open resetlogs command

In Oracle database, the “open resetlogs” command is used to open a database

• after performing incomplete recovery or
• after restoring a backup of the database.

Suppose you have a database that was backed up at time T1. After the backup was taken, changes were made to the database up to time T2. Now suppose that due to some issue, the database became corrupt and you had to restore it from the T1 backup. To bring the database up to date with the changes made after the backup was taken, you perform a recovery operation using redo logs generated between T1 and T2.

Once the recovery operation is complete, you would use the “open resetlogs” command to indicate that the database should be opened with a new redo log file. Here is an example SQL statement that you would use to open the database with resetlogs:

SQL> ALTER DATABASE OPEN RESETLOGS;

This command would create a new redo log file, reset the online redo log sequence numbers to 1, and update the control file and data dictionary to indicate that the database is now open with the new redo log file. After executing this command, the database would be fully recovered and ready for use.

The “open resetlogs” command is used to indicate that the recovery operation is complete and that the database should be opened with a new redo log file.

The “open resetlogs” command performs the following actions:

• It creates a new redo log file and resets the online redo log sequence numbers to 1.
• It updates the control file to reflect the new log file sequence number.
• It updates the data dictionary to indicate that the database is now open.

Note that the “open resetlogs” command should only be used after performing incomplete recovery or restoring a backup of the database. Using this command at any other time can result in data loss or corruption.

What are redologs ?

Imagine you are building a Lego castle, and as you build it, you keep a notebook where you write down all the pieces you use and where you put them. This way, if something goes wrong, you can look at your notebook and see what you did.

In a similar way, when you use a database like Oracle, the database keeps track of all the changes that are made to it in a file called a “redo log”. The redo log is like a notebook where the database writes down all the changes made to the database, such as adding new data, deleting data, or updating existing data.

The redo log is important because if something goes wrong with the database, such as a power failure or a software error, the database can use the redo log to “replay” all the changes made to the database since the last backup. This way, the database can recover all the changes that were made and bring itself up to date, just like you can use your notebook to rebuild your Lego castle if something goes wrong.

As changes are made to the database, the database writes the changes to the redo log files in a circular fashion. When the redo log file is full, the database switches to the next redo log file and continues writing changes to it. This process continues until all the redo log files have been used, at which point the database goes back to the beginning of the first redo log file and starts overwriting the oldest changes.

redo logs in an Oracle database are stored on disk in a location specified by the database administrator, and they are written to in a circular fashion as changes are made to the database.

In summary, the redo log is a file that keeps track of all the changes made to a database, and it is used to recover changes if something goes wrong with the database.