求分页后索引不能用问题，纠结?

SQL见下，没用分页时，在TOAD里执行计划都有用到索引，条件字段也都有索引。
1.当ROWNUM <= 20时，也是用到索引查询的，但一旦ROWNUM <= 40时，表payout_detail就报全表扫描了。
2.试着把内层的order by移出一层，无论怎么改都会用索引。怀疑是数据的问题，因在测试库上都没这问题，生产库频繁出现这问题，请大侠们帮分析下这问题？SELECT *
  FROM (SELECT temp.*, ROWNUM num
          FROM (select temp.wtid,
                       temp.wbid,
                       temp.PAYOUT_NUMBER,
                       temp.customer_number,
                       temp.processed_time,
                       temp.biz_type_code,
                       temp.general_biz_num,
                       temp.assigned_to,
                       pd.payout_notice_num,
                       p.party_name
                  from payout pt
                  join (select wt.id as wtid,
                              wb.id as wbid,
                              wb.PAYOUT_NUMBER as PAYOUT_NUMBER,
                              wb.customer_number as customer_number,
                              wt.processed_time as processed_time,
                              wb.biz_type_code,
                              wb.biz_event_number as general_biz_num,
                              wt.assigned_to
                         from task_inst wt
                         join task_info wb on wb.bp_task_id = wt.bp_task_id
                        where wb.biz_type_code = '7' 
                          and wt.assigned_to = '442000025'
                          and wt.processed_time <
                              to_date('2012-11-26 23:59:59',
                                      'yyyy-MM-dd HH24:mi:ss')
                          and wt.processed_time >=
                              to_date('2011-05-04 08:00:00',
                                      'yyyy-MM-dd HH24:mi:ss')
                        order by wt.id, wb.id) temp on pt.payout_app_form_num =
                                                       temp.payout_number
                  join party p on pt.customer_num = p.customer_num
                  join payout_detail pd on pt.payout_app_form_num =
                                           pd.payout_app_form_num) temp
         where ROWNUM <= 20) WHERE　num > 0