02)Briefly describe what are constraint states? (10 Points)
Enable Validate •If a constraint is in this state, no row violating the constraint can be inserted into the table. However, while the constraint is disabled such a row can be inserted. This row is known as an exception to the constraint. If the constraint is in the enable no validate state. •Violations resulting from data entered while the constraint was disabled remain. The rows that violate the constraint must be either updated or deleted in order for the constraint to put in the validated state. •When a constraint changes to enable validate from a disabled state, the table is locked and all data in the table is checked for conformity. This may cause DML operations such as a data load to wait, so it is advisable to move first form a disabled state to enable no validate, and then to enable validate. Transitions between these states are governed by the following rules: •ENABLE implies VALIDATE, unless NOVALIDTE is specified. •DISABLE implies NOVALIDATE, unless VALIDATE is specified. •VALIDATE and NOVALIDATE do not have default implications for the ENABLE and DISABLE states. When a unique or primary key moves from the DISABLE state to the ENABLE state and there is no existing index, a unique index is created automatically. Similarly, when a unique or primary key moves from ENABLE to DISABLE and it is enabled with a unique index, the unique index is dripped. •When any constraint is moved form the NOVALIDATE state to the VALIDATE state, all data must be checked. However, moving from
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- Spring '16
- administrator, Foreign key, Constraint, resource limits