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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"
namespace ROCKSDB_NAMESPACE {
// MergingIterator uses a min/max heap to combine data from point iterators.
// Range tombstones can be added and keys covered by range tombstones will be
// skipped.
//
// The following are implementation details and can be ignored by user.
// For merging iterator to process range tombstones, it treats the start and end
// keys of a range tombstone as two keys and put them into minHeap_ or maxHeap_
// together with regular point keys. Each range tombstone is active only within
// its internal key range [start_key, end_key). An `active_` set is used to
// track levels that have an active range tombstone. Take forward scanning
// for example. Level j is in active_ if its current range tombstone has its
// start_key popped from minHeap_ and its end_key in minHeap_. If the top of
// minHeap_ is a point key from level L, we can determine if the point key is
// covered by any range tombstone by checking if there is an l <= L in active_.
// The case of l == L also involves checking range tombstone's sequence number.
//
// The following (non-exhaustive) list of invariants are maintained by
// MergingIterator during forward scanning. After each InternalIterator API,
// i.e., Seek*() and Next(), and FindNextVisibleKey(), if minHeap_ is not empty:
// (1) minHeap_.top().type == ITERATOR
// (2) minHeap_.top()->key() is not covered by any range tombstone.
//
// After each call to SeekImpl() in addition to the functions mentioned above:
// (3) For all level i and j <= i, range_tombstone_iters_[j].prev.end_key() <
// children_[i].iter.key(). That is, range_tombstone_iters_[j] is at or before
// the first range tombstone from level j with end_key() >
// children_[i].iter.key().
// (4) For all level i and j <= i, if j in active_, then
// range_tombstone_iters_[j]->start_key() < children_[i].iter.key().
// - When range_tombstone_iters_[j] is !Valid(), we consider its `prev` to be
// the last range tombstone from that range tombstone iterator.
// - When referring to range tombstone start/end keys, assume it is the value of
// HeapItem::tombstone_pik. This value has op_type = kMaxValid, which makes
// range tombstone keys have distinct values from point keys.
//
// Applicable class variables have their own (forward scanning) invariants
// listed in the comments above their definition.
class MergingIterator : public InternalIterator {
public:
MergingIterator(const InternalKeyComparator* comparator,
InternalIterator** children, int n, bool is_arena_mode,
bool prefix_seek_mode,
const Slice* iterate_upper_bound = nullptr)
: is_arena_mode_(is_arena_mode),
prefix_seek_mode_(prefix_seek_mode),
direction_(kForward),
comparator_(comparator),
current_(nullptr),
minHeap_(MinHeapItemComparator(comparator_)),
pinned_iters_mgr_(nullptr),
iterate_upper_bound_(iterate_upper_bound) {
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;
}
// There must be either no range tombstone iterator or the same number of
// range tombstone iterators as point iterators after all iters are added.
// The i-th added range tombstone iterator and the i-th point iterator
// must point to the same LSM level.
// Merging iterator takes ownership of `iter` and is responsible for freeing
// it. One exception to this is when a LevelIterator moves to a different SST
// file or when Iterator::Refresh() is called, the range tombstone iterator
// could be updated. In that case, this merging iterator is only responsible
// for 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.
void Finish() {
if (!range_tombstone_iters_.empty()) {
assert(range_tombstone_iters_.size() == children_.size());
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;
// 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()).
pinned_heap_item_[i].tombstone_pik.type = kTypeMaxValid;
}
}
}
~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.
// pinned_heap_items_[level].type is updated based on `start_key`.
//
// If range_tombstone_iters_[level] is after iterate_upper_bound_,
// it is removed from the heap.
// @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());
// Maintains Invariant(phi)
if (start_key) {
pinned_heap_item_[level].type = HeapItem::Type::DELETE_RANGE_START;
ParsedInternalKey pik = range_tombstone_iters_[level]->start_key();
// iterate_upper_bound does not have timestamp
if (iterate_upper_bound_ &&
comparator_->user_comparator()->CompareWithoutTimestamp(
pik.user_key, true /* a_has_ts */, *iterate_upper_bound_,
false /* b_has_ts */) >= 0) {
if (replace_top) {
// replace_top implies this range tombstone iterator is still in
// minHeap_ and at the top.
minHeap_.pop();
}
return;
}
pinned_heap_item_[level].SetTombstoneKey(std::move(pik));
// Checks Invariant(active_)
assert(active_.count(level) == 0);
} else {
// allow end key to go over upper bound (if present) since start key is
// before upper bound and the range tombstone could still cover a
// range before upper bound.
