Best Practices for User Defined Functions in SQL Server

Types of User Defined Functions in SQL Server

SQL Server supports three main types of UDFs: scalar functions, inline table-valued functions, and multi-statement table-valued functions.

Each type has different performance characteristics and use cases.

• Scalar functions return a single value and can be used in expressions.
• Inline table-valued functions return a table result defined by a single SELECT statement.
• Multi-statement table-valued functions return a table and can contain multiple statements to build the result.

Best Practices for Writing User Defined Functions

Following best practices when writing UDFs helps ensure your database performs well and remains maintainable.

• Prefer inline table-valued functions over scalar or multi-statement functions for better query optimization.
• Avoid side effects: UDFs should not modify database state (no INSERT, UPDATE, DELETE).
• Keep functions simple and focused on a single task.
• Use schema binding when possible to prevent underlying table changes that break the function.
• Avoid using UDFs in WHERE clauses or JOIN conditions if they are scalar functions, as this can cause row-by-row execution.
• Test UDF performance with realistic data volumes and query plans.
• Use appropriate parameter data types and avoid unnecessary conversions.

Performance Considerations

Performance is a critical factor when using UDFs in SQL Server, especially in large or complex queries.

• Scalar functions often cause performance issues because they execute row-by-row and prevent parallelism.
• Inline table-valued functions are optimized as part of the calling query and generally perform better.
• Multi-statement table-valued functions can have hidden performance costs due to table variable usage and lack of statistics.
• Avoid calling UDFs repeatedly inside loops or large result sets.
• Consider rewriting complex UDF logic as stored procedures or using APPLY operators for better performance.

Example: Converting a Scalar Function to Inline Table-Valued Function

Here is an example showing how to convert a scalar function to an inline table-valued function to improve performance.