MSTVF vs. ITVF in SQL Server: When Should You Choose Inline Over Multi-Statement Functions?

SQL Server: Inline vs. Multi-Statement Table-Valued Functions – Performance Considerations

SQL Server offers two types of table-valued functions: multi-statement table-valued functions (MSTVFs) and inline table-valued functions (ITVFs). Both return tables, but their performance differs significantly.

A primary distinction lies in their compilation: ITVFs are compiled, resulting in a pre-optimized execution plan, while MSTVFs are interpreted, generating their execution plan at runtime. This often leads to superior performance for ITVFs, particularly when the function is repeatedly called with identical parameters.

Furthermore, ITVFs leverage table statistics, unlike MSTVFs. This advantage is crucial when the function involves filtering or joining large datasets.

In the provided examples, the MSTVF's reliance on a separate query for customer ID filtering hinders the optimizer's ability to utilize efficient join strategies. While optimizing the MSTVF to reduce multiple calls might minimize performance discrepancies, an equivalent ITVF or VIEW would generally still outperform it due to its access to table statistics.

Therefore, ITVFs are usually the preferred choice, offering better performance and simpler maintenance. However, MSTVFs remain necessary in situations involving dynamic parameters or requiring dynamic modification of intermediate results.

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