Snippet Name: PostgreSQL_FDW_Overview
Tags:
Created Date: 2025-06-16 13:12:42
Last Modified Date: 2025-05-31 10:32:20
Dependencies:
DBA Notes:
Sample Output:
Title: PostgreSQL FDW Overview
Author: user
can you give me some information about PostGres SQL and foreign data wrappers? is it a database connection to another postgres server similar to linked servers?
Author: ChatGPT
Yes, you're absolutely on the right track???**PostgreSQL Foreign Data Wrappers (FDWs)** are very much like **Linked Servers** in SQL Server. They allow a PostgreSQL server to connect to external data sources and query them as if they were local tables.
---
### ???? What is a Foreign Data Wrapper?
A **Foreign Data Wrapper (FDW)** is a PostgreSQL feature, defined by the SQL/MED (Management of External Data) standard, that enables **external data access through SQL**. Think of it as a plugin that acts as a bridge between PostgreSQL and another data source.
---
### ??? Common Use Cases
- Connect to **another PostgreSQL database**
- Connect to **MySQL, SQL Server, Oracle, or SQLite**
- Access **flat files**, **CSV**, or even **REST APIs**
- Query **foreign databases** without ETL or duplication
---
### ???? Connecting to Another PostgreSQL Server (postgres_fdw)
The most common FDW is `postgres_fdw`, which connects one PostgreSQL server to another. This is the one that's most like **Linked Servers** in SQL Server.
#### Example: Setting up a postgres_fdw
```sql
-- 1. Enable the extension
CREATE EXTENSION IF NOT EXISTS postgres_fdw;
-- 2. Create a foreign server reference
CREATE SERVER foreign_pg_server
FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (host '192.168.1.100', dbname 'remote_db', port '5432');
-- 3. Create a user mapping
CREATE USER MAPPING FOR current_user
SERVER foreign_pg_server
OPTIONS (user 'remote_user', password 'remote_password');
-- 4. Import specific foreign schema or table
IMPORT FOREIGN SCHEMA public
FROM SERVER foreign_pg_server
INTO local_schema;
-- Or manually define a table
CREATE FOREIGN TABLE local_schema.remote_table (
id integer,
name text
)
SERVER foreign_pg_server
OPTIONS (schema_name 'public', table_name 'actual_remote_table');
```
---
### ???? Behavior Similar to Linked Servers
- You can perform **SELECT**, and in some cases, **INSERT/UPDATE/DELETE** on the foreign data
- Joins between local and foreign tables are supported (but performance can vary)
- Data is **not copied**, but **queried live** from the remote source
- Requires **network connectivity** between the servers
---
### ???? Other Foreign Data Wrappers
You can also connect to:
- `mysql_fdw` ??? for MySQL/MariaDB
- `tds_fdw` ??? for SQL Server (via FreeTDS)
- `oracle_fdw` ??? for Oracle
- `csv_fdw` ??? for flat file CSVs
- `file_fdw` ??? read from files on disk
- `multicorn` ??? framework for Python-based FDWs (e.g., for APIs, MongoDB)
---
### ???? Things to Consider
- Performance depends on network latency and how well the FDW supports pushdown (filtering, joining remotely)
- Not all FDWs support write operations
- Security: remote credentials need to be stored (you can use `pgpass` or encrypted secrets)
- Not a replication tool???it's for live access, not syncing
---
Let me know if you want a working example to connect to a remote PostgreSQL instance or tips on FDWs to SQL Server or MySQL.
Author: user
can you give me an actual code examples for MySQL,SQL Server,cdv, and files for the fdw you mentions? assume i am on a development server that has mysql,sqlserver installed on classic ports, and that we have files store din d:\Data
Author: ChatGPT
Lowell is working with PostgreSQL Foreign Data Wrappers (FDWs) and has access to MySQL, SQL Server (on default ports), and files stored on `D:\Data` in a development server.
Author: ChatGPT
Model set context updated.
