05Apr 2021 by Grant Fritchey 1 Comment

Extended Events: Embrace the XML

SQL Server
While XML is, without a doubt, a giant pain in the bottom, sometimes, the best way to deal with Extended Events is to simply embrace the XML. Now, I know, just last week, I suggested ways to avoid the XML. I will freely admit, that is my default position. If I can avoid the XML, I will certainly do it. However, there are times where just embracing the XML works out nicely. Let's talk about it a little. Copy This Query I have a theory. It goes like this: There has only, ever, been a single XML query written from scratch. All other XML queries are just copied from that one and then edited to do what is necessary OK. Maybe that's not entirely true. In fact, I know it's…
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18Mar 2014 by Grant Fritchey 2 Comments

Finding Mistakes

SQL Server, T-SQL
Ever had that moment where you start getting errors from code that you've tested a million times? I had that one recently. I had this little bit of code for pulling information directly from query plans in cache: WITH XMLNAMESPACES(DEFAULT N'http://schemas.microsoft.com/sqlserver/2004/07/showplan'), QueryPlans AS ( SELECT RelOp.pln.value(N'@PhysicalOp', N'varchar(50)') AS OperatorName, RelOp.pln.value(N'@NodeId',N'integer') AS NodeId, RelOp.pln.value(N'@EstimateCPU', N'decimal(10,9)') AS CPUCost, RelOp.pln.value(N'@EstimateIO', N'decimal(10,9)') AS IOCost, dest.text FROM sys.dm_exec_query_stats AS deqs CROSS APPLY sys.dm_exec_sql_text(deqs.sql_handle) AS dest CROSS APPLY sys.dm_exec_query_plan(deqs.plan_handle) AS deqp CROSS APPLY deqp.query_plan.nodes(N'//RelOp') RelOp (pln) ) SELECT qp.OperatorName, qp.NodeId, qp.CPUCost, qp.IOCost, qp.CPUCost + qp.IOCost AS EstimatedCost FROM QueryPlans AS qp WHERE qp.text = 'some query or other in cache' ORDER BY EstimatedCost DESC; I've probably run this... I don't know how many times. But... I'm suddenly getting an error: Msg 8114, Level 16, State 5,…
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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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25Oct 2011 by Grant Fritchey 6 Comments

SQL Server XQuery Against Execution Plans

SQL Server, T-SQL
One of the greatest things about all the DMOs is how you can combine the information they present to you with execution plans. It allows you to see what a query is doing, even as it’s executing, because the plan is created first. I couldn’t possibly emphasize enough how important that’s going to be in your day-to-day troubleshooting. Even better is the fact that you’re going to be able to make use of XQuery to pull useful information out of the execution plans that are in cache. Or are you? Here’s a query to pull some information out of the procedure cache: SELECT  deps.type_desc, deps.last_execution_time, deps.execution_count, deps.total_logical_reads, dest.encrypted AS EncryptedText, dest.text, deqp.query_plan, deqp.encrypted AS EncryptedPlan FROM    sys.dm_exec_procedure_stats AS deps CROSS APPLY sys.dm_exec_sql_text(deps.sql_handle) AS dest CROSS APPLY sys.dm_exec_query_plan(deps.plan_handle) AS deqp WHERE  …
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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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30Nov 2009 by Grant Fritchey 6 Comments

SQL Server XQuery For Idiots

SQL Server, T-SQL
I'm still struggling with learning XQuery. My latest little revelation was small, but vital. The difference between: @inXML.nodes('/rss/channel/item') and @inXML.nodes('/rss[1]/channel[1]/item') Is the difference between a 10 minute query and a 4 second query. Kind of shocking really. My understanding is that XQuery assumes there are multiple possible paths that look like "/rss/channel" so it searches over and over again through the XML to see if it can find them. But by identifying it as a root, showing that only one possible path is available, it stops stumbling around in the dark and simply reads the data. It's a little thing, but it made an enormous difference. I've still got a long way to go in learning how to use XPath within XQuery. UPDATED: I modified the title so that it's…
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18Feb 2009 by Grant Fritchey 24 Comments

Refining the Missing Index Data

SQL Server, T-SQL
In my previous post I showed how you could determine if a query had missing index information, but it was difficult to get meaningful data out because I didn't know XQuery well enough. I spent some more time refining the query and here are the results. This new query will show some basic performance information gathered from the aggregate data in sys.dm_exec_query_stats. It combines this with the full data pulled from the Missing Indexes element in the XML of the execution plan. I've got it listing all the recommended columns and grouping. So this means that the performance data is repeated in order to allow for the full listing of groups & columns. It seems to work well. A couple of interesting points. My purpose is to provide a short-cut…
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12Feb 2009 by Grant Fritchey 11 Comments

Missing Index Information and Query Stats

SQL Server, T-SQL
So the goal was to find a way to pull information from sys.dm_exec_query_stats so that we could identify poor performing procedures that were in cache at the moment and combine it with missing index information from sys.dm_db_missing_index_details. We're combining these because we're working with a Microsoft Dynamics CRM database that is almost all ad hoc queries and lots of them are against tables with missing indexes. The hope was to identify necessary indexes merely by looking at the longest running queries. Unfortunately there is no way to combine data from the missing indexes set of DMV's and all the execution DMV's that show query stats, execution plan, etc. None of the missing index tables has a plan handle or a plan hash column that would allow you to combine that…
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