DB link的迁移（dblink中的密码未知）

    不知DBA有时候是否会遇到这样的尬尴局面，做某些Schema的迁移的时候，由于用到Public的db link，然而由于不知道db link中目标端账号的密码，因此无法在新环境重新创建DB link。

本次实验的思路是将视图dba_db_links的基表link$迁移到出来并替换到新环境。因为用户的password密文保存在link$基表中。

----首先交代两边环境，都是Linux上11.2.0.3的版本，在10.2.0.4上也测试过。

一边数据库seven，一边数据库ginna。假设在不知道scott的密码情况下将seven端的名为test的db link迁移至ginna端。

源端操作：

首先还在源端创建一个public 的db link 名为test。

seven >create public database link test connect to scott identified by tiger using 'ginna';

Database link created.

seven >select * from global_name@test;

GLOBAL_NAME
---------------------
GINNA

seven >select * from dba_db_links;

OWNER                          DB_LINK                        USERNAME                       HOST                           CREATED
------------------------------ ------------------------------ ------------------------------ ------------------------------ ---------
PUBLIC                         TEST                           SCOTT                          ginna                          06-MAR-15

创建中间表trans1，CTAS基表sys.link$。

seven >create table trans1 as select * from sys.link$;

Table created.

seven >select count(*) from trans1;

  COUNT(*)
----------
         1

然后在seven用户下创建第二个中间表trans2,（seven 有dba权限）。

seven >conn seven/oracle
Connected.
seven >create table trans2 as select * from sys.link$;   -----当然不能直接CTAS基表的。
create table trans2 as select * from sys.link$
                                         *
ERROR at line 1:
ORA-01031: insufficient privileges

seven >create table trans2 as select * from sys.trans1;

Table created.
seven >select count(*) from trans2;

  COUNT(*)
----------
         1
seven >

OK，中间表trans2创建成功。

目标端操作：

在目标端创建名为test2的Public db link。

ginna >show user
USER is "SYS"
ginna >select * from dba_db_links;

no rows selected

ginna >create public database link test2 connect to seven identified by oracle using 'seven';

Database link created.

ginna >select * from global_name@test2;

GLOBAL_NAME
----------------------
SEVEN

ginna >set line 200
ginna >col db_link for a30
ginna >col host for a30
ginna >select * from dba_db_links;

OWNER                          DB_LINK                        USERNAME                       HOST                           CREATED
------------------------------ ------------------------------ ------------------------------ ------------------------------ ---------
PUBLIC                         TEST2                          SEVEN                          seven                          06-MAR-15

通过test2的db link将seven端的trans2表CTAS到表trans3。

ginna >create table trans3 as select * from trans2@test2;

Table created.

ginna >select count(*) from trans3;

  COUNT(*)
----------
         1

truncate掉基表sys.link$,为了安全起见也可以先备份下基表，并将表trans3插入到基表link$中。

ginna >truncate table sys.link$;

Table truncated.

ginna >insert into sys.link$ select * from trans3;

1 row created.

ginna >commit;

Commit complete.

现在再查一下dba_db_links就可以发现名为test的db link已经成功迁移在ginna端。

ginna >select * from dba_db_links;

OWNER                          DB_LINK                        USERNAME                       HOST                           CREATED
------------------------------ ------------------------------ ------------------------------ ------------------------------ ---------
PUBLIC                         TEST                           SCOTT                          ginna                          06-MAR-15

测试db link 的有效性：

ginna >select * from global_name@test;

GLOBAL_NAME
------------------
GINNA

OK，现在还有一个问题，之前名为test2的db link现在还是否存在？是否能重建？

显然是不能的：

ginna >create public database link test2 connect to seven identified by oracle using 'seven';
create public database link test2 connect to seven identified by oracle using 'seven'
                            *
ERROR at line 1:
ORA-02011: duplicate database link name

前提需要刷新下shared pool。之后再重建就可以了。

ginna >alter system flush shared_pool;

System altered.

ginna >create public database link test2 connect to seven identified by oracle using 'seven';

Database link created.

ginna >select * from dba_db_links;

OWNER                          DB_LINK                        USERNAME                       HOST                           CREATED
------------------------------ ------------------------------ ------------------------------ ------------------------------ ---------
PUBLIC                         TEST                           SCOTT                          ginna                          06-MAR-15
PUBLIC                         TEST2                          SEVEN                          seven                          06-MAR-15

ginna >

当然如果不flush shared pool的话直接删除db link则会遭遇ora-600内部报错：

SQL> drop public database link test2;
drop public database link test2
                          *
ERROR at line 1:
ORA-00603: ORACLE server session terminated by fatal error
ORA-00600: internal error code, arguments: [16500], [kqdDBLinkUpdate], [3], [0], [TEST2], [], [], [], [], [], [], []
Process ID: 11165
Session ID: 152 Serial number: 359


SQL> 
SQL> create public database link test2 connect to seven identified by oracle using 'seven';
ERROR:
ORA-03114: not connected to ORACLE


SQL> conn / as sysdba
Connected.
SQL> create public database link test2 connect to seven identified by oracle using 'seven';

Database link created.

SQL> drop public database link test2;

Database link dropped.

SQL> 

当然我们也可以不创建test2这个db link，主要就是将trans1这张中间表迁移至ginna库就可以了，这有很多方法，可以直接使用exp/imp或者数据泵都可以。条条大道通罗马，方法有很多种，主要就是将seven端的sys.link$基表迁移至ginna端并替换掉ginna端的sys.link$。