Re: transaction isolation

Hi David,

David Cressey wrote:
>
> Ruud,
>
> > I would think the insert logically takes place at the moment it is
> > committed. Nothing undetermined about that.
>
> Agreed. But when does a "virtual snapshot" take place? I believe it's at
> the start of the transaction.
> In order to make reads repeatable, You have to have a virtual snapshot of
> the database (or at least of
> a subset of it) at the time a SELECT begins returning results.

Sure, but the point is: do we always want to make things repeatable?

>
> About your (unnamed) product... does it not support the "SET TRANSACTION"
> statement?

Yes, it does. My suspicion arose using the default setting, however. Also, the only settings that seem to be offered are READ COMMITTED, REPEATABLE READ and SERIALIZABLE. In my view that 's a choice between strict and very strict. Now this might not be a problem, but is is when you combine it with an implicit transaction start (at least in my opinion).

>
> Two products I know, Oracle RDBMS and Oracle Rdb, both support it.
> Although an implicit transaction will be started if you perform any
> operation that requires a transaction, I think it's better programming
> practice to make the start of a transaction explicit. I believe, from what
> you have written, that you would agree.

You are right, there.

>
> Oracle Rdb, unlike Oracle RDBMS, supports snapshot transactions, which can
> offer the highest level of consistency possible. If a snapshot transaction
> is started, nothing that had not been committed prior to the snapshot is
> visible.
> There are consequences as far as overhead is concerned.
>

> --
> Regards,
> David Cressey
> www.dcressey.com

Thanks again,

Ruud.

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Ruud de Koter                    HP OpenView Software Business Unit
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