使用DataPump的Sample参数来进行数据抽样

数据泵Datapump是Oracle推出的一项非常方便的数据逻辑备份还原工具。同传统的Exp/Imp相比，Datapump无论在性能、功能上都提供了很多优势进步。在实际工作中，大数据抽样是一个经常出现的业务需求。对一个海量数据表，也许抽取有代表性的数据量就可以完成相应工作。

在select语句中，我们的确可以使用sample语句去实现对应的一些功能。借助Datapump，我们也可以实现采样方式的数据表备份处理。

1、环境说明

我们选择Oracle 11gR2进行测试实验，具体版本为11.2.0.4。

SQL> select * from v$version;

BANNER

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

Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

PL/SQL Release 11.2.0.4.0 - Production

CORE    11.2.0.4.0    Production

TNS for Linux: Version 11.2.0.4.0 - Production

NLSRTL Version 11.2.0.4.0 – Production

创建数据表SM_TEST。

SQL> create table scott.sm_test as select * from dba_objects;

Table created

SQL> select count(*) from scott.sm_test;

  COUNT(*)

----------

133391

2、测试实验

对Expdp的Sample参数，官方的解释是很少的。

SAMPLE

Percentage of data to be exported.

Datapump是可以工作在三个模式下，全库模式、Schema模式和表模式。针对三种模式，我们都可以使用Sample参数来进行设置。下面通过最简单的比例值指定来设置采样比例。

[oracle@localhost ~]$ expdp \"/ as sysdba\" dumpfile=TEST_1.dmp tables=scott.sm_test sample=50

Export: Release 11.2.0.4.0 - Production on Mon Nov 23 13:50:02 2015

Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.

Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

With the Partitioning, OLAP, Data Mining and Real Application Testing options

Starting "SYS"."SYS_EXPORT_TABLE_01":  "/******** AS SYSDBA" dumpfile=TEST_1.dmp tables=scott.sm_test sample=50 

Estimate in progress using BLOCKS method...

Processing object type TABLE_EXPORT/TABLE/TABLE_DATA

Total estimation using BLOCKS method: 8 MB

Processing object type TABLE_EXPORT/TABLE/TABLE

. . exported "SCOTT"."SM_TEST"                          6.402 MB  66587 rows

Master table "SYS"."SYS_EXPORT_TABLE_01" successfully loaded/unloaded

******************************************************************************

Dump file set for SYS.SYS_EXPORT_TABLE_01 is:

  /u01/app/admin/sicsdb/dpdump/TEST_1.dmp

Job "SYS"."SYS_EXPORT_TABLE_01" successfully completed at Mon Nov 23 13:50:11 2015 elapsed 0 00:00:07

指定导出的数据表名称的表模式中，我们通过sample的百分比参数，来确定特定表的内容。从导出数据6万左右的情况，正好是13万总数据的50%，是合理的结果。

如果运行在schema工作模式下，直接指定比例份额，结果如何呢？

SQL> select count(*) from scott.emp;

  COUNT(*)

----------

        14

[oracle@localhost ~]$ expdp \"/ as sysdba\" dumpfile=TEST_2.dmp schemas=scott sample=50

Export: Release 11.2.0.4.0 - Production on Mon Nov 23 13:51:31 2015

Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.

Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

With the Partitioning, OLAP, Data Mining and Real Application Testing options

Starting "SYS"."SYS_EXPORT_SCHEMA_01":  "/******** AS SYSDBA" dumpfile=TEST_2.dmp schemas=scott sample=50 

Estimate in progress using BLOCKS method...

Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA

Total estimation using BLOCKS method: 8.093 MB

Processing object type SCHEMA_EXPORT/USER

Processing object type SCHEMA_EXPORT/SYSTEM_GRANT

Processing object type SCHEMA_EXPORT/ROLE_GRANT

Processing object type SCHEMA_EXPORT/DEFAULT_ROLE

Processing object type SCHEMA_EXPORT/PRE_SCHEMA/PROCACT_SCHEMA

Processing object type SCHEMA_EXPORT/TABLE/TABLE

Processing object type SCHEMA_EXPORT/TABLE/GRANT/OWNER_GRANT/OBJECT_GRANT

Processing object type SCHEMA_EXPORT/TABLE/INDEX/INDEX

Processing object type SCHEMA_EXPORT/TABLE/CONSTRAINT/CONSTRAINT

Processing object type SCHEMA_EXPORT/TABLE/INDEX/STATISTICS/INDEX_STATISTICS

Processing object type SCHEMA_EXPORT/VIEW/VIEW

Processing object type SCHEMA_EXPORT/TABLE/CONSTRAINT/REF_CONSTRAINT

Processing object type SCHEMA_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS

