• If a row has changes that were not committed at the start of query, the database server constructs a read- consistent image of the row by retrieving the before image of the changes from the undo segment and applying the changes to a copy of the row in memory.
Read Consistency Transaction Read Consistency • Read consistency is always provide from SQL statement. However, you can request read consistency for a read-only transaction by issuing the following command at the beginning of the transaction: SET TRANSACTION READ ONLY; • Or, you can request read consistency for a transaction performing DML by issuing the following command at the beginning of the transaction: SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; • In either case, the database server provides data that is consistent from the start of the transaction. Using SERIALIZABLE can have a negative impact on performance.
Types of Undo Segments • SYSTEM : Used for objects in the SYSTEM tablespace • Non-SYSTEM : Used for objects in other tablespace : • Auto Mode : Requires an UNDO tablespace • Manual Mode : • Private : Acquired by a single instance • Public : Acquired by any instance • Deferred : Used when tablespaces are taken offline immediate, temporary , or for recovery
Types of Undo Segments SYSTEM Undo Segment • The SYSTEM undo segment is created in the SYSTEM tablespace when a database is created. • This undo segment can be used only for changes made to objects in the SYSTEM tablespace. • The SYSTEM undo segment exists and works the same in both manual and auto mode.
Types of Undo Segments Non-SYSTEM Undo Segments • A database that has multiple tablespaces needs at least one Non- SYSTEM undo segment for manual mode or one UNDO tablespace for auto mode. Manual Mode • In manual mode, a non-system undo segment, which is created by the database administrator, can be used for changes made to objects in any non-system tablespace. • There are two types of non-system undo segments.
Types of Undo Segments Private • Private undo segments are segments that are brought online by an instance because they are listed in the parameter file. • They can be brought online explicitly by issuing an ALTER ROLLBACK SEGEMENT command.
Types of Undo Segments Public • Public undo segments form a pool of undo segments available in a database. • Public undo segments are normally used with the database Parallel Server to create a pool of undo segments that can be used by any of the Parallel Server instances. • Deferred Undo Segments Deferred undo segments may be created when a tablespace is brought offline. • They are used to roll back transactions when the tablespace is brought back online. • They are dropped automatically when they are no longer needed. • Because deferred undo segments are maintained by the database server, no maintenance is required on your part.
Automatic Undo Management : Concepts • Undo data is managed using an UNDO tablespaces.
You've reached the end of your free preview.
Want to read all 99 pages?
- Fall '15