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Integrity Constraint States
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As you saw in the previous section, integrity constraints are defined on tables to ensure that data that violates preset rules doesn t enter the tables. However, during times like data loading, you can t keep the integrity constraints in a valid state, as this will lead to certain problems. Oracle lets you disable constraints when necessary and enable them when you want. Let s examine the various ways you can alter the states of table constraints.
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Disabling Integrity Constraints
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During large data loads, using either the SQL*Loader or the Import utility, it may take a considerably longer time to load the data if you have to check for integrity violations for each row inserted into the table. A better strategy would be to disable the constraint, load the data, and worry about any possible insertion of bad data later on. After the load is completed, the constraints are brought into effect again by enabling them.
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The enabled state is Oracle s default constraint state.
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You can disable constraints in two ways: you can specify either the disable validate or the disable no validate constraint state, using the DISABLE VALIDATE or DISABLE NO VALIDATE command, respectively. The next sections briefly discuss these two ways of disabling constraints.
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When you use the DISABLE VALIDATE command, you re doing the following two things at once. First, by using the VALIDATE command, you re ensuring that all the data in the table satisfies the constraint. Second, by using the DISABLE command, you re doing away with the requirements of maintaining the constraint. Oracle drops the index on the constraint, but keeps it valid. Here s an example: SQL> ALTER TABLE sales_data ADD CONSTRAINT quantity_unique UNIQUE (prod_id,customer_id) DISABLE VALIDATE; When you issue the preceding SQL statement, Oracle ensures that only unique combinations of the unique key prod_id and customer_id exist in the table, but it will not maintain a unique index. Note that because I have chosen to keep the constraint in a disabled state, no DML is possible against the table. This option is really ideal for large data warehouse tables, which are normally used only for querying purposes.
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Disable No Validate State
Under the disable no validate constraint state, the constraint is disabled and there is no guarantee of the data meeting the constraint requirements, because Oracle does not perform constraint validation. This is essentially the same as a DISABLE constraint command.
Enable Validate State
This constraint state will have an enabled constraint that ensures that all data is checked to ensure compliance with the constraint. This state is exactly the same as the plain enabled state. The following example shows the use of this state: SQL> ALTER TABLE sales_data ADD CONSTRAINT sales_region_fk FOREIGN KEY (sales_region) REFERENCES region(region_id) ENABLE VALIDATE;
Enable No Validate State
Under this constraint state, all new inserts and updates will be checked for compliance. Because the existing data won t be checked for compliance, there s no assurance that the data already in the table meets the constraint requirements. You ll usually use this option when you re loading large tables and you have reason to believe that the data will satisfy the constraint. Here s an example: SQL> ALTER TABLE sales ADD CONSTRAINT sales_region_fk FOREIGN KEY (sales_region_id) REFERENCES time(time_id) ENABLE NOVALIDATE;
Rely Constraints
Data extraction, transformation, loading (ETL) steps are usually undertaken before loading data into data warehouse tables. If you have reason to believe that the data is good, you can save time during loading by disabling and not validating the constraints. You can use the ALTER TABLE command to disable the constraints with the RELY DISABLE NOVALIDATE option, as shown in the following example:
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SQL> ALTER TABLE sales ADD CONSTRAINT sales_region_fk FOREIGN KEY (sales_region_id) REFERENCES time(region_id) RELY DISABLE NOVALIDATE;
Deferrable and Immediate Constraints
In addition to specifying the type of validation of a constraint, you can specify when exactly this constraint is checked during the loading process. If you want the constraint to be checked immediately after each data modification occurs, choose the not deferrable option, which is, in fact, the default behavior in Oracle databases. If you want a one-time check of a constraint after the whole transaction is committed, choose the deferrable option. All constraints and foreign keys may be declared deferrable or not deferrable. If you choose the deferrable option, you have two further options. You can specify that the deferrable constraint is either initially deferred or initially immediate. In the former case, the database will defer checking until the transaction completes. If you choose the initially immediate option, the database checks the constraint before any data is changed. The following example shows how to specify this kind of constraint in the employee table: SQL> CREATE TABLE employee employee_id NUMBER, last_name VARCHAR2(30), first_name VARCHAR2(30), dept VARCHAR2(30) UNIQUE REFERENCES department(dept_name) DEFERRABLE INITIALLY DEFERRED; Oracle also provides a way of changing a deferrable constraint from immediate to deferred or vice versa with the following statements: SQL> SET CONSTRAINT constraint_name DEFERRED; SQL> SET CONSTRAINT constraint_name IMMEDIATE;
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