title: Performance suffers using distinct
body: <p>Following code gives me some trouble:</p>
  
  <pre><code>  /**
     *
     * finds all distinct values of HybridTypes for the Page Collection
     */
    def findDistinctTypeValues(): List[String] = {
      //cast all instances from Any to String
      val coll = dao.collection.distinct("hybridType") map (_.asInstanceOf[String])
      coll.toList
    }
  </code></pre>
  
  <p>The mongo console tells me that the corresponding query lasts about 4 seconds.
  The field is an indexed string.</p>
  
  <p>Is there anything im doing particularly wrong?</p>
  
  <p>Update: This is the output of the runcommand:</p>
  
  <pre><code>db.runCommand({distinct: 'pages',key:'hybridType'})
       "stats" : {
               "n" : 2492192,
               "nscanned" : 2492192,
               "nscannedObjects" : 0,
               "timems" : 4247,
               "cursor" : "BtreeCursor hybridType_1"
       },
       "ok" : 1
  </code></pre>
  
  <p>Looks like its performing  a full table scan? :/</p>