提示：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.
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：
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.
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));      ///       //计算待返回的结果数
       int ireturnnumrows = (iallnumrows        if(ireturnnumrows        {
              //获取逐条记录
              for(int i = 0; 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;    ///              int iquotient = iallnumrows/ireturnnumrows;      ///              int istartindex = rand()%(iremainder + 1);         ///              //获取逐条记录
        for(int iselectedindex = 0; 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               0:(rand()%(iallnumrows - return_query_host_num));
       int ilimitupper = (iallnumrows               iallnumrows:(ilimitlower + return_query_host_num);
       //计算待返回的结果数
       int ireturnnumrows = (iallnumrows                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       {
              //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;
}