View updating in relational databases.

Abstract

To properly realize view updates, the problem of view updating is analyzed in the context of mathematical logic. A view is a goal logically implied by its premises, viz. the constituent relations. Based on their logical interpretation, views can be classified as coexistent views and implicative views. A coexistent view is a view which, in addition to being implied by its premises, also implies its premises, whereas an implicative view is one which does not. View updates including insertions, deletions, and modifications are analyzed in each case. To provide a framework for view updates, database states are formulated as internal states and perceptible states in the system and user abstraction levels, respectively. This dissertation presents an innovative approach which makes view updating convenient, flexible, and reliable. Instead of guessing the user's intent, this approach exploits the information available, including view definitions and database contents, to complete the view update requested. An insertion into a coexistent view is translated into insertions of its constituent relations by an appropriate translating algorithm. A content-independent translating algorithm is presented to translate insertions of the class of attribute-preserving coexistent views, in which insertions can always be translated without referring to the database content, and a content-dependent translating algorithm is presented for the class of attribute-recoverable coexistent views, in which insertions can often be translated with the help of the database content. As an insertion into an implicative view must not be translated, it is instead carried out by being stored in a real relation associated with the view. View update anomalies arise in this approach but are resolved by the enforceable functional dependency set of the view. While a view deletion can be treated uniformly in both coexistent and implicative views, the difficulty lies in selecting the corresponding base relation deletions. This problem is solved by identifying and deleting nothing but the weak links. A view modification is logically considered as the combination of a view deletion and a view insertion and is consequently performed as a combination of the above approaches with some minor adjustments.