pureXML is the native XML storage capability in IBM DB2. It allows for whole XML documents to be stored in a DB2 table column, and for them to be easily queried via either SQL or XQuery or some combination of the two. Getting Started with DB2 has a decent introduction to both . There’s also schema validation, versioning, and other esoteric features.
I’ve played with it a fair bit and found it quite performant in my admittedly limited scenario of a couple thousand xml docs.