03Apr 2023 by Grant Fritchey No Comments

How Does SELECT * Affect Query Store?

SQL Server
I live for questions and recently, I had someone ask me, does using SELECT * affect Query Store. My immediate gut reaction was, hell no. Of course it doesn't. Then, yeah, I started thinking. It might. Let's test it and see. The Setup The trick here is to get a good setup. I need a query that's... interesting. Meaning, I need the query to do more than just SELECT * from a table. Although, let's start there: SELECT * FROM dbo.Customers AS c; GO 20 I ran it several times so that it will, for sure, get captured by Query Store (when on Auto, in 2022, one execution of a query may not be enough to see it captured by QS). Then, I took a look at the plan: SELECT…
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10Oct 2022 by Grant Fritchey No Comments

Check Every Metric

SQL Server
Recently, a person asked about the costs differences in an execution plan, referencing them as if they were performance measures. The key to understanding performance is to check every metric. When it comes to execution plans, I'm sure I've said this before, so please allow me to repeat myself. The cost numbers shown in an execution plan, which, barring a recompile, will be the same for an execution plan or an execution plan with runtime metrics (aka, estimated and actual plans), are not measures of performance. They do not represent actual metrics. Instead, they are calculations of a theoretical actual performance measurement. So, you can't look at two plans, with two costs, and say, "this plan will perform better." Instead, you can say, "this plan has a lower estimated cost."…
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03Oct 2022 by Grant Fritchey 2 Comments

Function Vs. Performance

SQL Server
Recently I was looking through DBA.StackExchange when I saw a pretty simple question that I decided to answer. I went off, set up a test database and some test tables and quickly wrote a query to answer the question. I got it all formatted pretty and was on my way to post it when I saw that another answer was already there. Yeah. Identical to mine. Almost line for line. Well, nuts. I know. I'll write a blog post. The Setup The original poster had two tables that, frankly, are badly designed. However, they share enough data that they are "related" if not relational. Here's the code: CREATE DATABASE Testing; GO USE Testing; GO CREATE TABLE Table_A ( ID INT IDENTITY(1, 1), Score INT ); CREATE TABLE Table_B ( FromPoint…
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14Jun 2021 by Grant Fritchey 2 Comments

Find Queries Using a Key Lookup Operator

SQL Server
While teaching about Extended Events and Execution Plans last week, Jason, one of the people in the class, asked: Is there a way in Extended Events to find queries using a Key Lookup operation? Sadly, the answer is no. However, you can query the Execution Plans in cache or in the Query Store to find this. Thanks for the question Jason. Here's your answer. Finding Key Lookups Since we can't have Extended Events just feed us the information, we have to query the plans. That means pulling out data from the XML. So, to find queries that are using the Key Lookup operator, we can do this: SELECT DB_NAME(detqp.dbid), SUBSTRING( dest.text, (deqs.statement_start_offset / 2) + 1, (CASE deqs.statement_end_offset WHEN -1 THEN DATALENGTH(dest.text) ELSE deqs.statement_end_offset END - deqs.statement_start_offset ) / 2…
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22Mar 2021 by Grant Fritchey 9 Comments

Query Store as an Upgrade Tool

SQL Server, Tools
There are a lot of uses for Query Store, but one of the most interesting is as an upgrade tool. We all know that upgrades in SQL Server can be more than a little bit nerve wracking. No matter how much you tested stuff in lower environments, deploying an update to production might result in performance issues as your code hits a regression. This is even more true when upgrading from versions of SQL Server prior to 2014 to anything 2014 and above. That's because of the new cardinality estimation engine introduced in 2014. Most queries won't notice it. Some queries will benefit from the better estimates. A few, problematic, queries will suffer. This is where Query Store can be used as an upgrade tool. The Steps We're going to…
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14Dec 2020 by Grant Fritchey 2 Comments

Getting Started Reading Execution Plans: Highest Cost Operator

SQL Server, T-SQL
Reading execution plans in SQL Server is just hard. There's a lot to learn and understand. I previously outlined the basics I use to get started when I'm looking at an execution plan for the first time. However, just those pointers are not enough. I want to explain a little further why and how those basic steps are how you get started reading execution plans. To begin with, instead of talking about the first operator, which I've detailed before, we'll talk about the highest cost operators. Highest Cost Operator Every execution plan within SQL Server includes what the optimizer has determined to be the estimated cost of each operation. All these estimated operator costs are tallied up, and that makes up the estimated cost of the whole execution plan. You…
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18May 2020 by Grant Fritchey 2 Comments

Can We Get Row Counts After Execution?

SQL Server, T-SQL
The general idea for this question came from dba.stackexchange.com: could we, and if we can, how, get row counts after execution. I was intrigued with the idea, so I ran some tests and did a little digging. I boiled it all down in the answer at the link, but I figured I could share a little here as well. Properly Retrieve Row Counts After Execution The right way to do this is obvious and simple. Before you need it, set up an Extended Events session. Done. The only question is what goes into the Session. First blush, sql_batch_completed and/or rpc_completed. Both will return a rows affected value. Although, interestingly, the row_count value is documented as rows returned. However, it's both. But, if you really want to get picky, batches and…
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03Mar 2020 by Grant Fritchey 10 Comments

A Rose By Any Other Name

SQL Server
There is only one kind of execution plan within SQL Server. I've said this several times on this blog. Now, I'd like you to go and read this excellent blog post by Hugo Kornelis. Hugo, Erin Stellato and I are working with Microsoft to hopefully, at long last, make this issue clear. In the grand scheme of life, like Shakespeare's rose, the name we use for execution plans doesn't really matter. However, the fact is, language really does matter. Clarity is so important in communications, of any kind. When we try to teach execution plans, the confusion caused by the old naming standards is one of the first things we have to spend time getting people to un-learn. I fully support this newly proposed naming standard: Execution PlanExecution Plan Plus…
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06Jan 2020 by Grant Fritchey 8 Comments

Execution Plans: First Operator

SQL Server, T-SQL
The first time you see a new execution plan that you're examining to fix a performance problem, something broken, whatever, you should always start by looking at the first operator. First Operator The first operator is easily discerned (with an exception). It's the very first thing you see in a graphical execution plan, at the top, on the left. It says SELECT in this case: This is regardless of how you capture the execution plan (with an exception). Whether you're looking at an execution plan from the plan cache, Query Store, or through SSMS, the execution plan, regardless of complexity, has this first operator. In this case, it says UPDATE: If you get an execution plan plus runtime metrics (previously referred to as an "actual" execution plan), you'll still see…
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19Nov 2018 by Grant Fritchey 1 Comment

Query Store and Plan Cache Plans Compared

SQL Server 2016, SQL Server 2017
Query Store plans and the plans in cache are identical, right? There won't be differences because the plan that is in cache is the plan that was used to execute the query. Similarly, the plan that is in the Query Store is the plan that was used to execute the query as well. Therefore, they will be the same. However, some small differences actually can show up. Differences Between Plans In order to compare the two plans, first, we need a query. Here's a stored procedure that I'm going to use to generate a plan that will be in cache and in the query store: CREATE PROC dbo.ProductTransactionHistoryByReference ( @ReferenceOrderID int ) AS BEGIN SELECT p.Name, p.ProductNumber, th.ReferenceOrderID FROM Production.Product AS p JOIN Production.TransactionHistory AS th ON th.ProductID = p.ProductID…
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