SQL Server 2012 SP2 Enterprise Edition. Users complaining of slowness. Monitoring tool shows highest wait event is `CXPACKET`. Instance settings * MAXDOP: 8 * Cost Threshold for Parallelism: 175 Ran `sp_BlitzCache` (from the Brent Ozar toolset, those guys rock) for further diagnosis and results show queries with cost under 175 going parallel. Anyone ever see Cost Threshold for Parallelism being ignored?
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Paul White
> *Anyone ever see Cost Threshold for Parallelism being ignored?* It is not being ignored. During the compilation process, the optimizer first considers a serial plan. If the estimated cost of that plan exceeds the Threshold, the optimizer goes on to look for a parallel plan. If the resulting parallel plan is costed below the best serial one, it will be chosen. So, the parallel plan will have a lower cost that the serial one (which you cannot see). It is perfectly possible for the final parallel plan to have an estimated cost below the Threshold - the point is *the best serial plan candidate exceeded the Threshold*. An example can be seen in my [blog post][1] on parallel plan bitmaps. [1]: https://sqlkiwi.blogspot.com/2011/07/bitmap-magic.html