NextGraph
/
rocksdb
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Improve documentation for MergingIterator (#11161)

Browse Source 
Summary:
Add some comments to try to explain how/why MergingIterator works. Made some small refactoring, mostly in MergingIterator::SkipNextDeleted() and MergingIterator::SeekImpl().

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11161

Test Plan:
crash test with small key range:
```
python3 tools/db_crashtest.py blackbox --simple --max_key=100 --interval=6000 --write_buffer_size=262144 --target_file_size_base=256 --max_bytes_for_level_base=262144 --block_size=128 --value_size_mult=33 --subcompactions=10 --use_multiget=1 --delpercent=3 --delrangepercent=2 --verify_iterator_with_expected_state_one_in=2 --num_iterations=10
```

Reviewed By: ajkr

Differential Revision: D42860994

Pulled By: cbi42

fbshipit-source-id: 3f0c1c9c6481a7f468bf79d823998907a8116e9e
oxigraph-8.1.1
Changyu Bi 2 years ago committed by Facebook GitHub Bot
parent 95d67f3646
commit d053926fa2
4 changed files with 504 additions and 167 deletions
Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL
Show Stats Download Patch File Download Diff File
  1. 22
      db/range_del_aggregator.cc
  2. 3
      db/range_del_aggregator.h
  3. 626
      table/merging_iterator.cc
  4. 14
      table/merging_iterator.h

22
db/range_del_aggregator.cc
Unescape View File

@ -30,6 +30,8 @@ TruncatedRangeDelIterator::TruncatedRangeDelIterator(
icmp_(icmp),
smallest_ikey_(smallest),
largest_ikey_(largest) {
// Set up bounds such that range tombstones from this iterator are
// truncated to range [smallest_, largest_).
if (smallest != nullptr) {
pinned_bounds_.emplace_back();
auto& parsed_smallest = pinned_bounds_.back();
@ -64,6 +66,8 @@ TruncatedRangeDelIterator::TruncatedRangeDelIterator(
//
// Therefore, we will never truncate a range tombstone at largest, so we
// can leave it unchanged.
// TODO: maybe use kMaxValid here to ensure range tombstone having
// distinct key from point keys.
} else {
// The same user key may straddle two sstable boundaries. To ensure that
// the truncated end key can cover the largest key in this sstable, reduce
@ -102,6 +106,24 @@ void TruncatedRangeDelIterator::Seek(const Slice& target) {
iter_->Seek(target);
}
void TruncatedRangeDelIterator::SeekInternalKey(const Slice& target) {
if (largest_ && icmp_->Compare(*largest_, target) <= 0) {
iter_->Invalidate();
return;
}
if (smallest_ && icmp_->Compare(target, *smallest_) < 0) {
// Since target < smallest, target < largest_.
// This seek must land on a range tombstone where end_key() > target,
// so there is no need to check again.
iter_->Seek(smallest_->user_key);
} else {
iter_->Seek(ExtractUserKey(target));
while (Valid() && icmp_->Compare(end_key(), target) <= 0) {
Next();
}
}
}
// NOTE: target is a user key, with timestamp if enabled.
void TruncatedRangeDelIterator::SeekForPrev(const Slice& target) {
if (smallest_ != nullptr &&

3
db/range_del_aggregator.h
Unescape View File

@ -49,6 +49,9 @@ class TruncatedRangeDelIterator {
// REQUIRES: target is a user key.
void Seek(const Slice& target);
// Seeks to the first range tombstone with end_key() > target.
void SeekInternalKey(const Slice& target);
// Seeks to the tombstone with the highest visible sequence number that covers
// target (a user key). If no such tombstone exists, the position will be at
// the latest tombstone that starts before target.

