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Home -> Community -> Mailing Lists -> Oracle-L -> Re: why does oracle recovery redo before undo?
"Optimal" recovery steps depend on the database architecture. By
design, Oracle has implemented a multi-versioning read and write
concurrency architecture to maximize data availability for many
simultaneous database users. In contrast, other database architectural
approaches block read access to data when an update is occuring - data
is not available for other users to read until the transaction is complete.
Chapter 1 of the Oracle Concepts documents the "Data Concurrency and Consistency" design. The Oracle Backup and Recovery Concepts documents the "Redo Application During Recovery" which includes a couple potential problems that can result if an instance failure occurs. Due to these potential problems, Oracle's cache recovery (roll forward) and transaction recovery (roll back) steps are "optimal" for the Oracle database architecture.
Have Fun :)
Ryan wrote:
>I know how it works. I know that it works. I can't find 'why'? I'm taking a university database class and my professor says the optimal way to recover is with undo before redo and so does my book.
>Anyone know why Oracle does it the other way?
>
>
-- Archives are at http://www.freelists.org/archives/oracle-l/ FAQ is at http://www.freelists.org/help/fom-serve/cache/1.html -----------------------------------------------------------------Received on Mon Apr 19 2004 - 22:39:04 CDT
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