Download Database Repairing and Consistent Query Answering by Leopoldo Bertossi, M. Tamer Ozsu PDF
By Leopoldo Bertossi, M. Tamer Ozsu
Integrity constraints are semantic stipulations database should still fulfill so as to be a suitable version of exterior truth. In perform, and for plenty of purposes, a database would possibly not fulfill these integrity constraints, and as a result it truly is stated to be inconsistent. although, and probably, a wide component to the database continues to be semantically right, in a feeling that should be made distinctive. After having supplied a proper characterization of constant facts in an inconsistent database, the usual challenge emerges of extracting that semantically right information, as question solutions. The constant info in an inconsistent database is mostly characterised because the facts that persists throughout the entire database circumstances which are constant and minimally fluctuate from the inconsistent example. these are the so-called upkeep of the database. specifically, the constant solutions to a question posed to the inconsistent database are these solutions that may be at the same time acquired from all of the database maintenance. As anticipated, the proposal of fix calls for an enough idea of distance that enables for the comparability of databases with appreciate to how a lot they range from the inconsistent example. in this foundation, the minimality on upkeep will be adequately formulated. during this monograph we current and speak about those basic recommendations, diversified fix semantics, algorithms for computing constant solutions to queries, and in addition complexity-theoretic effects regarding the computation of maintenance and doing constant question answering. desk of Contents: creation / The Notions of fix and constant solution / Tractable CQA and question Rewriting / Logically Specifying maintenance / choice difficulties in CQA: Complexity and Algorithms / maintenance and knowledge cleansing
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Extra resources for Database Repairing and Consistent Query Answering
2003b] The following instance is inconsistent wrt the FD R : A → B. 4 A 1 1 · n n There are n pairs of tuples that, in combination, violate the FD. , an exponential number in the size of the original instance. B 0 1 · 0 1 WHAT DO WE DO THEN? For the moment, and loosely speaking, (the problem of ) consistent query answering (CQA) will refer to the problem of deciding if a tuple is a consistent answer to a query or to the problem of computing all the consistent answers to a query. 9 already shows that solving this problem by computing all the repairs of an inconsistent instance D, followed by querying them to detect what they have in common, should be avoided whenever possible.
Actually, the residue of a literal L wrt to an IC in clausal form ψ : ∀x(L ¯ 1 ∨ · · · ∨ Ln ) can always be obtained by means of a resolution step between L and ψ. This is shown more clearly in the next example. 3 continued) The FD can be written in clausal form as follows: FD : ∀x∀y∀z (¬Students(x, y) ∨ ¬Students(x, y) ∨ y = z) . Now, if the query is Q(x, y) : Students(x, y), we can apply resolution with the query atom and the first disjunct of FD, which are both eliminated because they are complementary literals.
They are used to collect at the end, when the process has stabilized, the atoms (annotated with t ) that will form a repair. Those are the tuples that were originally in the database or were inserted, but never deleted. 5. Program constraints: ← S_(x, ta ), S_(x, fa ). ← Q_(x, ta ), Q_(x, fa ). 1. SPECIFYING REPAIRS WITH LOGIC PROGRAMS 37 They are used to filter out the models of the program where the combinations expressed as conjunctions in the bodies become true. In this case, models where a same tuple is both inserted and deleted.