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Author: Kevin Feasel

pg_prewarm and a Bug in PostgreSQL 16.8

Published 2026-04-09 by Kevin Feasel

Warda Bibi explains an issue:

Recently, we encountered a production incident where PostgreSQL 16.8 became unstable, preventing the application from establishing database connections. The same behavior was independently reproduced in a separate test environment, ruling out infrastructure and configuration issues. Further investigation identified the pg_prewarm extension as the source of the problem.

This blog post breaks down the failure, the underlying constraint, why it manifests only under specific configurations, and the corresponding short-term mitigation and long-term fix.

Click through for those details. The bug is no longer an issue as of PostgreSQL 16.10.

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Performing a GROUPBY in Excel

Published 2026-04-09 by Kevin Feasel

Ben Richardson explains a DAX function:

If you’ve ever built a summary table in Excel by writing five separate SUMIFS formulas – one for each region, product, or department – you’ll appreciate what GROUPBY does.

It collapses all of that into a single formula that updates automatically, sorts itself, and can even filter out rows you don’t care about.

Introduced to Microsoft 365 in late 2024, GROUPBY is one of the most practically useful additions Excel has had in years.

But it also comes with real limitations that most people gloss over.

Read on to learn more about the capability and one big catch.

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The Scope of ##MS_DatabaseManager##’s Exposure

Published 2026-04-09 by Kevin Feasel

Andreas Wolter expands upon a prior post:

In my previous post (SQL Server Privilege Escalation via DatabaseManager-role: Newly discovered Attack Paths Explained), I showed how a privilege escalation can be achieved by modifying a stored procedure in msdb, based on research shared by Emad Al-Mousa.

When I looked into this more closely, it became clear pretty quickly:
It is not just about a few specific procedures.

Mind you, Andreas is not some kind of shrinking violet, and he was involved in the creation of this role in the first place. There are approximately zero people outside of Microsoft (and the number inside I could probably count on one hand) I would trust more to understand the nature and scope of this issue. This is definitely worth reading.

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Optional SUBSTRING() Length in SQL Server 2025

Published 2026-04-09 by Kevin Feasel

Louis Davidson points out a neat update:

Sometimes along comes a feature that seems so obvious, so natural, that you wonder why it took so long for Microsoft to implement it. One of those features in SQL Server 2005 is the optional length parameter in the SUBSTRING function. It has long been one of those questions when you wrote a SUBSTRING expression when you wanted to go from the Nth character to the end of the string, how many characters do you want? And for the most part, it didn’t really matter.

But sometimes it did (especially when dealing with nvarchar(max) data.

I learned about this when putting together an update to my Teaching Old Dogs New Tricks presentation. This capability is pretty nifty and something I wish I had a while ago.

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Accelerated Database Recovery in tempdb for SQL Server 2025

Published 2026-04-08 by Kevin Feasel

Rebecca Lewis looks into a feature:

Two weeks ago I covered the Resource Governor changes in SQL Server 2025 — specifically, capping how much tempdb data space a workload group can consume. That was the data-file side. For the log side, SQL Server 2025 now lets you enable Accelerated Database Recovery (ADR) on tempdb. Enable it and cancelled queries stop grinding, the tempdb log stops bloating, and recovery gets faster. Sounds like an easy yes — but you’ve got to read the fine print.

Click through for that fine print.

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A Gotcha with pg_column_size()

Published 2026-04-08 by Kevin Feasel

Laetitia Avrot wants to know how large a column is:

Thanks to my colleague Ozair, who sent me a JIRA ticket saying “I need to drop that huge column, what are the consequences?” My first question was: how huge? That’s when the rabbit hole opened.

It looks simple. It is simple. Just use the administrative function pg_column_size(). Until you have toasted attributes. Then it gets interesting.

Read on for a demonstration.

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Shortcut Transformations now GA in Microsoft Fabric

Published 2026-04-08 by Kevin Feasel

Pernal Shah transforms some data:

Organizations today manage data across multiple storage systems, often in formats like CSV, Parquet, and JSON. While this data is readily available, turning it into analytics-ready tables typically requires building and maintaining complex ETL pipelines.

Shortcut transformations remove that complexity.

With Shortcut transformations, you can convert structured files referenced through OneLake shortcuts into Delta tables without building pipelines or writing code.

This currently works for CSV, Parquet, and JSON data and does cut out a very common step for raw-layer transformation.

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TOP(1) with Ties

Published 2026-04-08 by Kevin Feasel

Andy Brownsword can’t stop at one:

Having TOP (1) return multiple rows feels wrong… but that’s what WITH TIES can do.

For a long time I used patterns like this to get the first record in a group:

Andy goes on to explain how WITH TIES works in T-SQL, shows an alternative to using a common table expression + window function to narrow down to the first logical group, and digs into when you might not want to use that alternative.

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When Multi-Column Statistics (Mostly) Aren’t

Published 2026-04-08 by Kevin Feasel

Brent Ozar digs into multi-column statistics:

The short answer: in the real world, only the first column works. When SQL Server needs data about the second column, it builds its own stats on that column instead (assuming they don’t already exist), and uses those two statistics together – but they’re not really correlated.

For the longer answer, let’s take a large version of the Stack Overflow database, create a two-column index on the Users table, and then view the resulting statistics:

Click through for a dive into row estimation when you have multiple columns in a WHERE clause and how statistics come into play.

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