* The row data contains the actual Oracle rows and is just one of the important parts of every data block.

Figure 6-34. An Oracle data block model containing the three main block parts. Notice the block's free space can be decreased by growth from both the variable and row data areas.

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.