title: Which index would be used if there are multiple indexes containing the same fields?
body: <p>Take, for example, a <code>find()</code> that involves a field <code>a</code> and <code>b</code>, in that order. For example,</p>
  
  <pre><code>db.collection.find({'a':{'$lt':10},'b':{'$lt':5}})
  </code></pre>
  
  <p>I have two keys in my array of indexes for the collection:</p>
  
  <pre><code>[
  {
      "v" : 1,
      "key" : {
          "a" : 1,
          "b" : 1
      },
      "ns" : "x.test",
      "name" : "a_1_b_1"
  },
  {
      "v" : 1,
      "key" : {
          "a" : 1,
          "b" : 1,
          "c" : 1
      },
      "ns" : "x.test",
      "name" : "a_1_b_1_c_1"
  }
  ]
  </code></pre>
  
  <p>Is it <strong>guaranteed</strong> that mongo will use the first key since it more accurately matches the query, or does it randomly choose any of the two keys because they will both work?</p>