DBA Arena

Finding the root cause of sudden increase in the archive log generation on the database.

Find tables that have maximum number of changes during the period there was huge archive generation.

SELECT to_char(begin_interval_time,'YYYY_MM_DD HH24:MI') snap_time,
dhsso.object_name,
SUM(db_block_changes_delta)
FROM dba_hist_seg_stat dhss,
dba_hist_seg_stat_obj dhsso,
dba_hist_snapshot dhs
WHERE dhs.snap_id = dhss.snap_id
AND dhs.instance_number = dhss.instance_number
AND dhss.obj# = dhsso.obj#
AND dhss.dataobj# = dhsso.dataobj#
AND begin_interval_time BETWEEN to_date('2015_05_23 12','YYYY_MM_DD HH24')
AND to_date('2015_05_25 12','YYYY_MM_DD HH24')
GROUP BY to_char(begin_interval_time,'YYYY_MM_DD HH24:MI'),
dhsso.object_name order by SUM(db_block_changes_delta) desc;

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Cause:

Blocking sessions occur when one session holds an exclusive lock on an object and doesn't release it before another session wants to update the same data. This will block the second until the first one has done its work.
From the view of the user it will look like the application completely hangs while waiting for the first session to release its lock. You will often have to identify these sessions in order to improve your application performance to avoid as many blocking sessions as possible.

Solution:

The following query shows all the blocking sessions which can help you to identify the problem.

col WAIT_CLASS for a12;
select blocking_session,sid,serial#,wait_class,seconds_in_wait,status from v$session where blocking_session is not NULL order by blocking_session;
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