title: How to produce rows for non-existing time buckets?
body: <p>I have produced a table like this:</p>
  
  <pre><code>+----------+---------+---------+-------+
  | _time    | field_1 | field_2 | count |
  +----------+---------+---------+-------+
  | 08:00:00 | A       | 1       | 2     |
  +----------+---------+---------+-------+
  | 08:00:00 | B       | 1       | 4     |
  +----------+---------+---------+-------+
  | 08:00:03 | B       | 3       | 1     |
  +----------+---------+---------+-------+
  | 08:00:03 | A       | 2       | 3     |
  +----------+---------+---------+-------+
  </code></pre>
  
  <p>I want to know:
  what's average &amp; maximum count, per (field_1+field_2) combination, per second.
  The problem is the _time is missing some seconds, so the <code>stats count</code> result only give me the aggregated results on existing time buckets.</p>
  
  <p>How can I expand this table to include every missing time seconds, just by filling count=0, for each (field_1+field_2) combination? As long as I can do this, I can get the result simply by <code>stats avg(count) max(count) by field_1 field_2</code>.</p>