title: Mongo query is running endlessly
body: <p>Something went wrong and I don't understand why. I have a script that used to be working but all of the sudden if just stopped. It's basically a mongo query on a big Mongo collection (600gb+).</p>
  
  <p>Here's the query:</p>
  
  <pre><code>db.action_traces.findOne( {"block_time": {"$lt": "2018-07-15T00:00:00.000Z"} } ).pretty()
  </code></pre>
  
  <p>Originally I wasn't using findOne but I've restricted the results to one just in case I could help but the result is the same: nothing happens.</p>
  
  <p>If I just run a <code>find</code> query it goes fine.</p>
  
  <p>There is nothing showing up in the mongodb log and nothing seems relevant in the syslog either.</p>
  
  <p>There is clearly something wrong with Mongo though, as htop is showing me this:</p>
  
  <p><a href="https://i.stack.imgur.com/HVrE1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HVrE1.png" alt="Htop mongodb"></a></p>
  
  <p>The mongo process is fluctuating but is most of the time filling out one CPU thread to 100%.</p>
  
  <p>Can anyone help?</p>
  
  <p>Many thanks in advance,</p>