatus(HeapItem*);
// In backward direction, process a child that is not in the max heap.
// If valid, add to the min heap. Otherwise, check status.
void AddToMaxHeapOrCheckStatus(HeapItem*);
void SwitchToForward();
// Switch the direction from forward to backward without changing the
// position. Iterator should still be valid.
void SwitchToBackward();
IteratorWrapper* CurrentForward() const {
assert(direction_ == kForward);
assert(minHeap_.empty() || minHeap_.top()->type == HeapItem::ITERATOR);
return !minHeap_.empty() ? &minHeap_.top()->iter : nullptr;
}
IteratorWrapper* CurrentReverse() const {
assert(direction_ == kReverse);
assert(maxHeap_);
assert(maxHeap_->empty() || maxHeap_->top()->type == HeapItem::ITERATOR);
return !maxHeap_->empty() ? &maxHeap_->top()->iter : nullptr;
}
};
// Seek to fist key >= target key (internal key) for children_[starting_level:].
// Cascading seek optimizations are applied if range tombstones are present (see
// comment above Seek() for more).
//
// @param range_tombstone_reseek Whether target is some range tombstone
// end, i.e., whether this SeekImpl() call is a part of a "cascading seek". This
// is used only for recoding relevant perf_context.
void MergingIterator::SeekImpl(const Slice& target, size_t starting_level,
bool range_tombstone_reseek) {
// active range tombstones before `starting_level` remain active
ClearHeaps(false /* clear_active */);
ParsedInternalKey pik;
if (!range_tombstone_iters_.empty()) {
// pik is only used in InsertRangeTombstoneToMinHeap().
ParseInternalKey(target, &pik, false).PermitUncheckedError();
}
// TODO: perhaps we could save some upheap cost by add all child iters first
// and then do a single heapify.
for (size_t level = 0; level < starting_level; ++level) {
PERF_TIMER_GUARD(seek_min_heap_time);
AddToMinHeapOrCheckStatus(&children_[level]);
}
if (!range_tombstone_iters_.empty()) {
// Add range tombstones from levels < starting_level. We can insert from
// pinned_heap_item_ for the following reasons:
// - pinned_heap_item_[level] is in minHeap_ iff
// range_tombstone_iters[level]->Valid().
// - If `level` is in active_, then range_tombstone_iters_[level]->Valid()
// and pinned_heap_item_[level] is of type RANGE_DELETION_END.
for (size_t level = 0; level < starting_level; ++level) {
if (range_tombstone_iters_[level] &&
range_tombstone_iters_[level]->Valid()) {
assert(static_cast<bool>(active_.count(level)) ==
(pinned_heap_item_[level].type == HeapItem::DELETE_RANGE_END));
minHeap_.push(&pinned_heap_item_[level]);
} else {
assert(!active_.count(level));
}
}
// levels >= starting_level will be reseeked below, so clearing their active
// state here.
active_.erase(active_.lower_bound(starting_level), active_.end());
}
status_ = Status::OK();
IterKey current_search_key;
current_search_key.SetInternalKey(target, false /* copy */);
// Seek target might change to some range tombstone end key, so
// we need to remember them for async requests.
// (level, target) pairs
autovector<std::pair<size_t, std::string>> prefetched_target;
for (auto level = starting_level; level < children_.size(); ++level) {
{
PERF_TIMER_GUARD(seek_child_seek_time);
children_[level].iter.Seek(current_search_key.GetInternalKey());
}
PERF_COUNTER_ADD(seek_child_seek_count, 1);
if (!range_tombstone_iters_.empty()) {
if (range_tombstone_reseek) {
// This seek is to some range tombstone end key.
// Should only happen when there are range tombstones.
PERF_COUNTER_ADD(internal_range_del_reseek_count, 1);
}
if (children_[level].iter.status().IsTryAgain()) {
prefetched_target.emplace_back(
level, current_search_key.GetInternalKey().ToString());
}
auto range_tombstone_iter = range_tombstone_iters_[level];
if (range_tombstone_iter) {
range_tombstone_iter->Seek(current_search_key.GetUserKey());
if (range_tombstone_iter->Valid()) {
// insert the range tombstone end that is closer to and >=
// current_search_key. Strictly speaking, since the Seek() call above
// is on user key, it is possible that range_tombstone_iter->end_key()
// < current_search_key. This can happen when range_tombstone_iter is
// truncated and range_tombstone_iter.largest_ has the same user key
// as current_search_key.GetUserKey() but with a larger sequence
// number than current_search_key. Correctness is not affected as this
// tombstone end key will be popped during FindNextVisibleKey().
