Now that I've introduced ITLs and undo segment transaction tables, it's time to bring them together as a single working unit and present how ITLs change during transaction activity. This section should leave you with a very good understanding of how Oracle manages transaction concurrency, how Oracle creates read consistent blocks, and why the feared "snapshot too old" error occurs.

Figure 6-37 is not a model or a diagram. It is an actual block dump created by issuing the command alter system dump datafile 1 block 75847. At the time of the block dump, this block (1,75847) contains many rows and has three active transactions updating four different rows. The first and third transactions shown are updating one row each, while the second transaction is updating two rows.

Figure 6-37. Shown is a data block (1,75847) dumped in the midst of three concurrent transactions. There are three active transactions updating a total of four rows.