Oracle OCP 1Z0-050（32题）解析

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Oracle OCP 1Z0-050（32题）解析

QUESTION 32:

View the following SQL statements:

Transaction T1

INSERT INTO hr.regions VALUES (5,'Pole');

COMMIT;

Transaction T2

UPDATE hr.regions SET region_name='Poles' WHERE region_id = 5;

COMMIT;

Transaction T3

UPDATE hr.regions SET region_name='North and South Poles' WHERE region_id = 5;

You want to back out transaction T2. Which option would you use?

A. It is possible, but transaction T3 also backs out.

B. It is possible with the NOCASCADE_FORCE option.

C. It is not possible because it has conflicts with transaction T3.

D. It is possible with the NONCONFLICT_ONLY option.

Answer: B

解析：

参考Oracle官方文档：

https://docs.oracle.com/cd/B28359_01/appdev.111/b28419/d_flashb.htm#i997043

Backout dependent transactions:

• NOCASCADE - The user expects no dependency. If a dependency is found, this raises an error, with the first dependent transaction provided in the report.

• NOCASCADE_FORCE - The user forcibly backs out the given transactions without considering the dependent transactions. The RDBMS executes the UNDO SQL for the given transactions in reverse order of their commit times. If no constraints break, and the result is satisfactory, the user can either COMMIT the changes or else ROLL BACK.

• NONCONFLICT_ONLY - This option lets the user backout the changes to the non-conflicting rows of the given transactions. Note that a transaction dependency will happen due to a row conflict, whether either through WAW or primary/unique key constraints. If the user chooses to back out only the non-conflicting rows, this should not cause any problem with database consistency, although transaction atomicity is lost. As this is a recovery operation, the user can correct the data.

• CASCADE - This will completely remove the given transactions including their dependents in a post order fashion (reverse order of commit times).

Using DBMS_FLASHBACK, you can flash back to a version of the database at a specified wall-clock time or a specified system change number (SCN).