Torsten Grust

Accelerating XPath location steps

This work is a proposal for a database index structure that has been specifically designed to support the evaluation of XPath queries. As such, the index is capable to support all XPath axes (including ancestor, following, preceding-sibling, descendant-or-self, etc.). This feature lets the index stand out among related work on XML indexing structures which had a focus on regular path expressions (which correspond to the XPath axes children and descendant-or-self plus name tests). Its ability to start traversals from arbitrary context nodes in an XML document additionally enables the index to support the evaluation of path traversals embedded in XQuery expressions. Despite its flexibility, the new index can be implemented and queried using purely relational techniques, but it performs especially well if the underlying database host provides support for R-trees. A performance assessment which shows quite promising results completes this proposal.

MonetDB/XQuery: a fast XQuery processor powered by a relational engine

Relational XQuery systems try to re-use mature relational data management infrastructures to create fast and scalable XML database technology. This paper describes the main features, key contributions, and lessons learned while implementing such a system. Its architecture consists of (i) a range-based encoding of XML documents into relational tables, (ii) a compilation technique that translates XQuery into a basic relational algebra, (iii) a restricted (order) property-aware peephole relational query optimization strategy, and (iv) a mapping from XML update statements into relational updates. Thus, this system implements all essential XML database functionalities (rather than a single feature) such that we can learn from the full consequences of our architectural decisions. While implementing this system, we had to extend the state-of-the-art with a number of new technical contributions, such as loop-lifted staircase join and efficient relational query evaluation strategies for XQuery theta-joins with existential semantics. These contributions as well as the architectural lessons learned are also deemed valuable for other relational back-end engines. The performance and scalability of the resulting system is evaluated on the XMark benchmark up to data sizes of 11GB. The performance section also provides an extensive benchmark comparison of all major XMark results published previously, which confirm that the goal of purely relational XQuery processing, namely speed and scalability, was met.

Staircase join: teach a relational DBMS to watch its (axis) steps

Relational query processors derive much of their effectiveness from the awareness of specific table properties like sort order, size, or absence of duplicate tuples. This text applies (and adapts) this successful principle to database-supported XML and XPath processing: the relational system is made tree aware, i.e., tree properties like subtree size, intersection of paths, inclusion or disjointness of subtrees are made explicit. We propose a local change to the database kernel, the staircase join, which encapsulates the necessary tree knowledge needed to improve XPath performance. Staircase join operates on an XML encoding which makes this knowledge available at the cost of simple integer operations (e.g., +, ≤ ). We finally report on quite promising experiments with a staircase join enhanced main-memory database kernel.

Accelerating XPath evaluation in any RDBMS

This article is a proposal for a database index structure, the XPath accelerator, that has been specifically designed to support the evaluation of XPath path expressions. As such, the index is capable to support all XPath axes (including ancestor, following, preceding-sibling, descendant-or-self, etc.). This feature lets the index stand out among related work on XML indexing structures which had a focus on the child and descendant axes only. The index has been designed with a close eye on the XPath semantics as well as the desire to engineer its internals so that it can be supported well by existing relational database query processing technology: the index (a) permits set-oriented (or, rather, sequence-oriented) path evaluation, and (b) can be implemented and queried using well-established relational index structures, notably B-trees and R-trees.We discuss the implementation of the XPath accelerator on top of different database backends and show that the index performs well on all levels of the memory hierarchy, including disk-based and main-memory based database systems.

XQuery on SQL hosts

Relational database systems may be turned into efficient XML and XPath processors if the system is provided with a suitable relational tree encoding. This paper extends this relational XML processing stack and shows that an RDBMS can also serve as a highly efficient XQuery runtime environment. Our approach is purely relational: XQuery expressions are compiled into SQL code which operates on the tree encoding. The core of the compilation procedure trades XQuerys notions of variable scopes and nested iteration (FLWOR blocks) for equi-joins. The resulting relational XQuery processor closely adheres to the language semantics, e.g., it obeys node identity as well as document and sequence order, and can support XQuerys full axis feature. The system exhibits quite promising performance figures in experiments. Somewhat unexpectedly, we will also see that the XQuery compiler can make good use of SQLs OLAP functionality.

