oracle 优化方面的问题！！！

2个表都已经有了索引，我想走强制索引，这样的写法对么？？各位大侠帮助下啊。
tb_dw_subscriber_his_day表的索引是idx_dw_sub_his_day，
tb_dw_sub_ext_his_day表的索引是idx_dw_sub_ext_subkey，2个表现在都是百万级用户，后期会有千万级用户，已经有了分区，原来没索引要执行3个小时
现在想加上索引。怎么加呀？还有就是怎样看正确的执行计划呀？ select /*+ index(tb_dw_subscriber_his_day idx_dw_sub_his_day) parallel(a,4) parallel(b,4) use_hash(a,b)*/
             a.statis_date,
             a.msisdn,
             --nvl(child.parentnumber,a.msisdn),
             a.subscriberkey,
             a.activestop,
             a.activeflag,
             a.subcosid,
             a.languagetype,
             a.suspendstop,
             a.disablestop,
             a.timeenteractive,
             b.servicestatues,
             b.pso,
             b.acctgrpid,
             b.oldmainproudctid,
             a.poolenddate,
             a.deregisterflag,
            a.expiretime,
            a.freezeenddate,
            a.balance,
            a.usernettype,
            b.maxactivedays,
            b.promostarttime,
            b.promoendtime,
            b.promotionplanid,
            a.applytime,
            a.currentstate
                             from lbidw_airtel.tb_dw_subscriber_his_day        a,
                      lbidw_airtel.tb_dw_sub_ext_his_day      b
                 where  a.subscriberkey = b.subscriberkey
                 and a.statis_date=20120612
                 and b.statis_date=20120612
                      ;