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Curated SQL Posts

Generating Realistic-Looking Data With Markov Chains

Published 2018-07-16 by Kevin Feasel

Phil Factor shows how to use Markov chain generation in T-SQL to generate realistic-looking country names:

How did we do this? We started with a table that took each word, added two spaces at the beginning and a |, followed by two subsequent spaces, at the end. This allowed us to map the frequency of each three-letter combination in a collection of words. Any language is made up of common combinations of characters with a few wild exceptions. For words to look right, they must follow this distribution. This distribution will change in various parts of a word, so you need all this information.

So what would happen if, instead of feeding the name of countries into the batch, we do the names of people?

My favorite name from the list was Kuwatian Samoa.

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Windows Server 2019: USB-Based File Share Witness Support

Published 2018-07-16 by Kevin Feasel

Dave Bermingham introduces us to a new feature coming in Windows Server 2019:

I’m very excited to hear that coming in Windows Server 2019 there will be a few new features in regards to the File Share Witness for the Failover Cluster Quorum. The feature that many of my customers have been asking for about for many years is finally arriving…File Share Witness on a USB stick!

Okay, they didn’t really ask for that specifically, but many of my customers wanted to deploy a simple 2-node cluster in each store location, branch office, etc., and they didn’t want the added expense of a SAN to leverage a Disk Witness and weren’t to keen, or just didn’t have the connectivity, to rely on a Cloud Witness in Azure. Many of these customers just decided to forgo clustering, or they used an alternative clustering solution like the SIOS Protection Suite.

Now they have a viable alternative coming in Windows Server 2019. By leveraging a supported router, a USB disk inserted into the router can be configured with a file share that can be used as the witness. This eliminates the need for a 3rd server or internet connectivity.

I can’t see this being extremely useful in most scenarios, though that could be a lack of imagination on my part.

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Using Azure Blob Storage Archive Tier For Archival Data

Published 2018-07-16 by Kevin Feasel

Bob Pusateri shows us how to configure Azure Blob Storage Archive Tier:

Two of the products I use extensively for this purpose are Amazon Glacier and, more recently, Microsoft Azure Blob Storage Archive Tier. As happy as I’ve been with Amazon Glacier since its introduction in 2012, I always hoped Microsoft would offer a similar service. My wish came true in Fall of 2017 when an archive tier of Azure Blob Storage was announced. Rather than branding this capability as a new product, Microsoft decided to present it as a new tier of Azure Blob Storage, alongside the existing hot and cool storage tiers.

A noticeable difference from the hot and cool storage tiers is that the archive storage tier is only available on a per-blob basis. While a storage account can be configured to have all blobs placed in either the hot or cool tier by default once they are uploaded, the archive tier is not an option. Once a blob is uploaded, it must explicitly be moved into the archive tier. If one is using the Azure Portal to do this, there’s several clicks involved per blob. The free Azure Storage Explorer client is no better. While I found several third party tools that can upload files to the archive tier, none were free. At this point, I decided to write my own method using Powershell, which I am happy to share below.

Read on for the script.  A good use for Azure Blob Storage Archive Tier would be storing old database backups which you have to keep around for compliance purposes but rarely use.

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Diagnosing A SQL Server Error Using WinDbg

Published 2018-07-16 by Kevin Feasel

Dmitry Pilugin gives us a specific example of using WinDbg to track down a bug in SQL Server:

The important part is that the same stack with the same relative to a module addresses was in all the dumps.

To get an idea of what is going on, we may try to convert the relative address: Module(<sqlmodule>+hex), into a SQL Server’s method name. Probably the method’s name would be descriptive enough to give some information. This is the moment where WinDbg with public symbols may help.

You may download WinDbg from the here, and then download public symbols, you may use WinDbg documentation or here is an example of how to do it from Paul Randal, adopt it to your folders and version.

You should not debug a production server, because WinDbg will break any singe instruction execution, so make sure you have a test server with the exact same SQL Server version as production (in my case it was 13.0.4474.0).

I heartily second the comment that you never want to run the debugger in production.

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When Image Classifiers Look At Unknown Objects

Published 2018-07-13 by Kevin Feasel

Pete Warden explains that image classifiers aren’t magic:

As people, we’re used to being able to classify anything we see in the world around us, and we naturally expect machines to have the same ability. Most models are only trained to recognize a very limited set of objects though, such as the 1,000 categories of the original ImageNet competition. Crucially, the training process makes the assumption that every example the model sees is one of those objects, and the prediction must be within that set. There’s no option for the model to say “I don’t know”, and there’s no training data to help it learn that response. This is a simplification that makes sense within a research setting, but causes problems when we try to use the resulting models in the real world.

