Computed Columns And Indexes

Derik Hammer looks at computed columns with a bonus section on unique indexes (or maybe vice versa):

What can an index do that a constraint cannot?

  • Add non-key (INCLUDED) columns.
  • Data compression.
  • Lock management settings.

What can a constraint do that an index cannot?

The only potential benefit I was able to find, or think of, is that constraints can be disabled. If you could enable and disable a unique constraint, that could be one feature that the unique index does not. This is counter-intuitive knowing that the unique constraint is enforced with a unique index behind the scenes.

For me, the big benefit of unique constraints over unique indexes is that they provide a logical separation.  Unique constraints show up in the Keys section in Management Studio and let you say, “Yes, I made this thing because the data model requires uniqueness here.”  By contrast a unique index can tell the same thing, but could also say “for the subset of data in a filtered index, I can assume uniqueness” or “for performance reasons, this combination is unique, but its uniqueness is not important to the logical data model.”  I will happily admit that I’m splitting the hair pretty thin on this one and that in practice, there are benefits to using unique indexes across the board.

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