为什么这两种Oracle查询语句，执行计划有这么大的差异？

select
       d.statis_date,
 d.area_code,
 d.cell_sale_id,
 count(c.serv_number)
  from (select distinct (case
           when(a.whole_fee > 0 or a.word_fee>0) then
                             a.msisdn
                            when(a.msisdn = b.msisdn and (a.book_chpt_cnt > 0 or a.book_download_cnt > 0)) then
       a.msisdn
                            else
                             null
                         end) serv_number
          from masadw.tb_dw_ns_read_user_act_list a,
               masadw.tb_dw_ns_read_user_order_list b
         where a.msisdn = b.msisdn(+)
           and a.deal_date between vd_first_date and vd_date
           and b.deal_date between vd_first_date and vd_date) c
        masadw.tb_mk_sc_user_dtal d
  where c.serv_number = d.serv_number
    and c.user_status_id <> 'H'
    and c.statis_date = vd_date
  group by d.statis_date,
     d.area_code,
     d.cell_sale_id;

执行计划：
                                                 对象                                 耗费
-select statement,goal=all rows
 -sort group by
  -view                                        VM_NWVW_1
   -sort unique          
    -filter
     -filter
      -hash join outer
       -merge join cartesian
        -partition list iterator
         table access all                 TB_DW_NS_READ_USER_ACT_LIST                  10030
        -buffer sort
         -partition range single
          -partition list all
           table access all               TB_MK_SC_USER_DTAL                           64756033
       -partition list iterator
        table access full                 TB_DW_NS_READ_USER_ORDER_LIST                94268

select
       b.statis_date,
 b.area_code,
 b.cell_sale_id,
 count(case
    when a.new_stdt_count = '1' then
                a.serv_number
               else
                null
             end)
  from masadw.tb_kr_campus_user_dtal a,
       masadw.tb_mk_sc_user_dtal b
  where a.serv_number = b.serv_number
    and b.user_status_id <> 'H'
    and a.statis_date = vd_date
    and b.statis_date = vd_date
  group by b.statis_date,
     b.area_code,
     b.cell_sale_id;

执行计划：
                                                 对象                                   耗费
-select statement,goal=all rows
 -PX COORDINATOR
  -PX SEND QC(RANDOW)                            :TQ10003                              98152
   -SORT GROUP BY
    -PX RECEIVE
     -PX SEND HASH                               :TQ10002                              98152
      -SORT GROUP BY
       -HASH JOIN
        -BUFFER SORT
         -PX RECEIVE
          -PX SEND HASH                          :TQ10000                              31066
           -PARTITION LIST SINGLE
            TABLE ACCESS FULL                    TB_KR_CAMPUS_USER_DTAL                31066
        -PX RECEIVE
         -PX SEND HASH                           :TQ10001                              67082
          -PX BLOCK ITERATOR
           TABLE ACCESS FULL                     TB_MK_SC_USER_DTAL                    67082

         

         

select b.* from(select rownum num, a.* from users a)bwhere b.num>=7 and b.num<=9