Flashback Query in Oracle SQL
Key Concepts
Flashback Query is a powerful feature in Oracle SQL that allows users to query data as it existed at a specific point in the past. This feature is invaluable for auditing, troubleshooting, and data recovery. Understanding the following key concepts is essential for effectively using Flashback Query:
1. AS OF SCN
The AS OF SCN clause allows you to query data as it existed at a specific System Change Number (SCN). An SCN is a timestamp used by Oracle to track changes in the database.
Example:
To query data as it existed at SCN 123456:
SELECT * FROM Employees AS OF SCN 123456;
2. AS OF TIMESTAMP
The AS OF TIMESTAMP clause allows you to query data as it existed at a specific point in time. This is useful when you need to retrieve data from a specific date and time.
Example:
To query data as it existed on January 1, 2023, at 10:00 AM:
SELECT * FROM Employees AS OF TIMESTAMP TO_TIMESTAMP('2023-01-01-10:00:00', 'YYYY-MM-DD HH24:MI:SS');
3. VERSIONS BETWEEN
The VERSIONS BETWEEN clause allows you to query data across a range of SCNs or timestamps. This is useful for comparing data changes over a period.
Example:
To query data changes between SCN 123456 and SCN 234567:
SELECT * FROM Employees VERSIONS BETWEEN SCN 123456 AND 234567;
4. VERSIONS QUERY
The VERSIONS query provides detailed information about the versions of rows that existed within a specified time range. This includes the SCN, operation type, and other metadata.
Example:
To query versions of rows between two timestamps:
SELECT VERSIONS_STARTSCN, VERSIONS_ENDSCN, VERSIONS_OPERATION, *
FROM Employees
VERSIONS BETWEEN TIMESTAMP TO_TIMESTAMP('2023-01-01-10:00:00', 'YYYY-MM-DD HH24:MI:SS')
AND TO_TIMESTAMP('2023-01-01-11:00:00', 'YYYY-MM-DD HH24:MI:SS');
5. Flashback Data Archive
The Flashback Data Archive (FDA) is a feature that stores historical data for a specified retention period. This allows for more granular and long-term flashback queries.
Example:
To query data from the Flashback Data Archive:
SELECT * FROM Employees AS OF TIMESTAMP TO_TIMESTAMP('2023-01-01-10:00:00', 'YYYY-MM-DD HH24:MI:SS')
FLASHBACK ARCHIVE my_archive;
6. Flashback Transaction Query
The Flashback Transaction Query allows you to query the details of transactions that have been committed. This includes the operations performed, the SCN, and the affected rows.
Example:
To query the details of a transaction:
SELECT * FROM FLASHBACK_TRANSACTION_QUERY WHERE XID = '01020304';
7. Flashback Table
The Flashback Table feature allows you to restore a table to a previous state. This is useful for recovering from accidental data changes or deletions.
Example:
To flashback a table to a specific timestamp:
FLASHBACK TABLE Employees TO TIMESTAMP TO_TIMESTAMP('2023-01-01-10:00:00', 'YYYY-MM-DD HH24:MI:SS');
8. Flashback Drop
The Flashback Drop feature allows you to recover a table that has been dropped. Oracle stores dropped tables in the recycle bin, from which they can be restored.
Example:
To flashback a dropped table:
FLASHBACK TABLE Employees TO BEFORE DROP;
9. Flashback Database
The Flashback Database feature allows you to restore the entire database to a previous point in time. This is useful for recovering from catastrophic failures.
Example:
To flashback the database to a specific SCN:
FLASHBACK DATABASE TO SCN 123456;
10. Flashback Query Limitations
Flashback Query has certain limitations, such as the retention period for undo data and the need for sufficient undo tablespace. Understanding these limitations is crucial for effective use.
Example:
Ensure that the undo tablespace is large enough to retain data for the required period:
ALTER TABLESPACE UNDO_TBS ADD DATAFILE '/u01/app/oracle/oradata/ORCL/undo02.dbf' SIZE 1G;
11. Flashback Query Best Practices
Best practices for using Flashback Query include regularly monitoring undo tablespace usage, setting appropriate retention policies, and testing flashback capabilities.
Example:
Set a retention policy for undo data:
ALTER SYSTEM SET UNDO_RETENTION = 86400;
12. Flashback Query Use Cases
Flashback Query is used in various scenarios, including auditing data changes, troubleshooting application issues, and recovering from accidental data modifications.
Example:
Auditing data changes:
SELECT * FROM Employees VERSIONS BETWEEN TIMESTAMP TO_TIMESTAMP('2023-01-01-10:00:00', 'YYYY-MM-DD HH24:MI:SS')
AND TO_TIMESTAMP('2023-01-01-11:00:00', 'YYYY-MM-DD HH24:MI:SS');
13. Flashback Query and Data Guard
Flashback Query can be used in conjunction with Oracle Data Guard to provide high availability and disaster recovery solutions. This allows for seamless data recovery in case of primary database failure.
Example:
Flashback query on a standby database:
SELECT * FROM Employees AS OF TIMESTAMP TO_TIMESTAMP('2023-01-01-10:00:00', 'YYYY-MM-DD HH24:MI:SS')
AT STANDBY;