partial/diff updates?

[Liam Staskawicz]

When updating a document, is there any notion of submitting a partial  
update?  It seems like being able to specify that only some subset of  
the fields in a document should be updated would offer some efficiency  
benefits.  I guess I had in mind some scenario where CouchDB would  
create the updated record by merging the existing revision with the  
new info and saving the updated revision, but I'm still new to CouchDB  
so I don't have a good sense of whether this tramples on any important  
concepts.

Thanks for any thoughts.
Liam