Lesson 11	Taking it offline
Objective	Take tablespaces and data files offline.

Take tablespaces and Data Files Offline

Taking tablespaces and data files offline in Oracle is a critical operation for database maintenance, allowing a Database Administrator (DBA) to perform tasks such as restructuring, relocating data files, or troubleshooting without affecting the entire database. It is essential to understand the implications and procedures for performing these operations.

Taking Tablespaces Offline

General Syntax:
- The basic command to take a tablespace offline is:
```
ALTER TABLESPACE tablespace_name OFFLINE;
```
- This command makes the specified tablespace unavailable to users, but does not affect the rest of the database.
Considerations Before Taking Offline:
- Ensure that no active transactions are using the objects in the tablespace. Active transactions may prevent the tablespace from going offline.
- Inform users about the maintenance window to avoid disruption.
Taking Temporary Tablespaces Offline:
- Temporary tablespaces cannot be taken offline in the same manner as permanent tablespaces. If you need to perform maintenance on a temporary tablespace, consider creating a new temporary tablespace, assign it as the default temporary tablespace, and then drop the old one after the maintenance.
Immediate and Temporary Options:
- You can use the `IMMEDIATE` option to take a tablespace offline immediately, rolling back active transactions:
```
ALTER TABLESPACE tablespace_name OFFLINE IMMEDIATE;
```
- The `TEMPORARY` option allows the tablespace to go offline after all active transactions have completed.

Taking Data Files Offline

General Syntax:
- To take an individual data file offline, use the following command:
```
ALTER DATABASE DATAFILE 'datafile_path' OFFLINE;
```
- This operation is useful when a specific data file is corrupted or needs maintenance.
Implications:
- Taking a data file offline affects all objects in the data file. Ensure that the impact on database operations is understood and communicated.
Bringing Tablespaces and Data Files Online:
- Once maintenance is complete, bring the tablespace or data file back online using:
```
ALTER TABLESPACE tablespace_name ONLINE;
```
- Or for data files:
```
ALTER DATABASE DATAFILE 'datafile_path' ONLINE;
```

Best Practices and Considerations

Backup: Always perform a backup before taking tablespaces or data files offline, especially if you plan to perform actions that modify data or structure.
Monitoring: Monitor the database for any issues after bringing the tablespace or data files back online. Look out for any errors in the alert log or through Oracle Enterprise Manager.
Recovery: Be prepared for data recovery scenarios. If a data file is taken offline due to corruption, you may need to perform recovery operations before it can be brought online.
Documentation: Document the reasons for taking tablespaces or data files offline, the actions performed, and any issues encountered. This documentation is valuable for future reference and auditing purposes.

In summary, taking tablespaces and data files offline in Oracle is a strategic operation used for maintenance and troubleshooting. It must be performed with careful planning, clear communication with stakeholders, and a thorough understanding of the implications on the database environment. Proper backup and monitoring strategies are essential to ensure database integrity and availability.

Tablespace and data files may be taken offline. When they are offline, they become unavailable to database users. Oracle will take data files and their tablespaces offline automatically when they are damaged.
A tablespace may be taken offline for back up or to prevent access to the data within the tablespace. Data files may be taken offline to facilitate moving the files.

Taking a data file offline

The ALTER DATABASE statement is used to take a data file offline and to bring it back online again. The syntax looks like this:

ALTER DATABASE DATAFILE 'filename'
{ONLINE|OFFLINE [DROP]};

When you take a data file offline, Oracle does not checkpoint the file. Before the data file can be brought back online, the recovery process must be used to apply changes from the database redo log to make the file consistent with the other files in the database. Consequently, when the database is not in archivelog mode, Oracle makes it difficult to take a data file offline. You could use the DROP option to take a data file offline when the database is in noarchivelog mode, but you won't be able to bring it back online again; the only thing you can do afterward is to drop the data file.

Taking a tablespace offline

The ALTER TABLESPACE statement is used to take an entire tablespace offline. The syntax looks like this:

ALTER TABLESPACE tablespace_name 
{ONLINE|OFFLINE [NORMAL|IMMEDIATE|FOR RECOVER]};

The NORMAL option is the default, and this causes the tablespace to be checkpointed before being taken offline. The IMMEDIATE option takes the tablespace offline immediately, but you will need to recover the tablespace before it can be brought back online. The FOR RECOVER option is used only in preparation for a tablespace point-in-time recovery. In the next lesson, you will learn how to move data files to a new location.