Including Group-By in Query Optimization
Including Group-By in Query Optimization
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发表时间：
1994-09
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通讯作者：
S. Chaudhuri;Kyuseok Shim
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作者：
S. Chaudhuri;Kyuseok Shim
在现有的关系数据库系统中，始终延迟对组的处理和聚合功能的计算，直到执行所有连接。在本文中，我们提出了转换，使得可以将小组操作推到一个或多个连接的情况下，并有可能大大降低处理查询的成本。因此，应根据成本估算来确定小组的位置。我们解释了如何通过结合我们开发的贪婪的保守性启发式方法来修改传统的System-R样式优化器。我们证明，贪婪的保守启发式产品计划的应用比传统优化器生成的计划更好（或不糟糕）。我们的实验研究表明，计划质量的改善程度显着，而优化成本的增加仅增加。我们的技术还适用于以基于成本的方式降低重复消除的优化不同的查询。
In existing relational database systems, processing of group-by and computation of aggregate functions are always postponed until all joins are performed. In this paper, we present transformations that make it possible to push group-by operation past one or more joins and can potentially reduce the cost of processing a query significantly. Therefore, the placement of group-by should be decided based on cost estimation. We explain how the traditional System-R style optimizers can be modified by incorporating the greedy conservative heuristic that we developed. We prove that applications of greedy conservative heuristic produce plans that are better (or no worse) than the plans generated by a traditional optimizer. Our experimental study shows that the extent of improvement in the quality of plans is significant with only a modest increase in optimization cost. Our technique also applies to optimization of Select Distinct queries by pushing down duplicate elimination in a cost-based fashion.