Metrics vs Statistics
Here are the tuning metrics tables (SQL stats are not in “metric” tables per say)
(*DBA_HIST_…_HISTORY views are sort of confusing. AFAI remember they were storing alert history, but apparently they are used for adaptive thresholds – an area for future investigation)
I’ve noticed a number of people posting queries using DBA_HIST_SYSSTAT instead of DBA_HIST_SYSMETRIC_SUMMARY which leads me to believe that there is some confusion or lack of information on the metric tables.
Oracle 10g introduced metric tables which compute deltas and rates of statistics thus hugely simplifying the ability to answer simple questions like “what is the I/O rate on my databases right now.” This question, before 10g, was surprisingly tedious to answer. To answer the question one would have to query v$sysstat for example:
Select value from v$sysstat where name=’physical reads’;
but querying v$sysstat just once fails to answer the question but instead answers the question “How much I/O has been done since the database was started”. To answer the original question one would have to query v$sysstat twice and take the delta between the two values:
- Take value at time A
- Take value at time B
- Delta = (B-A)
- and/or get Rate = (B-A)/elapsed time
Getting these deltas and rates could be a pesky task especially working with a customer over the phone. Then 10g Oracle introduced metric tables which answer the questions in one single query using
Select VALUE , METRIC_UNIT,INTSIZE_CSEC from v$sysmetric where metric_name='Physical Reads Per Sec';
VALUE METRIC_UNIT INTSIZE_CSEC ---------- ----------------- ------------ 654.6736 Reads Per Second 5959 134.9835 Reads Per Second 1515
Notice that the query returns 2 rows. The first row is the the last minute (ie 59.59 seconds) and the second row is the last 15 seconds (ie 15.15 seconds). Oracle collects both the deltas and rates for 60 second and 15 second intervals.
Oracle has the average, maximum, minimum for the values for the last hour in
that one can query like:
select MAXVAL,MINVAL,AVERAGE,STANDARD_DEVIATION from V$SYSMETRIC_SUMMARY where metric_name='Physical Reads Per Sec'; MAXVAL MINVAL AVERAGE STANDARD_DEVIATION ---------- ---------- ---------- ------------------ 3.71784232 0 .076930034 .478529283
Also for the last hour Oracle stores the 60 second intervals and for the last 3 minutes the 15 second intervals in
One issue with using
- V$SYSMETRIC – last 15 and 60 seconds
- V$SYSMETRIC_SUMMARY – values last hour (last snapshot) like avg, max, min etc
- V$SYSMETRIC_HISTORY – last hour for 1 minute, last 3 mintes for 15 second deltas
- DBA_HIST_SYSMETRIC_SUMMARY – hour summaries for last week.
is becoming familiar with the statistics names which are different from v$sysstat. We can look at
For the group_names (statistic definitions)
- System Metrics Short Duration – 15 second delta stats (41 10gR2, 47 11gR2) – not sure I’d ever use short duration
- System Metrics Long Duration – 60 second delta stats (135 10gR2, 158 11gR2) – might as well just use long duration
For trending data over multiple days, the view DBA_HIST_SYSMETRIC_SUMMARY can be used. The view has a history of all theSystem Metrics Long Duration statistics. If you store multiple databases in the same AWR repository you can check the statistics available to a particular DBID with the view DBA_HIST_METRIC_NAME.
The view DBA_HIST_SYSMETRIC_SUMMARY can be queried easily for trending metrics, such as the simple query for bytes read by the database per second:
select dbid, to_char( begin_time ,'YYYY/MM/DD HH24:MI'), round(average) from dba_hist_sysmetric_summary where metric_name= 'Physical Read Total Bytes Per Sec' /* and DBID=[dbid if share repository] */ order by begin_time;
Compare the above query to the same query on DBA_HIST_SYSSTAT (note there are a lot of stats in v$sysstat)
with stats as ( select sn.dbid, st.stat_name, to_char(cast(begin_interval_time as date ), 'YYYY/MM/DD HH24:MI') btime, -- Lag (st.value) OVER( PARTITION BY st.stat_name,st.dbid ORDER BY sn.snap_id) Lag (st.value) OVER( PARTITION BY st.stat_name,st.dbid,st.startup_time ORDER BY sn.snap_id) value_beg, st.value value_end, (cast(end_interval_time as date) - cast(begin_interval_time as date )) * (24*60*60) delta from DBA_HIST_SYSSTAT st, DBA_HIST_SNAPSHOT sn where sn.snap_id=st.snap_id and sn.dbid=st.dbid and (st.stat_name= 'physical read total bytes') order by begin_interval_time ) select dbid, btime, round((value_end-value_beg)/delta) rate_per_sec from stats where (value_end-value_beg) > 0 ;
Its a bit disconcerting to note that the above two queries don’t return the exact same data on my laptop. If it was roughly the same that would be fine, and in general the stats are the similar but there are cases where they differ dramatically. I don’t see anything obvious in the way the queries are written. Possibly has to do with database bounces or the way the database is affected by the laptop’s sleep and hibernate modes. Will have to look into this farther.
One trick to make the data easy to load into Excel is to use the html output format and spool to a file with an “.html” extension
SET markup HTML on spool output.html
List of DBA_HIST views