title: BigQuery error "Response too large to return" when using COUNT(DISTINCT ...)
body: <p>I have a dataset with ~20M rows and I'm observing the following behavior.</p>
  
  <p>The query below returns the error "Response too large to return". The 'id' field is shared among multiple records and the 'field' field has some arbitrary value for each record. I would expect that the result set should only contain 10 rows, well below the query response limit.</p>
  
  <pre class="lang-sql prettyprint-override"><code>SELECT id, COUNT(DISTINCT field)
  FROM [my.dataset]
  GROUP BY id
  LIMIT 10
  </code></pre>
  
  <p>However, when the DISTINCT keyword is removed from the COUNT aggregation function, BigQuery returns 10 results as expected.</p>
  
  <pre class="lang-sql prettyprint-override"><code>SELECT id, COUNT(field)
  FROM [my.dataset]
  GROUP BY id
  LIMIT 10
  </code></pre>
  
  <p>I don't understand why the first query returns an error and the second completes successfully. Shouldn't both queries return the same number of rows?</p>