An access path virtual field is a field that is logically, rather than physically, present on an access path. Although the field does not reside on the based-on physical file, a view of it is available through the relations that exist for the access path.
It is possible to omit particular virtual fields from a particular access path. The virtual fields for an access path can be selected only from among fields that have been specified as virtual fields for the file.
For example, if Customer file has five fields (Customer no., Branch name, Branch no., Customer name, and Customer address), and if Customer file Refers To Branch file with only Branch name field as a virtual field, only the Branch name field is available as a virtual field on all access paths of Customer file.
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Branch |
Known by |
Branch no. |
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Branch |
Has |
Branch name |
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Branch |
Has |
Branch address |
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Customer |
Known by |
Customer no. |
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Customer |
Has |
Customer name |
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Customer |
Has |
Customer address |
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Customer |
Refers to |
Branch |
The Customer Refers to relation adds the field Branch No. to the Customer file. Virtualization allows you to add the field Branch name as a virtual field to the Customer file. You can include or drop this virtual field from any access paths over the Customer file.
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