title: Performance of count for mongodb
body: <p>I wonder the count operation in mongodb for large collection.</p>
  
  <p>When I do <code>db.collection.count()</code> without any criteria in a collection with ten million rows, it just take no more than 100ms.</p>
  
  <p>Why so fast, is it just read from the stat value of <code>db.collection.stats().count</code>?</p>