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For tables that have DEGREE DEFAULT, the degree of parallelism (DOP) is calculated by Oracle using number of CPUs and number of threads per CPU amongst other things. As I understand it, when costing execution plans Oracle will assume the default DOP for tables with DEGREE DEFAULT.

At execution time, however, if parallel_max_servers is less than the default DOP then the number of parallel servers will be restricted and - I presume - the final execution may not go as intended by the costed execution plan and a less parallel execution plan may be preferable.

Is this correct?

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parallel_max_servers is higher than the highest possible DOP by default - have you turned it down? –  Jack Douglas Oct 25 '11 at 20:11
    
Hi Jack - this is on a shared server managed by others. I presume they're looking to prevent a huge number of threads from kicking off. That's a great link though - hadn't come across that one before. Do you think my logic is correct? It assumes that parallel_max_servers is ignored when costing an execution plan. –  Nick Pierpoint Oct 26 '11 at 16:07
    
I think your logic is correct in that if the parallelism is limited to below what the CBO expected when it complied the plan then perhaps a different plan would have been better. –  Jack Douglas Oct 26 '11 at 17:59

1 Answer 1

It would be a very strange thing if parallel_max_servers is set to a value below cpu_count * parallel_threads_per_cpu. I would call that a mistake by the DBA even. Let me bring to your attention a new feature of 11g (you should always mention your version, by the way), called Automatic DOP, that I have introduced in a posting here: http://uhesse.wordpress.com/2009/11/24/automatic-dop-in-11gr2/

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