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Data Integrity and Error Handling in SQL Server 2005
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Implementing Check and Foreign Key Constraints to Improve Query Performance C a n check and foreign key constraints improve query performance Don t they just protect us against invalid data, and in doing so, somewhat degrade performance The answers to these questions are: Yes, they can and No, they don t. Because foreign keys and check constraints are declared rules, the query optimizer can use them to create more efficient query plans. This usually involves skipping some part of the query plan because the optmizer can see that, for example, because of a foreign key constraint, it is unnecessary to execute that particular part of the plan. The following code sample is a simple example of this behavior with a foreign key constraint. Consider the following two tables and the foreign key FKOrdersCustomers:
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CREATE TABLE Test.Customers ( CustomerID INT PRIMARY KEY ); CREATE TABLE Test.Orders ( OrderID INT PRIMARY KEY ,CustomerID INT NOT NULL CONSTRAINT FKOrdersCustomers REFERENCES Test.Customers (CustomerID) );
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If you execute the following query against the two tables, the query returns all orders that have a valid customer reference.
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SELECT o.* FROM Test.Orders AS o WHERE EXISTS (SELECT * FROM Test.Customers AS c WHERE c.CustomerID = o.CustomerID);
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The execution plan used by SQL Server to execute this query is shown in Figure 5-2. In the execution plan, you can see the Test.Customers table is not accessed; the only table being accessed is Test.Orders. This is because the query optimizer knows that the exists operator in this query is unnecessery to execute because the foreign key constraint requires all orders to refer to an existing customer, which is what is checked in the where clause.
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Figure 5-2 Actual Execution Plan in SQL Server Management Studio
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Turn off the foreign key by executing the following statement:
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ALTER TABLE Test.Orders NOCHECK CONSTRAINT FKOrdersCustomers;
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Lesson 1: Validating Data and Permissions
After executing the same query again, you get a new execution plan, as shown in Figure 5-3. The query optimizer executes the exists operator (in this case, the Nested Loops icon in the execution plan) to return only those orders that actually have a valid reference to the Test.Customers table. Because you turned off the foreign key constraint, SQL Server could not be sure that all orders actually have valid customer references and, therefore, had to execute the exists operator. For a large table, this can make a huge difference in execution time and input/output.
Figure 5-3 Actual Execution Plan in SQL Server Management Studio
Now turn on the foreign key again. Execute the statement below and rerun the query.
ALTER TABLE Test.Orders CHECK CONSTRAINT FKOrdersCustomers;
After running the query this time, you end up with the same plan again the plan shown in Figure 5-3. How can this be You turned the constraint back on, so now SQL Server should be sure that all orders have valid customer references. However, this is actually not the case. This is because the foreign key is considered not trusted. A constraint that is not trusted (which only applies to foreign key and check constraints) is not taken into account by the query optimizer. Your foreign key is not trusted because, while it was turned off, someone could have inserted or updated an order row with an invalid CustomerID. Turning the constraint back on does not verify existing data. You can verify that the foreign key is indeed not trusted by executing the following query:
SELECT name, is_not_trusted FROM sys.foreign_keys WHERE name = 'FKOrdersCustomers';
You will find that the is_not_trusted column contains the value 1, indicating that the constraint is not trusted. To make it trusted, you need to modify the earlier turn on statement by adding the WITH CHECK option to it, as shown in the following example:
ALTER TABLE Test.Orders WITH CHECK CHECK CONSTRAINT FKOrdersCustomers;
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Data Integrity and Error Handling in SQL Server 2005
This option tells SQL Server to verify that all rows in the table comply with the constraint prior to turning it back on. If any rows do not comply with the constraint, an error message is returned and the ALTER TABLE statement is rolled back. If you execute the query again, you will find that you are back to the first execution plan (the one shown in Figure 5-2) and, if you execute the query against sys.foriegn_keys again, you will find that the is_not_trusted column now returns the value 0. It is once again trusted. One last note on this implementation: you can change the script for the Test.Customers and Test.Orders tables, as shown in the following example, so that the CustomerID column in the Test.Orders table allows for null values that is, it is not declared with the NOT NULL constraint.
CREATE TABLE Test.Customers ( CustomerID INT PRIMARY KEY ); CREATE TABLE Test.Orders ( OrderID INT PRIMARY KEY ,CustomerID INT NULL CONSTRAINT FKOrdersCustomers REFERENCES Test.Customers (CustomerID) );
If you execute the same query against this table structure, you get the execution plan from Figure 5-3. This means that the exists operator is being executed even if you have the trusted foreign key constraint in place. To persuade the query optimizer to go back to the plan that didn t execute the exists operator, you need to change the query, as shown in the following example:
SELECT o.* FROM Test.Orders AS o WHERE EXISTS (SELECT * FROM Test.Customers AS c WHERE c.CustomerID = o.CustomerID) AND o.CustomerID IS NOT NULL;
This informs SQL Server that no orders with a CustomerID of NULL should be returned, which brings you back to the plan from Figure 5-2.
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