Author: ChatGPT
Great! Below are working PostgreSQL examples using **FDWs** to connect to:
- **MySQL** (`mysql_fdw`)
- **SQL Server** (`tds_fdw`)
- **CSV files** (`csv_fdw`)
- **Flat files** (`file_fdw`)
> ???? Assumptions:
> - You're on Windows or WSL with access to PostgreSQL
> - All services run on their **default ports**
> - You have administrative rights to install extensions
---
## ???? 1. MySQL FDW (`mysql_fdw`)
Install the FDW (if not already available):
```bash
-- In psql
CREATE EXTENSION IF NOT EXISTS mysql_fdw;
```
Create the connection and foreign table:
```sql
CREATE SERVER mysql_svr
FOREIGN DATA WRAPPER mysql_fdw
OPTIONS (host 'localhost', port '3306', database 'testdb');
CREATE USER MAPPING FOR CURRENT_USER
SERVER mysql_svr
OPTIONS (username 'mysql_user', password 'mysql_pwd');
CREATE FOREIGN TABLE public.mysql_users (
id int,
username text,
email text
)
SERVER mysql_svr
OPTIONS (table_name 'users');
```
---
## ???? 2. SQL Server FDW (`tds_fdw`)
Install `tds_fdw`:
- Get it from [https://github.com/tds-fdw/tds_fdw](https://github.com/tds-fdw/tds_fdw)
- Requires FreeTDS or ODBC driver installed
Enable the extension:
```sql
CREATE EXTENSION IF NOT EXISTS tds_fdw;
```
Set up connection:
```sql
CREATE SERVER sqlserver_svr
FOREIGN DATA WRAPPER tds_fdw
OPTIONS (servername 'localhost', port '1433', database 'AdventureWorks', msg_handler 'notice');
CREATE USER MAPPING FOR CURRENT_USER
SERVER sqlserver_svr
OPTIONS (username 'sa', password 'YourStrong!Passw0rd');
CREATE FOREIGN TABLE public.sqlserver_customers (
CustomerID int,
Name nvarchar(100)
)
SERVER sqlserver_svr
OPTIONS (schema_name 'dbo', table_name 'Customers');
```
---
## ???? 3. CSV Files FDW (`csv_fdw`)
Install from: [https://github.com/pgspider/csv_fdw](https://github.com/pgspider/csv_fdw)
Enable and set up:
```sql
CREATE EXTENSION IF NOT EXISTS csv_fdw;
CREATE SERVER csv_files
FOREIGN DATA WRAPPER csv_fdw;
CREATE FOREIGN TABLE public.sales_csv (
id int,
product text,
amount numeric
)
SERVER csv_files
OPTIONS (filename 'D:/Data/sales.csv', delimiter ',', null '', quote '"', header 'true');
```
---
## ???? 4. File FDW (`file_fdw`)
PostgreSQL's built-in `file_fdw` can only read **readable files on the PostgreSQL server filesystem**.
```sql
CREATE EXTENSION IF NOT EXISTS file_fdw;
CREATE SERVER file_server
FOREIGN DATA WRAPPER file_fdw;
CREATE FOREIGN TABLE public.products_file (
id int,
name text,
price numeric
)
SERVER file_server
OPTIONS (filename 'D:/Data/products.txt', format 'csv', header 'true', delimiter ',');
```
---
## ???? Permissions and Access Notes
- PostgreSQL must be able to **read files** (set correct permissions or run service as user with access).
- For SQL Server and MySQL, ports must be open and authentication must allow access from the dev PostgreSQL server.
---
Would you like a helper PowerShell script or SQL script to validate file paths or check service ports on your dev box?
Author: user
this was very enlightening, being able to see the similarities between the different sources. can you give me a query example for each, now, so i have the query syntax after having create dthe fdws?
Author: ChatGPT
Absolutely???now that you have the FDWs and foreign tables created, querying them is delightfully familiar. PostgreSQL makes it almost seamless, so you can use standard SQL `SELECT`, `JOIN`, `WHERE`, etc. Here are basic examples for each FDW type.
