查询优化代价估计中的直方图方法运用

Application of Histogram Method on Cost Estimate in Query Optimization

直方图是数据库系统中最常用的估算查询代价的一种工具,它将关系中的全部元组进行归类设桶,对每个桶分别进行大小统计,继而求和,使代价统计值更接近于真实。直方图方法在所有代价估计法中是最容易使用和最容易维护的,每一种直方图在代价估计精度上各不相同,使用中也各有其特点。结合实际课题应用,给出了将关系的连接转换为多种直方图的方法,以及如何利用直方图进行代价估计。

Histogram is a tool most commonly used in database system to estimate the cost of query.It categorizes and sets barrels to all tuples in the relations,calculates the size respectively for each barrel,and then summarizes the data,which makes the price value closer to the reality.Histogram method is the easiest in application and maintenance of all the methods for cost estimation.Each kind of histogram shows different features and varies in accuracy of cost estimation.In application of the actual project,this research works out a method that converts the connection of the relations to a variety of histograms and direct concrete steps to estimate the operation cost by using histograms.