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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).
#include "db/merge_helper.h"
#include <stdio.h>
#include <string>
#include "db/dbformat.h"
#include "monitoring/perf_context_imp.h"
#include "monitoring/statistics.h"
#include "rocksdb/comparator.h"
#include "rocksdb/db.h"
#include "rocksdb/merge_operator.h"
#include "table/internal_iterator.h"
namespace rocksdb {
MergeHelper::MergeHelper(Env* env, const Comparator* user_comparator,
const MergeOperator* user_merge_operator,
const CompactionFilter* compaction_filter,
Logger* logger, bool assert_valid_internal_key,
SequenceNumber latest_snapshot, int level,
Statistics* stats,
const std::atomic<bool>* shutting_down)
: env_(env),
stats_(stats) {
assert(user_comparator_ != nullptr);
if (user_merge_operator_) {
allow_single_operand_ = user_merge_operator_->AllowSingleOperand();
Status MergeHelper::TimedFullMerge(const MergeOperator* merge_operator,
const Slice& key, const Slice* value,
const std::vector<Slice>& operands,
std::string* result, Logger* logger,
Statistics* statistics, Env* env,
Slice* result_operand,
bool update_num_ops_stats) {
assert(merge_operator != nullptr);
if (operands.size() == 0) {
assert(value != nullptr && result != nullptr);
result->assign(value->data(), value->size());
return Status::OK();
if (update_num_ops_stats) {
MeasureTime(statistics, READ_NUM_MERGE_OPERANDS,
bool success;
Slice tmp_result_operand(nullptr, 0);
const MergeOperator::MergeOperationInput merge_in(key, value, operands,
MergeOperator::MergeOperationOutput merge_out(*result, tmp_result_operand);
// Setup to time the merge
StopWatchNano timer(env, statistics != nullptr);
// Do the merge
success = merge_operator->FullMergeV2(merge_in, &merge_out);
if ( {
// FullMergeV2 result is an existing operand
if (result_operand != nullptr) {
*result_operand = tmp_result_operand;
} else {
result->assign(, tmp_result_operand.size());
} else if (result_operand) {
*result_operand = Slice(nullptr, 0);
RecordTick(statistics, MERGE_OPERATION_TOTAL_TIME,
statistics ? timer.ElapsedNanos() : 0);
if (!success) {
RecordTick(statistics, NUMBER_MERGE_FAILURES);
return Status::Corruption("Error: Could not perform merge.");
return Status::OK();
// PRE: iter points to the first merge type entry
// POST: iter points to the first entry beyond the merge process (or the end)
// keys_, operands_ are updated to reflect the merge result.
// keys_ stores the list of keys encountered while merging.
// operands_ stores the list of merge operands encountered while merging.
// keys_[i] corresponds to operands_[i] for each i.
Status MergeHelper::MergeUntil(InternalIterator* iter,
RangeDelAggregator* range_del_agg,
const SequenceNumber stop_before,
const bool at_bottom) {
// Get a copy of the internal key, before it's invalidated by iter->Next()
// Also maintain the list of merge operands seen.
has_compaction_filter_skip_until_ = false;
bool first_key = true;
// We need to parse the internal key again as the parsed key is
// backed by the internal key!
// Assume no internal key corruption as it has been successfully parsed
// by the caller.
