Incorporating constraints in probabilistic XML

Constraints are important not just for maintaining data integrity, but also because they capture natural probabilistic dependencies among data items. A probabilistic XML database (PXDB) is the probability sub-space comprising the instances of a p-document that satisfy a set of constraints. In contrast to existing models that can express probabilistic dependencies, it is shown that query evaluation is tractable in PXDBs. The problems of sampling and determining well-definedness (i.e., whether the above sub-space is nonempty) are also tractable. Furthermore, queries and constraints can include the aggregate functions count, max, rain and ratio. Finally, this approach can be easily extended to allow a probabilistic interpretation of constraints.