To achieve maximum XML performance, bolstered indexing attributes in the technology will enable advanced search functions and a higher degree of filtering. IBM is also adding support for XPath and XQuery data models. This will allow users to create views that involve SQL and XQuery by sending the protocol through DB2's query optimizer for a unified query plan.
Craig Mullins points are more straightforward to answer since his comments don't jibe with the current state of the art in the XML world. He states that you can't query XML documents stored in databases but this is untrue. Almost three years ago, I was writing articles about querying XML documents stored in relational databases. Storing XML in a relational database doesn't mean it has to be stored in as an opaque binary BLOB or as a big, bunch of text which cannot effectively be queried. The next version of SQL Server will have extensive capabilities for querying XML data in relational database and doing joins across relational and XML data, a lot of this functionality is described in the article on XML Support in SQL Server 2005. As for XML not having a data model, I beg to differ. There is a data model for XML that many applications and people adhere to, often without realizing that they are doing so. This data model is the XPath 1.0 data model, which is being updated to handled typed data as the XQuery and XPath 2.0 data model.
OpenLink's list of supported standards is impressive and includes .Net, Mono, J2EE, XML Web Services (Simple Object Application Protocol, Web Services Description Language, WS-Security, Universal Description, Discovery and Integration), XML, XPath, XQuery, XSL-T, WebDav, HTTP, SMTP, LDAP, POP3, SQL-92, ODBC, JDBC and OLE-DB.