Until CLOLSEN performs a commit at 10:15,
the data in the table appears unchanged as of 10:00.
210 Oracle Database 11g DBA Handbook
If there is not enough undo space available to hold the previous values of changed rows, the
user issuing the select statement may receive an ???ORA-01555: Snapshot Too Old??? error. Later in
this chapter, we will discuss ways in which we can address this issue.
Database Recovery
Undo tablespaces are also a key component of instance recovery. The online redo logs bring both
committed and uncommitted transactions forward to the point in time of the instance crash; the
undo data is used to roll back any transactions that were not committed at the time of the crash or
instance failure.
Flashback Operations
The data in the undo tablespace is used to support the various types of Flashback options: Flashback
Table, Flashback Query, and the package DBMS_FLASHBACK. Flashback Table will restore a
table as of a point of time in the past, Flashback Query lets you view a table as of an SCN or time
in the past, and DBMS_FLASHBACK provides a programmatic interface for Flashback operations.
Flashback Data Archive, new to Oracle Database 11g, stores and tracks all transactions on a
specified table for a specified time period; in a nutshell, Flashback Data Archive stores undo data
for a specific table in a specific tablespace outside of the global undo tablespace. Also new to
Oracle Database 11g is Flashback Transaction Backout that can roll back an already committed
transaction and its dependent transactions while the database is online.
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