title: Cosmos DB Date Index Not Efficient
body: <p>I have a collection with a Date field that is populated by a C# application using a DateTime object. This field is serialized to the following format "2018-06-10T17:32:48.3285735Z".</p>
  
  <p>I haven't touched the Index Policy in the collection, so strings are using the Range index type. From what I've read in the documentation, that's the most efficient way to index dates, however, when I use the Date field in an ORDER BY clause, the query consumes at least 10x more RUs than if I were to query using the timestamp (_ts) number field. That means paying 10x more for this single collection.  </p>
  
  <p>To illustrate the issue:</p>
  
  <pre><code>SELECT TOP 100 * FROM c ORDER BY c.Date DESC
  //query consumes a minimum of 500 RUs
  
  SELECT TOP 100 * FROM c ORDER BY c._ts DESC
  //query consumes 50 RUs
  </code></pre>
  
  <p>Is this how it is supposed to work or am I missing something? I suspect that if this was the expected behavior, it would be emphasized in the index documentation, and storing dates as numbers would be highlighted as the best practice.</p>
  
  <p>EDIT:
  This is the index policy for the collection (I never changed it).</p>
  
  <pre><code>{
      "indexingMode": "consistent",
      "automatic": true,
      "includedPaths": [
          {
              "path": "/*",
              "indexes": [
                  {
                      "kind": "Range",
                      "dataType": "Number",
                      "precision": -1
                  },
                  {
                      "kind": "Range",
                      "dataType": "String",
                      "precision": -1
                  },
                  {
                      "kind": "Spatial",
                      "dataType": "Point"
                  }
              ]
          }
      ],
      "excludedPaths": []
  }
  </code></pre>