22Oct 2018 by Grant Fritchey 6 Comments

Why Did a Plan Get Removed From Cache?

SQL Server, SQL Server 2016, SQL Server 2017, T-SQL
I was recently asked if we could tell why a plan was removed from cache. If you read this blog, you know what I'm going to say next. I checked the extended events and there are actually two different events that will tell us information about a plan removed from cache; sp_cache_remove and query_cache_removal_statistics. Let's talk about how these work. Removed From Cache Just so we can see ALL the activity, I'm creating an Extended Events session that captures a little more than just the two events: CREATE EVENT SESSION PlanCacheRemoval ON SERVER ADD EVENT sqlserver.query_cache_removal_statistics (WHERE (sqlserver.database_name = N'AdventureWorks2017')), ADD EVENT sqlserver.rpc_completed (WHERE (sqlserver.database_name = N'AdventureWorks2017')), ADD EVENT sqlserver.rpc_starting (WHERE (sqlserver.database_name = N'AdventureWorks2017')), ADD EVENT sqlserver.sp_cache_hit (WHERE (sqlserver.database_name = N'AdventureWorks2017')), ADD EVENT sqlserver.sp_cache_insert (WHERE (sqlserver.database_name = N'AdventureWorks2017')), ADD EVENT…
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10Sep 2018 by Grant Fritchey 1 Comment

Estimated Plans and Forced Plans from Query Store

SQL Server 2016, SQL Server 2017, T-SQL
While all plans are estimated plans, there is still a difference between capturing an estimated plan and looking at a plan from the cache or from query store. Or is there? A question came up during a recent presentation; what happens to capturing an estimated plan when you're forcing plans? Let's find out. The answer is interesting. Estimated Plans Here's my stored procedure that I'll be using with AdventureWorks2017: CREATE OR ALTER PROC dbo.ProductTransactionHistoryByReference (@ReferenceOrderID INT) AS BEGIN SELECT p.Name, p.ProductNumber, th.ReferenceOrderID FROM Production.Product AS p JOIN Production.TransactionHistory AS th ON th.ProductID = p.ProductID WHERE th.ReferenceOrderID = @ReferenceOrderID; END; For reasons I'll explain in a bit, I'm going to free the procedure cache: ALTER DATABASE SCOPED CONFIGURATION CLEAR PROCEDURE_CACHE; Then, if I capture an estimated plan for two different values:…
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13Feb 2017 by Grant Fritchey 2 Comments

Export All Plans From Cache to a .SQLPLAN File

PowerShell
I was asked if there was an easy way to export all the plans from cache to a .SQLPLAN file. My immediate answer was, "I'm sure there's a PowerShell script out there somewhere." However, rather than do a Gingle search, I figured I'd quickly knock up an example. The Script I've gone minimal on the script. I'm creating a connection to the local instance, defining a command, and returning the data into a data set. From there, since the data set consists of a single column, I'm walking through them all to export out to a file: $Query = 'SELECT deqp.query_plan FROM sys.dm_exec_query_stats AS deqs CROSS APPLY sys.dm_exec_query_plan(deqs.plan_handle) AS deqp WHERE deqp.query_plan IS NOT NULL;' $SqlConnection = New-Object System.Data.SqlClient.SqlConnection $SqlConnection.ConnectionString = 'Server=ServerX\DOJO;Database=master;trusted_connection=true' $PlanQuery = new-object System.Data.SqlClient.SqlCommand $PlanQuery.CommandText = $Query $PlanQuery.Connection…
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24Aug 2015 by Grant Fritchey 9 Comments

Targeted Plan Cache Removal

Azure, T-SQL
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…
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30Apr 2012 by Grant Fritchey No Comments

Changing DB_CHAIN Can Clear the Plan Cache

T-SQL
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 sample script to show it in action. ALTER DATABASE Testing SET DB_CHAINING OFF; GO CREATE PROCEDURE x AS SELECT * FROM test.dbo.A AS a2; GO CREATE PROCEDURE y AS SELECT * FROM dbo.Table_1 AS t; GO EXEC dbo.x; EXEC dbo.y; SELECT deqs.creation_time FROM sys.dm_exec_query_stats AS deqs CROSS APPLY sys.dm_exec_sql_text(deqs.sql_handle) AS dest WHERE dest.text LIKE 'CREATE PROCEDURE x%' OR dest.text LIKE 'CREATE PROCEDURE y%'; ALTER DATABASE Testing SET DB_CHAINING ON; SELECT deqs.creation_time FROM sys.dm_exec_query_stats AS deqs…
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30Oct 2008 by Grant Fritchey No Comments

Procedure Cache and Dynamic Management Views

SQL Server, T-SQL
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 the old days. First, you can access the cache itself with the DMV, sys.dm_exec_cached_plans. This shows some of the data describing the plan in cache, but most importantly it provides the plan_handle. You need this for other joins later. You can also use sys.dm_exec_query_stats to get aggregated performance statistics about the plan. It also has the plan_handle and two things new to SQL Server 2008, the query_hash and the query_plan_hash, also known as query fingerprints.…
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