Beware Multi-Assignment dplyr::mutate() Statements

Kevin Feasel

2018-01-23

Bugs, R

John Mount hits on an issue when using dplyr backed by a database in R:

Notice the above gives an incorrect result: all of the x_i columns are identical, and all of the y_i columns are identical. I am not saying the above code is in any way desirable (though something like it does arise naturally in certain test designs). If this is truly “incorrect dplyr code” we should have seen an error or exception. Unless you can be certain you have no code like that in a database backed dplyr project: you can not be certain you have not run into the problem producing silent data and result corruption.

The issue is: dplyr on databases does not seem to have strong enough order of assignment statement execution guarantees. The running counter “delta” is taking only one value for the entire lifetime of the dplyr::mutate() statement (which is clearly not what the user would want).

Read on for a couple of suggested solutions.

ML Services Can Fill The Plan Cache

Kevin Feasel

2018-01-04

Bugs, R

I have a post talking about a bug in SQL Server:

For now, the workaround I have is to restart the SQL Server service occasionally. You can see that I have done it twice in the above screenshot. Our application is resilient to short database downtimes, so this isn’t a bad workaround for us; it’s just a little bit of an annoyance.

One thing to keep in mind if you are in this scenario is that if you are running ML Services hundreds of thousands of times a day, your ExtensibilityData folders might have a lot of cruft which may prevent the Launchpad service from starting as expected. I’ve had to delete all folders in \MSSQL14.MSSQLSERVER\MSSQL\ExtensibilityData\MSSQLSERVER01 after stopping the SQL Server service and before restarting it. The Launchpad service automatically does this, but if you have a huge number of folders in there, the service can time out trying to delete all of them.  In my experience at least, the other folders didn’t have enough sub-folders inside to make it worth deleting, but that may just be an artifact of how we use ML Services.

It’s very unlikely to affect most shops, as we only notice it after running sp_execute_external_script millions of times, and that’s pretty abnormal behavior.

SQL Server Feedback In A Post-Connect World

Kevin Feasel

2017-12-20

Bugs

Koen Verbeeck touts the new SQL Server feedback site:

After years of having to deal with Connect – the feedback platform of Microsoft – it is announced a successor has been found: feedback.azure.com. It’s not all about Azure, the link goes to the relevant portion of SQL Server. I’m glad for this change, as Connect could sometimes be a little … quirky. Especially the search function didn’t work properly. The new feedback site is based on UserVoice and it’s really easy to submit feedback. People submitting ideas for Power BI will be very familiar with the format. There are a couple of drawbacks:

  • You cannot specify many details (none to be exact, or you have to list them in the descriptions). OS version, SQL Server version, bitness, et cetera. On the other hand it makes the process of entering feedback a lot faster.

  • You cannot mark a feedback item as private so that only Microsoft can see it. This means it’s not exactly the place to dump your production data to show how a bug is bugging you (haha).

I’m not sure how much of an improvement this is, but at least it does serve the Power BI team well.

SQL Server 2017 CU2 Compatibility Mode Bug

Kevin Feasel

2017-12-06

Bugs

Tracy Boggiano has found a bug in SQL Server 2017:

This will be a very short blog post to make you aware of a bug in CU2 for all of those who I know have eagerly installed the newest CU for 2017.  A small bug I have found is that it changes your compatibility mode on the msdb database to 130.  All our servers were set to 140 and our nice server policy check alerts fired off and sent me 58 pages the day after I installed it in my development environment.  Well, I double checked before installing on QA today and sure enough, it changed it from 140 to 130.  So have your code ready to change it back after you install.

Click through for a script to fix the compatibility level.

Bug In Older Versions Of SQL Server 2012 & 2014

Kevin Feasel

2017-12-06

Bugs

Paul Randal explains a bug in versions of SQL Server 2012 and 2014:

There hasn’t been a case of it failing to reserve enough space until SQL Server 2012, when a bug was introduced. That bug was discovered by someone I was working with in 2015 (which shows just how rare the circumstances are), and at the time it was thought that the bug was confined to the log of tempdb filling up, rollback failing, and the server shutting down.