// Maintains Invariant(active_)
pinned_heap_item_[level].SetTombstoneKey(
range_tombstone_iters_[level]->end_key());
pinned_heap_item_[level].type = HeapItem::Type::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::Type::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::Type::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::Type::DELETE_RANGE_START) {
TEST_SYNC_POINT_CALLBACK("MergeIterator::PopDeleteRangeStart", nullptr);
// Invariant(rti) holds since
// range_tombstone_iters_[minHeap_.top()->level] is still valid, and
// parameter `replace_top` is set to true here to ensure only one such
// HeapItem is in minHeap_.
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::Type::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 this iterator
// reaches the end.
//
// If range tombstones are present, cascading seeks may be called (an
// optimization adapted from Pebble https://github.com/cockroachdb/pebble).
// Roughly, 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".
void Seek(const Slice& target) override {
// Define LevelNextVisible(i, k) to be the first key >= k in level i that is
// not covered by any range tombstone.
// After SeekImpl(target, 0), invariants (3) and (4) hold.
// For all level i, target <= children_[i].iter.key() <= LevelNextVisible(i,
// target). By the contract of FindNextVisibleKey(), Invariants (1)-(4)
// holds after this call, and minHeap_.top().iter points to the
// first key >= target among children_ that is not covered by any range
// tombstone.
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 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();
}
// Invariants (3) and (4) hold when after advancing current_.
// Let k be the smallest key among children_[i].iter.key().
// k <= children_[i].iter.key() <= LevelNextVisible(i, k) holds for all
// level i. After FindNextVisible(), Invariants (1)-(4) hold and
// minHeap_.top()->key() is the first key >= k from any children_ that is
// not covered by any range tombstone.
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 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:
// Represents an element in the min/max heap. Each HeapItem corresponds to a
// point iterator or a range tombstone iterator, differentiated by
// HeapItem::type.
struct HeapItem {
HeapItem() = default;
// corresponding point iterator
IteratorWrapper iter;
size_t level = 0;
// corresponding range tombstone iterator's start or end key value
// depending on value of `type`.
ParsedInternalKey tombstone_pik;
// Will be overwritten before use, initialize here so compiler does not
// complain.
enum class Type { ITERATOR, DELETE_RANGE_START, DELETE_RANGE_END };
Type type = Type::ITERATOR;
explicit HeapItem(size_t _level, InternalIteratorBase<Slice>* _iter)
: level(_level), type(Type::ITERATOR) {
iter.Set(_iter);
}
void SetTombstoneKey(ParsedInternalKey&& pik) {
// op_type is already initialized in MergingIterator::Finish().
tombstone_pik.user_key = pik.user_key;
tombstone_pik.sequence = pik.sequence;
}
};
class MinHeapItemComparator {
public:
explicit MinHeapItemComparator(const InternalKeyComparator* comparator)
: comparator_(comparator) {}
bool operator()(HeapItem* a, HeapItem* b) const {
if (LIKELY(a->type == HeapItem::Type::ITERATOR)) {
if (LIKELY(b->type == HeapItem::Type::ITERATOR)) {
return comparator_->Compare(a->iter.key(), b->iter.key()) > 0;
} else {
return comparator_->Compare(a->iter.key(), b->tombstone_pik) > 0;
}
} else {
if (LIKELY(b->type == HeapItem::Type::ITERATOR)) {
return comparator_->Compare(a->tombstone_pik, b->iter.key()) > 0;
} else {
return comparator_->Compare(a->tombstone_pik, b->tombstone_pik) > 0;
}
}
}
private:
const InternalKeyComparator* comparator_;
};
class MaxHeapItemComparator {
public:
explicit MaxHeapItemComparator(const InternalKeyComparator* comparator)
: comparator_(comparator) {}
bool operator()(HeapItem* a, HeapItem* b) const {
if (LIKELY(a->type == HeapItem::Type::ITERATOR)) {
if (LIKELY(b->type == HeapItem::Type::ITERATOR)) {
return comparator_->Compare(a->iter.key(), b->iter.key()) < 0;
} else {
return comparator_->Compare(a->iter.key(), b->tombstone_pik) < 0;
}
} else {
if (LIKELY(b->type == HeapItem::Type::ITERATOR)) {
return comparator_->Compare(a->tombstone_pik, b->iter.key()) < 0;
} else {
return comparator_->Compare(a->tombstone_pik, b->tombstone_pik) < 0;
}
}
}
private:
const InternalKeyComparator* comparator_;
};
using MergerMinIterHeap = BinaryHeap<HeapItem*, MinHeapItemComparator>;
using MergerMaxIterHeap = BinaryHeap<HeapItem*, MaxHeapItemComparator>;
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 (minHeap_.top()) is
// from a point iterator and is not covered by any range tombstone,
// or that there is no more keys (heap is empty). SeekImpl() may be called
// to seek to the end of a range tombstone as an optimization.
