Can foreign keys hurt query performance

Question

As I understood from this post, there are some scenarios where foreign keys can improve query performance.

I\'ve heard the opposite claim though, that because of refere

Answer 1

For INSERT/UPDATE/DELETE the short answer is, "Yes". The database will need to check that the referential integrity is intact and the creation/modification is allowed. Or in DELETE's case, there may be some cascading to be done.

For SELECTs, it's actually quite the contrary. Foreign Keys have a secret added benefit of showing you exactly where you're most likely to be doing complex JOINs and have very commonly used fields. This makes the job of indexing much easier, and you can pretty much guarantee that all of your FK fields should be indexed. This makes SELECTs much faster.