Maybe this can help: http://extensions.xwiki.org/xwiki/bin/view/Extension/Tag%20Application
If you’re referring to SOLR search, I see some info at http://extensions.xwiki.org/xwiki/bin/view/Extension/Solr%20Search%20Application#HFacetingonObjectProperties
I don’t know if it’s a feature OOB or if you need to enable it.