Oracle Core: Essential Internals for DBAs and Developers

C H A P T E R  3

Transactions and Consistency

Now You See Me, Now You Don’t

In Chapter 2 you saw how Oracle uses redo change vectors to describe changes to data, undo records to describe how to reverse out those changes, and redo (again) to describe how to create the undo records—and then (apart from a concurrency optimization introduced in Oracle Database 10g) applies the changes in near real time rather than “saving them up” to the moment you commit.

Chapter 2 also commented on the way that undo records allow changes to the data to be kept “invisible” until everyone is supposed to see them, and how we can also use undo records to roll back our work if we change our minds about the work we’ve done.

Finally, Chapter 2 pointed out that redo ...