There are a number of errors that you can face when working with database links in Oracle.

Below are a list of some of the more common ones and suggestions on how to troubleshoot oracle database link errors .

• Oracle10 Tip
• ORA-02021
• ORA-02080
• ORA-02085
• ORA-12154
• ORA-12224
• ORA-12305

Background

What happens during database link usage ?
Some information to help you troubleshoot oracle database link errors.

SQL> select * from dual@remotedb;

D
-
X

SQL> alter table dept@remotedb add column
     (manager varchar2(30));
alter table dept@remotedb add column
     (manager varchar2(30))
                 *
ERROR at line 1:
ORA-02021: DDL operations are not allowed on
a remote database

SQL> alter session close database link remotedb;
ERROR:
ORA-02080: database link is in use

SQL> commit;

Commit complete.

SQL> alter session close database link remotedb;

Session altered.

SQL> connect scott/tiger
Connected

SQL> create database link remotedb connect to scott
     identified by tiger using 'otherremotedb';

Database link created.

SQL> select * from dual@remotedb;
select * from dual@remotedb
                   *
ERROR at line 1:
ORA-02085 : database link REMOTEDB connects to
            OTHERREMOTEDB

SQL> show parameter global_names

NAME             TYPE        VALUE
---------------- ----------- ----------
global_names     boolean     TRUE

SQL> alter system set global_names=false
     scope=both;

System altered.

SQL> connect scott/tiger

Connected.
SQL> select * from dual@remotedb;

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SQL> select * from global_name;

GLOBAL_NAME
--------------------------------
REMOTEDB

SQL> create database link remotedb
      using 'remotedb';
Database link created.

SQL> select * from dual@remotedb;
ORA-2085 "database link REMOTEDB.WORLD.COM
         connects to REMOTEDB.WORLD.NET"

SQL> drop database link remotedb;
Database link dropped.

SQL> create database link remotedb.world.net
     using 'remotedb';
Database link created.

SQL> select * from dual@remotedb;

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-
X

SQL> connect / as sysdba
Connected

SQL> select * from global_name;

GLOBAL_NAME
------------
REMOTEDB

SQL>alter database rename global_name
    to remotedb.world.com;
Database altered.

SQL> select * from global_name;

GLOBAL_NAME
-------------------
REMOTEB.WORLD.COM

SQL> drop database link remotedb;
Database link dropped.

SQL> create database link remotedb
      using 'remotedb';
Database link created.

SQL> select * from dual@remotedb;

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-
X

Once you have created and made use of a database link in Oracle,
you might want to keep an eye on the number of concurrent open database links
in your database so you can tune the open_links initialization parameter.

Read more here on limiting concurrent open links.

You will need to query v$dblink to see how many links are open in your session:

SQL> select in_transaction, count(*) from v$dblink
     group by in_transaction;

IN_   COUNT(*)
--- ----------
YES          1

Here are some interesting columns and descriptions of v$dblink:

db_link           Db link name
owner_id          Owner name
logged_on         Is the database link currently logged on?
protocol          Dblink's communications protocol
open_cursors      Are there any cursors open for the db link ?
in_transaction    Is the db link part of a transaction which
                  has not been commited or rolled back yet ?
update_sent       Was there an update on the db link ?

To gather information on all database links in your database, query dba_db_links.
You will need dba privileges to see this view, the alternatives are user_db_links and all_db_links.

A user who does not have dba privileges can query all_db_links to see
which db links are available to him.

SQL> select * from dba_db_links;

OWNER      DB_LINK     HOST       CREATED
---------- ----------  ---------- ----------
SCOTT      REMOTEDB    remotedb   23-feb-2008 23:59:05

Some interesting columns:

owner           User who owns the db link,
                will state 'PUBLIC' if it is a public database link.
db_link         Db link name.
username        The username that was specified if it was hardcoded
                during the create statement, null if not specified
                during the create statement.
host            The tnsnames alias specified during the create
                statement.
created         Date and time of link creation.

Recommended reading:

Each session in your database has a limited number of database links that can be opened at the same time.

This number is determined by a database-wide initialization parameter: OPEN_LINKS.

The default value is set to 4.

If it is set to 0, distributed transactions are not allowed.

If you are expecting your transactions to have a maximum of 3 database links open concurrently, set this parameter to 3 or higher.

Do not set it too high, it is better for the application to close database links when no longer in use than to change the parameter to a high number.

OPEN_LINKS cannot be modified in real-time, so you will have to change it in the spfile or pfile and bounce the database.

SQL> alter system set open_links=10 scope=both;
alter system set open_links=0 scope=both
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter
cannot be modified

SQL> alter system set open_links=10 scope=spfile;

System altered.

If you are not sure how many database links are opened up concurrently by your session’s database application, you can query v$dblink.

SQL> select in_transaction, count(*) from v$dblink
     group by in_transaction;

IN_   COUNT(*)
--- ----------
YES          1

Recommended reading:

There are 2 ways you can hide the complexity involved with Oracle database links from your user:
create a synonym or a view.

This will allow your user to access the data she needs without worrying
about the syntax requirements of database links.

Your user may even be unaware of the fact that she is accessing some data in a remote database.

Note:
Database links are notoriously slow, not an optimal solution for applications needing real-time response.
Expect the database link to cause a delay in accessing the remote data.

Here is an example of creating a synonym for scott’s database link.

SQL> create  synonym rem_emp for emp@remotedb;

Synonym created.

