18Sep 2013 by Grant Fritchey 17 Comments

Finding Ad Hoc Queries with Query Hash

SQL Server, T-SQL
I was presenting a session on how to read execution plans when I received a question: Do you have a specific example of how you can use the query hash to identify similar query plans. I do, but I couldn't show it right then, so the person asking requested this blog post. If you're dealing with lots of application generated, dynamic or ad hoc T-SQL queries, then attempting to determine tuning opportunities, missing indexes, incorrect structures, etc., becomes much more difficult because you don't have a single place to go to see what's happening. Each ad hoc query looks different... or do they. Introduced in SQL Server 2008 and available in the standard Dynamic Management Objects (DMO), we have a mechanism to identify ad hoc queries that are similar in…
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23Jan 2013 by Grant Fritchey 2 Comments

Saving Execution Plans on Azure SQL Database

Azure
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 Azure interface. But, what happens if you want to share an execution plan with a friend, post it to an online forum, save it for later comparisons as part of troubleshooting bad parameter sniffing, track behaviors over time as statistics change, other purposes that I can't think of at the moment? To first answer this question, let me tell you how you would do these things in SQL Server Management Studio (SSMS). First, and most…
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02Jul 2012 by Grant Fritchey 12 Comments

Querying Information from the Plan Cache, Simplified

SQL Server, T-SQL
One of the great things about the Dynamic Management Objects (DMOs) that expose the information in plan cache is that, by their very nature, they can be queried. The plans exposed are in XML format, so you can run XQuery against them to pull out interesting information. For example, what if you wanted to see all the plans in cache that had a Timeout as the reason for early termination from the optimizer? It’d be great way to see which of your plans were less than reliable. You could so like this: 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…
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26Mar 2012 by Grant Fritchey 15 Comments

Execution Plan for a User Defined Function

SQL Server, T-SQL
When you execute a multi-statement user-defined function you may see an execution plan that looks something like this: It appears as if the cost of the UDF is free. This is especially true if you use the UDF in a query with other objects, such as joining it to actual tables. Since the optimizer always assumes a multi-statement UDF has a single row for statistics estimates, it' frequently displays a low cost. But you know that there’s more going on there, right? It’s a multi-statement UDF because it’s doing a lot of work, but that is not reflected in the execution plan.. or is it? What if we went after the cache? Let’s run this little query: SELECT deqp.query_plan, dest.text, SUBSTRING(dest.text, (deqs.statement_start_offset / 2) + 1, (deqs.statement_end_offset - deqs.statement_start_offset) /…
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25May 2011 by Grant Fritchey 26 Comments

Oh ****!

SQL Server
Did you ever accidentally close SQL Server Management Studio? And, in closing SSMS, did you get the prompt that says “Save changes to the following items?” And did you, completely unthinkingly, with a query you had just been working on, hit Cancel? Yeah, me neither. What kind of idiot does that…. OK. I confess. I just did that. Silly thing it was, but I had just spent at least 1/2 an hour working on a query and now it was gone…. or was it? I had just run the query and had been looking at the results when I closed SSMS. Initially, I panicked and started thinking about how I could get the data back (somewhere there’s a file I’ve heard). Then it occurred to me, I had just been…
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23May 2011 by Grant Fritchey 6 Comments

SQL Azure Query Tuning

Azure
SQL Azure is still SQL Server at the end of the day. This means it is entirely possible to write queries against SQL Azure that really… what’s a good word… stink. So what do you do? It’s all in the cloud. You couldn’t possibly tune the queries, right? Wrong. Many of the same tools that you have available to you, such as execution plans and dynamic management objects, are still available in SQL Azure. Let’s talk DMOs for a second. First off, don’t make the mistake I did of trying to run these outside the context of a specific database on SQL Azure. You’ll get extremely inconsistent results, trust me on this. Anyway, I did a quick run-down on some of the most used DMOs for performance tuning, the sys.dm_exec_*…
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06Apr 2011 by Grant Fritchey 4 Comments

SQL University: Index Usage

SQL Server, T-SQL
Greetings. Welcome once more to the Miskatonic University branch of SQL University. Does anyone know where to buy some camping equipment, cheap? I’ve been tagged to go an expedition to Antarctica and I need some cold weather gear a bit more substantial than my LL Bean boots. Evidently the last expedition found some caves in some mountains down there. Sounds like the perfect place to get away from all the crazy stuff that goes on here at Miskatonic. I mean, what could happen? Anyway, our last several talks have all been about indexes and indexing. One of the things that we haven’t talked about is how to tell if, how or when your indexes are being used. Starting with SQL Server 2005, and continuing to 2008 and R2, there has…
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05Jan 2011 by Grant Fritchey 4 Comments

Encryption and the Performance DMOs

T-SQL
Ever wonder what you can see in the performance oriented DMOs when stored procedures were encrypted? Me neither. But, I did get that question during my DMO presentation at the PASS Summit. I did not have an answer. I did get an answer from Johan Bijnens (twitter) from the audience, which I repeated without entirely knowing what I was saying. I decided that I ought to actually know the answer to that question, so here’s a little experiment. I'm going to create a simple stored procedure: CREATE PROCEDURE dbo.GetSalesDetails (@SalesOrderId INT) AS SELECT soh.AccountNumber, sod.LineTotal FROM Sales.SalesOrderHeader AS soh JOIN Sales.SalesOrderDetail AS sod ON soh.SalesOrderID = sod.SalesOrderID WHERE soh.SalesOrderID = @SalesOrderID When I create this procedure and run it, you can see the general performance of the query being run…
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04Oct 2010 by Grant Fritchey 1 Comment

How to Tell if Execution Plans are Reused

SQL Server, T-SQL
I try to watch the search phrases that point people to the blog because sometimes, you get a sense of what problems people are running into. The latest question or phrase I've seen a lot lately is along the lines of "how do you know if an execution plan is being reused." Since compiling an execution plan can be an extremely expensive operation, it's worth your time to understand how well a given plan is getting reused. If you've seen me present, I'll frequently talk about the application that had a query with an 86 table join. Recompiles on that thing were frequent and extremely costly. The only good news was, they were recompiles. If we weren't getting plan reuse it would have been an even worse system than it was. There are…
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18Mar 2010 by Grant Fritchey 5 Comments

Undocumented Virtual Column: %%lockres%

SQL Server, T-SQL
One of my development teams needed a mechanism for identifying the value of a key that was part of a lock (don't ask). I'd never tried doing that before. Obviously if you hit the DMV sys.dm_tran_locks you can see the hash of the key in the resource_description column. But how to pull the value back. After some research, I first found this excellent article by the late, great, Ken Henderson (I really wish he was still around). The article outlined, among other things, the use of an undocumented "virtual" column called %%lockres%%. Some more searching then uncovered this great article by James Rowland-Jones, AKA Claypole. He described how, in a very high volume system, he used %%lockres%% to identify the source of a deadlock as the internal mechanisms that SQL Server uses to manage locks, the hash…
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