Does making a column unique force an index to be created?

Question

In SQL Server 2005+ (I use both), does adding the UNIQUE constraint to a column automatically create an index, or should I still CREATE INDEX?

Answer 1

See this MSDN article:

The Database Engine automatically creates a UNIQUE index to enforce the uniqueness requirement of the UNIQUE constraint.

If you do create an index, you'll end up with two indexes, as this example demonstrates:

create table TestTable (id int)
alter table TestTable add constraint unique_id unique (id)
create unique index ix_TestTable_id on TestTable (id)

select * from sys.indexes where [object_id] = object_id('TestTable')

This will display two unique indexes on TestTable; and the HEAP that represents the table itself.

Answer 2

Yes, it does.

In fact, you can even create a CLUSTERED UNIQUE CONSTRAINT:

ALTER TABLE mytable ADD CONSTRAINT UX_mytable_col1 UNIQUE CLUSTERED (col1)

, which will make the table to be clustered on col1.

Almost all databases create an index for UNIQUE CONSTRAINT, otherwise it would be very hard to maintain it.

Oracle doesn't even distinguish between UNIQUE CONSTRAINT and UNIQUE INDEX: one command is just a synonym for another.

The only difference in Oracle is that a UNIQUE INDEX should have a user-supplied name, while a UNIQUE CONSTRAINT may be created with a system-generated name:

ALTER TABLE mytable MODIFY col1 UNIQUE

This will create an index called SYS_CXXXXXX.