03Jan 2022 by Grant Fritchey 4 Comments

Identifying Recompile Causes

SQL Server
Strictly speaking, a recompile isn't really a performance tuning problem. In fact, a lot of time, recompiles are desired because they reflect changes in statistics which are likely to need a new plan. However, you can get really excessive levels of recompiles because of a variety of different issues. So, identifying the causes can be a pain. Here's one way to get it done. Extended Events for Recompile Causes You knew I was going there. Seriously though, we know that, since SQL Server 2005, all recompiles are at the statement level (unless you call for a recompile of a procedure or create that proc with a RECOMPILE hint). So, capturing the recompiles and getting grotty on the details means capturing statements. Understand that this can be expensive, especially in terms…
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02Dec 2019 by Grant Fritchey 5 Comments

Tracking CPU Use Over Time

SQL Server
A question that I've seen come up frequently just recently is, how to track CPU use over time. Further, like a disk filling up, people want to know how to predict their CPU usage, so that they can easily decide "now is when I upgrade the hardware". Well, the bad news is, that ain't easy. CPU Use Over Time There are a bunch of ways to look at processor usage. The simplest, and probably most common, is to use the Performance Monitor counters such as '% Processor Time'. Query this, you can get an average of the processor usage at a moment in time. Ta-da! Fixed it. I thought you said this was hard Grant. Well, hang on. Are you running on a single processor machine? If so, cool, maybe…
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12Dec 2016 by Grant Fritchey 7 Comments

OPTIMIZE FOR Hints When Parameter Sniffing is Turned Off

Azure, SQL Server, SQL Server 2016, T-SQL
While presenting recently and talking about dealing with bad Parameter Sniffing, I got the question; what happens to OPTIMIZE FOR hints when parameter sniffing is disabled? This is my favorite kind of question because the answer is simple: I don't know. Parameter Sniffing For those who don't know, parameter sniffing is when SQL Server uses the precise values passed into a query as a parameter (this means stored procedures or prepared statements) to generate an execution plan from the statistics using the value from the parameter. Most of the time, parameter sniffing is either helping you, or is not hurting you. Sometimes, parameter sniffing turns bad and hurts you quite severely. Usually, but not always, this is because you either have severely skewed data (some data is very different than the rest, lots…
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24Feb 2016 by Grant Fritchey 6 Comments

Query Store and Recompile

Azure, SQL Server 2016, T-SQL
One of the many advantages of SQL Cruise is the ability to have enough time during a presentation to be able to answer questions from the people there in great detail. One question came up while I was showing the new functionality of Query Store (available soon in SQL Server 2016, available right now in Azure SQL Database). What happens to plan forcing when you have OPTION RECOMPILE on a query? Great question. I have a favorite procedure I use to illustrate the functionality of parameter sniffing: ALTER PROC dbo.AddressByCity @City NVARCHAR(30) AS SELECT a.AddressID, a.AddressLine1, a.AddressLine2, a.City, sp.Name AS StateProvinceName, a.PostalCode FROM Person.Address AS a JOIN Person.StateProvince AS sp ON a.StateProvinceID = sp.StateProvinceID WHERE a.City = @City; If this procedure is called with the value of 'Mentor' you get…
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