语句在没有rownum的情况执行3.5秒左右，加了rownum后(取前10行)就要10秒多，为什么加了rownum就怎么慢呢?
ORACLE10G
SQL语句如下：
select *
from (select ejwh0_.EJWHXH as col_0_0_,
cljbxx1_.cpys as col_1_0_,
cljbxx1_.cphm as col_2_0_,
cljbxx1_.yyzz as col_3_0_,
cljbxx1_.yyzh as col_4_0_,
yhxx3_.yhmc as col_5_0_,
xkxx5_.xkzz as col_6_0_,
xkxx5_.xkzh as col_7_0_,
yzjg7_.jgmc as col_8_0_,
yzry6_.xm as col_9_0_,
ejwh0_.WXDWMC as col_10_0_,
ejwh0_.WHRQ as col_11_0_,
ejwh0_.XCWHRQ as col_12_0_,
ejwh0_.WHLC as col_13_0_,
ejwh0_.XCWHLC as col_14_0_,
ejwh0_.HGZH as col_15_0_,
ejwh0_.ZJY as col_16_0_,
ejwh0_.BZ as col_17_0_
from T_SCJG_EJWH ejwh0_
left outer join T_XZXK_CL_JBXX cljbxx1_ on ejwh0_.FK_CLXH =
cljbxx1_.clxh
left outer join T_X