10Jan 2017 by Grant Fritchey 3 Comments

Query Optimizer and Data Definition Language Queries

SQL Server, SQL Server 2016, T-SQL
Data Definition Language queries don't go through the optimizer, right? While normally, my short answer to this question in the past would have been, yes. However, with testing comes knowledge and I want to be sure about the statement. I'm working with a team of people to completely rewrite the SQL Server Execution Plans book. We'll probably be published in April or May. It's going to be amazing. The structure will be completely different and the book will represent five years of additional knowledge in how execution plans work and how to read and interpret them since the last book was written. However, enough on that. Let's answer the question about Data Definition Language. First of all, we need to quickly define our terms. Data Definition Language (DDL) represents the syntax for queries that build…
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12Dec 2016 by Grant Fritchey 7 Comments

OPTIMIZE FOR Hints When Parameter Sniffing is Turned Off

Azure, SQL Server, SQL Server 2016, T-SQL
While presenting recently and talking about dealing with bad Parameter Sniffing, I got the question; what happens to OPTIMIZE FOR hints when parameter sniffing is disabled? This is my favorite kind of question because the answer is simple: I don't know. Parameter Sniffing For those who don't know, parameter sniffing is when SQL Server uses the precise values passed into a query as a parameter (this means stored procedures or prepared statements) to generate an execution plan from the statistics using the value from the parameter. Most of the time, parameter sniffing is either helping you, or is not hurting you. Sometimes, parameter sniffing turns bad and hurts you quite severely. Usually, but not always, this is because you either have severely skewed data (some data is very different than the rest, lots…
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05Dec 2016 by Grant Fritchey 3 Comments

Presentations for SQL Server Beginners

Azure, PASS, PowerShell, Professional Development, SQL Server, SQL Server 2016
[caption id="attachment_2548" align="alignleft" width="300"] Tired from all those blog posts[/caption] For my final blog post in the #gettingstarted, #iwanttohelp series, I decided to curate a set of presentations from the PASS Virtual Chapters. This content is available online. It's free. Most importantly for the person just getting started as a SQL Server data pro, it's good. I'm going to marry each of the presentations with my eleven blog posts in this series. The Importance of a Full Backup in SQL Server For this one I'm going to recommend Tim Radney's session Understanding SQL Server Backup and Restore. I know Tim personally and guarantee this is a good session. Why Is The Server Slow Jes Borland is a very close personal friend and an absolutely outstanding presenter (and person). She has…
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23Nov 2016 by Grant Fritchey 5 Comments

PowerShell to Test a Query

DevOps, PowerShell, SQL Server, SQL Server 2016, T-SQL
So you want to do some tuning, but you're not sure how to test a query on it's performance. Not a problem. Here's a very rough script that I use to do some recent testing. This script to test a query is post #11 of the #enterylevel #iwanttohelp effort started by Tim Ford (b|t). Read about it here. The Script The goal here is to load a bunch of parameter values from one table and then use those values to run a query to test it. To do this I connect up to my SQL Server instance, naturally. Then I retrieve the values I'm interested in. I set up the query I want to test. Finally a loop through the data set, calling the query once for each value. [reflection.assembly]::LoadWithPartialName("Microsoft.SqlServer.Smo") | out-null #…
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07Nov 2016 by Grant Fritchey 2 Comments

sp_executesql Is Not Faster Than an Ad Hoc Query

SQL Server, SQL Server 2016, T-SQL
This requires an immediate caveat. You should absolutely be using sp_executesql over any type of non-parameterized execution of T-SQL. You must parameterize your T-SQL because the lack of parameters in building up and executing strings is a classic SQL Injection attack vector. Using straight ad hoc T-SQL is an extremely poor coding choice because of SQL Injection, not because there is something that makes one method faster than the other. Yet, I see in performance checklists that you should be using sp_executesql over straight ad hoc T-SQL because it will perform faster. That statement is incorrect. Some Discussion Let me reiterate the caveat before we continue. I 100% advocate for the use of sp_executesql. This function is preferred over ad hoc SQL because, used properly (and isn't that usually one of the main problems,…
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03Nov 2016 by Grant Fritchey 1 Comment