InsertRangeTombstoneToMinHeap(
level, comparator_->Compare(range_tombstone_iter->start_key(),
pik) > 0 /* start_key */);
// current_search_key < end_key guaranteed by the Seek() and Valid()
// calls above. Only interested in user key coverage since older
// sorted runs must have smaller sequence numbers than this range
// tombstone.
//
// TODO: range_tombstone_iter->Seek() finds the max covering
// sequence number, can make it cheaper by not looking for max.
if (comparator_->user_comparator()->Compare(
range_tombstone_iter->start_key().user_key,
current_search_key.GetUserKey()) <= 0) {
// Since range_tombstone_iter->Valid(), seqno should be valid, so
// there is no need to check it.
range_tombstone_reseek = true;
// Current target user key is covered by this range tombstone.
// All older sorted runs will seek to range tombstone end key.
// Note that for prefix seek case, it is possible that the prefix
// is not the same as the original target, it should not affect
// correctness. Besides, in most cases, range tombstone start and
// end key should have the same prefix?
// If range_tombstone_iter->end_key() is truncated to its largest_
// boundary, the timestamp in user_key will not be max timestamp,
// but the timestamp of `range_tombstone_iter.largest_`. This should
// be fine here as current_search_key is used to Seek into lower
// levels.
current_search_key.SetInternalKey(
range_tombstone_iter->end_key().user_key, kMaxSequenceNumber);
}
}
}
}
// child.iter.status() is set to Status::TryAgain indicating asynchronous
// request for retrieval of data blocks has been submitted. So it should
// return at this point and Seek should be called again to retrieve the
// requested block and add the child to min heap.
if (children_[level].iter.status().IsTryAgain()) {
continue;
}
{
// Strictly, we timed slightly more than min heap operation,
// but these operations are very cheap.
PERF_TIMER_GUARD(seek_min_heap_time);
AddToMinHeapOrCheckStatus(&children_[level]);
}
}
if (range_tombstone_iters_.empty()) {
for (auto& child : children_) {
if (child.iter.status().IsTryAgain()) {
child.iter.Seek(target);
{
PERF_TIMER_GUARD(seek_min_heap_time);
AddToMinHeapOrCheckStatus(&child);
}
PERF_COUNTER_ADD(number_async_seek, 1);
}
}
} else {
for (auto& prefetch : prefetched_target) {
// (level, target) pairs
children_[prefetch.first].iter.Seek(prefetch.second);
{
PERF_TIMER_GUARD(seek_min_heap_time);
AddToMinHeapOrCheckStatus(&children_[prefetch.first]);
}
PERF_COUNTER_ADD(number_async_seek, 1);
}
}
}
// Returns true iff the current key (min heap top) should not be returned
// to user (of the merging iterator). This can be because the current key
// is deleted by some range tombstone, the current key is some fake file
// boundary sentinel key, or the current key is an end point of a range
// tombstone. Advance the iterator at heap top if needed. Heap order is restored
// and `active_` is updated accordingly.
// See FindNextVisibleKey() for more detail on internal implementation
// of advancing child iters.
//
// REQUIRES:
// - min heap is currently not empty, and iter is in kForward direction.
// - minHeap_ top is not DELETE_RANGE_START (so that `active_` is current).
bool MergingIterator::SkipNextDeleted() {
// 3 types of keys:
// - point key
// - file boundary sentinel keys
// - range deletion end key
auto current = minHeap_.top();
if (current->type == HeapItem::DELETE_RANGE_END) {
active_.erase(current->level);
assert(range_tombstone_iters_[current->level] &&
range_tombstone_iters_[current->level]->Valid());
range_tombstone_iters_[current->level]->Next();
if (range_tombstone_iters_[current->level]->Valid()) {
InsertRangeTombstoneToMinHeap(current->level, true /* start_key */,
true /* replace_top */);
} else {
minHeap_.pop();
}
return true /* current key deleted */;
}
if (current->iter.IsDeleteRangeSentinelKey()) {
// If the file boundary is defined by a range deletion, the range
// tombstone's end key must come before this sentinel key (see op_type in
// SetTombstoneKey()).
assert(ExtractValueType(current->iter.key()) != kTypeRangeDeletion ||
active_.count(current->level) == 0);
// LevelIterator enters a new SST file
current->iter.Next();
if (current->iter.Valid()) {
assert(current->iter.status().ok());
minHeap_.replace_top(current);
} else {
minHeap_.pop();
}
// Remove last SST file's range tombstone end key if there is one.
