基于代数的Transform查询优化策略

XQuery/Update中定义了一种特殊的查询--Transform查询.Transform查询类似于关系数据库中的假设查询,可以表示成假设查询的一般形式:"Q when{U}",即查询Q的查询结果是假设数据库执行了更新操作{U}以后的结果,而更新操作U实际不修改数据库的状态.Transform查询需要拷贝XML数据库中的结点,并对拷贝的结点执行更新操作,所以不影响数据库的状态.但该操作通常拷贝和更新了大量与查询结果无关的结点,因此如何减少拷贝与更新操作的代价是Transform查询处理优化的关键.提出了基于OrientXA的Transform查询优化方法,并在NativeXML数据库系统OrientX 3.0里实现和验证了该方法.

Algebra-Based Transform Query Optimization Strategy

XQuery/Update defines a special Transform query,which is similar to the hypothetical query in relation databases and can be expressed as:"Q when{U}".In other words,the results of query Q are same as the results after executing hypothetical update {U} on the original database,without actually updating database.The transform queries need to copy the nodes in XML database and then update copied nodes,so it doesn't affect the database.But transform queries will usually copy and update a lot of nodes which are useless for query Q and result in high cost.It is critical for query optimization to decrease the number of copied nodes and the update operation.In this paper,a set of rules for transform query optimization techniques are proposed based on OrientXA.They are implemented in OrientX3.0.