Re: Surrogate Keys: an Implementation Issue

"Bernard Peek" <bap_at_shrdlu.com> wrote in message news:xn0ep1e3sn11oo000_at_news.individual.net...
> In comp.databases.theory Paul Mansour wrote:
>
>
>> The second is perhaps more profound. Consider a rollback database, or
>> a database that must provide a complete audit trail of every change.
>> For example, the database must provide the answer to "who changed
>> this SS number from X to Y, and when did they change it? As far as I
>> can tell, if there is no way to answer this without an immutable
>> identifier. ( I suppose you could design the DB to handle specific
>> cases, but I'm interested in DBMS with native rollback and audit
>> trail support.)
>
> If what you think of as the key is mutable it's not a natural key.
> Natural keys aren't mutable, at all, ever.

Untrue! Natural keys are often mutable--especially compound keys.

>
> The problem here is that there isn't a usable natural key for
> identifying people. So all that's left are surrogates of varying
> quality. The SSN is a surrogate that works most of the time. If you
> assign someone a payroll number and tell tham that thay won't get paid
> unless they can quote it then you have a close to immutable key. But
> it's still a surrogate.
>
> I think the original post was part correct. Use a natural key when you
> have one. I'd just add to that, don't use a surrogate unless there is a
> compelling reason for it. Efficiency is unlikely to be a compelling
> argument in most cases.
>
>
There is always a compelling reason to use surrogates: natural keys can change. This makes it difficult--if not impossible--to detect changes to rows. For example, Bob is preparing to update a row--that is, he has read the row and is in the process of keying in a change. During that time, another process updated several rows in the same table. Unless the key is immutable, when Bob issues the update, there's no guarantee that the row he's updating is the same one that was read out. This problem is magnified if there are rows related via a foreign key constraint because it's possible for the referenced row to appear unchanged. So you're left with either maintaining an exclusive lock on the row until Bob returns from the golf outing, or adding additional columns and code in order to determine with certainty whether or not a change occurred between the time that a row was read and the time of the update.

Natural keys are necessary to maintain the integrity of the information stored in the database; surrogates are necessary to maintain the integrity of information during the time that it is in use by applications.
>
> --
> --
> Bernard Peek
> bap_at_shrdlu.com
>
Received on Mon Jul 24 2006 - 16:56:03 CEST