eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2023-03-17
24:1
24:18
10.4230/LIPIcs.ICDT.2023.24
article
A Simple Algorithm for Consistent Query Answering Under Primary Keys
Figueira, Diego
1
Padmanabha, Anantha
2
3
Segoufin, Luc
4
Sirangelo, Cristina
5
Univ. Bordeaux, CNRS, Bordeaux INP, LaBRI, UMR 5800, Talence, France
DI ENS, ENS, CNRS, PSL University, Paris, France
Inria, Paris, France
INRIA, ENS-Paris, PSL University, France
Université Paris Cité, CNRS, Inria, IRIF, F-75013, Paris, France
We consider the dichotomy conjecture for consistent query answering under primary key constraints. It states that, for every fixed Boolean conjunctive query q, testing whether q is certain (i.e. whether it evaluates to true over all repairs of a given inconsistent database) is either polynomial time or coNP-complete. This conjecture has been verified for self-join-free and path queries.
We propose a simple inflationary fixpoint algorithm for consistent query answering which, for a given database, naively computes a set Δ of subsets of database repairs with at most k facts, where k is the size of the query q. The algorithm runs in polynomial time and can be formally defined as:
1) Initialize Δ with all sets S of at most k facts such that S⊧ q.
2) Add any set S of at most k facts to Δ if there exists a block B (i.e., a maximal set of facts sharing the same key) such that for every fact a ∈ B there is a set S' ∈ Δ contained in S ∪ {a}. The algorithm answers "q is certain" iff Δ eventually contains the empty set. The algorithm correctly computes certainty when the query q falls in the polynomial time cases of the known dichotomies for self-join-free queries and path queries. For arbitrary Boolean conjunctive queries, the algorithm is an under-approximation: the query is guaranteed to be certain if the algorithm claims so. However, there are polynomial time certain queries (with self-joins) which are not identified as such by the algorithm.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol255-icdt2023/LIPIcs.ICDT.2023.24/LIPIcs.ICDT.2023.24.pdf
consistent query answering
primary keys
conjunctive queries