title: Why the parallel hint is not working in oracle 19c
body: <pre><code>SELECT /*+PARALLEL 8*/
          A.ACC_ID,
           COALESCE (B.PR_ID, NULL),
           COALESCE (B.DESC, NULL),
           COALESCE (B.CNT, 0)
      FROM (SELECT ACC_ID
              FROM ACC_ID_IN
             WHERE XXX = 1 AND YYY = 'ABCD') A
           LEFT OUTER JOIN
           (  SELECT /*+PARALLEL 8*/
                    A.ACC_ID,
                     B.PR_ID,
                     B.DESC,
                     COUNT (DISTINCT A.ACC_NBR) CNT
                FROM DB1.TABLE1 A,
                     TABLE2  B,
                     TABLE3  C
                     WHERE A.ACC_ID IN (8888888888)
                     AND A.P_ID = B.P_ID
                     AND A.P_ID = C.P_ID
                     AND A.START &lt;=TO_DATE (SUBSTR (A.ACC_ID, 1, 4) || '1231','YYYY/MM/DD')
                     AND A.END &gt; TO_DATE (SUBSTR (A.ACC_ID, 1, 4) || '1230','YYYY/MM/DD')
            GROUP BY A.ACC_ID, B.PR_ID, B.DESC
            ORDER BY A.ACC_ID, B.PR_ID, B.DESC) B
               ON A.ACC_ID = B.ACC_ID
     WHERE A.ACC_ID IN (8888888888)
  GROUP BY A.ACC_ID,
           COALESCE (B.PR_ID, NULL),
           COALESCE (B.DESC, NULL),
           COALESCE (B.CNT, 0)
  </code></pre>
  <p>The above query with parallel hints taking very long time to execute where as same query executed in secs when I removed the parallel hints. This issue observed when oracle is upgraded to 19c. Could you please help me to understand what's happening here and do I need to perform any settings to enable parallel hints as part of DB upgrade activity.</p>