Foreign Keys Pro

FAQ

What are MySQL Foreign Keys?

A foreign key is a column or group of columns in a table that links to a column or group of columns in another table. The foreign key places constraints on data in the related tables, which allows MySQL to maintain referential integrity.
Reference: MySQL Tutorial

Do Foreign Keys have an impact on performance?

This realistically can vary site to site based on many factors. It is always best to test in a development environment, but here is a good article on how it might help performance.