An XML wrapper queries an XML document in an on-the-fly manner so that only parent nodes in the document that satisfy the query are extracted and then unnested. The parent nodes and associated descendent nodes are located using XPath expressions contained as options in data definition language (DDL) statements. The parent nodes satisfying the query and associated descendent nodes are extracted and stored outside of a database according to a relational schema. The wrapper facilitates applications that use convention SQL queries and views to operate on that information stored according to the relational schema. The wrapper also responds to query optimizer requests for costs associated with queries against external data sources associated with the wrapper.