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Willkommen, Bienvenue, Welcome

December 14, 2017

It may be surprising to see a word of welcome posted in December 2017, on a blog that has posts going back as far as 2006. And yet, this is my very first new blog post on this site. All the content before this post was originally published at SQLblog.com. SQLblog.com is a fantastic website that hosts many bloggers. It has hosted me for almost twelve years as well. But it was time to move house, to create a new website for the content I have planned for the future. For now, this website is just my blog. But that…

Three new sessions in three months

September 14, 2017

Hugo Kornelis

Waking up from my blog-hibernation at this time is probably not ideal. I should spend my time on other things. For instance, on creating slide decks and demo code for the three completely new sessions that I will be delivering at various conferences within the next three months. So why am I still taking the time to write this? Because I am super excited about these new sessions, that’s why! Hash Match, the Operator The idea for this session is now almost two years old. I cannot remember why, during break time at another conference, I found myself wondering whether…

Misconceptions on parameter sniffing

November 3, 2016

Hugo Kornelis

In my previous post, I explained the basic of parameter sniffing, and then built on that to explain the lesser known mechanics of variable sniffing and cardinality sniffing. However, I also sneakily inserted a few comments on misconceptions about the performance impact of parameter sniffing, with the promise to explain in more detail later. Well … it is later now, so here is the promised explanation! Parameter sniffing’s bad rep Lots of people in the SQL Server field will claim that parameter sniffing is a bad feature. That is not the case. Just think about it for a moment –…

The sniffing database

October 23, 2016

Hugo Kornelis

Your SQL Server instances, like people with hay fever that forget to take their antihistamines during summer, is sniffing all the time. Sniffing is a trick employed by the optimizer in an attempt to give you better execution plans. The most common form of sniffing is parameter sniffing. Many people know about parameter sniffing, but there are a lot of misconceptions about this subject. I have heard people describe parameter sniffing as a bad thing, and I know people who claim that parameter sniffing is mostly good with some exceptions that they then call “bad parameter sniffing”. Neither of these…

Upcoming presentations. And an offer.

September 15, 2016

Hugo Kornelis

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It’s not my habit to announce my future presentations on this blog. But I sometimes make exceptions to this rule, and today is a good day for such an exception. Especially since I have been allowed to offer some money off for one of my upcoming presentations. I’ll fly to Copenhagen tomorrow, for SQL Saturday Demark, where I will present two sessions: a 60-minute talk (“T-SQL User-Defined Functions, or: Bad Performance Made Easy”) about how T-SQL user-defined functions can wreck your performance, and a 10-minute lightning talk (“Managing Execution Plans”) about the similarities between execution plans and my coworkers. Three…

The DIY guide for local-global aggregation

August 31, 2016

Hugo Kornelis

If you have read the title, you will not be surprised when I tell you that this blog is about a thing called “local-global aggregation”, and on how to do that yourself. So let’s start with the two obvious questions: what the heck is local-global aggregation anyway, and why the heck would you ever want to do it yourself? What is local-global aggregation The term local-global aggregation may sound scary, but it’s not that hard to understand. You might even already know the concept if you have ever heard the arguably most used explanation of parallelism. You have a jar…

How TOP wrecks performance (part 2)

June 25, 2016

Hugo Kornelis

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In my previous post, I showed how the vanilla version of TOP can really help performance by nudging the optimizer into picking an execution plan that is optimized for returning just a few rows instead of the entire result set. But I also showed that adding WITH TIES causes a cardinality misestimate, which may lead to suboptimal or downright terrible plan choices. Luckily, this is fixed in SQL Server 2014 and later, provided you use the new cardinality estimator. In this post I will show how the PERCENT option causes even worse performance, and how you can avoid that performance…

How TOP wrecks performance (part 1)

November 6, 2014

Hugo Kornelis

The TOP keyword in the SELECT clause may not be ANSI standard, but I am sure we have all used it. But do you know that it comes with two optional keywords? And have you used them? They are PERCENT and WITH TIES. TOP Let’s first look at the based functionality of TOP, and how it affects performance – which in fact, contrary to the title of this post, is usually quite good for performance. Here is a simple example, using the AdventureWorks2012 sample database (which can be downloaded from Codeplex). Let’s start with a very basic example: SELECT CustomerID,…

Give speakers feedback. It makes a difference.

October 25, 2014

Hugo Kornelis

I sometimes struggle with finding a catchy title for a blog post. It has to grab the attention, and clarify what the post is about. If you decided to spend time reading my words, I do not want you to finish ending “if I had known this was the content, I would have done something better with my time”. For this post, I think the title just sums up the entire post. If you are short on time, you can stop reading now. Seriously. Feedback from SQL Server Days Last week I received the feedback from my presentation at the…

Execution plans, laundry, and a giveaway

October 25, 2014

Hugo Kornelis

In just a week from now, SQL Saturday #337 will kick off in Portland, Oregon. And I will be there – the third time already for me to be speaking in Portland. For a European, Portland is not the most logical location. But the organization over there is a pretty smart bunch of people. They figure that being close to Seattle gives them a great opportunity – so whenever they get the chance, they will reserve a Saturday just before the PASS Summit in Seattle for their own event. And then they make sure to organize this event so extremely…

Willkommen, Bienvenue, Welcome

Three new sessions in three months

Misconceptions on parameter sniffing

The sniffing database

Upcoming presentations. And an offer.

The DIY guide for local-global aggregation

How TOP wrecks performance (part 2)

How TOP wrecks performance (part 1)

Give speakers feedback. It makes a difference.

Execution plans, laundry, and a giveaway

When it rains, it pours

Black Friday Charity summary

A quiet period

Good news, bad news

The final lap

Relational database design

Back home, but still a long way to go

Which execution plan icons are relevant today?

T-SQL Tuesday 162 – Execution plans according to ChatGPT

New trailers and an updated chapter

Hugo Kornelis

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