如何从结果集中获得随机结果

王朝other·作者佚名  2008-05-31
窄屏简体版  字體: |||超大  

从Oracle8i开始Oracle提供采样表扫描特性。

Oracle访问数据的基本方法有:

1.全表扫描

2.采样表扫描

全表扫描(Full table Scan)

全表扫描返回表中所有的记录。

执行全表扫描,Oracle读表中的所有记录,考查每一行是否满足WHERE条件。Oracle顺序的读分配给该表的每一个数据块,这样全表扫描能够受益于多块读.

每个数据块Oracle只读一次.

采样表扫描(sample table scan)

采样表扫描返回表中随机采样数据。

这种访问方式需要在FROM语句中包含SAMPLE选项或者SAMPLE BLOCK选项.

SAMPLE选项:

当按行采样来执行一个采样表扫描时,Oracle从表中读取特定百分比的记录,并判定是否满足WHERE子句以返回结果。

SAMPLE BLOCK选项:

使用此选项时,Oracle读取特定百分比的BLOCK,考查结果集是否满足WHERE条件以返回满足条件的纪录.

Sample_Percent:

Sample_Percent是一个数字,定义结果集中包含记录占总记录数量的百分比。

Sample值应该在[0.000001,99.999999]之间。

1.使用SAMPLE选项

SQL> select * from employee SAMPLE(30);

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO

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

7369 SMITH CLERK 7902 17-DEC-80 800 20

7788 SCOTT ANALYST 7566 19-APR-87 3000 20

7839 KING PRESIDENT 17-NOV-81 5000 10

Execution Plan

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

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=25 Bytes=2175)

1 0 TABLE Access (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=25 Bytes=2175)

Statistics

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

0 recursive calls

0 db block gets

5 consistent gets

0 physical reads

0 redo size

880 bytes sent via SQL*Net to client

503 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

3 rows processed

SQL> select * from employee SAMPLE(20);

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO

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

7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30

7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30

Execution Plan

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

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=16 Bytes=1392)

1 0 TABLE ACCESS (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=16 Bytes=1392)

Statistics

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

0 recursive calls

0 db block gets

5 consistent gets

0 physical reads

0 redo size

839 bytes sent via SQL*Net to client

503 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

2 rows processed

2.使用SAMPLE BLOCK选项

SQL> SELECT * FROM employee SAMPLE BLOCK (50);

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO

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

7369 SMITH CLERK 7902 17-DEC-80 800 20

7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30

7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30

7566 JONES MANAGER 7839 02-APR-81 2975 20

7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30

7698 BLAKE MANAGER 7839 01-MAY-81 2850 30

7782 CLARK MANAGER 7839 09-JUN-81 2450 10

7788 SCOTT ANALYST 7566 19-APR-87 3000 20

7839 KING PRESIDENT 17-NOV-81 5000 10

7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30

10 rows selected.

Execution Plan

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

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=41 Bytes=3567)

1 0 TABLE ACCESS (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=41 Bytes=3567)

Statistics

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

0 recursive calls

0 db block gets

4 consistent gets

0 physical reads

0 redo size

1162 bytes sent via SQL*Net to client

503 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

10 rows processed

SQL>

3.采样前n条记录的查询

也可以使用dbms_random包实现

SQL> select * from (

2 select * from employee

3 order by dbms_random.value )

4 where rownum <= 4;

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO

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

7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30

7839 KING PRESIDENT 17-NOV-81 5000 10

7369 SMITH CLERK 7902 17-DEC-80 800 20

7788 SCOTT ANALYST 7566 19-APR-87 3000 20

Execution Plan

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

0 SELECT STATEMENT Optimizer=CHOOSE

1 0 COUNT (STOPKEY)

2 1 VIEW

3 2 SORT (ORDER BY STOPKEY)

4 3 TABLE ACCESS (FULL) OF 'EMPLOYEE'

Statistics

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

0 recursive calls

0 db block gets

3 consistent gets

0 physical reads

0 redo size

927 bytes sent via SQL*Net to client

503 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

1 sorts (memory)

0 sorts (disk)

4 rows processed

对比一下SAMPLE选项

SQL> SELECT * FROM employee SAMPLE (40);

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO

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

7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30

7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30

7698 BLAKE MANAGER 7839 01-MAY-81 2850 30

7839 KING PRESIDENT 17-NOV-81 5000 10

7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30

Execution Plan

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

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=33 Bytes=2871)

1 0 TABLE ACCESS (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=33 Bytes=2871)

Statistics

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

0 recursive calls

0 db block gets

5 consistent gets

0 physical reads

0 redo size

961 bytes sent via SQL*Net to client

503 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

5 rows processed

SQL>

主要注重以下几点:

1.sample只对单表生效,不能用于表连接和远程表

2.sample会使SQL自动使用CBO

本文作者:

eygle,Oracle技术关注者,来自中国最大的Oracle技术论坛itpub.

www.eygle.com是作者的个人站点.你可通过Guoqiang.Gai@gmail.com来联系作者.欢迎技术探讨交流以及链接交换.

原文出处:

http://www.eygle.com/sql/How.To.Get.Random.Output.Of.Record.Set.htm

 
 
 
免责声明:本文为网络用户发布,其观点仅代表作者个人观点,与本站无关,本站仅提供信息存储服务。文中陈述内容未经本站证实,其真实性、完整性、及时性本站不作任何保证或承诺,请读者仅作参考,并请自行核实相关内容。
 
 
© 2005- 王朝網路 版權所有 導航