Using views to generate efficient evaluation plans for queries

Afrati, FN; Li, C; Ullman, JD

dc.contributor.author	Afrati, FN	en
dc.contributor.author	Li, C	en
dc.contributor.author	Ullman, JD	en
dc.date.accessioned	2014-03-01T01:27:32Z
dc.date.available	2014-03-01T01:27:32Z
dc.date.issued	2007	en
dc.identifier.issn	0022-0000	en
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/18497
dc.subject	Answering queries using views	en
dc.subject	CoreCover	en
dc.subject	Query performance	en
dc.subject.classification	Computer Science, Hardware & Architecture	en
dc.subject.classification	Computer Science, Theory & Methods	en
dc.subject.other	Algorithms	en
dc.subject.other	Mathematical models	en
dc.subject.other	Optimal systems	en
dc.subject.other	Optimization	en
dc.subject.other	Problem solving	en
dc.subject.other	Answering queries	en
dc.subject.other	CoreCover	en
dc.subject.other	Optimal rewritings	en
dc.subject.other	Query performance	en
dc.subject.other	Query languages	en
dc.title	Using views to generate efficient evaluation plans for queries	en
heal.type	journalArticle	en
heal.identifier.primary	10.1016/j.jcss.2006.10.019	en
heal.identifier.secondary	http://dx.doi.org/10.1016/j.jcss.2006.10.019	en
heal.language	English	en
heal.publicationDate	2007	en
heal.abstract	We study the problem of generating efficient, equivalent rewritings using views to compute the answer to a query. We take the closed-world assumption, in which views are materialized from base relations, rather than views describing sources in terms of abstract predicates, as is common when the open-world assumption is used. In the closed-world model, there can be an infinite number of different rewritings that compute the same answer, yet have quite different performance. Query optimizers take a logical plan (a rewriting of the query) as an input, and generate efficient physical plans to compute the answer. Thus our goal is to generate a small subset of the possible logical plans without missing an optimal physical plan. We first consider a cost model that counts the number of subgoals in a physical plan, and show a search space that is guaranteed to include an optimal rewriting, if the query has a rewriting in terms of the views. We also develop an efficient algorithm for finding rewritings with the minimum number of subgoals. We then consider a cost model that counts the sizes of intermediate relations of a physical plan, without dropping any attributes, and give a search space for finding optimal rewritings. Our final cost model allows attributes to be dropped in intermediate relations. We show that, by careful variable renaming, it is possible to do better than the standard ""supplementary relation"" approach, by dropping attributes that the latter approach would retain. Experiments show that our algorithm of generating optimal rewritings has good efficiency and scalability. © 2006 Elsevier Inc. All rights reserved.	en
heal.publisher	ACADEMIC PRESS INC ELSEVIER SCIENCE	en
heal.journalName	Journal of Computer and System Sciences	en
dc.identifier.doi	10.1016/j.jcss.2006.10.019	en
dc.identifier.isi	ISI:000246703900002	en
dc.identifier.volume	73	en
dc.identifier.issue	5	en
dc.identifier.spage	703	en
dc.identifier.epage	724	en