Active Session History at work

Performance Monitoring: Active Session History at work

Teaching an Oracle Database 10g Performance Tuning course this week, I introduced the 10g New Feature Active Session History (ASH) to the students. That was one major improvement – together with the Automatic Workload Repository (AWR) and the Automatic Database Diagnostic Monitor (ADDM) – of the 10g version. Way better than STATSPACK was before!

Imagine you are a DBA on a production system and get an emergency call like “The Database is dead slow!”. You are supposed to spot the cause as soon as possible. ASH kicks in here: We sample the Wait-Events of active sessions every second into the ASH-Buffer. It is accessed most comfortable with the Enterprise Manager GUI from the Performance Page (Button ASH Report there). Or with little effort from the command line like this:

--------------------------------------------- Top 10 CPU consumers in last 5 minutes-------------------------------------------SQL> select * from(select session_id, session_serial#, count(*)from v$active_session_historywhere session_state= 'ON CPU' and sample_time > sysdate - interval '5' minutegroup by session_id, session_serial#order by count(*) desc)where rownum <= 10;------------------------------------------------ Top 10 waiting sessions in last 5 minutes----------------------------------------------SQL> select * from(select session_id, session_serial#,count(*)from v$active_session_historywhere session_state='WAITING'  and sample_time >  sysdate - interval '5' minutegroup by session_id, session_serial#order by count(*) desc)where rownum <= 10;

These 2 queries should spot the most incriminating sessions of the last 5 minutes. But who is that and what SQL was running?

------------------------ Who is that SID?----------------------set lines 200col username for a10col osuser for a10col machine for a10col program for a10col resource_consumer_group for a10col client_info for a10SQL> select  serial#, username, osuser, machine, program, resource_consumer_group, client_infofrom v$session where sid=&sid;----------------------------- What did that SID do?---------------------------SQL> select distinct sql_id, session_serial# from v$active_session_historywhere sample_time >  sysdate - interval '5' minuteand session_id=&sid;-------------------------------------------------- Retrieve the SQL from the Library Cache:------------------------------------------------col sql_text for a80SQL> select sql_text from v$sql where sql_id='&sqlid';

You may spot the cause of the current performance problem in very short time with the shown technique. But beware: You need to purchase the Diagnostic Pack in order to be allowed to use AWR, ADDM and ASH