Answering Queries using Views over Probabilistic XML: Complexity and Tractability

We study the complexity of query answering using views in a probabilistic XML setting, identifying large classes of XPath queries -- with child and descendant navigation and predicates -- for which there are efficient (PTime) algorithms. We consider this problem under the two possible semantics for XML query results: with persistent node identifiers and in their absence. Accordingly, we consider rewritings that can exploit a single view, by means of compensation, and rewritings that can use multiple views, by means of intersection. Since in a probabilistic setting queries return answers with probabilities, the problem of rewriting goes beyond the classic one of retrieving XML answers from views. For both semantics of XML queries, we show that, even when XML answers can be retrieved from views, their probabilities may not be computable. For rewritings that use only compensation, we describe a PTime decision procedure, based on easily verifiable criteria that distinguish between the feasible cases -- when probabilistic XML results are computable -- and the unfeasible ones. For rewritings that can use multiple views, with compensation and intersection, we identify the most permissive conditions that make probabilistic rewriting feasible, and we describe an algorithm that is sound in general, and becomes complete under fairly permissive restrictions, running in PTime modulo worst-case exponential time equivalence tests. This is the best we can hope for since intersection makes query equivalence intractable already over deterministic data. Our algorithm runs in PTime whenever deterministic rewritings can be found in PTime.Comment: VLDB201

XPath: Looking Forward

The location path language XPath is of particular importance for XML applications since it is a core component of many XML processing standards such as XSLT or XQuery. In this paper, based on axis symmetry of XPath, equivalences of XPath 1.0 location paths involving reverse axes, such as anc and prec, are established. These equivalences are used as rewriting rules in an algorithm for transforming location paths with reverse axes into equivalent reverse-axis-free ones. Location paths without reverse axes, as generated by the presented rewriting algorithm, enable efficient SAX-like streamed data processing of XPath

Βελτιστοποίηση ερωτημάτων χρησιμοποιώντας σημασιολογία πολυσυνόλου και συνόλου-πολυσυνόλου σε περιβάλλον ετερογενών πηγών πληροφόρησης

