Figure 6-36. Diagram of an Oracle transaction table, which contains metadata about its associated undo segment. Each undo segment's transaction table consists of slots and also sequence numbers to distinguish transactions that have used the same slot, and also the transaction status. Additional information about the transaction is also stored. Transaction table size is limited by the database block size.

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.