At first glance, the maximum number of 255 ITLs may seem very limited, but consider this: Think of highest concurrency table, in the highest concurrency application, in the highest concurrency database system you administer. Perhaps there is a table that could possibly have 250 concurrent processes updating, deleting, and inserting rows. Now ask yourself how many processes will be concurrently updating, deleting, or inserting rows into a single block-not the entire table or extent, but in a single block. This is the number of ITLs that may be eventually created in a single block. Even with the highest concurrency applications, to have more than 255 concurrent transactions active in a single block is extremely unlikely. So a maximum of 255 ITLs is really not that limiting after all. However, if this does present a problem, you can reduce block concurrency by increasing the table's pct_free parameter or perhaps add a fixed-length column to reduce the number of rows that could possibly exist in the block.

As mentioned earlier in the undo segment header buffer busy wait section, each undo segment contains a structure called the transaction table located in its header block. Figure 6-36 is a diagram of an Oracle transaction table. As noted earlier, Oracle developers refer to its rows as slots. Every occupied slot is associated with a transaction that is storing or has stored its undo information in the undo segment. If a transaction has committed or rolled back, it is deemed an inactive transaction; otherwise, it is deemed an active transaction. Besides containing the slot number and the transaction status, each slot also contains a sequence number. To distinguish between different transactions that have used the same slot and to enable slot reuse, the sequence number can be incremented. The UBA is the undo block address, providing a direct link to the transaction's undo. The SCN is the transaction's system change number when the associated transaction began.

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.