Oracle中ROWNUM的使用技巧

Oracle中ROWNUM的使用技巧
作者： fuyuncat

來源： www.HelloDBA.com

ROWNUM是一種僞列，它會根據返回記錄生成一個序列化的數字。利用ROWNUM，我們可以生產一些原先難以實現的結果輸出，但因爲它是僞列的這個特殊性，我們在使用時也需要注意一些事項，不要掉入“陷阱”。下面就介紹一下它的使用技巧及注意事項。

1 特殊結果輸出
利用ROWNUM，我們可以做到一些特殊方式的輸出。

1.1 Top N結果輸出
我們如果希望取輸出結果的前面幾條數據，通過ROWNUM可以輕鬆實現：



SQL> select * from t_test4
2 where rownum <= 5;

USERNAME USER_ID CREATED
------------------------------ ---------- ---------
WOW 71 26-APR-07
CS2 70 15-JAN-07
3 69 01-NOV-06
DMP 68 12-OCT-06
PROFILER 67 05-SEP-06


但是，如果你希望對一個排序結果取Top N數據的話，使用ROWNUM存在一些“陷阱”，我們後面部分會介紹這些“陷阱”並且說明如何避免。

1.2 分頁查詢
利用ROWNUM對結果進行分頁，下面返回結果中的第6到第10條記錄：

SQL> select * from
2 (
3 select a.*, rownum as rn from css_bl_view a
4 where capture_phone_num = '(1) 925-4604800'
5 ) b
6 where b.rn between 6 and 10;

6 rows selected.


Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2770 Card=2183 Bytes =7166789)
1 0 VIEW (Cost=2770 Card=2183 Bytes=7166789)
2 1 COUNT
3 2 TABLE ACCESS (FULL) OF 'CSS_BL_VIEW' (Cost=2770 Card=2183 Bytes=1305434)


Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
29346 consistent gets
29190 physical reads
0 redo size
7328 bytes sent via SQL*Net to client
234 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
5 rows processed


另外一種實現方式：



SQL> select * from css_bl_view a
2 where capture_phone_num = '(1) 925-4604800'
3 and rownum <= 10
4 minus
5 select * from css_bl_view a
6 where capture_phone_num = '(1) 925-4604800'
7 and rownum <= 5
8 ;


Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=5920 Card=10 Bytes=8970)
1 0 MINUS
2 1 SORT (UNIQUE) (Cost=2960 Card=10 Bytes=5980)
3 2 COUNT (STOPKEY)
4 3 TABLE ACCESS (FULL) OF 'CSS_BL_VIEW' (Cost=2770 Card=2183 Bytes=1305434)
5 1 SORT (UNIQUE) (Cost=2960 Card=5 Bytes=2990)
6 5 COUNT (STOPKEY)
7 6 TABLE ACCESS (FULL) OF 'CSS_BL_VIEW' (Cost=2770 Card=2183 Bytes=1305434)


Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
62 consistent gets
50 physical reads
0 redo size
7232 bytes sent via SQL*Net to client
234 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
5 rows processed


第三種實現方式：



SQL> select * from
2 (
3 select a.*, rownum as rn from css_bl_view a
4 where capture_phone_num = '(1) 925-4604800'
5 and rownum <= 10
6 ) b
7 where b.rn > 5;


Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2770 Card=10 Bytes=32830)
1 0 VIEW (Cost=2770 Card=10 Bytes=32830)
2 1 COUNT (STOPKEY)
3 2 TABLE ACCESS (FULL) OF 'CSS_BL_VIEW' (Cost=2770 Card=2183 Bytes=1305434)


Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
35 consistent gets
30 physical reads
0 redo size
7271 bytes sent via SQL*Net to client
234 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
5 rows processed


這裏特地將三種實現方式的查詢計劃及統計數據打印出來，大家可以比較一下3中方式的性能。

1.3 利用ROWNUM做分組子排序
對於以下表T_TEST4的內容：



OWNER NAME
------------------------------------------------------
STRMADMIN STREAMS_QUEUE
APARKMAN JOB_QUEUE
SYS AQ$_AQ_SRVNTFN_TABLE_E
SYS AQ$_KUPC$DATAPUMP_QUETAB_E
APARKMAN AQ$_JMS_TEXT_E
STRMADMIN AQ$_STREAMS_QUEUE_TABLE_E
SYS AQ$_SCHEDULER$_EVENT_QTAB_E
…


