title: MongoDB index not helping query with multikey index
body: <p>I have a collection of documents with a multikey index defined.  However, the performance of the query is pretty poor for just 43K documents.  Is ~215ms for this query considered poor? Did I define the index correctly if nscanned is 43902 (which equals the total documents in the collection)?</p>
  
  <p>Document:</p>
  
  <pre><code>{
      "_id": {
          "$oid": "50f7c95b31e4920008dc75dc"
      },
      "bank_accounts": [
          {
              "bank_id": {
                  "$oid": "50f7c95a31e4920009b5fc5d"
              },
              "account_id": [
                  "ff39089358c1e7bcb880d093e70eafdd",
                  "adaec507c755d6e6cf2984a5a897f1e2"
              ]
          }
      ],
      "created_date": "2013,01,17,09,50,19,274089",
  }
  </code></pre>
  
  <p>Index:</p>
  
  <pre><code>{ "bank_accounts.bank_id" : 1 , "bank_accounts.account_id" : 1}
  </code></pre>
  
  <p>Query:</p>
  
  <pre><code>db.visitor.find({ "bank_accounts.account_id" : "ff39089358c1e7bcb880d093e70eafdd" , "bank_accounts.bank_id" : ObjectId("50f7c95a31e4920009b5fc5d")}).explain()
  </code></pre>
  
  <p>Explain:</p>
  
  <pre><code>{
      "cursor" : "BtreeCursor bank_accounts.bank_id_1_bank_accounts.account_id_1",
      "isMultiKey" : true,
      "n" : 1,
      "nscannedObjects" : 43902,
      "nscanned" : 43902,
      "nscannedObjectsAllPlans" : 43902,
      "nscannedAllPlans" : 43902,
      "scanAndOrder" : false,
      "indexOnly" : false,
      "nYields" : 0,
      "nChunkSkips" : 0,
      "millis" : 213,
      "indexBounds" : {
          "bank_accounts.bank_id" : [
              [
                  ObjectId("50f7c95a31e4920009b5fc5d"),
                  ObjectId("50f7c95a31e4920009b5fc5d")
              ]
          ],
          "bank_accounts.account_id" : [
              [
                  {
                      "$minElement" : 1
                  },
                  {
                      "$maxElement" : 1
                  }
              ]
          ]
      },
      "server" : "Not_Important"
  }
  </code></pre>