626
table/merging_iterator.cc
Unescape View File

@ -12,6 +12,43 @@
#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,
@ -49,30 +86,26 @@ class MergingIterator : public InternalIterator {
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_.
// 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 so that no further allocation is needed for HeapItem.
// 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;
@ -108,12 +141,18 @@ class MergingIterator : public InternalIterator {
// 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_ &&
@ -128,15 +167,16 @@ class MergingIterator : public InternalIterator {
return;
}
pinned_heap_item_[level].SetTombstoneKey(std::move(pik));
pinned_heap_item_[level].type = HeapItem::DELETE_RANGE_START;
// 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::DELETE_RANGE_END;
pinned_heap_item_[level].type = HeapItem::Type::DELETE_RANGE_END;
active_.insert(level);
}
if (replace_top) {
@ -156,12 +196,12 @@ class MergingIterator : public InternalIterator {
if (end_key) {
pinned_heap_item_[level].SetTombstoneKey(
range_tombstone_iters_[level]->end_key());
pinned_heap_item_[level].type = HeapItem::DELETE_RANGE_END;
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::DELETE_RANGE_START;
pinned_heap_item_[level].type = HeapItem::Type::DELETE_RANGE_START;
active_.insert(level);
}
if (replace_top) {
@ -177,9 +217,12 @@ class MergingIterator : public InternalIterator {
// so `active_` is updated accordingly.
void PopDeleteRangeStart() {
while (!minHeap_.empty() &&
minHeap_.top()->type == HeapItem::DELETE_RANGE_START) {
minHeap_.top()->type == HeapItem::Type::DELETE_RANGE_START) {
TEST_SYNC_POINT_CALLBACK("MergeIterator::PopDeleteRangeStart", nullptr);
// insert end key of this range tombstone and updates active_
// 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 */);
}
@ -191,7 +234,7 @@ class MergingIterator : public InternalIterator {
// so `active_` is updated accordingly.
void PopDeleteRangeEnd() {
while (!maxHeap_->empty() &&
maxHeap_->top()->type == HeapItem::DELETE_RANGE_END) {
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 */);
@ -246,44 +289,25 @@ class MergingIterator : public InternalIterator {
// 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.
// target after Seek(). If !Valid() and status().ok() then this iterator
// reaches the end.
//
// 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`.
// 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 {
assert(range_tombstone_iters_.empty() ||
range_tombstone_iters_.size() == children_.size());
// 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();
@ -311,7 +335,7 @@ class MergingIterator : public InternalIterator {
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
// 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_
@ -335,6 +359,12 @@ class MergingIterator : public InternalIterator {
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();
}
@ -354,7 +384,7 @@ class MergingIterator : public InternalIterator {
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
// 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_
@ -405,7 +435,6 @@ class MergingIterator : public InternalIterator {
// 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();
@ -436,29 +465,22 @@ class MergingIterator : public InternalIterator {
}
private:
// 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.
// 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 Type { ITERATOR, DELETE_RANGE_START, DELETE_RANGE_END };
Type type = ITERATOR;
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) {
@ -476,15 +498,16 @@ class MergingIterator : public InternalIterator {
public:
explicit MinHeapItemComparator(const InternalKeyComparator* comparator)
: comparator_(comparator) {}
bool operator()(HeapItem* a, HeapItem* b) const {
if (LIKELY(a->type == HeapItem::ITERATOR)) {
if (LIKELY(b->type == HeapItem::ITERATOR)) {
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::ITERATOR)) {
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;
@ -500,15 +523,16 @@ class MergingIterator : public InternalIterator {
public:
explicit MaxHeapItemComparator(const InternalKeyComparator* comparator)
: comparator_(comparator) {}
bool operator()(HeapItem* a, HeapItem* b) const {
if (LIKELY(a->type == HeapItem::ITERATOR)) {
if (LIKELY(b->type == HeapItem::ITERATOR)) {
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::ITERATOR)) {
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;
@ -522,20 +546,27 @@ class MergingIterator : public InternalIterator {
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 (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.
// 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);
@ -550,36 +581,55 @@ class MergingIterator : public InternalIterator {
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.
// 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. 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].
// 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 Seek() for meaning of
// "active".
// "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();
// 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.
// 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
@ -607,25 +657,93 @@ class MergingIterator : public InternalIterator {
IteratorWrapper* CurrentForward() const {
assert(direction_ == kForward);
assert(minHeap_.empty() || minHeap_.top()->type == HeapItem::ITERATOR);
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::ITERATOR);
assert(maxHeap_->empty() ||
maxHeap_->top()->type == HeapItem::Type::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).
// 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`.
//
// @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.
// 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
@ -638,6 +756,7 @@ void MergingIterator::SeekImpl(const Slice& target, size_t starting_level,
// 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]);
@ -650,15 +769,20 @@ void MergingIterator::SeekImpl(const Slice& target, size_t starting_level,
// - 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::DELETE_RANGE_END);
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
@ -701,45 +825,37 @@ void MergingIterator::SeekImpl(const Slice& target, size_t starting_level,
}
auto range_tombstone_iter = range_tombstone_iters_[level];
if (range_tombstone_iter) {
range_tombstone_iter->Seek(current_search_key.GetUserKey());
range_tombstone_iter->SeekInternalKey(
current_search_key.GetInternalKey());
// Invariants (rti) and (phi)
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().
// 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 Seek() and Valid()
// calls above. Only interested in user key coverage since older
// sorted runs must have smaller sequence numbers than this range
// tombstone.
// 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) {
// 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);
current_search_key.SetInternalKey(range_tombstone_iter->end_key());
}
}
}
@ -791,6 +907,8 @@ void MergingIterator::SeekImpl(const Slice& target, size_t starting_level,
// 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.
@ -801,11 +919,14 @@ bool MergingIterator::SkipNextDeleted() {
// - file boundary sentinel keys
// - range deletion end key
auto current = minHeap_.top();
if (current->type == HeapItem::DELETE_RANGE_END) {
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 */);
@ -820,41 +941,62 @@ bool MergingIterator::SkipNextDeleted() {
// SetTombstoneKey()).
assert(ExtractValueType(current->iter.key()) != kTypeRangeDeletion ||
active_.count(current->level) == 0);
// When entering a new file, old range tombstone iter 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.
// 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.
if (!minHeap_.empty() && minHeap_.top()->level == current->level &&
minHeap_.top()->type == HeapItem::DELETE_RANGE_END) {
// 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::ITERATOR);
assert(current->type == HeapItem::Type::ITERATOR);
// Point key case: check active_ for range tombstone coverage.
ParsedInternalKey pik;
ParseInternalKey(current->iter.key(), &pik, false).PermitUncheckedError();
@ -881,6 +1023,7 @@ bool MergingIterator::SkipNextDeleted() {
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 {
@ -900,7 +1043,7 @@ bool MergingIterator::SkipNextDeleted() {
}
// we can reach here only if active_ is empty
assert(active_.empty());
assert(minHeap_.top()->type == HeapItem::ITERATOR);
assert(minHeap_.top()->type == HeapItem::Type::ITERATOR);
return false /* current key not deleted */;
}
@ -924,7 +1067,8 @@ void MergingIterator::SeekForPrevImpl(const Slice& target,
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));
(pinned_heap_item_[level].type ==
HeapItem::Type::DELETE_RANGE_START));
maxHeap_->push(&pinned_heap_item_[level]);
} else {
assert(!active_.count(level));
@ -1029,7 +1173,7 @@ bool MergingIterator::SkipPrevDeleted() {
// - file boundary sentinel keys
// - range deletion start key
auto current = maxHeap_->top();
if (current->type == HeapItem::DELETE_RANGE_START) {
if (current->type == HeapItem::Type::DELETE_RANGE_START) {
active_.erase(current->level);
assert(range_tombstone_iters_[current->level] &&
range_tombstone_iters_[current->level]->Valid());
@ -1047,7 +1191,7 @@ bool MergingIterator::SkipPrevDeleted() {
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::DELETE_RANGE_START) {
maxHeap_->top()->type == HeapItem::Type::DELETE_RANGE_START) {
maxHeap_->pop();
active_.erase(current->level);
}
@ -1063,7 +1207,7 @@ bool MergingIterator::SkipPrevDeleted() {
}
return true /* current key deleted */;
}
assert(current->type == HeapItem::ITERATOR);
assert(current->type == HeapItem::Type::ITERATOR);
// Point key case: check active_ for range tombstone coverage.
ParsedInternalKey pik;
ParseInternalKey(current->iter.key(), &pik, false).PermitUncheckedError();
@ -1109,11 +1253,12 @@ bool MergingIterator::SkipPrevDeleted() {
}
assert(active_.empty());
assert(maxHeap_->top()->type == HeapItem::ITERATOR);
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);
@ -1137,6 +1282,7 @@ void MergingIterator::AddToMaxHeapOrCheckStatus(HeapItem* child) {
// 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();
@ -1275,12 +1421,172 @@ void MergingIterator::InitMaxHeap() {
}
}
// 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.
// 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
@ -1292,6 +1598,8 @@ inline void MergingIterator::FindNextVisibleKey() {
SkipNextDeleted()) {
PopDeleteRangeStart();
}
// Checks Invariant (1)
assert(minHeap_.empty() || minHeap_.top()->type == HeapItem::Type::ITERATOR);
}
inline void MergingIterator::FindPrevVisibleKey() {

