[XML-SIG] XML database options

[Josh Marcus]

Can I ask a slightly off-topic question?

Recently, I've been implementing an XML server
that is capable of storing XML documents in a
relational database in such a way that the
documents can be quickly queried.
I found an interesting paper that compares
the performance of alternative mapping
schemes ("A Performance Evalutation of
Alternative Mapping Schemes for Storing
XML Data in a Relational Database") and
decided to follow the scheme their experimentation
found most efficient -- with a few changes
to trade storage space for speed. 

I was just wondering:
  
   o  Is there an open source application that
      I can use for this?  
   o  If not, is there conventional wisdom
      regarding how one might go about storing
      and querying XML data (short of buying a
      commercial oo-db)?

Thanks,
--j