If I needed to unqiuely identify rows in a T-SQL database table I would set them as:
[id] INT NOT NULL IDENTITY(1,1) PRIMARY KEY
But if I need ID to be unique among several databases (on different computers) I go with:
[uid] UNIQUEIDENTIFIER NOT NULL
and then set an index on that column:
CREATE INDEX index_name ON [TableName] ([uid]);
So I was wondering how much of a performance hit would the database take by changing INT to UNIQUEIDENTIFIER?