Reinforcing the Importance of Statistics on Row Estimate

SQL Server, SQL Server 2016, T-SQL
I recently wrote an introductory post about the importance of statistics. I just received a reinforcement on how important they are during my own work. Bad Estimate I hit a weird problem while I was setting up a query to illustrate a point (blog to be published next week). Let's take the basis of the problem and explain it. I wanted data with distribution skew, so I ran this query to find out if there was a wide disparity between the top and bottom of the range: SELECT i.BillToCustomerID, COUNT(i.BillToCustomerID) AS TestCount FROM Sales.Invoices AS i GROUP BY i.BillToCustomerID ORDER BY TestCount ASC; Sure enough, the bottom of the range returned three (3) rows and the top returned 21,551. If I then run a query to retrieve just a few rows…
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01Nov 2016 by Grant Fritchey 5 Comments

Stored Procedures Are Not Faster Than Views

SQL Server, SQL Server 2016, T-SQL
A performance tuning tip I saw recently said, "Views don't perform as well as stored procedures." <sigh> Let's break this down, just a little. Definitions A view is nothing but a query. The definition given by Microsoft is that it's a virtual table that's defined by a query. It's a query that is used to mask data or perform a complex join or similar behaviors. Views are queries that get stored in the database. Views can be easily referred to as if they were a tables. That's it. I've written in the past about views, including how they can possibly perform poorly. A stored procedure is also a query, or a series of queries, or, a whole lot more. Microsoft's definition of a stored procedure basically defines it as programming object that can accept input through…
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24Oct 2016 by Grant Fritchey 10 Comments

A Sub-Query Does Not Hurt Performance

SQL Server, SQL Server 2016, T-SQL
The things you read on the internet, for example, "don't use a sub-query because that hurts performance." Truly? Where do people get these things? Let's Test It I've written before about the concept of cargo cult data professionals. They see one issue, one time, and consequently extrapolate that to all issues, all the time. It's the best explanation I have for why someone would suggest that a sub-query is flat out wrong and will hurt performance. Let me put a caveat up front (which I will reiterate in the conclusion, just so we're clear), there's nothing magically good about sub-queries just like there is nothing magically evil about sub-queries. You can absolutely write a sub-query that performs horribly, does horrible things, runs badly, and therefore absolutely screws up your system. Just as…
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17Oct 2016 by Grant Fritchey 32 Comments

SELECT * Does Not Hurt Performance

SQL Server, SQL Server 2016, T-SQL
I read all the time how SELECT * hurts performance. I even see where people have said that you just have to supply a column list instead of SELECT * to get a performance improvement. Let's test it, because I think this is bunkum. The Test I have here two queries: SELECT * FROM Warehouse.StockItemTransactions AS sit; --and SELECT sit.StockItemTransactionID, sit.StockItemID, sit.TransactionTypeID, sit.CustomerID, sit.InvoiceID, sit.SupplierID, sit.PurchaseOrderID, sit.TransactionOccurredWhen, sit.Quantity, sit.LastEditedBy, sit.LastEditedWhen FROM Warehouse.StockItemTransactions AS sit; I'm basically going to run this a few hundred times each from PowerShell. I'll capture the executions using Extended Events and we'll aggregate the results. The Results I ran the test multiple times because, funny enough, I kept seeing some disparity in the results. One test would show a clear bias for one method, another test would…
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10Oct 2016 by Grant Fritchey 2 Comments

Statistics Are Vital For Query Performance

SQL Server, SQL Server 2016
This is post #10 supporting  Tim Ford’s (b|t) initiative on #iwanttohelp, #entrylevel. Read about it here. When you send a query to your SQL Server database (and this applies to Azure SQL Database, APS, and Azure SQL Data Warehouse), that query is going to go through a process known as query optimization. The query optimization process figures out if you can use indexes to assist the query, whether or not it can seek against those indexes or has to use a scan, and a whole bunch of other stuff. The primary driving force in making these decisions are the statistics available on the indexes and on your tables. What Are Statistics Statistics are a mathematical construct to represent the data in your tables. Instead of scanning through the data each and every…
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