void FindNextVisibleKey();
void FindPrevVisibleKey();
// Advance this merging iterators to the first key >= `target` for all
// components from levels >= starting_level. All iterators before
// starting_level are untouched.
//
// @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 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_;
// HeapItem for all child point iterators.
// Invariant(children_): children_[i] is in minHeap_ iff
// children_[i].iter.Valid(), and at most one children_[i] is in minHeap_.
// TODO: We could use an autovector with a larger reserved size.
std::vector<HeapItem> children_;
// HeapItem for range tombstone start and end keys.
// pinned_heap_item_[i] corresponds to range_tombstone_iters_[i].
// Invariant(phi): If range_tombstone_iters_[i]->Valid(),
// pinned_heap_item_[i].tombstone_pik is equal to
// range_tombstone_iters_[i]->start_key() when
// pinned_heap_item_[i].type is DELETE_RANGE_START and
// range_tombstone_iters_[i]->end_key() when
// pinned_heap_item_[i].type is DELETE_RANGE_END (ignoring op_type which is
// kMaxValid for all pinned_heap_item_.tombstone_pik).
// pinned_heap_item_[i].type is either DELETE_RANGE_START or DELETE_RANGE_END.
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.
// Invariant(rti): pinned_heap_item_[i] is in minHeap_ iff
// range_tombstone_iters_[i]->Valid() and at most one pinned_heap_item_[i] is
// in minHeap_.
std::vector<TruncatedRangeDelIterator*> range_tombstone_iters_;
// Levels (indices into range_tombstone_iters_/children_ ) that currently have
// "active" range tombstones. See comments above MergingIterator for meaning
// of "active".
// Invariant(active_): i is in active_ iff range_tombstone_iters_[i]->Valid()
// and pinned_heap_item_[i].type == DELETE_RANGE_END.
std::set<size_t> active_;
bool SkipNextDeleted();
bool SkipPrevDeleted();
// Invariant: at the end of each InternalIterator API,
// current_ points to minHeap_.top().iter (maxHeap_ if backward scanning)
// or nullptr if no child iterator is valid.
// This follows from that current_ = CurrentForward()/CurrentReverse() is
// called at the end of each InternalIterator API.
IteratorWrapper* current_;
// If any of the children have non-ok status, this is one of them.
Status status_;
// Invariant: min heap property is maintained (parent is always <= child).
// This holds by using only BinaryHeap APIs to modify heap. One
// exception is to modify heap top item directly (by caller iter->Next()), and
// it should be followed by a call to replace_top() or pop().
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_;
// Used to bound range tombstones. For point keys, DBIter and SSTable iterator
// take care of boundary checking.
const Slice* iterate_upper_bound_;
// 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::Type::ITERATOR);
return !minHeap_.empty() ? &minHeap_.top()->iter : nullptr;
}
IteratorWrapper* CurrentReverse() const {
assert(direction_ == kReverse);
assert(maxHeap_);
assert(maxHeap_->empty() ||
maxHeap_->top()->type == HeapItem::Type::ITERATOR);
return !maxHeap_->empty() ? &maxHeap_->top()->iter : nullptr;
}
};
// Pre-condition:
// - Invariants (3) and (4) hold for i < starting_level
// - For i < starting_level, range_tombstone_iters_[i].prev.end_key() <
// `target`.
