这篇测试一下通过DBLink 访问含有Blob字段表的方法。 关于DBLINK 和 Lob 的说明，参考如下链接：
http://blog.csdn.net/tianlesoftware/article/details/4698642
删除Dblink 报错ORA-02024: database link not found 的解决方法
http://blog.csdn.net/tianlesoftware/article/details/6160196
http://blog.csdn.net/tianlesoftware/article/details/5070981
http://blog.csdn.net/tianlesoftware/article/details/6905406
一．模拟问题
1.1 在实例1上操作：
创建含有blob 的测试表：
/* Formatted on 2012/6/19 10:18:05 (QP5 v5.185.11230.41888) */
CREATE TABLE lob1
(
line NUMBER primary key,
text CLOB
);
INSERT INTO lob1
SELECT distinct line, text FROM all_source where rownum<500;
SELECT segment_name,
segment_type,
tablespace_name,
SUM (bytes) / 1024 / 1024 || 'M' AS "SIZE"
FROM user_segments
WHERE segment_name = 'LOB1'
GROUP BY segment_name, segment_type, tablespace_name;
LOB 表的信息如下：
SEGMENT_NAME SEGMENT_TYPE TABLESPACE_NAME SIZE --------------- ------------------ ------------------------------ ---------- LOB1 TABLE SYSTEM 9M SQL> set wrap off; SQL> select * from lob1 where rownum=1; LINE TEXT ---------- --------------------------------------------------------------------- 1 package STANDARD AUTHID CURRENT_USER is -- care
1.2 在实例2上操作
创建DBLINK：
CREATE PUBLIC DATABASE LINK lob_link CONNECT TO dave IDENTIFIED BY dave
USING '(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.2.222)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = dave)
)
)';
SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0 Production
TNS for Linux: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production
SQL> select count(*) from lob1@lob_link;
COUNT(*)
----------
58228
这边查询总记录数正常。但是当我们查询具体LOB字段里的内容的时候，就会报错，如下：
SQL> select * from lob1@lob_link where rownum=1; ERROR: ORA-22992: cannot use LOB locators selected from remote tables no rows selected [oracle@localhost ~]$ oerr ora 22992 22992, 00000, "cannot use LOB locators selected from remote tables" // *Cause: A remote LOB column cannot be referenced. // *Action: Remove references to LOBs in remote tables.
二．MOS 上的相关说明
2.1 ORA-22992 When TryingTo Select Lob Columns Over A Database Link [ID 119897.1]
在官网的这篇文章中，基于Oracle 8i的进行了说明，报错的原因是因为不支持这个特性。
(A)You cannot actually select a lob column (i.e. CLOB column) from a table
using remote database link. Thisis not a supported feature.
(B)Also, these are the INVALID operations on a LOB column:
--以下操作也不被支持。
1. SELECT lobcol from table1@remote_site;
2. INSERT INTO lobtable select type1.lobattr from table1@remote_site;
3. SELECT dbms_lob.getlength(lobcol) from table1@remote_site;
2.2 ORA-22992 in SQLUsing DBLINK and NVL2 function in 10g. [ID 427239.1]
在Oracle 9i/10g版本中，存在Bug.5185187 ，因此在dblink中使用NVL2 函数时，就会出现ORA-22992的错误。
该Bug 在Oracle 11gR2中已经修复。
也可以使用如下方法，来间接的解决这个问题：
Original SQL:
select nvl2('a', 'b','c' )from dual@test;
Modified SQL:
selectto_char(nvl2('a','b','c')) from dual@test;
2.3 SELECT with a LOB andDBLink Returns an ORA-22992: Cannot Use LOB Locators Selected from Remotetables [ID 1234893.1]
在这边文章里也提到了原因：because the use ofDBLinks and LOBs via the SELECT from PL/SQL is not supported.
在这篇文章里也提供了一些解决方法：
The followingNotes discuss this issue, and give code snippets as example to work-around therestriction using the INTO clause. Keep in mind, if working with large LOBsizes, it may be best to implement the work-around of using a MaterializedView.
--如果LOB字段很大，最好使用物化视图来解决这个问题。
Note 459557.1 (ORA-1406: FetchedColumn Value was Truncated When Selecting Remote Column into Local BLOBVariable) discusses using a Materialized View in the scenarios where the BLOBwill be larger than 2Meg.
