Figure 6-40 is the basis for this example. Suppose our query begins at SCN time 12330. At SCN time 12355, our query finally gets around to accessing buffer 7,678. However, we notice there is some ITL activity. Transaction 7.3.8 is currently active, and the buffer may have been changed since our query began. Transaction 5.2.6 is no longer active (Flag of C, has an SCN assigned, and Lck is 0), but its changes were committed after our query started and are reflected in this CU buffer. Both of these block changes mean the CU buffer has been changed after our query began at SCN time 12330 and cannot be used as is in our query. We need a version consistent copy, which is one taken back in time, at time 12330. Therefore, the CU buffer 7,678 must be cloned and undo applied, creating a SCN time 12330 CR buffer.

Figure 6-40. A diagram of a single data block with its three areas and three associated undo blocks with their relationships shown. Two transactions are active, changing three rows.

Before the cloning can occur, an unpopular free buffer must be found and then replaced with the 7,678 CU buffer. Our server process will acquire the LRU chain latch associated with its LRU and begin scanning, starting at the LRU end of the LRU chain, looking for an unpopular free buffer. Eventually, it will find an unpopular free buffer and replace it with a copy of CU buffer 7,678. And, of course, the CBC structure will need to be updated to reflect the cloned buffer's location in the buffer cache.