基于关系数据库的关键词查询,使得用户在不需要掌握结构化查询语言和数据库模式的情况下,可以方便地进行关系数据库查询.给定一个关键词查询,已有的方法通过数据库中的主外键关联,查询得到包含关键词的元组集合.但是,在很多实际应用中,...基于关系数据库的关键词查询,使得用户在不需要掌握结构化查询语言和数据库模式的情况下,可以方便地进行关系数据库查询.给定一个关键词查询,已有的方法通过数据库中的主外键关联,查询得到包含关键词的元组集合.但是,在很多实际应用中,元组集合的聚合结果对用户更有价值;研究了基于关系数据库的top-k聚合关键词查询,提出了基于递归的聚合单元枚举算法——基于递归的完全搜索(recursion-based full search,RFS).为了获得更好的查询性能,设计了新的排序方法、二维索引和快速搜索算法——基于输出的快速搜索(output-based quick search,OQS),从而可以高效地枚举top-k个聚合单元;在不同的数据集上进行了大量的实验,实验结果表明OQS算法具有良好的查询性能.展开更多
With the concept of super-atom, first principles calculations propose a new type of super stable cage clusters AlnH3n that are much more energetic stable than the well established clusters, AlnHn＋2. In the new cluste...With the concept of super-atom, first principles calculations propose a new type of super stable cage clusters AlnH3n that are much more energetic stable than the well established clusters, AlnHn＋2. In the new clusters, the aluminum core-frame acts as a super-atom with n vertexes and 2n A1-A1 edges, which allow to adsorb n hydrogen atoms at the top-site and 2n at the bridge-site. Using Al12H36 as the basic unit, stable chain structures, （Al12H36）m, have been constructed following the same connection mechanism as for （A1H3）n linear polymeric structures. Apart from high hydrogen percentage per molecule, calculations have shown that these new clusters possess large heat of formation values and their combustion heat is about 4.8 times of the methane, making them a promising high energy density material.展开更多
文摘基于关系数据库的关键词查询,使得用户在不需要掌握结构化查询语言和数据库模式的情况下,可以方便地进行关系数据库查询.给定一个关键词查询,已有的方法通过数据库中的主外键关联,查询得到包含关键词的元组集合.但是,在很多实际应用中,元组集合的聚合结果对用户更有价值;研究了基于关系数据库的top-k聚合关键词查询,提出了基于递归的聚合单元枚举算法——基于递归的完全搜索(recursion-based full search,RFS).为了获得更好的查询性能,设计了新的排序方法、二维索引和快速搜索算法——基于输出的快速搜索(output-based quick search,OQS),从而可以高效地枚举top-k个聚合单元;在不同的数据集上进行了大量的实验,实验结果表明OQS算法具有良好的查询性能.
基金This work was supported by the Swedish Research Council, Swedish National Infrastructure for Computing, the National Natural Science Foundation of China （No.10534010 and No.20925311）, the Funda-mental Research Funds for the Central Universities （No.201103255）, and the China Scholarship Council.
文摘With the concept of super-atom, first principles calculations propose a new type of super stable cage clusters AlnH3n that are much more energetic stable than the well established clusters, AlnHn＋2. In the new clusters, the aluminum core-frame acts as a super-atom with n vertexes and 2n A1-A1 edges, which allow to adsorb n hydrogen atoms at the top-site and 2n at the bridge-site. Using Al12H36 as the basic unit, stable chain structures, （Al12H36）m, have been constructed following the same connection mechanism as for （A1H3）n linear polymeric structures. Apart from high hydrogen percentage per molecule, calculations have shown that these new clusters possess large heat of formation values and their combustion heat is about 4.8 times of the methane, making them a promising high energy density material.
