Re: Unable to detect blocking session

New information: we didn't actually find the blocker using v$lock (or gv$lock). Someone finally looked at gv$session and saw that a sqlplus session had been idle for over an hour during the timeframe the application was experiencing issues. We had a script, below, that we were using to locate the root blocker since we have an application issue we're also trying to resolve. That script did NOT show the sqlplus session that appeared to be the blocker. We think that session was the blocker because as soon as the user exited the session, everything cleared almost immediately. We're trying to figure out a script that will find the root block, including sessions like this user's that ran an update statement, but didn't actually update any rows and he didn't do a commit. If someone has any suggestions, we would definitely like to hear them.

*ROOT BLOCKER SCRIPT* SELECT

        DISTINCT
        b.username||' -

        bb.blocker_sid,
        bb.blocker_sess_serial#,
        sb.inst_id,
        sb.username,
        sb.sql_id,
        sb.seconds_in_wait
      FROM gv$session_blockers bb
           JOIN gv$session sb
                ON bb.blocker_sid = sb.sid
                AND bb.blocker_sess_serial# = sb.serial#
     ) b
JOIN (SELECT
        bw.blocker_sid,
        bw.sid,
        bw.sess_serial#,
        sw.inst_id,
        sw.username,
        sw. sql_id,
        sw.seconds_in_wait
      FROM gv$session_blockers bw
           JOIN gv$session sw
                ON  bw.sid = sw.sid
                AND bw.inst_id = sw.inst_id
                AND bw.sess_serial# = sw.serial#
     ) w
        ON b.blocker_sid = w.blocker_sid

> Sandy, to add to what Rick's reply, the "SQL*Net message from client"
> indicates to me the user made an update and failed to commit it. I think
> Rick provided valid possibilities for EM but for why you could not directly
> query the information I think we would need to see what queries you used to
> try to find the blocker though from your final remarks you did use GV$LOCK
> and GV$SESSION plus sys.dbms_lock_allocated to find the blocker. Could the
> earlier attempt have been using the V$ version and so missed the blocker
> since it was on another instance? The sys.dbms_lock_allocated table would
> not be necessary to find the blocking session though it would identify
> which User Lock (UL) was involved if a UL was involved.
>
>
>
> Mark Powell
> Database Administration
> (313) 592-5148
>
>
> ------------------------------
> *From:* oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on
> behalf of Sandra Becker <sbecker6925_at_gmail.com>
> *Sent:* Wednesday, February 21, 2018 10:32:14 AM
> *To:* oracle-l
> *Subject:* Unable to detect blocking session
>
>
> Oracle EE 12.1.0.2 on RHEL 5.11
>
> We had a situation in our production environment yesterday where a user
> had a sqlplus session had an uncommitted "zero row" update on a table.
> This kept the actual application from processing orders using that same
> table. The sqlplus session was initiated from SQL*Plus Release
> 11.1.0.6.0. The wait event on the application session was holding a user
> lock, which was apparently blocked, with a wait event of "SQL*Net message
> from client". Once the user's sqlplus session was exited, all application
> sessions resumed normal functions without any intervention. The user who
> issues the update is a tier1 support person, so we can't lock out their
> access for such activites to prevent future occurrences.
>
> What we are trying to understand is why we were unable to see that the
> user's sqlplus session was blocking either through EM or through queries
> looking at gv$session and gv$session_blockers. We found the application
> locks by joining gv$lock, gv$session and sys.dbms_lock_allocated. Any
> ideas why we could see the blocking or suggestions on what we can look at
> so we don't miss it again?
>
>
> --
> Sandy B.
>
>

-- 
Sandy B.

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