how build "tagging" support controlling couchdb?

i'm controlling following perspective duty iterate over equipment database (in method tag), nonetheless i cruise opening unequivocally bad dataset large.
any approach?

def by_tag(tag):
return  '''
        function(doc) {
            (doc.tags.length > 0) {
                (var add-on doc.tags) {
                    (doc.tags[tag] == "%s") {
                        emit(doc.published, doc)
                    }
                }
            }
        };
        ''' % tag