Jim McLeod

As mentioned in my last post, it is possible to use RAISERROR WITH NOWAIT in order to immediately send a message back to the client.  This is useful for long, procedural (i.e., not set-based) stored procedures that loop over many different rows.

Consider the following stored procedure:

CREATE PROCEDURE dbo.InfoMsgTest
AS
    DECLARE @i int;
    SET @i = 1;
    WHILE @i < 100
    BEGIN
        RAISERROR('%d', 0, 1, @i) WITH NOWAIT;
        -- Do some processing!
        WAITFOR DELAY '00:00:01';
        SET @i = @i + 1;
    END
GO

This procedure is a simple loop that counts to 100.  Each time around the loop, a RAISERROR command is executed, passing out the value of @i.  Any message at all could be passed – you could include how many rows have been processed, how many to go, and what the primary key is of the current row.

On the client, consider the following C# console application.  All error handling has been removed, and I haven’t written any .NET code in two years, so your forgiveness is appreciated!

using System;
using System.Data.SqlClient;

namespace InfoMessages
{
    class Program
    {
        static void Main(string[] args)
        {
            SqlConnection conn = new SqlConnection(
                "Data Source=(local);Initial Catalog=AdventureWorks;" 
                    + "Integrated Security=SSPI;");
            conn.InfoMessage += 
                new SqlInfoMessageEventHandler(InfoMessage);
            conn.Open();
            SqlCommand cmd = new SqlCommand("exec dbo.InfoMsgTest", conn);
            cmd.CommandTimeout = 120;
            Console.WriteLine("Processing starting.");
            cmd.ExecuteReader();
            conn.Close();
            Console.WriteLine("Processing complete.");
        }

        private static void InfoMessage (object sender, 
            SqlInfoMessageEventArgs e)
        {
            Console.WriteLine("Percent completed: " + e.Message + "%");
        }
    }
}

Note that it is vital to use a cmd.ExecuteReader().  cmd.ExecuteNonQuery() will not fire the InfoMessage handler. 

And the output:

There you have it!  A GUI application shouldn’t be too much harder.  Little things like this can make the difference between having a responsive application that informs the user as to what is happening, versus a black box that appears to hang for 30 seconds while the stored procedure is executed.

(It’s T-SQL Tuesday #008 – Gettin’ Schooled)

I learn by doing, and by teaching.

Studies have shown that the best way to learn a topic is to teach it to someone else.  I agree wholeheartedly with this – you don’t really know a topic until you’ve had to put it into your own words and get someone else to understand.  Helping out people on the MSDN SQL Forums and the SQLServerCentral forums is a great way of learning.  It’s a very ad-hoc method, as there is no guarantee what you’ll be looking at on any particular day.  Although I might not know the answer, a well written question will pique my interest, and, as one of my strengths is researching how to do things with SQL Server, I’ll attempt to ferret out the answer.  This results in a deeper understanding for me, and (hopefully) a thankful person on the other end. 

Although helping completely unknown people on the Internet can be fun, it’s a lot more satisfying when helping in person, either through teaching courses, giving presentations at user groups, or one-on-one mentoring.  These require you to know the topic thoroughly up front, as there is much less of an opportunity to dart off to Books Online.

I don’t read too many SQL Server books anymore, with a few notable exceptions, such as the Inside SQL Server series, and the SQL Server MVP Deep Dives. These are highly recommended due to their deep technical nature.  The MVP Deep Dives is especially interesting, as it contains a wide range of topics about what MVPs find interesting, as opposed Books Online worded differently.  (This is not to bag authors – there’s definitely an audience for well written books – I’m just happy with Books Online!)  This is a very similar type of format to podcast interviews.  I don’t recall how many different times I’ve heard Paul and Kim go over the same material once more, but it’s always an entertaining listen!  With 90 minutes on a train each day, podcasts are quite useful, as long as they’re not dry technical topics.  Videocasts are not my thing as I rarely have the opportunity.

I keep up with blogs (thank you, Google Reader!) to see what current ideas are floating around, but it’s necessary to filter them – I don’t have time to read every blog post in detail, although many are deserving of that attention!  Instead, I’ll flick over the content to get a feel for the topic, and keep it in mind for later reference.  Blogs can be quite handy when searching, but it’s always worth remembering not to just blindly follow advice given.  Think through the offered steps and consider whether it makes sense before trying it out on your production system.

I believe in the value of certifications, although only as a supplement to experience.  I would love the opportunity to do the SQL Server MCM course as it appears to be an excellent test of all areas of SQL Server, but the wife and kids will insist on spending three weeks in Seattle!

If I had to pick one method of learning that I believe is optimal, I would choose mentoring.  It’s always important to have a mentor, even if you’re considered an expert, if only to bounce ideas off.  And it’s fantastic to give back by mentoring others.

A common myth is that the LocalSystem account has no access to networked resources, and so you may have trouble getting SQL Server to backup to remote locations.

Excusing the fact that running SQL Server or IIS as LocalSystem is not a best practice, it is still possible to connect to networked resources. This is done by the domain account DOMAIN\ComputerName$. For example, if my server SQL01 was on the COMPANY domain, there will be an account in Active Directory named COMPANY\SQL01$. Whenever a service running as LocalSystem attempts to connect remotely, it will attempt to use this account. The restriction is that the server must be in a domain – a workgroup will not cut it.

