Sunday, May 13, 2007

Plan Guides

In our production environment, we had a poorly performing stored procedure in terms of reads and CPU usage. A quick peek using the available dmv's gave us an insight into the plan and the parameters that were used to obtain that plan. That way, it was easy to simulate the behavior on a development environment.

Apparently, every morning, with a clean procedure cache, the procedure got called with certain parameters which resulted in a not very intuitive, but good plan with a cost of 3 reads. Subsequent calls with different parameters reused the cached plan, but that resulted in > 8000 logical reads and quite some CPU usage. A more intuitive plan resulting from typical parameters had a constant cost of about 8 logical reads for both types of parameters, which can give a really big performance boost, given the fact that the procedure gets called more than 40.000 times a day and delays a vital business process.

Because I'm not allowed to modify anything of the stored procedure (yep, it's CRM), I had to use a plan guide to 'guide' the engine to the more performant plan. There are several options: specifying the whole plan, add the parameters for the compilation, force recompile... I chose the parameter hint.

--solution, add plan guide
exec sp_create_plan_guide
@name = N'Plan for procedure p_GrantInheritedAccess UPDATE'
, @stmt = N'UPDATE poa1
SET InheritedAccessRightsMask = poa1.InheritedAccessRightsMask poa2.AccessRightsMask poa2.InheritedAccessRightsMask 0x08000000
FROM PrincipalObjectAccess poa1 JOIN PrincipalObjectAccess poa2 ON (poa1.PrincipalId = poa2.PrincipalId)
WHERE poa1.ObjectId = @ReferencingObjectId
AND poa1.ObjectTypeCode = @ReferencingObjectTypeCode
AND poa2.ObjectId = @ReferencedObjectId
AND poa2.ObjectTypeCode = @ReferencedObjectTypeCode'
, @type = N'OBJECT'
, @module_or_batch = N'p_GrantInheritedAccess'
, @params = NULL
, @hints = N'OPTION(OPTIMIZE FOR(@ReferencingObjectId=''D4F70FD5-41FA-DB11-9668-0011258C53FD'',@ReferencingObjectTypeCode=4200,@ReferencedObjectId=''2052793D-41FA-DB11-9668-0011258C53FD'',@ReferencedObjectTypeCode=4200))'
SELECT * FROM sys.plan_guides

The result of this simple operation is a clean and transparent solution. I haven't put it into production though, because it stays a last resort, but it's always good to know your options.

1 comment:

JG said...

Nice one, good tip.