如果我們希望結果按照OWNER進行分組後，再對每組中成員進行編號，結果類似如下：



OWNER NO NAME
------------------------------------------------------
APARKMAN 1 JOB_QUEUE
2 AQ$_JMS_TEXT_E
STRMADMIN 1 STREAMS_QUEUE
2 AQ$_STREAMS_QUEUE_TABLE_E
SYS 1 AQ$_AQ_SRVNTFN_TABLE_E
2 AQ$_KUPC$DATAPUMP_QUETAB_E
3 AQ$_SCHEDULER$_EVENT_QTAB_E
…


在沒有ROWNUM時要實現這樣的功能會很複雜，但通過ROWNUM我們可以輕鬆實現：



SQL> SELECT DECODE(ROWNUM-min_sno,0,a.owner,NULL) owner,DECODE(ROWNUM-min_sno,0,1,rownum+1-min_sno) sno, a.name
2 FROM (SELECT *
3 FROM t_test8
4 ORDER BY owner, name ) a,
5 (SELECT owner, MIN(rownum) min_sno
6 FROM( SELECT *
7 FROM t_test8
8 ORDER BY owner, name)
9 GROUP BY owner) b
10 WHERE a.owner=b.owner;

OWNER SNO NAME
------------------------------ ---------- ------------------------------
APARKMAN 1 JOB_QUEUE
2 AQ$_JMS_TEXT_E
STRMADMIN 1 STREAMS_QUEUE
2 AQ$_STREAMS_QUEUE_TABLE_E
SYS 1 AQ$_AQ_SRVNTFN_TABLE_E
2 AQ$_KUPC$DATAPUMP_QUETAB_E
3 AQ$_SCHEDULER$_EVENT_QTAB_E
4 AQ$_SCHEDULER$_JOBQTAB_E
5 AQ$_STREAMS_QUEUE_TABLE_E
6 AQ$_SYS$SERVICE_METRICS_TAB_E
7 AQ$_AQ_EVENT_TABLE_E
8 AQ$_AQ$_MEM_MC_E
9 AQ$_ALERT_QT_E
10 ALERT_QUE
11 AQ_EVENT_TABLE_Q
12 SYS$SERVICE_METRICS
13 STREAMS_QUEUE
14 SRVQUEUE
15 SCHEDULER$_JOBQ
16 SCHEDULER$_EVENT_QUEUE
17 AQ_SRVNTFN_TABLE_Q
SYSMAN 1 AQ$_MGMT_NOTIFY_QTABLE_E
2 MGMT_NOTIFY_Q
SYSTEM 1 DEF$_AQERROR
2 DEF$_AQCALL
3 AQ$_DEF$_AQERROR_E
4 AQ$_DEF$_AQCALL_E
WMSYS 1 AQ$_WM$EVENT_QUEUE_TABLE_E
2 WM$EVENT_QUEUE

29 rows selected.
2 性能
我們很多程序員在確認某個表中是否有相應數據時，喜歡加上ROWNUM=1，其思路就是隻要存在一條數據就說明有相應數據，查詢就可以直接返回了，這樣就能提高性能了。但是在10G之前，使用ROWNUM=1是不能達到預期的性能效果的，而是需要通過<2或<=1作爲過濾條件才能達到預期效果，看以下查詢計劃：



SQL> select * from t_test1
2 where object_id <100
3 and rownum = 1;


Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=37 Card=1 Bytes=86)
1 0 COUNT (STOPKEY)
2 1 TABLE ACCESS (BY INDEX ROWID) OF 'T_TEST1' (Cost=37 Card=89 Bytes=7654)
3 2 INDEX (RANGE SCAN) OF 'T_TEST1_PK' (UNIQUE) (Cost=2 Card=89)


Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
62 consistent gets
0 physical reads
0 redo size
654 bytes sent via SQL*Net to client
234 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

SQL> select * from t_test1
2 where object_id <100
3 and rownum <= 1;


Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=37 Card=1 Bytes=86)
1 0 COUNT (STOPKEY)
2 1 TABLE ACCESS (BY INDEX ROWID) OF 'T_TEST1' (Cost=37 Card=89 Bytes=7654)
3 2 INDEX (RANGE SCAN) OF 'T_TEST1_PK' (UNIQUE) (Cost=2 Card=89)


Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
654 bytes sent via SQL*Net to client
234 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

SQL> /


Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=37 Card=1 Bytes=86)
1 0 COUNT (STOPKEY)
2 1 TABLE ACCESS (BY INDEX ROWID) OF 'T_TEST1' (Cost=37 Card=89 Bytes=7654)
3 2 INDEX (RANGE SCAN) OF 'T_TEST1_PK' (UNIQUE) (Cost=2 Card=89)


Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
654 bytes sent via SQL*Net to client
234 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed


10G以後，這個問題就被修正了：



SQL> select * from t_test1
2 where rownum = 1;


Execution Plan
----------------------------------------------------------
Plan hash value: 536364188

------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 86 | 2 (0)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | TABLE ACCESS FULL| T_TEST1 | 1 | 86 | 2 (0)| 00:00:01 |
------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter(ROWNUM=1)


Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
4 consistent gets
1 physical reads
0 redo size
1201 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

SQL> select * from t_test1
2 where rownum <= 1;


Execution Plan
----------------------------------------------------------
Plan hash value: 536364188

------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 86 | 2 (0)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | TABLE ACCESS FULL| T_TEST1 | 1 | 86 | 2 (0)| 00:00:01 |
------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter(ROWNUM<=1)


Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1201 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed


3 ROWNUM的使用“陷阱”
由於ROWNUM是一個僞列，只有有結果記錄時，ROWNUM纔有相應數據，因此對它的使用不能向普通列那樣使用，否則就會陷入一些“陷阱”當中。

3.1 對ROWNUM進行>、>=、=操作
不能對ROWNUM使用>（大於1的數值）、>=（大於或等於1的數值）、=（大於或等於1的數值），否則無結果



SQL> select count(*) from css_bl_view a where rownum>0;

COUNT(*)
----------
361928


SQL> select count(*) from css_bl_view a
2 where rownum > 1;

COUNT(*)
----------
0


這是因爲：

1、ROWNUM是僞列，必須要要有返回結果後，每條返回記錄就會對應產生一個ROWNUM數值；

2、返回結果記錄的ROWNUM是從1開始排序的，因此第一條始終是1;



這樣，當查詢到第一條記錄時，該記錄的ROWNUM爲1，但條件要求ROWNUM>1，因此不符合，繼續查詢下一條；因爲前面沒有符合要求的記錄，因此下一條記錄過來後，其ROWNUM還是爲1，如此循環，就不會產生結果。上述查詢可以通過子查詢來替代：



SQL> select count(*)
2 from
3 (select BL_REF_CDE, rownum rn from css_bl_view)
4 where rn > 1;

COUNT(*)
----------
361927


我們可以通過以下方式來實現對ROWNUM的>、=的查詢：

查詢ROWNUM=5的數據：



SQL> select object_id,object_name
2 from (select object_id,object_name, rownum as rn from t_test1)
3 where rn = 5;

OBJECT_ID OBJECT_NAME
---------- ------------------------------
29 C_COBJ#


查詢ROWNUM > 25的數據：



SQL> select * from t_test4
2 minus
3 select * from t_test4
4 where rownum <= 25;

USERNAME USER_ID CREATED
------------------------------ ---------- ---------
DIP 19 21-NOV-05
OUTLN 11 21-NOV-05
PUBLIC 99999 18-JUL-07
SYS 0 21-NOV-05
SYSMAN 32 21-NOV-05
SYSTEM 5 21-NOV-05

6 rows selected.
3.2 ROWNUM和Order BY
要注意的是：在使用ROWNUM時，只有當Order By的字段是主鍵時，查詢結果纔會先排序再計算ROWNUM，下面OBJECT_ID是表T_TEST1的主鍵字段：



SQL> select object_id,object_name from t_test1
2 where rownum <= 5
3 order by object_id;

OBJECT_ID OBJECT_NAME
---------- ------------------------------
2 C_OBJ#
3 I_OBJ#
4 TAB$
5 CLU$
6 C_TS#


但是，對非主鍵字段OBJECT_NAME進行排序時，結果就混亂了：



SQL> select object_id,object_name from t_test1
2 where rownum <= 5
3 order by object_name;

