Figure 6-39. Shown is a data block (1,75847) dumped immediately after a simple select statement touched the block. Notice the transaction flags have changed from a --U- to a C---, indicating a block cleanout has occurred.

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.