PostgreSQL

PostgreSQL # root directory of your Database
├── Events
├── Federate
│   ├── Database_Server (shop_bluesafety_com)
│   │   ├── Server
│   │   │   ├── Events
│   │   │   ├── Functions
│   │   │   ├── Procedures
│   │   │   ├── Tables
│   │   │   └── Views
│   │   ├── Tables
│   │   ├── mapping.sql
│   │   └── server.sql
│   └── ...
├── Functions
├── Procedures
├── Trigger
├── Tables
└── Views

Federation of MySQL

-- 1) Install extension (needs mysql_fdw installed on the server)
CREATE EXTENSION IF NOT EXISTS mysql_fdw;

-- 2) Define remote server
CREATE SERVER mysql_svr
  FOREIGN DATA WRAPPER mysql_fdw
  OPTIONS (host 'mysql-host', port '3306');

-- 3) User mapping (use a MySQL user)
CREATE USER MAPPING FOR CURRENT_USER
  SERVER mysql_svr
  OPTIONS (username 'mysql_user', password 'mysql_pass');

-- 4) Create foreign table (example)
CREATE FOREIGN TABLE public.mysql_customers (
  id           integer,
  email        text,
  created_at   timestamp
)
SERVER mysql_svr
OPTIONS (dbname 'mysql_db', table_name 'customers');

-- Query it
SELECT * FROM public.mysql_customers WHERE id = 42;

-- For granting User
GRANT USAGE ON SCHEMA public TO db_user;
GRANT SELECT ON ALL TABLES IN SCHEMA public TO db_user;
ALTER DEFAULT PRIVILEGES IN SCHEMA public
GRANT SELECT ON TABLES TO db_user;