4: You have two tables with referential integrity enforced between them. You need to insert data to
the child table first because it is going to be a long transaction and data for the parent table will be
available in a later stage, which can be inserted as part of the same transaction.
View the Exhibit to examine the commands used to create tables.
Which action would you take to delay the referential integrity checking until the end of the
transaction?
A.Set the constraint to deferred before starting the transaction.
B.Alter the constraint to NOVALIDATE state before starting the transaction.
C.Enable the resumable mode for the session before starting the transaction.
D.Set the COMMIT_WAIT parameter to FORCE_WAIT for the session before starting the
transaction.
Correct Answers:A
Explanation:
DEFERRABLE and DEFERRED
You can temporarily set a constraint to DEFERRED so that a large set of data might be
processed without any constraint checking until after the transaction is completed.
To do this, create the constraint with the DEFERRABLE clause, as follows:
ALTER TABLE SHIPS DROP CONSTRAINT FK_SHIPS_PORTS;
ALTER TABLE SHIPS ADD CONSTRAINT FK_SHIPS_PORTS FOREIGN KEY (HOME_PORT_ID)
REFERENCES PORTS (PORT_ID) DEFERRABLE;
The default setting for constraints is NOT DEFERRABLE.
Once the constraint has been created as DEFERRABLE, you may issue this
statement during a session:
SET CONSTRAINT FK_SHIPS_PORTS DEFERRED;
Alternatively, you can set all constraints to DEFERRED:
SET CONSTRAINT ALL DEFERRED;
Once the SET CONSTRAINT command establishes a given constraint—or all
constraints—as DEFERRED, the current transaction will temporarily ignore the
deferred constraints, and accept rows of data that might violate those constraints.
However, once a commit event occurs, the constraint automatically changes state
from DEFERRED to IMMEDIATE, and the constraints will be applied. If any are
violated, the commit will fail and all data is rolled back.
At any time, you can restore the default behavior of constraints with this command:
SET CONSTRAINT FK_SHIPS_PORTS IMMEDIATE;
Alternatively, you can set all constraints to IMMEDIATE:
SET CONSTRAINT ALL IMMEDIATE;
Once set to IMMEDIATE, constraints go back to their default behavior, and are
checked after each DML statement is executed.
This approach is somewhat similar to disabling commands, only it’s a more
temporary state.