title: Why doesn't Redshift support the DOES EXIST correlated sub-query?
body: <p>RedShift is known to have a set of correlated sub-queries that it can decorrelate and thereby make the query run faster.</p>
  
  <p>Why can't Redshift decorrelate the common <code>DOES NOT EXIST</code> subquery? For example, the query below would be essentially:</p>
  
  <pre><code>SELECT ProductID,
         ProductName
  FROM   Products p
  WHERE  NOT EXISTS (SELECT *
                     FROM   [Order Details] od
                     WHERE  p.ProductId = od.ProductId) 
  </code></pre>
  
  <p>Can someone explain why Redshift can't decorrelate this query?</p>