84,348 research outputs found

    The complexity of acyclic conjunctive queries revisited

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    In this paper, we consider first-order logic over unary functions and study the complexity of the evaluation problem for conjunctive queries described by such kind of formulas. A natural notion of query acyclicity for this language is introduced and we study the complexity of a large number of variants or generalizations of acyclic query problems in that context (Boolean or not Boolean, with or without inequalities, comparisons, etc...). Our main results show that all those problems are \textit{fixed-parameter linear} i.e. they can be evaluated in time f(Q).db.Q(db)f(|Q|).|\textbf{db}|.|Q(\textbf{db})| where Q|Q| is the size of the query QQ, db|\textbf{db}| the database size, Q(db)|Q(\textbf{db})| is the size of the output and ff is some function whose value depends on the specific variant of the query problem (in some cases, ff is the identity function). Our results have two kinds of consequences. First, they can be easily translated in the relational (i.e., classical) setting. Previously known bounds for some query problems are improved and new tractable cases are then exhibited. Among others, as an immediate corollary, we improve a result of \~\cite{PapadimitriouY-99} by showing that any (relational) acyclic conjunctive query with inequalities can be evaluated in time f(Q).db.Q(db)f(|Q|).|\textbf{db}|.|Q(\textbf{db})|. A second consequence of our method is that it provides a very natural descriptive approach to the complexity of well-known algorithmic problems. A number of examples (such as acyclic subgraph problems, multidimensional matching, etc...) are considered for which new insights of their complexity are given.Comment: 30 page

    Culture and e-commerce: An exploration of the perceptions and attitudes of Egyptian internet users

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    This paper examines the perceptions and attitudes that Egyptian users hold towards electronic shopping sites. Internet sites are globally available, opening up huge potential markets for online retailers. However, it remains unclear whether sites designed for the US or European markets will be acceptable in other cultures. This paper describes an exploratory card sorting study conducted with Egyptian consumers. The study was designed to examine the e-commerce interface features that are most salient to this user group and to explore how these relate to user intentions to engage in internet shopping. The results support the role of site familiarity in predicting purchase intentions within this cultural setting
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