11Mar 2019 by Grant Fritchey 4 Comments

Missing Indexes in the Query Store

SQL Server 2016, SQL Server 2017, T-SQL, Tools
I've shown before how to use the DMVs that read the plan cache as a way to connect the missing indexes suggestions with specific queries, but the other place to find missing index suggestions is the Query Store. Pulling from the Query Store The plans stored in the Query Store are exactly the same as the plans stored within the plan cache. This means that the XML is available and you can try to retrieve information from it directly, just as we did with the missing index queries against the DMVs. Here's the query modified for the Query Store: WITH XMLNAMESPACES (DEFAULT 'http://schemas.microsoft.com/sqlserver/2004/07/showplan') SELECT qsqt.query_sql_text, rts.plan_id, rts.NumExecutions, rts.MinDuration, rts.MaxDuration, rts.AvgDuration, rts.AvgReads, rts.AvgWrites, qsp.QueryPlan, qsp.QueryPlan.value(N'(//MissingIndex/@Table)[1]', 'NVARCHAR(256)') AS TableName, qsp.QueryPlan.value(N'(//MissingIndex/@Schema)[1]', 'NVARCHAR(256)') AS SchemaName, qsp.QueryPlan.value(N'(//MissingIndexGroup/@Impact)[1]', 'DECIMAL(6,4)') AS ProjectedImpact, ColumnGroup.value('./@Usage', 'NVARCHAR(256)') AS ColumnGroupUsage, ColumnGroupColumn.value('./@Name',…
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23Jan 2019 by Grant Fritchey 2 Comments

All Day, Training Day at SQLBits

Azure, SQL Server 2016, SQL Server 2017, T-SQL
It's a somewhat late addition, but I have an all-day Training Day at SQLBits. It takes place on Thursday, February 28th. You can read all about it on the SQLBits web site. I want to take a moment here to expand on the information that we're going to cover. I think the abstract does a good job of conveying what we'll be doing all day, but I figured a little more detail won't hurt. Query Tuning is Hard This is the very first thing I talk about. Query tuning is hard. I've got a nearly 1,000 page book on the topic, which should give you an idea of just how much material there is to cover. With the training day I've decided to focus on the tools that Microsoft gives…
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29Oct 2018 by Grant Fritchey 14 Comments

Explicitly Drop Temporary Tables Or Wait For Cleanup?

SQL Server, SQL Server 2016, SQL Server 2017, T-SQL
I was recently asked if we are going to see performance differences if we explicitly drop temporary tables. I couldn't remember the specifics, but I said it actually didn't matter. However, that answer has bugged me, so I set up a quick test. Explicitly Drop Temporary Tables We could make this a crazy set of tests, but I wanted to keep things relatively simple. I created two procedures that create identical temporary tables. One drops the tables, the other doesn't: CREATE PROC dbo.BOMDrop AS CREATE TABLE #BOMData (BOMLevel SMALLINT, PerAssemblyQty DECIMAL(8, 2), ComponentName VARCHAR(50), ProductAssemblyName VARCHAR(50), UnitMeasure VARCHAR(50)); INSERT #BOMData (BOMLevel, PerAssemblyQty, ComponentName, ProductAssemblyName, UnitMeasure) SELECT bom.BOMLevel, bom.PerAssemblyQty, c.Name, pa.Name, um.Name FROM Production.BillOfMaterials AS bom JOIN Production.Product AS pa ON pa.ProductID = bom.ProductAssemblyID JOIN Production.Product AS c ON c.ProductID =…
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11Jun 2018 by Grant Fritchey 6 Comments

