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Show It was also necessary to work with a small database so that a control database could be created on the clinical computer system. The control database contains exactly the same set of data as the virtual database, and was used to evaluate the performance of the virtual database. 4.1 The Physical Database Management System A virtual database makes possible the transparent use of a low-cost storage medium to store the data of a database. This storage contains the physical file of a virtual database. The standard database management system to which virtual-database technology is applied cannot access this storage directly. If it could, there would be no need for a virtual database system. A database management system that can access the storage is required because the standard database management system cannot be used to manage the physical file. In the virtual database system for HELP, the physical file is stored on an IBM-compatible personal computer. A wide variety of database management systems are available for this type of personal computer. These database management systems provide the functionality required by the virtual database system. Due to the existence of off-the-shelf database products, it was not necessary to develop a database management system for the physical file. As noted in Chapter 3, a relational database management system that conforms to the SQL standard was selected to manage the physical files. The decision to use a database management system that is SQL compliant was not based on the functional requirements of the virtual database system. Any database management system that 36 |
