Annotation Propagation Revisited For Key Preserving Views

Abstract

This paper revisits the analysis of annotation propagation from source databases to views defined in terms of conjunctive (SPJ) queries. Given a source database D, an SPJ query Q, the view Q(D) and a tuple DV in the view, the view (resp. source) side-effect problem is to find a minimal set DD of tuples such that the deletion of DD from D results in the deletion of DV from Q(D) while minimizing the side effects on the view (resp. the source). A third problem, referred to as the annotation placement problem, is to find a single base tuple DD such that annotation in a field of DD propagates to DV while minimizing the propagation to other fields in the view Q(D). These are important for data provenance and the management of view updates. However important, these problems are unfortunately NP-hard for most subclasses of SPJ views.To make the annotation propagation analysis feasible in practice, we propose a key preserving condition on SPJ views, which requires that the projection fields of an SPJ view Q retain a key of each base relation involved in Q. While this condition is less restrictive than other proposals, it often simplifies the annotation propagation analysis. Indeed, for key-preserving SPJ views the annotation placement problem coincides with the view side-effect problem, and the view and source side-effect problems become tractable. In addition we generalize existing settings by allowing DV to be a group of tuples to be deleted, and investigate the insertion of tuples to the view. We show that group updates make the analysis harder: these problems become NP-hard for several subclasses of SPJ views. We also show that for SPJ views the source and view side-effect problems are NP-hard for single-tuple insertion, but are tractable for some subclasses of SPJ for group insertions, in the presence or in the absence of the key preservation condition.