Plan Metrics Without the Plan: Trace Flag 7412

SQL Server, SQL Server 2016, SQL Server 2017, T-SQL
I place a lot of emphasis on capturing actual execution plans because of the runtime metrics, but with Trace Flag 7412, we don't need the plan. This is great news, because capturing execution plans, even using extended events, is an expensive proposition. However, using either the query_thread_profile event, or, Trace Flag 7412, we can get the runtime metrics without the plan. Trace Flag 7412 Here's how it works.You can either be running the extended event, query_thread_profile (a debug event, but one documented and supported by Microsoft) or, enable the Trace Flag 7412. I like to use the extended event in a targeted fashion to easily see behaviors on a query without having to capture the plan. You can even capture the information and then combine it with an estimated plan…
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30Apr 2018 by Grant Fritchey 5 Comments

Automating Automatic Indexing in Azure SQL Database

Azure
I've been in love with the concept of a database as a service ever since I first laid eyes on Azure SQL Database. It just makes sense to me. Take away the mechanics of server management and database management. Focus on the guts of your database. Backups, consistency checks, these easily automated aspects can just be taken care of. Same thing goes with some, not all, but some, index management. Azure SQL Database can manage your indexes for you. I call it weaponizing Query Store. Anyway, I needed a way to automate this for the book I'm writing. I couldn't find any good examples online, so I built my own. Queries in Need of Automatic Indexing Because I want this to be as simple and repeatable as possible, I'm using…
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27Feb 2018 by Grant Fritchey 6 Comments

Adaptive Joins

T-SQL
I was surprised to find out that a lot people hadn't heard about the new join type, Adaptive join. So, I figured I could do a quick overview. Adaptive Join Behavior Currently the adaptive join only works with columnstore indexes, but according to Microsoft, at some point, they will also work with rowstore. The concept is simple. For larger data sets, frequently (but not always, let's not try to cover every possible caveat, it depends, right), a hash join is much faster than a loops join. For smaller data sets, frequently, a loops join is faster. Wouldn't it be nice if we could change the join type, on the fly, so that the most effective join was used depending on the data in the query. Ta-da, enter the adaptive join.…
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15Aug 2017 by Grant Fritchey 11 Comments

Bad Parameter Sniffing Decision Flow Chart

SQL Server, SQL Server 2016
Lots of people are confused by how to deal with bad parameter sniffing when it occurs. In an effort to help with this, I'm going to try to make a decision flow chart to walk you through the process. This is a rough, quite rough, first draft. I would love to hear any input. For this draft, I won't address the things I think I've left out. I want to see what you think of the decision flow and what you think might need to be included. Click on it to embiggen. Thanks to the attendees at my SQLSaturday Louisville pre-con for the great questions and the inspiration to get this done. Thank you in advance for any and all feedback.
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13Mar 2017 by Grant Fritchey 9 Comments

Why You Should Change the Cost Threshold for Parallelism

SQL Server, SQL Server 2016
I've written several times about the Cost Threshold for Parallelism and it's relationship to your execution plans, how to determine your execution plan cost, and even how to decide what value to set your Cost Threshold to. What I haven't explicitly addressed in extremely clear terms is why you should adjust your Cost Threshold for Parallelism. There are two reasons to modify this value. Cost Threshold for Parallelism Default Value The primary reason to change the Cost Threshold for Parallelism is because the default value is not a good choice for the vast majority of systems. The default value is 5. This means that when a query has an estimated cost greater than 5, it may get a parallel execution plan. Microsoft set the default value for the Cost Threshold…
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23Nov 2016 by Grant Fritchey 5 Comments

PowerShell to Test a Query

DevOps, PowerShell, SQL Server, SQL Server 2016, T-SQL
So you want to do some tuning, but you're not sure how to test a query on it's performance. Not a problem. Here's a very rough script that I use to do some recent testing. This script to test a query is post #11 of the #enterylevel #iwanttohelp effort started by Tim Ford (b|t). Read about it here. The Script The goal here is to load a bunch of parameter values from one table and then use those values to run a query to test it. To do this I connect up to my SQL Server instance, naturally. Then I retrieve the values I'm interested in. I set up the query I want to test. Finally a loop through the data set, calling the query once for each value. [reflection.assembly]::LoadWithPartialName("Microsoft.SqlServer.Smo") | out-null #…
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