// This means file boundary is before range tombstone end key,
// which could happen when a range tombstone and a user key
// straddle two SST files. Note that in TruncatedRangeDelIterator
// constructor, parsed_largest.sequence is decremented 1 in this case.
if (!minHeap_.empty() && minHeap_.top()->level == current->level &&
minHeap_.top()->type == HeapItem::DELETE_RANGE_END) {
minHeap_.pop();
active_.erase(current->level);
}
if (range_tombstone_iters_[current->level] &&
range_tombstone_iters_[current->level]->Valid()) {
InsertRangeTombstoneToMinHeap(current->level);
}
return true /* current key deleted */;
}
assert(current->type == HeapItem::ITERATOR);
// Point key case: check active_ for range tombstone coverage.
ParsedInternalKey pik;
ParseInternalKey(current->iter.key(), &pik, false).PermitUncheckedError();
if (!active_.empty()) {
auto i = *active_.begin();
if (i < current->level) {
// range tombstone is from a newer level, definitely covers
assert(comparator_->Compare(range_tombstone_iters_[i]->start_key(),
pik) <= 0);
assert(comparator_->Compare(pik, range_tombstone_iters_[i]->end_key()) <
0);
std::string target;
AppendInternalKey(&target, range_tombstone_iters_[i]->end_key());
SeekImpl(target, current->level, true);
return true /* current key deleted */;
} else if (i == current->level) {
// range tombstone is from the same level as current, check sequence
// number. By `active_` we know current key is between start key and end
// key.
assert(comparator_->Compare(range_tombstone_iters_[i]->start_key(),
pik) <= 0);
assert(comparator_->Compare(pik, range_tombstone_iters_[i]->end_key()) <
0);
if (pik.sequence < range_tombstone_iters_[current->level]->seq()) {
// covered by range tombstone
current->iter.Next();
if (current->iter.Valid()) {
minHeap_.replace_top(current);
} else {
minHeap_.pop();
}
return true /* current key deleted */;
} else {
return false /* current key not deleted */;
}
} else {
return false /* current key not deleted */;
// range tombstone from an older sorted run with current key < end key.
// current key is not deleted and the older sorted run will have its range
// tombstone updated when the range tombstone's end key are popped from
// minHeap_.
}
}
// we can reach here only if active_ is empty
assert(active_.empty());
assert(minHeap_.top()->type == HeapItem::ITERATOR);
return false /* current key not deleted */;
}
void MergingIterator::SeekForPrevImpl(const Slice& target,
size_t starting_level,
bool range_tombstone_reseek) {
// active range tombstones before `starting_level` remain active
ClearHeaps(false /* clear_active */);
InitMaxHeap();
ParsedInternalKey pik;
if (!range_tombstone_iters_.empty()) {
ParseInternalKey(target, &pik, false).PermitUncheckedError();
}
for (size_t level = 0; level < starting_level; ++level) {
PERF_TIMER_GUARD(seek_max_heap_time);
AddToMaxHeapOrCheckStatus(&children_[level]);
}
if (!range_tombstone_iters_.empty()) {
// Add range tombstones before starting_level.
for (size_t level = 0; level < starting_level; ++level) {
if (range_tombstone_iters_[level] &&
range_tombstone_iters_[level]->Valid()) {
assert(static_cast<bool>(active_.count(level)) ==
(pinned_heap_item_[level].type == HeapItem::DELETE_RANGE_START));
maxHeap_->push(&pinned_heap_item_[level]);
} else {
assert(!active_.count(level));
}
}
// levels >= starting_level will be reseeked below,
active_.erase(active_.lower_bound(starting_level), active_.end());
}
status_ = Status::OK();
IterKey current_search_key;
current_search_key.SetInternalKey(target, false /* copy */);
// Seek target might change to some range tombstone end key, so
// we need to remember them for async requests.
