Re: cdt glossary 0.1.1 [Transaction]

From: Brian Selzer <brian_at_selzer-software.com>
Date: Fri, 20 Apr 2007 14:38:12 GMT
Message-ID: <of4Wh.5037$2v1.792@newssvr14.news.prodigy.net>

"paul c" <toledobythesea_at_oohay.ac> wrote in message news:383Wh.100591$DE1.26701_at_pd7urf2no...

> Brian Selzer wrote:

>> "paul c" <toledobythesea_at_oohay.ac> wrote in message
>> news:x6SVh.98742$DE1.19705_at_pd7urf2no...
>>
>>>mAsterdam wrote:
>>>
>>>>paul c wrote:
>>
>> [snip]
>>
>>
>>>I wasn't suggesting it did. As far as I know, "order of work" is not a
>>>relational issue, nor is a transaction a relational concept any more than
>>>invoices are. Wondering why they aren't part of RT is like asking why
>>>the RM isn't based on a state machine.
>>>
>>
>>
>> So what would you call something that transforms one database state into
>> another? In 1970, Codd called them "state-changing transactions."
>> ...

>
> Codd subscribed to the idea of a data sub-language.  I think whenever he 
> used terms such as "states" and "transactions", he was talking about a 
> programming environment, not the RM.
>

Codd (1970) defined consistency using the terms "state" and the phrase "instantaneous value." And later emphasized it: "It is important to note that consistency as defined above is a property of the instantaneous state of a data bank..."

> ...

>>
>> I think that if a transaction contains more than one operation, then the
>> order in which each operation is evaluated is critical. 2 + 3 * 5 = 17,
>> not 25.
>>
>> After the following transaction,
>>
>> UPDATE r SET x = x + 5 WHERE k = 22,
>> UPDATE r SET x = x * 4 WHERE k = 22
>> ...

>
> If all I wanted to do was to add 5 to x and then multiply by 4, I would 
> expect my programming environment to give a single statement to the dbms, 
> not two.
>

Perhaps, but the updates above are pretty simple, how would you deal with a transaction consisting of multple updates where the tuples targeted by each overlap?

>> Is the result (x + 5) * 4 or (x * 4) + 5? Or is it x * 4, which is what
>> D&D's multiple assignment would produce?
>> ...

>
> I'm not in favour of encouraging the complexity that multiple assignment 
> requires a programmer to be aware of.  (I'm not even in favour of 
> assignment to mutable variables.  I realize most programmers are used to 
> them and expect them to be supported, but I don't care.)
>

I'm not sure what you mean by "mutable variables."

>> Do you limit a transaction so that only one transformation can occur per
>> relation? Per tuple? Per attribute value?
>> ...

>
> Any concept of a database that has two different values at the same time 
> is beyond me.
>

Who said anything about the database having two different values at the same time? The question involves a group of modifications to a database. The database wouldn't take on the new value until the entire transaction completed.

>

>> Should all constraints be checked after each operation? Only some? Or
>> should they all be deferred until the end? I mention this because the
>> result of one operation may leave the database in an inconsistent state,
>> making any subsequent operations suspect.
>> ...

>
> Any dbms that allows a programmer to introduce an inconsistency should be 
> recalled.
>

Transitory inconsistencies can occur, so long as they're resolved by the time that a transaction completes. A transaction executes in isolation, but the results of each operation in a transaction could be made visible to subsequent operations within the same transaction.

>> If it were possible to convert a group of statements into a single
>> operation, then there would be no need for order, there would only be one
>> transformation per relation, per tuple and per attribute value, and
>> constraints would only need to be checked once. Alas, this capability is
>> not available in any commercial system that I know of, and even D doesn't
>> doesn't support it.
>> ...

>
> So what?  (Most of IT is mistaken.  People usually prefer to re-invent old 
> wheels.)
>
> p 

Received on Fri Apr 20 2007 - 09:38:12 CDT