Redo and Undo - Expert Oracle Database Architecture - page 312

The DELETE
Again, undo is generated as a result of the DELETE , blocks are modified, and redo is sent over to the redo log buffer. This
is not very different from before. In fact, it is so similar to the UPDATE that we are going to move right on to the COMMIT .

The COMMIT
We've looked at various failure scenarios and different paths, and now we've finally made it to the COMMIT . Here,
Oracle will flush the redo log buffer to disk, and the picture will look like Figure 9-4 .

Block Buffer
Cache

Redo Log
Buffer

Redo

Undo Indexes Table T

Figure 9-4. State of the system after a COMMIT

The modified blocks are in the buffer cache; maybe some of them have been flushed to disk. All of the redo
necessary to replay this transaction is safely on disk and the changes are now permanent. If we were to read the data
directly from the data files, we probably would see the blocks as they existed before the transaction took place, as DBWn
most likely has not yet written them. That's OK—the redo log files can be used to bring those blocks up to date in the
event of a failure. The undo information will hang around until the undo segment wraps around and reuses those
blocks. Oracle will use that undo to provide for consistent reads of the affected objects for any session that needs them.

Commit and Rollback Processing
It is important to understand how redo log files might impact us as developers. We will look at how the different ways
we can write our code affect redo log utilization. We've already seen the mechanics of redo earlier in the chapter,
and now we'll look at some specific issues. You might detect many of these scenarios, but they would be fixed by the
DBA as they affect the database instance as a whole. We'll start with what happens during a COMMIT , and then get into
commonly asked questions and issues surrounding the online redo logs.

What Does a COMMIT Do?
As a developer, you should have a good understanding of exactly what goes on during a COMMIT . In this section, we'll
investigate what happens during the processing of the COMMIT statement in Oracle. A COMMIT is generally a very fast
operation, regardless of the transaction size. You might think that the bigger a transaction (in other words, the more