title: Change order of sortkey to descending
body: <p>We have a 2-node Redshift cluster with a table with around 100M records. We marked a timestamp column as the sortkey - because the queries are always time restricted. However, our use-case requires the results has to be sorted in the descending order (on the sortkey).</p>
  
  <p>After some benchmarking, we noticed that the average time taken  around 10s. However, when the reverse ordering was removed, the average time came down to under 1s.</p>
  
  <p>Is it possible to reverse the order of the sortkey to be of descending order? The official documentation doesn't seem to indicate that is possible. However I tried putting this while creating a new table:</p>
  
  <pre><code>sortkey(start_time DESC)
  </code></pre>
  
  <p>There were no errors but it doesn't seem to have any effect. </p>
  
  <p>EDIT: Added the result of EXPLAIN statement on the queries.</p>
  
  <ul>
  <li><p>The query with order_by ASC</p>
  
  <pre><code>  explain select * from kcdr_sr_desc where user_id=396747 and start_time &gt; '2016-01-01' and start_time &lt; '2016-07-01' order by start_time limit 20;
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>    XN Limit  (cost=0.00..10.86 rows=20 width=300)
      -&gt;  XN Merge  (cost=0.00..709235.56 rows=1306585 width=300)
         Merge Key: start_time
          -&gt;  XN Network  (cost=0.00..709235.56 rows=1306585 width=300)
         Send to leader
         -&gt;  XN Seq Scan on kcdr_sr_desc  (cost=0.00..709235.56 rows=1306585 width=300)
               Filter: ((user_id = 396747) AND (start_time &gt; '2016-01-01 00:00:00'::timestamp without time zone) AND (start_time &lt; '2016-07-01 00:00:00'::timestamp without time zone))
  </code></pre></li>
  <li><p>The query with order_by DESC</p>
  
  <pre><code> explain select * from kcdr_sr_desc where user_id=396747 and start_time &gt; '2016-01-01' and start_time &lt; '2016-07-01' order by start_time desc limit 20
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>  XN Limit  (cost=1000000841967.42..1000000841967.47 rows=20 width=300)
    -&gt;  XN Merge  (cost=1000000841967.42..1000000845233.88 rows=1306585 width=300)
    Merge Key: start_time
       -&gt;  XN Network  (cost=1000000841967.42..1000000845233.88 rows=1306585 width=300)
         Send to leader
         -&gt;  XN Sort  (cost=1000000841967.42..1000000845233.88 rows=1306585 width=300)
               Sort Key: start_time
               -&gt;  XN Seq Scan on kcdr_sr_desc  (cost=0.00..709235.56 rows=1306585 width=300)
                     Filter: ((user_id = 396747) AND (start_time &gt; '2016-01-01 00:00:00'::timestamp without time zone) AND (start_time &lt; '2016-07-01 00:00:00'::timestamp without time zone))
  </code></pre></li>
  </ul>