22Sep 2011 by Grant Fritchey 5 Comments

SQL Server Execution Plans

PASS, Professional Development, SQL Server
I write quite frequently about SQL Server Execution Plans. I started in that area just because that’s how you figure out what a query is doing and sooner or later, we all have to tune a query. I found I was doing it sooner and more frequently. When the opportunity came up to write a book , I jumped on it. Now I find myself presenting, rather frequently, on execution plans. One of the people I’ve learned from over the last several years is Gail Shaw (blog|twitter). I first saw Gail on stage at the PASS Summit, I think it was 2007. A co-worker of mine was picked, along with Gail, to go on stage for the Quiz Bowl. Gail was answering all the questions. If you go over to…
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20Sep 2011 by Grant Fritchey 17 Comments

Statistics in Execution Plans

SQL Server, T-SQL
I was presenting on execution plans when another question came up that I didn’t know the answer to immediately. Yes, I know you’ve seen that phrase before on this blog. I love presenting because you get exactly the kinds of questions that make you think and make you learn. I’m presenting, in part, to learn, just as much as I am to teach. It was the same with kenpo. The more I taught, the better I learned the art. Wait, this isn’t supposed to be a blog post about learning. This one is about statistics. The question was, does the execution plan have the statistics that were used by the optimizer to decide on the execution plan. And no, what was meant, was not does it show the estimated rows,…
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15Sep 2011 by Grant Fritchey 6 Comments

Probe Residual on Hash Match

SQL Server, T-SQL
I have to say, I only recently noticed this on a tool tip: and this in the property sheet: The bad news is, I noticed them while presenting. The worse news is, I said them out loud and then, inevitably, someone in the audience, I forget who it was, might have been Neil Hambly (blog|twitter) said, “What’s that?” Which left me standing there with a slack-jawed expression (one that comes naturally from years & years of practice). I didn’t know. I couldn't remember having seen one before. Here’s a query that you can run in AdventureWorks2008R2 to get a look at this critter: SELECT  soh.PurchaseOrderNumber, soh.AccountNumber, p.Name, sod.OrderQty, sod.LineTotal, cc.CardNumber FROM    Sales.SalesOrderHeader AS soh JOIN Sales.SalesOrderDetail AS sod ON soh.SalesOrderID = sod.SalesOrderID JOIN Production.Product AS p ON sod.ProductID = p.ProductID…
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22Aug 2011 by Grant Fritchey 3 Comments

Expert? Ha!

SQL Server, T-SQL
How do you define an expert? My personal definition: An expert is the person that is a chapter ahead of you in the book. Why am I talking about this? Just that I’m feeling more stupid than usual lately. In the last two weeks I’ve had people bring up through various discussions, documents, what have you, four different SQL Server trace flags that will affect how SQL Server builds execution plans and I’ve never heard of any of them before. I’ve never, ever, thought of myself as an expert in execution plans, despite having written a book about them. I just thought I had a good grasp on how they worked and I was willing to share. I didn’t know everything and never pretended to, but I thought I knew…
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15Jun 2011 by Grant Fritchey 9 Comments

Rebind and Rewind in Execution Plans

T-SQL
Ever looked at an execution plan? If you answered no, you can’t possibly have ever tried to tune a query, or, you’re doing it wrong. For every one else, no doubt you’ve looked at the tool tips or the property sheets of an operator and you’ve seen the Rebind & Rewind properties and wondered what the heck they mean. Me too. I learned as much as I could for the book on execution plans and I spent two pages describing it. Then, a little while ago, on the SQL Cruise, someone asked me to describe them and I was flummoxed. Specifically they said the explanation in the book was insufficiently clear, so I promised to put together a blog post on the topic, both to attempt to clarify my explanation…
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30May 2011 by Grant Fritchey 7 Comments

