Using “varchar” as the primary key? bad idea? or ok?

Question

Is it really that bad to use \"varchar\" as the primary key?

(will be storing user documents, and yes it can exceed 2+ billion documents)

Answer 1

If you are going to be joining to other tables, a varchar, particularly a wide varchar, can be slower than an int.

Additionally if you have many child records and the varchar is something subject to change, cascade updates can causes blocking and delays for all users. A varchar like a car VIN number that will rarely if ever change is fine. A varchar like a name that will change can be a nightmare waiting to happen. PKs should be stable if at all possible.

Next many possible varchar Pks are not really unique and sometimes they appear to be unique (like phone numbers) but can be reused (you give up the number, the phone company reassigns it) and then child records could be attached to the wrong place. So be sure you really have a unique unchanging value before using.

If you do decide to use a surrogate key, then make a unique index for the varchar field. This gets you the benefits of the faster joins and fewer records to update if something changes but maintains the uniquess that you want.

Now if you have no child tables and probaly never will, most of this is moot and adding an integer pk is just a waste of time and space.