The TX enqueue wait is arguably the most common enqueue wait. Personally, I think it's also the most fascinating. I want to drill down into this wait event because it will give you a much deeper understanding of how Oracle manages transaction concurrency, which is related to block cloning, undo, read consistency, and interested transactions lists. I introduced the undo segment's transaction table earlier in the chapter. Now we're going much, much deeper.

While the TX enqueue is known as the row-level lock enqueue, there are actually three reasons for a TX enqueue to be posted, and only one of those is actually a row-level lock. Figure 6-34 is very similar to Figure 6-1 shown at the beginning of this chapter. Every Oracle data block can be abstracted into three areas:

* The row data contains the actual Oracle rows and is just one of the important parts of every data block.