单选题
In the following essay, each blank has four choices. Choose
the best answer and write down on the answer sheet. How does
the developer of a large database system achieve {{U}} {{U}} 47
{{/U}} {{/U}}and data independence? How does the developer make certain that
stored items are {{U}} {{U}} 48 {{/U}} {{/U}}and do not simply
disappear in the immensities of the database, as if in quicksand?
One solution is to set up a {{U}} {{U}} 49 {{/U}}
{{/U}}system, in which all data (the internal level), all records and files
(the conceptual level), and all user interfaces (the external level) are
controlled by one central programming group. This may seem the most useful
solution. But worldwide enterprises such as the meteorological and satellite
tracking networks, IBM, and General Motors gather and record information at a
very large number of locations. To stay flexible and avoid top - heaviness, such
an organization might want to avoid excessive centralization. A
centralized system is {{U}} {{U}} 50 {{/U}} {{/U}}, but it may
not be responsive to user needs. This is particularly true in a large
organization - the worldwide meteorological network, for example, Data-gathering
methods in the Himalayas, the equatorial jungles are different in very possible
way. It is reasonable to believe that the best choices for the corresponding
data storage and access structures ought to be just as different. A centralized
database system would not {{U}} {{U}} 51 {{/U}} {{/U}}such
differences.