Implementing Hyperlog, a Graph-Based Database Language

We describe the implementation of Hyperlog , a graph-based declarative language which supports both queries and updates over a graph-based data model called the Hypernode Model. This model is capable of representing arbitrarily complex data structures by means of nested and recursively defined graphs , while the Hyperlog language is computationally complete. By requiring only a very small number of graphical constructs , Hyperlog is well-suited to non-expert database users. In this paper we describe the graphical aspects of the Hyperlog implementation including novel techniques for : representing and updating data , queries and programs ; representing and browsing the database ; and representing and browsing the output from queries. 1. Introduction M UCH RECENT database research has focussed on deductive [4] and object-oriented [12] databases , and also on graph-based representation and manipulation of data. Following the general direction of these trends , a graph-based data model called the Hypernode Model has been developed [14 , 15] which exhibits features of all three. The model is based on nested , and possibly recursively defined , graphs termed hypernodes. Hypernodes have unique labels which serve as object identifiers : this means that arbitrarily complex objects can be represented by a network of hypernodes. Hypernodes can contain the labels of other hypernodes in their node-set , including their own label : this means that the model provides inherent support for the nesting of information. The Hypernode Model comes equipped with a graph-based declarative language called Hyperlog which supports both queries and updates. A query in Hyperlog consists of a number of graphs (termed templates) which are matched against the hypernodes in the database and which generate graphical output. Database updates in Hyperlog are undertaken by means of programs. A Hyperlog program consists of a set of rules whose head and body are again sets of templates. The templates in the head of a rule indicate the updates to be undertaken for each match of the templates in the body of the rule. The evaluation of a program consists of a repeated matching of the bodies of its rules against the current database state until no more updates can be inferred .