PostgreSQL does not have a problem with this as long as the attributes are uniquely defined. We have xowiki with this query running on many versions of PostgreSQL without any problems; on my local machine, i have currently pg 9.0.1. Can it be that that you are using ]po[ and that ]po[ uses a modified datamodel? Just guessing.
Anyhow, if it helps you, i'll modify the query, being more specific never hurts.