Distributed databases principles and systems
Distributed databases principles and systems
Principles of database and knowledge-base systems, Vol. I
Principles of database and knowledge-base systems, Vol. I
Answering queries using views (extended abstract)
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In this paper we study the following problem. Given a database and a set of queries, we want to find, in advance, a set of views that can compute the answers to the queries, such that the size of the viewset (i.e., the amount of space, in bytes, required to store the viewset) is minimal on the given database. This problem is important for many applications such as distributed databases, data warehousing, and data integration. We explore the decidability and complexity of the problem for workloads of conjunctive queries. We show that results differ significantly depending on whether the workload queries have self-joins. If queries can have self-joins, then a disjunctive viewset can be a better solution than any set of conjunctive views. We show that the problem of finding a minimal-size disjunctive viewset is decidable, and give an upper bound on its complexity. If workload queries cannot have self-joins, there is no need to consider disjunctive viewsets, and we show that the problem is in NP. We describe a very compact search space of conjunctive views, which contains all views in at least one optimal disjunctive viewset. We give a dynamic-programming algorithm for finding minimal-size disjunctive viewsets for queries without self-joins, and discuss heuristics to make the algorithm efficient.