Deferred Constraint Checking

Default Column Values and Integrity Constraint Checking

Default values are included as part of an INSERT statement before the statement is parsed. Therefore, default column values are subject to all integrity constraint checking.

Deferred Constraint Checking

You can defer checking constraints for validity until the end of the transaction.

A constraint is deferred if the system checks that it is satisfied only on commit. If a deferred constraint is violated, then commit causes the transaction to roll back.

If a constraint is immediate (not deferred), then it is checked at the end of each statement. If it is violated, the statement is rolled back immediately.

If a constraint causes an action (for example, delete cascade), that action is always taken as part of the statement that caused it, whether the constraint is deferred or immediate.

Constraint Attributes

You can define constraints as either deferrable or not deferrable, and either initially deferred or initially immediate. These attributes can be different for each constraint. You specify them with keywords in the CONSTRAINT clause:

Constraints can be added, dropped, enabled, disabled, or validated. You can also modify a constraint’s attributes.

21-24 Oracle9i Database Concepts

Deferred Constraint Checking

See Also:

SET CONSTRAINTS Mode

The SET CONSTRAINTS statement makes constraints either DEFERRED or IMMEDIATE for a particular transaction (following the ANSI SQL92 standards in both syntax and semantics). You can use this statement to set the mode for a list of constraint names or for ALL constraints.

The SET CONSTRAINTS mode lasts for the duration of the transaction or until another SET CONSTRAINTS statement resets the mode.

SET CONSTRAINTS ... IMMEDIATE causes the specified constraints to be checked immediately on execution of each constrained statement. Oracle first checks any constraints that were deferred earlier in the transaction and then continues immediately checking constraints of any further statements in that transaction, as long as all the checked constraints are consistent and no other SET CONSTRAINTS statement is issued. If any constraint fails the check, an error is signaled. At that point, a COMMIT causes the whole transaction to roll back.

The ALTER SESSION statement also has clauses to SET CONSTRAINTS IMMEDIATE or DEFERRED. These clauses imply setting ALL deferrable constraints (that is, you cannot specify a list of constraint names). They are equivalent to making a SET CONSTRAINTS statement at the start of each transaction in the current session.

Making constraints immediate at the end of a transaction is a way of checking whether COMMIT can succeed. You can avoid unexpected rollbacks by setting constraints to IMMEDIATE as the last statement in a transaction. If any constraint fails the check, you can then correct the error before committing the transaction.

The SET CONSTRAINTS statement is disallowed inside of triggers.

SET CONSTRAINTS can be a distributed statement. Existing database links that have transactions in process are told when a SET CONSTRAINTS ALL statement occurs, and new links learn that it occurred as soon as they start a transaction.

Data Integrity 21-25