2）Oracle官方文檔中關(guān)于ESTIMATE參數(shù)的描述
參考鏈接：http://download.oracle.com/docs/cd/B19306_01/server.102/b14215/dp_export.htm#sthref105
引用內(nèi)容如下：
ESTIMATE

Default: BLOCKS

Purpose

Specifies the method that
Export will use to estimate how
much disk space each table in the export job will consume (in bytes).
The estimate is printed in the log file and displayed on the client\\\’s
standard output device. The estimate is for table row data only; it does
not include metadata.

Syntax and Description

ESTIMATE={BLOCKS | STATISTICS}

BLOCKS – The estimate is calculated by multiplying the
number of database blocks used by the source objects, times the
appropriate block sizes.

STATISTICS – The estimate is calculated using statistics
for each table. For this method to be as accurate as possible, all
tables should have been analyzed recently.

Restrictions

If the Data Pump export job involves compressed tables, the default
size estimation given for the compressed table is inaccurate when ESTIMATE=BLOCKS
is used. This is because the size estimate does not reflect that the
data was stored in a compressed form. To get a more accurate size
estimate for compressed tables, use ESTIMATE=STATISTICS.

Example

The following example shows a use of the ESTIMATE
parameter in which the estimate is calculated using statistics for the employees
table:

> expdp hr/hr TABLES=employees ESTIMATE=STATISTICS DIRECTORY=dpump_dir1
 DUMPFILE=estimate_stat.dmp

2.顯式指定ESTIMATE參數(shù)為BLOCKS
ora10g@secDB /expdp$ expdp sec/sec directory=expdp_dir dumpfile=sec.dmp ESTIMATE=BLOCKS

Export: Release 10.2.0.3.0 – 64bit Production on Saturday, 10 April, 2010 6:23:26

Copyright (c) 2003, 2005, Oracle.  All rights reserved.

Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 – 64bit Production
With the Partitioning, Oracle Label Security, OLAP and Data Mining Scoring Engine options
Starting SEC.SYS_EXPORT_SCHEMA_01:  sec/******** directory=expdp_dir dumpfile=sec.dmp ESTIMATE=BLOCKS
Estimate in progress using BLOCKS method…
Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA
.  estimated SEC.TEST_SEC                                2 MB
.  estimated SEC.TEST_SEC1                               2 MB
.  estimated SEC.TEST_SEC2                               2 MB
Total estimation using BLOCKS method: 6 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/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/TABLE/COMMENT
. . exported SEC.TEST_SEC                            1.018 MB   11716 rows
. . exported SEC.TEST_SEC1                           1.018 MB   11717 rows
. . exported SEC.TEST_SEC2                           1.018 MB   11718 rows
Master table SEC.SYS_EXPORT_SCHEMA_01 successfully loaded/unloaded
******************************************************************************
Dump file set for SEC.SYS_EXPORT_SCHEMA_01 is:
  /expdp/sec.dmp
Job SEC.SYS_EXPORT_SCHEMA_01 successfully completed at 06:23:37

此時預(yù)估單位是按照數(shù)據(jù)庫塊進行統(tǒng)計的，相比最終導(dǎo)出的大小來說不是很準(zhǔn)確，不過有一定的參考意義。

3.指定ESTIMATE參數(shù)為STATISTICS
ora10g@secDB /expdp$ expdp sec/sec directory=expdp_dir dumpfile=sec.dmp ESTIMATE=STATISTICS

Export: Release 10.2.0.3.0 – 64bit Production on Saturday, 10 April, 2010 6:22:02

Copyright (c) 2003, 2005, Oracle.  All rights reserved.

Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 – 64bit Production
With the Partitioning, Oracle Label Security, OLAP and Data Mining Scoring Engine options
Starting SEC.SYS_EXPORT_SCHEMA_01:  sec/******** directory=expdp_dir dumpfile=sec.dmp ESTIMATE=STATISTICS
Estimate in progress using STATISTICS method…
Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA
.  estimated SEC.TEST_SEC                            9.558 KB
.  estimated SEC.TEST_SEC1                           9.558 KB
.  estimated SEC.TEST_SEC2                           9.558 KB
Total estimation using STATISTICS method: 28.67 KB
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/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/TABLE/COMMENT
. . exported SEC.TEST_SEC                            1.018 MB   11716 rows
. . exported SEC.TEST_SEC1                           1.018 MB   11717 rows
. . exported SEC.TEST_SEC2                           1.018 MB   11718 rows
Master table SEC.SYS_EXPORT_SCHEMA_01 successfully loaded/unloaded
******************************************************************************
Dump file set for SEC.SYS_EXPORT_SCHEMA_01 is:
  /expdp/sec.dmp
Job SEC.SYS_EXPORT_SCHEMA_01 successfully completed at 06:22:14

