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Category: Bugs

High CPU with SQL Server Database Mail

Published 2024-12-12 by Kevin Feasel

Vitaly Bruk deals with a CPU issue:

Today, I investigated an interesting issue.

One of my clients called me and complained about high CPU on his server. Server with 0 load at this time frame.

Using my favorite “Activity” script and the sys.dm_os_ring_buffers DMV, I clearly saw that the CPU is being used by a non-SQL server service. Next, I’ve asked to connect SQL Server machine and opened a task manager.

Surprise, surprise! The CPU was being used by the SQL Server process! Well, kind of…

The moral of the story is to keep your systems patched.

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Query Hash Value Changes

Published 2024-11-25 by Kevin Feasel

Kendra Little digs into a problem:

This is the worst bug I’ve found in SQL Server to date. Previously, my top find was SQL Server Online Index Rebuild sometimes happens offline without warning. This one has taken top slot because it makes my life more difficult on a daily basis.

Background: SQL Server generates a query_hash for each query. This is stored in sys.query_store_query and it’s one of the primary ways you can identify what a query is across different Query Stores, or even the same Query Store over time, as surrogate query_id values get reset if Query Store is cleared or data ages on. The query_hash is a “Zobrist hash over the shape of the individual query, based on the bound (input) logical query tree. Query hints aren’t included as part of the hash.” (Source)

Except that’s wrong.

Read the whole thing.

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AWS DMS and a LOB Bug

Published 2024-11-25 by Kevin Feasel

Richard O’Riordan fixes an issue:

The table over in our Postgres cluster is similar except for the data type “text” being used instead of “varchar”. All kind of boring so far, but what we noticed that on some very rare occasions the “largevalue” column was empty over in the PostgreSQL database even though for that row it was populated in SQL Server.

This seemed odd to me, like you would expect if there was some error inserting the row on the PostgreSQL side then since it is all done within a transaction that it would either all succeed or all fail, how would the row be partially inserted, i.e. missing this text value.

Read on for the story and several tests of how things work.

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The Pain of fn_xe_file_target_read_file

Published 2024-10-21 by Kevin Feasel

Tom Zika plants a flag:

I haven’t had a rant post in a while. There is a saying: “Anything before the word ‘but’ is ignored”. I love Extended Events, but … reading the extended event file is so much pain.

It feels like there is a conspiracy between Microsoft and Big Pharma SQL Monitoring because the best analytics tool available in SQL Server (and I mean Extended Events and Query Store of course) have the worst GUI and supporting tools. I’m focusing on XE in this post.

Read on for Tom’s rant. To add on to it, the feedback item Tom links to now has a new update from Dimitri Furman, indicating (based on how I’m reading it) that the fix will be in SQL vNext, not SQL Server 2022.

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The State of the ANY Aggregate Transformation

Published 2024-09-16 by Kevin Feasel

Paul White covers an aggregate operator:

SQL Server provides a way to select any one row from a group of rows, provided you write the statement using a specific syntax. This method returns any one row from each group, not the minimum, maximum or anything else. In principle, the one row chosen from each group is unpredictable.

The general idea of the required syntax is to logically number rows starting with 1 in each group in no particular order, then return only the rows numbered 1. The outer statement must not select the numbering column for this query optimizer transformation (SelSeqPrjToAnyAgg) to work.

Read on for information about this internal operator, a bug that existed in it for a long time, and the current state of fixes.

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The Brokenness of TABLESAMPLE

Published 2024-09-10 by Kevin Feasel

Paul White walks us through some issues:

Initial testing went well, which was a pleasant surprise. Soon enough though, errors started to appear in the tool’s output. That’s not entirely unexpected since ensuring consistent results under high concurrency tends to expose all sorts of niggly edge cases. It’s still an annoyance because debugging edge cases in trigger code can be tricky and laborious.

What was a surprise though was the nature of the error messages.

Read on for the full story. Paul has also created a feedback issue covering a problem with the function.

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“General Failure” and Query Store Compilation Times

Published 2024-08-14 by Kevin Feasel

Kendra Little warns us not to let a person named General Failure run your military:

This post demonstrates two related bugs with plan forcing in Query Store which increase the likelihood of slower query execution and application timeouts in SQL Server environments.

These bugs are most likely to impact you if:

  • You use the Automatic Plan Correction feature in SQL Server, which automatically forces query plans.
  • Anyone manually forces query plans with Query Store.
  • You have slow storage, which can increase your likelihood of having longer compilation times.

The General Purpose tier of Azure SQL Managed Instance and Azure SQL Database feature both slow storage and Automatic Plan Correction enabled by default. So, weirdly enough, your risks of suffering from this problem are high if you are an Azure SQL customer.

In the words of the great John Madden, that’s a heckuva bug.

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Losing .NET Code with SSIS 2016 Package Deployment

Published 2024-08-02 by Kevin Feasel

Andy Leonard goes on a quest:

…when one of our earliest SSIS framework customers reached out to share – in the very nicest way possible – that Fail Application on Package Failure in our SSIS framework was “not working.” My response was, “Great Scott!”

The customer is running SQL Server 2016. That’s fine for EDNA’s SSIS frameworks because we have framework versions that work with several versions of SSIS and SQL Server from 2012 forward. I maintain a collection of virtual servers built using the tools available for each version, including the operating systems and versions of Visual Studio, SSIS, and SQL Server.

Read on for the context, the story, and what Andy was able to do about it.

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Date Calculation Bug in Power Query ODBC Code

Published 2024-04-03 by Kevin Feasel

Meagan Longoria files a report:

I was working on an imported Power BI semantic model, adding some fiscal year calculations to my date table. The date table was sourced from a view in Databricks Unity Catalog. I didn’t have access to add more fields to the view, so I was adding the fields in Power Query first, with plans to request they be added to the view in the future. I got some unexpected results, which turned into a bug being logged for the ODBC code for Power Query.

If you are only analyzing data in the last 20 years, you won’t see this bug. But if you are doing long-term analysis including years before 2000, you might just run into it.

Read on to see the bug, how you can replicate it, and three workarounds you can use to avoid it.

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GUID Conversion and the Serverless SQL Pool

Published 2024-02-22 by Kevin Feasel

Reitse Eskens hits a weird error:

One of the transformations is to change one primary key column from integer to GUID. This is something you can do with some trickery you’ll see in the code. But what I found was that, even though the primary key is unique, the GUID’s weren’t. And then the fun starts digging into the why…

Read on for the research Reitse performed. I don’t even have a good guess for this, it’s so weird. It feels like a bug but it’s weird regardless.

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