Re: Surrogate Keys: an Implementation Issue

"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message news:X1gxg.15744$pu3.340040_at_ursa-nb00s0.nbnet.nb.ca...
> Brian Selzer wrote:
>
>> "Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
>> news:XQ5xg.14298$pu3.333248_at_ursa-nb00s0.nbnet.nb.ca...
>>
>>>Brian Selzer wrote:
>>>
>>>>"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.
>>>
>>>So? Bob issues an update statement to change some attribute identified by
>>>a logical identifier. As long as the data Bob is entering is correct, why
>>>should he care what happens to any other attributes?
>>
>> He may be overwriting a change made by another user.
>
> The only relevant concern is whether the update is correct. If the update
> is correct, it matters not at all that it overwrites something, which is
> presumably no longer correct.
>
>
>>> 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.
>>>
>>>Or you can just write better applications that don't update anything that
>>>didn't change. By introducing some 'under-the-covers' identifying
>>>attribute, you create a risk that Bob will change some data identified by
>>>a familiar logical identifier and some other process in the meantime will
>>>associate that identifier with a different surrogate. Your application
>>>will then record the updates against the wrong logical identifier.
>>
>> You're missing the point. The change Bob's making may change the row,
>> but because several changes occurred to the table while Bob was keying in
>> his change, the row he's about to change may represent some other entity
>> altogether.
>
> Using logical identity and natural keys, that's not possible. The key that
> Bob specifies identifies the entity. And if Bob updates the entire row,
> the entire row needs updating. I draw your attention, in particular, to my
> observation that one can just write better applications that don't update
> anything that didn't change.

Of course it's possible. The key that Bob specifies identifies the entity at the time of the read from the database. By the time that the write occurs, that same key now refers to a different entity. The logical identity in the database value that existed at the time of the read is different from the logical identity in the database value that exists at the time of the write even though their atomic values are identical. Logical identity within the value of a database at a specific point in time can be represented by a natural key. In order to span multiple database values with an arbitrary number of intermediate values, there must exist on each relation a key whose value is guaranteed not to change during the time that Bob is keying in his change--even if he decides to go on vacation. Since a natural key value can change a number of times in the interval between the database value that existed at the time of the read and the database value at the time of the write and since in a concurrent environment these changes may have originated by a number of users, the natural key value at the time of the read may not necessarily refer to the same entity at the time of the write. The logical identity of an entity represented by a key value is relative to the contents of the relation value to which the entity belongs. The introduction of an undefined number of intermediate relation values (up to one for each intermediate database value) between the time that a row is read and the time that it is written makes it impossible to determine whether or not the key value about to be written refers to the same entity for which the key was read.

>
> The only time Bob's change could update the wrong entity is if the
> application Bob is using identifies the 'row' using a surrogate key that
> Bob cannot see.
>

I don't think you understand the problem. See above.

>
>> The Relational Model doesn't take into account duration
>
> You have yet to establish that it needs to.
>
>
> , so the value of a
>> database at the time of a read is not necessarily the same as that at the
>> time of a write
>
> This only matters if one writes shitty applications with sloppy code. See
> my earlier posts.
>

No. The integrity of the database should not depend on the quality of application code. The database should reject changes that would cause it to become corrupt, so shitty applications and sloppy code are not relevant to this discussion.

>
> , nor is the meaning of a key with a specific atomic value at
>> the time of a read necessarily the same as the meaning of a key with the
>> same atomic value at the time of a write.
>
> Your statement is only possible if the value of the key is hidden from the
> user. Thus, you invented the problem by introducing a surrogate.
> Otherwise, values do not change their meanings. They have the same meaning
> for all time.
>

The problem only exists when there is no surrogate, so your statement makes no sense.

Atomic values don't change their meaning. But an attribute value can.

>
> Surrogates provide the continuity
>> across database states that the Relational Model lacks.
>
> I disagree. Hidden surrogates re-introduce all the problems of
> location-based updates. The use of logical identity through exposed values
> obviates your entire argument. Apparently, you lack the ability to
> recognize that fact.
>

I don't think that surrogates should be hidden--at least not from applications. They should be hidden from end-users, however.

Logical identity does not necessarily span database values. Apparently, you lack the ability to recognize that fact.

>
>>>It seems to me you just went to a lot of trouble to invent a problem that
>>>never existed in the first place.
>>
>> I didn't invent this problem.
>
> I disagree. Had you not introduced a hidden surrogate, the problem would
> never have existed.
>

Duh! The problem stems from the lack of a surrogate and the inherent volatility of natural keys.

>
> I've run across it several times. It's
>> extremely difficult to diagnose and even more difficult to repair, since
>> it lets garbage get into the database.
>
> Shitty applications with sloppy code force garbage into the database. Stop
> writing them.
>

I didn't write them: I fixed them. Had I written them, they would have used surrogates and thus would not have exhibited this behavior. But it has noting to do with how poor an application is. Integrity should be handled by the database, not the application.

>
>>>>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.
>>>
>>>I disagree entirely.
Received on Tue Jul 25 2006 - 13:58:25 CEST