I caught the end of a Twitter conversation the other day between Wes Brown (Blog | @WesBrownSQL), Paul Randal (blog | @PaulRandal), and Aaron Bertrand (blog | @aaronbertrand) talking about shrinking tempdb and the potential issues.
I’ve never thought about it before, but someone investigated and the question popped up on Twitter. Nioholas Cain (blog | @SirSQL) wrote a nice blog post on the events, and it explains how he came to the question (a full disk).
The short answer is that you can cause corruption in tempdb if there is activity taking place when you shrink.
So don’t shrink tempdb!
At least not without quiescing SQL Server. If you don’t know what that means, don’t shrink tempdb on your active server. In case you don’t know, it means allowing active connections to drain off and not allowing new ones. There’s a KB article on this.
select TOP 10 *
from users a
inner join Banned b
on a.username = b.username
and got this lovely message.
I’d seen that message before, so I knew what was wrong. The collations for the two tables were inconsistent. Since this was a database that was upgraded from another version of SQL, and uses objects from a third party, I wasn’t surprised that a specific collation was used. I had created the “b” table myself, using database defaults, and they didn’t match the object.
The fix is easy, add a COLLATE DATABASE_DEFAULT to the join condition to force a specific collation on the field. I could easily have added a COLLATE Latin1_General_CI_AS as well, but since I knew that the second field was database defaults, I did this:
select TOP 10 *
from users a
inner join Banned b
on a.username COLLATE DATABASE_DEFAULT = b.username
In SQL Server, filegroups are a management technique that I don’t see many people using. It’s amazing how many people ask questions about filegroups on the discussion forums, often unsure of how they fit into a well architected SQL Server. I have tended to use filegroups mostly as a space management technique, when I need to add more disks to my server, but they can be used in many more places.
We continue to grow our data sizes all the time. While many databases are still measured in the single digits of gigabytes (or smaller), it is fairly common to find many database servers with over a terabyte of disk space. Our disks grow larger and larger, but it seems that data grows faster than disks, requiring larger storage subsystems all the time.
While our storage grows larger, the tolerance for delays shrinks and demands for better performance increase. That means that data professionals need to be more cognizant of not only how their code is written, but also how they design storage. Tiering storage is one idea that I think has merit, but one that requires some planning.
In SQL Server, we can’t split a table across filegroups. Or can we? We can partition a table (Enterprise Edition and higher), which can allow us to manage performance and storage appropriately. There is also the recommended practice of only having system objects in the primary partition and using separate filegroups for user data. That allows you to bring a partial database online, again, in Enterprise Edition only, while you restore different filegroups.
This isn’t the first thing I would recommend you learn about SQL Server, but as you advance your knowledge, you should better understand when and how filegroups can help you. You will use them at some point and being comfortable with a filegroup restore is one of the skills that separates the accidental DBA from the data professional.
I recently had someone post this after I made a comment about someone taking responsibility for their database server.
“So Steve, what would you recommend then for those poor souls? As being one of them it drives me nuts to constantly be told I can’t get more training as it’s not my true job position yet I’m responsible for making the databases work. “
I’ve been in this spot a few times, in and out of IT. I’ve been tossed into cooking or bartending jobs without training and had to learn quickly how to do the job, and I’ve had the same thing happen in IT. In a few cases the companies knew it was a bad situation and they eventually got me training, in others they didn’t. Here’s the advice I have for you.
Ultimately you are responsible for the job. Tough love, but you’re being paid for that job, so you are responsible. That means you have to learn how the technology in your environment works and how to solve the problems you have.
First, get your resume up to date. Make sure it is ready for submission, and you are prepared to get fired every week. Keep an eye on the job market and save some extra money, because to me, the financial security for my family comes first.
Second, learn to restore data and then make sure you have backups in place. This is secondary because if you have a failure quick, you want to be ready to get a new job. But ultimately no matter what breaks or doesn’t work, getting data back first is crucial.
As you go through all of this, you might be fighting fires. So while you practice restores or document the environment, you might be trying to fix things and asking questions of others, but invest the time to get yourself into a solid position.
I’d also talk to my boss regularly. Every time I found a place I didn’t know something, I’d make a note and let my boss know this is a hole. Maybe I can learn it, maybe I can’t, but I could use help. That might be the best ROI for conferences. Go, make friends, get contacts that can fill your knowledge holes. Or find consultants you can call.
Voice of the DBA 