并行查询下查询执行计划的选择

查询是数据库系统的主要负载，其效率决定了数据库性能的好坏。一个查询存在多种执行计划，当前，查询优化器只能按照数据库系统的配置参数，静态地为查询选择一个较优的执行计划。并行查询间存在复杂多变的资源争用，很难通过配置参数准确反映，而且同一执行计划在不同情景下的效率并不一致。并行查询下执行计划的选择需考虑查询间的相互影响——查询交互。基于此，提出了一种在并行查询下度量查询受查询交互影响大小的标准QIs。针对并行查询下查询执行计划的选择，还提出了一种动态地为查询选择执行计划的方法TRating，该方法通过比较查询组合中按不同执行计划执行的查询受查询交互影响的大小，选择受查询交互影响较小的执行计划作为该查询的较优执行计划。实验结果表明，TRating方法为查询选择较优执行计划的准确率达61%，相比查询优化器提高了25%；而且在为查询选择次优执行计划时，其准确率也高达69%。

Query execution plan selection under concurrent query

Query is the main workload of a database system， and its efficiency determines the performance of the database system. There are multiple execution plans for a query， and the existing query optimizers can only statically select a better execution plan for a query according to the configuration parameters of the database system. There are complex and variable resource contentions between concurrent queries， and such contentions are difficult to be reflected accurately through configuration parameters； besides， the efficiency of the same execution plan is not consistent in different scenarios. The selection of the execution plans for concurrent queries needs to consider the influence between queries — query interaction. Based on the above， a metric for measuring the influence of query interaction on the query under concurrent query called QIs （Query Interactions） was proposed. For the selection of query execution plan under concurrent query， a method called TRating （Time Rating） was proposed to dynamically select the execution plan for the query. In the method， the influence of query interaction on the queries executed with different plans in the query combination was measured， and the plan with small influence of query interaction was selected as the better execution plan for the query. Experimental results show that TRating can select a better execution plan for the query with an accuracy of 61%， which is 25% higher than that of the query optimizer； and the accuracy of the proposed method is as high as 69% when selecting suboptimal execution plan for the query.