title: Filtering on objects.all() isn't behaving lazy in Django
body: <p>Django querysets are supposed to be lazy but it seams as though the following is not. </p>
  
  <p>I have the following (5 seconds) slow code (on 200,000 records):<br>
  1. <code>qs = MyCars.objects.all()</code><br>
  2. <br>
  3. <code>qs = qs.filter( car_id__gt=0 )</code></p>
  
  <p>The following filtering is fast:<br>
  A. <code>qs = MyCars.objects.filter( car_id__gt=0 )</code></p>
  
  <p>What am I missing? (And for reasons I must include line 1).</p>
  
  <p>To make it more interesting. If I on line 2 add:<br>
  2. <code>qs2 = qs</code><br>
  And then on line 7:<br>
  7. <code>qs2 = qs2.filter( car_id__gt=0 )</code><br>
  Then it is fast. <br>
  So, I thought I'll use qs2 to filter on line 3 but then it is slow. </p>