此時的統(tǒng)計結(jié)果比較離譜，是何原因？
真實的原因是在使用ESTIMATE=STATISTICS參數(shù)預(yù)估時，數(shù)據(jù)來源是表的分析數(shù)據(jù)，如果表未曾分析過或分析數(shù)據(jù)不準(zhǔn)確就會導(dǎo)致此處的數(shù)據(jù)失真。

4.在數(shù)據(jù)庫中對sec用戶做一下分析，然后再完成一次數(shù)據(jù)導(dǎo)出
1）對sec用戶進行分析
sys@ora10g> exec dbms_stats.gather_schema_stats(OWNNAME=>\\\’SEC\\\’,ESTIMATE_PERCENT=>10,DEGREE=>4,cascade=>true);

PL/SQL procedure successfully completed.

2）再次導(dǎo)出sec用戶下的數(shù)據(jù)
ora10g@secDB /expdp$ expdp sec/sec directory=expdp_dir dumpfile=sec.dmp ESTIMATE=STATISTICS

Export: Release 10.2.0.3.0 – 64bit Production on Saturday, 10 April, 2010 6:30:20

Copyright (c) 2003, 2005, Oracle.  All rights reserved.

Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 – 64bit Production
With the Partitioning, Oracle Label Security, OLAP and Data Mining Scoring Engine options
Starting SEC.SYS_EXPORT_SCHEMA_01:  sec/******** directory=expdp_dir dumpfile=sec.dmp ESTIMATE=STATISTICS
Estimate in progress using STATISTICS method…
Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA
.  estimated SEC.TEST_SEC2                           993.6 KB
.  estimated SEC.TEST_SEC1                           993.6 KB
.  estimated SEC.TEST_SEC                            993.5 KB
Total estimation using STATISTICS method: 2.910 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/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/TABLE/COMMENT
Processing object type SCHEMA_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS
. . exported SEC.TEST_SEC2                           1.018 MB   11718 rows
. . exported SEC.TEST_SEC1                           1.018 MB   11717 rows
. . exported SEC.TEST_SEC                            1.018 MB   11716 rows
Master table SEC.SYS_EXPORT_SCHEMA_01 successfully loaded/unloaded
******************************************************************************
Dump file set for SEC.SYS_EXPORT_SCHEMA_01 is:
  /expdp/sec.dmp
Job SEC.SYS_EXPORT_SCHEMA_01 successfully completed at 06:30:35

OK，此時的預(yù)估數(shù)據(jù)與最后的生成數(shù)據(jù)大小較為接近。

5.不指定ESTIMATE參數(shù)
ora10g@secDB /expdp$ expdp sec/sec directory=expdp_dir dumpfile=sec.dmp

Export: Release 10.2.0.3.0 – 64bit Production on Saturday, 10 April, 2010 7:26:54

Copyright (c) 2003, 2005, Oracle.  All rights reserved.

Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 – 64bit Production
With the Partitioning, Oracle Label Security, OLAP and Data Mining Scoring Engine options
Starting SEC.SYS_EXPORT_SCHEMA_01:  sec/******** directory=expdp_dir dumpfile=sec.dmp
Estimate in progress using BLOCKS method…
Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA
Total estimation using BLOCKS method: 6 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/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/TABLE/COMMENT
Processing object type SCHEMA_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS
. . exported SEC.TEST_SEC                            1.018 MB   11716 rows
. . exported SEC.TEST_SEC1                           1.018 MB   11717 rows
. . exported SEC.TEST_SEC2                           1.018 MB   11718 rows
Master table SEC.SYS_EXPORT_SCHEMA_01 successfully loaded/unloaded
******************************************************************************
Dump file set for SEC.SYS_EXPORT_SCHEMA_01 is:
  /expdp/sec.dmp
Job SEC.SYS_EXPORT_SCHEMA_01 successfully completed at 07:27:05

在不指定ESTIMATE參數(shù)時，默認(rèn)會使用BLOCKS進行估算，不過，具體的數(shù)據(jù)庫對象進估算被省略，僅有以下信息提示：
Total estimation using BLOCKS method: 6 MB

6.小結(jié)
雖然ESTIMATE參數(shù)提供給我們一種預(yù)估大小的功能，不過在使用EXPDP進行數(shù)據(jù)備份時，還是盡量不要使用，畢竟所有額外的操作都是有代價的。

Good luck.

secooler
10.04.10

— The End —

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