Figure 6-33. Shown is a standard v$session_wait-based report (in particular, the OSM swswp.sql script) with three sessions involved in a TM enqueue wait. In this particular situation, session 4393 has the table locked and is not waiting on the enqueue. Session 4388 is next in the queue, followed by session 4383.

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: