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dynamic management views

TIL: Broken Log Chains

Something that always bites me in the butt from time to time is restoring a database in full recovery model and forgetting to do a full right away.  Then I find out shortly when my log backups start failing and spamming me with error messages.  I’m fairly certain this has happened to you, as well.  It’s an easy thing to forget, especially when you’re dealing with non-critical databases.

Humans are typically bad at remembering things.  That’s why we make task lists, have calendar reminders, document processes, and script for automation.  We either have to remind ourselves to do something or (my preference) build something that’s smart enough to correct our oversight.  So when I found a way to do this with broken log chains, I was pretty happy.

The key was a new dynamic management view (DMV) I discovered.  Most people who know me know I love the DMVs and all the information they can provide.  My new discovery is the sys.database_recovery_status view, which provides recovery info for all the databases.  The key field is the last_log_backup_lsn field, which will be NULL if the log chain has somehow been broken.

What’s cool about this is now we have a programmatic way we can use to validate if a full backup needs to be performed.  By adding a quick check to a log backup script, we can make sure we never have a broken log chain.  A quick and dirty way would be:

if (select last_log_backup_lsn from sys.database_recovery_status where database_id = db_id(‘foo’)) is NULL
    backup database [foo] to disk=<<path to full backup>>

backup log [foo] to disk=<<path to log backup>>

This is obviously the most simple approach you can use, but the pattern is the important piece.  By combining this with your standard backup scripts (please tell me you have standard backup scripts), you can always ensure that you never get an error because you forgot a full backup.

Because your script will remember for you.


Standing on the Shoulders of Giants

When I went to SQL Rally last week, I fully expected to be learning from many of the presenters there. The excitement about being immersed in all that awesomesauce is infectious. What I didn’t expect is that I would take my turn at teaching, handing out information like candy to twelve year olds. It was one of those times where I was talking shop with some other folks while we were all taking a session off. The group started chatting about performance tuning(something I geek out about) and I shared out some of the fundamental links, blogs, and tools I use when I get the inevitable “My query is slow!” call.

Now, just to be clear, none of the information I shared was “mine”. The beauty of the SQL Server community is that we have so much great talent and everyone is willing to help educate. What follows here is a list of my core tools and information links, resources that I use on almost a daily basis. I also see them as the start of a journey, because none of this is the last word on a particular subject. If you’re like me you’ll continue to browse the Google ‘verse with what you pick up here.

Brent Ozar:

Full disclosure, I’m a Brent Ozar fanboy. I started reading his blog about a year or so ago and he’s made me laugh and taught me a whole heck of a lot. It’s because of him that I started blogging and getting involved in the community.

Glenn Berry:

Glenn Berry is a local Denver MVP and I see him from time to time at the Denver SQL User’s Group meetings. Not only has he provided these great DMV queries, but he really understands hardware and has some great posts on the nitty-gritty of your machines.

General Community Resources

These two sites are invaluable resources for articles and tutorials on getting stuff done. When I was figuring out how to implement partitioning, I did a lot of reading on both of these sites.


Both of these docs live on my laptop and are constantly getting referenced. My only complaint about the waits and queues white paper is that it’s a little stale and I hope Microsoft does an updated one for Denali when it’s released.

If you don’t have a cheat sheet with your go to resources, hopefully this list gives you a good start. Or you can add this to your currently existing list. I have found each one of these links invaluable to my day to day work, not only enriching my general knowledge but giving me the tools to troubleshoot many of the performance problems that come my way.

T-SQL Tuesday #17: Using APPLY

I confess, I’m one of those guys who really didn’t know much about APPLY. Sure, I saw it in the occasional query I’d find on the ‘tubes, but I never really dug into what it was and why this piece of syntax was so important. Fortunately, this month’s T-SQL Tuesday has rolled around with an opportunity for me to do a little learning, so off to the Google-mobile!

Of course, you can easily find the MSDN article explaining apply, but there’s a much more detailed one I found on Simple Talk by Robert Sheldon. Conceptually, it’s pretty simple. APPLY, in both its CROSS APPLY and OUTER APPLY forms, allows you to call a table valued function for each row in your data set. In a lot of ways it’s like a join, but instead of two tables linked on a common field, you’re linking your query with a computed set of rows that you return with your function.

Now, I wish I could say I had some cool query to share, but tend to use APPLY in some simple ways to get query information. Here’s a query that makes use of the dynamic management functions dm_exec_query_plan and dm_exec_sql_text that gets a lot of work from me:

	er.session_id, database_name,
from sys.dm_exec_requests er
	join sys.databases d on (er.database_id = d.database_id)
	CROSS APPLY sys.dm_exec_query_plan(er.plan_handle) eqp
	CROSS APPLY sys.dm_exec_sql_text(er.sql_handle) est

Using this, you can get your text and your query plan for all queries currently running on your server. I like to use the query plans to troubleshoot long running queries, and the wait types help you identify the queries you might want to look at first.

Enjoy the other T-SQL Tuesday posts out there and huge thanks to Matt Velic(blog|twitter) for hosting this month!