---
### ???? **MySQL FDW** ??? Query from remote `users` table
```sql
-- Simple query
SELECT id, username, email
FROM public.mysql_users
WHERE email LIKE '%@example.com';
-- Join with a local table
SELECT l.id, l.comment, r.email
FROM local_comments l
JOIN public.mysql_users r ON l.user_id = r.id;
```
---
### ???? **SQL Server FDW** ??? Query from `Customers` in AdventureWorks
```sql
-- Simple query
SELECT CustomerID, Name
FROM public.sqlserver_customers
WHERE Name ILIKE '%bike%';
-- Join with a local PostgreSQL table
SELECT o.order_id, c.Name
FROM local_orders o
JOIN public.sqlserver_customers c ON o.customer_id = c.CustomerID;
```
---
### ???? **CSV FDW** ??? Read from `sales.csv`
```sql
-- Simple aggregate
SELECT product, SUM(amount) AS total_sales
FROM public.sales_csv
GROUP BY product
ORDER BY total_sales DESC;
```
---
### ???? **File FDW** ??? Query from `products.txt`
```sql
-- List all products over $100
SELECT id, name, price
FROM public.products_file
WHERE price > 100
ORDER BY price DESC;
```
---
### ???? Common Notes for All
- The syntax is **identical to querying local tables**.
- You can use `JOIN`, `GROUP BY`, `ORDER BY`, and even **subqueries**.
- Pushdown support varies. For instance, `postgres_fdw` pushes down most things (filters, joins), while `file_fdw` reads the whole file then filters in memory.
---
Let me know if you'd like:
- Examples of `INSERT`, `UPDATE`, or `DELETE` (where supported)
- View creation from foreign tables
- A test script to validate foreign tables and log execution time (for perf testing)
Would you also want to try wrapping these in **materialized views** or **importing snapshots** for offline use?
Author: user
so unlike sql server linked servers, where i can use sp_tables_ex to query the available tables, to me, this appears more like SQL server External tables, where i need to create a one to one mapping of source to the foreign table, which is effectively a view. is my understanding correct?
Author: ChatGPT
Yes, **your understanding is exactly right**, and you're making a really insightful comparison here.
---
### ??? Let???s Break It Down:
| Feature | SQL Server Linked Server | SQL Server External Table (PolyBase) | PostgreSQL Foreign Data Wrapper (FDW) |
|-------------------------------|-------------------------------------------|----------------------------------------------|----------------------------------------------|
| **Object discovery** | `sp_tables_ex`, `OPENQUERY` on-the-fly | Manual DDL (`CREATE EXTERNAL TABLE`) | Manual DDL (`CREATE FOREIGN TABLE` or `IMPORT FOREIGN SCHEMA`) |
| **Ad hoc querying** | Yes (`OPENQUERY`, `SELECT * FROM [srv].[db].[schema].[table]`) | No | No (must define a foreign table) |
| **DDL mapping required** | No | Yes | Yes |
| **Pushdown support** | Partial | Strong (depends on PolyBase backend) | Varies by FDW (strong for `postgres_fdw`) |
| **Writable?** | Yes (depends on configuration) | No | Yes/No (depends on FDW) |
| **Analogy to FDW** | ??? Not equivalent | ??? Similar in concept | ??? More similar to external tables |
---
### ???? Key Observations:
- With **FDWs**, you **must define each foreign table**, either manually with `CREATE FOREIGN TABLE` or automatically using `IMPORT FOREIGN SCHEMA` (if supported by the FDW, like `postgres_fdw`).
- This is **closer to SQL Server External Tables** (PolyBase) than classic Linked Servers.
- There???s **no generic metadata browsing like `sp_tables_ex`** or `INFORMATION_SCHEMA.TABLES` across the FDW boundary unless the FDW explicitly supports it (some don???t).
- If you want to explore the remote schema **programmatically**, you'd need to:
- Connect to the remote DB directly and inspect its metadata
- Or use tools to generate `CREATE FOREIGN TABLE` scripts from remote catalog views
---
### ???? Example: Automating Foreign Table Creation
```sql
-- For postgres_fdw
IMPORT FOREIGN SCHEMA public
FROM SERVER foreign_pg_server
INTO local_foreign_schema;
```
Other FDWs don???t always support `IMPORT FOREIGN SCHEMA`, so you may need a **code generator** (Python, PowerShell, or bash) to automate `CREATE FOREIGN TABLE` statements based on the remote catalog.
---
### ???? View Analogy
You're also right that **a foreign table is functionally like a view**:
- It doesn???t store data.
- It maps directly to a remote structure.
- You can wrap it in a real `VIEW` for abstraction or joins.
---
Let me know if you want help building a script that:
- Connects to a remote MySQL or SQL Server database
- Inspects its schema
- Generates `CREATE FOREIGN TABLE` statements for PostgreSQL FDWs
This is a great way to bootstrap FDWs in environments with many tables.