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3 Cards in this Set
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Oracle Flashback Query
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Oracle Flashback Query lets you view and repair historical data. You can perform queries on the database as of a certain wall clock time or user-specified system change number (SCN).
Flashback Query uses the Oracle Database multiversion read-consistency capabilities to restore data by applying undo as needed. Oracle Database 11g automatically tunes a parameter called the undo retention period. The undo retention period indicates the amount of time that must pass before old undo information—that is, undo information for committed transactions—can be overwritten. The database collects usage statistics and tunes the undo retention period based on these statistics and on undo tablespace size. Using Flashback Query, you can query the database as it existed this morning, yesterday, or last week. The speed of this operation depends only on the amount of data being queried and the number of changes to the data that need to be backed out. |
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Automatic Undo Retention
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After a transaction is committed, undo data is no longer needed for rollback or transaction recovery purposes. However, for consistent read purposes, long-running queries may require this old undo information for producing older images of data blocks. Furthermore, the success of several Oracle Flashback features can also depend upon the availability of older undo information. For these reasons, it is desirable to retain the old undo information for as long as possible. If the undo tablespace has space available for new transactions, then old undo information can be retained. When available space in the tablespace becomes short, the database begins to overwrite old undo information for transactions that have been committed.
Oracle Database automatically tunes the system to provide the best possible undo retention for the current undo tablespace. The database collects usage statistics and tunes the undo retention period based on these statistics and the undo tablespace size. If the undo tablespace is configured with the AUTOEXTEND option, with maximum size not specified, undo retention tuning is slightly different. In this case, the database tunes the undo retention period to be slightly longer than the longest-running query, if space allows. |
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Interval Literals
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Interval Literals specify a period of time. The interval can be expressed in terms of years and months, or in terms of days, hours, minutes and seconds. The two supported interval literals are Year to Month and Day to Second
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