The actual value represented by 'zzz' above must be based on a separate (physical) database lookup for each attribute value $attr0 in the collection ( i.e. the values of attr1 is a (non-trivial lookup) function of attr0.

Is this a scenario in which multiple queries would have to be made based on the number of documents in the collection ?

Perhaps there is some slick way of including a call out to some method to such that the 'zzz' above is replaced with something like: JavaModule.doLookup($attr0) ? 

I'm executing xquery from a jruby application so either the ruby or the java client can be used (perhaps that is somewhat irrelevant). 

Thanks in advance for any suggestions,

Buzz