At time T4, row 136's value is 115. Any access to this block that started at time T4 or later will see a value of 115. However, even after the transaction commits, if a query started before time T4, a read-consistent view of buffer 142 must be created using buffer 142 at time T4 and undo buffer 210. I am leaving out quite a bit of detail (covered in Chapter 6), because the point I want to make here is that all this read-consistency work is referencing standard Oracle undo segment buffers, not highly optimized in memory structures.

Figure 7-16. How traditional undo segment are used, as well as the various key components (data buffer, undo buffer, redo buffer, and online redo log) change as a transaction progresses

* Time T1: For space reasons, time T1 is not shown in Figure 7-17, but it's the same as in Figure 7-16. At time T1, buffer 142 contains row 136, which has a column value of 105.