Oracle分页查询格式8

Oracle10g的新功能GROUP BY STOPKEY，使得Oracle10g解决了上一篇文章中提到的GROUP BY操作无法分页的问题。

在10g以前，Oracle的GROUP BY操作必须完全执行完，才能将结果返回给用户。但是Oracle10g增加了GROUP BY STOPKEY执行路径，使得用户在执行GROUP BY操作时，可以根据STOPKEY随时中止正在运行的操作。

这使得标准分页函数对于GROUP BY操作重新发挥了作用。

SQL> CREATE TABLE T AS SELECT ROWNUM ID, A.* FROM DBA_OBJECTS A;

表已创建。

SQL> CREATE INDEX IND_T_CREATED ON T (CREATED);

索引已创建。

SQL> ALTER TABLE T MODIFY CREATED NOT NULL;

表已更改。

SQL> ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';

会话已更改。

SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS(USER, 'T')

PL/SQL 过程已成功完成。

SQL> SET AUTOT ON

SQL> SET TIMING ON

SQL> SELECT * 

2 FROM 

3 (

4 SELECT ROWNUM RN, A.* 

5 FROM 

6 (

7 SELECT CREATED, COUNT(*) 

8 FROM T 

9 GROUP BY CREATED

10 ) A

11 WHERE ROWNUM < 20

12 ) 

13 WHERE RN >= 10;

RN CREATED COUNT(*)

---------- ------------------- ----------

10 2005-12-19 17:07:57 50

11 2005-12-19 17:07:58 36

12 2005-12-19 17:08:24 10

13 2005-12-19 17:08:25 49

14 2005-12-19 17:08:26 66

15 2005-12-19 17:08:27 62

16 2005-12-19 17:08:28 81

17 2005-12-19 17:08:29 82

18 2005-12-19 17:08:33 1

19 2005-12-19 17:08:35 3

已选择10行。

已用时间: 00: 00: 00.04

执行计划

----------------------------------------------------------

Plan hash value: 3639065582

------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|

-------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 19 | 665 | 1 (0)|

|* 1 | VIEW | | 19 | 665 | 1 (0)| 

|* 2 | COUNT STOPKEY | | | | |

| 3 | VIEW | | 973 | 21406 | 1 (0)| 

|* 4 | SORT GROUP BY STOPKEY| | 973 | 7784 | 1 (0)| 

| 5 | INDEX FULL SCAN | IND_T_CREATED | 984 | 7872 | 1 (0)| 

------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("RN">=10)

2 - filter(ROWNUM<20)

4 - filter(ROWNUM<20)

统计信息

----------------------------------------------------------

1 recursive calls

0 db block gets

67 consistent gets

0 physical reads

0 redo size

730 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

1 sorts (memory)

0 sorts (disk)

10 rows processed

SQL> SELECT * 

2 FROM 

3 (

4 SELECT ROWNUM RN, A.* 

5 FROM 

6 (

7 SELECT CREATED, COUNT(*) 

8 FROM T 

9 GROUP BY CREATED

10 ) A

11 ) 

12 WHERE RN >= 10

13 AND RN < 20;

RN CREATED COUNT(*)

---------- ------------------- ----------

10 2005-12-19 17:09:27 34

11 2005-12-19 17:09:31 29

12 2005-12-19 17:09:40 29

13 2005-12-19 17:09:58 11

14 2005-12-19 17:10:06 6

15 2005-12-19 17:10:12 48

16 2005-12-19 17:10:20 24

17 2005-12-19 17:10:37 8

18 2005-12-19 17:10:40 2

19 2005-12-19 17:10:49 2

已选择10行。

已用时间: 00: 00: 00.06

执行计划

----------------------------------------------------------

Plan hash value: 4036621539

------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|

-----------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 973 | 34055 | 14 (36)|

|* 1 | VIEW | | 973 | 34055 | 14 (36)|

| 2 | COUNT | | | | |

| 3 | VIEW | | 973 | 21406 | 14 (36)|

| 4 | HASH GROUP BY | | 973 | 7784 | 14 (36)|

| 5 | INDEX FAST FULL SCAN| IND_T_CREATED | 50359 | 393K| 10 (10)|

------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("RN">=10 AND "RN"<20)

统计信息

----------------------------------------------------------

1 recursive calls

0 db block gets

73 consistent gets

0 physical reads

0 redo size

724 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)

10 rows processed

通过上面的执行计划可以看到，无论使用标准分页函数，还是使用其他方式，Oracle采用的都是10g特有的执行计划GROUP BY STOPKEY和HASH GROUP BY，与9i的GROUP BY相比较，二者的执行效率都很高，不过利用了标准分页模式，Oracle可以将STOPKEY推入到查询最内层，使得Oracle在扫描的开始阶段就将结果集限制住。从查询的逻辑读和执行时间上也可以看出来，GROUP
BY STOPKEY的执行效率更高一些。

从这一点上看，Oracle10g的新功能确实使得查询效率得到提高。