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Don Burleson Blog 







Oracle Parallel Automatic Tuning tips

Oracle Database Tips by Donald BurlesonJune 6, 2015

Note that this parameter has been deprecated.  In 11g release 2 and beyond, see the parallel_degree_policy parameter.

Oracle parallel query (OPQ) is a great way to speed full full-scan operations, but Oracle parallelism can be problematic if it is not implemented properly.

Oracle provides a general method for implementing parallelism with the parallel_automatic_tuning=true parameter setting, but there are cases when using PAT can actually degrade your Oracle performance.  The Oracle docs note:

"With parallel automatic tuning enabled, Oracle determines parameter settings for each environment based on the number of CPUs on your system and the value set for PARALLEL_THREADS_PER_CPU.

The default values Oracle sets for parallel execution processing when PARALLEL_AUTOMATIC_TUNING is TRUE are usually optimal for most environments. In most cases, Oracle's automatically derived settings are at least as effective as manually derived settings."

I disagree.

The parallel_automatic_tuning parameter feature was created for DSS and Data Warehouse environments using the all_rows optimizer mode on servers with many CPU processors to ensure that the common full table scans are parallelized to an optimal DEGREE.

However, parallel_automatic_tuning is not always appropriate for OLTP and online systems, where setting parallel_automatic_tuning may change the SQL optimizers perception of the costs of full scan operations, causing indexes not to be used.

For non-warehouse environments, I do not recommend setting parallel_automatic_tuning, or turning on parallel query at the system or table level.  I get the best performance my manually adding the "parallel" hint to query where I want the full can to be done in parallel.



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