Abstract
Consistent query answering (CQA) aims to find meaningful answers to queries when databases are inconsistent, i.e., do not conform to their specifications. Such answers must be certainly true in all repairs, which are consistent databases whose difference from the inconsistent one is minimal, according to some measure. This task is often computationally intractable, and much of CQA research concentrated on finding islands of tractability. Nevertheless, there are many relevant queries for which no efficient solutions exist, which is reflected by the limited practical applicability of the CQA approach. To remedy this, one needs to devise a new CQA framework that provides explicit guarantees on the quality of query answers. However, the standard notions of repair and certain answers are too coarse to permit more elaborate schemes of query answering. Our goal is to provide a new framework for CQA based on revised definitions of repairs and query answering that opens up the possibility of efficient approximate query answering with explicit guarantees. The key idea is to replace the current declarative definition of a repair with an operational one, which explains how a repair is constructed, and how likely it is that a consistent instance is a repair. This allows us to define how certain we are that a tuple should be in the answer. Using this approach, we study the complexity of both exact and approximate CQA. Even though some of the problems remain hard, for many common classes of constraints we can provide meaningful answers in reasonable time, for queries going far beyond the standard CQA approach.
Highlights
Consistent query answering (CQA) is an elegant idea introduced in the late 1990s by Arenas, Bertossi, and Chomicki [1] that has been extensively studied since
The key elements of the CQA approach are the notion of repair of an inconsistent database D, that is, a consistent database whose difference with D is somehow minimal, and the notion of query answering based on certain answers
Since there could be many repairs, finding certain answers is most commonly CONP-hard, even for conjunctive queries [3, 9]. This led to a large body of work on showing dichotomy results; see, e.g., [6, 7], classifying all query answering into tractable and CONP-hard cases as the ultimate goal of the CQA endeavor
Summary
Consistent query answering (CQA) is an elegant idea introduced in the late 1990s by Arenas, Bertossi, and Chomicki [1] that has been extensively studied since. We believe that the ultimate goal of a practically applicable CQA approach should be efficient approximate query answering with explicitly stated guarantees. Our goal is to replace the current declarative approach to repairs with an operational one that explains the process of constructing a repair As it gives us a finer understanding of why an instance is a repair, it leads to more refined ways of answering queries, by letting us define how certain we are that a tuple should be in the answer. This in turn opens up the possibility of efficient approximate consistent query answering. This is a short version of [2]
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