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Top-k ranking for an uncertain database is to rank tuples in it so that the best k of them can be determined. The problem has been formalized under the unified approach based on parameterized ranking functions (PRFs) and the possible world semantics. Given a PRF, one can always compute the ranking function values of all the tuples to determine the top-k tuples, which is a formidable task for large databases. In this paper, we present a general approach to pruning for the framework based on PRFs. We show a mathematical manipulation of possible worlds which reveals key insights in the part of computation that may be pruned and how to achieve it in a systematic fashion. This leads to concrete pruning methods for a wide range of ranking functions. We show experimentally the effectiveness of our approach.

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Boron nitride (BN) fibers were fabricated on a large scale through the melt‐drawn technique from low‐cost boric acid, NH3, and N2. Evolution of structure and properties of BN fibers during the fabrication process was studied by Fourier transform infrared (FT‐IR), X‐ray diffraction (XRD), scanning electron microscope (SEM), and X‐ray photoelectron spectroscopy (XPS). The mechanical properties of BN fibers were tested and analyzed. The results shown that both the mechanical properties and the crystallinity of BN fibers slightly increased with the temperature from 450 to 850°C, due to the combination of the fused‐B3N3. For BN fibers heat‐treated at 850 or 1000°C, the tensile strength (σR) and elastic modulus (E) were strongly increased because of the increase in crystallization of the BN phase. The meso‐hexagonal BN fibers with a diameter of 5.0 μm were fabricated at 1750°C, of which the tensile strength (σR) and elastic modulus (E) are 1200 MPa and 85 GPa, respectively. BN fibers with excellent mechanical properties and proper diameters were obtained by nitriding of green fibers during their conversion into ceramic.