01Aug 2016 by Grant Fritchey 1 Comment

There Is No Difference Between Table Variables, Temporary Tables and Common Table Expressions

SQL Server, SQL Server 2016, SQL Server 2017, T-SQL
I actually saw the above statement posted online. The person making the claim further stated that choosing between these three constructs was "personal preference" and didn't change at all the way SQL Server would choose to deal with them in a query. Let's immediately say, right up front, the title is wrong. Yes, there are very distinct differences between these three constructs. Yes, SQL Server will absolutely deal with these three constructs in different ways. No, picking which one is correct in a given situation is not about personal preference, but rather about the differences in behavior between the three. To illustrate just a few of the differences between these three constructs, I'll use variations of this query: SELECT * FROM Sales.Orders AS o JOIN Sales.OrderLines AS ol ON ol.OrderID = o.OrderID WHERE ol.StockItemID = 227; The…
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18Jul 2016 by Grant Fritchey 14 Comments

Common Table Expression, Just a Name

SQL Server, SQL Server 2016, T-SQL
The Common Table Expression (CTE) is a great tool in T-SQL. The CTE provides a mechanism to define a query that can be easily reused over and over within another query. The CTE also provides a mechanism for recursion which, though a little dangerous and overused, is extremely handy for certain types of queries. However, the CTE has a very unfortunate name. Over and over I've had to walk people back from the "Table" in Common Table Expression. The CTE is just a query. It's not a table. It's not providing a temporary storage space like a table variable or a temporary table. It's just a query. Think of it more like a temporary view, which is also just a query. Every time I explain this, there are people who don't…
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11Jul 2016 by Grant Fritchey 26 Comments

Same Query, Different Servers, Different Performance. Now What?

SQL Server, SQL Server 2016, T-SQL
Based on the number of times I see this question on forums, it must be occurring all the time. You have two different servers that, as far as you know, are identical in terms of their options and setup (although not necessarily in terms of power, think a test or pre-production system versus production). On these servers you have a database on each that, as far as you know, is the same as the other in terms of options, objects, maybe even data (although, this does mean that you have unmasked production information in your QA environment, which potentially means you're going to jail, might want to address this, especially now that I've told you about it, mens rea, you're welcome). On each database you run, as far as you know, the exact same query (whether…
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24May 2016 by Grant Fritchey 3 Comments

On the Buckeye Blitz!

T-SQL
In just a few weeks I'll be doing the Buckeye Blitz. That is, a tour of user groups in Ohio (aka, the Buckeye state, after a tree, not a sports team), one per day across a week. Here's how it breaks down: Cleveland: 6/13 Toledo: 6/14 Columbus: 6/15 Cincinnati: 6/16 I'll be talking on this topic at each of the groups: Change Your Habits: Tips to Tune Your T-SQL T-SQL proides many different ways to accomplish the same task, and as you might expect, some ways are better than others. In this session, you will learn specific techniques, that when followed make you a better T-SQL developer. The session is jam-packed with practical examples and is designed for administrators and developers who want to bring their T-SQL skills to the…
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12May 2016 by Grant Fritchey 9 Comments

Use The Correct Data Type

SQL Server, SQL Server 2016, T-SQL
Blog post #5 in support of Tim Ford’s (b|t) #iwanttohelp, #entrylevel. Read about it here. Saying that you should use the correct data type seems like something that should be very straight forward. Unfortunately it's very easy for things to get confusing. Let's take a simple example from AdventureWorks. If I run this query: SELECT a.ModifiedDate FROM Person.Address AS a WHERE a.AddressID = 42; The output looks like this: 2009-01-20 00:00:00.000 Normal right? You see the year, the month and the day followed by the time in hours, minutes, and seconds as a decimal. Ah, but there is an issue. This query is supposed to be for the reporting system, and the business only cares about the date that the values in the Person.Address table have been modified, so they don't want…
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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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12Apr 2016 by Grant Fritchey 8 Comments

A View Is Not A Table

Azure, SQL Server, SQL Server 2016, T-SQL
Blog post #4 in support of Tim Ford’s (b|t) #iwanttohelp, #entrylevel In SQL Server, in the T-SQL you use to query it, a view looks just like a table (I'm using the AdventureWorks2014 database for all these examples): SELECT * FROM Production.vProductAndDescription AS vpad;   SELECT vpad.Name, vpad.Description, vpmi.Instructions FROM Production.vProductAndDescription AS vpad JOIN Production.Product AS p ON p.ProductID = vpad.ProductID JOIN Production.vProductModelInstructions AS vpmi ON vpmi.ProductModelID = p.ProductModelID WHERE vpad.ProductID = 891 AND vpad.CultureID = 'fr'; The above query actually combines two views and a table. This is what is commonly referred to as a "code smell". A code smell is a coding practice that works, but that can lead to problems. In this case, we're talking about performance problems. The performance problems when using views to join to…
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05Apr 2016 by Grant Fritchey 3 Comments

Views and Simplification

SQL Server, SQL Server 2016, T-SQL
I've been getting lots of questions on views lately. Must be something in the water. Because SQL Server allows you to treat a view as if it was a table, lots of people pretty much assume that it is a table since they get to treat it that way. The thing is, a view is not a table. It's a query. Let's explore this just a little bit. Here's a relatively straight forward view: CREATE VIEW dbo.PersonInfo AS SELECT a.AddressLine1, a.City, a.PostalCode, a.SpatialLocation, p.FirstName, p.LastName, be.BusinessEntityID, bea.AddressID, bea.AddressTypeID FROM Person.Address AS a JOIN Person.BusinessEntityAddress AS bea ON a.AddressID = bea.AddressID JOIN Person.BusinessEntity AS be ON bea.BusinessEntityID = be.BusinessEntityID JOIN Person.Person AS p ON be.BusinessEntityID = p.BusinessEntityID; GO I can query this view like this: SELECT * FROM dbo.PersonInfo AS pni…
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29Feb 2016 by Grant Fritchey 4 Comments

Wait Statistics in Azure SQL Database

Azure, T-SQL
You need to be aware that you're going to see differences when you're working with Azure SQL Database when it comes to wait statistics. If you're running a v12 Azure SQL Database (and if you're not, go fix it), you can run a query against sys.dm_os_wait_stats. Here's the output for the TOP 10 waits ordered by wait time on one of my databases: Everything you're used to seeing, right? Well... not quite. This is Azure SQL Database. So, let's use sys.dm_db_wait_stats, a DMO that is only available within Azure. This lists waits by database. The TOP 10 here are: You'll notice that these results are wildly different from those above. What we're looking is largely a server versus a database, but not completely. I mean that sys.dm_os_wait_stats is showing the waits for the…
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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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