We consider the problem of storing and accessing documents (SGML and HTML, in particular) using database technology. To specify the database image of documents, we use structuring schemas that consist in grammars annotated with database programs. To query documents, we introduce an extension of OQL, the ODMG standard query language for object databases. Our extension (named OQL-doc) allows to query documents without a precise knowledge of their structure using in particular generalized path expressions and pattern matching. This allows us to introduce in a declarative language (in the style of SQL or OQL), navigational and information retrieval styles of accessing data. Query processing in the context of documents and path expressions leads to challenging implementation issues. We extend an object algebra with new operators to deal with generalized path expressions. We then consider two essential complementary optimization techniques: 1. we show that almost standard database optim...

Several alternatives to manage large XML document collections exist, ranging from file systems over relational or other database systems to specifically tailored XML repositories. In this paper we give a tour of Natix, a database management system designed from scratch for storing and processing XML data. Contrary

In the past few years, query languages featuring generalized path expressions have been proposed. These languages allow the interrogation of both data and structure. They are powerful and essential for a number of applications. However, until now, their evaluation has relied on a rather naive and inefficient algorithm. In this paper, we extend an object algebra with two new operators and present some interesting rewriting techniques for queries featuring generalized path expressions. We also show how a query optimizer can integrate the new techniques. 1 Introduction In the past few years there has been a growing interest in query languages featuring generalized path expressions (GPE) [BRG88, KKS92, CACS94, QRS + 95]. With these languages, one may issue queries on data without exact knowledge of its structure. A GPE queries data and structure at the same time. Although very useful for standard database applications, these languages are vital for new applications dedicated, for insta...

This paper describes XPeer, a zero-administration system for sharing and querying XML data. The system allows users to share XML data without significant human intervention, and to pose XQuery FLWR queries against them.The proposed system can be used in any application field, being a general purpose XML p2p DBMS, even though its main application is the management of resource descriptions in GRID environments.

A new method for efficiently evaluating queries with aggregate functions is presented. More specifically, we introduce a class of aggregate queries where traditional query evaluation strategies in general require O(n ) time and space in the size of the (at most two) input relations. For this class of aggregate queries our approach needs at most O(n log n) time and linear space. Further, our approach deals not only with relations but with general bulk types like sets, bags, and lists.

We present algebraic equivalences that allow to unnest nested algebraic expressions for order-preserving algebraic operators. We illustrate how these equivalences can be applied successfully to unnest nested queries given in the XQuery language. Measurements illustrate the performance gains possible by unnesting.

XML has reached a widespread diffusion as a language for representing nearly any kind of data source, from relational databases to digital movies. Due to the growing interest toward XML, many tools for storing, processing, and querying XML data have appeared in the last two years. Three main problems affect XML query processing: path expression evaluation, nested query resolution, and preservation of document order. These issues, which are related to the hierarchical structure of XML and to the features of current XML query languages, require compile-time as well as run-time solutions. This paper describes a query algebra for XML data. The main purpose of this algebra, which forms the basis for the Xtasy database management system, is to combine good optimization properties with a good expressive power that allows it to model significant fragments of current XML query languages; in particular, explicit support is given to efficient path expression evaluation, nested query resolution, and order preservation.

We investigate the optimization and evaluation of queries with universal quantification in the context of the object-oriented and object-relational data models. The queries are classified into 16 categories depending on the variables referenced in the so-called range and quantifier predicates. For the three most important classes we enumerate the known query evaluation plans and devise some new ones. These alternative plans are primarily based on anti-semijoin, division, generalized grouping with count aggregation, and set difference. In order to evaluate the quality of the many different evaluation plans a thorough performance analysis on some sample database configurations was carried out. The quantitative analysis reveals that---if applicable---the anti-semijoin-based plans are superior to all the other alternatives, even if we employ the most sophisticated division algorithms. Furthermore, exploiting object-oriented features, anti-semijoin plans can be derived even when this is not...

Queries formulated in a nested way are very common in XQuery. Unfortunately, their evaluation is usually very inefficient when done in a straightforward fashion. We present a framework for handling nested queries that is based on unnesting the queries after having translated them into an algebra. We not only present a collection of algebraic equivalences, but also supply a strategy on how to use them effectively. The full potential of the approach is demonstrated by applying our rewrites to actual queries and showing that performance gains of several orders of magnitude are possible.

. Inter-object references are one of the key concepts of object-relational and object-oriented database systems. In this work, we investigate alternative techniques to implement inter-object references and make the best use of them in query processing, i.e., in evaluating functional joins. We will give a comprehensive overview and performance evaluation of all known techniques for simple (singlevalued) as well as multi-valued functional joins. Furthermore, we will describe special order-preserving functionaljoin techniques that are particularly attractive for decision support queries that require ordered results. While most of the presentation of this paper is focused on object-relational and object-oriented database systems, some of the results can also be applied to plain relational databases because index nested-loop joins along key/foreign-key relationships, as they are frequently found in relational databases, are just one particular way to execute a functional join. Key words: O...