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Oracle Database Tips by Donald BurlesonMay 29, 2015


Question:  I want to find the SQL statements that are generating the largest amount of redo log activity.  I don't see a correlation between v$session and the redo log activity.  How can I find the DML session hogs?


Answer:  Yes, you are correct that there is no direct connection between DML activity and redo log activity, but you can inspect v$session and v$bgprocess and get an idea which sessions are doing lots of DML.  The SQL (DML) statements updates data blocks and the DBWR process writes these block back to the database, causing redo.

Another indirect way to see the top DML causing redo activity is to use Oracle LogMiner, which re-constructs the DML statements based upon the redo log entries.

For advanced DBA's, there are also scripts in the the Oracle scripts download.  

This script queries the v$statname, v$sesstat, v$session, and v$bgprocess views.  This script will show SQL with high updates to the database:

   round(100 * total_user_io/total_io,2) tot_io_pct
     b.sid sid,
     nvl(b.username, username,
     sum(value) total_user_io
     sys.v_$statname c, 
     sys.v_$sesstat a,
     sys.v_$session b,
     sys.v_$bgprocess p
   see code depot for full script
      a.statistic#=c.statistic# and
      p.paddr (+) = b.paddr and
      b.sid=a.sid and in ('physical reads',
                 'physical writes',
                 'physical writes direct',
                 'physical reads direct',
                 'physical writes direct (lob)',
                 'physical reads direct (lob)')
group by
      b.sid, nvl(b.username,,
      sum(value) total_io
      sys.v_$statname c,
      sys.v_$sesstat a
      a.statistic#=c.statistic# and in ('physical reads',
                 'physical writes',
                 'physical writes direct',
                 'physical reads direct',
                 'physical writes direct (lob)',
                 'physical reads direct (lob)'))
order by
      3 desc;

If you are a DBA that wants to see the total I/O picture (in other words, both logical and physical I/O), then use the totpctio.sql  query instead.

See other session I/O detection scripts here.


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