Now I realize this is interesting and all that, but I also understand some readers may think this block dump and ITL stuff is not all that relevant. But I beg to differ. Not only have you gained a fuller understanding of the TX enqueue, but you have also clearly seen how Oracle implements its patented row-level locking scheme, In the next section, you will see how ITLs are involved with the "snapshot too old" error and with buffer cloning!

Earlier in this chapter, I introduced block cloning as it relates to CBC latch contention. Now I will delve deeper into exactly how Oracle does this using ITLs, undo blocks, SCNs, and other interesting Oracle tidbits.

If you recall, when a server process locates a desired buffer and discovers a required row has changed since its query began, it must create a back-in-time image of the buffer. This is known as a consistent read (CR) buffer of the current (CU) buffer. Once the buffer is copied, the appropriate undo is applied, bringing the copied buffer back in time until the CR buffer has been properly cloned. Now let's step through this process.