Optimizer Timeouts with XQuery

SQL Server
I was looking at performance of a database and I noticed a few of the plans were very large and timing out in the optimizer. This made me wonder, just how many of them were timing out? This sounds like a job for XQuery! There’s really nothing to it. Once you start plucking stuff out of the execution plans using XQuery, it’s kind of hard to stop. So here’s my little bit of code. WITH XMLNAMESPACES(DEFAULT N'http://schemas.microsoft.com/sqlserver/2004/07/showplan'),  QueryPlans AS  (  SELECT  RelOp.pln.value(N'@StatementOptmEarlyAbortReason', N'varchar(50)') AS TerminationReason,         RelOp.pln.value(N'@StatementOptmLevel', N'varchar(50)') AS OptimizationLevel,         --dest.text,         SUBSTRING(dest.text, (deqs.statement_start_offset / 2) + 1,                   (deqs.statement_end_offset - deqs.statement_start_offset)                   / 2 + 1) AS StatementText,         deqp.query_plan,         deqp.dbid,         deqs.execution_count,         deqs.total_elapsed_time,         deqs.total_logical_reads,         deqs.total_logical_writes FROM    sys.dm_exec_query_stats AS deqs         CROSS APPLY sys.dm_exec_sql_text(deqs.sql_handle)…
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14Mar 2011 by Grant Fritchey 1 Comment

ANSI Connection Settings

SQL Server, T-SQL
It’s reasonably well known that you can get different execution plans if you change the ANSI connection settings. But the question comes up, fairly often, how do you know what the settings are. It’s actually surprisingly simple. They’re stored right inside the execution plan, but they’re in one of the operators that most people ignore. Heck, I used to ignore this operator. Which operator is it you ask? Let’s find out. Let’s use AdventureWorks2008R2 (because I’m lazy). We’ll call one of the stored procedures there like so: EXEC dbo.uspGetBillOfMaterials @StartProductID = 0, -- int     @CheckDate = '2011-03-10 02:31:39' – datetime If you execute this with “Include Actual Execution Plan” enabled you’re likely to end up with the following execution plan: Don’t worry about the fact that you can’t really…
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07Mar 2011 by Grant Fritchey 1 Comment

Execution Plan Stability

SQL Server
I’ve talked before about one of the primary things that the Query Optimizer team at Microsoft tries to avoid, regressions. Basically, they want a plan that worked well in SQL Server 2005 to work well in SQL Server 2008 R2. What’s more, they want everything to work well between service packs, updates and cumulative updates. Ever wonder how they do it? Well, they cheat. OK, that’s mean and not entirely accurate, but it gets the idea across. No, what they do is, identify when they have a breaking change, when they’ve got a special cumulative update or service pack that fixes some bad behavior, but that can cause plans to “break,” they wall it off. Note, in most cases, this “breakage” is actually a question of plans working correctly, but…
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27Sep 2010 by Grant Fritchey 3 Comments

Parameter Name Size And Performance

SQL Server, T-SQL
I saw an odd statement the other day, "The size of the name of the parameter does not affect performance." My first thought was, "Well, duh!" But then, I had one of those, "Ah, but are you sure" thoughts. And you know what, I wasn't sure. If size of the parameter name did affect performance, I figured, the one sure place where that would be evident is in the size of the execution plan. Right? I mean, if there was an impact on memory, and hence on performance, that's probably where you'd see evidence of it. I wrote two queries: DECLARE @ThisIsAVeryVeryLongParameterNameThatIsTrulyRidiculousButItIllustratesThePointThatParameterLengthDoesNotAffectPerformance int SET @ThisIsAVeryVeryLongParameterNameThatIsTrulyRidiculousButItIllustratesThePointThatParameterLengthDoesNotAffectPerformance = 572 SELECT soh.SalesOrderID ,sod.SalesOrderDetailID FROM Sales.SalesOrderHeader AS soh JOIN Sales.SalesOrderDetail AS sod ON soh.SalesOrderID = sod.SalesOrderID WHERE soh.SalesOrderID = @ThisIsAVeryVeryLongParameterNameThatIsTrulyRidiculousButItIllustratesThePointThatParameterLengthDoesNotAffectPerformance DECLARE @v int SET…
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12Jan 2010 by Grant Fritchey 4 Comments

TSQL Tuesday #2

SQL Server, T-SQL
I've wracked my brain for some bit of puzzle that I could present as part of TSQL Tuesday #2 and I finally came up with a bit of something. When you're looking at an execution plan for a query, you know that this represents SQL Servers best attempt at a good execution plan. But, it may not represent the best possible plan. Or, it could be the only possible plan. The puzzle is, how do you know what you're looking at? Is this a trivial plan, meaning it's the only possible execution method for the query? Is this plan fully optimized, or did the optimizer go through it's prescribed cycles and simply take the best plan it had generated up to that point? These questions can be answered directly from information…
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