Implementing a declarative string query language with string restructuring

Abstract

We describe the design and implementation of a declarative database query language for manipulating character strings. The language can be used to create logical predicates expressing structural properties of strings and relations between several strings. The predicates can be used to query strings in databases, and by leaving variables uninstan-tiated, also to generate new strings not contained in the database. A full working system was implemented as an extension of an object-oriented database management system and its query language. The declarative expressions are evaluated by first performing a compilation transforming them to nondeterministic finite state automata and then by simulating these automata using a depth-first search engine. The system checks the safety of each string-manipulation query in advance to preclude infinite ones. This safety checking provides also a compile-time loop-checking mechanism for the search engine, improving its efficiency. © Springer-Verlag Berlin Heidelberg 1998.