Aggregates

1. Can we create aggregates on Hierarchy ?

2. Can we compress the data in aggregates/ will the data in aggregates are compressed?

3. What is the Thumb rule of aggregates?

4. By creating aggregates how much percentage increase query performance ?

5. How to find which aggregates are more usage &which are less usage?

6. How to find which aggregates are in regular usage?

7. Aggr are created on char info object but how to find out based on which char we need to create?

Please explain me in a Detail manner.