// - For i < starting_level, if i in active_, then
// range_tombstone_iters_[i]->start_key() < `target`.
//
// Post-condition:
// - Invariants (3) and (4) hold for all level i.
// - (*) target <= children_[i].iter.key() <= LevelNextVisible(i, target)
// for i >= starting_level
// - (**) target < pinned_heap_item_[i].tombstone_pik if
// range_tombstone_iters_[i].Valid() for i >= starting_level
//
// Proof sketch:
// Invariant (3) holds for all level i.
// For j <= i < starting_level, it follows from Pre-condition that (3) holds
// and that SeekImpl(-, starting_level) does not update children_[i] or
// range_tombstone_iters_[j].
// For j < starting_level and i >= starting_level, it follows from
// - Pre-condition that range_tombstone_iters_[j].prev.end_key() < `target`
// - range_tombstone_iters_[j] is not updated in SeekImpl(), and
// - children_[i].iter.Seek(current_search_key) is called with
// current_search_key >= target (shown below).
// When current_search_key is updated, it is updated to some
// range_tombstone_iter->end_key() after
// range_tombstone_iter->SeekInternalKey(current_search_key) was called. So
// current_search_key increases if updated and >= target.
// For starting_level <= j <= i:
// children_[i].iter.Seek(k1) and range_tombstone_iters_[j]->SeekInternalKey(k2)
// are called in SeekImpl(). Seek(k1) positions children_[i] at the first key >=
// k1 from level i. SeekInternalKey(k2) positions range_tombstone_iters_[j] at
// the first range tombstone from level j with end_key() > k2. It suffices to
// show that k1 >= k2. Since k1 and k2 are values of current_search_key where
// k1 = k2 or k1 is value of a later current_search_key than k2, so k1 >= k2.
//
// Invariant (4) holds for all level >= 0.
// By Pre-condition Invariant (4) holds for i < starting_level.
// Since children_[i], range_tombstone_iters_[i] and contents of active_ for
// i < starting_level do not change (4) holds for j <= i < starting_level.
// By Pre-condition: for all j < starting_level, if j in active_, then
// range_tombstone_iters_[j]->start_key() < target. For i >= starting_level,
// children_[i].iter.Seek(k) is called for k >= target. So
// children_[i].iter.key() >= target > range_tombstone_iters_[j]->start_key()
// for j < starting_level and i >= starting_level. So invariant (4) holds for
// j < starting_level and i >= starting_level.
// For starting_level <= j <= i, j is added to active_ only if
// - range_tombstone_iters_[j]->SeekInternalKey(k1) was called
// - range_tombstone_iters_[j]->start_key() <= k1
// Since children_[i].iter.Seek(k2) is called for some k2 >= k1 and for all
// starting_level <= j <= i, (4) also holds for all starting_level <= j <= i.
//
// Post-condition (*): target <= children_[i].iter.key() <= LevelNextVisible(i,
// target) for i >= starting_level.
// target <= children_[i].iter.key() follows from that Seek() is called on some
// current_search_key >= target for children_[i].iter. If current_search_key
// is updated from k1 to k2 when level = i, we show that the range [k1, k2) is
// not visible for children_[j] for any j > i. When current_search_key is
// updated from k1 to k2,
// - range_tombstone_iters_[i]->SeekInternalKey(k1) was called
// - range_tombstone_iters_[i]->Valid()
// - range_tombstone_iters_[i]->start_key().user_key <= k1.user_key
// - k2 = range_tombstone_iters_[i]->end_key()
// We assume that range_tombstone_iters_[i]->start_key() has a higher sequence
// number compared to any key from levels > i that has the same user key. So no
// point key from levels > i in range [k1, k2) is visible. So
// children_[i].iter.key() <= LevelNextVisible(i, target).
//
// Post-condition (**) target < pinned_heap_item_[i].tombstone_pik for i >=
// starting_level if range_tombstone_iters_[i].Valid(). This follows from that
// SeekInternalKey() being called for each range_tombstone_iters_ with some key
// >= `target` and that we pick start/end key that is > `target` to insert to
// minHeap_.
