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Lawson speaks on Oracle parallel query

In this excellent article by Chris Lawson, author of the bestselling “The Art and Science of Oracle Performance Tuning”, we see an excellent discussion on tuning with Oracle Parallel query (OPQ):

 

http://www.oraclemagician.com/mag/magic7.pdf

 

Most Oracle professionals know that setting parallelism a the system level (parallel_automatic_tuning parameter), or session-level parallelism (the alter session force parallel query command) can be dangerous because the cost-based SQL optimizer will assign a smaller cost to full-table scans, causing index to cease being used.  Most Oracle folks only invoke parallelism at the SQL-level, using the parallel hint (e.g. select /*+ parallel (c, 31) */).  Lawson notes:

 

Our experiments showed that forcing parallelism at the session level causes extra extents to be used for each slave, whether the Insert or Merge command is used.

 

The extra extents may be a problem only when extent sizes are way too high. In addition, if you don’t alter your session to force parallelism, this may never be a problem for you.
 


 

 
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