title: Improve querying fields exist in MongoDB
body: <p>I'm in progress with estimation of MongoDB for our customers. Per requirements we need associate with some entity <code>ent</code> variable set of name-value pairs.</p>
  
  <pre><code>db.ent.insert({'a':5775, 'b':'b1'})
  db.ent.insert({'c':'its a c', 'b':'b2'})
  db.ent.insert({'a':7557, 'c':'its a c'})
  </code></pre>
  
  <p>After this I need intensively query <code>ent</code> for presence of fields:</p>
  
  <pre><code>db.ent.find({'a':{$exists:true}})
  db.ent.find({'c':{$exists:false}})
  </code></pre>
  
  <p>Per MongoDB <a href="http://www.mongodb.org/display/DOCS/Advanced+Queries" rel="noreferrer">docs</a>: </p>
  
  <blockquote>
    <p>$exists is not very efficient even with an index, and esp. with {$exists:true} since it will effectively have to scan all indexed values.</p>
  </blockquote>
  
  <p>Can experts there provide more efficient way (even with shift the paradigm) to deal fast with vary name-value pairs</p>