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Targeted Plan Cache Removal

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A lot of times you’ll hear how people are experiencing sudden, intermittent, poor performance on a query, bad parameter sniffing at work, so they’ll fix it by running the following code:

DBCC FREEPROCCACHE();

BOOM!

Yeah, you just nuked the cache on your server because you wanted to take out a single terrorist query. Now, yes, that problematic query is going to recompile and hopefully have a better execution plan. Also, so are all the other queries on your system. That spike in CPU and the slow-down all your business people are experiencing… Your fault for going nuclear.

Instead of a nuke, why not use a sniper rifle to just remove the one problematic plan. Here’s a little piece of code to help out:

DECLARE @PlanHandle VARBINARY(64);

SELECT  @PlanHandle = deps.plan_handle
FROM    sys.dm_exec_procedure_stats AS deps
WHERE   deps.object_id = OBJECT_ID('dbo.SomeProcedureName');

IF @PlanHandle IS NOT NULL
    BEGIN
        DBCC FREEPROCCACHE(@PlanHandle);
    END
GO

Take a look at the documentation for FREEPROCCACHE. You can target specific plans using a plan_handle, a sql_handle, or even a resource governor pool by passing pool_name. I take advantage of that with this query to pull the plan_handle from sys.dm_exec_procedure_stats. You could use T-SQL text too, you’d just have to add in sys.dm_exec_query_text to one of the other DMOs that has the plan_handle or sql_handle such as sys.dm_exec_requests or sys.dm_exec_query_stats. However you choose to do it, you can use a targeted approach to remove plans from cache.

Let’s take the nukes off the table.


Want to really get into talking about the plan cache, query tuning, parameter sniffing and the rest? I have two upcoming all day pre-conference seminars. There’s still room at IT/Dev Connections in Las Vegas on September 14th. Click here to register. Also, at Connections, I’m hoping to be able to read your execution plans, so bring the really scary ones. Later that same week, I’ll be at SQL Saturday San Diego and will do a pre-con there on query tuning. Click here to get your seat.

 

The post Targeted Plan Cache Removal appeared first on Home Of The Scary DBA.

The Scary DBA

I have twenty+ years experience in IT. That time was spent in technical support, development and database administration. I work forRed Gate Software as a Product Evangelist. I write articles for publication at SQL Server Central, Simple-Talk, PASS Book Reviews and SQL Server Standard. I have published two books, ”Understanding SQL Server Execution Plans” and “SQL Server 2008 Query Performance Tuning Distilled.” I’m one of the founding officers of the Southern New England SQL Server Users Group and its current president. I also work on part-time, short-term, off-site consulting contracts. In 2009 and 2010 I was awarded as a Microsoft SQL Server MVP. In the past I’ve been called rough, intimidating and scary. To which I usually reply, “Good.” You can contact me through grant -at- scarydba dot kom (unobfuscate as necessary).

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