Advances in Database Technology — EDBT 2000

While we can take as a fact “the Web changes everything”, we argue that “XML is the means” for such a change to make a significant step forward. We therefore regard XML-related research as the most promising and challenging direction for the community of database researchers. In this paper, we approach XML-related research by taking three progressive perspectives. We first consider XML as a data representation standard (in the small), then as a data interchange standard (in the large), and finally as a basis for building a new repository technology. After a broad and necessarily coarse-grain analysis, we turn our focus to three specific research projects which are currently ongoing at the Politecnico di Milano, concerned with XML query languages, with active document management, and with XML-based specifications of Web sites.

FERRY: database-supported program execution

We demonstrate the language Ferry and its editing, compilation, and execution environment FerryDeck. Ferrys type system and operations match those of scripting or programming languages; its compiler has been designed to emit (bundles of) compliant and efficient SQL:1999 statements. Ferry acts as glue that permits a programming style in which developers access database tables using their programming languages own syntax and idioms -- the Ferry-expressible fragments of such programs may be executed by a relational database back-end, i.e., close to the data. The demonstrator FerryDeck implements compile-and-execute-as-you-type interactivity for Ferry and offers a variety of (graphical) hooks to explore and inspect this approach to database-supported program execution.

Why off-the-shelf RDBMSs are better at XPath than you might expect

To compensate for the inherent impedance mismatch between the relational data model (tables of tuples) and XML (ordered, unranked trees), tree join algorithms have become the prevalent means to process XML data in relational databases, most notably the TwigStack[6], structural join[1], and staircase join[13] algorithms. However, the addition of these algorithms to existing systems depends on a significant invasion of the underlying database kernel, an option intolerable for most database vendors. Here, we demonstrate that we can achieve comparable XPath performance without touching the heart of the system. We carefully exploit existing database functionality and accelerate XPath navigation by purely relational means: partitioned B-trees bring access costs to secondary storage to a minimum, while aggregation functions avoid an expensive computation and removal of duplicate result nodes to comply with the XPath semantics. Experiments carried out on IBM DB2 confirm that our approach can turn off-the-shelf database systems into efficient XPath processors.

Avalanche-safe LINQ compilation

We report on a query compilation technique that enables the construction of alternative efficient query providers for Microsofts Language Integrated Query (LINQ) framework. LINQ programs are mapped into an intermediate algebraic form, suitable for execution on any SQL:1999-capable relational database system. This compilation technique leads to query providers that (1) faithfully preserve list order and nesting, both being core features of the LINQ data model, (2) support the complete family of LINQs Standard Query Operators, (3) bring database support to LINQ to XML where the original provider performs in-memory query evaluation, and, most importantly, (4) emit SQL statement sequences whose size is only determined by the input querys result type (and thus independent of the database size). A sample query scenario uses this LINQ provider to marry database-resident TPC-H and XMark data---resulting in a unique query experience that exhibits quite promising performance characteristics, especially for large data instances.

A SQL: 1999 code generator for the pathfinder xquery compiler

The Pathfinder XQuery compiler has been enhanced by a new code generator that can target any SQL:1999-compliant relational database system(RDBMS). This code generator marks an important next step towards truly relational XQuery processing, a branch of database technology that aims to turn RDBMSs into highly efficient XML and XQuery processors without the need to invade the relational database kernel. Pathfinder, a retargetable front-end compiler, translates input XQuery expressions into DAG-shaped relational algebra plans. The code generator then turns these plans into sequences of either SQL:1999 statements or view definitions which jointly implement the (sometimes intricate) XQuery semantics. In a sense, this demonstration thus lets relational algebra and SQL swap their traditional roles in database query processing. The result is a code generator that (1) supports an almost complete dialect of XQuery, (2) can target any RDBMS with a SQL:1999 language interface, and (3) exhibits quite promising performance characteristics when run against high-volume XML data as well as complex XQuery expressions.