随机的获取记录
在某些数据库的应用中, 我们并不是要获取所有的满足条件的记录,而只是要随机挑选出满足条件的记录. 这种情况常见于数据业务的统计分析,从大容量数据库中获取小量的数据的场合.
有两种方法可以做到
1. 常规方法,首先查询出所有满足条件的记录,然后随机的挑选出部分记录.这种方法在满足条件的记录数很多时效果不理想.
2. 使用limit语法,先获取满足条件的记录条数, 然后在sql查询语句中加入limit来限制只查询满足要求的一段记录. 这种方法虽然要查询两次,但是在数据量大时反而比较高效.
示例代码如下:
| //1.常规的方法 //性能瓶颈,10万条记录时,执行查询140ms, 获取结果集500ms,其余可忽略 int CDBManager::QueryHostCache(MYSQL* connecthandle, char * channelid, int ISPtype, CDBManager::CHostCacheTable * &hostcache) { char selectSQL[SQL_LENGTH]; memset(selectSQL, 0, sizeof(selectSQL)); sprintf(selectSQL,"select * from HostCache where ChannelID = '%s' and ISPtype = %d", channelid, ISPtype); if(mysql_real_query(connecthandle, selectSQL, strlen(selectSQL)) != 0) //检索 return 0; //获取结果集 m_pResultSet = mysql_store_result(connecthandle); if(!m_pResultSet) //获取结果集出错 return 0; int iAllNumRows = (int)(mysql_num_rows(m_pResultSet)); ///<所有的搜索结果数 //计算待返回的结果数 int iReturnNumRows = (iAllNumRows <= RETURN_QUERY_HOST_NUM)? iAllNumRows:RETURN_QUERY_HOST_NUM; if(iReturnNumRows <= RETURN_QUERY_HOST_NUM) { //获取逐条记录 for(int i = 0; i<iReturnNumRows; i++) { //获取逐个字段 m_Row = mysql_fetch_row(m_pResultSet); if(m_Row[0] != NULL) strcpy(hostcache[i].sessionid, m_Row[0]); if(m_Row[1] != NULL) strcpy(hostcache[i].channelid, m_Row[1]); if(m_Row[2] != NULL) hostcache[i].ISPtype = atoi(m_Row[2]); if(m_Row[3] != NULL) hostcache[i].externalIP = atoi(m_Row[3]); if(m_Row[4] != NULL) hostcache[i].externalPort = atoi(m_Row[4]); if(m_Row[5] != NULL) hostcache[i].internalIP = atoi(m_Row[5]); if(m_Row[6] != NULL) hostcache[i].internalPort = atoi(m_Row[6]); } } else { //随机的挑选指定条记录返回 int iRemainder = iAllNumRows%iReturnNumRows; ///<余数 int iQuotient = iAllNumRows/iReturnNumRows; ///<商 int iStartIndex = rand()%(iRemainder + 1); ///<开始下标 //获取逐条记录 for(int iSelectedIndex = 0; iSelectedIndex < iReturnNumRows; iSelectedIndex++) { mysql_data_seek(m_pResultSet, iStartIndex + iQuotient * iSelectedIndex); m_Row = mysql_fetch_row(m_pResultSet); if(m_Row[0] != NULL) strcpy(hostcache[iSelectedIndex].sessionid, m_Row[0]); if(m_Row[1] != NULL) strcpy(hostcache[iSelectedIndex].channelid, m_Row[1]); if(m_Row[2] != NULL) hostcache[iSelectedIndex].ISPtype = atoi(m_Row[2]); if(m_Row[3] != NULL) hostcache[iSelectedIndex].externalIP = atoi(m_Row[3]); if(m_Row[4] != NULL) hostcache[iSelectedIndex].externalPort = atoi(m_Row[4]); if(m_Row[5] != NULL) hostcache[iSelectedIndex].internalIP = atoi(m_Row[5]); if(m_Row[6] != NULL) hostcache[iSelectedIndex].internalPort = atoi(m_Row[6]); } } //释放结果集内容 mysql_free_result(m_pResultSet); return iReturnNumRows; } //2.使用limit版 int CDBManager::QueryHostCache(MYSQL * connecthandle, char * channelid, unsigned int myexternalip, int ISPtype, CHostCacheTable * hostcache) { //首先获取满足结果的记录条数,再使用limit随机选择指定条记录返回 MYSQL_ROW row; MYSQL_RES * pResultSet; char selectSQL[SQL_LENGTH]; memset(selectSQL, 0, sizeof(selectSQL)); sprintf(selectSQL,"select count(*) from HostCache where ChannelID = '%s' and ISPtype = %d", channelid, ISPtype); if(mysql_real_query(connecthandle, selectSQL, strlen(selectSQL)) != 0) //检索 return 0; pResultSet = mysql_store_result(connecthandle); if(!pResultSet) return 0; row = mysql_fetch_row(pResultSet); int iAllNumRows = atoi(row[0]); mysql_free_result(pResultSet); //计算待取记录的上下范围 int iLimitLower = (iAllNumRows <= RETURN_QUERY_HOST_NUM)? 0:(rand()%(iAllNumRows - RETURN_QUERY_HOST_NUM)); int iLimitUpper = (iAllNumRows <= RETURN_QUERY_HOST_NUM)? iAllNumRows:(iLimitLower + RETURN_QUERY_HOST_NUM); //计算待返回的结果数 int iReturnNumRows = (iAllNumRows <= RETURN_QUERY_HOST_NUM)? iAllNumRows:RETURN_QUERY_HOST_NUM; //使用limit作查询 sprintf(selectSQL,"select SessionID, ExternalIP, ExternalPort, InternalIP, InternalPort " "from HostCache where ChannelID = '%s' and ISPtype = %d limit %d, %d" , channelid, ISPtype, iLimitLower, iLimitUpper); if(mysql_real_query(connecthandle, selectSQL, strlen(selectSQL)) != 0) //检索 return 0; pResultSet = mysql_store_result(connecthandle); if(!pResultSet) return 0; //获取逐条记录 for(int i = 0; i<iReturnNumRows; i++) { //获取逐个字段 row = mysql_fetch_row(pResultSet); if(row[0] != NULL) strcpy(hostcache[i].sessionid, row[0]); if(row[1] != NULL) hostcache[i].externalIP = atoi(row[1]); if(row[2] != NULL) hostcache[i].externalPort = atoi(row[2]); if(row[3] != NULL) hostcache[i].internalIP = atoi(row[3]); if(row[4] != NULL) hostcache[i].internalPort = atoi(row[4]); } //释放结果集内容 mysql_free_result(pResultSet); return iReturnNumRows; } |
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