26Nov 2018 by Grant Fritchey 5 Comments

Forcing a Plan That Has a Plan Guide

SQL Server 2016, SQL Server 2017, T-SQL
The question that came up during a recent class I was teaching was: What if you have a plan guide to get the plan you want, but then decide, instead of using the plan guide, you'll just force the plan? Ummmm…. No idea. Let's test it. First, Create a Plan Guide I have a couple of queries I use to teach about how statistics affects plan choice, so we'll use that here. I'm going to also define and create a plan guide that makes this plan use a small row count for all queries against it: CREATE OR ALTER PROC dbo.AddressByCity @City NVARCHAR(30) AS SELECT a.AddressID, a.AddressLine1, a.AddressLine2, a.City, sp.Name AS StateProvinceName, a.PostalCode FROM Person.Address AS a JOIN Person.StateProvince AS sp ON a.StateProvinceID = sp.StateProvinceID WHERE a.City = @City; GO…
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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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17Sep 2018 by Grant Fritchey 5 Comments

Query Store and Log Backups

T-SQL
A question that came up recently around Query Store is what happens when there are log backups in use on the database. Let's talk about it. Query Store and Log Backups The core of the answer is very simple. Query Store, like any other data written to a database, whether a system table or a user table, is a logged operation. So, when you backup the database, you're backing up Query Store data. When you backup the logs, you're also backing up Query Store data. A point in time will include all the data written to the Query Store at that point. However, that's the kicker. At what point was the Query Store information written to disk? By default, there's a fifteen minute cycle before the Query Store moves the…
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10Sep 2018 by Grant Fritchey 1 Comment

Estimated Plans and Forced Plans from Query Store

SQL Server 2016, SQL Server 2017, T-SQL
While all plans are estimated plans, there is still a difference between capturing an estimated plan and looking at a plan from the cache or from query store. Or is there? A question came up during a recent presentation; what happens to capturing an estimated plan when you're forcing plans? Let's find out. The answer is interesting. Estimated Plans Here's my stored procedure that I'll be using with AdventureWorks2017: CREATE OR ALTER 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 WHERE th.ReferenceOrderID = @ReferenceOrderID; END; For reasons I'll explain in a bit, I'm going to free the procedure cache: ALTER DATABASE SCOPED CONFIGURATION CLEAR PROCEDURE_CACHE; Then, if I capture an estimated plan for two different values:…
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26Mar 2018 by Grant Fritchey 6 Comments

Query Store and Automated Cleanup

SQL Server 2016, SQL Server 2017
Query Store has mechanisms for automatically cleaning your data. It is possible to cause them to break down. While presenting a session about the Query Store recently, I was asked what happened if you set the size of the Query Store below the amount of data currently in the store. I didn't know the answer, so we tried it. Things got a little weird. Bryan Hundley of Marathon Consulting asked the question, so Bryan, this blog post is for you. Automated Cleanup There are actually two kinds of automated cleanup inside the Query Store. First, you have a time-based cleanup. By default it keeps queries that have been accessed within the last 30 days. Anything older, it tosses. If you have the stale_query_threshold_days (all in sys.database_query_store_options) set to 0, it…
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12Mar 2018 by Grant Fritchey 6 Comments

Execution Plans and the GDPR

SQL Server, SQL Server 2016, SQL Server 2017
What? Execution plans and the GDPR? Is this it? Have I completely lost it? Well, no, not on this topic, keep reading so I can defend myself. GDPR and Protected Data The core of the GDPR is to ensure the privacy and protection of a "natural person's" information. As such, the GDPR defines what personal data is and what processing means (along with a bunch of additional information). It all comes down to personally identifying (PI) data, how you store it, and how you process it. More importantly, it's about the right for the individual, the natural person, to control their information, up to and including the right to be forgotten by your system. OK. Fine. And execution plans? Execution Plans and PI Data If you look at an execution…
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18Dec 2017 by Grant Fritchey 8 Comments

Query Store Wait Statistics with sys.query_store_wait_stats

Azure, SQL Server 2017
The second best thing to questions that people ask is when I sit down to write a book. It's so easy to miss things in the day-to-day grind of doing work. Then, late at night, you're working on a chapter, so you read up on the documentation to ensure that you're not missing anything. Of course, then you find, yes, you are missing something. In my case, sys.query_store_wait_stats. sys.query_store_wait_stats. If you follow the link above, it'll give you what you need to know, but, I figured I'd provide a little more clarity because I think there are some pitfalls in using this data. I love Query Store (do a search to see all the exploration I've done with it). One of my favorite things is the time intervals. It breaks…
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24Apr 2017 by Grant Fritchey No Comments

Query Hash Values, Plan Guides and the Query Store

SQL Server 2016
I was eating dinner with Hugo Kornelis and we started talking about query hash values. You know, like everyone does at dinner. As we talked about it, I suddenly thought about both Plan Guides and the Query Store. I wondered what happened to the query hash values in that case? Thus are blog posts born. Query Hash and Plan Guides The behavior of the query hash itself is fairly straight forward. The text of the query is run through a hashing algorithm within SQL Server and a value comes out, so these two queries: SELECT * FROM Sales.SalesOrderDetail AS sod JOIN Sales.SalesOrderHeader AS soh ON sod.SalesOrderID = soh.SalesOrderID; SELECT * FROM Sales.SalesOrderDetail AS sod JOIN Sales.SalesOrderHeader AS soh ON sod.SalesOrderID = soh.SalesOrderID OPTION (FORCE ORDER); Result in two different query…
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07Feb 2017 by Grant Fritchey 4 Comments

Does Query Store Pre-Allocate Space

SQL Server 2016, T-SQL
I love the questions I get while I'm presenting because they force me to think and learn. The question in the title is one I received recently. The answer, now that I'm not standing in front of people, is easy. Of course the space is not pre-allocated. Query Store tables are just system tables. They have a limit on how big they can grow (100mb by default), but that space isn't going to be pre-allocated in any way. The space will just get used as and when it's needed, just like any other system table. However, don't take my word for it, let's prove that. The Test Testing whether or not enabling Query Store is straight forward. Here's a query that should give us information rather quickly: CREATE DATABASE QSTest; GO USE…
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28Nov 2016 by Grant Fritchey 1 Comment

Database Clone

SQL Server 2016, T-SQL
There are a bunch of ways you could create a database clone. Backup and restore is one method. Export/Import is another. There are even third party tools that will help with that. However, each of these has a problem. It's moving all the data, not just once, but twice. You move the data when you export it and you move it again when import it (same thing with backup and restore). That makes these methods slow for larger databases. How can you create a database clone without moving the data multiple times? Don't Move the Data At All New with SQL Server 2016, expanded in SP1, and added to SQL Server 2014 SP2 is a new command, DBCC CLONEDATABASE. This is like a dream come true. The use is extremely…
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