Working with the supported foreign data wrappers for Amazon RDS for PostgreSQL - Amazon Relational Database Service

Working with the supported foreign data wrappers for Amazon RDS for PostgreSQL

A foreign data wrapper (FDW) is a specific type of extension that provides access to external data. For example, the oracle_fdw extension allows your RDS for PostgreSQL DB cluster to work with Oracle databases. As another example, by using the PostgreSQL native postgres_fdw extension you can access data stored in PostgreSQL DB instances external to your RDS for PostgreSQL DB instance.

Following, you can find information about several supported PostgreSQL foreign data wrappers.

Using the log_fdw extension to access the DB log using SQL

RDS for PostgreSQL supports the log_fdw extension, which you can use to access your database engine log using a SQL interface. The log_fdw extension provides two functions that make it easy to create foreign tables for database logs:

  • list_postgres_log_files – Lists the files in the database log directory and the file size in bytes.

  • create_foreign_table_for_log_file(table_name text, server_name text, log_file_name text) – Builds a foreign table for the specified file in the current database.

All functions created by log_fdw are owned by rds_superuser. Members of the rds_superuser role can grant access to these functions to other database users.

By default, the log files are generated by Amazon RDS in stderr (standard error) format, as specified in log_destination parameter. There are only two options for this parameter, stderr and csvlog (comma-separated values, CSV). If you add the csvlog option to the parameter, Amazon RDS generates both stderr and csvlog logs. This can affect the storage capacity on your instance, so you need to be aware of the other parameters that affect log handling. For more information, see Setting the log destination.

One benefit of generating csvlog logs is that the log_fdw extension lets you build foreign tables with the data neatly split into several columns. To do this, your instance needs to be associated with a custom DB parameter group so that you can change the setting for log_destination. For more information about how to do so, see Working with parameters on your RDS for PostgreSQL DB instance.

The following example assumes that the log_destination parameter includes cvslog.

To use the log_fdw extension

  1. Get the log_fdw extension.

    postgres=> CREATE EXTENSION log_fdw; CREATE EXTENSION
  2. Create the log server as a foreign data wrapper.

    postgres=> CREATE SERVER log_server FOREIGN DATA WRAPPER log_fdw; CREATE SERVER
  3. Select all from a list of log files.

    postgres=> SELECT * FROM list_postgres_log_files() ORDER BY 1;

    A sample response is as follows.

    file_name | file_size_bytes ------------------------------+----------------- postgresql.log.2016-08-09-22.csv | 1111 postgresql.log.2016-08-09-23.csv | 1172 postgresql.log.2016-08-10-00.csv | 1744 postgresql.log.2016-08-10-01.csv | 1102 (4 rows)
  4. Create a table with a single 'log_entry' column for the selected file.

    postgres=> SELECT create_foreign_table_for_log_file('my_postgres_error_log', 'log_server', 'postgresql.log.2016-08-09-22.csv');

    The response provides no detail other than that the table now exists.

    ----------------------------------- (1 row)
  5. Select a sample of the log file. The following code retrieves the log time and error message description.

    postgres=> SELECT log_time, message FROM my_postgres_error_log ORDER BY 1;

    A sample response is as follows.

    log_time | message ----------------------------------+--------------------------------------------------------------------------- Tue Aug 09 15:45:18.172 2016 PDT | ending log output to stderr Tue Aug 09 15:45:18.175 2016 PDT | database system was interrupted; last known up at 2016-08-09 22:43:34 UTC Tue Aug 09 15:45:18.223 2016 PDT | checkpoint record is at 0/90002E0 Tue Aug 09 15:45:18.223 2016 PDT | redo record is at 0/90002A8; shutdown FALSE Tue Aug 09 15:45:18.223 2016 PDT | next transaction ID: 0/1879; next OID: 24578 Tue Aug 09 15:45:18.223 2016 PDT | next MultiXactId: 1; next MultiXactOffset: 0 Tue Aug 09 15:45:18.223 2016 PDT | oldest unfrozen transaction ID: 1822, in database 1 (7 rows)

Using the postgres_fdw extension to access external data

You can access data in a table on a remote database server with the postgres_fdw extension. If you set up a remote connection from your PostgreSQL DB instance, access is also available to your read replica.

To use postgres_fdw to access a remote database server

  1. Install the postgres_fdw extension.

    CREATE EXTENSION postgres_fdw;
  2. Create a foreign data server using CREATE SERVER.

    CREATE SERVER foreign_server FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host 'xxx.xx.xxx.xx', port '5432', dbname 'foreign_db');
  3. Create a user mapping to identify the role to be used on the remote server.

    CREATE USER MAPPING FOR local_user SERVER foreign_server OPTIONS (user 'foreign_user', password 'password');
  4. Create a table that maps to the table on the remote server.

