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We're using CDC to capture changes made to a production table. The changed rows are being exported out to a data warehouse (informatica). I know that the __$update_mask column stores what columns were updated in a varbinary form. I also know that I can use a variety of CDC functions to find out from that mask what those columns were.

My question is this. Can anyone define for me the logic behind that mask so that we can identify the columns that were changed over in the warehouse? Since we're processing outside of the server we don't have easy access to those MSSQL CDC functions. I would rather just break down the mask myself in code. Performance of the cdc functions on the SQL end is problematic for this solution.

In short, I'd like to identify changed columns by hand from the __$update_mask field.

Update:

As an alternate sending a human readable list of changed columns over to the warehouse was also accepatable. We found this could be performed with performance far greater than our original approach.

The CLR answer to this question below meets this alternative and includes details of interpreting the mask for future visitors. However the accepted answer using XML PATH is the fastest yet for the same final result.

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2 Answers

So, after some research we decided to still do this on the SQL side before handing off to the data warehouse. But we're taking this much improved approach (based on our needs and new understanding of how the mask works).

We get a list of the column names and their ordinal positions with this query. The return comes back in an XML format so that we can pass off to SQL CLR.

DECLARE @colListXML varchar(max);

SET @colListXML = (SELECT column_name, column_ordinal
    FROM  cdc.captured_columns 
    INNER JOIN cdc.change_tables 
    ON captured_columns.[object_id] = change_tables.[object_id]
    WHERE capture_instance = 'dbo_OurTableName'
    FOR XML Auto);

We then pass that XML block as a variable and the mask field to a CLR function that returns a comma delimted string of the columns that changed per the _$update_mask binary field. This clr function interrogates the mask field for change bit for each column in the xml list and then returns it's name from the related ordinal.

SELECT  cdc.udf_clr_ChangedColumns(@colListXML,
        CAST(__$update_mask AS VARCHAR(MAX))) AS changed
    FROM cdc.dbo_OurCaptureTableName
    WHERE NOT __$update_mask IS NULL;

The c# clr code looks like this: (compiled into an assembly called CDCUtilities)

using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;

public partial class UserDefinedFunctions
{
    [Microsoft.SqlServer.Server.SqlFunction]
    public static SqlString udf_clr_cdcChangedColumns(string columnListXML, string updateMaskString)
    {
        /*  xml of column ordinals shall be formatted as follows:

            <cdc.captured_columns column_name="Column1" column_ordinal="1" />                
            <cdc.captured_columns column_name="Column2" column_ordinal="2" />                

        */

        System.Text.ASCIIEncoding encoding=new System.Text.ASCIIEncoding();
        byte[] updateMask = encoding.GetBytes(updateMaskString);

        string columnList = "";
        System.Xml.XmlDocument colList = new System.Xml.XmlDocument();
        colList.LoadXml("<columns>" + columnListXML + "</columns>"); /* generate xml with root node */

        for (int i = 0; i < colList["columns"].ChildNodes.Count; i++)
        {
            if (columnChanged(updateMask, int.Parse(colList["columns"].ChildNodes[i].Attributes["column_ordinal"].Value)))
            {
                columnList += colList["columns"].ChildNodes[i].Attributes["column_name"].Value + ",";
            }
        }

        if (columnList.LastIndexOf(',') > 0)
        {
            columnList = columnList.Remove(columnList.LastIndexOf(','));   /* get rid of trailing comma */
        }

        return columnList;  /* return the comma seperated list of columns that changed */
    }

    private static bool columnChanged(byte[] updateMask, int colOrdinal)
    {
        unchecked  
        {
            byte relevantByte = updateMask[(updateMask.Length - 1) - ((colOrdinal - 1) / 8)];
            int bitMask = 1 << ((colOrdinal - 1) % 8);  
            var hasChanged = (relevantByte & bitMask) != 0;
            return hasChanged;
        }
    }
}

And the function to the CLR like this:

CREATE FUNCTION [cdc].[udf_clr_ChangedColumns]
       (@columnListXML [nvarchar](max), @updateMask [nvarchar](max))
RETURNS [nvarchar](max) WITH EXECUTE AS CALLER
AS 
EXTERNAL NAME [CDCUtilities].[UserDefinedFunctions].[udf_clr_cdcChangedColumns]

We then append this column list to the rowset and pass off to the data warehouse for analysis. By using the query and the clr we avoid having to use two function calls per row per change. We can skip right to the meat with results customized for our change capture instance.

Thanks to this stackoverflow post suggested by Jon Seigel for manner in which to interpret mask.

In our experience with this approach we are able to get a list of all changed columns from 10k cdc rows in under 3 seconds.

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Thanks for returning with a solution, I might have use for that soon. –  Mark Storey-Smith Mar 5 '13 at 21:04
    
Check out my NEW answer before you do. As cool as the CLR is... we found an even better way. Good luck. –  RThomas Mar 7 '13 at 18:33
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up vote 2 down vote accepted

And the moral of the story is... test, try other things, think big, then small, always assume there is a better way.

As scientifically interesting as my last answer was. I decided to try one other approach. I remembered I could do concat with the XML PATH('') trick. Since I knew how to get the ordinal of each changed column from the captured_column list from the previous answer I thought it would be worth testing if the MS bit function would work better that way for what we needed.

SELECT __$update_mask ,
        ( SELECT    CC.column_name + ','
          FROM      cdc.captured_columns CC
                    INNER JOIN cdc.change_tables CT ON CC.[object_id] = CT.[object_id]
          WHERE     capture_instance = 'dbo_OurTableName'
                    AND sys.fn_cdc_is_bit_set(CC.column_ordinal,
                                              PD.__$update_mask) = 1
        FOR
          XML PATH('')
        ) AS changedcolumns
FROM    cdc.dbo_MyTableName PD

It's way cleaner than (though not as fun as) all that CLR, returns the approach back to native SQL code only. And, drum roll.... returns the same results in less than a second. Since the production data is 100 times bigger every second counts.

I'm leaving the other answer up for scientific purposes - but for now, this is our correct answer.

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