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.

* Xid: This is the transaction ID, which is a concatenation of the transaction table ID (in Figure 6-37, 0003), the transaction's transaction table's slot number (in Figure 6-37, 00d), and the sequence number (in Figure 6-37, 0000318). The transaction ID is important, as it is used to ensure seemingly related undo information is truly related.