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Day: December 7, 2018

The Intuition Behind Principal Component Analysis

Published 2018-12-07 by Kevin Feasel

Holger von Jouanne-Diedrich gives us an intuition behind how principal component analysis (PCA) works:


Principal component analysis (PCA) is a dimension-reduction method that can be used to reduce a large set of (often correlated) variables into a smaller set of (uncorrelated) variables, called principal components, which still contain most of the information.
PCA is a concept that is traditionally hard to grasp so instead of giving you the n’th mathematical derivation I will provide you with some intuition.
Basically PCA is nothing else but a projection of some higher dimensional object into a lower dimension. What sounds complicated is really something we encounter every day: when we watch TV we see a 2D-projection of 3D-objects!

Click through for the rest of the story.

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Thoughts On Exclusive Locks

Published 2018-12-07 by Kevin Feasel

Louis Davidson shares some thoughts on exclusive locks in SQL Server:


You will find that the SELECT statement executes, ignoring the exclusive lock, because it is not a write lock, and the data on the page has not been changed.
The main reason people try to do this is to force access to a row in a single threaded manner. For example, building their own sequence number, either in a row they update, or by trying to do MAX() on all of the data in a table to make sure only one reader gets the same value.
This is generally a bad idea, since locking an entire table is a generally bad idea, but if you needed to block readers, you can couple the XLOCK with a PAGLOCK. So, change the first reader to:

BEGIN TRANSACTION;
SELECT *
FROM   Demo.Test WITH (XLOCK,PAGLOCK);

As Louis points out in the summary, locking is complicated.  Having a good understanding of the locking model will serve you well, though.

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NoSQL? No! MoSQL

Published 2018-12-07 by Kevin Feasel

Steve Jones points out a bit of a shift at Google:


Google is doing more SQL, or at least shifting towards relational SQL databases as a way of storing data. At least, some of their engineers see this as a better way to store data for many problems. Since I’m a relational database advocate, I found this to be interesting.
When Google first started to publish information on BigTable and other new ways of dealing with large amounts of data, I felt that these weren’t solutions I’d use or problems that many people had. The idea of Map Reduce is interesting and certainly applicable to the problem space Google had of a global database of sites, but that’s not a problem I’ve ever encountered. Instead, most of the struggles I’ve had with relational systems are still better addressed in a relational system.

Read the whole thing.  Note that this is slightly different than Feasel’s Law, as Steve is focusing more on the consistency side of things rather than the interface.

Also, just going to leave this here:

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When Azure SQL DB Tells You You’re Hitting Your DTU Limits

Published 2018-12-07 by Kevin Feasel

Brent Ozar shows us how to use sp_BlitzFirst to determine if you’re hitting DTU limits:


You’ve got an Azure SQL DB, and your queries are going slow. You’re wondering, “Am I hitting the performance limits? Is Microsoft throttling my queries?”
There’s an easy way to check: run sp_BlitzFirst. sp_BlitzFirst is our free performance health check stored procedure that analyzes a lot of common performance issues and then gives you a prioritized list of reasons why your server might be slow right now.

Click through for an example.  The same signal can mean “it’s time to move to a higher level in your tier,” “it’s time to move up to the next tier,” or “it’s time to tune that cursor in a cursor in a cursor which performs several scans of a very large table for each operation.”

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Finding The SQL Server Port With T-SQL

Published 2018-12-07 by Kevin Feasel

Jack Vamvas shows us how to find the port SQL Server is listening on using T-SQL:


Question: Without going into the SQL Server Configuration manager via the GUI is there a command oriented method to extract the port number SQL Server is listening on?
 
Answer: There are a few different methods to extract the port number without going into the Configuration interface.
Method 1 – use xp_readererrorlog

Read on for an explanation of this technique as well as links to a couple other methods.  I hadn’t thought about using the error log as a source, but it works.

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