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
  • aggregate 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.

Aggregate Pushdown#

The FDW pushes common aggregate queries down to SQL Server so the aggregation runs remotely and only the final result rows are transferred to Postgres. This is much faster than fetching every row and aggregating locally, especially over large tables.

Supported aggregates — count(*), count(col), count(distinct col), sum(col), avg(col), min(col), max(col).

Supported shapes — scalar aggregates, group by over plain columns, with or without a where clause. Pushdown also works when the foreign table option is a sub-query.

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-- All of these run as a single aggregate query on SQL Server:
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select count(*) from mssql.users;
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select id, sum(amount) from mssql.users group by id;
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select count(distinct name) from mssql.users where id = 42;

count(*) and count(col) are translated to SQL Server's count_big so the result fits Postgres' bigint without overflow. Each aggregate is also wrapped in a cast(... as <sql_server_type>) so values come back in the exact type Postgres expects (for example, sum over a bigint column is cast to numeric, matching Postgres' sum(bigint) → numeric rule).

Cases that are not pushed down — the query still returns the correct result, but the aggregation happens in Postgres after fetching the rows:

• The query has a having clause
• The aggregate has a filter (where …) clause
• A distinct modifier is used on anything other than count
• The aggregate's argument is not a plain column (for example sum(a + 1))
• A group by item is not a plain column (for example group by id + 1)
• The aggregate function is not in the list above (for example stddev, string_agg)

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;

Aggregate Query Examples#

These examples assume an orders table on SQL Server and a matching foreign table on Postgres:

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-- Run on SQL Server
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create table orders (
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  id        bigint,
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  user_id   bigint,
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  amount    numeric(18,2),
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  status    varchar(20)
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);
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insert into orders (id, user_id, amount, status) values
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  (1, 42, 100.00, 'paid'),
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  (2, 42,  50.00, 'paid'),
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  (3, 43, 200.00, 'pending'),
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  (4, 43,  75.00, 'paid'),
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  (5, 44, 300.00, 'paid');

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-- Foreign table on Postgres
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create foreign table mssql.orders (
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  id      bigint,
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  user_id bigint,
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  amount  numeric(18,2),
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  status  text
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)
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  server mssql_server
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  options (
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    table 'orders'
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  );

Each query below runs a single aggregate query against SQL Server and returns just the result rows:

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-- Total order count
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select count(*) from mssql.orders;
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-- Total revenue from paid orders
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select sum(amount) from mssql.orders where status = 'paid';
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-- Per-user order count and revenue
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select user_id, count(*) as orders, sum(amount) as revenue
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from mssql.orders
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group by user_id
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order by user_id;
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-- Smallest and largest order
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select min(amount), max(amount) from mssql.orders;
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-- Number of distinct users who placed an order
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select count(distinct user_id) from mssql.orders;
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-- Average order value per status
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select status, avg(amount) as avg_amount
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from mssql.orders
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group by status;