title: Self join in big query is running very slowly, am I following best practices?
body: <p>I'm creating a table of the number of overlapping commentators between Reddit subreddits via the following self join: </p>
  
  <pre><code>SELECT t1.subreddit, t2.subreddit, COUNT(*) as NumOverlaps
  FROM [fh-bigquery:reddit_comments.2015_05] t1
  JOIN [fh-bigquery:reddit_comments.2015_05] t2
  ON t1.author=t2.author
  WHERE t1.subreddit&lt;t2.subreddit
  GROUP BY t1.subreddit, t2.subreddit;
  </code></pre>
  
  <p>Typical queries I have made on this data set in Big Query finish very quickly (&lt; 1 min) but this query has run for over an hour and still not finished. This data has 54,504,410 rows and 22 columns. </p>
  
  <p>Am I missing obvious speed ups that I should be implementing to make this query run quickly? Thanks! </p>