28Jun 2016 by Grant Fritchey 6 Comments

CASE Statement in GROUP BY

SQL Server, SQL Server 2016
Set based operations means you should put everything into a single statement, right? Well, not really. People seem to think that having two queries is really bad, so when faced with logical gaps, they just cram them into the query they have. This is partly because SQL Server and T-SQL supports letting you do this, and it's partly because it looks like a logical extension of code reuse to arrive at a query structure that supports multiple logic chains. However, let's explore what happens when you do this on particular situation, a CASE statement in a GROUP BY clause. You see this a lot because a given set of data may be needed in slightly different context by different groups within the company. Like many of my example queries, this…
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31May 2016 by Grant Fritchey 3 Comments

Precedence Goes to Query Store or Plan Guide?

SQL Server 2016
While presenting at SQLDay in Wroclaw, Poland, on the Query Store, I was asked a pretty simple question, which takes precedence, the Query Store or a Plan Guide? One of my favorite answers to questions is "I don't know" because it gives me the opportunity to learn. Let's figure this one out together. I'll post the code to recreate this experiment within AdventureWorks at the end of the article. I'm doing this because the code for forcing execution plans using Plan Guides can be pretty doggone long (you may need to generate your own XML from a plan on your own system, fair warning). I have a stored procedure that I use frequently to demonstrate parameter sniffing and bad parameter sniffing, AddressByCity (listed below). This query when passed the value…
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16May 2016 by Grant Fritchey 8 Comments

Query Store, Forced Plans, and New Plans

Azure, SQL Server 2016
I love questions. I recently received one about new plans in the Query Store (available in Azure SQL Database now and in SQL Server 2016 after June 1). Let's say you have selected a plan that you want to force. You set it up. Now, let's say the plan ages out of cache or even goes through a recompile. During the recompile, due to out of date statistics or skew in the statistics, you would, under normal circumstances, get a new plan. However, with Query Store and plan forcing, the plan that's going to be used is the plan that is being forced. But, does that other plan, the one not used, get stored in Query Store? I have no idea. Let's find out. The Setup To start with, a small stored procedure…
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03May 2016 by Grant Fritchey 5 Comments

Implicit Conversion and Performance

SQL Server, SQL Server 2016, T-SQL
Letting SQL Server change data types automatically can seriously impact performance in a negative way. Because a calculation has to be run on each column, you can't get an index seek. Instead, you're forced to use a scan. I can demonstrate this pretty simply. Here's a script that sets up a test table with three columns and three indexes and tosses a couple of rows in: CREATE TABLE dbo.ConvertTest ( BigIntColumn BIGINT NOT NULL, IntColumn INT NOT NULL, DateColumn VARCHAR(30) ); CREATE INDEX BigIntIndex ON dbo.ConvertTest (BigIntColumn); CREATE INDEX IntIndex ON dbo.ConvertTest (IntColumn); CREATE INDEX DateIndex ON dbo.ConvertTest (DateColumn); WITH Nums AS (SELECT TOP (1000000) ROW_NUMBER() OVER (ORDER BY (SELECT 1 )) AS n FROM master.sys.all_columns ac1 CROSS JOIN master.sys.all_columns ac2 ) INSERT INTO dbo.ConvertTest (BigIntColumn, IntColumn, DateColumn ) SELECT Nums.n, Nums.n,…
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24Feb 2016 by Grant Fritchey 6 Comments

Query Store and Recompile

Azure, SQL Server 2016, T-SQL
One of the many advantages of SQL Cruise is the ability to have enough time during a presentation to be able to answer questions from the people there in great detail. One question came up while I was showing the new functionality of Query Store (available soon in SQL Server 2016, available right now in Azure SQL Database). What happens to plan forcing when you have OPTION RECOMPILE on a query? Great question. I have a favorite procedure I use to illustrate the functionality of parameter sniffing: 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; If this procedure is called with the value of 'Mentor' you get…
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22Feb 2016 by Grant Fritchey 3 Comments

Why Is The Server Slow?