14
table/merging_iterator.h
Unescape View File

@ -24,7 +24,7 @@ template <class TValue>
class InternalIteratorBase;
using InternalIterator = InternalIteratorBase<Slice>;
// Return an iterator that provided the union of the data in
// Return an iterator that provides the union of the data in
// children[0,n-1]. Takes ownership of the child iterators and
// will delete them when the result iterator is deleted.
//
@ -36,11 +36,15 @@ extern InternalIterator* NewMergingIterator(
const InternalKeyComparator* comparator, InternalIterator** children, int n,
Arena* arena = nullptr, bool prefix_seek_mode = false);
// The iterator returned by NewMergingIterator() and
// MergeIteratorBuilder::Finish(). MergingIterator handles the merging of data
// from different point and/or range tombstone iterators.
class MergingIterator;
// A builder class to build a merging iterator by adding iterators one by one.
// User should call only one of AddIterator() or AddPointAndTombstoneIterator()
// exclusively for the same builder.
// A builder class to for an iterator that provides the union of data
// of input iterators. Two APIs are provided to add input iterators. User should
// only call one of them exclusively depending on if range tombstone should be
// processed.
class MergeIteratorBuilder {
public:
// comparator: the comparator used in merging comparator
@ -50,7 +54,7 @@ class MergeIteratorBuilder {
const Slice* iterate_upper_bound = nullptr);
~MergeIteratorBuilder();
// Add iter to the merging iterator.
// Add point key iterator `iter` to the merging iterator.
void AddIterator(InternalIterator* iter);
// Add a point key iterator and a range tombstone iterator.

Write Preview
Loading…
Cancel
Save