MSSQL

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:

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create extension if not exists wrappers with schema extensions;

Enable the SQL Server Wrapper#

Enable the mssql_wrapper FDW:

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create foreign data wrapper mssql_wrapper
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  handler mssql_fdw_handler
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  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.

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-- Save your SQL Server connection string in Vault and retrieve the created `key_id`
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select vault.create_secret(
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  'Server=localhost,1433;User=sa;Password=my_password;Database=master;IntegratedSecurity=false;TrustServerCertificate=true;encrypt=DANGER_PLAINTEXT;ApplicationName=wrappers',
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  'mssql',
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  'MS SQL Server connection string for Wrappers'
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);

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.

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:

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

Create a schema#

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

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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,

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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#

Usage#

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

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#

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:

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-- Run below SQLs on SQL Server to create source table
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create table users (
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  id bigint,
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  name varchar(30),
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  dt datetime2
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);
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-- Add some test data
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insert into users(id, name, dt) values (42, 'Foo', '2023-12-28');
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insert into users(id, name, dt) values (43, 'Bar', '2023-12-27');
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insert into users(id, name, dt) values (44, 'Baz', '2023-12-26');

Then create and query the foreign table in PostgreSQL:

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

Remote Subquery Example#

Create a foreign table using a subquery:

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