提高MySQL数据库查询效率的三个实用技巧（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;
}