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Curated SQL Posts

Determining Why Constraints Are Untrusted

Published 2022-06-24 by Kevin Feasel

Tom Zika adopts a zero-trust constraint architecture:

Okay, so you went through the effort of fixing them, but the next day your constraints are not trusted again. What gives?

If you are sure none of the DBAs or developers is doing this to spite you, the most common culprit is a BULK INSERT or bulk copy tool (bcp).

One of its parameters is -h(hints)
and one of those hints is CHECK_CONSTRAINTS

Read on to see how this can mess everything up, as well as how you can track and fix it. There are cases, particularly in extremely high-write systems, where you don’t necessarily need the constraint to exist but want it to be there for documentation purposes. In that case, the constraints are usually disabled rather than simply untrusted. The other time I see people purposefully using untrusted constraints is that old data is garbage and essentially unfixable but they want new data to be correct. Most of the time, though, constraints are untrusted because nobody noticed the problem.

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Connecting to Azure SQL DB over VPN

Published 2022-06-24 by Kevin Feasel

Reitse Eskens has some routing issues:

To make sure the on-premises connection uses the VPN and the private endpoint, we need to make sure the on-premises DNS (it’s always DNS) recognizes the traffic and redirects it to the VPN connection. But whatever we tried on the firewall, the traffic kept going the wrong way. It did have something to do with the on-premises DNS setup in the end.

When we tried to connect to the Azure SQL instance on IP-address, it threw an error because the instance wasn’t found. You can only connect to it with the FQDN (dbname.database.windows.net)

Click through to see what the problem was and how Reitse solved it.

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SSIS — RPC Server is Unavailable

Published 2022-06-24 by Kevin Feasel

Jon Morisi does some troubleshooting:

I just spent a long slog sorting out why I could not connect to my SSIS instance remotely.  I work in a very secure environment requiring network approval for any and all ports.  According to the following article, I was under the impression that a request to open incoming traffic on port 135, to a specific IP, would allow SQL Server Management Studio, on that specific IP, to connect remotely to SSIS:

https://docs.microsoft.com/en-us/sql/sql-server/install/configure-the-windows-firewall-to-allow-sql-server-access?redirectedfrom=MSDN&view=sql-server-ver16#BKMK_ssis

After opening port 135, I was receiving the error message in the title of this article:

If you find yourself in this situation, read on to see how Jon was able to solve the problem.

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Git Native Support for Databricks Workflows

Published 2022-06-23 by Kevin Feasel

Vaibhav Sethi and Roland Faeustlin make an announcement:

We are happy to announce native support for Git in Databricks Workflows, which enables our customers to build reliable production data and ML workflows using modern software engineering best practices. Customers can now use a remote Git reference as the source for tasks that make up a Databricks Workflow, for example, a notebook from the main branch of a repository on GitHub can be used in a notebook task. By using Git as the source of truth, customers eliminate the risk of accidental edits to production code. They also remove the overhead of maintaining a production copy of the code in Databricks and keeping it updated, and improve reproducibility as each job run is tied to a commit hash. Git support for Workflows is available in Public Preview and works with a wide range of Databricks supported Git providers including GitHub, Gitlab, Bitbucket, Azure Devops and AWS CodeCommit.

Read on to see how it works.

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Understanding the Poisson Distribution

Published 2022-06-23 by Kevin Feasel

Achim Zeileis shows off my favorite statistical distribution:

The Poisson distribution has many distinctive features, e.g., both its expectation and variance are equal and given by the parameter λλ. Thus, E(Y)=λE(Y)=λ and Var(Y)=λVar(Y)=λ. Moreover, the Poisson distribution is related to other basic probability distributions. Namely, it can be obtained as the limit of the binomial distribution when the number of attempts is high and the success probability low. Or the Poisson distribution can be approximated by a normal distribution when λλ is large. See Wikipedia (2002) for further properties and references.

Here, we leverage the distributions3 package (Hayes et al. 2022) to work with the Poisson distribution in R. In distributions3, Poisson distribution objects can be generated with the Poisson() function. Subsequently, methods for generic functions can be used print the objects; extract mean and variance; evaluate density, cumulative distribution, or quantile function; or simulate random samples.

Read on for a detailed tutorial. H/T R-bloggers.

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Saving and Loading a Keras Model

Published 2022-06-23 by Kevin Feasel

Jason Brownlee made it to a savepoint in time:

Given that deep learning models can take hours, days and even weeks to train, it is important to know how to save and load them from disk.

In this post, you will discover how you can save your Keras models to file and load them up again to make predictions.

After reading this tutorial you will know:

– How to save model weights and model architecture in separate files.

– How to save model architecture in both YAML and JSON format.

– How to save model weights and architecture into a single file for later use.

Read on for an updated step-by-step tutorial.

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Setting Breakpoints in Powershell Scripts

Published 2022-06-23 by Kevin Feasel

Patrick Gruenauer does a bit of debugging:

The Set-PSBreakPoint cmdlet sets a breakpoint in a script. When you are troubleshooting a script it could be helpful to know what’s going on in a particualar step or workflow. In this blog bost I will give you an overview and the basics you can build on to troubleshoot and investigate your script. Let’s jump in.

One of these years, I’m finally going to learn command-line debugging. I grew up in the IDE era and so never took the time to learn that skill.

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Working with xp_cmdshell

Published 2022-06-23 by Kevin Feasel

Hadi Fadlallah takes us through xp_cmdshell:

In brief, xp_cmdshell is a system stored procedure in SQL Server. It allows executing Windows shell commands from the SQL Server environment. While commands are passed as an input string, the shell’s output is returned as rows of text.

The xp_cmdshell takes two parameters; one required and one optimal:

Hadi does a good job of showing us what security is in place protecting malicious use of xp_cmdshell and how you can add a person to the list of users.

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Improving Join Performance with Skewed Datasets in Spark

Published 2022-06-22 by Kevin Feasel

Ajay Gupta gets into the topic of join performance:

Performing Joins on Skewed Datasets: A Dataset is considered to be skewed for a Join operation when the distribution of join keys across the records in the dataset is skewed towards a small subset of keys. For example when 80% of records in the datasets contribute to only 20% of Join keys.

Implications of Skewed Datasets for Join: Skewed Datasets, if not handled appropriately, can lead to stragglers in the Join stage (Read this linked story to know more about Stragglers). This brings down the overall execution efficiency of the Spark job. Also, skewed datasets can cause memory overruns on certain executors leading to the failure of the Spark job. Therefore, it is important to identify and address Join-based stages where large skewed datasets are involved.

Read on for five techniques which may help you out.

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