Blog - SQLServerFast

A picture that shows several soccer goals, aligned in rows.

Rowgoals, part 1: How do rowgoals work?

January 17, 2026

For my second vlog, I decided to talk about rowgoals. First an explanation of what they are, then an overview of some obvious and some not so obvious cases where the optimizer will use a rowgoal, and finally a warning about cases where this normally beneficial feature might hurt instead of help. And then, after recording and editing, I had a video of over 35 minutes. I felt that this might be too long for the format of these vlogs. But that is just my assumption. So I decided to verify, by opening this poll on LinkedIn (which, by the…

Storage structures 1 – On-disk rowstore

January 15, 2026

Hugo Kornelis

No Comments

When a query is slow, it is often caused by inefficient access to the data. So our tuning work very frequently comes down to figuring out how data was read, and then massaging our queries or database structures to get SQL Server to access the data in a more efficient way. So we look at scans, seeks, and lookups. We know that scans are good when we access most of the data. Or, in the case of an ordered scan, to prevent having to sort the data. We know that seeks are preferred when there is a filter in the…

Self-promo: Precon opportunities

January 14, 2026

Hugo Kornelis

No Comments

I am normally not very good at promoting myself. I always forget to use the social media banners that conference organizes helpfully send me. And while I do have a list of upcoming speaking engagements on this site, I don’t really push it in your face at every opportunity. But the organizers of SeaQL 2026 pushed me. They asked me to record a video, to explain what attendees can expect when they sign up for my precon. And then, the fantastic Tonie Huizer went one step further, and created a YouTube short out of my promo video! The obvious next…

Problems purchasing videos?

January 10, 2026

Hugo Kornelis

No Comments

I started the SQLServerFast Execution Plan Video Training in 2020. At that time, I chose Vimeo as the platform to use for hosting and selling my videos. The major reason for this choice was their advertised ability to sell video access to customers across the world, so that everyone, regardless of location, can buy access to my videos and learn to read execution plans. Not in India! Two months ago, I received an email from someone who lives in India. When he tried to buy access to my videos, he received an error message: “This video is not for your…

A dark figure called "danger", armed with a knife, is chasing a running DBA.

A new experiment for 2026

January 1, 2026

Hugo Kornelis

It is January 1st, 2026. So let me first use this opportunity to wish all my readers a very good 2026, with lots of performance gains in your servers, lots of new insights into SQL Server, … but most of all, lots of love and happiness in your personal lives. Video blogging? It started when Erik Darling tempted me, even though I doubt he remembers. He has been vlogging for a long time already, and he basically dared me to try it too. I didn’t. Not at that time. But the seed was planted. During this year, I had several…

A deep dive into hash tables, part 4

December 23, 2025

Hugo Kornelis

No Comments

This is the last part of my blog series about the internals of hash tables in SQL Server. In part 1, I introduced a trick to get data returned in the order of the hash table, that I used in part 2 to gain an understanding of its internals. I then built on that in part 3 to find examples of hash collisions. With that, the discussion of the in-memory structure of a hash table is over. But what if we run out of memory and have to spill to tempdb? How is the spilled data stored on disk? I…

T-SQL Tuesday 191 – String parsing (and why I hate intelligent keys)

October 21, 2025

Hugo Kornelis

Welcome to the delayed 191st T-SQL Tuesday. Our host is, once more, Steve Jones. This time, he asks us to write about our favorite string parsing. Which is an odd thing to ask: nobody ever said “I like doing string manipulation in T-SQL”. So instead of doing what Steve asked, I will provide some background, and they show some tricks that might make the task a bit less unpleasant. Why intelligent keys are anything but intelligent I am currently most known for my performance tuning and execution plan work. But when I started working with database, I actually came from…

Black Friday 2025

October 11, 2025

Hugo Kornelis

No Comments

It’s October. Almost time for the annual Black Friday tradition, where companies prove how large their regular margins are by giving deep discounts to all their customers. Since the margin on my SQLServerFast Execution Plan Video Training is 100% (if we conveniently fail to count the hundreds of hours of labour I put in), I can also jump this bandwagon. I have done so in previous years, and by now I guess my regulars almost expect me to. So, let’s not disappoint people. Like previous years, during the entire month of November 2025, all video purchases are discounted by a…

A deep dive into hash tables, part 3

September 27, 2025

Hugo Kornelis

No Comments

Welcome back! In the previous parts, I first showed how a Hash Match (Left Outer Join) can give insight in the order of data in a hash table, and then used that trick to obtain and verify some interesting insights into the internal structure of such a table. It’s now time to see if this same trick can also be used to find hash collisions. The foundation for this part are these two conclusions from the previous parts: Unmatched rows from the left (top) input are returned bucket by bucket, with rows that are in the same bucket returned in…

T-SQL Tuesday 190 – Mastering a technical skill

September 9, 2025

Hugo Kornelis

No Comments

It’s time for T-SQL Tuesday again. Our host for edition 190 is Todd Kleinhans, and he asks us to write about mastering technical skills. I like that topic. Most of all, of course, because I spend a lot of time teaching, which is for most people the other side of mastering new skills, or improving existing skills. But also because I believe that we all live in a world of continuous learning. Those who do not learn will fall behind. Gradually at first, then ever faster. Of course, that does leave us with the choice of which technical skill to…

Rowgoals, part 1: How do rowgoals work?

Storage structures 1 – On-disk rowstore

Self-promo: Precon opportunities

Problems purchasing videos?

A new experiment for 2026

A deep dive into hash tables, part 4

T-SQL Tuesday 191 – String parsing (and why I hate intelligent keys)

Black Friday 2025

A deep dive into hash tables, part 3

T-SQL Tuesday 190 – Mastering a technical skill

Relating unrelated rows

Snapshot isolation: A threat for integrity? (Part 1)

Snapshot isolation: A threat for integrity? (Part 2)

Snapshot isolation: A threat for integrity? (Part 3)

Fun with ambiguous table names

Snapshot isolation: A threat for integrity? (Part 4)

The prime number challenge – great waste of time!

Just stuff it!

The table scan from hell

The Beatles versus the Stones

Hugo Kornelis

Subscribe via Email

Meta

Archives