I. Introduction
1. Overview of Flashback Table feature in Oracle Database
Flashback Table is an important feature of Oracle Database, allowing users to restore previous versions of a table in the database easily and quickly. This feature helps users to efficiently and reliably recover deleted or altered data without having to restore the entire database from a single backup. Flashback Table also allows users to review the history of the table in the database, giving them a better understanding of the evolution of the data and helping in analysis and error checking. This is especially useful in applications requiring high flexibility and reliability.
2. Explain the importance and benefits of this feature
The Flashback Table feature is one of the most important features of Oracle Database, as it allows users to recover data quickly and efficiently. The benefits of this feature include:
- Easy and fast data recovery: With Flashback Table, users can restore previous versions of a table easily and quickly, saving time and effort compared to recovery. entire database from a backup.
- Ability to recover deleted or altered data: Flashback Table enables users to recover deleted or altered data efficiently, helping them avoid data loss scenarios and minimize risks in the process of database management.
- Review table history: This feature makes it possible for users to review the history of tables in the database, thereby helping them better understand the evolution of data and helping in analysis and control. error check.
- Flexibility: Flashback Table allows users to flexibly restore previous versions of the table, thereby helping them customize the data to their needs without having to restore the entire database from a backup.
In short, Oracle Database’s Flashback Table feature is a useful tool to help users manage data efficiently and reliably.
II. How to enable Flashback Table
To enable Flashback Table feature for a table in Oracle Database, you can follow these steps:
- Make sure your database is running in ARCHIVELOG mode. Otherwise, you need to enable this mode before using Flashback Table feature
- Check if the Flashback feature is enabled. You can check using the following command:
SELECT flashback_on FROM v$database;If the result is “YES” , this means Flashback is enabled
If it is “NO”, run the following command to activate it:
alter database flashback on;and set the time you want to store the flashback log files (default is 1440 minutes = 24 hours)
alter system set db_flashback_retention_target= 2880; - Enable Flashback Table for the table to use by using the following command:
ALTER TABLE table_name ENABLE ROW MOVEMENT;Note that the ROW MOVEMENT feature must be enabled before you can use the Flashback Table feature.
- Enable Flashback Table for the table to use by using the following command:
ALTER TABLE table_name FLASHBACK ON; - Check if the Flashback Table feature is enabled for the table. You can check using the following command:
SELECT flashback_enabled FROM user_tables WHERE table_name = 'table_name';If the result is “YES” , this means that the Flashback Table feature has been enabled for the table. After enabling the Flashback Table feature for the table, you can use Oracle Database commands and tools to restore the previous data of the table when needed.
III. Where Flashback Table will fail
- Can’t use flashback table feature for SYS . schema objects
- If the table to be flashback has subtables referenced, you need to flashback the child table first.
- If actions such as upgrade, move, sever the table, add constraints to the table, or add the table to the cluster are performed after the point of flashback, the flashback operation will not be performed.
- Changing or deleting columns in a table is not supported in flashback tables.
- Changing the column encryption key will cause the flashback table to fail.
- Adding, deleting, merge, split, coalesce or truncate partition or subpartition is only supported for range partitions. Other partition types will not be supported.
- Objects that depend on the table (indexes, triggers) will be restored but the names will remain the same as they were in the recyclebin. You need to change their names again after you have performed the flashback.
IV. Example using Flashback table to recover data in oracle database
This example describes using the Flashback Table feature to restore data from the Employees table in an Oracle database. First, we will create the Employees table and add some data to it:
1 2 3 4 5 6 7 8 9 10 11 12 13 | CREATE TABLE Employees ( ID NUMBER(10), Name VARCHAR2(50), Department VARCHAR2(50), Salary NUMBER(10), Hire_Date DATE ); INSERT INTO Employees VALUES (1, 'John Smith', 'Marketing', 5000, TO_DATE('01/01/2022', 'MM/DD/YYYY')); INSERT INTO Employees VALUES (2, 'Jane Doe', 'Sales', 6000, TO_DATE('02/15/2022', 'MM/DD/YYYY')); INSERT INTO Employees VALUES (3, 'Bob Johnson', 'Engineering', 7000, TO_DATE('03/20/2022', 'MM/DD/YYYY')); |
Next, we will enable the Flashback Table feature on the Employees table:
1 2 3 | ALTER TABLE Employees ENABLE ROW MOVEMENT; -- kích hoạt row movement |
Suppose we then delete the employee’s data with ID 2 as follows:
1 2 3 | DELETE FROM Employees WHERE ID = 2; |
Now, to restore the employee’s data with ID 2, we use the Flashback Table feature as follows:
1 2 | flashback table Employees to timestamp to_timestamp('12-DEC-2022 11.14.50','dd-MON-YY hh24.MI.SS'); |
After executing the above command, the employee’s data with ID 2 will be restored back in the Employees table. We can double check by running the following command:
1 2 3 4 5 6 7 | ID Name Department Salary Hire_Date --- ----------- ------------ ------- ---------- 1 John Smith Marketing 5000 01/01/2022 2 Jane Doe Sales 6000 02/15/2022 3 Bob Johnson Engineering 7000 03/20/2022 |
V. Summary
The Flashback Table feature in Oracle Database allows to restore data in a table to a specific time in the past without having to restore the entire database. Using this feature ensures flexibility and saves time for database administrators. To use the Flashback Table feature, first enable the feature for the table, and then use the FLASHBACK TABLE command to restore the table’s data. However, using this feature also has some limitations, including not being able to restore objects belonging to the SYS schema, not being able to flashback the child table before the parent table, and not being able to flashback if there are actions like upgrade, move, truncate, add constraints, change columns, add or remove partitions…