提高MySQL查询效率的三个技巧（2）

2、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;
}