Unindexed Foreign Keys Can Make Performance Worse.

I know. You, dear reader, saw that title and you came in here because you’re furious. You want foreign key relationships configured in all of your tables to prevent bad data from getting in.

But you gotta make sure to index them, too.

Let’s take the Stack Overflow database – I’m using the 50GB Stack Overflow 2013 version for this post. I’m going to start with no nonclustered indexes whatsoever, and then I’m going to add in foreign keys between the Users table and several tables where users add content, like Badges, Comments, Posts, and Votes:

/* Implement foreign keys: */