title: Indexing with mongodb: bad performance / indexOnly=false
body: <p>I have a mongodb on a 8GB linux machine running. Currently it's in test-mode so there are very few other requests coming in if any at all.</p>
  
  <p>I have a colelction items with 1 million documents in it. I am creating an index on the fields: PeerGroup and CategoryIds (which is an array of 3-6 elements which will yield in an multi key): <code>db.items.ensureIndex({PeerGroup:1, CategoryIds:1}</code>.</p>
  
  <p>When I am querying </p>
  
  <pre><code>db.items.find({"CategoryIds" : new BinData(3,"xqScEqwPiEOjQg7tzs6PHA=="), "PeerGroup" : "anonymous"}).explain()
  </code></pre>
  
  <p>I have the following results:</p>
  
  <pre><code>{
      "cursor" : "BtreeCursor PeerGroup_1_CategoryIds_1",
      "isMultiKey" : true,
      "n" : 203944,
      "nscannedObjects" : 203944,
      "nscanned" : 203944,
      "nscannedObjectsAllPlans" : 203944,
      "nscannedAllPlans" : 203944,
      "scanAndOrder" : false,
      "indexOnly" : false,
      "nYields" : 1,
      "nChunkSkips" : 0,
      "millis" : 680,
      "indexBounds" : {
              "PeerGroup" : [
                      [
                              "anonymous",
                              "anonymous"
                      ]
              ],
              "CategoryIds" : [
                      [
                              BinData(3,"BXzpwVQozECLaPkJy26t6Q=="),
                              BinData(3,"BXzpwVQozECLaPkJy26t6Q==")
                      ]
              ]
      },
      "server" : "db02:27017"
  </code></pre>
  
  <p>}</p>
  
  <p>I think 680ms is not that very fast. Or is this acceptable? 
  Also, why does it say "indexOnly:false" ?</p>