title: postgres count or sum ignoring dirty stuff
body: <p>I have an index where the information that has to be summed up is in the index. Index looks like this <code>(key,sumable_attribute)</code>.</p>
  
  <p>Now if I say: </p>
  
  <pre><code>SELECT sum(sumable_attribute) from table where key between 1 and 1000000
  </code></pre>
  
  <p>It would be perfect if we can run along the linked leafs of the b-tree and collect the sumable_attributes.</p>
  
  <p>BUT: MVCC tells us to check the visibility of the data... I don't care if my result isn't 100% correct, as long as it is fast. MyISAM kind of fast...</p>
  
  <p>How do I tell postgres to ignore this?</p>