Foreign Keys Help Performance

I have always put FKs into my database for data integrity purposes. I’ve worked on enough applications that didn’t have FKs, or any RI in place and it was always a nightmare when the application broke down or there were enhancements that allowed duplicates, orphans, or other data integrity problems.

However I ran across an old post form Grant Fritchey that shows Foreign Keys do more than that. They can actually help performance because the SQL Server database engine knows that there is data in the related tables that matches because of the FK relationship.

Does that matter?

If you read Grant’s post, and you should, it shows two different queries of the same data, but one has FKs enabled. That results in a much smaller execution plan, hitting fewer tables. I took Grant’s test and added one more twist.

I ran both queries in the same batch, with the execution plan. Guess what I found? Check out this image:

query1

Guess which query has FKs and which one doesn’t? If you read Grant’s post, you’ll realize the first one has the FKs, but more importantly, if you look at the relative percentages of the batches, you see that there’s a 9x difference in resources.

Use FKs. They do more than protect data, they speed things up.

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