True Session Wait Activity in Oracle 10g

Catching a session
waiting on a resource used to be hit or miss. Let’s take a look at how Oracle
has helped give us a better mousetrap for seeing the waits experienced by

It used to be that a DBA
would sit in front of the terminal screen and continually hit the ENTER key to
watch activity in the V$SESSION_WAIT view to try and get a glimpse of the
bottleneck and wait activity for connected sessions. This in itself was a very
tedious task and honestly I would often get paranoid that I would miss some
valuable information if I did not hit the ENTER key fast enough and thus I
would never see the important wait event. Yes there was the V$SESSION_EVENT
view that gave us a total for all waits but we never knew when the wait might
have occurred. DBAs very quickly got smarter and started to sample the data in
the V$SESSION_WAIT table and write it out to another table or dump it to an
external list file. This was fine but the overhead of actually creating and
then running the task that would populate these statistics could get high, and
management of the task could get tedious.

To the Rescue

With Oracle 10g this has
all been nicely taken over for us. Oracle will now sample data through internal
kernel code for statistics similar to what was seen in the V$SESSION_WAIT view and
store it in a limited amount of buffer space, 2 Meg per CPU. We can view these
statistics by querying the new V$ACTIVE_SESSION_HISTORY view. The
V$ACTIVE_SESSION_HISTORY view contains one row of sampled activity for each
session that was active, either on the CPU or actually waiting for a resource. As
this buffer gets full, because of the activity on the system, Oracle will move
the captured statistics to disk as part of the Automatic Workload Repository
(AWR) snapshot mechanism and it will now be available through the
DBA_HIST_ACTIVE_SESS_HISTORY view. When querying for current information,
depending on the activity of your system, you may need to go to both views to
construct a valid picture of what has happened. The important thing to remember
here is that this is historical activity of the waits that have occurred for
SQL executed and we can now go back in time and look at what was the true cause
of performance bottlenecks. Be forewarned that since the statistics are stored
in the rotating buffer and are only moved to the DBA_HIST_ACTIVE_SESS_HISTORY
through a snapshot, you may loose statistics. In addition, when
the snapshot mechanism (AWR) does kick in and does its work to move database
it only takes a sampling of the data in V$ACTIVE_SESS_HISTORY for the snapshot.
Furthermore, when AWR does kick in, it
only takes a sampling of the data in V$ACTIVE_SESS_HISTORY for the snapshot.
What this means is that you need to either catch what you want in the
V$ACTIVE_SESS_HISTORY view before a snapshot or have your own scrapping utility
again. For this reason, this article will only be concerned with querying off
of the V$ACTIVE_SESS_HISTORY view to show a few ways you might want to query
for information that had not been available before.

What resource is currently in high demand?

While we have always been able to look at the system or a
currently running session as a whole and see the resources that are in most use
through the V$SYSTEM_EVENT and V$SESSTAT views, we really have not ever had a
good way to determine for a current time frame what the resources being used
were. This query will give you for the last 30 minutes those resources that are
in high demand on your system.

1 select active_session_history.event,
2 sum(active_session_history.wait_time +
3 active_session_history.time_waited) ttl_wait_time
4 from v$active_session_history active_session_history
5 where active_session_history.sample_time between sysdate – 60/2880 and sysdate
6* group by active_session_history.event
7 order by 2

—————————— ————-
SQL*Net message to client 4
null event 5
LGWR wait for redo copy 161
ksfd: async disk IO 476
direct path read 30025
latch: cache buffers chains 33258
direct path write 93564
log file sequential read 178662
db file sequential read 458653
log file sync 612660
control file single write 1127626
read by other session 2024242
db file parallel write 2278618
control file parallel write 3091888
enq: CF – contention 4108238
rdbms ipc reply 4283877
db file scattered read 4357653
class slave wait 5123780
control file sequential read 6971659
rdbms ipc message 11899157
jobq slave wait 14732974
log file parallel write 15310721
log buffer space 21405250
SQL*Net message from client 26272575
log file switch completion 66115558
enq: HW – contention 100841479
buffer busy waits 114070065

What user is waiting the most?

