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Re: Surrogate Keys: an Implementation Issue

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Wed, 26 Jul 2006 16:33:51 GMT
Message-ID: <PPMxg.21411$pu3.353655@ursa-nb00s0.nbnet.nb.ca>


Brian Selzer wrote:

> "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.

Are you stupid or just ignorant? What you state above can only happen if one assumes one is using a hidden surrogate. If one uses explicit values that Bob interacts with, then the value identifies a single entity. That identity does not change.

   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.

You are a crank. Plonk.

[longwinded idiotic nonsense snipped]

>>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.

I don't think you understand relatively plain english. I won't bother directing you to anything. Since you were incapable to understand it the first time, I have little hope you ever will.

>>>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.

With all due respect, the dbms can only do so much. If a willful ignorant like yourself is determined enough, he can write shitty sloppy code that forces garbage into the dbms. All he has to do is make updates that were never requested by users exactly as you propose when constructing your straw man.

   The database should reject changes that would cause it to > become corrupt, so shitty applications and sloppy code are not relevant to > this discussion.

To answer your stupidity, I would first have to teach you what an external predicate is. Since that is extremely basic and since you have failed to learn that before spouting nonsense, you don't deserve any such remedial lesson.

You are a vociferous ignoramus. It suffices to observe that fact.

[remaining nonsense snipped without further effort] Received on Wed Jul 26 2006 - 11:33:51 CDT

Original text of this message

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