// original_key_is_iter variable is just caching the information:
// original_key_is_iter == (iter->key().ToString() == original_key)
bool original_key_is_iter = true;
std::string original_key = iter->key().ToString();
// Important:
// orig_ikey is backed by original_key if keys_.empty()
// orig_ikey is backed by keys_.back() if !keys_.empty()
ParsedInternalKey orig_ikey;
ParseInternalKey(original_key, &orig_ikey);
Status s;
bool hit_the_next_user_key = false;
for (; iter->Valid(); iter->Next(), original_key_is_iter = false) {
if (IsShuttingDown()) {
return Status::ShutdownInProgress();
ParsedInternalKey ikey;
assert(keys_.size() == merge_context_.GetNumOperands());
if (!ParseInternalKey(iter->key(), &ikey)) {
// stop at corrupted key
if (assert_valid_internal_key_) {
assert(!"Corrupted internal key not expected.");
return Status::Corruption("Corrupted internal key not expected.");
} else if (first_key) {
assert(user_comparator_->Equal(ikey.user_key, orig_ikey.user_key));
first_key = false;
} else if (!user_comparator_->Equal(ikey.user_key, orig_ikey.user_key)) {
// hit a different user key, stop right here
hit_the_next_user_key = true;
} else if (stop_before && ikey.sequence <= stop_before) {
// hit an entry that's visible by the previous snapshot, can't touch that
// At this point we are guaranteed that we need to process this key.
if (ikey.type != kTypeMerge) {
// hit a put/delete/single delete
// => merge the put value or a nullptr with operands_
// => store result in operands_.back() (and update keys_.back())
// => change the entry type to kTypeValue for keys_.back()
// We are done! Success!
// If there are no operands, just return the Status::OK(). That will cause
// the compaction iterator to write out the key we're currently at, which
// is the put/delete we just encountered.
if (keys_.empty()) {
return Status::OK();
// TODO(noetzli) If the merge operator returns false, we are currently
// (almost) silently dropping the put/delete. That's probably not what we
// want. Also if we're in compaction and it's a put, it would be nice to
// run compaction filter on it.
const Slice val = iter->value();
const Slice* val_ptr = (kTypeValue == ikey.type) ? &val : nullptr;
std::string merge_result;
s = TimedFullMerge(user_merge_operator_, ikey.user_key, val_ptr,
merge_context_.GetOperands(), &merge_result, logger_,
stats_, env_);
// We store the result in keys_.back() and operands_.back()
// if nothing went wrong (i.e.: no operand corruption on disk)
if (s.ok()) {
// The original key encountered
original_key = std::move(keys_.back());
orig_ikey.type = kTypeValue;
UpdateInternalKey(&original_key, orig_ikey.sequence, orig_ikey.type);
// move iter to the next entry
return s;
} else {
// hit a merge
// => if there is a compaction filter, apply it.
// => check for range tombstones covering the operand
// => merge the operand into the front of the operands_ list
// if not filtered
// => then continue because we haven't yet seen a Put/Delete.
// Keep queuing keys and operands until we either meet a put / delete
// request or later did a partial merge.
Slice value_slice = iter->value();
// add an operand to the list if:
// 1) it's included in one of the snapshots. in that case we *must* write
// it out, no matter what compaction filter says
// 2) it's not filtered by a compaction filter
CompactionFilter::Decision filter =
ikey.sequence <= latest_snapshot_
? CompactionFilter::Decision::kKeep
: FilterMerge(orig_ikey.user_key, value_slice);
if (filter != CompactionFilter::Decision::kRemoveAndSkipUntil &&
range_del_agg != nullptr &&
RangeDelAggregator::RangePositioningMode::kForwardTraversal)) {
filter = CompactionFilter::Decision::kRemove;
if (filter == CompactionFilter::Decision::kKeep ||
filter == CompactionFilter::Decision::kChangeValue) {
if (original_key_is_iter) {
// this is just an optimization that saves us one memcpy
} else {
if (keys_.size() == 1) {
// we need to re-anchor the orig_ikey because it was anchored by
// original_key before
ParseInternalKey(keys_.back(), &orig_ikey);
if (filter == CompactionFilter::Decision::kKeep) {
value_slice, iter->IsValuePinned() /* operand_pinned */);
} else { // kChangeValue
// Compaction filter asked us to change the operand from value_slice
// to compaction_filter_value_.
merge_context_.PushOperand(compaction_filter_value_, false);
} else if (filter == CompactionFilter::Decision::kRemoveAndSkipUntil) {
// Compaction filter asked us to remove this key altogether
// (not just this operand), along with some keys following it.
has_compaction_filter_skip_until_ = true;
return Status::OK();
if (merge_context_.GetNumOperands() == 0) {
// we filtered out all the merge operands
return Status::OK();
// We are sure we have seen this key's entire history if we are at the
// last level and exhausted all internal keys of this user key.