SQL> select count(*) from rem_emp;

  COUNT(*)
----------
        14

The other method is to create a view on your remote database’s data.

SQL> create view v_emp as select * from emp@remotedb;

View created.

SQL> select count(*) from v_emp;

  COUNT(*)
----------
        14

Recommended reading:

Dropping a database link from your Oracle database is as easy as doing the following:

drop database link remotedb;
or
drop public database link remotedb;

You will need no other system privilege other than the ‘create database link’ privilege granted directly
to your username or granted via a role to drop your own database link.

It is not possible to drop a database link belonging to another user.

If you try to specify another schema’s database link by qualifying it with a name,
Oracle will just look for a private database link in your schema with a name
which includes the other schema’s name and will not find it.

To drop a public database link, you will need the system privilege ‘drop public database link’.

For example:

SQL> connect / as sysdba
Connected.

SQL> drop database link scott.remotedb;
drop database link scott.remotedb
                   *
ERROR at line 1:
ORA-02024: database link not found

SQL> connect scott/tiger
Connected.
SQL> drop database link scott.remotedb;
drop database link scott.remotedb
                   *
ERROR at line 1:
ORA-02024: database link not found

SQL> drop database link remotedb;

Database link dropped.

Recommended reading:

The syntax is :

select <column list> from <table>@<dblink name>;

For example:

SQL> select * from dept@remotedb;

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        40 OPERATIONS     BOSTON

The database link is opened during the select (or other DML transaction) and remains open for the duration of the session.
After you close a session, the links that were active in the session are automatically closed.

Close a db link

To explicitly close the database link , use the command below:

SQL> alter session close database link remotedb;

Session altered.

Using db links in other DML statements

You can use insert/update/delete statements just as easily with database links

SQL> select * from dept@remotedb;

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        40 OPERATIONS     BOSTON

SQL> insert into dept@remotedb (deptno,dname,loc)
  2  values (50,'MARKETING','BOISE');

1 row created.

SQL> commit;

Commit complete.

SQL> select * from dept@remotedb;

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        40 OPERATIONS     BOSTON
        50 MARKETING      BOISE

SQL> update dept@remotedb set loc = 'LONDON'
     where deptno = 50;

1 row updated.

SQL> commit;

Commit complete.

SQL> select * from dept@remotedb;

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        40 OPERATIONS     BOSTON
        50 MARKETING      LONDON

SQL> delete from dept@remotedb
     where dname = 'MARKETING';

1 row deleted.

SQL> commit;

Commit complete.

SQL> select * from dept@remotedb;

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        40 OPERATIONS     BOSTON

Note that DDL operations are not allowed through a database link:

SQL> alter table dept@remotedb
     add column (manager varchar2(30));

alter table dept@remotedb add column
(manager varchar2(30))
                 *
ERROR at line 1:
ORA-02021: DDL operations are not allowed
           on a remote database

Recommended reading:

CREATE {PUBLIC} DATABASE LINK
{CONNECT TO
IDENTIFIED BY }
USING ‘ ‘;

Privileges needed

When creating an oracle database link, you can decide to create it privately,
for use only by the owner of the database link,
or publicly, for use by all other database users who have the right privileges.

If you do not specify the word ‘PUBLIC’ in the create statement, the database link will be created as a private one by default.
Once a public database link has been created, it will be owned by a special user named ‘PUBLIC’.

The second decision is to create it with a hard-coded username and password, or without access credentials.
If you create a public database link without access credentials, any user accessing it will do so with his own username and password.

Take care when creating a public dblink with a hardcoded username and password, it is a security risk.

Creation

Now that you are ready to create it, you must provide a database link name
(this is what your db link will be called).

One common problem with dblinks is that Oracle will not allow you to create a db link whose name is different from the global name
of the remote database if the local database’s ‘GLOBAL_NAMES’ initialization parameter is set to ‘TRUE’.

Lastly, you must specify a valid tnsnames alias which Oracle will use to connect to the remote database.
To test if your tnsnames.ora alias is valid, try to access it through sql*plus on your local server.

To be able to create a database link, you will need to have the system privilege
‘CREATE DATABASE LINK’
or
‘CREATE PUBLIC DATABASE LINK’
granted to you.

The remote user will need to have at least ‘CREATE SESSION’ privileges.

Examples:
Let’s create a private database link using a hard-coded username and password.

SQL>Create database link remotedb connect to scott identified by tiger using ‘remotedb.world.com’;
Database link created.

The same db link without a username/password:

SQL>create database link remotedb using ‘remotedb.world.com’;
Database link created.

Here’s an example to create a public database link with a username and password:

SQL> create public database link remotedb connect to scott identified by tiger using ‘remotedb.world.com’;
Database link created.

Here’s a public database link without a username and password.

SQL>create public database link remotedb using ‘remotedb.world.com’;
Database link created.

Recommended reading:

The connection can be made to a database hosted on the same machine or remotely on another server.

When you query data on a different database, you are essentially using a distributed query.
It is also possible to execute distributed transactions through database links like insert, update and delete.

The authentication used in a database link is either the link’s owner or a hard-coded username and password, depending on the way the link was created.

A database link in oracle can be either private (for use only by the link’s owner) or public (accessible to any user with the correct privileges).

Connectivity is established via Sql*Net, using an alias defined in the local database server’s tnsnames.ora

Have a look at the tutorials on this site  to find out more about oracle database links :

Dblinks category