Transaction tables are relevant to performance analysts because they supply the transaction number. Every transaction has an associated transaction number, and that number is based on the transaction's transaction table entry. The transaction number consists of three parts concatenated together. The first part is the transaction table number, the second is the slot number, and finally the associated sequence number. For example, the first transaction shown in Figure 6-36 has a transaction number of 00100.000.0007. The connection between ITLs and the transaction table is that every ITL entry is related to a specific transaction and contained within the ITL entry is the transaction number, such as 00100.000.00007. Now it's time to complete our ITL discussion.

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.