levelDB是谷歌的開源key-value存儲系統,性能很高、設計思想很妙、使用起來也很簡單。但像絕大多數的No Sql數據庫一樣,只適合做單一的查詢,不能勝任複雜的關係組合查詢。在實際項目中,我們應用levelDB,需要針對其進行一段時間的數據檢索。於是,通過時間加減,來完成的這項功能,在此貼出簡要代碼,僅供參考。
http://vcsky.net by havenzhao
先看數據插入的函數:
int InsertLevelDB(const char* key, const char* content)
{
//const char* Content = "2011-03-18 10:15:10,1.16591,2.27857,3.37197,4.45305,3.37197,1.16591,2.27857,3.37197,4.45305,5.52507,5.52507,4.45305,4.45305,4.45305,4.45305";
leveldb::DB* db = NULL;
leveldb::Options options;
options.create_if_missing = true;
options.write_buffer_size = 8 * 1024* 1024;
leveldb::Status status = leveldb::DB::Open(options, "c:/tmp/testdb", &db);
leveldb::WriteOptions wo;
wo.sync = false;
if (status.ok())
{
// cout << "write key: " << key << endl;
leveldb::Status s = db->Put(wo, key, content);
}
else
{
printf("Insert LevelDB error\r\n");
}
delete db;
return 0;
}
其中,key的形成,通過device_id+時間戳(time_t類型)
void GetKey(int device_id, time_t time_stamp, char* key)
{
char id[20];
_itoa_s(device_id, id, 10);
strcpy_s(key, 50, id);
char *timeStamp = TimeToChar(time_stamp); //自己實現的函數
strcat_s(key, 50, timeStamp);
}
根據時間起止點,檢索數據,主要是time_t與struct tm的特性,通過時間的加減,形成key值,從而得到value值,存放到value集合中。
int QueryData(int device_id, time_t begin_time, time_t end_time, vector<string>& history_data)
{
leveldb::DB* db = NULL;
leveldb::Options options;
options.create_if_missing = true;
options.write_buffer_size = 8 * 1024* 1024;
leveldb::Status status = leveldb::DB::Open(options, "c:/tmp/testdb", &db);
if (!status.ok())
{
delete db;
printf("Insert LevelDB error\r\n");
return -1;
}
time_t cur_time = begin_time ;
struct tm *beginTime, *curTime;
curTime = localtime(&begin_time);
char key[100];
string content;
while(cur_time <= end_time) //循環得到key,取出value
{
curTime->tm_sec += 1;
cur_time = mktime(curTime);
memset(key, 0, sizeof(key));
GetKey(device_id, cur_time, key);
leveldb::ReadOptions ro;
leveldb::Status s = db->Get(ro, key, &content);
if (!content.empty())
{
history_data.push_back(content.c_str());
}
}
delete db;
return 0;
}
以上是大體思路,得益於key值中包含了時間,才得以實現的範圍檢索。 http://vcsky.net by havenzhao