* C--- means the transaction has been committed, row data has been consolidated, and the ITL entry in the row data has been removed. Any block touch can (but does not always) trigger the change to this flag value, including a select statement. I realize this is difficult to believe, so I will demonstrate it shortly. This seemingly delayed change is commonly referred to as a delayed block cleanout, or simply a block cleanout.

* Lck: This is the number of rows the transaction has, at some point, locked in this block. A value greater than 0 does not indicate a row(s) is currently locked. If the number is 2, as is the case with the second transaction shown in Figure 6-37, the transaction is related to two rows. The lock value remains until the flag changes to a C---. This means after a transaction has committed and is no longer deemed active (flag of --U-), the Lck value can be greater than zero.

* Scn/Fsc: The SCN is the system change number and is used to determine when the transaction ended (committed or rolled back). Notice in Figure 6-37 that the SCN has not been assigned, but after the transaction commits, as shown in Figure 6-38, the SCN is set. The SCN is important when determining if undo retrieval is necessary when creating a read consistent version of the buffer; that is, creating a buffer clone. The FSC refers to the free space credit. It is used by uncommitted transactions when an update or delete operation caused a row to shrink in length. Oracle will preserve this free space in case the transaction rolls back and the space needs to be refilled. If the free space was used for something else and then the transaction rolled back, the row may need to be migrated!