Functions and Missing Indexes – SQL in Sixty Seconds 204

In this blog post, we will discuss the relationship between Functions and Missing Indexes Hints in SQL Server. Functions can reduce query performance, especially when used on a column in the WHERE clause. However, we will explore even further to understand the significant impact on query hints. If you prefer a visual guide, you can watch my YouTube video embedded below.

Functions and Query Performance

Using functions on columns in the WHERE clause can significantly impact query performance. When a function is applied to a column, SQL Server often cannot use existing indexes efficiently. This often leads to full table scans instead of index seeks, as the function’s computation needs to be performed on every row. Consequently, the query optimizer finds it challenging to leverage indexes, which can drastically slow down the query performance. You can also watch a blog on this topic over Function in the WHERE Clause – SQL in Sixty Seconds 201.

Functions and Missing Indexes Hints

SQL Server provides missing index hints to help improve query performance. These hints suggest indexes that, if created, could optimize query execution. However, when functions are used on columns in the WHERE clause, SQL Server may not provide these helpful hints. The function’s usage obscures the column’s true value, making it difficult for the optimizer to analyze how an index could be beneficial.

Without these missing index hints, you lose a valuable starting point for performance optimization. Missing index hints are often crucial as they guide you on where to create indexes that can significantly improve query performance. By not receiving these hints, you’re left without clear guidance on potential optimizations, making it harder to identify and implement necessary indexes.

Conclusion – Functions and Missing Indexes

Understanding the relationship between functions and missing indexes is essential for tuning SQL Server queries. Functions in the WHERE clause can prevent index usage and hinder SQL Server from providing missing index hints, leading to slower performance. By avoiding functions on indexed columns and utilizing SQL Server’s missing index suggestions, you can significantly improve query performance.

Question: How do you handle functions in your SQL queries? Share your thoughts over here!

Reference: Pinal Dave (

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1 Comment. Leave new

  • Hi Pinal,
    Thank you for your articles.
    Could you please write an article about using OPTIMIZE_FOR_SEQUENTIAL_KEY for sequential uniqueidentifiers. Can it improve performance?


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