The new explain plans after upgrading to 12c were driving me crazy, but after some reading and debugging I found out that PL/SQL Developer is showing inactive steps from the adaptative plan.

Example:

[Linked Image]

The Hash join doesn't make any sense.

Using the dbms_xplan.display_cursor(format => 'allstats last adaptive') I get:

[Linked Image]

Where we can see that the Hash Join is inactive, so PL/SQL Developer is incorrectly showing the inactive steps.

Hope you fix this in the next release.

Regards,
Manuel