OBJECT_ID OBJECT_NAME
---------- ------------------------------
28 CON$
29 C_COBJ#
20 ICOL$
44 I_USER1
15 UNDO$

SQL> select count(*) from t_test1
2 where object_name < 'CON$';

COUNT(*)
----------
21645


出現這種混亂的原因是：Oracle先按物理存儲位置（rowid）順序取出滿足rownum條件的記錄，即物理位置上的前5條數據，然後在對這些數據按照Order By的字段進行排序，而不是我們所期望的先排序、再取特定記錄數。



如果需要對非主鍵字段排序再去前n條數據，我們可以以以下方式實現：



SQL> select object_id,object_name
2 from (select object_id,object_name from t_test1
3 order by object_name)
4 where rownum <= 5;

OBJECT_ID OBJECT_NAME
---------- ------------------------------
35489 /1000e8d1_LinkedHashMapValueIt
35490 /1000e8d1_LinkedHashMapValueIt
21801 /1005bd30_LnkdConstant
21802 /1005bd30_LnkdConstant
17205 /10076b23_OraCustomDatumClosur


3.3 排序分頁
當對存在重複值的字段排序後再分頁輸出，我們很容易會陷入到另外一個“陷阱”。



請看以下例子，我們希望對T_TEST1的OWNER字段排序後，以每頁輸出10個結果的方式分頁輸出：



SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select owner, object_name from t_test1 order by owner) a
4 where rownum <= 10)
5 where rn >= 1;

OWNER OBJECT_NAME
------------------------------ ------------------------------
AFWOWNER AFWADAPTER
AFWOWNER AFWADAPTERCONFIGURATION
AFWOWNER AFWADAPTERCONFIGURATION_IDX1
AFWOWNER AFWADAPTERFQN_PK
AFWOWNER AFWADAPTERCONFIGURATION_PK
AFWOWNER AFWADAPTERCONFIGURATION_IDX2
AFWOWNER AFWSERVERCODE_PK
AFWOWNER AFWSERVER
AFWOWNER AFWADAPTERLOOKUP_IDX1
AFWOWNER AFWADAPTERLOOKUP

10 rows selected.

SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select owner, object_name from t_test1 order by owner) a
4 where rownum <= 20)
5 where rn >= 11;

OWNER OBJECT_NAME
------------------------------ ------------------------------
AFWOWNER AFWTOKENSTATUSCODE_PK
AFWOWNER AFWTOKENSTATUS
AFWOWNER AFWTOKENADMIN_IDX1
AFWOWNER AFWTOKENADMINCODE_PK
AFWOWNER AFWTOKENADMIN
AFWOWNER AFWTOKEN
AFWOWNER AFWSERVERCONFIGURATION_PK
AFWOWNER AFWSERVERCONFIGURATION
AFWOWNER AFWSERVER
AFWOWNER AFWADAPTERLOOKUP

10 rows selected.


仔細比較結果，你會發現“AFWSERVER”、“AFWADAPTERLOOKUP”在兩次分頁結果中都出現了。但是OBJECT_NAME在每個OWNER中的值是唯一的，說明這個輸出結果是錯誤的，我們又陷入了一個“陷阱”。這是怎麼回事呢，請先看下上述語句的查詢計劃：



SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select owner, object_name from t_test1 order by owner) a
4 where rownum <= 20)
5 where rn >= 11;

10 rows selected.


Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=205 Card=20 Bytes=94
0)

1 0 VIEW (Cost=205 Card=20 Bytes=940)
2 1 COUNT (STOPKEY)
3 2 VIEW (Cost=205 Card=30670 Bytes=1042780)
4 3 SORT (ORDER BY STOPKEY) (Cost=205 Card=30670 Bytes=858760)
5 4 TABLE ACCESS (FULL) OF 'T_TEST1' (Cost=42 Card=30670 Bytes=858760)


看下這個“陷阱”是怎麼形成的。從查詢計劃上，我們可以注意到，對於子查詢

select a.*, rownum as rn from
(select owner, object_name from t_test1 order by owner) a
where rownum <= 20
優化器採用了“SORT (ORDER BY STOPKEY)”。



