The package dbms_advisor can be used to analyze the undo tablespace usage, as
we did in Chapter 6; we will investigate this package in more detail and show how Oracle
Enterprise Manager Database Control can make it easy to perform the analysis.
The last major section of this book will review the different types of Oracle Flashback features
that rely on an adequately sized undo tablespace to recover from a number of different user error
scenarios. All the major Flashback features at the query, table, or transaction level are covered in
this section; Flashback Database is covered in Chapter 14.
Rollback segments from previous Oracle releases were hard to manage and were usually
sized too large or too small by most DBAs; Oracle strongly recommends that all new databases
use Automatic Undo Management and that databases upgraded from a previous version of Oracle
be converted to using Automatic Undo Management. We won??™t cover any aspects of manual
undo management here except for how to migrate from rollback segments to automatic undo.
Transaction Basics
A transaction is a collection of SQL DML statements that is treated as a logical unit; the failure of
any of the statements in the transaction implies that none of the other changes made to the database
in the transaction should be permanently saved to the database. Once the DML statements in the
transaction have successfully completed, the application or SQL*Plus user will issue a commit to
make the changes permanent.
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