oracle 语句优化。有关联条件，但表在连接时还是merge join，建成临时表或变换表位置变成hash join 是什么原因
    在oracle中代码如下

  select t2.home_county, count(distinct t1.user_id)
  from user_t2 t2
 inner join vpn_t1 t1 on t1.sum_month = 201212
                               and t1.inure_date <=
                                   to_date(20121231, 'yyyymmdd')
                               and t1.expire_date >=
                                   to_date(20121231, 'yyyymm')
                               and t1.user_id = t2.user_id
 inner join team_t3 t3 on t3.change_inure_date <=
                                       to_date(20121231, 'yyyymmdd')
                                   and t3.change_expire_date >
                                       to_date(20121231, 'yyyymmdd')
                                   and t3.inure_time <=
                                       to_date(20121231, 'yyyymmdd') + 1
                                   and t3.expire_time >=
                                       to_date(20121231, 'yyyymmdd')
                                   and t3.team_sub_type = 31
                                   and t3.team_id = t1.vpn