// (level, target) pairs
autovector<std::pair<size_t, std::string>> prefetched_target;
for (auto level = starting_level; level < children_.size(); ++level) {
{
PERF_TIMER_GUARD(seek_child_seek_time);
children_[level].iter.SeekForPrev(current_search_key.GetInternalKey());
}
PERF_COUNTER_ADD(seek_child_seek_count, 1);
if (!range_tombstone_iters_.empty()) {
if (range_tombstone_reseek) {
// This seek is to some range tombstone end key.
// Should only happen when there are range tombstones.
PERF_COUNTER_ADD(internal_range_del_reseek_count, 1);
}
if (children_[level].iter.status().IsTryAgain()) {
prefetched_target.emplace_back(
level, current_search_key.GetInternalKey().ToString());
}
auto range_tombstone_iter = range_tombstone_iters_[level];
if (range_tombstone_iter) {
range_tombstone_iter->SeekForPrev(current_search_key.GetUserKey());
if (range_tombstone_iter->Valid()) {
InsertRangeTombstoneToMaxHeap(
level, comparator_->Compare(range_tombstone_iter->end_key(),
pik) <= 0 /* end_key */);
// start key <= current_search_key guaranteed by the Seek() call above
// Only interested in user key coverage since older sorted runs must
// have smaller sequence numbers than this tombstone.
if (comparator_->user_comparator()->Compare(
current_search_key.GetUserKey(),
range_tombstone_iter->end_key().user_key) < 0) {
range_tombstone_reseek = true;
current_search_key.SetInternalKey(
range_tombstone_iter->start_key().user_key, kMaxSequenceNumber,
kValueTypeForSeekForPrev);
}
}
}
}
// child.iter.status() is set to Status::TryAgain indicating asynchronous
// request for retrieval of data blocks has been submitted. So it should
// return at this point and Seek should be called again to retrieve the
// requested block and add the child to min heap.
if (children_[level].iter.status().IsTryAgain()) {
continue;
}
{
// Strictly, we timed slightly more than min heap operation,
// but these operations are very cheap.
PERF_TIMER_GUARD(seek_max_heap_time);
AddToMaxHeapOrCheckStatus(&children_[level]);
}
}
if (range_tombstone_iters_.empty()) {
for (auto& child : children_) {
if (child.iter.status().IsTryAgain()) {
child.iter.SeekForPrev(target);
{
PERF_TIMER_GUARD(seek_min_heap_time);
AddToMaxHeapOrCheckStatus(&child);
}
PERF_COUNTER_ADD(number_async_seek, 1);
}
}
} else {
for (auto& prefetch : prefetched_target) {
// (level, target) pairs
children_[prefetch.first].iter.SeekForPrev(prefetch.second);
{
PERF_TIMER_GUARD(seek_max_heap_time);
AddToMaxHeapOrCheckStatus(&children_[prefetch.first]);
}
PERF_COUNTER_ADD(number_async_seek, 1);
}
}
}
// See more in comments above SkipNextDeleted().
// REQUIRES:
// - max heap is currently not empty, and iter is in kReverse direction.
// - maxHeap_ top is not DELETE_RANGE_END (so that `active_` is current).
bool MergingIterator::SkipPrevDeleted() {
// 3 types of keys:
// - point key
// - file boundary sentinel keys
// - range deletion start key
auto current = maxHeap_->top();
if (current->type == HeapItem::DELETE_RANGE_START) {
active_.erase(current->level);
assert(range_tombstone_iters_[current->level] &&
range_tombstone_iters_[current->level]->Valid());
range_tombstone_iters_[current->level]->Prev();
if (range_tombstone_iters_[current->level]->Valid()) {
InsertRangeTombstoneToMaxHeap(current->level, true /* end_key */,
true /* replace_top */);
} else {
maxHeap_->pop();
}
return true /* current key deleted */;
}
if (current->iter.IsDeleteRangeSentinelKey()) {
// LevelIterator enters a new SST file
current->iter.Prev();
if (current->iter.Valid()) {
assert(current->iter.status().ok());
maxHeap_->replace_top(current);
} else {
maxHeap_->pop();
}
if (!maxHeap_->empty() && maxHeap_->top()->level == current->level &&
maxHeap_->top()->type == HeapItem::DELETE_RANGE_START) {
maxHeap_->pop();
active_.erase(current->level);
}
if (range_tombstone_iters_[current->level] &&
range_tombstone_iters_[current->level]->Valid()) {
InsertRangeTombstoneToMaxHeap(current->level);
}
return true /* current key deleted */;
}
assert(current->type == HeapItem::ITERATOR);
// Point key case: check active_ for range tombstone coverage.
