Undo tablespace,Redo Tablespace
In interview questions are asked like what is undo,what it contains?
Undo records are used to:
Roll back transactions when a ROLLBACK statement is issued
Recover the database
Provide read consistency
Analyze data as of an earlier point in time by using Oracle Flashback Query
Recover from logical corruptions using Oracle Flashback features
When a ROLLBACK statement is issued, undo records are used to undo changes that were made to the database by the uncommitted transaction. During database recovery, undo records are used to undo any uncommitted changes applied from the redo log to the datafiles. Undo records provide read consistency by maintaining the before image of the data for users who are accessing the data at the same time that another user is changing it.
Overview of Automatic Undo Management
Oracle provides a fully automated mechanism, referred to as automatic undo management, for managing undo information and space. In this management mode, you create an undo tablespace, and the server automatically manages undo segments and space among the various active sessions.
You set the UNDO_MANAGEMENT initialization parameter to AUTO to enable automatic undo management. A default undo tablespace is then created at database creation. An undo tablespace can also be created explicitly. The methods of creating an undo tablespace are explained in "Creating an Undo Tablespace".
When the instance starts, the database automatically selects the first available undo tablespace. If no undo tablespace is available, then the instance starts without an undo tablespace and stores undo records in the SYSTEM tablespace. This is not recommended in normal circumstances, and an alert message is written to the alert log file to warn that the system is running without an undo tablespace.
If the database contains multiple undo tablespaces, you can optionally specify at startup that you want to use a specific undo tablespace. This is done by setting the UNDO_TABLESPACE initialization parameter, as shown in this example:
When automatic undo management is enabled, there is always a current undo retention period, which is the minimum amount of time that Oracle Database attempts to retain old undo information before overwriting it. Old (committed) undo information that is older than the current undo retention period is said to be expired. Old undo information with an age that is less than the current undo retention period is said to be unexpired.
Oracle Database automatically tunes the undo retention period based on undo tablespace size and system activity. You can specify a minimum undo retention period (in seconds) by setting the UNDO_RETENTION initialization parameter. The database makes its best effort to honor the specified minimum undo retention period, provided that the undo tablespace has space available for new transactions. When available space for new transactions becomes short, the database begins to overwrite expired undo. If the undo tablespace has no space for new transactions after all expired undo is overwritten, the database may begin overwriting unexpired undo information. If any of this overwritten undo information is required for consistent read in a current long-running query, the query could fail with the snapshot too old error message.
The following points explain the exact impact of the UNDO_RETENTION parameter on undo retention:
The UNDO_RETENTION parameter is ignored for a fixed size undo tablespace. The database may overwrite unexpired undo information when tablespace space becomes low.
For an undo tablespace with the AUTOEXTEND option enabled, the database attempts to honor the minimum retention period specified by UNDO_RETENTION. When space is low, instead of overwriting unexpired undo information, the tablespace auto-extends. If the MAXSIZE clause is specified for an auto-extending undo tablespace, when the maximum size is reached, the database may begin to overwrite unexpired undo information.
Retention Guarantee
To guarantee the success of long-running queries or Oracle Flashback operations, you can enable retention guarantee. If retention guarantee is enabled, the specified minimum undo retention is guaranteed; the database never overwrites unexpired undo data even if it means that transactions fail due to lack of space in the undo tablespace. If retention guarantee is not enabled, the database can overwrite unexpired undo when space is low, thus lowering the undo retention for the system. This option is disabled by default.
What Is Undo?
Every Oracle Database must have a method of maintaining information that is used to roll back, or undo, changes to the database. Such information consists of records of the actions of transactions, primarily before they are committed. These records are collectively referred to as undo.Undo records are used to:
Roll back transactions when a ROLLBACK statement is issued
Recover the database
Provide read consistency
Analyze data as of an earlier point in time by using Oracle Flashback Query
Recover from logical corruptions using Oracle Flashback features
When a ROLLBACK statement is issued, undo records are used to undo changes that were made to the database by the uncommitted transaction. During database recovery, undo records are used to undo any uncommitted changes applied from the redo log to the datafiles. Undo records provide read consistency by maintaining the before image of the data for users who are accessing the data at the same time that another user is changing it.
Overview of Automatic Undo Management
Oracle provides a fully automated mechanism, referred to as automatic undo management, for managing undo information and space. In this management mode, you create an undo tablespace, and the server automatically manages undo segments and space among the various active sessions.
You set the UNDO_MANAGEMENT initialization parameter to AUTO to enable automatic undo management. A default undo tablespace is then created at database creation. An undo tablespace can also be created explicitly. The methods of creating an undo tablespace are explained in "Creating an Undo Tablespace".
When the instance starts, the database automatically selects the first available undo tablespace. If no undo tablespace is available, then the instance starts without an undo tablespace and stores undo records in the SYSTEM tablespace. This is not recommended in normal circumstances, and an alert message is written to the alert log file to warn that the system is running without an undo tablespace.
If the database contains multiple undo tablespaces, you can optionally specify at startup that you want to use a specific undo tablespace. This is done by setting the UNDO_TABLESPACE initialization parameter, as shown in this example:
Undo Retention
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.When automatic undo management is enabled, there is always a current undo retention period, which is the minimum amount of time that Oracle Database attempts to retain old undo information before overwriting it. Old (committed) undo information that is older than the current undo retention period is said to be expired. Old undo information with an age that is less than the current undo retention period is said to be unexpired.
Oracle Database automatically tunes the undo retention period based on undo tablespace size and system activity. You can specify a minimum undo retention period (in seconds) by setting the UNDO_RETENTION initialization parameter. The database makes its best effort to honor the specified minimum undo retention period, provided that the undo tablespace has space available for new transactions. When available space for new transactions becomes short, the database begins to overwrite expired undo. If the undo tablespace has no space for new transactions after all expired undo is overwritten, the database may begin overwriting unexpired undo information. If any of this overwritten undo information is required for consistent read in a current long-running query, the query could fail with the snapshot too old error message.
The following points explain the exact impact of the UNDO_RETENTION parameter on undo retention:
The UNDO_RETENTION parameter is ignored for a fixed size undo tablespace. The database may overwrite unexpired undo information when tablespace space becomes low.
For an undo tablespace with the AUTOEXTEND option enabled, the database attempts to honor the minimum retention period specified by UNDO_RETENTION. When space is low, instead of overwriting unexpired undo information, the tablespace auto-extends. If the MAXSIZE clause is specified for an auto-extending undo tablespace, when the maximum size is reached, the database may begin to overwrite unexpired undo information.
Retention Guarantee
To guarantee the success of long-running queries or Oracle Flashback operations, you can enable retention guarantee. If retention guarantee is enabled, the specified minimum undo retention is guaranteed; the database never overwrites unexpired undo data even if it means that transactions fail due to lack of space in the undo tablespace. If retention guarantee is not enabled, the database can overwrite unexpired undo when space is low, thus lowering the undo retention for the system. This option is disabled by default.
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