Fortunately, the transaction numbers do match! The undo in undo block 2,70 was changed at SCN time 12340, which is after our query started, so we apply the undo to our CR buffer. The next undo link is empty, so there is no more undo to consider applying.

Referring back to our ITL entries, there are no more ITLs to consider, so we have completed our block clone. Any server process can now access CR buffer 7,678 consistent at time 12330.

It should now be obvious why ITLs are so important and that Oracle's read-consistent model, while extremely powerful and necessary and efficient, is still relatively expensive because it has the potential to consume a lot of CPU and IO, slowing application response time. Oracle is very aware of this, and in Oracle Database 10g Release 2, it started using in-memory optimized structures to temporarily store undo information. These objects are not segments and are therefore not subjected to segment-related overhead like CBC and LRU chain activity. In memory, undo is stored in the shared pool, which is covered in Chapter 7.