During interview last week, I couldn't explain properly the following 3 questions:

1)Users complaining that a particular query was ok till last week...now it is very slow..he has not changed anything. As a
DBA how u go about it?

Ans: I told execution plan might have changed.....but not able to
explain the steps to rectify.....

Can anyone give detailed steps to take?

2)Regarding gets and misses of V$librarycache and v$rowcache...

He asked these views uses only memory or data to get 'gets &misses' information.....

Frankly I don't understand the question itself...

3)How to make a query (with where clause) to use Full table scan(means not to use index)?