When we talk about performance tuning in SQL Server, the first thing that comes to mind is indexes. Everyone knows clustered indexes, nonclustered indexes, and included columns. But very few people ever bother with a small, often overlooked gem introduced back in SQL Server 2008: the filtered index. It is one of those features that sits quietly in the documentation, rarely used in the wild, and yet it can make the difference between a sluggish dashboard query and a blazing fast one.
In this article, we will explore what filtered indexes are, why they exist, how