Residing in every Oracle data block's variable data area are structures called interested transaction lists (ITLs). These structures are largely responsible for Oracle's row-level lock and read consistency capabilities. From a highly abstracted perspective, you can think of ITLs like check boxes, as illustrated in Figure 6-35. Each check box is related to a specific transaction. If you want to update a row, but that locked row is already associated with another transaction's ITL, you will receive a TX enqueue wait-truly, a row-level lock.

Figure 6-35. A highly abstracted Oracle data block variable data area highlighting the three interested transaction lists (ITLs). The ITLs are deeply involved with row-level locking; that is, concurrency control.

Each Oracle data block is created with a specific number of ITLs. The initial number of ITLs is controlled by the table's initrans space parameter and can be queried from the dba_tables view's ini_trans column. Starting with Oracle9i, the default ini_trans value is 1, yet a simple block dump clearly shows two ITLs are always created. With two ITLs, a single block can be subjected to two concurrently active transactions.