Capturing missing tuples and missing valuesCapturing missing tuples and missing values
Faculty of Sciences. Mathematics and Computer Science
Advanced Database Research and Modeling (ADReM)
2016New York, N.Y., 2016
ACM transactions on database systems. - New York, N.Y., 1976, currens
41(2016):2, 47 p.
University of Antwerp
Databases in real life are often neither entirely closed-world nor entirely open-world. Databases in an enterprise are typically partially closed, in which a part of the data is constrained by master data that contains complete information about the enterprise in certain aspects. It has been shown that, despite missing tuples, such a database may turn out to have complete information for answering a query. This article studies partially closed databases from which both tuples and attribute values may be missing. We specify such a database in terms of conditional tables constrained by master data, referred to as c-instances. We first propose three models to characterize whether a c-instance T is complete for a query Q relative to master data. That is, depending on how missing values in T are instantiated, the answer to Q in T remains unchanged when new tuples are added. We then investigate three problems, to determine (a) whether a given c-instance is complete for a query Q, (b) whether there exists a c-instance that is complete for Q relative to master data available, and (c) whether a c-instance is a minimal-size database that is complete for Q. We establish matching lower and upper bounds on these problems for queries expressed in a variety of languages in each of the three models for specifying relative completeness.