On 19 Feb 2002, tom.mcclelland_at_mondas.com wrote:
> damorgan <dan.morgan_at_ci.seattle.wa.us> wrote in message
>> 1. No multiversioning.
> Sadly in my ignorance I don't know what this is. In my other platforms
> I am presumably suffering because of its absence. You didn't name lots
> of other great features of Oracle, that I would love if it were the
> *only* platform that I were trying to develop against, eg materialised
> views, the ability to use a sub query where a table-name goes in a
> query without building a view first, function-based indexes, and many
> others. Lots of great features don't make a product perfect however,
> and my experience of porting to Oracle has been around 100 times more
> painful than the other ports of my product that I've done.
Basically, all databases will have a rollback or undo capability.
Oracle goes through great but very clever steps to use this rollback
facility in a very powerful way. User A starts a transaction and
updates row X. After this transaction starts, User B queries for row X.
Oracle sees that this row has been updated but not committed. It
therefore performs a rollback on this data for User B only. User B gets
the before picture of row X. All other users will get the same
treatment. Therefore, Oracle is maintain multiple versions of the same
data. This makes it so that writers do not block readers. SQL Server
will not allow User B to see Row X until it has been committed.
--
Galen deForest Boyer
Sweet dreams and flying machines in pieces on the ground.
Received on Fri Mar 01 2002 - 21:19:15 CST
