title: Slowness with WMSYS.WM_CONCAT function
body: <p>When I use the Oracle function WMSYS.WM_CONCAT,it my SQL executes very slowly. How can I speed it up? The query is:</p>
  
  <pre><code>SELECT T.A,
         T.B,
         WMSYS.WM_CONCAT(T.C),
         WMSYS.WM_CONCAT(T.D) 
    FROM ( SELECT A,B,C,D FROM TABLE_NAME ) T 
   GROUP BY T.A ,T.B
  </code></pre>
  
  <p>The table contains about two million rows and has indexes on related columns.</p>