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Unique Constraints vs Unique Indexes

Erik Darling calls out unique key constraints:

I do love appropriately applied uniqueness. It can be helpful not just for keeping bad data out, but also help the optimizer reason about how many rows might qualify when you join or filter on that data.

The thing is, I disagree a little bit with how most people set them up, which is by creating a unique constraint.

Data modeling Kevin wants to use unique key constraints because that’s the correct thing to do. Implementation Kevin uses unique nonclustered indexes for the reasons Erik describes. Not mentioned in Erik’s post but potentially relevant is that operations on unique nonclustered indexes can be done online, whereas unique key constraint operations (creation and alteration via drop+create) are offline.