一个SQL语句优化，急！！！
SQL：
select fexp_bill_no, bill_type, fexp_remark
from (select fexp_bill_no,
case
when exp.FEXP_FORWARD_FLAG = 'N ' then exp.fexp_rp_flag
when exp.FEXP_FORWARD_FLAG = 'Y ' and exp.fexp_rp_flag = '0 ' then '5 '
when exp.FEXP_FORWARD_FLAG = 'Y ' and exp.fexp_rp_flag = '1 ' then '6 '
end bill_type,
fexp_remark,
row_number() over(partition by exp.fexp_bill_no order by exp.fexp_charge_id, exp.fexp_currency_code) rn
from fexpense exp
where exp.fexp_cancel_flag = 'N ' and
exp.fexp_remark is not null)
where rn = 1
fexpense表的数据为638721条，查出记录数为2261条，执行时间为2.641s,大家有什么好的优化方法(建立了对fexp_bill_no,fexp_charge_id,fexp_currency_code的联合索引，更慢，要7S多)
[解决办法]
換種方法試試呢
select fexp_bill_no, bill_type, fexp_remark
from
(
select fexp_bill_no,
case
when exp.FEXP_FORWARD_FLAG = 'N ' then exp.fexp_rp_flag
when exp.FEXP_FORWARD_FLAG = 'Y ' and exp.fexp_rp_flag = '0 ' then '5 '
when exp.FEXP_FORWARD_FLAG = 'Y ' and exp.fexp_rp_flag = '1 ' then '6 '
end bill_type,
fexp_remark
from fexpense exp
where exp.fexp_cancel_flag = 'N ' and
exp.fexp_remark is not null and
rowid in
(select rowid from(select rowid from fexpense t group by t.fexp_bill_no order by t.fexp_charge_id,t.fexp_currency_code ) where rownum <=1)
[解决办法]
考虑建个fexp_cancel_flag+fexp_remark的索引，建完索引对表作个分析。
