Re: cdt glossary 0.1.1 [Transaction]

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

> Brian Selzer wrote:

>
> 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..."

> ...

>
> 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.

>

>
> 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