User errors occur when, either due to an error in application logic or a manual mistake, data in your database is changed or deleted incorrectly.

For example, a user logs in to the wrong database and drops a database table. User errors are estimated to be the greatest single cause of database downtime.

A media failure occurs when a problem external to the database prevents it from reading from or writing to a file during normal operations.

Typical media failures include disk failures and the deletion of database files. Media failures are less common than user or application errors, but your backup and recovery strategy should prepare for them.

RMAN performs certain actions when you open the database with the RESETLOGS option.

The action performed are as follows:

• Archives the current online redo logs (if they are accessible) and then erases the contents of the online redo logs and resets the log sequence number to 1.

  For example, if the current online redo logs are sequence 1000 and 1001 when you open RESETLOGS, then the database archives logs 1000 and 1001 and then resets the online redo logs to sequence 1 and 2.

• Creates the online redo log files if they do not currently exist.

• Initializes redo thread records and online redo log records in the control file to the beginning of the new database incarnation.

  More specifically, the database sets the redo thread status to closed, sets the current thread sequence in the redo thread records to 1, sets the thread checkpoint of each redo thread to the beginning of log sequence 1, chooses one redo log from each thread and initialize its sequence to 1, and so on.

• Updates all current data files and online redo logs and all subsequent archived redo logs with a new RESETLOGS SCN and time stamp.

Because the database does not apply an archived redo log to a data file unless the RESETLOGS SCN and time stamps match, the RESETLOGS requirement prevents you from corrupting data files with archived logs that are not from direct parent incarnations of the current incarnation. The relationship among incarnations is explained more fully in the following section.

In previous releases, it was recommended that you back up the database immediately after the OPEN RESETLOGS. Because you can now easily recover a pre-RESETLOGS backup like any other backup, making a new database backup is optional. To perform recovery through RESETLOGS you must have all archived logs generated after the most recent backup and at least one control file (current, backup, or created).

Database incarnations can stand in the following relationships to each other:

• The current incarnation is the one in which the database is currently operating.

• The incarnation from which the current incarnation branched following an OPEN RESETLOGS operation is the parent incarnation of the current incarnation.

• The parent of the parent incarnation is an ancestor incarnation. Any parent of an ancestor incarnation is also an ancestor of the current incarnation.

• The direct ancestral path of the current incarnation begins with the earliest incarnation and includes only the branches to an ancestor of the current incarnation, the parent incarnation, or the current incarnation.

An incarnation number is used to uniquely tag and identify a stream of redo. Figure 14-1 illustrates a database that goes through several incarnations, each with a different incarnation number.

Figure 14-1 Database Incarnation History

Description of "Figure 14-1 Database Incarnation History"

Incarnation 1 of the database starts at SCN 1 and continues through SCN 1000 to SCN 2000. Suppose that at SCN 2000 in incarnation 1, you perform a point-in-time recovery back to SCN 1000, and then open the database with the RESETLOGS option. Incarnation 2 now begins at SCN 1000 and continues to SCN 3000. In this example, incarnation 1 is the parent of incarnation 2.

Suppose that at SCN 3000 in incarnation 2, you perform a point-in-time recovery to SCN 2000 and open the database with the RESETLOGS option. In this case, incarnation 2 is the parent of incarnation 3. Incarnation 1 is an ancestor of incarnation 3.

When DBPITR or Flashback Database has occurred in database, an SCN can refer to multiple points in time, depending on which incarnation is current. For example, SCN 1500 in Figure 14-1 could refer to an SCN in incarnation 1 or 2.

You can use the RESET DATABASE TO INCARNATION command to specify that SCNs are to be interpreted in the frame of reference of a specific database incarnation. The RESET DATABASE TO INCARNATION command is required when you use FLASHBACK, RESTORE, or RECOVER to return to an SCN in a noncurrent database incarnation. However, RMAN executes the RESET DATABASE TO INCARNATION command implicitly with Flashback, as explained in "Resetting the Database Incarnation in the Recovery Catalog".

A pluggable database (PDB) incarnation is a subincarnation of the multitenant container database (CDB) and is expressed as (database_incarnation, pdb_incarnation).

For example, if the CDB is incarnation 5, and a PDB is incarnation 3, then the fully specified incarnation number of the PDB is (5, 3). The initial incarnation of a PDB is 0. Subsequent incarnations are unique but not always sequential numbers.

The V$PDB_INCARNATION view contains information about all PDB incarnations. Use the following query to display the current incarnation of a PDB:

SELECT PDB_INCARNATION# FROM V$PDB_INCARNATION
   WHERE STATUS = 'CURRENT' AND CON_ID = PDB_container_id;

When a database goes through multiple incarnations, some backups can become orphaned backups. Orphaned backups are backups created during incarnations of the database that are not in the direct ancestral path.

Assume the scenario shown in Figure 14-1. If incarnation 3 is the current incarnation, then the following backups are orphaned:

• All backups from incarnation 1 after SCN 1000

• All backups from incarnation 2 after SCN 2000

In contrast, the following backups are not orphaned because they are in the direct ancestral path:

• All backups from incarnation 1 before SCN 1000

• All backups from incarnation 2 before SCN 2000

• All backups from incarnation 3

You can use orphaned backups when you intend to restore the database to an SCN not in the direct ancestral path. RMAN can restore backups from parent and ancestor incarnations and recover to the current time, even across OPEN RESETLOGS operations, if a continuous path of archived logs exists from the earliest backups to the point to which you want to recover. If you restore a control file from an incarnation in which the changes represented in the backups had not been abandoned, then RMAN can also restore and recover orphaned backups.

Orphan PDB backups can result when you perform point-in-time recovery on a pluggable database (PDB) and then open the PDB using the RESETLOGS option.

After recovering a PDB to a specified point-in-time, when you open the PDB using the RESETLOGS option, a new incarnation of the PDB is created. Orphan PDB backups are backups that were created when the SCN or time value was between the SCN or time to which the PDB was recovered and the SCN or time at which the PDB was opened in RESETLOGS mode. The END_RESETLOGS_SCN column of the V$PDB_INCARNATION view contains the SCN at which the PDB is opened in RESETLOGS mode.