How Do Foreign Keys Impact Query Performance in SQL Server?

Foreign Key and Query Performance

This article investigates the role of foreign keys (FKs) in optimizing database queries.

Query Performance Analysis

Consider a query that joins two tables, Products and ProductCategories, based on their CategoryId. While the query returns the expected Clustered Index Seek for the ProductCategories table, it performs a Clustered Index Scan on the Products table. This raises the question: why does the FK not improve the query performance for the Products table?

To improve performance, an index was created on the Products.CategoryId column. Subsequent execution plan analysis revealed that both tables now perform Index Seeks, and the estimated subtree cost was significantly reduced.

FK's Role in Query Optimization

While FKs primarily serve as relationship enforcers, they do not directly enhance query performance. In SQL Server, FKs do not inherently create associated indexes. For optimal performance, indexes should be explicitly defined on FK columns.

Indexing FK Columns

To optimize lookup times, it is recommended to create indexes on all FK columns across tables, ensuring efficient data retrieval and reducing the cost of queries that rely on these relationships.

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