SQL Server

Microsoft SQL Server is a proprietary relational database management system developed by Microsoft.

The SQL Server Wrapper allows you to read data from Microsoft SQL Server within your Postgres database.

Preparation

Before you can query SQL Server, you need to enable the Wrappers extension and store your credentials in Postgres.

Enable Wrappers

Make sure the wrappers extension is installed on your database:

create extension if not exists wrappers with schema extensions;

Enable the SQL Server Wrapper

Enable the mssql_wrapper FDW:

create foreign data wrapper mssql_wrapper
  handler mssql_fdw_handler
  validator mssql_fdw_validator;

Store your credentials (optional)

By default, Postgres stores FDW credentials inside pg_catalog.pg_foreign_server in plain text. Anyone with access to this table will be able to view these credentials. Wrappers is designed to work with Vault, which provides an additional level of security for storing credentials. We recommend using Vault to store your credentials.

-- Save your SQL Server connection string in Vault and retrieve the `key_id`
insert into vault.secrets (name, secret)
values (
  'mssql',
  'Server=localhost,1433;User=sa;Password=my_password;Database=master;IntegratedSecurity=false;TrustServerCertificate=true;encrypt=DANGER_PLAINTEXT;ApplicationName=wrappers'
)
returning key_id;

The connection string is an ADO.NET connection string, which specifies connection parameters in semicolon-delimited string.

Supported parameters

All parameter keys are handled case-insensitive.

Parameter	Allowed Values	Description
Server	`<string>`	The name or network address of the instance of SQL Server to which to connect. Format: `host,port`
User	`<string>`	The SQL Server login account.
Password	`<string>`	The password for the SQL Server account logging on.
Database	`<string>`	The name of the database.
IntegratedSecurity	false	Windows/Kerberos authentication and SQL authentication.
TrustServerCertificate	true, false	Specifies whether the driver trusts the server certificate when connecting using TLS.
Encrypt	true, false, DANGER_PLAINTEXT	Specifies whether the driver uses TLS to encrypt communication.
ApplicationName	`<string>`	Sets the application name for the connection.

Connecting to SQL Server

We need to provide Postgres with the credentials to connect to SQL Server. We can do this using the create server command:

With VaultWithout Vault

create server mssql_server
  foreign data wrapper mssql_wrapper
  options (
    conn_string_id '<key_ID>' -- The Key ID from above.
  );

create server mssql_server
  foreign data wrapper mssql_wrapper
  options (
    conn_string 'Server=localhost,1433;User=sa;Password=my_password;Database=master;IntegratedSecurity=false;TrustServerCertificate=true;encrypt=DANGER_PLAINTEXT;ApplicationName=wrappers'
  );

Create a schema

We recommend creating a schema to hold all the foreign tables:

create schema if not exists mssql;

Options

The full list of foreign table options are below:

table - Source table or view name in SQL Server, required.

This can also be a subquery enclosed in parentheses, for example,

table '(select * from users where id = 42 or id = 43)'

Entities

SQL Server Tables

This is an object representing SQL Server tables and views.

Ref: Microsoft SQL Server docs

Operations

Object	Select	Insert	Update	Delete	Truncate
table/view	✅	❌	❌	❌	❌

Usage

create foreign table mssql.users (
  id bigint,
  name text,
  dt timestamp
)
  server mssql_server
  options (
    table 'users'
  );

Notes

Supports both tables and views as data sources
Can use subqueries in the table option
Query pushdown supported for:
- where clauses
- order by clauses
- limit clauses
See Data Types section for type mappings between PostgreSQL and SQL Server

Query Pushdown Support

This FDW supports where, order by and limit clause pushdown.

Supported Data Types

Postgres Type	SQL Server Type
boolean	bit
char	tinyint
smallint	smallint
real	float(24)
integer	int
double precision	float(53)
bigint	bigint
numeric	numeric/decimal
text	varchar/char/text
date	date
timestamp	datetime/datetime2/smalldatetime
timestamptz	datetime/datetime2/smalldatetime

Limitations

This section describes important limitations and considerations when using this FDW:

Large result sets may experience slower performance due to full data transfer requirement
Only supports specific data type mappings between Postgres and SQL Server
Only support read operations (no INSERT, UPDATE, DELETE, or TRUNCATE)
Windows authentication (Integrated Security) not supported
Materialized views using these foreign tables may fail during logical backups

Examples

Basic Example

First, create a source table in SQL Server:

-- Run below SQLs on SQL Server to create source table
create table users (
  id bigint,
  name varchar(30),
  dt datetime2
);

-- Add some test data
insert into users(id, name, dt) values (42, 'Foo', '2023-12-28');
insert into users(id, name, dt) values (43, 'Bar', '2023-12-27');
insert into users(id, name, dt) values (44, 'Baz', '2023-12-26');

Then create and query the foreign table in PostgreSQL:

create foreign table mssql.users (
  id bigint,
  name text,
  dt timestamp
)
  server mssql_server
  options (
    table 'users'
  );

select * from mssql.users;

Remote Subquery Example

Create a foreign table using a subquery:

create foreign table mssql.users_subquery (
  id bigint,
  name text,
  dt timestamp
)
  server mssql_server
  options (
    table '(select * from users where id = 42 or id = 43)'
  );

select * from mssql.users_subquery;