Relational query optimizers are not always robust. They depend on statistics and cost models which are often inaccurate, and sometimes absent. They fail to detect correlations, and cannot efficiently handle the large search space of big queries. Those challenges and their impact on the quality of the chosen plan are aggravated in the context of XML. In fact, in XML, it is harder to collect and maintain representative statistics. Moreover, the search space of plans is usually larger than that of relational queries, due to the higher number of joins in a typical XQuery. ROX, our Run-time...
Relational query optimizers are not always robust. They depend on statistics and cost models which are often inaccurate, and sometimes absent. They fa...
