Foreign Data Wrappers and DBLink: Bridges for PostgreSQL Cross-Database Communication

Here's a general overview of the steps involved in setting up FDW:

• Install postgres_fdw: This extension usually comes bundled with PostgreSQL, but you might need to enable it manually.
• Define the remote server: Provide details like hostname, port, and database name for the external database you want to connect to.
• Create user mappings: Set up credentials to ensure secure connections between your local and remote databases.
• Create an external table: Define a local representation of the remote table, specifying its schema and columns.
• Write your queries: Compose SQL queries that access data from both local and remote tables using the defined external table name.

It's important to consider that FDW can introduce some performance overhead compared to querying a single database. Additionally, FDW and DBLink don't allow for foreign key relationships between tables residing in different databases.

-- Enable FDW extension (if not already installed)
CREATE EXTENSION IF NOT EXISTS postgres_fdw;

-- Define the remote server
CREATE SERVER remote_server FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (
  host 'your_remote_host',
  port '5432',
  dbname 'remote_database',
  user 'remote_user',
  password 'remote_password'
);

-- Create user mapping (assuming same username and password on both servers)
CREATE USER MAPPING FOR CURRENT USER SERVER remote_server;

-- Define an external table referencing the remote table
CREATE FOREIGN TABLE my_remote_table (
  id int PRIMARY KEY,
  name varchar(50)
)
SERVER remote_server
OPTIONS (schema_name 'public', table_name 'users');

-- Now you can query data from the remote table like a local one
SELECT * FROM my_remote_table;

Using DBLink:

-- Enable DBLink extension (if not already installed)
CREATE EXTENSION IF NOT EXISTS dblink;

-- Execute a query on the remote database (assuming Oracle)
SELECT * FROM dblink(
  'dbname=remote_oracle_db user=oracle_user password=oracle_password@(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=remote_oracle_host)(PORT=1521))(CONNECT_DATA=(SID=oracle_sid)))',
  'SELECT * FROM users'
) AS remote_users(id int, username varchar(50));

The best method for your situation depends on factors like:

• Frequency of cross-database queries: For frequent queries, FDW or materialized views might be better.
• Performance requirements: FDW can introduce overhead, while materialized views require maintenance.
• Data consistency needs: Real-time consistency might favor replication or application-side logic.
• Database types involved: DBLink works best for PostgreSQL connections, while federation tools offer broader compatibility.