Oracle 排序规则

<pre name="code" class="html">SQL> select * from t1 where id>=1 and id<=20;

ID	   A1	      A2	 A3
---------- ---------- ---------- ----------
1	   1	      1 	 a1
2	   2	      2 	 a2
3	   3	      3 	 a3
4	   4	      4 	 a4
5	   5	      5 	 a5
6	   6	      6 	 a6
7	   7	      7 	 a7
8	   8	      8 	 a8
9	   9	      9 	 a9
10	   10	      10	 a10
11	   11	      11	 a11
12	   12	      12	 a12
13	   13	      13	 a13
14	   14	      14	 a14
15	   15	      15	 a15
16	   16	      16	 a16
17	   17	      17	 a17
18	   18	      18	 a18
19	   19	      19	 a19
20	   20	      20	 a20

已选择20行。

SQL>  explain plan for select id from t1 where rownum<20;

已解释。

SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3581814200

---------------------------------------------------------------------------------
| Id  | Operation	 | Name 	| Rows	| Bytes | Cost (%CPU)| Time	|
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |		|    19 |   209 |     1   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY	 |		|	|	|	     |		|
|   2 |   INDEX FULL SCAN| SYS_C0022200 |    19 |   209 |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------

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

1 - filter(ROWNUM<20)

已选择14行。

INDEX FULL SCAN 返回数据有序:

SQL> select id from t1 where rownum<20;

ID
----------
1
10
100
1000
10000
1001
1002
1003
1004
1005
1006
1007
1008
1009
101
1010
1011
1012
1013

已选择19行。

不走索引,随机读:
SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3836375644

---------------------------------------------------------------------------
| Id  | Operation	   | Name | Rows  | Bytes | Cost (%CPU)| Time	  |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	  |    19 |   836 |	2   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY	   |	  |	  |	  |	       |	  |
|   2 |   TABLE ACCESS FULL| T1   |    19 |   836 |	2   (0)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
---------------------------------------------------

1 - filter(ROWNUM<20)

已选择14行。

SQL> select * from t1 where rownum<20;

ID	   A1	      A2	 A3
---------- ---------- ---------- ----------
495	   495	      495	 a495
496	   496	      496	 a496
497	   497	      497	 a497
498	   498	      498	 a498
499	   499	      499	 a499
500	   500	      500	 a500
501	   501	      501	 a501
502	   502	      502	 a502
503	   503	      503	 a503
504	   504	      504	 a504
505	   505	      505	 a505

ID	   A1	      A2	 A3
---------- ---------- ---------- ----------
506	   506	      506	 a506
507	   507	      507	 a507
508	   508	      508	 a508
509	   509	      509	 a509
510	   510	      510	 a510
511	   511	      511	 a511
512	   512	      512	 a512
513	   513	      513	 a513

已选择19行

排序规则:

SQL> select id from t1 where rownum<20;

ID
----------
1
10
100
1000
10000
1001
1002
1003
1004
1005
1006
1007
1008
1009
101
1010
1011
1012
1013

已选择19行。

SQL> select * from ( select id from t1 where rownum<20) order by id;

ID
----------
1
10
100
1000
10000
1001
1002
1003
1004
1005
1006
1007
1008
1009
101
1010
1011
1012
1013

已选择19行。

SQL> select * from (select * from t1 order by id ) where rownum<20;

ID	   A1	      A2	 A3
---------- ---------- ---------- ----------
1	   1	      1 	 a1
10	   10	      10	 a10
100	   100	      100	 a100
1000	   1000       1000	 a1000
10000	   10000      10000	 a10000
1001	   1001       1001	 a1001
1002	   1002       1002	 a1002
1003	   1003       1003	 a1003
1004	   1004       1004	 a1004
1005	   1005       1005	 a1005
1006	   1006       1006	 a1006
1007	   1007       1007	 a1007
1008	   1008       1008	 a1008
1009	   1009       1009	 a1009
101	   101	      101	 a101
1010	   1010       1010	 a1010
1011	   1011       1011	 a1011
1012	   1012       1012	 a1012
1013	   1013       1013	 a1013

已选择19行。

SQL>  select * from (select * from t1 where id<100 order by id) where rownum<20;

ID	   A1	      A2	 A3
---------- ---------- ---------- ----------
1	   1	      1 	 a1
10	   10	      10	 a10
11	   11	      11	 a11
12	   12	      12	 a12
13	   13	      13	 a13
14	   14	      14	 a14
15	   15	      15	 a15
16	   16	      16	 a16
17	   17	      17	 a17
18	   18	      18	 a18
19	   19	      19	 a19
2	   2	      2 	 a2
20	   20	      20	 a20
21	   21	      21	 a21
22	   22	      22	 a22
23	   23	      23	 a23
24	   24	      24	 a24
25	   25	      25	 a25
26	   26	      26	 a26

已选择19行。