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.
// Invariant(children_) for level < starting_level
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) {
// Restores Invariants(rti), (phi) and (active_) for level <
// starting_level
if (range_tombstone_iters_[level] &&
range_tombstone_iters_[level]->Valid()) {
// use an iterator on active_ if performance becomes an issue here
if (active_.count(level) > 0) {
assert(pinned_heap_item_[level].type ==
HeapItem::Type::DELETE_RANGE_END);
// if it was active, then start key must be within upper_bound,
// so we can add to minHeap_ directly.
minHeap_.push(&pinned_heap_item_[level]);
} else {
assert(pinned_heap_item_[level].type ==
HeapItem::Type::DELETE_RANGE_START);
// this takes care of checking iterate_upper_bound, but with an extra
// key comparison if range_tombstone_iters_[level] was already out of
// bound. Consider using a new HeapItem type or some flag to remember
// boundary checking result.
InsertRangeTombstoneToMinHeap(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->SeekInternalKey(
current_search_key.GetInternalKey());
// Invariants (rti) and (phi)
if (range_tombstone_iter->Valid()) {
// If range tombstone starts after `current_search_key`,
// we should insert start key to heap as the range tombstone is not
// active yet.
InsertRangeTombstoneToMinHeap(
level, comparator_->Compare(range_tombstone_iter->start_key(),
pik) > 0 /* start_key */);
// current_search_key < end_key guaranteed by the SeekInternalKey()
// and Valid() calls above. Here we only need to compare user_key
// since if target.user_key ==
// range_tombstone_iter->start_key().user_key and target <
// range_tombstone_iter->start_key(), no older level would have any
// key in range [target, range_tombstone_iter->start_key()], so no
// keys in range [target, range_tombstone_iter->end_key()) from older
// level would be visible. So it is safe to seek to
// range_tombstone_iter->end_key().
//
// 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) {
range_tombstone_reseek = true;
// 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?
current_search_key.SetInternalKey(range_tombstone_iter->end_key());
}
}
}
}
// 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.
// When false is returned, if minHeap is not empty, then minHeap_.top().type
// == ITERATOR
//
// 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::Type::DELETE_RANGE_END) {
// Invariant(active_): range_tombstone_iters_[current->level] is about to
// become !Valid() or that its start key is going to be added to minHeap_.
active_.erase(current->level);
assert(range_tombstone_iters_[current->level] &&
range_tombstone_iters_[current->level]->Valid());
range_tombstone_iters_[current->level]->Next();
// Maintain Invariants (rti) and (phi)
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);
// When entering a new file, range tombstone iter from the old file is
// freed, but the last key from that range tombstone iter may still be in
// the heap. We need to ensure the data underlying its corresponding key
// Slice is still alive. We do so by popping the range tombstone key from
// heap before calling iter->Next(). Technically, this change is not needed:
// if there is a range tombstone end key that is after file boundary
// sentinel key in minHeap_, the range tombstone end key must have been
// truncated at file boundary. The underlying data of the range tombstone
// end key Slice is the SST file's largest internal key stored as file
// metadata in Version. However, since there are too many implicit
// assumptions made, it is safer to just ensure range tombstone iter is
// still alive.
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.
// Maintains Invariant(rti) that at most one
// pinned_heap_item_[current->level] is in minHeap_.
if (range_tombstone_iters_[current->level] &&
range_tombstone_iters_[current->level]->Valid()) {
if (!minHeap_.empty() && minHeap_.top()->level == current->level) {
assert(minHeap_.top()->type == HeapItem::Type::DELETE_RANGE_END);
minHeap_.pop();
// Invariant(active_): we are about to enter a new SST file with new
// range_tombstone_iters[current->level]. Either it is !Valid() or its
// start key is going to be added to minHeap_.
active_.erase(current->level);
} else {
// range tombstone is still valid, but it is not on heap.
