Multi-Statement vs. Inline Table-Valued Functions in SQL Server: Which Performs Better?

SQL Server Table-Valued Functions: MSTVFs vs. ITVFs – A Performance Analysis

Multi-statement table-valued functions (MSTVFs) and inline table-valued functions (ITVFs) in SQL Server, while superficially similar, exhibit significant performance differences. Understanding these differences is crucial for optimal query performance.

Function Structure and Optimization

MSTVFs function like stored procedures returning a table, while ITVFs are essentially single SELECT statements. This structural difference impacts how the SQL Server query optimizer handles them. ITVFs are treated as views, allowing the optimizer to utilize table statistics for efficient query planning. Conversely, MSTVFs behave like table variables; the entire SELECT statement is materialized, limiting the optimizer's ability to leverage statistics.

Performance Implications: A Detailed Look

ITVFs generally outperform MSTVFs, especially in complex queries with multiple joins and filters. The optimizer's ability to effectively utilize statistics with ITVFs translates to faster execution times. This advantage becomes more pronounced as query complexity increases.

Parameterization and Optimization Trade-offs

MSTVFs support parameterization, enabling dynamic filtering. However, this flexibility comes at a cost. The optimizer must evaluate the function repeatedly for each distinct parameter value, impacting performance. ITVFs, lacking parameterization, are more efficient when filtering isn't necessary.

Illustrative Example

Consider the previously mentioned MSTVF:

<code class="language-sql">CREATE FUNCTION MyNS.GetLastShipped(@CustomerID INT)
RETURNS @CustomerOrder TABLE
...</code>

Rewriting it as an ITVF:

<code class="language-sql">CREATE FUNCTION MyNS.GetLastShipped()
RETURNS @CustomerOrder TABLE
...</code>

In queries with multiple joins, the ITVF version will generally perform better because the optimizer only needs to call the function once.

Choosing the Right Function Type

Due to their superior performance and simpler syntax, ITVFs are generally preferred. However, MSTVFs remain valuable in situations demanding parameterization. The optimal choice depends entirely on the specific query requirements and the need for dynamic filtering. Understanding the performance trade-offs is key to making an informed decision.

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