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2、隨機(jī)的獲取記錄
在某些數(shù)據(jù)庫的應(yīng)用中, 我們并不是要獲取所有的滿足條件的記錄,而只是要隨機(jī)挑選出滿足條件的記錄. 這種情況常見于數(shù)據(jù)業(yè)務(wù)的統(tǒng)計(jì)分析,從大容量數(shù)據(jù)庫中獲取小量的數(shù)據(jù)的場合.
有兩種方法可以做到：
- 常規(guī)方法,首先查詢出所有滿足條件的記錄,然后隨機(jī)的挑選出部分記錄.這種方法在滿足條件的記錄數(shù)很多時(shí)效果不理想.
- 使用limit語法,先獲取滿足條件的記錄條數(shù), 然后在sql查詢語句中加入limit來限制只查詢滿足要求的一段記錄. 這種方法雖然要查詢兩次,但是在數(shù)據(jù)量大時(shí)反而比較高效.
示例代碼如下：
//1.常規(guī)的方法
//性能瓶頸,10萬條記錄時(shí),執(zhí)行查詢140ms, 獲取結(jié)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) //檢索
return 0;
//獲取結(jié)果集
m_pResultSet = mysql_store_result(connecthandle);
if(!m_pResultSet) //獲取結(jié)果集出錯(cuò)
return 0;
int iAllNumRows = (int)(mysql_num_rows(m_pResultSet)); ///<所有的搜索結(jié)果數(shù)
//計(jì)算待返回的結(jié)果數(shù)
int iReturnNumRows = (iAllNumRows <= RETURN_QUERY_HOST_NUM)? iAllNumRows:RETURN_QUERY_HOST_NUM;
if(iReturnNumRows <= RETURN_QUERY_HOST_NUM)
{
//獲取逐條記錄
for(int i = 0; i<iReturnNumRows; i++)
{
//獲取逐個(gè)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
{
//隨機(jī)的挑選指定條記錄返回
int iRemainder = iAllNumRows%iReturnNumRows; ///<余數(shù)
int iQuotient = iAllNumRows/iReturnNumRows; ///<商
int iStartIndex = rand()%(iRemainder + 1); ///<開始下標(biāo)
//獲取逐條記錄
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]);
}
}
//釋放結(jié)果集內(nèi)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)
{
//首先獲取滿足結(jié)果的記錄條數(shù),再使用limit隨機(jī)選擇指定條記錄返回
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) //檢索
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);
//計(jì)算待取記錄的上下范圍
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);
//計(jì)算待返回的結(jié)果數(shù)
int iReturnNumRows = (iAllNumRows <= RETURN_QUERY_HOST_NUM)?
iAllNumRows:RETURN_QUERY_HOST_NUM;
//使用limit作查詢
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) //檢索
return 0;
pResultSet = mysql_store_result(connecthandle);
if(!pResultSet)
return 0;
//獲取逐條記錄
for(int i = 0; i<iReturnNumRows; i++)
{
//獲取逐個(gè)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]);
}
//釋放結(jié)果集內(nèi)容
mysql_free_result(pResultSet);
return iReturnNumRows;
}
出處：陳敏的Blog
責(zé)任編輯：bluehearts
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