Why did the plan change
I know I am going to be hounded for this, but my question is truly a generic one. But if you want specifics, then consider it to be a 10.2.0.4 database on linux machine.
My question is how do I find out why a plan changed for a query. I know the probable causes can be data changes (which is not the case I am looking into) or stats change or index rebuild. But my question is how do I pin point the cause of a plan change (what views/reports can help). I know the plan has changed, now what caused it keeping in mind its not a data change and how do I find it out with the help of DBA_HIST or V$ views.
When in doubt ...go to the basics!!