提高MySQL查询效率的三个技巧

时间：2008-05-19 来源：剑心通明

　MySQL由于它本身的小巧和操作的高效, 在数据库应用中越来越多的被采用.我在开发一个P2P应用的时候曾经使用MySQL来保存P2P节点,由于P2P的应用中,结点数动辄上万个,而且节点变化频繁,因此一定要保持查询和插入的高效.以下是我在使用过程中做的提高效率的三个有效的尝试.

　　使用statement进行绑定查询

　　使用statement可以提前构建查询语法树,在查询时不再需要构建语法树就直接查询.因此可以很好的提高查询的效率. 这个方法适合于查询条件固定但查询非常频繁的场合.

　　使用方法是:

　　绑定, 创建一个MYSQL_STMT变量,与对应的查询字符串绑定,字符串中的问号代表要传入的变量,每个问号都必须指定一个变量.
　　查询, 输入每个指定的变量, 传入MYSQL_STMT变量用可用的连接句柄执行.

　　代码如下:

//1.绑定 bool CDBManager::BindInsertStmt(MYSQL * connecthandle) { //作插入操作的绑定 MYSQL_BIND insertbind[FEILD_NUM]; if(m_stInsertParam == NULL) m_stInsertParam = new CHostCacheTable; m_stInsertStmt = mysql_stmt_init(connecthandle); //构建绑定字符串 char insertSQL[SQL_LENGTH]; strcpy(insertSQL, "insert into HostCache(SessionID, ChannelID, ISPType, " "ExternalIP, ExternalPort, InternalIP, InternalPort) " "values(?, ?, ?, ?, ?, ?, ?)"); mysql_stmt_prepare(m_stInsertStmt, insertSQL, strlen(insertSQL)); int param_count= mysql_stmt_param_count(m_stInsertStmt); if(param_count != FEILD_NUM) return false; //填充bind结构数组, m_sInsertParam是这个statement关联的结构变量 memset(insertbind, 0, sizeof(insertbind)); insertbind[0].buffer_type = MYSQL_TYPE_STRING; insertbind[0].buffer_length = ID_LENGTH /* -1 */; insertbind[0].buffer = (char *)m_stInsertParam->sessionid; insertbind[0].is_null = 0; insertbind[0].length = 0; insertbind[1].buffer_type = MYSQL_TYPE_STRING; insertbind[1].buffer_length = ID_LENGTH /* -1 */; insertbind[1].buffer = (char *)m_stInsertParam->channelid; insertbind[1].is_null = 0; insertbind[1].length = 0; insertbind[2].buffer_type = MYSQL_TYPE_TINY; insertbind[2].buffer = (char *)&m_stInsertParam->ISPtype; insertbind[2].is_null = 0; insertbind[2].length = 0; insertbind[3].buffer_type = MYSQL_TYPE_LONG; insertbind[3].buffer = (char *)&m_stInsertParam->externalIP; insertbind[3].is_null = 0; insertbind[3].length = 0; insertbind[4].buffer_type = MYSQL_TYPE_SHORT; insertbind[4].buffer = (char *)&m_stInsertParam->externalPort; insertbind[4].is_null = 0; insertbind[4].length = 0; insertbind[5].buffer_type = MYSQL_TYPE_LONG; insertbind[5].buffer = (char *)&m_stInsertParam->internalIP; insertbind[5].is_null = 0; insertbind[5].length = 0; insertbind[6].buffer_type = MYSQL_TYPE_SHORT; insertbind[6].buffer = (char *)&m_stInsertParam->internalPort; insertbind[6].is_null = 0; insertbind[6].is_null = 0; //绑定 if (mysql_stmt_bind_param(m_stInsertStmt, insertbind)) return false; return true; }

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; }

使用连接池管理连接.

　　在有大量节点访问的数据库设计中,经常要使用到连接池来管理所有的连接.

　　一般方法是:建立两个连接句柄队列,空闲的等待使用的队列和正在使用的队列.

　　当要查询时先从空闲队列中获取一个句柄,插入到正在使用的队列,再用这个句柄做数据库操作,完毕后一定要从使用队列中删除,再插入到空闲队列.

　　设计代码如下:

//定义句柄队列 typedef std::list<MYSQL *> CONNECTION_HANDLE_LIST; typedef std::list<MYSQL *>::iterator CONNECTION_HANDLE_LIST_IT; //连接数据库的参数结构 class CDBParameter { public: char *host; ///<主机名 char *user; ///<用户名 char *password; ///<密码 char *database; ///<数据库名 unsigned int port; ///<端口，一般为0 const char *unix_socket; ///<套接字，一般为NULL unsigned int client_flag; ///<一般为0 }; //创建两个队列 CONNECTION_HANDLE_LIST m_lsBusyList; ///<正在使用的连接句柄 CONNECTION_HANDLE_LIST m_lsIdleList; ///<未使用的连接句柄 //所有的连接句柄先连上数据库,加入到空闲队列中,等待使用. bool CDBManager::Connect(char * host /* = "localhost" */, char * user /* = "chenmin" */, \ char * password /* = "chenmin" */, char * database /* = "HostCache" */) { CDBParameter * lpDBParam = new CDBParameter(); lpDBParam->host = host; lpDBParam->user = user; lpDBParam->password = password; lpDBParam->database = database; lpDBParam->port = 0; lpDBParam->unix_socket = NULL; lpDBParam->client_flag = 0; try { //连接 for(int index = 0; index < CONNECTION_NUM; index++) { MYSQL * pConnectHandle = mysql_init((MYSQL*) 0); //初始化连接句柄 if(!mysql_real_connect(pConnectHandle, lpDBParam->host, lpDBParam->user, lpDBParam->password,\ lpDBParam->database,lpDBParam->port,lpDBParam->unix_socket,lpDBParam->client_fla)) return false;

//加入到空闲队列中 m_lsIdleList.push_back(pConnectHandle); } } catch(...) { return false; } return true; } //提取一个空闲句柄供使用 MYSQL * CDBManager::GetIdleConnectHandle() { MYSQL * pConnectHandle = NULL; m_ListMutex.acquire(); if(m_lsIdleList.size()) { pConnectHandle = m_lsIdleList.front(); m_lsIdleList.pop_front(); m_lsBusyList.push_back(pConnectHandle); } else //特殊情况,闲队列中为空,返回为空 { pConnectHandle = 0; } m_ListMutex.release(); return pConnectHandle; } //从使用队列中释放一个使用完毕的句柄,插入到空闲队列 void CDBManager::SetIdleConnectHandle(MYSQL * connecthandle) { m_ListMutex.acquire(); m_lsBusyList.remove(connecthandle); m_lsIdleList.push_back(connecthandle); m_ListMutex.release(); } //使用示例,首先获取空闲句柄,利用这个句柄做真正的操作,然后再插回到空闲队列 bool CDBManager::DeleteHostCacheBySessionID(char * sessionid) { MYSQL * pConnectHandle = GetIdleConnectHandle(); if(!pConnectHandle) return 0; bool bRet = DeleteHostCacheBySessionID(pConnectHandle, sessionid); SetIdleConnectHandle(pConnectHandle); return bRet; } //传入空闲的句柄,做真正的删除操作 bool CDBManager::DeleteHostCacheBySessionID(MYSQL * connecthandle, char * sessionid) { char deleteSQL[SQL_LENGTH]; memset(deleteSQL, 0, sizeof(deleteSQL)); sprintf(deleteSQL,"delete from HostCache where SessionID = '%s'", sessionid); if(mysql_query(connecthandle,deleteSQL) != 0) //删除 return false; return true; }