What is a linked server query?

A linked server query is a Transact-SQL (T-SQL) statement that retrieves or modifies data stored on a linked server. Linked servers themselves are configurations within a database management system (like SQL Server) that allow a local database instance to execute commands against OLE DB data sources on remote servers. This capability enables you to implement distributed databases that can seamlessly fetch and update data across disparate systems as if they were local.

Understanding Linked Servers and Their Queries

Linked servers are powerful tools for integrating data across different database instances and even different database platforms (e.g., SQL Server to Oracle, MySQL, or others accessible via OLE DB providers). They are particularly useful for scenarios requiring database sharding without needing to create custom application code or directly load from remote data sources.

What is a Linked Server?

A linked server establishes a connection to another database instance or data source. Once configured, this connection allows the local SQL Server to interact with the remote data source using standard T-SQL queries. This means you can join tables from your local server with tables on a remote server, execute stored procedures on the remote server, and even perform updates or inserts across the link.

Key Characteristics:

• Distributed Data Access: Facilitates accessing data spread across multiple physical servers.
• Heterogeneous Data Sources: Can connect to various database systems (SQL Server, Oracle, MySQL, etc.) as long as an OLE DB provider is available.
• Simplified Data Integration: Reduces the complexity of integrating data from disparate sources into a single application view.

Why Use Linked Server Queries?

Linked server queries offer several compelling advantages for database administrators and developers:

• Cross-Database Reporting: Generate reports that combine data from multiple, distinct database systems.
• Data Migration & ETL: Extract, Transform, Load (ETL) processes can pull data directly from source systems into a data warehouse or staging area.
• Distributed Transactions: Participate in distributed transactions that span multiple servers, ensuring data consistency across systems.
• Database Sharding: A good solution for database sharding, allowing data to be partitioned across multiple servers without complex custom application logic.
• Data Synchronization: Keep data consistent between different environments (e.g., production and reporting databases).

How Linked Server Queries Work

When you execute a linked server query, the local SQL Server instance acts as a client, passing the query to the remote linked server. The remote server then processes the query and returns the results to the local server. The communication typically happens over network protocols, managed by the OLE DB provider associated with the linked server.

Types of Linked Server Queries

There are primarily three methods to query data through a linked server:

1. Four-Part Naming Convention: The most common and straightforward method.
2. OPENQUERY Function: Executes a passthrough query directly on the linked server.
3. OPENDATASOURCE Function: Similar to OPENQUERY but allows specifying the ad hoc connection details directly within the query, without a pre-configured linked server.

1. Four-Part Naming Convention

This method uses a fully qualified name to refer to an object on a linked server, following the format: [linked_server_name].[database_name].[schema_name].[object_name].

Example:

SELECT
    p.ProductID,
    p.Name,
    od.OrderQty
FROM
    [YourLocalDB].[dbo].[Products] AS p
JOIN
    [RemoteSQLServer].[AdventureWorks2019].[Sales].[SalesOrderDetail] AS od
    ON p.ProductID = od.ProductID
WHERE
    od.OrderQty > 5;

Pros:

• Easy to write and understand.
• SQL Server's query optimizer can attempt to push down parts of the query to the remote server, potentially improving performance.

Cons:

• Performance can be unpredictable if the optimizer can't effectively push down operations.
• Requires the linked server to be pre-configured.

2. OPENQUERY Function

The OPENQUERY function executes a specified passthrough query on the linked server. The query within OPENQUERY is treated as a single string and is sent as-is to the remote server for execution.

Syntax:

OPENQUERY (linked_server, 'query')

Example:

SELECT
    CustomerID,
    OrderDate,
    TotalDue
FROM
    OPENQUERY(RemoteSQLServer, 'SELECT CustomerID, OrderDate, TotalDue FROM AdventureWorks2019.Sales.SalesOrderHeader WHERE OrderDate >= ''2014-01-01''');

Pros:

• Optimal Performance: The remote server executes the query entirely, often leading to better performance, especially for complex filtering or aggregations.
• Precise Control: Allows full control over the query executed on the remote server.
• Can bypass the local optimizer's limitations when dealing with remote data.

Cons:

• The query string must be valid SQL for the remote server's database type.
• Less readable for very complex queries as the remote query is a string.
• Requires the linked server to be pre-configured.

3. OPENDATASOURCE Function

Similar to OPENQUERY, OPENDATASOURCE allows ad hoc connections to remote data sources without needing a pre-configured linked server. It specifies the OLE DB provider and connection string directly within the query.

Syntax:

OPENDATASOURCE ('provider_name', 'connection_string')

Example:

SELECT
    *
FROM
    OPENDATASOURCE('SQLNCLI', 'Server=REMOTE_SERVER_IP;UID=MyUser;PWD=MyPassword;Database=AdventureWorks2019').AdventureWorks2019.Sales.SalesOrderHeader;

Pros:

• No need to pre-configure a linked server (useful for one-off tasks).
• Flexible for connecting to various data sources.

Cons:

• Security Risk: Directly exposes sensitive connection information (like passwords) within the query, which is generally discouraged.
• Can be cumbersome for frequent use.
• Not all providers support OPENDATASOURCE.

Security Considerations

Linked servers, by their nature, involve connecting to other systems, which introduces security implications.

• Authentication: Configure secure authentication methods (e.g., Windows Authentication, specific SQL Server logins mapped to remote logins). Avoid using sa or overly permissive accounts.
• Permissions: Grant only the necessary permissions to the linked server login on the remote server.
• Encryption: Ensure communication between servers is encrypted, especially over public networks.
• AllowInProcess: Be cautious with AllowInProcess provider options, as they can sometimes lead to instability or security vulnerabilities.

Best Practices and Performance Tips

Best Practice	Description
Use OPENQUERY	For complex queries involving filtering or aggregation on the remote server, OPENQUERY is generally more efficient as it executes the query remotely.
Filter Early	Always try to filter data on the remote server before bringing it over the network to minimize data transfer.
Minimize Data Transfer	Select only the columns and rows you need. Avoid SELECT *.
Index Remote Tables	Ensure tables on the linked server are appropriately indexed for the queries being run against them.
Monitor Performance	Use SQL Server Profiler or Extended Events to monitor linked server query performance and identify bottlenecks.
Secure Credentials	Use specific, least-privilege logins for linked servers. Avoid embedding credentials directly in code (e.g., with OPENDATASOURCE).
Connection Timeouts	Configure appropriate connection and query timeouts for linked servers to prevent long-running or stalled queries from impacting the local server.

Conclusion

A linked server query is a fundamental mechanism for interacting with remote databases and other data sources, facilitating data integration, reporting, and distributed database architectures. By understanding the different query methods and adhering to best practices, developers can leverage linked servers effectively to build robust and efficient data solutions.