SQL Clone
SQLServerCentral is supported by Redgate
Log in  ::  Register  ::  Not logged in

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:


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.

Filed under: Blog Tagged: Performance, syndicated, T-SQL

The Voice of the DBA

Steve Jones is the editor of SQLServerCentral.com and visits a wide variety of data related topics in his daily editorial. Steve has spent years working as a DBA and general purpose Windows administrator, primarily working with SQL Server since it was ported from Sybase in 1990. You can follow Steve on Twitter at twitter.com/way0utwest


Posted by Glenn Berry on 2 May 2011

Nice example, Steve.  I used these posts to beat on my developers once again, since they don't like FKs very much...

Leave a Comment

Please register or log in to leave a comment.