Oracle下建立dblink时的权限问题

[oracle@oracle000 ~]$ sqlplus gao/gao
[uniread] Loaded history (10 lines)

SQL*Plus: Release 11.2.0.3.0 Production on 木 10月 17 09:30:27 2013

Copyright (c) 1982, 2011, Oracle.  All rights reserved.



Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
に接続されました。

SQL> create database link gaolink connect to gao identified by gao using 'mydb'; 
create database link gaolink connect to gao identified by gao using 'mydb'; 
create database link gaolink connect to gao identified by gao using 'mydb'
                     *
行1でエラーが発生しました。:
ORA-01031: 権限が不足しています。


SQL> 

===============================================================================


[oracle@oracle000 ‾]$ sqlplus / as sysdba
[uniread] Loaded history (13 lines)

SQL*Plus: Release 11.2.0.3.0 Production on 木 10月 17 14:17:58 2013

Copyright (c) 1982, 2011, Oracle.  All rights reserved.



Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
に接続されました。
SQL> create database link gaolink connect to gao identified by gao using 'mydb'; 

データベース・リンクが作成されました。

SQL>