[gaius]

Was this a big deal in the past?

In MySQL, yes. It lacked the concept of foreign keys.

[kondro]

Lack of foreign keys does not effect performance.

In fact, having foreign keys is a net-negative performance-wise because it means that each foreign key must be checked for every insert/update/delete.

[branko_d]

Foreign keys can improve performance, in some cases:

https://stackoverflow.com/a/8154375/533120

[gaius]

Errm, we're talking about performance of joins

[ComputerGuru]

Joins will use the regular index matching it against the join condition just like a where clause would.

[kondro]

They don't help with that either.

[Sacho]

Perhaps parent is conflating indexes with constraints? I believe InnoDB creates indexes automatically with foreign keys.