{"type":"ScholarlyArticle","title":"Counting Query Answers over a DL-Lite Knowledge Base","authors":[{"name":"Diego Calvanese","slug":"diego-calvanese","given":"Diego","family":"Calvanese","affiliation":"Free University of Bozen- Bolzano , Italy; Umeå University , Sweden"},{"name":"Julien Corman","slug":"julien-corman","given":"Julien","family":"Corman","affiliation":"Free University of Bozen- Bolzano , Italy"},{"name":"Davide Lanti","slug":"davide-lanti","given":"Davide","family":"Lanti","affiliation":"Free University of Bozen- Bolzano , Italy"},{"name":"Simon Razniewski","slug":"simon-razniewski","given":"Simon","family":"Razniewski","affiliation":"Max-Planck-Institut für Informatik , Germany"}],"abstract":"Counting answers to a query is an operation supported by virtually all database management systems.\r\nIn this paper we focus on counting answers over a Knowledge Base (KB), which may be viewed as a database enriched with background knowledge about the domain under consideration.\r\nIn particular, we place our work in the context of Ontology-Mediated Query Answering/Ontology-based Data Access (OMQA/OBDA), where the language used for the ontology is a member of the DL-Lite family and the data is a (usually virtual) set of assertions.\r\nWe study the data complexity of query answering, for different members of the DL-Lite family that include number restrictions, and for variants of conjunctive queries with counting that differ with respect to their shape (connected, branching, rooted).\r\nWe improve upon existing results by providing PTIME and coNP lower bounds, and upper bounds in PTIME and LOGSPACE.\r\nFor the LOGSPACE case, we have devised a novel query rewriting technique into first-order logic with counting.","doi":"10.24963/ijcai.2020/230","url":"https://ijcai.org/proceedings/2020/230","pdf":"https://ijcai.org/proceedings/2020/0230.pdf","markdown":"https://ijcai.org/proceedings/2020/230.md","bibtex":"https://ijcai.org/proceedings/2020/bibtex/230","volume":{"slug":"2020","title":"Proceedings of the Twenty-Ninth International Joint Conference on Artificial Intelligence","short_title":"IJCAI 2020","url":"https://ijcai.org/proceedings/2020/","year":2020,"location":"Yokohama","dates":"January 2021 (postponed from 11-17 July 2020)","editors":"Christian Bessiere","isbn":"978-0-9992411-6-5","publisher":"International Joint Conferences on Artificial Intelligence Organization","doi_prefix":"10.24963/ijcai.2020"},"section":"Knowledge Representation and Reasoning","track":"Main track","number":230,"pages":{"first":1658,"last":1666},"keywords":["Knowledge Representation and Reasoning: Description Logics and Ontologies","Multidisciplinary Topics and Applications: Databases","Knowledge Representation and Reasoning: Computational Complexity of Reasoning"],"videos":[{"label":"Video","url":"https://videos.ijcai20.org/25229/file_25229.m3u8","length_sec":281,"provider":"hls"}],"references":[{"position":1,"raw":"[Artale et al., 2009] Alessandro Artale, Diego Calvanese, Ro- man Kontchakov, and Michael Zakharyaschev. 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