I have to be certain to delete from OrderDetail first, then Order (to maintain the RI) and then load Order first then OrderDetail. No big deal in this simple example, but what if I’m dealing with a dozen tables? Or I’m only re-loading the parent (Order)?
The easiest thing to do is to disable the foreign key, load your data, and then re-enable the foreign key. You might be tempted to skip that last step but don’t. RI is very important and in fact a trusted foreign key can be used by the optimizer to improve your query plan. It’s easy to say that the application doesn’t have bugs that cause problems with referential integrity. It’s not like you’re ever going insert an OrderDetail without an Order right? Unfortunately it’s far to easy for mistakes to happen. Maybe not in the application, it might be a mistake in an update meant to fix something else. Our job is to protect the data, and RI is an important part of that. So by all means disable a foreign key to help with a load but make sure you turn it back on when you are done.
If a foreign key constraint isn’t trusted, the optimizer won’t be able to assume relational integrity, and so it’s possible that the optimizer could make sub-par choices when joining tables with a foreign key constraint.