“SORT (ORDER BY STOPKEY)”不需要對所有數據進行排序，而是隻要找出結果集中的按特定順序的最前N條記錄，一旦找出了這N條記錄，就無需再對剩下的數據進行排序，而直接返回結果。這種算法我們可以視爲是“快速排序”算法的變種。快速排序算法的基本思想是：先將數據分2組集合，保證第一集合中的每個數據都大於第二個集合中每個數據，然後再按這個原則對每個集合進行遞歸分組，直到集合的單位最小。在進行“SORT (ORDER BY STOPKEY)”時，首先找出N條數據（這些數據並沒有做排序）放在第一組，保證第一組的數據都大於第二組的數據，然後只對第一組數據進行遞歸。

可以看到，基於這樣的算法基礎上，如果N的數值不同，數據的分組也不同（如N=20時，第一次分組比例爲12:8，然後繼續遞歸；當N=10時，第一次分組比例爲3:7 … …），這樣，在數據的排序字段值都相等時，輸出結果的順序就會因爲N值不同而不同。



知道原因後，我們可以通過以下幾種方法來避免這個“陷阱”。

1、讓查詢計劃避免“SORT (ORDER BY STOPKEY)”，採用“SORT (ORDER BY)”，使數據排序不受ROWNUM的影響。但這樣會使所有數據都做排序：



SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select owner, object_name, rowid from t_test1 order by owner) a)
4 where rn <= 10
5 and rn >= 1;

OWNER OBJECT_NAME
------------------------------ ------------------------------
AFWOWNER AFWADAPTER
AFWOWNER AFWADAPTERCONFIGURATION
AFWOWNER AFWADAPTERCONFIGURATION_IDX2
AFWOWNER AFWADAPTERCONFIGURATION_PK
AFWOWNER AFWADAPTERCONFIGURATION_IDX1
AFWOWNER AFWADAPTERFQN_PK
AFWOWNER AFWADAPTERLOOKUP_IDX1
AFWOWNER AFWSERVERCODE_PK
AFWOWNER AFWSERVERCONFIGURATION_IDX1
AFWOWNER AFWTOKENTYPECODE_PK

10 rows selected.

SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select owner, object_name, rowid from t_test1 order by owner) a)
4 where rn <= 20
5 and rn >= 11;

OWNER OBJECT_NAME
------------------------------ ------------------------------
AFWOWNER AFWTOKENTYPE
AFWOWNER AFWTOKENSTATUSCODE_PK
AFWOWNER AFWTOKENSTATUS
AFWOWNER AFWTOKENADMIN_IDX1
AFWOWNER AFWTOKENADMINCODE_PK
AFWOWNER AFWTOKENADMIN
AFWOWNER AFWTOKEN
AFWOWNER AFWSERVERCONFIGURATION_PK
AFWOWNER AFWTOKEN_PK
AFWOWNER AFWTOKEN_IDX6

10 rows selected.

SQL> set autot trace
SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select owner, object_name, rowid from t_test1 order by owner) a)
4 where rn <= 20
5 and rn >= 11;

10 rows selected.


Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=237 Card=30670 Bytes=1441490)
1 0 VIEW (Cost=237 Card=30670 Bytes=1441490)
2 1 COUNT
3 2 VIEW (Cost=237 Card=30670 Bytes=1042780)
4 3 SORT (ORDER BY) (Cost=237 Card=30670 Bytes=1073450)
5 4 TABLE ACCESS (FULL) OF 'T_TEST1' (Cost=42 Card=30670 Bytes=1073450)


2、在排序時，加上一個或多個字段（如主鍵字段、ROWID），使排序結果具有唯一性：



SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select owner, object_name, rowid from t_test1 order by owner, object_id) a
4 where rownum <= 10)
5 where rn >= 1;

OWNER OBJECT_NAME
------------------------------ ------------------------------
AFWOWNER AFWADAPTER
AFWOWNER AFWADAPTERFQN_PK
AFWOWNER AFWADAPTERCONFIGURATION
AFWOWNER AFWADAPTERCONFIGURATION_PK
AFWOWNER AFWADAPTERCONFIGURATION_IDX1
AFWOWNER AFWADAPTERCONFIGURATION_IDX2
AFWOWNER AFWADAPTERLOOKUP
AFWOWNER AFWADAPTERLOOKUP_IDX1
AFWOWNER AFWSERVER
AFWOWNER AFWSERVERCODE_PK

10 rows selected.

SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select owner, object_name, rowid from t_test1 order by owner, object_id) a
4 where rownum <= 20)
5 where rn >= 11;

OWNER OBJECT_NAME
------------------------------ ------------------------------
AFWOWNER AFWSERVERCONFIGURATION
AFWOWNER AFWSERVERCONFIGURATION_PK
AFWOWNER AFWSERVERCONFIGURATION_IDX1
AFWOWNER AFWTOKEN
AFWOWNER AFWTOKEN_PK
AFWOWNER AFWTOKEN_IDX1
AFWOWNER AFWTOKEN_IDX2
AFWOWNER AFWTOKEN_IDX3
AFWOWNER AFWTOKEN_IDX4
AFWOWNER AFWTOKEN_IDX5

10 rows selected.

SQL> set autot trace
SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select owner, object_name, rowid from t_test1 order by owner, object_id) a
4 where rownum <= 20)
5 where rn >= 11;

10 rows selected.


Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=253 Card=20 Bytes=940)
1 0 VIEW (Cost=253 Card=20 Bytes=940)
2 1 COUNT (STOPKEY)
3 2 VIEW (Cost=253 Card=30670 Bytes=1042780)
4 3 SORT (ORDER BY STOPKEY) (Cost=253 Card=30670 Bytes=1196130)
5 4 TABLE ACCESS (FULL) OF 'T_TEST1' (Cost=42 Card=306
70 Bytes=1196130)


3、對排序字段建立索引，並強制使用索引。這樣就能利用索引已經建立好的排序結果：

SQL> create index t_test1_idx1 on t_test1(owner);

Index created.

SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select /*+index(t T_TEST1_IDX1)*/owner, object_name from t_test1 t order by owner) a
4 where rownum <= 10)
5 where rn >= 1
6 ;

OWNER OBJECT_NAME
------------------------------ ------------------------------
AFWOWNER AFWADAPTER
AFWOWNER AFWADAPTERCONFIGURATION
AFWOWNER AFWADAPTERCONFIGURATION_IDX1
AFWOWNER AFWADAPTERCONFIGURATION_IDX2
AFWOWNER AFWADAPTERCONFIGURATION_PK
AFWOWNER AFWADAPTERFQN_PK
AFWOWNER AFWADAPTERLOOKUP
AFWOWNER AFWADAPTERLOOKUP_IDX1
AFWOWNER AFWSERVER
AFWOWNER AFWSERVERCODE_PK

10 rows selected.

SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select /*+index(t T_TEST1_IDX1)*/owner, object_name from t_test1 t order by owner) a
4 where rownum <= 20)
5 where rn >= 11;

OWNER OBJECT_NAME
------------------------------ ------------------------------
AFWOWNER AFWSERVERCONFIGURATION
AFWOWNER AFWSERVERCONFIGURATION_IDX1
AFWOWNER AFWSERVERCONFIGURATION_PK
AFWOWNER AFWTOKEN
AFWOWNER AFWTOKENADMIN
AFWOWNER AFWTOKENADMINCODE_PK
AFWOWNER AFWTOKENADMIN_IDX1
AFWOWNER AFWTOKENSTATUS
AFWOWNER AFWTOKENSTATUSCODE_PK
AFWOWNER AFWTOKENTYPE

10 rows selected.

SQL> set autot trace
SQL> select owner, object_name from
2 (select a.*, rownum as rn from
3 (select /*+index(t T_TEST1_IDX1)*/owner, object_name from t_test1 t order by owner) a
4 where rownum <= 20)
5 where rn >= 11;

10 rows selected.


Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=414 Card=20 Bytes=940)
1 0 VIEW (Cost=414 Card=20 Bytes=940)
2 1 COUNT (STOPKEY)
3 2 VIEW (Cost=414 Card=30670 Bytes=1042780)
4 3 TABLE ACCESS (BY INDEX ROWID) OF 'T_TEST1' (Cost=414Card=30670 Bytes=858760)
5 4 INDEX (FULL SCAN) OF 'T_TEST1_IDX1' (NON-UNIQUE) (
Cost=26 Card=30670)



以上就是ROWNUM的使用技巧及其注意事項，希望編程成員正確使用ROWNUM，也希望DBA遇到相關問題能迅速定位。