ParsedInternalKey pik;
ParseInternalKey(current->iter.key(), &pik, false).PermitUncheckedError();
if (!active_.empty()) {
auto i = *active_.begin();
if (i < current->level) {
// range tombstone is from a newer level, definitely covers
assert(comparator_->Compare(range_tombstone_iters_[i]->start_key(),
pik) <= 0);
assert(comparator_->Compare(pik, range_tombstone_iters_[i]->end_key()) <
0);
std::string target;
AppendInternalKey(&target, range_tombstone_iters_[i]->start_key());
// This is different from SkipNextDeleted() which does reseek at sorted
// runs >= level (instead of i+1 here). With min heap, if level L is at
// top of the heap, then levels <L all have internal keys > level L's
// current internal key, which means levels <L are already at a different
// user key. With max heap, if level L is at top of the heap, then levels
// <L all have internal keys smaller than level L's current internal key,
// which might still be the same user key.
SeekForPrevImpl(target, i + 1, true);
return true /* current key deleted */;
} else if (i == current->level) {
// By `active_` we know current key is between start key and end key.
assert(comparator_->Compare(range_tombstone_iters_[i]->start_key(),
pik) <= 0);
assert(comparator_->Compare(pik, range_tombstone_iters_[i]->end_key()) <
0);
if (pik.sequence < range_tombstone_iters_[current->level]->seq()) {
current->iter.Prev();
if (current->iter.Valid()) {
maxHeap_->replace_top(current);
} else {
maxHeap_->pop();
}
return true /* current key deleted */;
} else {
return false /* current key not deleted */;
}
} else {
return false /* current key not deleted */;
}
}
assert(active_.empty());
assert(maxHeap_->top()->type == HeapItem::ITERATOR);
return false /* current key not deleted */;
}
void MergingIterator::AddToMinHeapOrCheckStatus(HeapItem* child) {
if (child->iter.Valid()) {
assert(child->iter.status().ok());
minHeap_.push(child);
} else {
considerStatus(child->iter.status());
}
}
void MergingIterator::AddToMaxHeapOrCheckStatus(HeapItem* child) {
if (child->iter.Valid()) {
assert(child->iter.status().ok());
maxHeap_->push(child);
} else {
considerStatus(child->iter.status());
}
}
// Advance all non current_ child to > current_.key().
// We advance current_ after the this function call as it does not require
// Seek().
// Advance all range tombstones iters, including the one corresponding to
// current_, to the first tombstone with end_key > current_.key().
// TODO: potentially do cascading seek here too
void MergingIterator::SwitchToForward() {
ClearHeaps();
Slice target = key();
for (auto& child : children_) {
if (&child.iter != current_) {
child.iter.Seek(target);
// child.iter.status() is set to Status::TryAgain indicating asynchronous
// request for retrieval of data blocks has been submitted. So it should
// return at this point and Seek should be called again to retrieve the
// requested block and add the child to min heap.
if (child.iter.status() == Status::TryAgain()) {
continue;
}
if (child.iter.Valid() && comparator_->Equal(target, child.key())) {
assert(child.iter.status().ok());
child.iter.Next();
}
}
AddToMinHeapOrCheckStatus(&child);
}
for (auto& child : children_) {
if (child.iter.status() == Status::TryAgain()) {
child.iter.Seek(target);
if (child.iter.Valid() && comparator_->Equal(target, child.key())) {
assert(child.iter.status().ok());
child.iter.Next();
}
AddToMinHeapOrCheckStatus(&child);
}
}
// Current range tombstone iter also needs to seek for the following case:
// Previous direction is backward, so range tombstone iter may point to a
// tombstone before current_. If there is no such tombstone, then the range
// tombstone iter is !Valid(). Need to reseek here to make it valid again.
