The Magic Of Sampling

Nathan LeClaire reminds us of an important story that statisticians have been telling us for a couple centuries:

It starts slowly. Maybe your home-grown centralized logging cluster becomes more difficult to operate, demanding unholy amounts of engineer time every week. Maybe engineers start to find that making a query about production is a “go get a coffee and come back later” activity. Or maybe monitoring vendors offer you a quote that elicits a response ranging anywhere from curses under the breath to blood-curdling screams of terror.

The multi-headed beast we know as Scale has reared its ugly visage.

As some of you may have already guessed from the title, I’m going to discuss one way to solve this problem, and why it might not be as bad as you might think.

Take some of your precious information and throw it in the garbage. In lots of cases, you can just drop those writes on the floor as long as your observability stack is equipped to handle it.

In other words, sample.

Read on for a couple of methods.  One thing I’ve taken a fancy to is collecting the first N of a particular type of message and keeping track of how often that message appears.  If you get the same error for every row in a file, then you might really only need to see that one time and the number of times it happened.  Or maybe you want to see a few of them to ensure that they’re really the same error and not two separate errors which are getting reported together due to insufficient error separation.

Quickly Computing Area Under The Curve

Jean-Francois Puget has a fast method for computing Area Under the Curve in Python:

When the target only takes two values we have a binary classification problem at hand.  Example of binary classification are very common. For instance fraud detection where examples are credit card transactions, features are time, location, amount, merchant id, etc., and target is fraud or not fraud.  Spam detection is also a binary classification where examples are emails, features are the email content as a string of words, and target is spam or not spam.  Without loss of generality we can assume that the target values are 0 and 1, for instance 0 means no fraud or no spam, whiloe 1 means fraud or spam.

For binary classification, predictions are also binary.  Therefore, a prediction is either equal to the target, or is off the mark.  A simple way to evaluate model performance is accuracy: how many predictions are right? For instance, if our test set has 100 examples in it, how many times is the prediction correct?  Accuracy seems a logical way to evaluate performance: a higher accuracy obviously means a better model.  At least this is what people think when they are exposed to the first time to binary classification problems.  Issue is that accuracy can be extremely misleading.

Read Jean-Francois’ explanation and scroll down for the Python sample.

Spotting SQL Agent Job Differences

Rob Sewell shows us how to compare SQL Agent jobs across Availability Group replicas:

On the plane home from PAS Summit I was sat next to someone who had also attended and when he saw on my laptop that I was part of the SQL Community we struck up a conversation. He asked me how he could compare SQL Agent Jobs across availability group replicas to ensure that they were the same.

He already knew that he could use Copy-DbaAgentJob from dbatools to copy the jobs between replicas and we discussed how to set up an Agent job to accomplish this. The best way to run an Agent Job with a PowerShell script is described here

Maybe I’m out in la-la land, but wouldn’t this be a perfect use case for MSX/TSX?

Offline Installation Of SQL Server 2017 ML Services

Jan Mulkens shows how to install SQL Server 2017 Machine Learning Services when your the server hosting SQL Server doesn’t have outbound internet access:

That’s when you remember it… Your server isn’t connected to the internet!
Pretty normal, but in your enthusiasm you completely forgot that SQL Server needs to download some binaries for the R and Python components you so desperately want on your precious machine!

Luckily, the installer comes to your rescue and shows you where to download those binaries it needs.
Turns out however… This link only is for one R component and the installer won’t let you pass to the next screen!

Read on for the answer.

Actual Execution Plan Enhancements

Pedro Lopes points out some additional data available in the properties section when you generate an actual execution plan:

Looking at the actual execution plan is one of the most used performance troubleshooting techniques. Having information on elapsed CPU time and overall execution time, together with session wait information in an actual execution plan allows a DBA to use showplan to troubleshoot issues away from the server, and be able to correlate and compare different types of waits that result from query or schema changes.

A few months ago we had introduced exposed in SSMS some of the per-operator statistics, such as CPU and elapsed time per thread. More recently, we have introduced overall query CPU and elapsed time tracking for statistics showplan xml (both in ms). These can be found in the root node of an actual plan. Available using the latest versions of SSMS v17, when used with SQL Server 2012 SP4SQL Server 2016 SP1 and SQL Server 2017. For SQL Server 2014 it will become available in a future Service Pack.

Also be sure to check out Geoff Patterson’s Connect item asking that the execution plan results show the top ten waits in descending order rather than ascending order.  That’s the appropriate ordering in my mind:  show me the most important things first.

Mislabeled Column In dm_os_sys_memory

Lonny Niederstadt points out that the definition of a column in the sys.dm_os_sys_memory DMV is incorrect:

Based on the column names and values above, seems natural to think:
total_page_file_kb – available_page_file_kb = used page file kb
11027476 kb – 3047668 kb = 7979808 kb

Holy cow! Is my laptop using nearly as much paging space as there is RAM on the laptop??
Weird. If something forced that much paging space use relative to RAM on the laptop… I certainly wouldn’t expect system_memory_state_desc = ‘Available physical memory is high’!!

Read on for an explanation of what the columns actually mean.

The Difficulties Of Memory-Optimized Tables

Michael J. Swart relays a cautionary table around using In-Memory OLTP:

We’re leaving the feature behind for a few reasons. There’s an assumption we relied on for the sardine servers: Databases that contain no data and serve no activity should not require significant resources like disk space or memory. However, when we turned on In Memory OLTP by adding the filegroup for the memory-optimized data, we found that the database began consuming memory and disk (about 2 gigabytes of disk per database). This required extra resources for the sardine servers. So for example, 1000 databases * 2Gb = 2Tb for a server that should be empty.

Another reason is that checkpoints began to take longer. Checkpoints are not guaranteed to be quick, but on small systems they take a while which impacts some of our Continuous Integration workflows.

Read the whole thing.  This technology definitely does not fit all use cases, and there are some painful limitations.  If it does fit, however, you’ll wonder how you lived without it.

Handling IoT Traffic With SQL Server

Perry Skountrianos builds a reference architecture for handling nearly one and a half million rows per second with SQL Server:

The following sample demonstrates the high scale and performance of SQL Database, with the ability to insert 1.4 million rows per second by using a non-durable memory-optimized table to speed up data ingestion, while managing the In-Memory OLTP storage footprint by offloading historical data to a disk-based Columnstore table for real time analytics. One of the customers already leveraging Azure SQL Database for their entire IoT solution is Quorum International Inc., who was able to double their key database’s workload while lowering their DTU consumption by 70%.

If you hit on the right scenario, memory-optimized tables can be great.

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