title: Picking up a good shard key for time based query in MongoDB
body: <p>I have a collection with the following document:</p>
  
  <pre class="lang-js prettyprint-override"><code>{
      "_id" : ObjectId("535e194aba863da3118cdf8a"),
      "device_id" : "1080000008",
      "others" : [ 
          {
              "mileage" : "0.0",
              "event" : "5",
              "satellite" : "8",
              "altitude" : "0",
              "heading" : "290"
          }
      ],
      "speed" : 68,
      "lat" : 1.3209,
      "lng" : 103.89139,
      "dateTime" : ISODate("2014-04-28T17:03:05.000Z"),
      "output_status" : 0,
      "street_name" : "JALAN AFIFI",
      "device_type" : "VT10",
      "__v" : 0
  }
  </code></pre>
  
  <p>I have 2 shards, A and B, and i want shard A to contain the latest documents based on DateTime and shard B to contain documents that is later than 48 hours.</p>
  
  <p>Is it possible in MongoDB? Or are there better sharding combinations that i can pick, like lat/lng? or should the shard key follow the index key?</p>