Efficient way to evaluate uncorrelated path-based row sources with xml storage

Abstract

The approaches described herein provide an efficient way to process certain kinds of queries that retrieve XML data stored in an object-relational database. A technique determines whether a query comprises a plurality of iteration expressions that exhibit a particular relationship to one another, and if so, re-writes the query and produces an optimized query execution plan. The query execution plan specifies an XPath-based row source with caching enabled. A very small cache structure is used, and reference locators are stored in the cache that point into binary-encoded data rather than copying XMLType data into the cache. Row data may be cached on the first use of the path-based row source and re-used in subsequent requests for rows. Once there is a subsequent request for rows from the optimized path-based row source, the cache may be reorganized to efficiently produce the rows of the row source.