At GitHub we do not use foreign keys

From https://github.com/github/gh-ost/issues/331#issuecomment-266027731

At GitHub we do not use foreign keys, ever, anywhere.

Personally, it took me quite a few years to make up my mind about whether foreign keys are good or evil, and for the past 3 years I’m in the unchanging strong opinion that foreign keys should not be used. Main reasons are:

• FKs are in your way to shard your database. Your app is accustomed to rely on FK to maintain integrity, instead of doing it on its own. It may even rely on FK to cascade deletes (shudder).
  When eventually you want to shard or extract data out, you need to change & test the app to an unknown extent.
• FKs are a performance impact. The fact they require indexes is likely fine, since those indexes are needed anyhow. But the lookup made for each insert/delete is an overhead.
• FKs don’t work well with online schema migrations.