FLASHBACK

Syntax

flashback::=

Text description of flashback

Purpose

Performs a Flashback Database operation, returning the database to (or to just before) target time, as specified by time, SCN or log sequence number.

The result of using flashback database is generally similar to a database point-in-time recovery performed with RECOVER, except for the following principal differences:

• You do not need to restore a backup.
• RMAN uses flashback logs to undo changes to a point before the target time or SCN, and then uses archived redo logs to recover the database forward to make it consistent. RMAN automatically restores from backup any archived logs that are needed. (Flashback logs are stored as Oracle-managed files in the flash recovery area, and cannot be created if no flash recovery area is configured.)
• The FLASHBACK command does not start modifying the database until it has made sure that it has all the files and resources that it needs.
• Some NOLOGGING changes will be reflected in the flashback which would not be reflected in the results of a point-in-time recovery, because flashback database uses backed-up block images as the basis for undoing changes to your current datafiles.

Because FLASHBACK DATABASE does not require you to restore a backup, it is usually much faster than incomplete recovery.

Restrictions and Usage Notes

• You can run this command from the RMAN prompt or from within a RUN command.
• The target database must be in ARCHIVELOG mode.
• The target database must be a Release 10g database.
• You must be connected to the target database.
• You must have enabled the flashback functionality before the target time for flashback, using the SQL statement ALTER DATABASE ... FLASHBACK ON. You can query this status in V$DATABASE.FLASHBACK_ON.
• The target database must be mounted with a current control file, that is, the control file cannot be a backup or have been re-created.
• The flash recovery area must be enabled (that is, DB_RECOVERY_FILE_DEST must be set). Flashback logs can only be stored in the flash recovery area.
• RMAN performs restore failover when unable to restore an archvied redo log file for use in a Flashback Database operation. See Oracle Database Backup and Recovery Advanced User's Guide for details on restore failover.
• The point to which RMAN can flash back the database depends on the setting of the DB_FLASHBACK_RETENTION_TARGET initialization parameter, and upon the actual retention of flashback logs permitted by available disk space. If there is not enough space in the flash recovery area for the retention of other required files, flashback logs may be deleted to make room. If the FLASHBACK command does not have enough flashback data to return the database to the requested SCN or time, then RMAN issues an error and does not modify the database.
• If insufficient flashback data is available for a subset of your datafiles, then you can take these datafiles offline and then run FLASHBACK on those for which there is sufficient flashback log data. Afterwards, you can perform point-in-time recovery on those files where there was not sufficient flashback log data.
• RMAN issues an error if you attempt to perform Flashback Database on online tablespaces on which flashback was disabled using the SQL statement ALTER TABLESPACE ... FLASHBACK OFF.
• You must OPEN RESETLOGS after running FLASHBACK DATABASE. If datafiles are not flashed back because they are offline, then the RESETLOGS may fail with an error. In that case you must do one of the following:
  • Flash back the affected datafiles to the same time or SCN as the other datafiles.
  • Run RESTORE and then RECOVER to bring the affected datafiles to the same time or SCN as the rest of the database.
  • Take the datafiles offline and then drop them.
• RMAN never flashes back data for temporary tablespaces.
• You cannot run FLASHBACK DATABASE to a time before an OPEN RESETLOGS.
• If a datafile has changed status between the current SCN and the SCN to which you are flashing back, then the FLASHBACK command behaves differently depending on the nature of the status change. Refer to Table 2-3.
• If the FLASHBACK command fails or is interrupted, then the database is left mounted. You can issue another FLASHBACK command or run RECOVER to return the database to its original state.
• When you have completed a flashback operation, you may wish to open the database read-only and run some queries to see if you have achieved the desired result. If you are not satisfied with your flashback operation, you can perform RECOVER DATABASE to re-apply all changes and bring the database back to its state when you started the flashback operation. You can then attempt flashb ack again. If you are satisfied, you can either perform an OPEN RESETLOGS to abandon all changes after the target time for the flashback, or you can export lost data, use RECOVER DATABASE to return your database to its state before the flashback database operation, and then re-import the lost data.
• When performing a FLASHBACK DATABASE operation, your database may not be left at the SCN most immediately before the target time you specify. There are events other than transactions which cause the SCN for your database to be updated. If you use the FLASHBACK DATABASE TO form of the command and there is a transaction associated with your specified SCN, the database after the flashback operation will include all changes up to and including that transaction. Otherwise, all changes up to but not including that transaction will be included in your datafiles, whether you use the FLASHBACK DATBASE TO or FLASHBACK DATABASE TO BEFORE form of the command. Changes after the specified target time or SCN are never applied.