// NOTE: !iter->Valid() does not necessarily mean we hit the
// beginning of a user key, as versions of a user key might be
// split into multiple files (even files on the same level)
// and some files might not be included in the compaction/merge.
// There are also cases where we have seen the root of history of this
// key without being sure of it. Then, we simply miss the opportunity
// to combine the keys. Since VersionSet::SetupOtherInputs() always makes
// sure that all merge-operands on the same level get compacted together,
// this will simply lead to these merge operands moving to the next level.
// So, we only perform the following logic (to merge all operands together
// without a Put/Delete) if we are certain that we have seen the end of key.
bool surely_seen_the_beginning = hit_the_next_user_key && at_bottom;
if (surely_seen_the_beginning) {
// do a final merge with nullptr as the existing value and say
// bye to the merge type (it's now converted to a Put)
assert(kTypeMerge == orig_ikey.type);
assert(merge_context_.GetNumOperands() >= 1);
assert(merge_context_.GetNumOperands() == keys_.size());
std::string merge_result;
s = TimedFullMerge(user_merge_operator_, orig_ikey.user_key, nullptr,
merge_context_.GetOperands(), &merge_result, logger_,
stats_, env_);
if (s.ok()) {
// The original key encountered
// We are certain that keys_ is not empty here (see assertions couple of
// lines before).
original_key = std::move(keys_.back());
orig_ikey.type = kTypeValue;
UpdateInternalKey(&original_key, orig_ikey.sequence, orig_ikey.type);
} else {
// We haven't seen the beginning of the key nor a Put/Delete.
// Attempt to use the user's associative merge function to
// merge the stacked merge operands into a single operand.
s = Status::MergeInProgress();
if (merge_context_.GetNumOperands() >= 2 ||
(allow_single_operand_ && merge_context_.GetNumOperands() == 1)) {
bool merge_success = false;
std::string merge_result;
StopWatchNano timer(env_, stats_ != nullptr);
merge_success = user_merge_operator_->PartialMergeMulti(
&merge_result, logger_);
stats_ ? timer.ElapsedNanosSafe() : 0);
if (merge_success) {
// Merging of operands (associative merge) was successful.
// Replace operands with the merge result
keys_.erase(keys_.begin(), keys_.end() - 1);
return s;
MergeOutputIterator::MergeOutputIterator(const MergeHelper* merge_helper)
: merge_helper_(merge_helper) {
it_keys_ = merge_helper_->keys().rend();
it_values_ = merge_helper_->values().rend();
void MergeOutputIterator::SeekToFirst() {
const auto& keys = merge_helper_->keys();
const auto& values = merge_helper_->values();
assert(keys.size() == values.size());
it_keys_ = keys.rbegin();
it_values_ = values.rbegin();
void MergeOutputIterator::Next() {
CompactionFilter::Decision MergeHelper::FilterMerge(const Slice& user_key,
const Slice& value_slice) {
if (compaction_filter_ == nullptr) {
return CompactionFilter::Decision::kKeep;
if (stats_ != nullptr) {
auto ret = compaction_filter_->FilterV2(
level_, user_key, CompactionFilter::ValueType::kMergeOperand, value_slice,
&compaction_filter_value_, compaction_filter_skip_until_.rep());
if (ret == CompactionFilter::Decision::kRemoveAndSkipUntil) {
if (user_comparator_->Compare(*compaction_filter_skip_until_.rep(),
user_key) <= 0) {
// Invalid skip_until returned from compaction filter.
// Keep the key as per FilterV2 documentation.
ret = CompactionFilter::Decision::kKeep;
} else {
total_filter_time_ += filter_timer_.ElapsedNanosSafe();
return ret;
} // namespace rocksdb