how to generate barcode in c# FiGURe 3-1 An actual execution plan in SSMS in Visual C#

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FiGURe 3-1 An actual execution plan in SSMS
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Now 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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After executing the same query again, you get a new execution plan, as shown in Figure 3-2. The 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. Therefore, it had to execute the EXISTS operator. For a large table, this can make a huge difference in performance.
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FiGURe 3-2 An actual execution plan in SSMS when the foreign key constraint is turned off
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Tables, Data Types, and Declarative Data Integrity
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Now turn on the foreign key again by executing the following statement, then rerun the query:
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ALTER TABLE Test.Orders CHECK CONSTRAINT FKOrdersCustomers;
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After running the query this time, you end up with the same plan again the plan shown in Figure 3-2. 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 to be not trusted . The optimizer does not take into account a constraint that is not trusted (which applies only to foreign key and check constraints). Your foreign key is no longer 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:
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SELECT name, is_not_trusted FROM sys.foreign_keys WHERE name = 'FKOrdersCustomers';
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You 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;
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 find that you are back to the first execution plan (the one shown in Figure 3-1) and, if you execute the query against the sys.foreign_keys catalog view again, you find that the is_not_trusted column now returns the value 0. The constraint 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) );
Lesson 2: Declarative Data Integrity
If you execute the same query against this table structure, you get the execution plan from Figure 3-2. This means that the EXISTS operator is being executed even if you have the trusted foreign key constraint in place. To persuade the 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 3-1. Pr actice
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