Voice of the DBA

Category: Blog

  • The Difference Between Change Tracking and Change Data Capture

    Change Tracking (CT) and Change Data Capture (CDC) were both added to SQL Server in 2008. At first it seems like these two items ought to be synonyms, but they’re separate features. They are similar, but there are some differences, and you might choose to use them in different situations.

    Change Tracking (CT)

    CT is not as well known as CDC, and I see it talked about less. This is really a feature that allows the net changes made to data to be easily returned from a query. This only lets you know that a particular row has changed since your last query. You have no idea

    • how many times it’s changed
    • the various change values over time

    The queries you run will return a table that lets you know which rows have changed since the last check, and then let you know the type of DML change. You need to join this table with the source table to actually get the data.

    This is really useful for those applications that cache data and periodically query to update their caches. Lots of .NET frameworks allow this, and it’s a great way to limit the load on your database server.

    Change Data Capture (CDC)

    CDC is more well known, and seemed like a great tool when I first saw it, but like many useful enhancements, there is a bit of complexity that you have to work through in order to understand and use this feature.

    CDC is a little more complex to implement, and it creates a bit more data in your database. You get a change table that is a copy of your table, along with a few additional columns that contain metadata. For each DML operation, you get a row(s) added to the change table. Inserts get one row (new data). Deletes get one row (old data) and updates get two rows (old and new data).

    This gives you lots of history and information about your table, but it’s a lot of data. The amount can grow quickly in a busy database, so you need to be sure that you extract the information you need and prune the change tables periodically.

    Which One?

    Which feature should you use? Is CT better than  CDC? They work differently, and they capture different amounts of data. There’s an entry in BOL that compares them, and you should understand the differences. However you really need to spend time working with both to make a decision about which one meets your needs.

    Pick a table or two, enable one, test with some workload changes, then evaluate. Then repeat with the other. You might find that you need to use CT in some places, and CDC in others, depending on the downstream processing of the data.

  • T-SQL Tuesday #30 – Ethics

    TSQL2sDay150x150This month the topic is hosted by Chris Shaw, and his topic is ethics.

    The blog party is the second Tuesday of each month. There is a theme and it is hosted by a different blogger every month. I’ve got a complete list of topics on my blog if you want to read about the past entries.

    It was started by Adam Machanic and if you want to host one, let him know. It can be fun, though a little busy as you compile a summary of the posts from the month.

    Ethics

    I thought this was a great topic, and I’ve actually written on it before. Chris asks a few questions about ethics and they are good ones. I’d like to think that most people have a good code of ethics, but I also think many people haven’t really thought through what their ethics are on this situation. You should do your job, and do what you’re told, but I think breaking the law, or performing an action that violates your moral principles is a no-no.

    However when you get into more subtle issues, like the one Chris talks about with regard to security, what do you do?

    Overall you have to follow your moral compass and use The Test.

    • If you have any doubt about what you’ve been asked to do, seek a second opinion, and get a confirmation from your boss (or their boss) in writing.
    • If you are sure it’s wrong, don’t do it, give you reasons, and let them get someone else to do it.
    • If it’s illegal, you should report it.

    I know sometimes this puts you in a bad situation, and you may fear for your job. That’s natural, and understandable, especially if you are the sole breadwinner for your family.

    However you also have to live with your actions, and they reflect on your soul daily. I would ask that you make the right decision, even when it’s not the most popular, profitable, or palatable one.

  • Finding the Default Trace File

    A post more for me than for anyone else, since I’ll look for something in the default trace and I often need this snippet of code:

    select path 
 from sys.traces 
 where is_default = 1

    That returns something like this:

    path

    ————————————————————————-

    C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\Log\log_92.trc

    This is the current file being used by the trace, which allows me to then look into the file for some event.

    From here, I usually start running a query like this:

    select 
   e.name as eventclass
 , t.textdata
 , t.starttime
 , t.error 
 , t.hostname
 , t.ntusername
 , t.ntdomainname
 , t.clientprocessid
 , t.applicationname
 , t.loginname
 , t.spid
 from fn_trace_gettable('C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\Log\log_92.trc', default) t
  inner join sys.trace_events e 
     on t.eventclass = e.trace_event_id 
  where eventclass = xx

    In this query, I take the output from the first query and use that as the FROM file and then include an event class number in the WHERE clause. I needed this today, running a check for the latest DBCC, and so I used the class 116.

    You can get a list of event classes here: Trace Event Classes

  • Living a Dream

    It was a little over a year ago that I stood on stage at the Rocky Mountain Tech Trifecta and gave a keynote speech in which I said these words:

    “My wife works in the mobility/cellular phone industry right now, but she really wants to train horses for a living.  In the short term, which might be the next few years, she’s a little "stuck" in her current job. However we spend time and money every year improving her skills, and I support her in doing the "professional development" she needs to do in order to be a horse trainer some day. I know she’ll get there at some point, and I am sure she’ll be ready for her next career with all the work and training she is doing now.”

    As I write this today, my wife is finishing her last day of work in corporate America. She resigned a month ago, and today, May 4th, is her last day working for her company. She had a farewell dinner with her boss last night. She is packing up her equipment and shipping it back. She’s having final calls with co-workers and saying good-bye.

    Her dream has been to work with horses for a living. She has been working part time for years boarding and training horses and people to work with them, and we’ve been working for years to try and adjust our life so she could work on her dream full time.

    That starts tonight, as one job ends and she commits to building her business full time, and with a passion that amazes me. I have no doubt this will be a success for her, and for our family.

    We know there will be an adjustment, and finances will get tighter, but we all think it’s a trade that is worth making. Life is short, very short, and you only get to live it once. We have lost a number of friends in the last year, all too young to leave this Earth, and it’s helped to make us realize that it’s important we don’t delay this change any longer than we need to, even if it’s uncomfortable.

    Life is short, but we never know how short it will be for each of us. Any of us could leave this life at any time. Remember that each day, as you choose your path in life.