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Curated SQL Posts

When MAX becomes TOP

Published 2021-11-16 by Kevin Feasel

Forrest McDaniel tries out a few aggregations:

I can’t tell you how tempted I was to call this MAXTOP. Thinking about it still causes stray giggles to bubble up, as I gleefully envision sharing this info with eager young DBAs at a SQL Saturday, then ushering them off to a session on parallelism. Thankfully I’m not that evil.

The real conclusion is that SQL Server is programmed to be very clever. Despite the cleverness though, details matter, and the cleverness often falls short. What’s that warm glow I feel inside? Ah yes, job security.

Definitely worth a read, and Forrest also shares the repro scripts.

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Viewing Site-to-Site VPN Logs in Azure

Published 2021-11-16 by Kevin Feasel

Denny Cherry troubleshoots a site-to-site VPN issue:

Recently I needed to view the logs from an Azure Site to Site VPN to see why it wasn’t working as expected. When Azure Site to Site VPNs aren’t working as expected the GUI falls apart quickly for troubleshooting.

Log Analytics is where this problem gets solved. Log Analytics is going to allow you to see basically everything that the Azure Network Gateway is doing. Setting the feed up to Log Analytics isn’t as straightforward as it could be, but it is documented in this post.

Read on for some sample queries.

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Fundamentals of Query Store

Published 2021-11-16 by Kevin Feasel

Deepthi Goguri provides an intro to Query Store:

Introduced in SQL Server 2016, the Query Store feature will store the Query plans, Queries, resource consumption information along with the run time stats information for your queries. This feature is not automatically enabled on your databases. It is a database-level option only. You need to enable the Query Store on the database to capture the queries and query information. You cannot enable the Query store on master or tempdb databases. The information captured by the Query Store is stored in the database you enabled the feature on, in the internal tables created in the PRIMARY filegroup and this cannot be changed. (Microsoft docs reference).

One thing I would mention that Deepthi didn’t cover is, if you tried out Query Store a while ago and ran into problems, try again on the latest CUs of SQL Server. There have been a lot of bugfixes and optimizations which have made it a much more useful product.

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Replacing p-values with Bootstrapped Confidence Intervals

Published 2021-11-15 by Kevin Feasel

Florent Buisson has an interesting post on avoiding p-value calculations:

And indeed, I worked with highly-skilled data scientists who had a very sharp understanding of statistics. But after years of designing and analyzing experiments, I grew dissatisfied with the way we communicated results to decision-makers. I felt that the over-reliance on p-values led to sub-optimal decisions. After talking to colleagues in other companies, I realized that this was a broader problem, and I set up to write a guide to better data analysis. In this article, I’ll present one of the biggest recommendations of the book, which is to ditch p-values and use Bootstrap confidence intervals instead.

I’m a committed Bayesian (or at least a Bayesian who should be committed—depends on who you ask), so I’d consider this a big step forward.

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Monotonic Constraints on Random Forests

Published 2021-11-15 by Kevin Feasel

Michael Mayer has some interesting R and Python code for us:

On ML competition platforms like Kaggle, complex and unintuitively behaving models dominate. In this respect, reality is completely different. There, the majority of models do not serve as pure prediction machines but rather as fruitful source of information. Furthermore, even if used as prediction machine, the users of the models might expect a certain degree of consistency when “playing” with input values.

A classic example are statistical house appraisal models. An additional bathroom or an additional square foot of ground area is expected to raise the appraisal, everything else being fixed (ceteris paribus). The user might lose trust in the model if the opposite happens.

One way to enforce such consistency is to monitor the signs of coefficients of a linear regression model. Another useful strategy is to impose monotonicity constraints on selected model effects.

Certain types of regression algorithm make this easy, but random forest? Not so much. That’s where Michael steps in.

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Creating a Clock with Powershell

Published 2021-11-15 by Kevin Feasel

Jeffery Hicks knows what time it is (it’s clobberin’ time):

I’ve published a new project to the PowerShell Gallery. This is something that I needed, and maybe you do as well. Even though I have the typical clock running in the Windows taskbar, I have an ultrawide monitor so it isn’t always easy to read. I had been running the xclock app from WSL which was nice. But realized what I really wanted was a WPF-based clock that would display a formatted date-time string on my desktop.

Now there’s a PSClock similar to xclock, and you can check it out.

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Using ConcatenateX in Power BI

Published 2021-11-15 by Kevin Feasel

Reza Rad describes a DAX function:

It happens often in Power BI calculations and reports that you need to concatenate a list of values from a column. You can do this concatenation in Power Query or DAX. However, if the concatenation needs to be done dynamically. ConcatenateX is a very helpful DAX function to achieve such results. It is very helpful to understand what happens in the virtual tables in DAX too. In this article and video, I’ll explain what ConcatenateX is and how it works in Power BI and DAX.

Click through for a video, as well as a detailed explanation in blog post format.

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Pokey Performance with EXISTS

Published 2021-11-15 by Kevin Feasel

Erik Darling reminds us that even good things can go bad:

Look, I really like EXISTS and NOT EXISTS. I do. They solve a lot of problems.

This post isn’t a criticism of them at all, nor do I want you to stop using them. I would encourage you to use them more, probably.

But there’s some stuff you need to be aware of when you use them, whether it’s in control-flow logic, or in queries.

But do read on to see a specific type of issue you can run into with a left semi join.

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