184 σ.Στην συγκεκριμένη διατριβή, μελετάμε ανάπτυξη τεχνικών βελτιστοποίησης ερωτημάτων με την χρήση όψεων, σε σχεσιακές και XML βάσεις δεδομένων. Ειδικότερα, επικεντρωνόμαστε στα ακόλουθα βασικά προβλήματα βελτιστοποίησης ερωτημάτων: την περιεκτικότητα ερωτημάτων, την αναδιατύπωση ερωτημάτων και την επιλογή όψεων. Στις σχεσιακές βάσεις δεδομένων, επικεντρωνόμαστε στα συζευκτικά ερωτήματα (εν συντομία CQs), που αντιστοιχούν σε SQL ερωτήματα με χρήση των τελεστών select, project και join. Επίσης, χρησιμοποιούμε σημασιολογίες πολυσυνόλου (οι βασικές σχέσεις και οι απαντήσεις των ερωτημάτων είναι πολυσύνολα) και συνόλου-πολυσυνόλου (οι βασικές σχέσεις είναι σύνολα, ενώ οι απαντήσεις είναι πολυσύνολα) για να περιγράψουμε, θεωρητικά, την σημασιολογία της SQL. Για ερωτήματα σε XML δεδομένα χρησιμοποιούμε την γλώσσα XPath, και ειδικότερα επικεντρωνόμαστε στις τρεις βασικές υποκλάσεις της γλώσσας, που σχηματίζεται από την χρήση δύο από τα τρία βασικά συστατικά: wildcard ετικέτες (*), ακμές απογόνου (//) και κλαδιά ([ ]). Στο πλαίσιο της περιεκτικότητας ερωτημάτων μελετάμε το πρόβλημα, καθώς και την πολυπλοκότητα του, για βασικές υποκλάσεις των CQs. Για την γενική κλάση των CQs το πρόβλημα παραμένει ανοικτό εδώ και μια δεκαετία. Επιπλέον, μελετάμε τα προβλήματα περιεκτικότητας και ισοδυναμίας για ενώσεις XPath ερωτημάτων. Για την αναδιατύπωση CQ ερωτημάτων, περιγράφουμε βασικές συνθήκες που πρέπει να πληρούν οι όψεις έτσι ώστε να υπάρχει μία ισοδύναμη αναδιατύπωση. Για τα XPath ερωτήματα που σχηματίζονται από // και *, δείχνουμε ότι η χρήση του τελεστή ένωσης απαιτείται για την εύρεση ισοδύναμης αναδιατύπωσης. Το πρόβλημα επιλογής όψεων μελετάται για CQ ερωτήματα, όπου επικεντρωνόμαστε στον περιορισμό του χώρου αναζήτησης βέλτιστων λύσεων. Ειδικότερα, δείχνομαι ότι εάν η επιλογή του συνόλου όψεων γίνεται βάσει συγκεκριμένων συνθηκών (ως προς την μορφή των όψεων), τότε εξασφαλίζεται η εύρεση τουλάχιστον μίας βέλτιστης λύσης για το πρόβλημα. Έπειτα, επικεντρωνόμενοι σε υποκλάσεις των CQ ερωτημάτων, δείχνουμε ότι για ένα σύνολο ερωτημάτων αλυσίδας, και για τις δύο σημασιολογίες, όψεις που ορίζονται, και αυτές, από ερωτήματα αλυσίδας δεν επαρκούν, πάντα, για την εύρεση βέλτιστης λύσης. Στην περίπτωση, όμως, των ερωτημάτων μονοπατιού, και θεωρώντας σημασιολογία πολυσυνόλου, δείχνουμε ότι οι όψεις που ορίζονται από ερωτήματα μονοπατιού μας εξασφαλίζουν την εύρεση τουλάχιστον μίας βέλτιστης λύσης για το πρόβλημα επιλογής όψεων.In this thesis, we investigate techniques for query optimization using a set of views, considering both relational and XML databases. In particular, we focus on three fundamental problems of query optimization; which are the query containment, the query rewriting and the view selection. For relational databases we focus on the class of select-project-join SQL queries with equality comparisons, a.k.a. conjunctive queries (CQs for short). We consider two kinds of semantics to theoretically approximate the SQL semantics: the bag (multiple occurrences of the same tuple are allowed in both base relations and answers of queries) and bag-set semantics (the base relations are sets and the operators are liable for bag-results). For XML databases, we focus on XPath. Especially, we focus on the major fragments of XPath which contain two of the constructs: wildcard, descendant edge and branches. Query containment under both bag and bag-set semantics is investigated through a detailed analysis of special cases of CQs. The complexity in each case is given, as well. For the general case, the problem remains open for more than a decade. Moreover, we give necessary and sufficient conditions for deciding both containment and equivalence for unions of XPath queries; a problem which was not investigated in depth, in the past. The problem of finding an equivalent rewriting is also investigated for both relational and XPath queries. In particular, for relational queries, we describe the requirements that a set of views have to satisfy in order to give an equivalent rewriting of a CQ under both bag and bag-set semantics. In the case of XML databases, we investigate the problem of rewriting an XPath query using multiple views, and prove that in the case that the query contains both descendant edges and wildcards, the union operator may be required for finding an equivalent rewriting. The view selection is investigated for workloads of CQs under both bag and bag-set semantics. Especially, we aim to limit the search space of candidate viewsets. We start with the general case, where we give a tight condition that candidate views can satisfy and still the search space does contain at least one optimal solution. Then we study special cases. We show that for chain query workloads under both bag and bag-set semantics, taking only chain views may miss optimal solution, whereas, if we further limit the queries to be path queries, then under bag semantics, path views suffice.Ματθαίος Γ. Δαμίγο

Ensuring Query Compatibility with Evolving XML Schemas

During the life cycle of an XML application, both schemas and queries may change from one version to another. Schema evolutions may affect query results and potentially the validity of produced data. Nowadays, a challenge is to assess and accommodate the impact of theses changes in rapidly evolving XML applications. This article proposes a logical framework and tool for verifying forward/backward compatibility issues involving schemas and queries. First, it allows analyzing relations between schemas. Second, it allows XML designers to identify queries that must be reformulated in order to produce the expected results across successive schema versions. Third, it allows examining more precisely the impact of schema changes over queries, therefore facilitating their reformulation

A performant XQuery to SQL translator

We describe a largely complete and efficient XQuery to SQL translation for XML publishing. Our translation supports the entire XQuery language, except for functions, if statements and upwards navigation axes. The system has three important properties. First, it preserves the correct XQuery semantics. This is accomplished by first translating XQuery into core-XQuery, using a complete XQuery implementation, Galax. Second, we optimize the resulting SQL queries. We develop a comprehensive framework for optimizing the XQuery to SQL translation, which is effective for a wide range of XQuery workloads. Third, our translation is platform independent. Our system achieves high degree of efficiency on a wide range of relational systems. This paper reports an extensive experimental validation on several XQuery workloads, using MySQL, PostgreSQL, and SQL Server, and compares this approach with five native XQuery engines: Galax (the newer, optimized version), Saxon, QizOpen, IMDB and Quexo

