Voice of the DBA

Tag: indexing

  • Enabling an Index: #SQLNewBlogger

    I don’t do a lot of work with disabled index, but I learned how to re-enable one today, which was a surprise to me. This short post covers how this works.

    The Scenario

    Imagine that you have an index on a table. In my case, I created this index:

    CREATE INDEX LoggerNCI ON dbo.Logger (LogID)

    I can then disable this index with the following code:

    ALTER INDEX LoggerNCI ON dbo.Logger DISABLE

    I had assumed that ENABLE would be the opposite, but SQL Prompt taught me this wasn’t an option. I checked the docs, and sure enough, it’s not ENABLE.

    It’s resume. This code turns the index back on and updates it.

    ALTER INDEX LoggerCI ON dbo.Logger REBUILD

    I can also use either of these items:

    CREATE INDEX LoggerNCI ON dbo.Logger (logid) WITH DROP_EXISTING

DBCC DBREINDEX(Logger, LoggerNCI)

    Interesting short moment the other day as I realized there are a few options here.

    SQL New Blogger

    While playing with this, I realized that I didn’t know all the ways this worked, so I spent 10 minutes after I’d worked with the code to put this together.

    A nice short way to showcase some learning.

  • Mastering Index Tuning–Day 3

    This is a short series of posts on the courses I took with Brent Ozar. I actually completed the courses in the past, but I wrote notes and wanted to revisit the way things went.

    This post looks at the Mastering Index Tuning class. Other  posts are:

    Day 3

    As with Day 2, we begin with reviewing the labs from yesterday. These were harder labs, and Brent spent time looking at how he solved the labs, referencing parts of solutions some people had. This took awhile, with a break in the middle.

    As usual, we can ask questions and discuss the solutions in Slack, which Brent keeps an eye on.

    We start the lectures with artisanal food, which Brent does enjoy. Hand crafted items from the chef, which felt funny since my car killed something and left an organ of some sort in the bathroom.

    The analogy is that there are artisanal indexes, like those on computed columns, indexed views, and filtered indexes. These are items that can help in specific situations, but in general we don’t want to use them.

    I like that Brent brings in the experience they’ve had with clients, noting that some of these features don’t work well.

    The afternoon lab is fix some really bad reporting procedures with indexes (regular or artisanal) or changing code. I know I can’t always change code in databases, but this gives us a chance to try things. I ended up changing some code, but not much. The lab review after lunch was interesting, as Brent had a strange result with the last proc. Looking forward to seeing his debugging of this later.

    The lecture after lunch moves to the end of D.E.A.T.H, heaps. I hate heaps, so this was interesting. Brent agrees with me, you really need a CI on the table. Maybe there are some reasons to not use one in a situation, but most of you need to just add a key.

    The last part of the afternoon looks at the impact of CIs and then constraints and FKs. The CI part is interesting. I see lots of people talking about how to decide on this. I tend to lean towards Brent’s view, which he’s presented on and it’s in the class. Take the class if you want to learn (I don’t want to republish here).

    For FKs/constraints, the module had lots of discussion. People think about FKs in interesting ways. I’ll have to re-watch this as I got busy in the middle with other stuff and missed some lecture.

    The final lab is a big one. Use all the skills from the three days of the class. Restore the db, run a setup that messes up indexes, then fix things. It was a challenge, and I burned about 12 minutes deduping and eliminating indexes, then about 20 coming up with more to add. The creation took quite some time, so I never really got around to tuning, and since this is only part of my day, I had to stop. I do have some real work to do.

    The final lab solution goes up the day after, and what Brent came up with was interesting. I liked watching the videos later to see how he approached the issues and solved them. I like that there wasn’t “one” solution, and he talks about how we might solve the lab that would be different than production.

    That’s important, and it’s something that I appreciated in this class. I know better, but it’s always good to be reminded that the class is a game, a model of what could happen, but in the real world, these are just tools that might help, but could hurt. Judgment is still needed.

    The Aftermath

    One thing I like about this class, which I’ve missed in some live classes, is that I can re watch sections of the class later. The class page has a list of all the lectures and labs, with each containing a video. Some might be from my class, some from previous ones. Since this is delivered and recorded in a modular fashion, Brent can update sections over time.

    I went back to watch the first Artisanal index module, as I was distracted that morning by something at work. That was a nice benefit.

