* 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!

Continuing with the example shown in Figure 6-37, suppose the first two transactions (ITLs x01 and x02) commit, making their transactions inactive. The third transaction, ITL x03, has not yet committed. Immediately after the first two transactions commit, the same block dump command, alter system dump datafile 1 block 75847 is issued, with the results shown in Figure 6-38. Notice the flag has changed, and an SCN has been assigned to the transaction.

Figure 6-38. Shown is a data block (1,75847) dumped immediately after the first two transactions committed and the third transaction remaining active. Notice the committed inactive transaction flags have changed from ---- to --U-.