SQL Server, SQL Server 2016, T-SQL
This is blog post #2 in support of Tim Ford's (b|t) #iwanttohelp, #entrylevel. If you haven't been working in SQL Server for very long, you may not have got this phone call yet, but you will: Hi, yeah, the server is slow. Thanks. Bye. Let's pretend for a moment that you know which server they're referring to (because just finding out that piece of information can be a challenge). Now what? The list of tools and mechanisms within SQL Server for gathering metrics is extremely long: Performance Monitor Dynamic Management Views & Functions System Views Extended Events Trace Events Activity Monitor Data Collector Execution Plans 3rd Party Tools I'm leaving out lots of stuff in that list. So where do you start when you get this phone call? Where is the server slow?…
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25Jan 2016 by Grant Fritchey 7 Comments

Finding Your Query in Query Store

Azure, SQL Server 2016, T-SQL
Query Store is pretty amazing. I'm loving working with it. I think it's likely to change how query tuning will be done in the future. Lots of people are probably going to just use the reports and tools in SQL Server Management Studio. However, a pretty healthy chunk of us will start using the system views in order to programmatically access the information stored in Query Store. One of the first things you're going to want to do is track down your query. The primary views you'll want are sys.query_store_query and sys.query_store_query_text. They join together based on the query_text_id. Let's take four scenarios and see if we can retrieve the correct query. First up, an ad hoc query: SELECT e.NationalIDNumber, p.LastName, p.FirstName, a.City, bea.AddressTypeID FROM HumanResources.Employee AS e JOIN Person.BusinessEntityAddress AS…
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09Sep 2015 by Grant Fritchey 16 Comments

Yes, Foreign Keys Help Performance

SQL Server
I created this example several years ago that illustrates how foreign key constraints can help performance. It's a contrived example. Granted. I feel like it illustrates the point. However, over the years, people have questioned one aspect of it. The optimizer uses the foreign keys to figure out which tables can be eliminated from the query, making for a more efficient plan and making the query run faster. The pushback has always been, "Yeah, Grant, but nobody writes T-SQL where they include extra tables that they don't need." My initial response, after I stop laughing, is to point out any number of ORM tools. But, you know what, let's assume that's correct. No one would ever create a giant catch-all view that has all their JOINs in one place so they don't have…
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05May 2015 by Grant Fritchey 15 Comments

Effects of Persisted Columns on Performance

T-SQL
I live for questions. And my favorite questions are the ones where I'm not completely sure of the answer. Those are the questions that make me stop presenting in order to take a note so I can try to answer the question later, usually in a blog post. Guess where we are today? I was asked at SQL Bits in London about the direct impact of the PERSISTED operator on calculated columns, both inserts and selects. I didn't have a specific answer, so I wrote it down for later (and asked the, self-described, persisting Dane, to email me to remind me. He did, so I put together a few tests to try to answer his question. First, I created three tables: CREATE TABLE dbo.PersistTest ( PersistTestID INT IDENTITY(1,1) NOT NULL PRIMARY…
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06Apr 2015 by Grant Fritchey 8 Comments

Constraints and SELECT Statements

Azure, SQL Server, T-SQL
I've posted previously about how a foreign key constraint can change how a SELECT query behaves. Logically that just makes sense. But other types of constraints don't affect execution plans do they? Yes. Let's take this constraint as an example: ALTER TABLE Sales.SalesOrderDetail WITH CHECK ADD  CONSTRAINT CK_SalesOrderDetail_UnitPrice CHECK  ((UnitPrice>=(0.00))) That will ensure that no values less than zero can slip in there. We can even validate it: INSERT Sales.SalesOrderDetail (SalesOrderID, CarrierTrackingNumber, OrderQty, ProductID, SpecialOfferID, UnitPrice, UnitPriceDiscount, rowguid, ModifiedDate ) VALUES (60176, -- SalesOrderID - int N'XYZ123', -- CarrierTrackingNumber - nvarchar(25) 1, -- OrderQty - smallint 873, -- ProductID - int 1, -- SpecialOfferID - int -22, -- UnitPrice - money 0.0, -- UnitPriceDiscount - money NEWID(), -- rowguid - uniqueidentifier GETDATE() -- ModifiedDate - datetime ); Will give me…
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