Abstract: A new cost model of distributed database query based on the study of availability semi-joins is put forward in this paper，which can describe the cost of distributed database query precisely by including selection of relation copies，query order of multi-join，determination of performance sites，transaction of data and cost of local computation.Then the cost model is analyzed，designed in detail and realized.

Key words: net cost, query time, local cost, cost function

摘要： 在收益半连接的研究基础上，对查询执行代价模型进行研究，提出一种包括数据副本选择、多连接查询次序、操作站点选择、数据传输及局部数据处理等因素的代价模型，能准确地描述当前分布式数据库查询执行计划的代价，更具有实际意义。

关键词: 网络传输代价, 查询时间, 局部处理代价, 代价函数