// This should only happen if the range tombstone is over iterator
// upper bound.
assert(iterate_upper_bound_ &&
comparator_->user_comparator()->CompareWithoutTimestamp(
range_tombstone_iters_[current->level]->start_key().user_key,
true /* a_has_ts */, *iterate_upper_bound_,
false /* b_has_ts */) >= 0);
}
}
// LevelIterator enters a new SST file
current->iter.Next();
// Invariant(children_): current is popped from heap and added back only if
// it is valid
if (current->iter.Valid()) {
assert(current->iter.status().ok());
minHeap_.push(current);
}
// Invariants (rti) and (phi)
if (range_tombstone_iters_[current->level] &&
range_tombstone_iters_[current->level]->Valid()) {
InsertRangeTombstoneToMinHeap(current->level);
}
return true /* current key deleted */;
}
assert(current->type == HeapItem::Type::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();
// Invariant (children_)
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::Type::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::Type::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::Type::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
maxHeap_->pop();
// Remove last SST file's range tombstone key if there is one.
if (!maxHeap_->empty() && maxHeap_->top()->level == current->level &&
maxHeap_->top()->type == HeapItem::Type::DELETE_RANGE_START) {
maxHeap_->pop();
active_.erase(current->level);
}
current->iter.Prev();
if (current->iter.Valid()) {
assert(current->iter.status().ok());
maxHeap_->push(current);
}
if (range_tombstone_iters_[current->level] &&
range_tombstone_iters_[current->level]->Valid()) {
InsertRangeTombstoneToMaxHeap(current->level);
}
return true /* current key deleted */;
}
assert(current->type == HeapItem::Type::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::Type::ITERATOR);
return false /* current key not deleted */;
}
void MergingIterator::AddToMinHeapOrCheckStatus(HeapItem* child) {
// Invariant(children_)
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
// TODO: show that invariants hold
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.iter.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.iter.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.iter.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>(MaxHeapItemComparator(comparator_));
}
}
// Assume there is a next key that is not covered by range tombstone.
// Pre-condition:
// - Invariants (3) and (4)
// - There is some k where k <= children_[i].iter.key() <= LevelNextVisible(i,
// k) for all levels i (LevelNextVisible() defined in Seek()).
//
// Define NextVisible(k) to be the first key >= k from among children_ that
// is not covered by any range tombstone.
// Post-condition:
// - Invariants (1)-(4) hold
// - (*): minHeap_->top()->key() == NextVisible(k)
//
// Loop invariants:
// - Invariants (3) and (4)
// - (*): k <= children_[i].iter.key() <= LevelNextVisible(i, k)
//
// Progress: minHeap_.top()->key() is non-decreasing and strictly increases in
// a finite number of iterations.
// TODO: it is possible to call SeekImpl(k2) after SeekImpl(k1) with
// k2 < k1 in the same FindNextVisibleKey(). For example, l1 has a range
// tombstone [2,3) and l2 has a range tombstone [1, 4). Point key 1 from l5
// triggers SeekImpl(4 /* target */, 5). Then point key 2 from l3 triggers
// SeekImpl(3 /* target */, 3).
// Ideally we should only move iterators forward in SeekImpl(), and the
// progress condition can be made simpler: iterator only moves forward.
//
// Proof sketch:
// Post-condition:
// Invariant (1) holds when this method returns:
// Ignoring the empty minHeap_ case, there are two cases:
// Case 1: active_ is empty and !minHeap_.top()->iter.IsDeleteRangeSentinelKey()
// By invariants (rti) and (active_), active_ being empty means if a
// pinned_heap_item_[i] is in minHeap_, it has type DELETE_RANGE_START. Note
// that PopDeleteRangeStart() was called right before the while loop condition,
// so minHeap_.top() is not of type DELETE_RANGE_START. So minHeap_.top() must
// be of type ITERATOR.
// Case 2: SkipNextDeleted() returns false. The method returns false only when
// minHeap_.top().type == ITERATOR.
//
// Invariant (2) holds when this method returns:
// From Invariant (1), minHeap_.top().type == ITERATOR. Suppose it is
// children_[i] for some i. Suppose that children_[i].iter.key() is covered by
// some range tombstone. This means there is a j <= i and a range tombstone from
// level j with start_key() < children_[i].iter.key() < end_key().
