转自：http://www.cnblogs.com/PiedPiper/archive/2006/02/28/339919.html

一直以为以下这种语句（执行一）就是隐式游标，跟显式定义游标（执行二）运行速度差不了多少。为方便简单起见，我通常都是用执行一的方式来写循环操作，数据量较小没有觉得有什么，只到有一天，一个海量数据的更新运行了一晚上都没出来结果，才知道自己犯了多大的错误：
执行一：
begin
    for c in (
     select ca.org_code, m.material_code, sum(cd.num) ammount
             from mas_admin.ASC_STOCK_CHANGE_BILL c
         join mas_admin.ASC_STOCK_CHANGE_DETAIL cd on c.STOCK_CHANGE_BILL_ID =
                                     cd.STOCK_CHANGE_BILL_ID
          join mas_admin.asc_materiel m on m.materiel_id = cd.MATERIAL_ID --物料表
          join mas_admin.asc_stock s on s.STOCK_ID = c.STOCK_ID
          join mas_admin.asc_agent ca on ca.ORG_ID = s.org_id
           where c.ENABLE_FLAG = 'Y'
            and s.STOCK_TYPE_ID = 1
             and c.BILL_STATE = 3
            and substr(ca.org_code, 1, 3) <> 'BAK'
           and c.STOCK_CHANGE_REASON_ID=3 -- 3=系统错误
           group by ca.org_code, m.material_code
  ) loop
       update asc_ca_stock_good_usable
    set ADJUST_NUM_ERROR=c.ammount
    where ORG_CODE=c.org_code and MATERIAL_CODE=c.MATERIAL_CODE and action_date=curr_date;
      end loop;
    commit; 
    end;
执行二：
 declare
    var_org_code varchar2(50);
    var_material_code varchar2(50);
    var_ammount number;
    cursor c is
       select ca.org_code, m.material_code, sum(cd.num) ammount
             from mas_admin.ASC_STOCK_CHANGE_BILL c
         join mas_admin.ASC_STOCK_CHANGE_DETAIL cd on c.STOCK_CHANGE_BILL_ID =
                                     cd.STOCK_CHANGE_BILL_ID
          join mas_admin.asc_materiel m on m.materiel_id = cd.MATERIAL_ID --物料表
          join mas_admin.asc_stock s on s.STOCK_ID = c.STOCK_ID
          join mas_admin.asc_agent ca on ca.ORG_ID = s.org_id
           where c.ENABLE_FLAG = 'Y'
            and s.STOCK_TYPE_ID = 1
             and c.BILL_STATE = 3
            and substr(ca.org_code, 1, 3) <> 'BAK'
           and c.STOCK_CHANGE_REASON_ID=3 -- 3=系统错误
           group by ca.org_code, m.material_code;
begin
    open c;
    loop
       fetch c into var_org_code,var_material_code,var_ammount;
       exit when c%notfound;
       update asc_ca_stock_good_usable
    set ADJUST_NUM_ERROR=var_ammount
    where ORG_CODE=var_org_code and MATERIAL_CODE=var_material_code and action_date=trunc(sysdate);
      commit;
      end loop;    
    end;

执行一10多分钟算不完，执行二只需0.2秒