title: mongodb not using indexes
body: <p>I have a collection with these indexes:</p>
  
  <pre><code>db.colaboradores.getIndexKeys()
  [ { "_id" : 1 }, { "nome" : 1 }, { "sobrenome" : 1 } ]
  </code></pre>
  
  <p>and a query like</p>
  
  <pre><code>db.colaboradores.find({_id: ObjectId("5040e298914224dca3000006")}).explain();
  </code></pre>
  
  <p>thatworks fine with index</p>
  
  <pre><code>{
      "cursor" : "BtreeCursor _id_",
      "nscanned" : 0,
      "nscannedObjects" : 0,
      "n" : 0,
      "millis" : 0,
  }
  </code></pre>
  
  <p>but when run:</p>
  
  <pre><code>db.colaboradores.find({nome: /^Administrador/}).explain()
  </code></pre>
  
  <p>mongodb do not use indexes any more:</p>
  
  <pre><code>{
      "cursor" : "BtreeCursor nome_1",
      "nscanned" : 10000,
      "nscannedObjects" : 10000,
      "n" : 10000,
      "millis" : 25,
  }
  </code></pre>
  
  <p>any solutions?
  Thanks!</p>