1. A computer implemented method, the method implemented on a processor and a memory programmed for decomposing an XML document for storage in a relational database, the method comprising: receiving a mapping document, the mapping document describing components of the XML document that are to be decomposed, the mapping document being in the form of a set of related XML schema documents augmented with at least one annotation that describes a mapping of the components to specified relational tables and columns in the relational database; transforming the mapping document into a first data structure and at least one second data structure for decomposing the XML document, the transforming comprising: i) parsing the set of related XML schema documents to produce the first data structure, the first data structure representing a data model associated with the set of related XML schema documents, wherein the first data structure comprises: a) at least one node that represents schema components comprising model groups, particles, and element and attribute declarations, and b) at least one edge that connect the at least one node according to relationships defined in the set of related XML schema documents; and ii) parsing the at least one annotation to obtain mapping information that maps the components to the specified relational tables and columns in the database; and iii) creating the at least one second data structure, each of the at least one second data structures being created for a corresponding mapped relational table in the relational database based on the mapping information; and making the first and second data structures persistent for use with a subsequent decomposition operation that decomposes the XML document, wherein making the data structure persistent includes storing the first and second data structures in the database, wherein storing the first and second data structures comprises assigning and storing a unique identifier to the first and second data structures, the unique identifier used in later retrieving the first and second data structures from the database for a subsequent decomposition of a second XML document in response to receiving the unique identifier, such that the retrieved first and second data structures can be used and need not be transformed from a second mapping document for the second XML document.