title: What's the best Mongo index strategy that includes a date range
body: <p>I have the following schema: </p>
  
  <pre><code>{
    a: string;
    b: date;
    c: number;
  }
  </code></pre>
  
  <p>My query is </p>
  
  <pre><code>find({
    a: 'some value',
    b: {
      $gte: new Date('some date')
    }
  })
  .sort({ 
   c: -1
  });
  </code></pre>
  
  <p>I have an index that is: </p>
  
  <pre><code>{ a: 1, b: 1, c: 1 }
  </code></pre>
  
  <p>But it's not using this index.  </p>
  
  <p>I have several other indexes, and when analyzing my explain(), it shows it's employing multiple other indexes to accomplish my query. </p>
  
  <p>I believe since my "b" query is a date range, that's not considered an equality condition, so maybe that index won't work?</p>
  
  <p>Should I have two indexes:
  { a: 1, c: 1} and separately { b: 1 }</p>