Working with Oracle databases by using the oracle_fdw extension - Amazon Relational Database Service
Turning on the oracle_fdw extensionExample: Using a foreign server linked to an Amazon RDS for Oracle databaseWorking with encryption in transitUnderstanding the pg_user_mappings view and permissions

Working with Oracle databases by using the oracle_fdw extension

To access an Oracle database from your RDS for PostgreSQL DB instance you can install and use the oracle_fdw extension. This extension is a foreign data wrapper for Oracle databases. To learn more about this extension, see the oracle_fdw documentation.

The oracle_fdw extension is supported on RDS for PostgreSQL 12.7, 13.3, and higher versions.

Turning on the oracle_fdw extension

To use the oracle_fdw extension, perform the following procedure.

To turn on the oracle_fdw extension

  • Run the following command using an account that has rds_superuser permissions.

    CREATE EXTENSION oracle_fdw;

Example: Using a foreign server linked to an Amazon RDS for Oracle database

The following example shows the use of a foreign server linked to an Amazon RDS for Oracle database.

To create a foreign server linked to an RDS for Oracle database

  1. Note the following on the RDS for Oracle DB instance:

    • Endpoint

    • Port

    • Database name

  2. Create a foreign server.

    test=> CREATE SERVER oradb FOREIGN DATA WRAPPER oracle_fdw OPTIONS (dbserver '//endpoint:port/DB_name');
CREATE SERVER

  3. Grant usage to a user who doesn't have rds_superuser privileges, for example user1.

    test=> GRANT USAGE ON FOREIGN SERVER oradb TO user1;
GRANT

  4. Connect as user1, and create a mapping to an Oracle user.

    test=> CREATE USER MAPPING FOR user1 SERVER oradb OPTIONS (user 'oracleuser', password 'mypassword');
CREATE USER MAPPING

  5. Create a foreign table linked to an Oracle table.

    test=> CREATE FOREIGN TABLE mytab (a int) SERVER oradb OPTIONS (table 'MYTABLE');
CREATE FOREIGN TABLE

  6. Query the foreign table.

    test=>  SELECT * FROM mytab;
a
---
1
(1 row)

If the query reports the following error, check your security group and access control list (ACL) to make sure that both instances can communicate.

ERROR: connection for foreign table "mytab" cannot be established
DETAIL: ORA-12170: TNS:Connect timeout occurred

Working with encryption in transit

PostgreSQL-to-Oracle encryption in transit is based on a combination of client and server configuration parameters. For an example using Oracle 21c, see About the Values for Negotiating Encryption and Integrity in the Oracle documentation. The client used for oracle_fdw on Amazon RDS is configured with ACCEPTED, meaning that the encryption depends on the Oracle database server configuration.

If your database is on RDS for Oracle, see Oracle native network encryption to configure the encryption.

Understanding the pg_user_mappings view and permissions

The PostgreSQL catalog pg_user_mapping stores the mapping from an RDS for PostgreSQL user to the user on a foreign data (remote) server. Access to the catalog is restricted, but you use the pg_user_mappings view to see the mappings. In the following, you can find an example that shows how permissions apply with an example Oracle database, but this information applies more generally to any foreign data wrapper.

In the following output, you can find roles and permissions mapped to three different example users. Users rdssu1 and rdssu2 are members of the rds_superuser role, and user1 isn't. The example uses the psql metacommand \du to list existing roles.

test=>  \du
                                                               List of roles
    Role name    |                         Attributes                         |                          Member of
-----------------+------------------------------------------------------------+-------------------------------------------------------------
 rdssu1          |                                                            | {rds_superuser}
 rdssu2          |                                                            | {rds_superuser}
 user1           |                                                            | {}

All users, including users that have rds_superuser privileges, are allowed to view their own user mappings (umoptions) in the pg_user_mappings table. As shown in the following example, when rdssu1 tries to obtain all user mappings, an error is raised even though rdssu1rds_superuser privileges:

test=> SELECT * FROM pg_user_mapping;
ERROR: permission denied for table pg_user_mapping

Following are some examples.

test=> SET SESSION AUTHORIZATION rdssu1;
SET
test=> SELECT * FROM pg_user_mappings;
 umid  | srvid | srvname | umuser | usename    |            umoptions
-------+-------+---------+--------+------------+----------------------------------
 16414 | 16411 | oradb   |  16412 | user1      |
 16423 | 16411 | oradb   |  16421 | rdssu1     | {user=oracleuser,password=mypwd}
 16424 | 16411 | oradb   |  16422 | rdssu2     |
 (3 rows)

test=> SET SESSION AUTHORIZATION rdssu2;
SET
test=> SELECT * FROM pg_user_mappings;
 umid  | srvid | srvname | umuser | usename    |            umoptions
-------+-------+---------+--------+------------+----------------------------------
 16414 | 16411 | oradb   |  16412 | user1      |
 16423 | 16411 | oradb   |  16421 | rdssu1     |
 16424 | 16411 | oradb   |  16422 | rdssu2     | {user=oracleuser,password=mypwd}
 (3 rows)

test=> SET SESSION AUTHORIZATION user1;
SET
test=> SELECT * FROM pg_user_mappings;
 umid  | srvid | srvname | umuser | usename    |           umoptions
-------+-------+---------+--------+------------+--------------------------------
 16414 | 16411 | oradb   |  16412 | user1      | {user=oracleuser,password=mypwd}
 16423 | 16411 | oradb   |  16421 | rdssu1     |
 16424 | 16411 | oradb   |  16422 | rdssu2     |
 (3 rows)

Because of implementation differences between information_schema._pg_user_mappings and pg_catalog.pg_user_mappings, a manually created rds_superuser requires additional permissions to view passwords in pg_catalog.pg_user_mappings.

No additional permissions are required for an rds_superuser to view passwords in information_schema._pg_user_mappings.

Users who don't have the rds_superuser role can view passwords in pg_user_mappings only under the following conditions:

  • The current user is the user being mapped and owns the server or holds the USAGE privilege on it.

  • The current user is the server owner and the mapping is for PUBLIC.