Re: Wait Event "PX Deq: Execution Msg"

This event appears when a PQ slave has nothing to do, but is not allowed to go idle.

One scenario where you can see large values for the event is when (for example) you have a large data set coming out of a parallel ORDER BY.

The last layer of PX slaves in the query will receive a ranged set of rows and sort them. The QC will then request ALL the rows from the first PX slave, then the second, then the third and so on.

When the first slave has supplied all its rows, it will go into "PX Deq: Execution Msg" waiting for the QC to tell it do die - which happens only after the QC has got all the rows from all the slaves and passed them to the front end.

This means that it is a usually an idle event - but it may be a symptom of excessively large queries choosing an inappropriate execution path.

Jonathan Lewis
http://www.jlcomp.demon.co.uk

Author of:
Practical Oracle 8i: Building Efficient Databases

Next Seminar - Australia - July/August
http://www.jlcomp.demon.co.uk/seminar.html

Host to The Co-Operative Oracle Users' FAQ http://www.jlcomp.demon.co.uk/faq/ind_faq.html

-----Original Message-----
To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com> Date: 17 May 2002 01:22

|Hello,
|
|I am hoping someone can help me with an elusive wait event called 'PX
Deq:
|Execution Msg'. As you can see from the statspack "Top 5 Wait
Events" this
|event accounts for almost 75% of the wait time. I have been looking
in past
|Oracle-L posts, Google, Oracle FAQ, OTN, Metalink, Oracle manuals,
etc. I
|can find almost nothing about this event. What I did find leads me
to
|believe may be an idle event and that it is the second event 'PX Deq
Credit:
|send blkd' that may be the more troublesome one. However I am still
|confused about both. Can someone help set me straight or tell me
where
|there might be some more information about these 2 events?
|
|Thanks,
|Nancy
|
|Top 5 Wait Events
|~~~~~~~~~~~~~~~~~ Wait
%
|Total
|Event Waits Time (cs)
Wt
|Time
|-------------------------------------------- ------------ -----------
- -----
|--
|PX Deq: Execution Msg 42,144
8,639,803
|74.48
|PX Deq Credit: send blkd 15,339
2,878,443
|24.81
|log file parallel write 41,934
30,787
|.27
|db file parallel write 8,776
25,046
|.22
|db file sequential read 24,767
18,255
|.16
|

-- 
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-- 
Author: Jonathan Lewis
  INET: jonathan_at_jlcomp.demon.co.uk

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