Steps to connect Oracle from PostgreSQL

Connecting to Oracle from PostgreSQL involves several steps, and it often requires the use of a Foreign Data Wrapper (FDW) in PostgreSQL. FDWs allow PostgreSQL to access and query data from external databases, including Oracle. Here's a general guide:

### Prerequisites:

1. **Oracle Database Setup:**

   - Ensure that you have the necessary credentials (username, password) and connection details for your Oracle database.

2. **PostgreSQL Setup:**

   - Make sure you have PostgreSQL installed and running.

3. **Oracle Instant Client (Optional):**

   - If not installed, you may need to install Oracle Instant Client on the machine where PostgreSQL is running. This client provides necessary libraries for connecting to Oracle.

### Steps:

1. **Install Oracle FDW Extension:**

   - Install the `oracle_fdw` extension for PostgreSQL. This extension allows PostgreSQL to connect to Oracle databases.

   ```sql

   -- Install the extension

   CREATE EXTENSION oracle_fdw;

   ```

2. **Create Oracle Server Definition:**

   - Create a server definition in PostgreSQL that represents your Oracle database.

   ```sql

   CREATE SERVER oracle_server

   FOREIGN DATA WRAPPER oracle_fdw

   OPTIONS (dbserver '//oracle_server:1521/ORCLCDB');

   ```

   - Adjust the `dbserver` option to match your Oracle server details.

3. **Create User Mapping:**

   - Create a user mapping for the PostgreSQL user who will be accessing the Oracle server.

   ```sql

   CREATE USER MAPPING FOR postgres

   SERVER oracle_server

   OPTIONS (user 'oracle_username', password 'oracle_password');

   ```

   - Replace `postgres` with the actual PostgreSQL username and provide the Oracle username and password.

4. **Create Foreign Table:**

   - Create a foreign table in PostgreSQL that represents a table in your Oracle database.

   ```sql

   CREATE FOREIGN TABLE oracle_table (

      column1 datatype,

      column2 datatype,

      -- ... other columns ...

   )

   SERVER oracle_server

   OPTIONS (table_name 'your_oracle_table');

   ```

   - Replace `oracle_table` with the actual table name in your Oracle database.

5. **Query Oracle Data:**

   - You can now query data from Oracle within your PostgreSQL database.

   ```sql

   SELECT * FROM oracle_table;

   ```

### Notes:

- Ensure that the Oracle client libraries are in the library search path. You may need to set the `LD_LIBRARY_PATH` environment variable to include the path to Oracle Instant Client libraries.

- Make sure that network connectivity between the PostgreSQL server and Oracle database is established. Firewall settings may need to be adjusted.

- Adjust connection details, such as host, port, and database name, according to your Oracle setup.

- Consult the documentation for the specific version of `oracle_fdw` you are using for any additional configuration options.

This is a simplified guide, and you may need to adapt the steps based on your specific Oracle and PostgreSQL setups. Always refer to the official documentation for `oracle_fdw` and PostgreSQL for the most accurate and up-to-date information.