// - If range_tombstone_iters_[j]->Valid(), by Invariants (rti) and (phi),
// pinned_heap_item_[j] is in minHeap_, and pinned_heap_item_[j].tombstone_pik
// is either start or end key of this range tombstone. If
// pinned_heap_item_[j].tombstone_pik < children_[i].iter.key(), it would be at
// top of minHeap_ which would contradict Invariant (1). So
// pinned_heap_item_[j].tombstone_pik > children_[i].iter.key().
// By Invariant (3), range_tombstone_iters_[j].prev.end_key() <
// children_[i].iter.key(). We assume that in each level, range tombstones
// cover non-overlapping ranges. So range_tombstone_iters_[j] is at
// the range tombstone with start_key() < children_[i].iter.key() < end_key()
// and has its end_key() in minHeap_. By Invariants (phi) and (active_),
// j is in active_. From while loop condition, SkipNextDeleted() must have
// returned false for this method to return.
// - If j < i, then SeekImpl(range_tombstone_iters_[j']->end_key(), i)
// was called for some j' < i and j' in active_. Note that since j' is in
// active_, pinned_heap_item_[j'] is in minHeap_ and has tombstone_pik =
// range_tombstone_iters_[j']->end_key(). So
// range_tombstone_iters_[j']->end_key() must be larger than
// children_[i].iter.key() to not be at top of minHeap_. This means after
// SeekImpl(), children_[i] would be at a key > children_[i].iter.key()
// -- contradiction.
// - If j == i, children_[i]->Next() would have been called and children_[i]
// would be at a key > children_[i].iter.key() -- contradiction.
// - If !range_tombstone_iters_[j]->Valid(). Then range_tombstone_iters_[j]
// points to an SST file with all range tombstones from that file exhausted.
// The file must come before the file containing the first
// range tombstone with start_key() < children_[i].iter.key() < end_key().
// Assume files from same level have non-overlapping ranges, the current file's
// meta.largest is less than children_[i].iter.key(). So the file boundary key,
// which has value meta.largest must have been popped from minHeap_ before
// children_[i].iter.key(). So range_tombstone_iters_[j] would not point to
// this SST file -- contradiction.
// So it is impossible for children_[i].iter.key() to be covered by a range
// tombstone.
//
// Post-condition (*) holds when the function returns:
// From loop invariant (*) that k <= children_[i].iter.key() <=
// LevelNextVisible(i, k) and Invariant (2) above, when the function returns,
// minHeap_.top()->key() is the smallest LevelNextVisible(i, k) among all levels
// i. This is equal to NextVisible(k).
//
// Invariant (3) holds after each iteration:
// PopDeleteRangeStart() does not change range tombstone position.
// In SkipNextDeleted():
// - If DELETE_RANGE_END is popped from minHeap_, it means the range
// tombstone's end key is < all other point keys, so it is safe to advance to
// next range tombstone.
// - If file boundary is popped (current->iter.IsDeleteRangeSentinelKey()),
// we assume that file's last range tombstone's
// end_key <= file boundary key < all other point keys. So it is safe to
// move to the first range tombstone in the next SST file.
// - If children_[i]->Next() is called, then it is fine as it is advancing a
// point iterator.
// - If SeekImpl(target, l) is called, then (3) follows from SeekImpl()'s
// post-condition if its pre-condition holds. First pre-condition follows
// from loop invariant where Invariant (3) holds for all levels i.
// Now we should second pre-condition holds. Since Invariant (3) holds for
// all i, we have for all j <= l, range_tombstone_iters_[j].prev.end_key()
// < children_[l].iter.key(). `target` is the value of
// range_tombstone_iters_[j'].end_key() for some j' < l and j' in active_.
// By Invariant (active_) and (rti), pinned_heap_item_[j'] is in minHeap_ and
// pinned_heap_item_[j'].tombstone_pik = range_tombstone_iters_[j'].end_key().
// This end_key must be larger than children_[l].key() since it was not at top
// of minHeap_. So for all levels j <= l,
// range_tombstone_iters_[j].prev.end_key() < children_[l].iter.key() < target
//
// Invariant (4) holds after each iteration:
// A level i is inserted into active_ during calls to PopDeleteRangeStart().
// In that case, range_tombstone_iters_[i].start_key() < all point keys
// by heap property and the assumption that point keys and range tombstone keys
// are distinct.
