Category: Constraints

Daniel Janik explains what happens when you don’t have trusted foreign key constraints:

Why is it untrusted? Perhaps we disabled the check to load data and neglected to re-enable it?

No matter what the reason is the next part is not as simple. This is for two reasons.

1. The data in the child table may not be valid. Since the key was not being checked I may have data in my table that isn’t represented in the parent.

2. The syntax is a bit silly. As Mike Byrd in Austin, TX says, Microsoft studders. The syntax to reenable is “CHECK CHECK”. Let’s look at how we reenable the Address key check.

Read on for pros and cons of disabling (or not trusting) foreign key constraints.

Michael Swart has a script to check start versus end dates to ensure there is a valid check constraint for date ranges:

Don’t blindly run scripts that you got from some random guy’s blog. Even if that someone is me. That’s terribly irresponsible.

But this query may be useful if you do want to look for a very specific, simple kind of constraint that may match your business specs. These constraints are just suggestions and may not match your business rules. For example, when I run this query on Adventureworks, I get one “missing” check constraint for HumanResources.Shift(StartTime, EndTime) and when I look at the contents of the Shift table, I get this data:

If you wish non-blindly to run a script you got from that guy’s blog, click through for the script and more details, including a discussion of false positives.

Steve Jones shows how to rename a primary key constraint:

When you compare that with the same table in another database, what’s the likelihood that you’ll have the PK named PK__OrderDet__D3B9D30C7D677BB4? Probably pretty low.

This means that if you are looking to deploy changes, and perhaps compare the deployment from one database to the next, you’ll think you have different indexes. Most comparison tools will then want to change the index on your target server, which might be using this technique. Or the choice might be something that performs much worse.

What we want to do is get this named the same on all databases. In this case, the easiest thing to do with rename the constraint on all systems. This is easy to do with sp_rename, which is better than dropping and rebuilding the index.

Do read this and avoid renaming a constraint the bad way.

Kenneth Fisher discusses foreign key constraints:

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.

Shane O’Neill discusses primary key and foreign key constraints:

If you check the two definitions for Primary key and Foreign key you’ll see that, even though they are both called keys, they serve two different purposes; namely identifying rows and enforcing links.

And those two purposes are not mutually exclusive!

A column/column combo that identifies a row can also be used to enforce a link back to another table (or itself, as shown above with Foreign keys).

The assumption, that if you were one then you couldn’t be the other, was incorrect. If your business rules call for it, don’t let a column being one type of key stop it from being the other.

Not included but still important are unique constraints.