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_XZXK_YH_YHXX yhxx3_ on cljbxx1_.FK_YHXH =

yhxx3_.yhxh

left outer join T_XZXK_YHXK_XKXX xkxx5_ on cljbxx1_.FK_YHXKXH =

xkxx5_.yhxkxh

left outer join T_YZRY_JBXX yzry6_ on ejwh0_.FK_LRR = yzry6_.ryxh

left outer join T_YZJG_JBXX yzjg7_ on ejwh0_.FK_YZJG =

yzjg7_.yzjgxh

left outer join T_YZRY_JBXX yzry8_ on ejwh0_.FK_XGR = yzry8_.ryxh

where cljbxx1_.FK_YHXKXH = xkxx5_.yhxkxh

and xkxx5_.xkzz = '锦'

and (cljbxx1_.cphm like '%A%')

and cljbxx1_.yyzz = '锦'

order by ejwhxh)

where rownum <= 10

执行计划：

SELECT STATEMENT, GOAL = ALL_ROWS                        Cost=2046        Cardinality=10        Bytes=7380

COUNT STOPKEY

VIEW        Object owner=TEPS_DB                Cost=2046        Cardinality=10        Bytes=7380

NESTED LOOPS

NESTED LOOPS                        Cost=2046        Cardinality=10        Bytes=1740

NESTED LOOPS OUTER                        Cost=2026        Cardinality=10        Bytes=1530

NESTED LOOPS OUTER                        Cost=2006        Cardinality=10        Bytes=1380

NESTED LOOPS OUTER                        Cost=1996        Cardinality=10        Bytes=1220

NESTED LOOPS                        Cost=1986        Cardinality=10        Bytes=1100

TABLE ACCESS BY INDEX ROWID        Object owner=TEPS_DB        Object name=T_SCJG_EJWH        Cost=220        Cardinality=2402260        Bytes=163353680

INDEX FULL SCAN        Object owner=TEPS_DB        Object name=PK_EJWHXHCost=5        Cardinality=882

TABLE ACCESS BY INDEX ROWID        Object owner=TEPS_DB        Object name=T_XZXK_CL_JBXX        Cost=2        Cardinality=1        Bytes=42

INDEX UNIQUE SCAN        Object owner=TEPS_DB        Object name=PK_T_XZXK_CL_JBXX        Cost=1        Cardinality=1

TABLE ACCESS BY INDEX ROWID        Object owner=TEPS_DB        Object name=T_YZRY_JBXX        Cost=1        Cardinality=1        Bytes=12

INDEX UNIQUE SCAN        Object owner=TEPS_DB        Object name=PK_T_YZRY_JBXX        Cost=0        Cardinality=1

TABLE ACCESS BY INDEX ROWID        Object owner=TEPS_DB        Object name=T_YZJG_JBXX        Cost=1        Cardinality=1        Bytes=16

INDEX UNIQUE SCAN        Object owner=TEPS_DB        Object name=PK_T_YZJG_JBXX        Cost=0        Cardinality=1

TABLE ACCESS BY INDEX ROWID        Object owner=TEPS_DB        Object name=T_XZXK_YH_YHXX        Cost=2        Cardinality=1        Bytes=15

INDEX UNIQUE SCAN        Object owner=TEPS_DB        Object name=PK_T_XZXK_YH_YHXX        Cost=1        Cardinality=1

INDEX UNIQUE SCAN        Object owner=TEPS_DB        Object name=PK_XZXK_YHXK_XKXX        Cost=1        Cardinality=1

TABLE ACCESS BY INDEX ROWID        Object owner=TEPS_DB        Object name=T_XZXK_YHXK_XKXX        Cost=2        Cardinality=1        Bytes=21

Statistics：

bytes received via SQL*Net from client        3701

bytes sent via SQL*Net to client        2108

consistent gets        2819932

consistent gets - examination        1825569

consistent gets direct        0

consistent gets from cache        2819932

db block gets        0

db block gets direct        0

db block gets from cache        0

lob reads        0

lob writes        0

parse count (failures)        1

parse count (hard)        1

parse count (total)        1

physical read bytes        0

physical reads        0

recursive calls        1

redo size        0

sorts (disk)        0

sorts (memory)        0

sorts (rows)        0