  Table 2-3 How FLASHBACK Responds to Datafile Operations

  If this datafile operation occurred during the flashback window ...	Then the FLASHBACK command ...
  Added	Removes the datafile record from the control file.
  Dropped	Adds the datafile to the control file, but marks it as offline and does not flash it back. You can then restore and recover the datafile to the same time or SCN.
  Renamed	Ignores the renaming. The datafile retains its current name.
  Resized	May fail. You can take the datafile offline and then rerun the FLASHBACK command. The datafile will not be flashed back. You can then restore and recover the datafile to the same time or SCN.
  Taken offline	Ignores the operation. The datafile retains its current online status.
  Brought online	Ignores the operation. The datafile retains its current offline status.
  Made read-only or read-write	Changes the status of the datafile in the control file.

Keywords and Parameters

Syntax Element	Description
DEVICE TYPE deviceSpecifier	Allocates automatic channels for the specified device type only. For example, if you configure automatic disk and tape channels, and issue FLASHBACK ... DEVICE TYPE DISK, then RMAN allocates only disk channels. RMAN may need to restore redo logs from backup during the flashback database process. Changes between the last flashback log and the target time must be re-created based on the archived redo log. If no automatic channels are allocated for tape and a needed redo log is on tape, the FLASHBACK operation will fail. See Also: "deviceSpecifier"
DATABASE	Returns the database to the specified point. Query OLDEST_FLASHBACK_SCN and OLDEST_FLASHBACK_TIME in V$FLASHBACK_DATABASE_LOG to display the approximate lowest SCN and time to which you can flash back. View the current database SCN in V$DATABASE.CURRENT_SCN.
TO SCN = integer	Returns the database to the point up to (and including) the specified SCN.
TO BEFORE SCN = integer	Returns the database to its state just before the specified SCN. Any changes at an SCN lower than that specified are applied, but if there is a change associated with the specified SCN it is not applied.
TO SEQUENCE = integer THREAD = integer	Specifies a redo log sequence number and thread as an upper limit. RMAN applies changes up to (and including) the last change in the log with the specified sequence and thread number.
TO BEFORE SEQUENCE = integer [THREAD = integer]	Specifies a redo log sequence number and thread as an upper limit. RMAN applies changes up to (but not including) the last change in the log with the specified sequence and thread number.
TO TIME = 'date_string'	Returns the database to its state at the specified time. You can use any SQL DATE expressions to convert the time to the current format, for example, FLASHBACK DATABASE UNTIL TIME 'SYSDATE-7'.
TO BEFORE TIME = 'date_string'	Similar to the TO TIME clause, but returns the database to its state including all changes up to but not including changes at the specified time.

Examples

FLASHBACK DATABASE to a Specific SCN: Example

The following command flash back the database to a particular SCN:

RMAN> FLASHBACK DATABASE TO SCN 46963;

FLASHBACK DATABASE to One Hour Ago: Example

The following command flashes the database by 1/24 of a day, or one hour:

RMAN> FLASHBACK DATABASE TO TIMESTAMP (SYSDATE-1/24);

FLASHBACK DATABASE to a Specific Time: Example

The following command uses SQL date conversion functions to specify the target time:

RMAN> FLASHBACK DATABASE TO TIMESTAMP
   TO_TIMESTAMP('2002-03-11 16:00:00', 'YYYY-MM-DD HH24:MI:SS');