title: PERCENTILE_CONT() returns same value regardless of input parameter
body: <p>I would like to get the 5th, 50th, 95th percentile of a table</p>
  
  <pre><code>SELECT col1, col2, col3, AVG(col4), STD(col4), 
      PERCENTILE_CONT(0.05) WITHIN GROUP (ORDER BY col4) 
          OVER (PARTITION BY col1, col2, col3) as 5th_percentile, 
      PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY col4)  
          OVER (PARTITION BY col1, col2, col3) as 50th_percentile, 
      PERCENTILE_CONT(0.95) WITHIN GROUP (ORDER BY col4)  
          OVER (PARTITION BY col1, col2, col3) as 95th_percentile
  FROM table
  GROUP BY col1, col2, col3
  LIMIT 100
  </code></pre>
  
  <p>What I end up getting back is 5th_percentile == 50th_percentile == 95th_percentile</p>
  
  <pre><code>AVG(col4)   STD(col4)   5th_percentile   50th_percentile  95th_percentile
  300.000000  0.000000    300.000000       300.000000       300.000000
  67.076600   16.968851   82.031792        82.031792        82.031792
  66.166136   11.452172   78.348846        78.348846        78.348846
  544.262809  68.269014   605.797302       605.797302       605.797302
  22.523138   1.820358    24.000000        24.000000        24.000000
  </code></pre>
  
  <p>Whats going on?</p>
  
  <p>Edit: The db is MemSQL</p>