Semantics and efficient evaluation of partial tree-pattern queries on XML

Current applications export and exchange XML data on the web. Usually, XML data are queried using keyword queries or using the standard structured query language XQuery the core of which consists of the navigational query language XPath. In this context, one major challenge is the querying of the data when the structure of the data sources is complex or not fully known to the user. Another challenge is the integration of multiple data sources that export data with structural differences and irregularities. In this dissertation, a query language for XML called Partial Tree-Pattern Query (PTPQ) language is considered. PTPQs generalize and strictly contain Tree-Pattern Queries (TPQs) and can express a broad structural fragment of XPath. Because of their expressive power and flexibility, they are useful for querying XML documents the structure of which is complex or not fully known to the user, and for integrating XML data sources with different structures. The dissertation focuses on three issues. The first one is the design of efficient non-main-memory evaluation methods for PTPQs. The second one is the assignment of semantics to PTPQs so that they return meaningful answers. The third one is the development of techniques for answering TPQs using materialized views. Non-main-memory XML query evaluation can be done in two modes (which also define two evaluation models). In the first mode, data is preprocessed and indexes, called inverted lists, are built for it. In the second mode, data are unindexed and arrives continuously in the form of a stream. Existing algorithms cannot be used directly or indirectly to efficiently compute PTPQs in either mode. Initially, the problem of efficiently evaluating partial path queries in the inverted lists model has been addressed. Partial path queries form a subclass of PTPQs which is not contained in the class of TPQs. Three novel algorithms for evaluating partial path queries including a holistic one have been designed. The analytical and experimental results show that the holistic algorithm outperforms the other two. These results have been extended into holistic and non-holistic approaches for PTPQs in the inverted lists model. The experiments show again the superiority of the holistic approach. The dissertation has also addressed the problem of evaluating PTPQs in the streaming model, and two original efficient streaming algorithms for PTPQs have been designed. Compared to the only known streaming algorithm that supports an extension of TPQs, the experimental results show that the proposed algorithms perform better by orders of magnitude while consuming a much smaller fraction of memory space. An original approach for assigning semantics to PTPQs has also been devised. The novel semantics seamlessly applies to keyword queries and to queries with structural restrictions. In contrast to previous approaches that operate locally on data, the proposed approach operates globally on structural summaries of data to extract tree patterns. Compared to previous approaches, an experimental evaluation shows that our approach has a perfect recall both for XML documents with complete and with incomplete data. It also shows better precision compared to approaches with similar recall. Finally, the dissertation has addressed the problem of answering XML queries using exclusively materialized views. An original approach for materializing views in the context of the inverted lists model has been suggested. Necessary and sufficient conditions have been provided for tree-pattern query answerability in terms of view-to-query homomorphisms. A time and space efficient algorithm was designed for deciding query answerability and a technique for computing queries over view materializations using stack- based holistic algorithms was developed. Further, optimizations were developed which (a) minimize the storage space and avoid redundancy by materializing views as bitmaps, and (b) optimize the evaluation of the queries over the views by applying bitwise operations on view materializations. The experimental results show that the proposed approach obtains largely higher hit rates than previous approaches, speeds up significantly the evaluation of queries without using views, and scales very smoothly in terms of storage space and computational overhead

Path constraints in semistructured data

International audienceWe consider semistructured data as multirooted edge-labelled directed graphs, and path inclusion constraints on these graphs. A path inclusion constraint pnot precedes, equalsq is satisfied by a semistructured data if any node reached by the regular query p is also reached by the regular query q. In this paper, two problems are mainly studied: the implication problem and the problem of the existence of a finite exact model. - We give a new decision algorithm for the implication problem of a constraint pnot precedes, equalsq by a set of bounded path constraints pinot precedes, equalsui where p, q, and the pi's are regular path expressions and the ui's are words, improving in this particular case, the more general algorithms of S. Abiteboul and V. Vianu, and N. Alechina et al. In the case of a set of word equalities ui≡vi, we provide a more efficient decision algorithm for the implication of a word equality u≡v, improving the more general algorithm of P. Buneman et al. We prove that, in this case, implication for nondeterministic models is equivalent to implication for (complete) deterministic ones. - We introduce the notion of exact model: an exact model of a set of path constraints Click to view the MathML source satisfies the constraint pnot precedes, equalsq if and only if this constraint is implied by Click to view the MathML source. We prove that any set of constraints has an exact model and we give a decidable characterization of data which are exact models of bounded path inclusion constraints sets