Do Foreign Keys Speed Up Queries?

Foreign Keys and Query Performance

While foreign keys play a crucial role in enforcing referential integrity, their impact on query performance is often misunderstood. Contrary to common belief, foreign keys themselves do not significantly enhance query performance.

In the provided scenario, a query joins two tables, Products and ProductCategories, using the CategoryId field as the join condition. Despite the foreign key relationship, the query plan shows a cluster index scan on table Products. This is because SQL Server does not automatically create indexes based on foreign keys.

To improve the performance of this query, an index should be created explicitly on the Products.CategoryId field. After creating the index, the execution plan shows index seeks on both tables, significantly reducing the estimated subtree cost.

Therefore, the answers to the questions posed are as follows:

• Does foreign key improve query performance?

  • No. Foreign keys are primarily for data integrity, not performance optimization.

• Should I create index on all FK columns in all tables?

  • Yes. To improve query performance on foreign key fields, indexes should be created explicitly on those columns. This allows for faster lookups and reduces the likelihood of full table scans.

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