Re: a union is always a join!
From: paul c <toledobythesea_at_oohay.ac>
Date: Wed, 04 Mar 2009 23:47:09 GMT
Message-ID: <1qErl.15295$Db2.6812_at_edtnps83>
>
> Let's look at "whenever nothing is happening" in more detail. I submit
> that, at a point in time where there are no transactions in progress, that
> "nothing is happening". Further, I submit that, if serializability is the
> criterion for concurrency management, then the DBMS ensures that the view
> of the database is as if "nothing is happening", during a transaction,
> except for the actions of that transaction. Every other transaction can be
> seen as being completed "in the past" or beginning "in the future".
> ...
Date: Wed, 04 Mar 2009 23:47:09 GMT
Message-ID: <1qErl.15295$Db2.6812_at_edtnps83>
Walter Mitty wrote:
> "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".
> ...
well put. What's more, IYAM, assuming a very strict implementation such as two-phase lock protocol where a transaction ends when the first "lock" is released (not that I advocate 2PL), nothing is happening during any transaction and IYAM this would apply even for a dbms with temporal support. Received on Thu Mar 05 2009 - 00:47:09 CET