    The Final Word

    This was a great class. I haven’t been to a real class across multiple days in awhile, and I think the format of some lecture, a lab (with interactivity), and then a review of the lab, was great.

    The lectures were interesting, and I learned a few things. The labs were challenging, designed to force you to work within constraints to tune something. Indexing is often a place where you can make changes and rapidly affect your system. The effects could be good or bad, so you need to be sure you are proceeding in a methodical fashion and also capturing metrics on the changes.

    If you’re interested in the class, you can visit the Mastering Index Tuning page to learn more and purchase the class.

  • Mastering Index Tuning–Day 2

    This is a short series of posts on the courses I took with Brent Ozar. I actually completed the courses in the past, but I wrote notes and wanted to revisit the way things went.

    This post looks at the Mastering Index Tuning class. Other  posts are:

    Day 2

    The day starts by looking at homework from last lab. How does Brent do this? One nice thing is that Brent limits the time here for himself. He solves the indexing lab, but stops before some people would. He explains this as he tackles indexing like this. Make some changes, but set a time limit. Then see deploy them and evaluate again after some time.

    We get to watch how he’d solve the lab, and I popped open my VM to check what I’d done. After all, it had been like 16 hours. I saved each lab work in a file on my VM, which was good. I could reference the order in which I’d done things and since I’d save the before/after stats, I could compare with Brent.

    The rest of the day was similar to day 1, with a lecture, a short lab, more lecture, and a long lab over lunch. During this day, we looked at blocking, which is one of those areas where many people have issues. Brent has built a lab that creates blocking, so we can see it happening in our instances.

    While there are different ways you can clear blocking, the challenge here is to use indexes to get rid of blocking. This isn’t the best way to clear blocking, but it is an option, and again, the challenge here is to focus on indexes.

    This was a better day for me, getting into the swing and rhythm of the class as well as starting to feel challenged. I focused more on the labs here, which I wish I’d done a bit more of this on the first day.

    The lab at the end of the day was more complex, though I didn’t have time to get it all done. I got a first pass that seemed to solve some of the issues, and I had to stop since other work was calling. Still, a good day.

    If you’re interested in the class, you can visit the Mastering Index Tuning page to learn more and purchase the class.

  • Interviews: Covering index

    Performance seems to be a part of every DBAs job. It should be a part of every developer’s job, but I understand the focus to build software is different and many developers aren’t sure how much a feature or option they work on will be used. They don’t think too much about performance because at small scales, perhaps performance doesn’t matter a lot these days with multiple cores and many GB of RAM. I’d disagree with that, but I understand the perspective.

    What’s interesting to me is that in many interviews I’ve had over the years, I’ve not often been quizzed on specific scenarios that might help improve performance for queries. It seems many interviewers like to ask trivia questions such as the difference between a clustered and non-clustered index or what configuration knobs should I change. Code questions might be how to find duplicates in a table or deal with NULLs in aggregates.

    I hope that’s changing and more interviewers are giving candidates scenarios, and looking for ways to ensure that candidates think about performance. I hope someone asks how a candidate looks at an execution plan or structures code to remove RBAR concerns. I wondered about this since I chatted with someone recently that had been asked about covering indexes in an interview and was struggling to understand how a covering index might be different than a clustered, nonclustered, filtered, or other type of index.

    This might seem like a trick question, and perhaps it is for some interviewers. After all, if you don’t know, you might correctly guess what this means, but will you know this isn’t a special type of index? Perhaps you’ll try to bluff your way through the answer and hope you get close.

    Answering interview questions should be a discussion, as your answer can reveal as much about how you work as it can about your knowledge. Do you probe to understand the boundaries of the question or confirm that you know what’s being asked? Do you admit when you don’t know something or perhaps reason out loud? Do you give details that make sense, or do you launch into a lecture?

    In this case, you should understand that this index covers a query. This isn’t a type of index, but rather a characteristic of an index against a particular query (or set of queries). To learn more, you might want to read a bit about adding included columns or how to evaluate your choice. If you don’t know much about indexing, get started with an introduction or go through our Stairway to Indexing.

    Future versions of SQL Server will include automatic index tuning, but I think we’ll always need to balance the number of indexes we have. We’ll also often want to build some indexes that will ensure the system works fairly well when it’s first installed. That means data professionals (DBAs ans developers) need to understand how indexes work and ensure they can choose a good index most of the time.

    Steve Jones

    The Voice of the DBA Podcast

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