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Tracking Oracle blocking sessions

 

Question:  I have transient locking problems in what appear to be blocking sessions.  They come and go so fast that I cannot always see them.  If there a way to track blocking sessions in Oracle?

Answer:  Sadly, Oracle time-series tables (STATSPACK and AWR) do not track blocking session and waiting sessions. but you can use the extra-cost ASH components can track blocking sessions for one hour into the past. 

For full scripts, download the Oracle script collection.   Also see these scripts for finding RAC blocking sessions.

As a blocking session alert mechanism, you can schedule a job  to check every 5 minutes for a blocking session (using this script to find blocking sessions with v$session) and then trigger a procedure to locate the exact data block where the blocking/waiting occurred.  This automated blocking tracing can then be e-nailed to the DBA for subsequent investigation and repair.

Here is a query that gives us a list of blocking sessions and the sessions that they are blocking:

select
   blocking_session,
   sid,
   serial#,
   wait_class,
   seconds_in_wait
from
   v$session
where
   blocking_session is not NULL
order by
   blocking_session;
 
BLOCKING_SESSION        SID    SERIAL# WAIT_CLASS           SECONDS_IN_WAIT
---------------- ---------- ---------- -------------------- --------
             148        135      61521 Idle                              64

In this case, we find that session 148 is blocking session 135 and has been for 64 seconds.


For complete scripts for detecting blocking session, see the Oracle script collection.

Also see these related notes on finding Oracle blocking sessions:




 

 

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