The paper develops Editor, a language for manipulating semi-structured documents, such as the ones typically available on the Web. Editor programs are based on two simple ideas, taken from text editors: "search" instructions are used to select regions of interest in a document, and "cut & paste" to restructure them. We study the expressive power and the complexity of these programs. We show that they are computationally complete, in the sense that any computable document restructuring can be expressed in Editor. We also study the complexity of a safe subclass of programs, showing that it captures exactly the class of polynomial-time restructurings. The language has been implemented in Java, and is currently used in the Araneus project as a basis for a wrapper--generation toolkit. 1 Introduction It is well known that databases provide robust technology for querying highly structured data in a flexible and efficient way. Recently, the manipulation of less structured information has als...

A minimal and complete unification procedure for a theory with individual and sequence variables, free constants and free fixed and flexible arity function symbols is described and a brief overview of an extension with pattern-terms is given.

In order to enable the database programmer to reason about relations over strings of arbitrary length, we introduce Alignment Calculus, a modal extension of relational calculus. In addition to relations, a state in the model consists of a two-dimensional array where the strings are aligned on top of each other. The basic modality in the language (a transpose, or "slide") rearranges this alignment, and more complex formulae can be formed using a syntax reminiscent of regular expressions, in addition to the usual connectives and quantifiers. It turns out that the computational counterpart of the string-based portion of the logic is the class of multitape two-way finite state automata, which are devices particularly well suited for the implementation of string matching. A computational counterpart of the full logic is obtained from relational algebra by performing selection with these devices. Safety of formulae in Alignment Calculus implies that new strings generated from old ones have t...

We study analogs of classical relational calculus in the context of strings. We start by studying string logics. Taking a classical model-theoretic approach, we fix a set of string operations and look at the resulting collection of definable relations. These form an algebra - a class of n-ary relations for every n, closed under projection and Boolean operations. We show that by choosing the string vocabulary carefully, we get string logics that have desirable properties: computable evaluation and normal forms. We identify five distinct models and study the differences in their model-theory and complexity of evaluation. We identify a subset of these models which have additional attractive properties, such as finite VC dimension and quantifier elimination. Once you have a logic,

Many new database applications involve querying of graph data. In this paper, we present an objectoriented graph data model, and an OQL like graph query language, GOQL. The data model and the language are illustrated in the application domain of multimedia presentation graphs. We then discuss the query processing techniques for GOQL, more specifically, the translation of GOQL into an operatorbased language, called O-Algebra, extended with operators to deal with paths and sequences. We also discuss different approaches for efficient implementations of algebra operators for paths and sequences. 1 Introduction Many database applications such as hypertext applications, geographic information systems, world wide web searching, and heterogeneous information integration, etc., require modeling and querying of graph data ([Guti94, GBPV 94, MeMM 96, BDHS 96, AQMWW 96, AM 98, FFKLS 98, LSBBOO98]). In this paper, we present a data model, and an OQL-like query language GOQL, for querying graphs. ...

We study relational calculi with support for string operations. Most prior proposals were based on adding the operation of concatenation to rst-order logic. Such an extension is problematic as the relational calculus becomes computationally complete, which in turn implies strong limits on the ability to perform optimization and static analysis of properties such as query safety. In contrast, we look at extensions of relational calculus that have nice expressiveness, decidability, and safety properties, while corresponding to sets of string operations used in SQL. We start with an extension based on the string ordering and LIKE predicates. We then extend this basic model to include string length comparison. While both of these share some of the attractive properties of relational calculus (low data complexity for generic queries, eective syntax for safe queries, correspondence with an algebra), there is a large gap between these calculi in expressive power and complexity. The smaller...

An ordering for terms with sequence variables and flexible arity symbols is presented. The ordering coincides with the lexicographic extension of multiset path ordering on terms without sequence variables. It is shown that the classical strict superposition calculus with ordering and equality constraints can be used as a refutationally complete proving method for well-constrained sets of clauses with sequence variables and flexible arity symbols.

In this paper we pursue the study of Alignment Calculus, a declarative string database query language that supports both string querying and restructuring. This language is aimed for applications such as molecular biology databases, where the basic data type is a string, and the queries are combinatorial in nature. The declarative nature of our language does, however, require some additional effort in its implementation. Here we solve this problem by first defining a domain independent syntactic subset of the full language and then developing a query evaluation mechanism for this sublanguage. This mechanism then handles the required restructuring operations in a finite manner.

A string database is simply a collection of tables, the columns of which contain strings over some given alphabet. We address in this paper the issue of designing a simple, user friendly query language for string databases. We focus on the language FO(ffl), which is classical first order logic extended with a concatenation operator, and where quantifiers range over the set of all strings. We wish to capture all string queries, i.e., well-typed and computable mappings involving a notion of string genericity. Unfortunately, unrestricted quantification may allow some queries to have infinite output. This leads us to study the "safety" problem for FO(ffl), that is, how to build syntactic and/or semantic restrictions so as to obtain a language expressing only queries with finite output, hopefully all string queries. We introduce a family of such restrictions and study their expressivness and complexity. We prove that none of these languages express all string queries. We prov...

This paper develops a query language for sequence databases, such as genome databases and text databases. Unlike relational data, queries over sequential data can easily produce infinite answer sets, since the universe of sequences is infinite, even for a finite alphabet. The challenge is to develop query languages that are both highly expressive and finite. This paper develops such a language. It is a subset of a recently developed logic called Sequence Datalog [22]. Sequence Datalog distinguishes syntactically between subsequence extraction and sequence construction. Extraction creates sequences of bounded length, and leads to safe recursion; while construction can create sequences of arbitrary length, and leads to unsafe recursion. In this paper, we develop syntactic restrictions for Sequence Datalog that allow sequence construction but preserve finiteness. The main idea is to use safe recursion to control and limit unsafe recursion. The main results are a finite language, called We...