Lesson 8

This module discussed how to recover datafiles for an ARCHIVELOG database in some special situations. Although these problems do not occur frequently during routine database operation, they still deserve your attention. You must be able to recognize the problems when they occur and provide workable solutions. Having completed this module, you should be able to:

1. Recover a datafile with no backup
2. Recover a file in backup mode
3. Clear a corrupted online redo log file
4. Perform a recovery with inactive online redo logs
5. Recreate online redo log files
6. Obtain recovery status information

This module introduced you to the following term:

1. PGA: The Program/Process Global Area (PGA) is a memory region that contains data and controls information for a single server process or a single background process. The information contained in the PGA could be sort data, session information, cursor state, or stack space. The PGA is allocated when a process is created and de-allocated when the process is terminated.

Performing an "Inactive Redo Log Recovery" in Oracle Database 19c is a critical procedure that is undertaken when a database needs to be recovered to a point in time using archived redo logs and online redo logs, especially when certain online redo logs have not been applied to the database due to various reasons such as a database crash. The steps outlined below must be executed with precision to ensure data integrity and system reliability.

1. Assess the Current Log Status: Begin by evaluating the status of your redo logs. Connect to the Oracle database using an administrative account, and execute the following SQL query to identify the status of all redo log groups:

   SELECT group#, status FROM v$log;
   

   Identify any logs marked as 'INACTIVE'. These are the logs that are not currently in use by the LGWR (Log Writer) process but contain valuable data that needs to be applied to the database.
2. Ensure Database is in MOUNT State: The database must be started in a MOUNT state and not open. This can be achieved by starting the Oracle instance without mounting the database and then mounting it manually:

   STARTUP MOUNT;
   

3. Identify Necessary Archived Logs: Use the `V$ARCHIVED_LOG` view to identify the sequence numbers of the archived redo logs that are necessary for the recovery process. This can be done by querying the view for logs that have not been applied:

   SELECT sequence#, applied FROM v$archived_log WHERE applied = 'NO';
   

4. Apply Inactive Redo Logs: Begin the recovery process by applying the inactive redo logs. Use the `RECOVER DATABASE` command, specifying 'UNTIL CANCEL' to manually select and apply each required archived log:

   RECOVER DATABASE UNTIL CANCEL;
   

   
   Oracle will prompt for the archived redo log files to apply. Provide the full path to each required log file when prompted.
5. Terminate the Recovery Process: Once all necessary redo logs have been applied, and the database has been brought to the desired point in time, terminate the recovery process by typing 'CANCEL' at the next prompt for a redo log file.
6. Open the Database with RESETLOGS: After the recovery process is completed and cancelled, open the database with the `RESETLOGS` option. This is crucial as it resets the log sequence and allows the database to start afresh with new log files:

   ALTER DATABASE OPEN RESETLOGS;
   

7. Verify Database Integrity: Finally, conduct thorough checks to ensure the integrity and consistency of the database. Use tools like `DBVERIFY` or SQL queries against `DBA_OBJECTS` to check for any object corruption or invalid objects:
   

   SELECT object_name FROM dba_objects WHERE status = 'INVALID';
   

8. Backup the Database: After a successful recovery, it is paramount to take a full backup of the database. This serves as a safeguard against any future data loss and ensures that there is a recent restore point available.

This procedure requires meticulous attention to detail and a deep understanding of Oracle database internals. It is recommended that these steps be performed by experienced database administrators to avoid inadvertent data loss or corruption.

Click the Quiz link below to review your understanding of infrequent recovery situations and redo log maintenance.
Redo Log Maintenance - Quiz