    CREATE FOREIGN TABLE foreign_table ( id integer NOT NULL, data text) SERVER foreign_server OPTIONS (schema_name 'some_schema', table_name 'some_table');

Working with MySQL databases by using the mysql_fdw extension

To access a MySQL-compatible database from your RDS for PostgreSQL DB instance, you can install and use the mysql_fdw extension. This foreign data wrapper lets you work with RDS for MySQL, Aurora MySQL, MariaDB, and other MySQL-compatible databases. The connection from RDS for PostgreSQL to the MySQL database is encrypted on a best-effort basis, depending on the client and server configurations. However, you can enforce encryption if you like. For more information, see Using encryption in transit with the extension.

The mysql_fdw extension is supported on Amazon RDS for PostgreSQL version 14.2, 13.6, and higher releases. It supports selects, inserts, updates, and deletes from an RDS for PostgreSQL DB to tables on a MySQL-compatible database instance.

Setting up your RDS for PostgreSQL DB to use the mysql_fdw extension

Setting up the mysql_fdw extension on your RDS for PostgreSQL DB instance involves loading the extension in your DB instance and then creating the connection point to the MySQL DB instance. For that task, you need to have the following details about the MySQL DB instance:

  • Hostname or endpoint. For an RDS for MySQL DB instance, you can find the endpoint by using the Console. Choose the Connectivity & security tab and look in the "Endpoint and port" section.

  • Port number. The default port number for MySQL is 3306.

  • Name of the database. The DB identifier.

You also need to provide access on the security group or the access control list (ACL) for the MySQL port, 3306. Both the RDS for PostgreSQL DB instance and the RDS for MySQL DB instance need access to port 3306. If access isn't configured correctly, when you try to connect to MySQL-compatible table you see an error message similar to the following:

ERROR: failed to connect to MySQL: Can't connect to MySQL server on 'hostname.aws-region.rds.amazonaws.com:3306' (110)

In the following procedure, you (as the rds_superuser account) create the foreign server. You then grant access to the foreign server to specific users. These users then create their own mappings to the appropriate MySQL user accounts to work with the MySQL DB instance.

To use mysql_fdw to access a MySQL database server

  1. Connect to your PostgreSQL DB instance using an account that has the rds_superuser role. If you accepted the defaults when you created your RDS for PostgreSQL DB instance, the user name is postgres, and you can connect using the psql command line tool as follows:

    psql --host=your-DB-instance.aws-region.rds.amazonaws.com --port=5432 --username=postgres –-password
  2. Install the mysql_fdw extension as follows:

    postgres=> CREATE EXTENSION mysql_fdw; CREATE EXTENSION

After the extension is installed on your RDS for PostgreSQL DB instance, you set up the foreign server that provides the connection to a MySQL database.

To create the foreign server

Perform these tasks on the RDS for PostgreSQL DB instance. The steps assume that you're connected as a user with rds_superuser privileges, such as postgres.

  1. Create a foreign server in the RDS for PostgreSQL DB instance:

    postgres=> CREATE SERVER mysql-db FOREIGN DATA WRAPPER mysql_fdw OPTIONS (host 'db-name.111122223333.aws-region.rds.amazonaws.com', port '3306'); CREATE SERVER
  2. Grant the appropriate users access to the foreign server. These should be non-administrator users, that is, users without the rds_superuser role.

    postgres=> GRANT USAGE ON FOREIGN SERVER mysql-db to user1; GRANT

PostgreSQL users create and manage their own connections to the MySQL database through the foreign server.

Example: Working with an RDS for MySQL database from RDS for PostgreSQL

Suppose that you have a simple table on an RDS for MySQL DB instance. Your RDS for PostgreSQL users want to query (SELECT), INSERT, UPDATE, and DELETE items on that table. Assume that the mysql_fdw extension was created on your RDS for PostgreSQL DB instance, as detailed in the preceding procedure. After you connect to the RDS for PostgreSQL DB instance as a user that has rds_superuser privileges, you can proceed with the following steps.

  1. On the RDS for PostgreSQL DB instance, create a foreign server:

    test=> CREATE SERVER mysqldb FOREIGN DATA WRAPPER mysql_fdw OPTIONS (host 'your-DB.aws-region.rds.amazonaws.com', port '3306'); CREATE SERVER
  2. Grant usage to a user who doesn't have rds_superuser permissions, for example, user1:

    test=> GRANT USAGE ON FOREIGN SERVER mysqldb TO user1; GRANT
  3. Connect as user1, and then create a mapping to the MySQL user:

    test=> CREATE USER MAPPING FOR user1 SERVER mysqldb OPTIONS (username 'myuser', password 'mypassword'); CREATE USER MAPPING
  4. Create a foreign table linked to the MySQL table:

    test=> CREATE FOREIGN TABLE mytab (a int, b text) SERVER mysqldb OPTIONS (dbname 'test', table_name ''); CREATE FOREIGN TABLE
  5. Run a simple query against the foreign table:

    test=> SELECT * FROM mytab; a | b ---+------- 1 | apple (1 row)
  6. You can add, change, and remove data from the MySQL table. For example:

    test=> INSERT INTO mytab values (2, 'mango'); INSERT 0 1

    Run the SELECT query again to see the results:

    test=> SELECT * FROM mytab ORDER BY 1; a | b ---+------- 1 | apple 2 | mango (2 rows)

Using encryption in transit with the extension

The connection to MySQL from RDS for PostgreSQL uses encryption in transit (TLS/SSL) by default. However, the connection falls back to non-encrypted when the client and server configuration differ. You can enforce encryption for all outgoing connections by specifying the REQUIRE SSL option on the RDS for MySQL user accounts. This same approach also works for MariaDB and Aurora MySQL user accounts.