if (!range_tombstone_iters_.empty()) {
ParsedInternalKey pik;
ParseInternalKey(target, &pik, false /* log_err_key */)
.PermitUncheckedError();
for (size_t i = 0; i < range_tombstone_iters_.size(); ++i) {
auto iter = range_tombstone_iters_[i];
if (iter) {
iter->Seek(pik.user_key);
// The while loop is needed as the Seek() call above is only for user
// key. We could have a range tombstone with end_key covering user_key,
// but still is smaller than target. This happens when the range
// tombstone is truncated at iter.largest_.
while (iter->Valid() &&
comparator_->Compare(iter->end_key(), pik) <= 0) {
iter->Next();
}
if (range_tombstone_iters_[i]->Valid()) {
InsertRangeTombstoneToMinHeap(
i, comparator_->Compare(range_tombstone_iters_[i]->start_key(),
pik) > 0 /* start_key */);
}
}
}
}
direction_ = kForward;
assert(current_ == CurrentForward());
}
// Advance all range tombstones iters, including the one corresponding to
// current_, to the first tombstone with start_key <= current_.key().
void MergingIterator::SwitchToBackward() {
ClearHeaps();
InitMaxHeap();
Slice target = key();
for (auto& child : children_) {
if (&child.iter != current_) {
child.iter.SeekForPrev(target);
TEST_SYNC_POINT_CALLBACK("MergeIterator::Prev:BeforePrev", &child);
if (child.iter.Valid() && comparator_->Equal(target, child.key())) {
assert(child.iter.status().ok());
child.iter.Prev();
}
}
AddToMaxHeapOrCheckStatus(&child);
}
ParsedInternalKey pik;
ParseInternalKey(target, &pik, false /* log_err_key */)
.PermitUncheckedError();
for (size_t i = 0; i < range_tombstone_iters_.size(); ++i) {
auto iter = range_tombstone_iters_[i];
if (iter) {
iter->SeekForPrev(pik.user_key);
// Since the SeekForPrev() call above is only for user key,
// we may end up with some range tombstone with start key having the
// same user key at current_, but with a smaller sequence number. This
// makes current_ not at maxHeap_ top for the CurrentReverse() call
// below. If there is a range tombstone start key with the same user
// key and the same sequence number as current_.key(), it will be fine as
// in InsertRangeTombstoneToMaxHeap() we change op_type to be the smallest
// op_type.
while (iter->Valid() &&
comparator_->Compare(iter->start_key(), pik) > 0) {
iter->Prev();
}
if (iter->Valid()) {
InsertRangeTombstoneToMaxHeap(
i, comparator_->Compare(range_tombstone_iters_[i]->end_key(),
pik) <= 0 /* end_key */);
}
}
}
direction_ = kReverse;
if (!prefix_seek_mode_) {
// Note that we don't do assert(current_ == CurrentReverse()) here
// because it is possible to have some keys larger than the seek-key
// inserted between Seek() and SeekToLast(), which makes current_ not
// equal to CurrentReverse().
current_ = CurrentReverse();
}
assert(current_ == CurrentReverse());
}
void MergingIterator::ClearHeaps(bool clear_active) {
minHeap_.clear();
if (maxHeap_) {
maxHeap_->clear();
}
if (clear_active) {
active_.clear();
}
}
void MergingIterator::InitMaxHeap() {
if (!maxHeap_) {
maxHeap_ = std::make_unique<MergerMaxIterHeap>(comparator_);
}
}
// Repeatedly check and remove heap top key if it is not a point key
// that is not covered by range tombstones. SeekImpl() is called to seek to end
// of a range tombstone if the heap top is a point key covered by some range
// tombstone from a newer sorted run. If the covering tombstone is from current
// key's level, then the current child iterator is simply advanced to its next
// key without reseeking.
inline void MergingIterator::FindNextVisibleKey() {
// When active_ is empty, we know heap top cannot be a range tombstone end
// key. It cannot be a range tombstone start key per PopDeleteRangeStart().
