Finding Memory-Rich Queries

Matthew McGiffen wants to find the queries which demand the largest memory grants:

I had a server that looked like it had been suffering from memory contention. I wanted to see what queries were being run that had high memory requirements. The problem was that it wasn’t happening right now – I needed to be able to see what had happened over the last 24 hours.

Enter Query Store. In the run-time stats captured by Query Store are included details relating to memory.

Click through for a script which retrieves this data over a time frame.

Query Store Required Permissions

Andreas Wolter takes us through exactly which permissions are required for Query Store to work:

Typically, there are 3 aspects to the work with the Query Store, which can be reflected in roles:
1) Configuration – turning Query Store on and off, clearing the contents, flushing its contents to disk and changing its settings.
2) Viewing the reports or using the DMVs to analyze the Query Store contents (queries, plans and wait statistics) to gain insights but not necessarily having the authority to change anything
3) Actively change Plans by forcing or un-forcing, based on the information obtained from (2)

This is a nice overview of the problem and a fair amount of the solution.

Changing Query Store Report Interval

Arun Sirpal wants to change the report interval for a Query Store report:

While not specific to SQL Server 2019 (I was using this version to do some testing) I was struggling to find how to change the time period of analysis for the Query Store reports within SSMS.

This is not a ground breaking post but hopefully a helpful one! So, I load up the “Top 25 resource consumers” report and by default it will show data for the past hour. So what do you do, or should I say what do you click to change the time interval for the report?

Read on for the two screenshots which answer this question for you.

Best Practices for Query Store

Erin Stellato ties together a number of best practices around Query Store:

I’m a huge fan of Query Store, which regular readers may know, but there’s a need to write a bit more about Query Store best practices.  This isn’t a “you must use this feature” post, this is a “here is what you must know if you want to use this feature” post.

I have a lot of content about Query Store, but maybe what’s really important gets lost amongst everything else.  Glenn prompted me to write this, after I worked with two customers last week that ran into issues related to their configuration of Query Store.  Listed below are the things you must know before you enable Query Store.  If you already have Query Store enabled and running, I recommend reviewing the information to ensure you are following best practices.

Click through for the full set of practices and links to additional details.

Usability Issues with Query Store

Lonny Niederstadt has a new series on usability “soft spots” with Query Store. Part one looks at plan identifiers:

Yeah. That’s a lotta plans in the right-hand legend.  22 of them.  In a not very helpful order.  In fact… though I’ve tried to figure it out, I don’t know what type of order is used for that right-hand legend.  It’s not chronological.  It’s not based on duration which is the metric displayed by the graph. I dunno.

Part two looks at “morally equivalent plans”:

Let’s refresh the “Tracked Queries” activity.

Ohhhh.  I forced plan_id 2 (in the purple box below) but what showed up was plan_id 3220 (in the yellow box below).

Lonny promises more, so keep on the lookout.

Finding High-Variance Memory Grants

Erin Stellato shows how you can use Query Store to track the variance of memory grant requests:

One of the more perplexing problems to troubleshoot in SQL Server can be those related to memory grants. Some queries need more memory than others to execute, based on what operations need to be performed (e.g. sort, hash). SQL Server’s optimizer estimates how much memory is needed, and the query must obtain the memory grant in order to start executing. It holds that grant for the duration of query execution – which means if the optimizer overestimates memory you can run into concurrency issues. If it underestimates memory, then you can see spills in tempdb. Neither is ideal, and when you simply have too many queries asking for more memory than is available to grant, you’ll see RESOURCE_SEMAPHORE waits. There are multiple ways to attack this issue, and one of my new favorite methods is to use Query Store.

Click through for a demonstration.

Query Store Changes in CTP 3.0

Milos Radivojevic notes some changes in Query Store with SQL Server 2019 CTP 3.0:

In addition to default value, the minimum number of query executions in the AUTOquery_capture_mode for storing the query, its plan(s) and runtime statistics in SQL Server 2019 has been increased from 3 to 30. That means, Query Store does not store anything for first 29 query executions. It reserves query_ids, but it starts storing execution plan and runtime stats from 30thexecution in a single day.

These look like reasonable changes to me.

New Query Store Functionality in 2019

Erin Stellato is excited about SQL Server 2019 CTP 3.0:

Friends, CTP 3.0 dropped today, and it includes some changes for Query Store in SQL Server 2019!  I am so excited!!  I’ve downloaded it and have WideWorldImporters installed and have a lot of testing planned, but if you’re impatient, guess what?  The documentation is already updated!  If you check out the ALTER DATABASE SET page you will see that Query Store now has a new option for QUERY_CAPTURE_MODE: CUSTOM.  For those of you with ad hoc workloadsthis will help.

Read on to see how it can help.

Recompile Hints and Query Store

Erin Stellato answers two questions relating to recompile hints and whether those queries show up in Query Store:

Last week in our IEPTO2 class I was asked about queries with OPTION (RECOMPILE) and Query Store. Specifically: Do queries that have the OPTION (RECOMPILE) hint go into Query Store, AND do queries in a stored procedure created with the RECOMPILE option go into Query Store? I knew the answer to the first question, and was pretty sure I know the answer to the second one, but I wanted to test to verify. Let’s take a look.

Erin gives you a tl;dr version but I’m going to ask you to read the whole thing anyhow.

Forcing Plans with Table Variables

Grant Fritchey takes us through an interesting scenario around Query Store:

This weekend I was in Stockholm in Sweden, talking Query Store and plan forcing with Steinar Anderson, when he mentioned the problems he had while forcing plans that had table variables in them.

Don’t panic. Of course you can force a plan with a table variable, most of the time. Steinar had a fairly focused problem. Before I go on to explain the issue, let me be really clear, Steinar figured out the issue all on his own. When he outlined the problem, I saw immediately what his conclusion was going to be. What’s spurring this blog post is that Steinar said he’d searched on the internet and no one had talked about the issue yet. So, let’s talk about it.

Read on for the problem as well as solution.


August 2019
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