// If SeekImpl(target, l) is called, then there is a range_tombstone_iters_[j]
// where target = range_tombstone_iters_[j]->end_key() and children_[l]->key()
// < target. By loop invariants, (3) and (4) holds for levels.
// Since target > children_[l]->key(), it also holds that for j < l,
// range_tombstone_iters_[j].prev.end_key() < target and that if j in active_,
// range_tombstone_iters_[i]->start_key() < target. So all pre-conditions of
// SeekImpl(target, l) holds, and (4) follow from its post-condition.
// All other places either in this function either advance point iterators
// or remove some level from active_, so (4) still holds.
//
// Look Invariant (*): for all level i, k <= children_[i] <= LevelNextVisible(i,
// k).
// k <= children_[i] follows from loop `progress` condition.
// Consider when children_[i] is changed for any i. It is through
// children_[i].iter.Next() or SeekImpl() in SkipNextDeleted().
// If children_[i].iter.Next() is called, there is a range tombstone from level
// i where tombstone seqno > children_[i].iter.key()'s seqno and i in active_.
// By Invariant (4), tombstone's start_key < children_[i].iter.key(). By
// invariants (active_), (phi), and (rti), tombstone's end_key is in minHeap_
// and that children_[i].iter.key() < end_key. So children_[i].iter.key() is
// not visible, and it is safe to call Next().
// If SeekImpl(target, l) is called, by its contract, when SeekImpl() returns,
// target <= children_[i]->key() <= LevelNextVisible(i, target) for i >= l,
// and children_[<l] is not touched. We know `target` is
// range_tombstone_iters_[j]->end_key() for some j < i and j is in active_.
// By Invariant (4), range_tombstone_iters_[j]->start_key() <
// children_[i].iter.key() for all i >= l. So for each level i >= l, the range
// [children_[i].iter.key(), target) is not visible. So after SeekImpl(),
// children_[i].iter.key() <= LevelNextVisible(i, target) <=
// LevelNextVisible(i, k).
//
// `Progress` holds for each iteration:
// Very sloppy intuition:
// - in PopDeleteRangeStart(): the value of a pinned_heap_item_.tombstone_pik_
// is updated from the start key to the end key of the same range tombstone.
// We assume that start key <= end key for the same range tombstone.
// - in SkipNextDeleted()
// - If the top of heap is DELETE_RANGE_END, the range tombstone is advanced
// and the relevant pinned_heap_item_.tombstone_pik is increased or popped
// from minHeap_.
// - If the top of heap is a file boundary key, then both point iter and
// range tombstone iter are advanced to the next file.
// - If the top of heap is ITERATOR and current->iter.Next() is called, it
// moves to a larger point key.
// - If the top of heap is ITERATOR and SeekImpl(k, l) is called, then all
// iterators from levels >= l are advanced to some key >= k by its contract.
// And top of minHeap_ before SeekImpl(k, l) was less than k.
// There are special cases where different heap items have the same key,
// e.g. when two range tombstone end keys share the same value). In
// these cases, iterators are being advanced, so the minimum key should increase
// in a finite number of steps.
inline void MergingIterator::FindNextVisibleKey() {
PopDeleteRangeStart();
// PopDeleteRangeStart() implies heap top is not DELETE_RANGE_START
// active_ being empty implies no DELETE_RANGE_END in heap.
// So minHeap_->top() must be of type ITERATOR.
while (
!minHeap_.empty() &&
(!active_.empty() || minHeap_.top()->iter.IsDeleteRangeSentinelKey()) &&
SkipNextDeleted()) {
PopDeleteRangeStart();
}
// Checks Invariant (1)
assert(minHeap_.empty() || minHeap_.top()->type == HeapItem::Type::ITERATOR);
}
inline void MergingIterator::FindPrevVisibleKey() {
PopDeleteRangeEnd();
// PopDeleteRangeEnd() implies heap top is not DELETE_RANGE_END
// active_ being empty implies no DELETE_RANGE_START in heap.
// So maxHeap_->top() must be of type ITERATOR.
while (
!maxHeap_->empty() &&
(!active_.empty() || maxHeap_->top()->iter.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,
const Slice* iterate_upper_bound)
: 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, iterate_upper_bound);
}
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