title: How could predicate pushing on an inline view slow down a query?
body: <p>I have inherited a somewhat messy query that I am working on refactoring to be performant.  </p>
  
  <p>During this process, one of the things I did due to personal preference was change all of the ANSI-99 join syntax from the "inner join" and "left outer join" statements to be predicates in the query.  I noticed two very strange things that I would appreciate an explanation on.</p>
  
  <ol>
  <li>Changing the joins from the "INNER JOIN..." syntax has changed the explain plan.  With the ANSI 99 syntax, oracle did full table scans on the columns that were being joined.  After changing the join syntax, it now does predicate pushing.  Why would the join syntax change the explain plan?</li>
  <li>The predicate pushing on the inline view has actually slowed down the query by a very significant margin.  The query that was running (prior to altering the joins) at around 3 sec.  Now it is taking 9 sec.  To be honest, I fairly new to reading explain plans so it's entirely possible that the restructuring of the query has slowed it down for a different reason.  But ultimately my question is: "Is it possible for predicate pushing on indexed columns to slow down a query so substantially? If so, why?" </li>
  </ol>
  
  <p>Thanks for the replies, and my apologies if this isn't very clear...</p>