Time travel with Oracle database’s Flashback Queries

Oracle’s database management system offers a feature known as Flashback Queries, allowing users to peek into the past and retrieve data as it existed at a previous point in time. This functionality can eliminate the need for manual data restoration from backups, making it a useful asset for both developers and database administrators.

Enabling Flashback Queries

Before using Flashback Queries, ensure that the database has the required configuration. Firstly, confirm that the database’s DB_FLASHBACK_RETENTION_TARGET parameter is appropriately set. This parameter defines the period for which historical data is retained. Adjust it based on your organization’s data retention policies. Before making changes, you can check its current value:

SHOW PARAMETER DB_FLASHBACK_RETENTION_TARGET;

Use the ALTER SYSTEM command to set the parameter. For example, to set it to retain data for 7 days:

ALTER SYSTEM SET DB_FLASHBACK_RETENTION_TARGET=10080 SCOPE=BOTH;

10080 is the retention period in minutes (7 days × 24 hours × 60 minutes). Please note that setting the parameter to a higher value consumes more space in the flashback recovery area, so consider your storage constraints. SCOPE=BOTH ensures that the change persists across database restarts, i.e. it changes the value both in memory and in the server parameter file.

To enable Flashback Queries for a specific table, execute the ALTER TABLE command with the FLASHBACK option:

ALTER TABLE table_name FLASHBACK ARCHIVE;

This setup allows Oracle to maintain historical changes for the specified table.

Using Flashback Queries

Consider a scenario where an employee accidentally updates critical data in the employees table. With Flashback Queries, you can rectify the mistake:

SELECT * FROM employees AS OF TIMESTAMP TO_TIMESTAMP('2023-11-10 15:00:00', 'YYYY-MM-DD HH24:MI:SS');

This query retrieves the data from the employees table as it existed before the erroneous update.

You can also recover dropped tables if they are still within the retention period:

FLASHBACK TABLE orders TO BEFORE DROP;

This command restores the dropped table and its data.

Flashback Queries offer a mechanism to navigate through time within a database, providing a simple way to recover historical data or inspect changes. They stand as a useful asset in the arsenal of database administrators and developers, fostering greater confidence in managing data.