Targeted Plan Cache Removal

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, […]

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Generating Estimated Plan and the Plan Cache

Does generating an Estimated Plan cause that plan to be loaded into the plan cache? No.   What? Still here? You want more? Proof? Fine. Let’s first run this bit of code (but please, not on your production server): DBCC FREEPROCCACHE(); That will remove all plans from cache. Now, let’s take this query and generate […]

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Simple Parameterization and Data Types

Simple paramaterization occurs when the optimizer determines that a query would benefit from a reusable plan, so it takes the hard coded values and converts them to a parameter. Great stuff. But… Let’s take this example. Here’s a very simple query: SELECT ct.* FROM Person.ContactType AS ct WHERE ct.ContactTypeID = 7; This query re...

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Saving Execution Plans on Azure SQL Database

In my previous post showing how to get to execution plans in the Database Management Portal, I showed that it’s pretty easy to put a query in a query window and get the execution plan. This allows you to understand query behavior in order to tune your T-SQL or your data structures, all through the […]

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How to Drop One Plan from Cache

While presenting this weekend at SQL Saturday #117 in Columbus, OH (great event, if you missed it, you missed it), I had what I thought was a little piece of throw-away code, but several people from the audience asked about it. Here it is: DBCC FREEPROCCACHE(0x05000700618F532C40E190CE000000000000000000000000) ; Not much to it is there? The trick [&...

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Changing DB_CHAIN Can Clear the Plan Cache

If you make changes to the settings of a database, it can cause the procedure cache to be cleared. Microsoft has documented changes that cause this for all procs within a database (scroll down to just above the examples). But guess what, if you change the DB_CHAINING option, it clears the cache too. Here’s a […]

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Ad Hoc Queries Don't Reuse Execution Plans: Myth or Fact

Another frequently heard story is that stored procedures get and reuse execution plans, but ad hoc queries do not. A lot of people believe this and say as much online and in the real world. Unlike my last myth, this time, I’m going to give you the DBA answer to this question. It depends. There […]

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Procedure Cache and Dynamic Management Views

I’m just going through the chapter on the procedure cache in the new book and I’m having a blast playing with the dynamic management views and functions that let you access the procedure cache. It’s just too much fun and way too easy to get detailed information about the queries in the system, not like […]

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nHibernate Recompiles and Execution Plans

One little potential for performance problems that we’ve seen comes out of how nHibernate generates it’s parameratized queries. It limits the length of any parameter to the length of the column, but if the length of that parameter is less than the column, it uses tha smaller length when declaring the variable. This results in […]

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