Back when I was at Jetpac, we had a lot of trouble convincing people that the ground-breaking AlexNet model was a big leap forward because every time we handed over a demo phone running the network, they would point it at their faces and it would predict something like “Oxygen mask” or “Seat belt”. This was because the ImageNet competition categories didn’t include any labels for people, but most of the photos with mask and seatbelt labels included faces along with the objects. Another embarrassing mistake came when they would point it at a plate and it would predict “Toilet seat”! This was because there were no plates in the original categories, and the closest white circular object in appearance was a toilet.

Read the whole thing.

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Replicating Data In HDFS Between Clusters

Published 2018-07-13 by Kevin Feasel

Murali Ramasami and Niru Anisetti have an article showing how to use the Hortonworks Data Lifecycle Manager to set up replication between two Hadoop clusters:

Data Lifecycle Manager (DLM) delivers on the promise of location-agnostic, secure replication by encapsulating and copying data seamlessly across physical private storage and public cloud environments. This empowers businesses to deliver the right data in the right environment to power the right use cases.

DLM v1.1 provides a complete solution to replicate data, metadata and security policies between on-premises and in cloud. It also supports data movement for data-at-rest and data-in-motion – whether the data is encrypted using a single key or multiple keys on both source and target clusters. DLM supports HDFS and Apache Hive dataset replication.

With DLM infrastructure administrators can manage their data, metadata and security management on-prem and in-cloud using a single-pane of glass that is built on open source technology. Business users can consume their workload outputs in the cloud with data-source-abstraction. DLM also enables business to reduce their capital expenditures and enjoy the benefits of flexibility and elasticity that cloud provides.

Click through for a demo.  May HDFS replication have as long a life and slightly less vitriol than SQL Server replication.

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Microsoft Research Open Data Sets

Published 2018-07-13 by Kevin Feasel

David Smith notes that there are several data sets that Microsoft Research has made available:

Other data sets of note include:

  • A collection of 38M tweets related to the 2012 US election

  • 3-D capture data from individuals performing a variety of hand gestures

  • Infer.NET, a framework for running Bayesian inference in graphical models

  • Images for 1 million celebrities, and associated tags

  • MS MARCO, is a new large-scale dataset for reading comprehension and question answering

Click through for more information, and then check out the data sets.

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Comparing System Metadata Between SQL Server Versions

Published 2018-07-13 by Kevin Feasel

Aaron Bertrand shows how he finds hidden features in new SQL Server builds:

One of the areas I like to focus on is new features in SQL Server. Under both MVP and Microsoft Partner programs, I get to see a lot of builds of SQL Server that don’t make it to the public, and documentation for these builds is typically sparse. In order to get a head start on testing things out, I often need to explore on my own. And so I wrote some scripts for that, which I’ve talked about in previous blog posts:

When I install a new version of SQL Server (be it a cumulative update, the final service pack for a major version, or the first CTP of vNext), there are two steps:

  1. Create a linked server to the build that came before it

  2. Create local synonyms referencing the important catalog views in the linked server

It’s a good way to get a glimpse at which features devs are currently working on but haven’t enabled yet.

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Why .NET And Java Have StringBuilders

Published 2018-07-13 by Kevin Feasel

Randolph West walks us through a performance troubleshooting issue with a twist:

So we branch the the code in source control, and start writing a helper class to manage the data for us closer to the application. We throw in a SqlDataAdapter, use the Fill() method to bring back all the rows from the query in one go, and then use a caching layer to keep it in memory in case we need it again. SQL Server’s part in this story has now faded into the background. This narrow table consumes a tiny 8 MB of RAM, and having two or more copies in memory isn’t the end of the world for our testing. So far, so good again.

We run the new code, first stepping through to make sure that it still does what it used to, massaging here and there so that in the end, a grid is populated on the application with the results of the query. Success! We then compile it in Release mode, and run it without any breakpoints to do some performance testing.

And then we find that it runs at exactly the same speed to produce exactly the same report, using our caching and SqlDataAdapter, and we’ve wasted another hour of our time waiting for the grid and report. Where did we go wrong?

As people get better at tuning, we start to make assumptions based on prior experience.  That, on net, is a good thing, but as Randolph shows, those assumptions can still be wrong.

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The Problems With NOLOCK

Published 2018-07-13 by Kevin Feasel

Rob Farley demonstrates the downside of the READ UNCOMMITTED isolation level:

I’m going to create a table and insert exactly 1 million rows. This particular table will be a clustered index, and will contain 1 million GUIDs.

Next I prove that there a million rows.

Now without inserting or deleting any rows, I’m going to shuffle them.

And if while this is happening, I count the rows in a different session, I have to wait for that query to finish.

Read on to see what happens when someone gets the idea of running the select query with NOLOCK.

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