Voice of the DBA

Category: Blog

  • Boulder SQL Server User’s Group – Common SQL Server Mistakes

    The first iteration of my presentation on Common SQL Server Mistakes, based on the various things I’ve seen in the forums over the years as well as written about on this blog, was last night.

    It went fairly well, though not a lot of questions from the group and I ended up running ahead of schedule. Afterwards we had a nice debate on triggers, which I need to blog about as well.

    Good getting up to Boulder, or Broomfield in this case, for the meeting. It’s at Level 3, which is a great facility, and I’m hoping that we can hold another event there at some point.

    I’ve uploaded the presentation to SQLServerCentral if you are interested in seeing the deck.

  • Common SQL Server – Not Indexing FKs

    This series looks at Common SQL Server mistakes that I see many people making in SQL Server.

    Foreign Keys

    It’s way too often that I see people building databases without including declared referential integrity (DRI) in their databases. Even when I see people setting a primary key on tables, it seems that often they ignore foreign keys and creating linkages between tables that link them together.

    However, even when people have declared a FK, they often don’t create an index on that column. Perhaps they assume that SQL Server will create the index like it does for PKs, but it does not.

    If I create these two tables and join them with a FK:

    CREATE TABLE [dbo].[Products](
        [ProductID] [int] NOT NULL,
        [ProductName] [varchar](50) NULL,
    CONSTRAINT [PK_Products] PRIMARY KEY CLUSTERED
    (
        [ProductID] ASC
    )
    ) ON [PRIMARY]

    GO
    CREATE TABLE [dbo].[ProductDetails](
        [ProductDetailID] [int] NOT NULL,
        [ProductID] [int] NULL,
        [SKU] [varchar](50) NULL,
        [Price] [numeric](18, 2) NULL,
    CONSTRAINT [PK_ProductDetails] PRIMARY KEY CLUSTERED
    (
        [ProductDetailID] ASC
    )
    ) ON [PRIMARY]
    GO
    ALTER TABLE [dbo].[ProductDetails]  WITH CHECK ADD  CONSTRAINT [FK_ProductDetails_Products] FOREIGN KEY([ProductID])
    REFERENCES [dbo].[Products] ([ProductID])
    GO

    ALTER TABLE [dbo].[ProductDetails] CHECK CONSTRAINT [FK_ProductDetails_Products]
    GO

    If I go and check indexes on ProductDetails, I find that there is only one index, the index for the PK.

    FKIndex_a

    Why is this a problem? It’s because of performance. We should realize that indexes speed up performance by reducing the amount of work that SQL Server has to do.

    With FK columns, what I’ve often found with child tables is that I know the value of the FK column I am searching for and don’t need to join with the parent table. However without an index on the FK column, this query requires a table scan.

    select
    sku
    , price
    from ProductDetails pd
    where pd.ProductID = 3

    If you are creating FKs in your database, don’t forget to index them where appropriate.

    Auto Creation

    I’ve seen some people ask why SQL Server doesn’t automatically create indexes on those FK columns. I am torn on this, but I like the 80/20 rle. If 80% of the tables would benefit from it, I think it should be done. I am leaning towards some intelligent mechanism to do this.

    The main issue is that you might not want just an index on the FK column. You might want some sort of covering index that includes columns in addition to the FK column to prevent key/bookmark lookups to the clustered index. If you can avoid those, you can drastically increase performance.

    There is also the chance that with your query load, you never use these indexes. That can be horrible for performance as well since there is overhead to maintain these indexes on all insert/update/delete operations.

    The Advice

    Look at the queries that are coming into your database. Check the missing index DMVs and if you find that the FK columns are being used, index them.

    If you’re not sure, or don’t know how to look for missing indexes, here’s a reference.

  • Database Mirroring Connection Strings – Automatic Failover

    I saw a post recently from Hugo Shebbeare that reminded me of something that I’ve seen asked often on the forums. One thing that I preach to people is that they should use their blog to show what they know, and in this case I want to do that. Also, I remind people to give credit to the inspiration, as I’ve done with the link to Hugo above.

    Database mirroring has automatic failover if you have newer SQL Server clients. Those of you with Vista or Windows 7 should be fine, though XP might need an update. SQL Server 2005 started distributing a client that would handle automatic failover, and it does this through the connection string. Here’s a typical one:

    Server=DBServer01;Database=Sales;Connection Timeout=30;Integrated Security=SSPI;Failover Partner=DBServer02

    In this string we have our main database instance (DBServer01) and the database (Sales). Our mirror server is called DBServer02, and the database name on this server would be the same. Note that you can use IP or named instances as in DBServer01\Sales as well for the connection strings.

    If the client has issues connecting to the primary, when the Connection Timeout passes, it will try to connect to the mirror server and start working there. Note that if you had a transaction in progress when the failure occurred, your application has to reconnect to the other server and resubmit the batch.

  • Copyright

    This is actually amazing. A talk on what YouTube does with copyright and how they view it, from TED. It’s interesting, and there are two things in there to watch. Note, that I caught this link from Jeff Atwood (@CodingHorror) on his blog.

    The first thing is that YouTube scans over 100 years of video every day. 100 years! That’s freaking amazing. That is an incredible amount of video processing that’s taking place, and a low of power being spent to enforce copyright. Forget about the legal issues for a minute and think about the technology. That is just amazing.

    The second thing in the talk is that content owners can, and are choosing, to not block this content, but rather use it to generate some benefits from them. They ought to be able to receive some benefits, and I agree, but knocking down some kid’s video because he used a copyrighted song seems silly. Especially as in the case of the wedding video, you never know what will go viral and what benefits you’ll get. Including selling more copies of your stuff.

    However the thing that worries me is that we are now not looking out for Fair Use. If a copyright holder wants to block their content, can we still get our 30sec of use of it in a video we make? Or a mashup?

    7231[1] The copyright office doesn’t necessarily define the fair use guidelines, but I think they need to. We ought to have some idea of what portion of audio, video, imagery, and text can be used, with citation. Even with automatic citation, that may benefit the copyright holder.

    We publish books here through Simple Talk Publishing. We typically offer books for free as marketing materials here on the site, and also sell them on Amazon and other retailers. Regularly I find these republished, or even for sale, on the Internet on other sites. I understand people wanting to share, but ultimately it’s a problem for us if all our books become too widely distributed for free. I know there are a lot of debates on piracy, and I’m not completely sure where I stand. I don’t actively worry about piracy, but I do act to have things removed when I find it.

    We need to do something here to both protect the copyright holders, but also allow people to reuse ideas and build on them. I certainly wouldn’t mind someone republishing portions of my work, or building their own knowledge publications based on it, just don’t wholesale copy what I’ve done without giving me any credit.

    I think YouTube has a good balance here, though I’d like to see more efforts from content owners to share and allow others to build on their work.