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rocksdb/db/merge_helper.h

98 lines
3.8 KiB

#ifndef MERGE_HELPER_H
#define MERGE_HELPER_H
#include "db/dbformat.h"
#include "rocksdb/slice.h"
#include "rocksdb/statistics.h"
#include <string>
#include <deque>
namespace leveldb {
class Comparator;
class Iterator;
class Logger;
class MergeOperator;
class MergeHelper {
public:
MergeHelper(const Comparator* user_comparator,
const MergeOperator* user_merge_operator,
Logger* logger,
bool assert_valid_internal_key)
: user_comparator_(user_comparator),
user_merge_operator_(user_merge_operator),
logger_(logger),
assert_valid_internal_key_(assert_valid_internal_key),
keys_(),
operands_(),
success_(false) {}
// Merge entries until we hit
// - a corrupted key
// - a Put/Delete,
// - a different user key,
// - a specific sequence number (snapshot boundary),
// or - the end of iteration
// iter: (IN) points to the first merge type entry
// (OUT) points to the first entry not included in the merge process
// stop_before: (IN) a sequence number that merge should not cross.
// 0 means no restriction
// at_bottom: (IN) true if the iterator covers the bottem level, which means
// we could reach the start of the history of this user key.
void MergeUntil(Iterator* iter, SequenceNumber stop_before = 0,
bool at_bottom = false, shared_ptr<Statistics> stats=nullptr);
// Query the merge result
// These are valid until the next MergeUntil call
// If the merging was successful:
// - IsSuccess() will be true
// - key() will have the latest sequence number of the merges.
// The type will be Put or Merge. See IMPORTANT 1 note, below.
// - value() will be the result of merging all the operands together
// - The user should ignore keys() and values().
//
// IMPORTANT 1: the key type could change after the MergeUntil call.
// Put/Delete + Merge + ... + Merge => Put
// Merge + ... + Merge => Merge
//
// If the merge operator is not associative, and if a Put/Delete is not found
// then the merging will be unsuccessful. In this case:
// - IsSuccess() will be false
// - keys() contains the list of internal keys seen in order of iteration.
// - values() contains the list of values (merges) seen in the same order.
// values() is parallel to keys() so that the first entry in
// keys() is the key associated with the first entry in values()
// and so on. These lists will be the same length.
// All of these pairs will be merges over the same user key.
// See IMPORTANT 2 note below.
// - The user should ignore key() and value().
//
// IMPORTANT 2: The entries were traversed in order from BACK to FRONT.
// So keys().back() was the first key seen by iterator.
// TODO: Re-style this comment to be like the first one
bool IsSuccess() { return success_; }
Slice key() { assert(success_); return Slice(keys_.back()); }
Slice value() { assert(success_); return Slice(operands_.back()); }
const std::deque<std::string>& keys() { assert(!success_); return keys_; }
const std::deque<std::string>& values() {
assert(!success_); return operands_;
}
private:
const Comparator* user_comparator_;
const MergeOperator* user_merge_operator_;
Logger* logger_;
Iterator* iter_; // in: the internal iterator, positioned at the first merge entry
bool assert_valid_internal_key_; // enforce no internal key corruption?
// the scratch area that holds the result of MergeUntil
// valid up to the next MergeUntil call
std::deque<std::string> keys_; // Keeps track of the sequence of keys seen
std::deque<std::string> operands_; // Parallel with keys_; stores the values
bool success_;
};
} // namespace leveldb
#endif