From：11gR2
http://download.Oracle.com/docs/cd/E11882_01/server.112/e17118/pseudocolumns007.htm#SQLRF50953
Foreach row, ORA_ROWSCN returns the conservative upper bound system change number(SCN) of the most recent change to the row in the current session. Thispseudocolumn is useful for determining approximately when a row was lastupdated.
Itis not absolutely precise, because Oracle tracks SCNs by transaction committedfor the block in which the row resides. You can obtain a more fine-grainedapproximation of the SCN by creating your tables with row-level dependencytracking. Refer to CREATE TABLE ... NOROWDEPENDENCIES| ROWDEPENDENCIES for more information on row-level dependency tracking.
ROWDEPENDENCIES if you want to enable row-level dependency tracking.This setting is useful primarily to allow for parallel propagation inreplication environments. It increases the size of each row by 6 bytes.
NOROWDEPENDENCIES if you do not want table to use the row-leveldependency tracking feature. This is the default.
而在创建table时指定为rowdependencies时，
如：
tab 0, row 1, @0×1f88
tl: 12 fb: –H-FL– lb: 0×2 cc: 1
tab 0, row 1, @0×1f88
tl: 12 fb: –H-FL– lb: 0×0 cc: 1
这是同一个row的dump 信息，第一次dscn 为0. 此时信息是从itl的Scn/Fsc中获得的，。具体这块后面会实验证明。
关于block dump 和 itl 说明，参考：
http://www.linuxidc.com/Linux/2011-08/40286.htm
You cannot use this pseudocolumn in a query to a view.However, you can use it to refer to the underlying table when creating a view.You can also use this pseudocolumn in the WHERE clause of an UPDATE or DELETEstatement.
ORA_ROWSCN is not supported for Flashback Query.Instead, use the version query pseudocolumns, which are provided explicitly forFlashback Query. Refer to the SELECT ... flashback_query_clausefor information on Flashback Query and "VersionQuery Pseudocolumns" for additional information on thosepseudocolumns.
Restriction on ORA_ROWSCN: This pseudocolumn is notsupported for external tables.
Example：
Thefirst statement below uses the ORA_ROWSCN pseudocolumn to get the system changenumber of the last operation on the employees table.
SELECT ORA_ROWSCN, last_name
FROM employees
WHERE employee_id = 188;
The second statement uses the pseudocolumnwith the SCN_TO_TIMESTAMP function to determine the timestamp of the operation:
SELECTSCN_TO_TIMESTAMP(ORA_ROWSCN), last_name
FROM employees
WHERE employee_id = 188;
ORA_ROWSCN伪列是Oracle10g引入的，可以查询表中记录最后变更的SCN。这个新的伪列在某些环境下会非常有用，比如执行乐观锁定，或者增量数据抽取的时候。但是，默认情况下，每行记录的ORA_ROWSCN是基于Block的，除非在建表的时候执行开启行级跟踪(createtable … rowdependencies)。
需要select ... for update做悲观锁定的时候，通过使用ORA_ROWSCN可以改成乐观锁定。一开始select数据的时候将ORA_ROWSCN查出来，修改后如果要写回数据库之前再比对下最新的ORA_ROWSCN就可以知道这期间数据是否有发生变化。
每次抽取后记录最大的ORA_ROWSCN，下次抽取再基于上一次的SCN来获得最近修改过的数据即可。在10g之前，很多系统要实现增量数据抽取，要么通过解析日志，要么加触发器，要么就在表上加一个时间截字段。
ORA_ROWSCN其实就是第三种方式，只是这个字段由Oracle来维护，这样可以避免一些应用绕过时间截去更新其他字段带来的问题。不过，如果系统中使用了逻辑备库或者streams等逻辑复制的方案，而数据抽取又是基于逻辑备库的话，ORA_ROWSCN就可能对抽取后的数据分析有影响了，因为通过这个得到的时间是逻辑备库上记录变更的时间，而不是源库的时间了。
当然，如果纯粹只是做数据抽取，而不需要使用这个时间来做分析，还是问题不大的，但还是要考虑一旦逻辑备库出现故障需要重做的，则这个增量抽取要怎么来处理的问题。