Again, the power in this SQL query is that we are now able
to determine what user is consuming the most resource at a point in time,
independent of the total resources that the user has used. You can now, with
this query, answer the question of who is waiting the most for resources at a
point in time. If a user calls you up on the phone and says they are
experiencing delays, you can use this query to verify that they are actually
waiting in the database for a result set for a given time period. This SQL is
written for a 30-minute interval from current system time so you may need to

1 select sesion.sid,
2 sesion.username,
3 sum(active_session_history.wait_time +
4 active_session_history.time_waited) ttl_wait_time
5 from v$active_session_history active_session_history,
6 v$session sesion
7 where active_session_history.sample_time between sysdate – 60/2880 and sysdate
8 and active_session_history.session_id = sesion.sid
9 group by sesion.sid, sesion.username
10* order by 3

—– ———- ————-
135 SCOTT 91167481
149 SCOTT 107409491
153 SCOTT 110796799

What SQL is currently using the most resources?

This query will get you started in the proper direction of
zeroing in on what SQL statement is consuming the most resource wait times on
your system. No longer do you have to go to the V$SQLAREA and try to pick out
the top 10 or 20 SQL hogs on your system by disk reads or executions. Now you
really know what SQL statements are consuming resources and waiting the most.
These are the SQL that you really need to tune because the fact that a SQL
statement performs 20,000 reads does not mean that it is a bottleneck in your

1 select active_session_history.user_id,
2 dba_users.username,
3 sqlarea.sql_text,
4 sum(active_session_history.wait_time +
5 active_session_history.time_waited) ttl_wait_time
6 from v$active_session_history active_session_history,
7 v$sqlarea sqlarea,
8 dba_users
9 where active_session_history.sample_time between sysdate – 60/2880 and sysdate
10 and active_session_history.sql_id = sqlarea.sql_id
11 and active_session_history.user_id = dba_users.user_id
12 group by active_session_history.user_id,sqlarea.sql_text, dba_users.username
13* order by 4
——- —— —————————————————– ————-
57 SCOTT insert into sys.sourcetable (select * from sys.source$) 304169752

What object is currently causing the highest resource waits?

This is a great query. Now we can actually see which objects
a SQL statement is hitting. Moreover, if you take a further look at the V$ACTIVE_SESSION_HISTORY
view you will see that you can tailor this query to report on the actual blocks
that are being accessed within the objects for the SQL statement. This is great
help in determining if you may need to reorg your object or redistribute to
reduce the contention on that object.

1 select dba_objects.object_name,
2 dba_objects.object_type,
3 active_session_history.event,
4 sum(active_session_history.wait_time +
5 active_session_history.time_waited) ttl_wait_time
6 from v$active_session_history active_session_history,
7 dba_objects
8 where active_session_history.sample_time between sysdate – 60/2880 and sysdate
9 and active_session_history.current_obj# = dba_objects.object_id
10 group by dba_objects.object_name, dba_objects.object_type, active_session_history.event
11* order by 4
————– ———— —————————— ————-
SOURCE$ TABLE ksfd: async disk IO 23
SOURCETABLE TABLE ksfd: async disk IO 453
SOURCETABLE TABLE latch: cache buffers chains 33258
SOURCETABLE TABLE db file scattered read 82593
SOURCETABLE TABLE db file sequential read 111202
SOURCETABLE TABLE control file parallel write 137237
SOURCETABLE TABLE read by other session 165501
SOURCETABLE TABLE log file sync 612660
SOURCE$ TABLE log buffer space 932308
SOURCETABLE TABLE control file sequential read 1428575
SOURCE$ TABLE log file switch completion 1856281
SOURCE$ TABLE read by other session 1858741
SOURCE$ TABLE db file scattered read 4275060
SOURCETABLE TABLE rdbms ipc reply 4283877
SOURCETABLE TABLE log buffer space 14152000
SOURCE$ TABLE enq: HW – contention 25483656
SOURCETABLE TABLE log file switch completion 59228080
SOURCETABLE TABLE enq: HW – contention 75357823
SOURCETABLE TABLE buffer busy waits 114055403

The new information that Oracle has given us through the
V$ACTIVE_SESSION_HISTORY view is invaluable. Now we can really zero in on those
resources, session, users, SQL statements, and objects that are causing our
systems to be resource intensive. By reducing the resource usage, you can in
fact tune your systems better.


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James Koopmann

James Koopmann
James Koopmann
James Koopmann has fourteen years of database design, development and performance tuning experience. In addition, he has extensive database administration experience in Oracle and other relational databases in production environments, specializing in performance tuning of database engines and SQL based applications. Koopmann is an accomplished author with several technical papers in various Oracle related publications such as Oracle Magazine, Oracle Professional and SQL>UPDATE_RMOUG. He is a featured author and database expert for DatabaseJournal, a member of the editorial review committee for Select Journal (The Magazine for the International Oracle Users Group), an Oracle Certified Professional DBA and noted speaker at local Oracle User Groups around the country.
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