PopDeleteRangeStart();
while (!minHeap_.empty() &&
(!active_.empty() || minHeap_.top()->IsDeleteRangeSentinelKey()) &&
SkipNextDeleted()) {
PopDeleteRangeStart();
}
}
inline void MergingIterator::FindPrevVisibleKey() {
PopDeleteRangeEnd();
while (!maxHeap_->empty() &&
(!active_.empty() || maxHeap_->top()->IsDeleteRangeSentinelKey()) &&
SkipPrevDeleted()) {
PopDeleteRangeEnd();
}
}
InternalIterator* NewMergingIterator(const InternalKeyComparator* cmp,
InternalIterator** list, int n,
Arena* arena, bool prefix_seek_mode) {
assert(n >= 0);
if (n == 0) {
return NewEmptyInternalIterator<Slice>(arena);
} else if (n == 1) {
return list[0];
} else {
if (arena == nullptr) {
return new MergingIterator(cmp, list, n, false, prefix_seek_mode);
} else {
auto mem = arena->AllocateAligned(sizeof(MergingIterator));
return new (mem) MergingIterator(cmp, list, n, true, prefix_seek_mode);
}
}
}
MergeIteratorBuilder::MergeIteratorBuilder(
const InternalKeyComparator* comparator, Arena* a, bool prefix_seek_mode)
: first_iter(nullptr), use_merging_iter(false), arena(a) {
auto mem = arena->AllocateAligned(sizeof(MergingIterator));
merge_iter =
new (mem) MergingIterator(comparator, nullptr, 0, true, prefix_seek_mode);
}
MergeIteratorBuilder::~MergeIteratorBuilder() {
if (first_iter != nullptr) {
first_iter->~InternalIterator();
}
if (merge_iter != nullptr) {
merge_iter->~MergingIterator();
}
}
void MergeIteratorBuilder::AddIterator(InternalIterator* iter) {
if (!use_merging_iter && first_iter != nullptr) {
merge_iter->AddIterator(first_iter);
use_merging_iter = true;
first_iter = nullptr;
}
if (use_merging_iter) {
merge_iter->AddIterator(iter);
} else {
first_iter = iter;
}
}
void MergeIteratorBuilder::AddPointAndTombstoneIterator(
InternalIterator* point_iter, TruncatedRangeDelIterator* tombstone_iter,
TruncatedRangeDelIterator*** tombstone_iter_ptr) {
// tombstone_iter_ptr != nullptr means point_iter is a LevelIterator.
bool add_range_tombstone = tombstone_iter ||
!merge_iter->range_tombstone_iters_.empty() ||
tombstone_iter_ptr;
if (!use_merging_iter && (add_range_tombstone || first_iter)) {
use_merging_iter = true;
if (first_iter) {
merge_iter->AddIterator(first_iter);
first_iter = nullptr;
}
}
if (use_merging_iter) {
merge_iter->AddIterator(point_iter);
if (add_range_tombstone) {
// If there was a gap, fill in nullptr as empty range tombstone iterators.
while (merge_iter->range_tombstone_iters_.size() <
merge_iter->children_.size() - 1) {
merge_iter->AddRangeTombstoneIterator(nullptr);
}
merge_iter->AddRangeTombstoneIterator(tombstone_iter);
}
if (tombstone_iter_ptr) {
// This is needed instead of setting to &range_tombstone_iters_[i]
// directly here since the memory address of range_tombstone_iters_[i]
// might change during vector resizing.
range_del_iter_ptrs_.emplace_back(
merge_iter->range_tombstone_iters_.size() - 1, tombstone_iter_ptr);
}
} else {
first_iter = point_iter;
}
}
InternalIterator* MergeIteratorBuilder::Finish(ArenaWrappedDBIter* db_iter) {
InternalIterator* ret = nullptr;
if (!use_merging_iter) {
ret = first_iter;
first_iter = nullptr;
} else {
for (auto& p : range_del_iter_ptrs_) {
*(p.second) = &(merge_iter->range_tombstone_iters_[p.first]);
}
if (db_iter && !merge_iter->range_tombstone_iters_.empty()) {
// memtable is always the first level
db_iter->SetMemtableRangetombstoneIter(
&merge_iter->range_tombstone_iters_.front());
}
merge_iter->Finish();
ret = merge_iter;
merge_iter = nullptr;
}
return ret;
}
} // namespace ROCKSDB_NAMESPACE
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