Consistent Query Answering for Primary Keys and Conjunctive Queries with Counting
arxiv(2022)
摘要
The problem of consistent query answering for primary keys and self-join-free
conjunctive queries has been intensively studied in recent years and is by now
well understood. In this paper, we study an extension of this problem with
counting. The queries we consider count how many times each value occurs in a
designated (possibly composite) column of an answer to a full conjunctive
query. In a setting of database repairs, we adopt the semantics of [Arenas et
al., ICDT 2001] which computes tight lower and upper bounds on these counts,
where the bounds are taken over all repairs. Ariel Fuxman defined in his PhD
thesis a syntactic class of queries, called C_forest, for which this
computation can be done by executing two first-order queries (one for lower
bounds, and one for upper bounds) followed by simple counting steps. We use the
term "parsimonious counting" for this computation. A natural question is
whether C_forest contains all self-join-free conjunctive queries that admit
parsimonious counting. We answer this question negatively. We define a new
syntactic class of queries, called C_parsimony, and prove that it contains all
(and only) self-join-free conjunctive queries that admit parsimonious counting.
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关键词
conjunctive queries,consistent query,primary keys
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