Blocks of rollback segments are cached in the System Global Area just like
blocks of tables and indexes. If rollback segment blocks are unused for a period
of time, they may be aged out of the cache and written to disk.
System Change Number (SCN)
To preserve the integrity of the data in the database and enforce any type of serialization,
it is critical to keep track of the order in which actions were performed.
Oracle uses the System Change Number as an absolute determinant of the order
of transactions.
Oracle Concurrency Features | 193
The SCN is a logical timestamp that tracks the order in which transactions
begin. Oracle uses the SCN information in the redo log to reproduce transactions
in the original and correct order when applying redo. Oracle also uses the
SCN to determine when to clean up information in rollback segments that are
no longer needed, as you will see in the following sections.
Since Oracle Database 10g, there is a pseudocolumn on each row that
contains the SCN, ORA_ROWSCN. You can use this to quickly determine
if a row has been updated since it was retrieved by comparing
the value read from this pseudocolumn at the start of a transaction
with the value read from this pseudocolumn at the end of the
transaction.
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