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Relational Database Management Systems are currently suggested to be
used as a base for document retrieval systems. This is because
commercially available relational DBMSs provide effective tools for
managing databases and, especially, the underlying relational model
supports ad hoc queries which are characteristic of document
retrieval systems. When SQL is directly used for ad hoc queries
navigation through the underlying database scheme has to be provided
by the enquirer, which could be very difficult. This article
presents the achievable simplifications when document retrieval
systems will be based on the universal relation model. When the
search condition of a query is too fuzzy, the cardinality of the
answer set may become very large. Some retrieval systems thus
provide the capability of refining answer sets; this allows
referring to an existing answer set when issuing an additional query
to reduce the number of qualifying tuples. There are two different
methods to refine a given answer set: one method produces a new
query based on the predicates of both queries, the other method
creates an intermediate answer set based on the new query and
provides the refined answer set via suitable set manipulations. The
results are in general different. Focusing on queries on universal
relation systems, we describe the above problem in more detail and
provide criteria under which the above methods produce the same
result.
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