大对象对数据库导出/导入性能的影响

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Export and Import of Table with LOB Columns (like CLOB and BLOB) has Slow Performance

Doc ID:Note:281461.1Type:PROBLEM Last Revision Date:02-SEP-2004Status:PUBLISHED

The information in this article applies to:
- Oracle Server - Enterprise Edition - Version: 8.1.7.0 to 10.2.0.0
- Oracle Server - Personal Edition - Version: 8.1.7.0 to 10.2.0.0
- Oracle Server - Standard Edition - Version: 8.1.7.0 to 10.2.0.0
This problem can occur on any platform.

SYMPTOMS
--------

An export or import of a table with a Large Object (LOB) column,
has a slower performance than an export or import of a table without LOB
columns:

-- create two tables: TESTTAB1 with a VARCHAR2 column, and TESTTAB2 with a
-- CLOB column:
connect / as sysdba
create table scott.testtab1 (nr number, txt varchar2(2000));
create table scott.testtab2 (nr number, txt clob);

-- populate both tables with the same 500,000 rows:
declare
x varchar2(50);
begin
for i in 1..500000 loop
   x := 'This is a line with the number: ' || i;
   insert into scott.testtab1 values(i,x);
   insert into scott.testtab2 values(i,x);
   commit;
end loop;
end;
/

-- export both tables:
% exp system/manager file=exp_testtab1.dmp tables=scott.testtab1 direct=y
% exp system/manager file=exp_testtab1a.dmp tables=scott.testtab1
% exp system/manager file=exp_testtab2.dmp tables=scott.testtab2

               No CLOB      No CLOB    With CLOB
                DIRECT CONVENTIONAL       column
          ------------ ------------ ------------
8.1.7.4.0        0:13         0:20         7:49
9.2.0.4.0        0:14         0:18         7:37
9.2.0.5.0        0:12         0:15         7:03
10.1.0.2.0        0:16         0:31         7:15

Note:
These performance results should not be considered as a benchmark
of the performance between different Oracle versions, as the test
databases were located on different machines with different hardware,
and the databases had a different parameter configuration.
The main objective of these results is to give an indication of the
difference in the time that is needed to export a table with a LOB
column, and a table without a LOB column.

-- cleanup after running the test:
connect / as sysdba
drop table scott.testtab1;
drop table scott.testtab2;

CHANGES
-------

You recently created tables that have Large Object (LOB) columns.

CAUSE
-----

This is expected behavior. The rows of a table with a LOB column are fetched
one row at a time. Also note that rows in tables that contain objects and
LOBs will be exported using conventional path, even if direct path was
specified.
Also during import, the rows of tables containing LOB columns are inserted
individually.

FIX
---

Although the performance of the export cannot be improved directly, possible
alternative solutions are:

1. If not required, do not use LOB columns.
or:
2. Use Transport Tablespace export instead of full/user/table level export.
or:
3. Upgrade to Oracle10g and use Export DataPump and Import DataPump.

我的测试：

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Tue Jan 29 10:26:06 CST 2008

Export: Release 9.2.0.6.0 - Production on Tue Jan 29 10:26:06 2008

Connected to: Oracle9i Enterprise Edition Release 9.2.0.6.0 - 64bit Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.6.0 - Production
Export done in AL32UTF8 character set and AL16UTF16 NCHAR character set

About to export specified tables via Direct Path ...
Current user changed to SCOTT
. . exporting table TESTTAB1 500000 rows exported
Export terminated successfully without warnings.
Tue Jan 29 10:26:11 CST 2008

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50万条记录，无lob对象，direct路径导出，耗时5秒

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Export: Release 9.2.0.6.0 - Production on Tue Jan 29 10:26:11 2008

About to export specified tables via Conventional Path ...
Current user changed to SCOTT
. . exporting table TESTTAB1 500000 rows exported
Export terminated successfully without warnings.
Tue Jan 29 10:26:18 CST 2008

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50万条记录，无lob对象，Conventional路径导出，耗时7秒

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Export: Release 9.2.0.6.0 - Production on Tue Jan 29 10:26:18 2008

About to export specified tables via Conventional Path ...
Current user changed to SCOTT
. . exporting table TESTTAB2 500000 rows exported
Export terminated successfully without warnings.
Tue Jan 29 10:29:19 CST 2008

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50万条记录，含lob对象，只能Conventional路径导出，耗时181秒