Integrity on large sites

[Naz Gassiep]

I'm working in a project at the moment that is using MySQL, and people keep making
assertions like this one:

"*Really* big sites don't ever have referential integrity. Or if the few spots they do
(like with financial transactions) it's implemented on the application level (via, say,
optimistic locking), never the database level."

A large DB working with no RI would give me nightmares. Is it really true that large sites
turn RI off to improve performance? Am I just being naive in thinking that everyone runs
their DBs with RI in production?