Extended Stored Procedures

Extended stored procedures are procedures in SQL Server that can be executed from a client application or from within another stored procedure. They are implemented as Dynamic Link Libraries (DLLs) and provide a way to extend the functionality of SQL Server by calling functions written in languages such as C or C++. This allows for integration with external components, operating system services, and other applications.

Overview

Extended stored procedures are called using the sp_OACreate, sp_OAMethod, sp_OAGetProperty, sp_OASetProperty, and sp_OADestroy system stored procedures. They offer a powerful mechanism for:

  • Interacting with the operating system (e.g., file system operations, registry access).
  • Performing complex computations or data manipulation not easily achievable with T-SQL alone.
  • Integrating with third-party libraries or applications.
Security Note: Extended stored procedures can pose significant security risks if not properly managed. They run within the SQL Server process and can potentially compromise the server or its data. Use them with caution and only from trusted sources.

Commonly Used Extended Stored Procedures

While most extended stored procedures are custom-built, some are provided by SQL Server for specific tasks. Here are a few examples:

Procedure Name Description
xp_cmdshell Executes a Microsoft Windows command shell. Use with extreme caution.
xp_fileexist Checks for the existence of a file.
xp_regread Reads a value from the Windows registry.
xp_sendmail Sends an e-mail message. (Deprecated in favor of Database Mail).

Creating Extended Stored Procedures

Developing custom extended stored procedures involves writing a DLL in C or C++ that exposes specific entry points for SQL Server to call. These entry points typically adhere to a defined interface, allowing SQL Server to pass parameters and receive results.

Key Steps:

  1. Develop the DLL: Write your C/C++ code with the desired functionality.
  2. Expose Entry Points: Implement the required functions with specific naming conventions (e.g., xp_your_procedure_name).
  3. Register with SQL Server: Use sp_addextendedproc to register your DLL with SQL Server.
  4. Grant Permissions: Ensure appropriate permissions are granted to users who need to execute the extended stored procedure.

For detailed information on the API and development guidelines, refer to the official Microsoft documentation for the specific version of SQL Server you are using.

Considerations and Best Practices

  • Security: Always validate the source of extended stored procedures. Limit their use to essential tasks and grant permissions judiciously.
  • Performance: Poorly written extended stored procedures can degrade SQL Server performance.
  • Error Handling: Implement robust error handling within your DLL to prevent unexpected behavior.
  • Alternatives: Explore native T-SQL features, CLR integration, or SQL Server Integration Services (SSIS) before resorting to extended stored procedures for tasks that can be accomplished otherwise.

Extended stored procedures are a powerful tool for extending SQL Server's capabilities, but they should be employed thoughtfully due to their potential security and performance implications.

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