title: How to query data efficiently in large mongodb collection?
body: <p>I have one big mongodb collection (3-million docs, 50 GigaBytes), and it would be very slow to query the data even I have created the indexs. </p>
  
  <pre><code>db.collection.find({"C123":1, "C122":2})
  </code></pre>
  
  <p>e.g. the query will be timeout or will be extreme slow (10s at least), even if I have created the separate indexes for <code>C123</code> and <code>C122</code>. </p>
  
  <p>Should I create more indexs or increase the physical memory to accelerate the querying? </p>