View determinacy for preserving selected information in data transformations

When transforming data one often wants certain information in the data source to be preserved, i.e.,we identify parts of the source data and require these parts to be transformed without loss of information. We characterize the preservation of selected information in terms of the notions of invertibility and query preservation, in a setting when transformations are specified as a view image (a set of queries), and source information is selected by a query Q. We investigate the problem for determining whether transformations image preserve the information selected by Q. (1) We show that the notion of invertibility coincides with view determinacy studied for query rewriting. (2) We establish the undecidability of the problem when either Q or image is in DATALOG or first-order logic, for invertibility and query preservation. (3) When Q and image are conjunctive queries (image), the problem is as hard as view determinacy for image queries and image views, an open problem. Nevertheless, we provide complexity bounds of the problem, either in ptime or np-complete, when image ranges over subclasses of image (i.e., image, SC, image), and when Q is assumed to be a minimal image query or not. (4) We show that image is complete for image rewriting when image is image, SC or image, i.e.,every image query can be rewritten in terms of image, SC or image views using a query in image.