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... breaks down selection, arithmetics, and predicate evaluation into separate algebra operators. In earlier work, we found a similar representation also suited to express, e.g., the semantics of XQuery =-=[11]-=-. In the context of the current work, the notation turns out to have nice correspondences to the data flow in a hardware circuit and helps detecting opportunities for parallel evaluation. Figure 1 ill...

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...ational algebra plan for XQuery query Q 1. Annotations in ○ explained below. our relational encoding of variables and iteration. A full and formal account of the compilation technique may be found in =-=[7, 8]-=-. Oncetherelationalrepresentationofanitemsequenceis fixed, the encoding of a variable that gets bound to this sequence by an XQuery for-clause may be derived rather straightforwardly. In Q1 ,variable$...

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...fficiency and scalability. Here, we will review those bits of Pathfinder which are affected by order indifference. The subsequent developments directly plug into prior descriptions of this technology =-=[10, 11]-=-. Encoding trees and item sequences. Pathfinder can operate with any schemaoblivious XML tree encoding that assigns document order-preserving node identifiers. Viable candidates are, e.g., ORDPATH lab...

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...on of the size of each domain capture enough information to estimate the cardinality of relational XQuery plans. We will present our approach as follows. Section 2 recapitulates all relevant aspects of relational XQuery processing, followed by the principles of relational XQuery cardinality estimation in Section 3. In Section 4, we add support for structural (XPath-) and value-based queries and illustrate the cardinality inference process for Query Q1 in Section 5. Sections 6 and 7 discuss related work and wrap up. 2. RELATIONAL XQUERY We map XQuery syntax to relational plans via loop lifting [11], a compilation technique—originally developed for the XQuery compiler Pathfinder [10]—that derives algebraic queries from the compositional XQuery dialect shown in Table 1. The compiler’s target language is a table algebra (Table 2) that has been designed to ease query analysis—the focus in this work—as well as efficient execution on modern SQL-style database engines [11]. To these ends, we work with rather restricted operator literal values document order (e1 << e2) sequences (e1 , e2) node identity (e1 is e2) variables ($v) arithmetics (+, -, . . . ) let $v := e1 return e2 comparisons (eq, ...

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... exploit the properties of XML. There have been several initiatives to extend the relational data model to facilitate the XML data model and once again the XPath Accelerator has been at the forefront =-=[4]-=- [1] [15]. In [10], the key issue of whether the ordered XML data model can be efficiently represented by the relational tabular data model is examined and the authors propose three new encoding metho...

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...ces, its workload, etc. If there is a relational representation of this knowledge, the formulation of respective trust policies – we illustrate in Section 4 - is straightforward. Previous work, e.g., =-=[7]-=-, has already discussed the relational representation of hierarchies including operators for navigation. We borrow from this work to facilitate specification of contexts and facets in trust policies. ...

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...zed on top of existing XQuery processors. In Section 5, we present a syntactic fragment of XQuery that guarantees distributivity. Further, if we adopt a relational view of the XQuery semantics (as in =-=[16, 17]-=-), distributivity can be elegantly and uniformly tested on the familiar algebraic level. In Section 6, we present the algebraic counterpart of the inflationary fixed point operator on top of a relatio...

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... [BGvK + 06] pursue the primary goal to also embrace the complete dynamic semantics of XQuery (expression evaluation and runtime aspects) with the help of relational database systems. In earlier work =-=[GT04]-=-, we have shown that relational algebra makes for a suitable target language in an XQuery compiler. This purely relational approach to XQuery inherits the scalability advantages of the underlying rela...

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...ational algebra plan for XQuery query Q 1. Annotations in ○ explained below. our relational encoding of variables and iteration. A full and formal account of the compilation technique may be found in =-=[7, 8]-=-. Once the relational representation of an item sequence is fixed, the encoding of a variable that gets bound to this sequence by an XQuery for-clause may be derived rather straightforwardly. In Q 1, ...

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...tional encoding—the XPath accelerator—applying the ideas presented in [5]. Item sequences, on the other hand, may be transformed using techniques originally developed for a mapping from XQuery to SQL =-=[6, 7]-=-. The inference rules described in these papers form a mapping from XQuery Core to an (almost) standard relational algebra. MIL perfectly supports these operators and also allows for a tight integrati...