Oracle中ROWNUM的使用技巧 [English] 作者： fuyuncat 来源： HdhCmsTestHelloDBA测试数据 日期： 2009-02-14 14:54:36 ROWNUM是一种伪列，它会根据返回记录生成一个序列化的数字。利用ROWNUM，我们可以生产一些原先难以实现的结果 输出，但因为它是伪列的这个特殊性，
Oracle中ROWNUM的使用技巧
[English]
作者： fuyuncat
来源： HdhCmsTestHelloDBA测试数据
日期： 2009-02-14 14:54:36
ROWNUM是一种伪列，它会根据返回记录生成一个序列化的数字。利用ROWNUM，我们可以生产一些原先难以实现的结果 输出，但因为它是伪列的这个特殊性，我们在使用时也需要注意一些事项，不要掉入“陷阱”。下面就介绍一下它的使用技巧及注意事项。
1 特殊结果 输出
利用ROWNUM，我们可以做到一些特殊方式的 输出。
1.1 Top N 结果 输出
我们如果希望取 输出结果的前面几条数据，通过ROWNUM可以轻松实现：
SQL> select * from t_test4
2 where rownum
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，如果你希望对一个排序结果取Top N数据的话，使用ROWNUM存在一些“陷阱”，我们后面部分会介绍这些“陷阱”并且说明如何避免。
1.2 分页查询
利用ROWNUM对结果进行分页，下面返回结果中的第6到第10条记录：
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
另外一种实现方式：
SQL> select * from css_bl_view a
2 where capture_phone_num = '(1) 925-4604800'
3 and rownum
4 minus
5 select * from css_bl_view a
6 where capture_phone_num = '(1) 925-4604800'
7 and rownum
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
第三种实现方式：
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
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
这里特地将三种实现方式的查询计划及统计数据打印出来，大家可以比较一下3中方式的性能。
1.3 利用 ROWNUM 做分组子排序
对于以下表T_TEST4的内容：
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$_EVENT_QTAB_E
…
如果我们希望结果按照OWNER进行分组后，再对每组中成员进行编号，结果类似如下：
OWNER NO NAME
------------------------------------------------------
APARKMAN 1 JOB_QUEUE
2 AQ$_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
…
在没有ROWNUM时要实现这样的功能会很复杂，但通过ROWNUM我们可以轻松实现：
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
我们很多程序员在确认某个表中是否有相应数据时，喜欢加上ROWNUM=1，其思路就是只要存在一条数据就说明有相应数据，查询就可以直接返回了，这样就能提高性能了。但是在10G之前，使用ROWNUM=1是不能达到预期的性能效果的，而是需要通过
SQL> select * from t_test1
2 where object_id
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
3 and rownum
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以后，这个问题就被修正了：
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
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
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”
由于ROWNUM是一个伪列，只有有结果记录时，ROWNUM才有相应数据，因此对它的使用不能向普通列那样使用，否则就会陷入一些“陷阱”当中。
3.1 对 ROWNUM 进行 > 、 >= 、 = 操作
不能对ROWNUM使用>（大于1的数值）、>=（大于或等于1的数值）、=（大于或等于1的数值），否则无结果
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
这是因为：
1、ROWNUM是伪列，必须要要有返回结果后，每条返回记录就会对应产生一个ROWNUM数值；
2、返回结果记录的ROWNUM是从1开始排序的，因此第一条始终是1;
这样，当查询到第一条记录时，该记录的ROWNUM为1，但条件要求ROWNUM>1，因此不符合，继续查询下一条；因为前面没有符合要求的记录，因此下一条记录过来后，其ROWNUM还是为1，如此循环，就不会产生结果。上述查询可以通过子查询来替代：
SQL> select count(*)
2 from
3 (select BL_REF_CDE, rownum rn from css_bl_view)
4 where rn > 1;
COUNT(*)
----------
361927
我们可以通过以下方式来实现对ROWNUM的>、=的查询：
查询ROWNUM=5的数据：
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#
查询ROWNUM > 25的数据：
SQL> select * from t_test4
2 minus
3 select * from t_test4
4 where rownum
