title: Slow performance for dual table and views after upgrading from 10g to 19c
body: <p>After upgrading the Oracle database from 10g to 19c, I'm experienced the performance issue as querying in dual table and views on a large database when using update/ insert statement. I've to wait for more than a hour when comparing the performance of views in Oracle database 10g.</p>
  <pre><code>--use Views
  update table1
  set break_show ='Y'
  where developer in (select developer_dis from view1 where rn1 &lt;=10 or rn2 &lt;=10);
  
  --use Table
  update table1
  set break_show ='Y'
  where developer in (select developer_dis from table2 where rn1 &lt;=10 or rn2 &lt;=10);
  </code></pre>
  <p>I had to set EXECUTE IMMEDIATE 'alter session set optimizer_features_enable=&quot;10.2.0.4&quot;';, and this fixed by bad performance.</p>
  <p>What should I do to find out the real reason that the dual table and views are running slow.</p>
  <p>Any help would be appreciated.</p>