Re: a union is always a join!

"Brian Selzer" <brian_at_selzer-software.com> wrote in message news:pi_rl.24249$ZP4.5099_at_nlpi067.nbdc.sbc.com...
>
> "Walter Mitty" <wamitty_at_verizon.net> wrote in message
> news:Ziyrl.1235$%u5.1161_at_nwrddc01.gnilink.net...
>>
>> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
>> news:wLsrl.14888$as4.3977_at_nlpi069.nbdc.sbc.com...
>>>
>>
>>>> that states what is happening. Whenever nothing is happening, what has
>>>> been the case /is/ the case. For example, if Joe has been second in
>>>> line
>>
>> Let's look at "whenever nothing is happening" in more detail. I submit
>> that, at a point in time where there are no transactions in progress,
>> that "nothing is happening". Further, I submit that, if serializability
>> is the criterion for concurrency management, then the DBMS ensures that
>> the view of the database is as if "nothing is happening", during a
>> transaction, except for the actions of that transaction. Every other
>> transaction can be seen as being completed "in the past" or beginning "in
>> the future".
>>
>> So, if the transactions can be serialized, each transaction sees the
>> database as if "nothing is happening". If you agree with all that, then
>> why isn't your point moot? If you don't agree, where don't you agree?
>
> My point has little if anything at all to do with transactions and
> concurrency control. Those belong to implementations. My point is that
> relational calculus, or any equivalent mechanism such as relational
> algebra, while necessary for describing database updates, is not
> sufficient for that purpose because it can only apply to a single
> database, not two successive databases. The mechanism of updating the
> database cannot be reduced to mere algebraic expressions, but instead to
> asserting, in the context of what has been the case, just what in the
> world is different and exactly how. Let me explain.
>

So, what does "whenever nothing is happening" mean in the context of the point you are making? Received on Fri Mar 06 2009 - 16:05:28 CET