We determine when a join/outerjoin query can be expressed unambiguously as a query graph, without an explicit specification of the order of evaluation. To do so, we first characterize the set of expression trees that implement a given join/outerjoin query graph, and investigate the existence of transformations among the various trees. Our main theorem is that a join/outerjoin query is freely reorderable if the query graph derived from it falls within a particular class, every tree that "implements" such a graph evaluates to the same result.
The result has applications to language design and query optimization. Languages that generate queries within such a class do not require the user to indicate priority among join operations, and hence may present a simplified syntax. And it is unnecessary to add extensive analyses to a conventional query optimizer in order to generate legal reorderings for a freely-reorderable language.
我们确定何时可以明确将联接/外部联接查询表示为查询图,而无需明确指定评估顺序。为此,我们首先表征实现给定联接/外联接查询图的表达式树集,并研究各种树之间存在转换的情况。我们的主要定理是,如果联接/外部联接查询的派生图属于特定类,则“联接” /“外部联接”查询可以自由地重新排序,“实现”这种图的每棵树都将得出相同的结果。 P>
结果可应用于语言设计和查询优化。在此类中生成查询的语言不需要用户指示联接操作之间的优先级,因此可以提供简化的语法。而且,不必为常规的查询优化器添加大量分析以生成可自由重排序的语言的合法重排序。 P>
机译:优化树模式以查询图形和树结构数据
机译:技术观点:优化树模式以查询图形和树结构数据
机译:使用MapReduce查找图上关键字查询的最小k紧凑树集
机译:使用K2-Trees对压缩图进行分布式查询处理
机译:支持HD树中多维范围查询的路由算法的设计与实现。
机译:PEPR GeneChip数据仓库以及带有图形界面的动态时间序列查询工具(SGQT)的实现
机译:使用维度图评估树结构数据的查询*