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Category: Query Tuning

Techniques for Unpivoting Data in SQL Server

Published 2024-10-01 by Kevin Feasel

Jared Westover performs a technique showdown:

A few weeks ago, I helped someone combine multiple integer columns into a single column. The common term for this process is unpivoting or transposing. The table they worked with had millions of rows, so they needed the most efficient method to accomplish this task. If you search online, you’ll find several suggestions for doing this. But which performs best? Is one easier to maintain than the others?

Click through for a review of three separate techniques: using the UNPIVOT operator, using UNION ALL, and using CROSS APPLY. The dataset was relatively small, but even at that size, CROSS APPLY did a good job. But I won’t spoil too much here.

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Window Functions and Running Totals

Published 2024-09-30 by Kevin Feasel

Steve Jones makes a comparison:

Often I see running totals that are written in SQL using a variety of techniques. Many pieces of code were written in pre-2012 techniques, prior to window functions being introduced.

After SQL Server 2012, we had better ways to write a total. In this case, let’s see how much better. This is based on an article showing how you might convert code from the first query to the second. This is a performance analysis of the two techniques are different scales..

Steve shows a very fancy version of the self-join technique, which is actually even slower than using a cursor for this work. The fastest variant on the technique was something nicknamed the ‘quirky update’ technique, but it relied on an accidental property of how clustered indexes worked on temp tables in SQL Server and was not something Microsoft ever officially supported, meaning that any service pack, cumulative update, or hotfix might have broken your code and the best you’d get is an indifferent shrug.

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Parameter Sensitive Plan Optimization and Memory Grant Feedback

Published 2024-09-24 by Kevin Feasel

Deborah Melkin has a video for us:

I’m doing something new – instead of a full post with lots of text, I’m doing a video! While it’s not as polished as others in the community, I didn’t want making it perfect get in the way of getting this done. I’m hoping to do more of these so I will definitely be upping in my game in the production department in the future.

But it’s really the content that I wanted to focus on. It’s a quick look at Parameter Sensitive Plan Optimization and Memory Grant Feedback and what they can do. This is just an introduction to seeing them in action and understanding how we can use that to help tune our queries long term.

Click through for the video.

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Parsing SQL Server Query Stats in Powershell

Published 2024-09-24 by Kevin Feasel

Andy Levy builds a cmdlet:

A couple weeks ago, DrData asked

With SET STATISTICS IO ON, is there really no way to show the TOTALS at the bottom? There are some nice tools like StatisticsParser but it sure would be nice to see the totals of all values right at the bottom, especially when there are many tables.

The task at hand in the office was a need to do the same thing, but with SET STATISTICS TIME ON. So I got to thinking…I can PowerShell my way through this, right?

Andy, being The Man, shows us exactly how to do this.

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ISNULL vs COALESCE in SQL Server

Published 2024-09-13 by Kevin Feasel

Erik Darling has a video for us:

A Difference Between ISNULL And COALESCE You Might Care About In SQL Server

There’s nothing for me to snip as the graf. I don’t often link to videos without any sort of text accompaniment, but it’s been too long since I’ve linked to Erik and this was an interesting topic.

Bonus points for using “case expression” instead of the more common but technically incorrect “case statement.”

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Implicit Conversions in SQL Server

Published 2024-09-12 by Kevin Feasel

Vlad Drumea explains what it means implicitly to convert:

If you’re here, you most likely know what a data type conversion is, but, in short, it’s the operation of converting a value from one data type to another.

There are two types of conversions in SQL Server:

  • explicit – which are done by explicitly applying the CAST and CONVERT functions on a column, variable, or value.
  • implicit – when CAST and CONVERT are not used explicitly, but SQL Server ends up doing the conversation behind the scenes due to two distinct data types being compared.

Read on to learn more about which types of implicit conversion are relevant for performance and what you can do instead.

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Query Start Times in Query Store

Published 2024-09-11 by Kevin Feasel

Hugo Kornelis describes an issue:

I was hired by a customer who had a very annoying issue with the daily data load of their data warehouse. The volume of data to be loaded is high and they were already struggling to finish the load before business opens. But that was not their biggest issue. The biggest problem, the real pain point that they hired me for, is that at unpredictable moments, the load would run much longer than normal, pushing it well into business hours. They wanted me to find out what caused those irregular delays, and find a way to stop them from happening.

Read on to learn more about the issue itself, as well as a discrepancy in what Query Store showed. Hugo also points out that the quick-and-easy solution may not be the right solution.

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Troubleshooting a Performance Issue using Query Store

Published 2024-09-03 by Kevin Feasel

Edwin Sarmiento shows a practical application of Query Store:

In a previous tip on SQL Server 2016 Query Store Introduction, we’ve seen how to enable Query Store and find out the top resource consumers. We’ve experienced high CPU utilization recently and wanted to know the root cause and how to fix it. How can we use Query Store to achieve this?

Read on for the scenario and how Edwin diagnosed and fixed the issue.

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Performance Profile of Fast-Forward Cursors

Published 2024-09-03 by Kevin Feasel

Hugo Kornelis continues a deep dive into cursors:

One of the things that has always bothered me about the fast forward cursor type is the shocking lack of documentation of what it does exactly. Okay, the name suggests that it is fast. But is it really? When I first looked at cursor performance (granted, a long time ago), I found that a static cursor was actually faster than a fast forward cursor for the same query. So… fastish forward?

The name also suggests that this cursor is forward only. That is indeed the case. At least they got that right in the naming.

Read on to learn more about this cursor property, as well as how it compares to static and dynamic cursors. Hugo ends on a spicy take you won’t want to miss.

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Computed Columns and Wide Index Updates

Published 2024-08-30 by Kevin Feasel

Paul White takes us through a performance scenario:

Update execution plans are not something the T-SQL statement writer has much control over. You can affect the data reading side of the plan with query rewrites and hints, but there’s not nearly as much tooling available to affect the writing side of the plan.

Update processing can be extremely complex and reading data-changing execution plans correctly can also be difficult. Many important details are hidden away in obscure and poorly documented properties, or simply not present at all.

In this X article, I want to you show a particularly bad update plan example. It has value in and of itself, but it will also give me a chance to describe some less well-known SQL Server details and behaviours.

Read on for the full story.

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