For MySQL user accounts configured to REQUIRE SSL, the connection attempt fails if a secure connection can't be established.

To enforce encryption for existing MySQL database user accounts, you can use the ALTER USER command. The syntax varies, depending on the MySQL version, as shown in the following table. For more information, see ALTER USER in MySQL Reference Manual.

MySQL 5.7, MySQL 8 MySQL 5.6

ALTER USER 'user'@'%' REQUIRE SSL;

GRANT USAGE ON *.* to 'user'@'%' REQUIRE SSL;

For more information about the mysql_fdw extension, see the mysql_fdw documentation.

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.

Working with SQL Server databases by using the tds_fdw extension

You can use the PostgreSQL tds_fdw extension to access databases that support the tabular data stream (TDS) protocol, such as Sybase and Microsoft SQL Server databases. This foreign data wrapper lets you connect from your RDS for PostgreSQL DB instance to databases that use the TDS protocol, including Amazon RDS for Microsoft SQL Server. For more information, see tds-fdw/tds_fdw documentation on GitHub.

The tds_fdw extension is supported on Amazon RDS for PostgreSQL version 14.2, 13.6, and higher releases.

Setting up your RDS for PostgreSQL DB to use the tds_fdw extension

In the following procedures, you can find an example of setting up and using the tds_fdw with an RDS for PostgreSQL DB instance. Before you can connect to a SQL Server database using tds_fdw, you need to get the following details for the instance:

  • Hostname or endpoint. For an RDS for SQL Server DB instance, you can find the endpoint by using the Console. Choose the Connectivity & security tab and look in the "Endpoint and port" section.

  • Port number. The default port number for Microsoft SQL Server is 1433.

  • Name of the database. The DB identifier.

You also need to provide access on the security group or the access control list (ACL) for the SQL Server port, 1433. Both the RDS for PostgreSQL DB instance and the RDS for SQL Server DB instance need access to port 1433. If access isn't configured correctly, when you try to query the Microsoft SQL Server you see the following error message:

ERROR: DB-Library error: DB #: 20009, DB Msg: Unable to connect: Adaptive Server is unavailable or does not exist (mssql2019.aws-region.rds.amazonaws.com), OS #: 0, OS Msg: Success, Level: 9

To use tds_fdw to connect to a SQL Server database

  1. Connect to your PostgreSQL DB instance using an account that has the rds_superuser role:

    psql --host=your-DB-instance.aws-region.rds.amazonaws.com --port=5432 --username=test –-password
  2. Install the tds_fdw extension:

    test=> CREATE EXTENSION tds_fdw; CREATE EXTENSION

After the extension is installed on your RDS for PostgreSQL DB instance, you set up the foreign server.

To create the foreign server

Perform these tasks on the RDS for PostgreSQL DB instance using an account that has rds_superuser privileges.

  1. Create a foreign server in the RDS for PostgreSQL DB instance:

    test=> CREATE SERVER sqlserverdb FOREIGN DATA WRAPPER tds_fdw OPTIONS (servername 'mssql2019.aws-region.rds.amazonaws.com, port '1433', database 'tds_fdw_testing'); CREATE SERVER
  2. Grant permissions to a user who doesn't have rds_superuser role privileges, for example, user1:

    test=> GRANT USAGE ON FOREIGN SERVER sqlserverdb TO user1;
  3. Connect as user1 and create a mapping to a SQL Server user:

    test=> CREATE USER MAPPING FOR user1 SERVER sqlserverdb OPTIONS (username 'sqlserveruser', password 'password'); CREATE USER MAPPING
  4. Create a foreign table linked to a SQL Server table:

    test=> CREATE FOREIGN TABLE mytab (a int) SERVER sqlserverdb OPTIONS (table 'MYTABLE'); CREATE FOREIGN TABLE
  5. Query the foreign table:

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

Using encryption in transit for the connection

The connection from RDS for PostgreSQL to SQL Server uses encryption in transit (TLS/SSL) depending on the SQL Server database configuration. If the SQL Server isn't configured for encryption, the RDS for PostgreSQL client making the request to the SQL Server database falls back to unencrypted.

You can enforce encryption for the connection to RDS for SQL Server DB instances by setting the rds.force_ssl parameter. To learn how, see Forcing connections to your DB instance to use SSL. For more information about SSL/TLS configuration for RDS for SQL Server, see Using SSL with a Microsoft SQL Server DB instance.