Oracle Data Base (DB) Link Introduction

Business Need:

Database Links are used to connect from one database to a remote database without exposing/repeating connection details to the end user.

Oracle DB 12g is used for these SQLs. But the SQLs will be applicable most of the recent versions.

DB Link Creation:

Here is the command to create DB Link:

CREATE DATABASE LINK DBLINK_NAME

CONNECT TO REMOTE_SCHEMA IDENTIFIED BY REMOTE_PWD

USING 'REMOTE_TNSSTRING';

To use the DB Link, the REMOTE_TNSSTRING needs to be defined in the DB Server’s TNSNAMES.ORA file, you are connecting to. Otherwise this will give TNS Error.

 

DB Link Creation without TNS Definition:

Here is command to create DB Link to a remote database whose TNS String is not defined in the server:

CREATE DATABASE LINK DBLINK_NAME

    CONNECT TO REMOTE_SCHEMA IDENTIFIED BY REMOTE_PWD

    USING '(DESCRIPTION=

                (ADDRESS=(PROTOCOL=TCP)(HOST=<mymachine.mydomain.com>)(PORT=<1521>))

                (CONNECT_DATA=(SERVICE_NAME=<service or sid name>))

            )';

<mymachine.mydomain.com> à Domain Name or IP Address of Remote Database

<1521> à Port Number of Remote Database

<service or sid name> à Service Name of Remote Database

 

DB Link Usage:

Tables within DB Link can be accessed using the format <table name> @ <DB Link name>, just like any local table (Eg: USERS@DEV).

If frequently used, create a local synonym:

CREATE SYNONYM RTUSERS FOR USERS@DBLINK_NAME;

So the below SQLs work in the same way:

SELECT * FROM USERS@DBLINK_NAME;

SELECT * FROM RTUSERS;

  

Other Notes/Considerations:

Few points to add before using DB Links: 

1. Direct DB Links to Production might cause data security policy violation. Be careful over sensitive data exposure.
2. Query Optimization is poor across DB Links. A few steps might help improve performance over DB Links:
3. If a single table is accessed frequently, use a local temp table and fill the required data in the local table and use. DB Link to be used only in the first selection (to store data in temp table)
4. Use query-in-query format to choose remote data, if possible

Eg: Only 1 order type will be present in remote table. Use “(SELECT COL1, COL2 FROM ORDERS WHERE ORDER_TYPE = 21) X” instead of Order Type in the main query.

  

Keywords:

Oracle, DB Link, Database Link, Remote Database, TNSNAMES, Query, SQL, SQLPLUS