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Traditional optimizers fail to pick good execution plans, when faced with increasingly complex queries and large data sets. This failure is even more acute in the context of XQuery, due to the structured nature of the XML language. To overcome the vulnerabilities of traditional optimizers, we have previously proposed ROX, a Run-time Optimizer for XQueries, which interleaves optimization and execution of full tables. ROX has proved to be robust, even in the presence of strong correlations, but it has one limitation: it uses full materialization of intermediate results making it unsuitable for pipelined systems. Therefore, this paper proposes ROX-sampled, a variant of ROX, which executes small data samples, thus generating smaller intermediates. We conduct extensive experiments which proved that ROX-sampled is comparable to ROX in performance, and that it is still robust against correlations. The main benefit of ROX-sampled is that it allows the large number of pipelined databases to import the ROX idea into their optimization paradigm.

We demonstrate ROX, a run-time optimizer of XQueries, that focuses on finding the best execution order of XPath steps and relational joins in an XQuery. The problem of join ordering has been extensively researched, but the proposed techniques are still unsatisfying. These either rely on a cost model which might result in inaccurate estimations, or explore only a restrictive number of plans from the search space. ROX is developed to tackle these problems. ROX does not need any cost model, and defers query optimization to run-time intertwining optimization and execution steps. In every optimization step, sampling techniques are used to estimate the cardinality of unexecuted steps and joins to make a decision which sequence of operators to process next. Consequently, each execution step will provide updated and accurate knowledge about intermediate results, which will be used during the next optimization round. This demonstration will focus on: (i) illustrating the steps that ROX follows and the decisions it makes to choose a good join order, (ii) showing ROX’s robustness in the face of data with different degree of correlation, (iii) comparing the performance of the plan chosen by ROX to different plans picked from the search space, (iv) proving that the run-time overhead needed by ROX is restricted to a small fraction of the execution time.

Fifty-six patients were diagnosed to have mitral valve prolapse (MVP) syndrome by auscultatory criteria, confirmed by cross-sectional echocardiography. Complete physical examination and x-ray of the bony thorax revealed bony deformities which were subdivided into four main groups according to the predominant deformity. Shallow chest with pectus excavatum, straight back, kyphoscoliosis and elliptical chest in marfanoid patients were reported in the cases studied. It is concluded that musculoskeletal abnormalities have to be considered as nonauscultatory features of MVP. Therefore, any patient with musculoskeletal deformity has to be screened for MVP by cross-sectional echocardiography to prevent life-threatening complications.

Relationships have been developed for the calculation of the viscosity of a polar gas at atmospheric pressure from macroscopic parameters available for the substance. The molecular force constants for the Stockmayer potential determined by Monchick and Mason from viscosity data were related to the critical constants of the substance and the parameters ω and × defined through the vapor pressure. For 10 polar gases, an average deviation of approximately 3.0% was obtained between experimental viscosities and values calculated with the relationships of this study. The relationships were also utilized to calculate diffusion coefficients for a number of polar binary mixtures. On a mis au point des equations pour calculer la viscosite d'un gaz polaire, a la pression atmospherique, a partir des parametres macroscopiques dont on disposait a son sujet. On a etabli une relation entre les constantes des forces moleculaires pour le potentiel de Stockmayer (que Monchick et Mason avaient determinees) et les constantes critiques du gaz ainsi que les parametres ω et × definis par la pression de vapeur. On a obtenu, dans le cas de 10 gaz polaires, un ecart moyen d'environ 3.0% entre les viscosites determinees experimentalement et les valeurs calculees avec les equations precitees. On a aussi utilise les relations en question pour calculer les coefficients de diffusion dans le cas d'un certain nombre de melanges binaires et polaires.