A recent example of where this came in handy recently was an IIS installation that called Crystal Reports, which ran using System DSNs to connect to the database. These DSNs were configured to use a SQL login with no password. This worked quite well, until the security was tightened, and the SQL login was to be given a password. This then caused every report using the DSN to prompt the user for a password, as System DSNs cannot store passwords. Possible solutions were to change all 100+ reports to File DSNs (and protect them well, as the password would be stored in plain text inside the DSN). Alternatively, the System DSN could be modified to log in using Windows Authentication.

As IIS was running as LocalSystem, the initial attempt was to change this to be a domain account. Unfortunately, however, many security issues appeared, particularly with some versions of Internet Explorer. Allowing the DOMAIN\ServerName$ account limited access to the database removed the reliance on the insecure SQL login, and avoided a lot of rework.

As mentioned earlier, it is a better practice to use a dedicated domain account, but this technique may be useful in a pinch.

It is a common practice to rebuild indexes frequently in order to improve SQL Server performance.  The problem with rebuilding indexes is that you need to have space inside the data file to hold the index currently being rebuilt.  This means that a 5 GB index will require an additional 5 GB of space.  The subsequent problem here is that when the operation is over, the database will appear to have 5 GB of free space, and the DBA might decide to shrink the database (a bad idea, as this will re-fragment the data file).

A potential solution, for those building non-clustered indexes offline, is to first disable the non-clustered index.  A disabled index is akin to dropping the index, but keeping the index definition. After an index is disabled, it must be rebuilt before it can be used, as SQL Server has no way of knowing how many, or which, rows were inserted, updated, or deleted during the period the index was disabled.

This means that a 5 GB index can be rebuilt in place, using the same 5 GB.  The operation may be a little bit slower, and temporary space (either in TempDB or the current database) will be needed to re-sort the index (rather than base it off the current “live” copy of the index, as there is none), but it removes the requirement for the data file to have enough empty space to hold a second copy of the index.

Note that this only applies to non-clustered indexes, as disabling a clustered index will result in the entire table being unavailable, and all non-clustered indexes disabled. Ideally, clustered indexes will be based on a small ever-increasing clustered key, which will greatly reduce the need to ever de-fragment the non-clustered indexes (although there are cases, for example when the row is initially inserted very small (due to variable length columns), and then later updated to populate the variable length columns, widening the row and causing it to not fit on the existing page, forcing a page split).

While this may be a useful technique to avoid data file growth, I would worry about an environment that does not have sufficient working room to rebuild the largest non-clustered index.

Example:

IF EXISTS (SELECT * FROM sys.databases WHERE name = 'IndexRebuildTest')
    USE MASTER
    DROP DATABASE IndexRebuildTest
GO
CREATE DATABASE IndexRebuildTest
GO
USE IndexRebuildTest
GO

-- Force the transaction log to grow
ALTER DATABASE [IndexRebuildTest] MODIFY FILE ( NAME = N'IndexRebuildTest_log'
     , SIZE = 51200KB )
GO

CREATE TABLE t1 (
      i int IDENTITY
    , UniqueID UNIQUEIDENTIFIER DEFAULT newid()
    , c CHAR(1000)
    , CONSTRAINT pk_t1 PRIMARY KEY (i)
)
GO
CREATE UNIQUE NONCLUSTERED INDEX ncix_UniqueID ON t1(UniqueID) INCLUDE (c)
GO

SET NOCOUNT ON
GO

INSERT INTO t1 DEFAULT VALUES
GO 10000

-- Note that sp_spaceused reports the database size including transaction log.
EXEC sp_spaceused @updateusage = N'TRUE'

We have an 80 MB database –  but 50 MB of that is the transaction log.  The data is about 12 MB, and the index is 18 MB – quite a large, fragmented index!

ALTER INDEX ncix_UniqueID ON t1 REBUILD WITH (SORT_IN_TEMPDB = ON)
GO
EXEC sp_spaceused @updateusage = N'TRUE'

Now, our database has grown to 91 MB – an increase of 11 MB.  We’ve also experienced a shrinking in the size of the index due to the rebuild, saving 6 MB.

Let’s run the first listing again (to reset the database size back to 80 MB), and then try disabling the index first.  Note that the 18 MB currently being taken by the index is immediately released when the index is disabled.

ALTER INDEX ncix_UniqueID ON t1 DISABLE
GO
ALTER INDEX ncix_UniqueID ON t1 REBUILD WITH (SORT_IN_TEMPDB = ON)
GO
EXEC sp_spaceused @updateusage = N'TRUE'

There you have it.  The database is still 80MB (it fluctuates slightly, depending on how fragmented in the index is each time we reset the database), and there is 6 MB of unallocated space – which is the savings from de-fragmenting the index.

Again, this primarily only useful for rebuilding offline – the index will be unavailable while it’s disabled.  If you do have 2005/2008 Enterprise Edition, you could rebuild online and at least have the table available during the rebuild, but the index will not be.  (And if you do have Enterprise Edition, forking out another $2000 for extra disk space won’t be an issue!)