Note 119897.1 (ORA-22992 When Trying To SelectLob Columns Over A Database Link) states this feature is not supported but thenat the end of the Note it states starting with 10.2 there is a work-around ofusing LONG and LONG RAW types.
Note 436707.1 (Ora-22992 has a workaround in10gR2) illustrates a work-around (using LONG RAW) as mentioned was possible inthe previous note.
Note 796282.1 (Workaround for ORA-22992)provides 3 work-arounds.
三．解决方法
3.1 将接收的lob 存入char（CBob）或者raw（BLob）本地变量
MOS文章：
Ora-22992 has a workaround in 10gR2 [ID 436707.1]
Workaround for ORA-22992 [ID 796282.1]
Starting from 10g the select from alob object through a database link is supportedby receiving the LOB objects into variables defined as CHAR orRAW.
--从Oracle10g开始，dblink 的select可以被本地的char或raw 类型变量接收。
（1） Selecting a CLOB objectthrough the dblink:
set serveroutput on
declare
my_ad varchar(2000);
BEGIN
SELECT obj INTO my_ad FROM test@torem where id=1;
dbms_output.put_line(my_ad);
END;
/
我这里的测试环境是CBLOB，示例如下：
SQL> declare
2 my_ad varchar(2000);
3 BEGIN
4 SELECT text INTO my_ad FROMlob1@lob_link where rownum=1;
5 dbms_output.put_line(my_ad);
6 END;
/
7
package STANDARD AUTHIDCURRENT_USER is -- careful onthis line;
SED edit occurs!
PL/SQL procedure successfully completed.
--这里输出了我们CLOB里的内容。
（2）Selecting a BLOB object through thedblink:
declare
my_ad raw(50);
BEGIN
SELECT obj INTO my_ad FROM test2@torem where id=1;
END;
/
3.2 使用物化视图
MOS 文章： ORA-1406: Fetched Column Value was Truncated When Selecting RemoteColumn into Local BLOB Variable [ID 459557.1]
在3.1 中，我们可以使用本地变量来接收dblink中LOB的值，但是这个方法是有限制的。当我们接收的LOB 字段值大于32KB值，就会报如下错误：
ORA-01406 :fetched column value was truncated
"If the LOBis greater than 32KB - 1 in size, then PL/SQL will raise a truncation error andthe contents of the buffer are undefined."
This means thatwe are not able to retrieve BLOBs columns greater than 32KB - 1 in size througha database link.
The restrictionstill holds good for 11g from Oracledocumention SecureFiles and LargeObjects Developer's Guide
3.2.1 测试LOB字段长度
--在远程端创建过程：
create or replace procedure get_bloblengthas
blob_loc blob;
blob_length number;
begin
select <lob_column>
into blob_loc
from <table_name>
where name ='<critira>';
blob_length := dbms_lob.getlength(blob_loc);
dbms_output.put_line('Length of the Column : ' || to_char(blob_length));
end;
--在本地调用过程：
exec get_bloblength@repb
如果返回值大于32KB，我们就可以使用物化视图了。
3.2.2 创建物化视图
SQL> create materializedview lobmv1 as select line,text from lob1@lob_link;
--查询物化视图：CLOB正常显示
SQL> set wrap off;
SQL> select * from lobmv where rownum<5;
LINE TEXT
-------------------------------------------------------------------------------
5
8 type NUMBER is NUMBER_BASE;
9 subtype FLOAT is NUMBER; --NUMBER(126)
11 subtype "DOUBLEPRECISION" is FLOAT;
3.3 将含有LOB字段的表复制到本地的全局临时表
我们在本地创建一张和dblink远程端相同的全局临时表，然后在查询临时表：
--创建临时表：
create global temporary table lob2
(
line number primary key,
text clob
)
on commit delete rows;
--插入数据：
SQL> insert into lob2 select line,text from lob1@lob_link;
499 rows created.
SQL> select * from lob2 where rownum<5;
LINE TEXT
---------- ---------------------------------------------------------------------
5
8 type NUMBER is NUMBER_BASE;
9 subtype FLOAT is NUMBER; -- NUMBER(126)
11 subtype "DOUBLE PRECISION" is FLOAT;
SQL> commit;
Commit complete.
SQL> select * from lob2 where rownum<5;
no rows selected
--提交之后数据就被删除了，这个是临时表的属性。
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