. . exported "SCOTT"."SM_TEST"                          6.406 MB  66611 rows

. . exported "SCOTT"."DEPT"                              5.859 KB      1 rows

. . exported "SCOTT"."EMP"                              8.359 KB      9 rows

. . exported "SCOTT"."SALGRADE"                          5.835 KB      3 rows

. . exported "SCOTT"."BONUS"                                0 KB      0 rows

Master table "SYS"."SYS_EXPORT_SCHEMA_01" successfully loaded/unloaded

******************************************************************************

Dump file set for SYS.SYS_EXPORT_SCHEMA_01 is:

  /u01/app/admin/sicsdb/dpdump/TEST_2.dmp

Job "SYS"."SYS_EXPORT_SCHEMA_01" successfully completed at Mon Nov 23 13:51:48 2015 elapsed 0 00:00:16

采样比例在Schema下所有数据表范围内生效，所有数据表基本上以50%的比例进行采样处理。

最后，如果我们导出多张数据表，只对某些数据表进行采样处理，那么如何指定sample参数呢？

[oracle@localhost ~]$ expdp \"/ as sysdba\" dumpfile=TEST_3.dmp schemas=scott sample=scott.sm_test:30

Export: Release 11.2.0.4.0 - Production on Mon Nov 23 13:52:50 2015

Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.

Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

With the Partitioning, OLAP, Data Mining and Real Application Testing options

Starting "SYS"."SYS_EXPORT_SCHEMA_01":  "/******** AS SYSDBA" dumpfile=TEST_3.dmp schemas=scott sample=scott.sm_test:30 

Estimate in progress using BLOCKS method...

Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA

Total estimation using BLOCKS method: 4.987 MB

Processing object type SCHEMA_EXPORT/USER

Processing object type SCHEMA_EXPORT/SYSTEM_GRANT

Processing object type SCHEMA_EXPORT/ROLE_GRANT

Processing object type SCHEMA_EXPORT/DEFAULT_ROLE

Processing object type SCHEMA_EXPORT/PRE_SCHEMA/PROCACT_SCHEMA

Processing object type SCHEMA_EXPORT/TABLE/TABLE

Processing object type SCHEMA_EXPORT/TABLE/GRANT/OWNER_GRANT/OBJECT_GRANT

Processing object type SCHEMA_EXPORT/TABLE/INDEX/INDEX

Processing object type SCHEMA_EXPORT/TABLE/CONSTRAINT/CONSTRAINT

Processing object type SCHEMA_EXPORT/TABLE/INDEX/STATISTICS/INDEX_STATISTICS

Processing object type SCHEMA_EXPORT/VIEW/VIEW

Processing object type SCHEMA_EXPORT/TABLE/CONSTRAINT/REF_CONSTRAINT

Processing object type SCHEMA_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS

. . exported "SCOTT"."SM_TEST"                          3.861 MB  40146 rows

. . exported "SCOTT"."DEPT"                              5.960 KB      5 rows

. . exported "SCOTT"."EMP"                              8.562 KB      14 rows

. . exported "SCOTT"."SALGRADE"                          5.859 KB      5 rows

. . exported "SCOTT"."BONUS"                                0 KB      0 rows

Master table "SYS"."SYS_EXPORT_SCHEMA_01" successfully loaded/unloaded

******************************************************************************

Dump file set for SYS.SYS_EXPORT_SCHEMA_01 is:

  /u01/app/admin/sicsdb/dpdump/TEST_3.dmp

Job "SYS"."SYS_EXPORT_SCHEMA_01" successfully completed at Mon Nov 23 13:53:00 2015 elapsed 0 00:00:09

从上面实验中，如果只是针对特定数据表进行采样处理，需要通过数据表名:采样比例方式来指定。

3、结论

根据笔者的猜测，sample参数在expdp中主要是传导到select语句的sample语句来实现。