title: Mongo indexes are not being used when add one more property
body: <p>I'm running two different queries to my database, however, one is running really fast, calling/using all relevant indexes, and the other one, which just adding one more field to search on, is super slow. Please let me know if you have any idea how to solve it.</p>
  <p>This one is running super fast:</p>
  <pre><code>db.orders.find(
  {
          '$or': 
              [ 
                  { order_id: { '$regex': '^.*8664149', '$options': 'i' } }, 
                  { ref_number: { '$regex': '^.*8664149', '$options': 'i' } }, 
                  { 'ls.ref_order_number': { '$regex': '^.*8664149', '$options': 'i' } }, 
                  { tracking_number: { '$regex': '^.*8664149', '$options': 'i' }}, 
                  { 'customer.first_name': { '$regex': '^8664149', '$options': 'i' } }, 
                  { 'customer.last_name': { '$regex': '^8664149', '$options': 'i' }  } 
                   ]
          })
  </code></pre>
  <p>However, this one, where i also search on retailer name, is running very slow and its also not calling the index that i created on the retailer field:</p>
  <pre><code>db.orders.find(
  {
          retailer: ObjectId(&quot;57a8d0b9eb656789de793&quot;),
          '$or': 
              [ 
                  { order_id: { '$regex': '^.*8664149', '$options': 'i' } }, 
                  { ref_number: { '$regex': '^.*8664149', '$options': 'i' } }, 
                  { 'ls.ref_order_number': { '$regex': '^.*8664149', '$options': 'i' } }, 
                  { tracking_number: { '$regex': '^.*8664149', '$options': 'i' }}, 
                  { 'customer.first_name': { '$regex': '^8664149', '$options': 'i' } }, 
                  { 'customer.last_name': { '$regex': '^8664149', '$options': 'i' }  } 
                   ]
          })
  </code></pre>
  <p>Thanks for the help</p>