Three basic requirements that determine AWR snapshot usability

I am a newbie to Oracle and I know that oracle uses two AWR snapshots to create an AWR report. What are three basic requirements that dete开发者_StackOverflow社区rmine whether an AWR snapshot is usable? Or in other words a snapshot can be used for generating an AWR report.

I also have another question. Out of the AWR tables (not the DBA_HIST views) which is the main snapshot table. I want to know which is the most important AWR table for snapshots.

Any help will be greatly appreciated.

Make sure the snapshots cover a representative work load.

A snapshot of an idle database during the night is of little use when troubleshooting a busy reporting database during office hours.

One of the requirements that I can think of is that the snapshot interval should not be too small. If it is you won't get a clear picture of database activity.