Wait events with v$system views Oracle Tips by Burleson Consulting

What waits are occurring in your database?

In our final example of the power of the V$ tables, we can see what waits are happening in our database. Waits are conditions where a session is waiting for something to happen. Waits can be caused by a number of things from slow disks, to locking situations (like the one we saw above) to various kinds of internal Oracle contention.

See my notes on Oracle wait event scripts and see Oracle code depot for full wait event monitoring scripts.

Waits come in two main kinds of flavors, system-level and session level. The system-level waits represent a high level summary of all session-level waits. Session-level waits then are session specific waits for specific sessions.

System waits come in different wait classes. Classes such as �idle waits� have no real impact on the database at all in most cases (there are some rare exceptions). You can see the current wait class waits by querying the v$system_wait_class view as seen in this example:

Note that in this output we find that the idle wait class far outweighs the other wait event classes, which is often the case for healthy databases. We do see some other waits of interest; particularly the application and concurrency waits have some time accumulated. Let�s see what waits are causing us problems.

To do this, we drill down to the next level of wait events, using the v$system_event view. This gives us more detailed wait event information, and we can associate the waits with our wait classes as we have done in the following SQL:

In this report we find that the top problem appears to be Enqueue waits, that's the enq: TX  row lock contention event is an enqueue. There are a huge number of events in Oracle Database 10g (811 in my 10g database!) so you can't possibly know what each one means. They are all documented in the Oracle reference guide. Also, if you go out to a search engine such as Google and search for the event, you will often find someone who has had problems with it, and you will find lots of help in correcting the problem.

In this case an Enqueue wait has to do with the blocking locks we looked at earlier in this chapter. So in Oracle an enqueue is just another word for lock. If we saw that this was the big problem, I'd start monitoring my sessions to try to figure out what is causing locking issues.   We can drill down even further into the session level if we like. In this case, let's see if anyone is causing locking still:

Note the time_waited and average_wait columns are in Centaseconds in this view.  If we wanted to see the time in seconds we would have to divide the time by 100. Hence, we can see that we have waited for this blocking lock for 4 seconds, up until now.

As we see, SID 110 is blocked right now!! Quickly, let�s see who is blocking this session by re-running our earlier query:

This is clearly a problem. We see that the block continues to be held (now 246 seconds into the blocking event!). We need to go find out who SID 161 is and run them out of town. We do that by returning to the query at the beginning of this section with a slight modification:

Guy Harrison notes this query for wait events using the v$sys_time_model ASH table:

COLUMN wait_class format a20
COLUMN name       format a30
COLUMN time_secs  format 999,999,999,999.99
COLUMN pct        format 99.99

SELECT   
   wait_class,
   NAME,
   ROUND (time_secs, 2) time_secs,
   ROUND (time_secs * 100 / SUM (time_secs) OVER (), 2) pct
FROM
   (SELECT
      n.wait_class,
      e.event NAME,
      e.time_waited / 100 time_secs
    FROM
      v$system_event e,
      v$event_name n
    WHERE
       n.NAME = e.event AND n.wait_class <> 'Idle'
    AND
       time_waited > 0
    UNION
    SELECT
      'CPU',
      'server CPU',
      SUM (VALUE / 1000000) time_secs
    FROM
      v$sys_time_model
    WHERE
      stat_name IN ('background cpu time', 'DB CPU'))
ORDER BY
   time_secs DESC;

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