Oracle Database 11 g - Online Public Access Catalog

CHAPTER 3 ■ DATABASE ADMINISTRATION 125Note that the default value of nocascade expects that the transaction pending a backouthas no dependent transactions.TRANSACTION_BACKOUT ReportsYou can examine the DBA_FLASHBACK_TRANSACTION_STATE and DBA_FLASHBACK_TRANSACTION_REPORT views for details about the transaction backout operation. Thetransaction_backout procedure populates both of these views. If a transaction appears in theDBA_FLASHBACK_TRANSACTION_STATE view, it means the transaction has been successfullybacked out of the database. For each backed-out transaction, the DBA_FLASHBACK_TRANSACTION_REPORT view provides a detailed report.You can also use Database Control to perform a transaction backout operation if you don’twant to use the DBMS_FLASHBACK package directly.Database Administration New FeaturesOracle Database 11g adds several important weapons to the toolkit of DBAs to facilitate theday-to-day management of the database as well as providing additional object managementcapabilities. In the following sections, we discuss the most important of these enhancements.Virtual ColumnsSometimes you might want to store data in a column based on the evaluation of an expression.Oracle Database 11g provides a new type of column you can include in a table, called a virtualcolumn. Virtual columns are similar to normal table columns, with two major differences:• You can’t write to a virtual column.• A virtual column is populated by the evaluation of an expression.You can derive the virtual column from evaluating an expression on columns from the sametable, from constants, or as the result of evaluating a SQL or PL/SQL function. You’ll see the valuesunder a virtual column only when you query that column, but the values aren’t permanently storedon disk, as is the case for normal columns. The values in a virtual column are calculated only whenit’s queried by dynamically computing a function or a column expression.You can do the following things with a virtual column:• Use them in both DDL and DML statements.• Collect statistics on them.• Define indexes on them. The indexes you define are similar to function-based indexesyou create on normal columns. Behind the scenes, Oracle creates a function-basedindex on the virtual column when you create an index on that column.