两个Oracle查询语句，执行计划怎么有这么大的差异？
select /*parallel(a, 12)*/
  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 /*parallel(b, 12)*/
  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

------解决方案-----