ADDM report dba_hist_snapshot

Question

How does ADDM report work? When I take two snapshots one for

START: 05-OCT-2011 09:00:00, 
END  : 05-OCT-2011 10:00:00 

and another one for

START: 06-OCT-2011 09:00:00, 
END  : 06-OCT-2011 10:00:00, 

what does it do?

Does it show the difference between 05 and 06 October during that hours(9,10)?

Or it will show you the whole interval from 05-OCT-2011 09:00:00 to 06-OCT-2011 10:00:00 compared to some baseline? (Note: I do not have any baseline..is there any default baseline?)

I was confused a little bit when I saw the dates on the generated report, there was written:

Analysis Period: from 05-OCT-2011 10:01 to 06-OCT-2011 10:00

Can you explain me? Thank you very much

Answer 1

The AWR repository stores the snapshots you see listed in DBA_HIST_SNAPSHOT. The END_TIME is when the Snapshot data was recorded. This is the raw data about performance on your DB instance. They are usually taken every hour and retained for 7 days.

2 snapshots are required to generate a report. They don't really mean much on their own. In this case for the 2 snaps you specified you are getting a report that covers the END time of the 1st snapshot until the END time of the 2nd snapshot.

As far as the times on a generated report not being exactly at :00 I am thinking some rounding might be going on. The snapshots are triggered as close to top of the hour as possible, but it may take a few seconds to actually record the data. If you look at FLUSH_ELAPSED time in DBA_HIST_SNAPSHOT you will see that some data recording operations take a few seconds, some may take a minute or more. This is why you are seeing 10:01 instead of exactly 10:00 (the END of the 1st snap).