View selection for real conjunctive queries

Afrati, F; Chirkova, R; Gergatsoulis, M; Pavlaki, V

dc.contributor.author	Afrati, F	en
dc.contributor.author	Chirkova, R	en
dc.contributor.author	Gergatsoulis, M	en
dc.contributor.author	Pavlaki, V	en
dc.date.accessioned	2014-03-01T01:27:33Z
dc.date.available	2014-03-01T01:27:33Z
dc.date.issued	2007	en
dc.identifier.issn	0001-5903	en
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/18505
dc.subject	Conjunctive Queries	en
dc.subject	Query Evaluation	en
dc.subject	Satisfiability	en
dc.subject.classification	Computer Science, Information Systems	en
dc.subject.other	AGGREGATE QUERIES	en
dc.title	View selection for real conjunctive queries	en
heal.type	journalArticle	en
heal.identifier.primary	10.1007/s00236-007-0046-z	en
heal.identifier.secondary	http://dx.doi.org/10.1007/s00236-007-0046-z	en
heal.language	English	en
heal.publicationDate	2007	en
heal.abstract	Given a query workload, a database and a set of constraints, the view-selection problem is to select views to materialize so that the constraints are satisfied and the views can be used to compute the queries in the workload efficiently. A typical constraint, which we consider in the present work, is to require that the views can be stored in a given amount of disk space. Depending on features of SQL queries (e.g., the DISTINCT keyword) and on whether the database relations on which the queries are applied are sets or bags, the queries may be computed under set semantics, bag-set semantics, or bag semantics. In this paper we study the complexity of the view-selection problem for conjunctive queries and views under these semantics. We show that bag semantics is the ""easiest to handle"" (we show that in this case the decision version of view selection is in NP), whereas under set and bag-set semantics we assume further restrictions on the query workload (we only allow queries without self-joins in the workload) to achieve the same complexity. Moreover, while under bag and bag-set semantics filtering views (i.e., subgoals that can be dropped from the rewriting without impacting equivalence to the query) are practically not needed, under set semantics filtering views can reduce significantly the query-evaluation costs. We show that under set semantics the decision version of the view-selection problem remains in NP only if filtering views are not allowed in the rewritings. Finally, we investigate whether the cgalg algorithm for view selection introduced in Chirkova and Genesereth (Linearly bounded reformulations of conjunctive databases, pp. 987-1001, 2000) is suitable in our setting. We prove that this algorithm is sound for all cases we examine here, and that it is complete under bag semantics for workloads of arbitrary conjunctive queries and under bag-set semantics for workloads of conjunctive queries without self-joins. © 2007 Springer-Verlag.	en
heal.publisher	SPRINGER	en
heal.journalName	Acta Informatica	en
dc.identifier.doi	10.1007/s00236-007-0046-z	en
dc.identifier.isi	ISI:000249014500001	en
dc.identifier.volume	44	en
dc.identifier.issue	5	en
dc.identifier.spage	289	en
dc.identifier.epage	321	en