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时，只有当Order By的字段是主键时，查询结果才会先排序再计算ROWNUM，下面OBJECT_ID是表T_TEST1的主键字段：
SQL> select object_id,object_name from t_test1
2 where rownum
3 order by object_id;
OBJECT_ID OBJECT_NAME
---------- ------------------------------
2 C_OBJ#
3 I_OBJ#
4 TAB$
5 CLU$
6 C_TS#
但是，对非主键字段OBJECT_NAME进行排序时，结果就混乱了：
SQL> select object_id,object_name from t_test1
2 where rownum
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
COUNT(*)
----------
21645
出现这种混乱的原因是：Oracle先按物理存储位置（rowid）顺序取出满足rownum条件的记录，即物理位置上的前5条数据，然后在对这些数据按照Order By的字段进行排序，而不是我们所期望的先排序、再取特定记录数。
如果需要对非主键字段排序再去前n条数据，我们可以以以下方式实现：
SQL> select object_id,object_name
2 from (select object_id,object_name from t_test1
3 order by object_name)
4 where rownum
OBJECT_ID OBJECT_NAME
---------- ------------------------------
35489 /1000e8d1_LinkedHashMapValueIt
35490 /1000e8d1_LinkedHashMapValueIt
21801 /1005bd30_LnkdConstant
21802 /1005bd30_LnkdConstant
17205 /10076b23_OraCustomDatumClosur
3.3 排序分页
当对存在重复值的字段排序后再分页 输出，我们很容易会陷入到另外一个“陷阱”。
请看以下例子，我们希望对T_TEST1的OWNER字段排序后，以每页 输出10个结果的方式分页 输出：
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
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
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.
仔细比较结果，你会发现“AFWSERVER”、“AFWADAPTERLOOKUP”在两次分页结果中都出现了。但是OBJECT_NAME在每个OWNER中的值是唯一的，说明这个 输出结果是错误的，我们又陷入了一个“陷阱”。这是怎么回事呢，请先看下上述语句的查询计划：
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
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)
看下这个“陷阱”是怎么形成的。从查询计划上，我们可以注意到，对于子查询
select a.*, rownum as rn from
(select owner, object_name from t_test1 order by owner) a
where rownum
优化器采用了“SORT (ORDER BY STOPKEY)”。
“SORT (ORDER BY STOPKEY)”不需要对所有数据进行排序，而是只要找出结果集中的按特定顺序的最前N条记录，一旦找出了这N条记录，就无需再对剩下的数据进行排序，而直接返回结果。这种算法我们可以视为是“快速排序”算法的变种。快速排序算法的基本思想是：先将数据分2组集合，保证第一集合中的每个数据都大于第二个集合中每个数据，然后再按这个原则对每个集合进行递归分组，直到集合的单位最小。在进行“SORT (ORDER BY STOPKEY)”时，首先找出N条数据（这些数据并没有做排序）放在第一组，保证第一组的数据都大于第二组的数据，然后只对第一组数据进行递归。
可以看到，基于这样的算法基础上，如果N的数值不同，数据的分组也不同（如N=20时，第一次分组比例为12:8，然后继续递归；当N=10时，第一次分组比例为3:7 … …），这样，在数据的排序字段值都相等时， 输出结果的顺序就会因为N值不同而不同。
知道原因后，我们可以通过以下几种方法来避免这个“陷阱”。
1、让查询计划避免“SORT (ORDER BY STOPKEY)”，采用“SORT (ORDER BY)”，使数据排序不受ROWNUM的影响。但这样会使所有数据都做排序：
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
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
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
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、在排序时，加上一个或多个字段（如主键字段、ROWID），使排序结果具有唯一性：
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
5 where rn >= 1;
OWNER OBJECT_NAME
------------------------------ ------------------------------
AFWOWNER AFWADAPTER
AFWOWNER AFWADAPTERFQN_PK
AFWOWNER AFWADAPTERCONFIGURATION
AFWOWNER AFWADAPTERCONFIGURATION_PK
AFWOWNER &nbs
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