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背景

rocksdb的merge主要是为了解决读&写需要两步的操作。例如定义一个累加器，总得先把之前的值读出来才能加。

下面给两个例子，大家可以直接用。

AboutAddMerge

#include <iostream>
#include <rocksdb/db.h>
#include <rocksdb/table.h>
#include <rocksdb/options.h>#include <rocksdb/filter_policy.h>#include <rocksdb/iostats_context.h>
#include <rocksdb/trace_reader_writer.h>
#include "utilities/merge_operators.h"
#include "util/coding.h"using namespace rocksdb;using namespace std;
rocksdb::DB* db;
rocksdb::Options option;void OpenDB() {option.create_if_missing = true;option.compression = rocksdb::CompressionType::kNoCompression;option.merge_operator =MergeOperators::CreateUInt64AddOperator();auto s = rocksdb::DB::Open(option, "./db", &db);if (!s.ok()) {cout << "open faled :  " << s.ToString() << endl;exit(-1);}cout << "Finish open !"<< endl;
}void DoWrite() {int j = 0;string key = std::to_string(j);std::string value;char buf[8];rocksdb::Status s;EncodeFixed64(buf, 15);db->Merge(rocksdb::WriteOptions(),key, std::string(buf,8));EncodeFixed64(buf, 33);s = db->Merge(rocksdb::WriteOptions(),key, std::string(buf,8));EncodeFixed64(buf, 77);s = db->Merge(rocksdb::WriteOptions(),key, std::string(buf,8));if (!s.ok()) {cout << "Merge value failed: " << s.ToString() << endl;exit(-1);}cout << "Finish merge !" << endl;s = db->Get(rocksdb::ReadOptions(), key, &value);if (!s.ok()) {cout << "Get after only merge is failed " << s.ToString() << endl;exit(-1);}cout << "Get merge value " << value.size() << " " << DecodeFixed64(value.data()) << endl;
}int main() {OpenDB();DoWrite();return 0;
}

最终打印结果

Get merge value 8 155

AboutAppendMerge

#include <rocksdb/db.h>
#include <rocksdb/iostats_context.h>
#include <rocksdb/options.h>
#include <rocksdb/table.h>
#include <rocksdb/trace_reader_writer.h>#include <iostream>#include "utilities/merge_operators.h"
using namespace rocksdb;rocksdb::DB* db;void OpenDB() {rocksdb::Options option;option.create_if_missing = true;// 默认会用 逗号分隔 不同的mergeoption.merge_operator = MergeOperators::CreateStringAppendOperator();auto s = rocksdb::DB::Open(option, "./db", &db);if (!s.ok()) {std::cout << "open faled :  " << s.ToString() << std::endl;exit(-1);}std::cout << "Finish open !" << std::endl;
}void DoWrite() {std::string key = "mykey5";std::string value;rocksdb::WriteOptions writeOptions;db->Merge(writeOptions, key, "1");db->Merge(writeOptions, key, "3");db->Merge(writeOptions, key, "    ");db->Merge(writeOptions, key, "abc");rocksdb::Status s = db->Merge(writeOptions, key, "6");if (!s.ok()) {std::cout << "Merge value failed: " << s.ToString() << std::endl;exit(-1);}s = db->Get(rocksdb::ReadOptions(), key, &value);if (!s.ok()) {std::cout << "Get after only merge is failed " << s.ToString() << std::endl;exit(-1);}std::cout << "Get merge value " << key << " " << value << std::endl;
}int main() {OpenDB();DoWrite();return 0;
}

最后打印结果

Get merge value mykey5 1,3,    ,abc,6

CMakeList.TXT

add_executable(AboutAppendMerge app/AboutAppendMerge.cpp)
target_link_libraries(AboutAppendMerge ${ROCKSDB_SHARED_LIB})add_executable(AboutAddMerge app/AboutAddMerge.cpp)
target_link_libraries(AboutAddMerge ${ROCKSDB_SHARED_LIB})

参考资料

https://blog.csdn.net/Z_Stand/article/details/119703176