However, just last week I was contacted by someone running SQL Server 2012 SP3 who’d seen similar symptoms but for a user database this time, and the user database went into recovery.

Read on for details and make sure those SQL Servers are patched.

Love Your Bugs

Kevin Feasel

2017-11-14

Bugs

Allison Kaptur has an appreciation for strange bugs:

I love this bug because it shows that bitflips are a real thing that can happen, not just a theoretical concern. In fact, there are some domains where they’re more common than others. One such domain is if you’re getting requests from users with low-end or old hardware, which is true for a lot of laptops running Dropbox. Another domain with lots of bitflips is outer space – there’s no atmosphere in space to protect your memory from energetic particles and radiation, so bitflips are pretty common.

You probably really care about correctness in space – your code might be keeping astronauts alive on the ISS, for example, but even if it’s not mission-critical, it’s hard to do software updates to space. If you really need your application to defend against bitflips, there are a variety of hardware & software approaches you can take, and there’s a very interesting talk by Katie Betchold about this.

This is an interesting approach, although I fear I may not have the temperament to love my bugs so much as despise their existence.

Locale Problems With SSAS

Koen Verbeeck diagnoses an awful Analysis Services error:

This really didn’t make any sense. However, in one of the Discover Begin/End events, the same number appeared again: 8192 (this time explicitly marked as locale identifier). Hmmm, I had problems with weird locales before. I dug into my system, and yes, the English (Belgium) locale was lingering around. I removed it from my system and lo and behold, I could log into SSAS with SSMS again. Morale of the story: if you get weird errors, make sure you have a normal locale on your machine because apparently the SQL Server client tools go bonkers.

Worth reading the whole thing.  And also maybe just using en-US for all locales; at least that one gets tested…

Bugs With Backup Compression And TDE

Parikshit Savjani provides recommendations on combining backup compression with Transparent Data Encryption:

In past months, we discovered some edge scenarios related to backup compression for TDE databases causing backups or restores to fail, hence our recommendations have been

  • Avoid using striped backups with TDE and backup compression.

  • If your database has virtual log files (VLFs) larger than 4GB then do not use backup compression with TDE for your log backups. If you don’t know what a VLF is, start here.

  • Avoid using WITH INIT for now when working with TDE and backup compression. Instead, use WITH FORMAT.

  • Avoid using backup checksum with TDE and backup compression

Brent Ozar explains the risk:

When you install a new version of SQL Server, you get new features – and sometimes, you’re not told about them. For example, when 2016’s TDE compression came out, nobody told you, “If you back up across multiple files, your backups might suddenly be compressed.” You didn’t know that you had a new thing to test – after all, I don’t know a lot of DBAs who have the time to test that the new version of SQL Server successfully performs restores. They restore their production databases into the new version, test a few things, and declare victory – but testing restores FROM the new version’s backups isn’t usually on that list.

Keep up to date on those patches.

Incomplete Execution Plan Wait Stats

Brent Ozar has found an issue with query-level wait stats in SQL Server 2016 SP1:

That sounds amazing! Our server only had 609 milliseconds of wait time altogether, none of which was spent waiting on storage! Our storage must be super-blazing fast, and there’s practically no way to tune it to wait less than 609 milliseconds, right?

Spoilers:  it’s not that fast.

Partitioned Views With Memory-Optimized Tables

Ned Otter ran into an issue building partitioned views from a combination of disk-based and memory-optimized tables:

Let’s assume that we have two tables that belong to a partitioned view. Both tables can be memory-optimized, or one table can be memory-optimized, and the other on-disk.

Success condition

an UPDATE occurs to a row in a table, and the UPDATE does not change where the row would reside, i.e. does not cause it to “move” to another table, based on defined CONSTRAINTS

Failure conditions:

   a. UPDATE occurs to a row in the memory-optimized table that causes it to move to either another memory-optimized table, or a on-disk table

   b. UPDATE occurs to a row in the on-disk table that causes it to move to the memory-optimized table

Read the whole thing.  The good news is that if you’re splitting in a way that doesn’t require updating from one type of table to the other, partitioned views work fine.

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