Friday, 1 February 2013

T-SQL Merge in SSIS as SCD alternative

About two year ago I did a post about alternatives for the poorly performing SCD Transformation in SSIS. And even in SSIS 2012 it is still unusably slow for larger data sets. In that post I showed a couple of alternatives, but I didn't workout the T-SQL Merge solution.

A small recap of the alternatives in that post:
  1. The SSIS Dimension Merge SCD from Pragmatic Works which is according to the developers about a 100 times faster.
  2. And there are lot of third party components that can do the trick like CozyRoc Table Difference ($), the Pragmatic Works Upsert Destination ($), Konesans Checksum Transformation and our own Checksum Transformation.
  3. And of course the standard Lookup Transformation solution which works quite well for 2005.
  4. But if you have SSIS 2008 or later, then you could also use the TSQL Merge statement. Let's elaborate that solution.
Before we start, there is also a MERGE Destination that uses a T-SQL Merge statement, but it is still beta (since 2008) and the reviews aren't that good. I did a small test my self, but stopped when I got some meaningless error messages.

T-SQL Merge solution
For this example I use a text file with client information that I want to use to populate my Client dimension. To keep it simple I just use 4 columns: BusinessKey, Title, Firstname and Lastname.

1) Variables
For logging purposes we need to add three integer variables: staged, inserts, updates:
Variables for logging

2) Staging
Next step is to load the flat file with clients into a staging table with a Data Flow Task. The staging table is truncated before loading and the number of records in the Data Flow Task is stored in the variable staged with a rowcount transformation. It should look something like this below:

3) Execute SQL Task
Now we have two tables (staging and dimension) and we need to merge them:
Staging and dimension

For this we need to add an Execute SQL Task.
Execute SQL Task

4) Execute SQL Task - General
Edit the task and set the ResultSet to Single row, select the OLE DB connection and enter the MERGE query. The MERGE query below does INSERTS and UPDATES only. We don't delete records from the dimension table. For logging purposes I added a count of the number of inserts and updates.
General pane

The query to use:
-- Create a table variable to store the number of updates and inserts
DECLARE @tableVar TABLE (MergeAction VARCHAR(20))

-- The actual MERGE statement
MERGE  dwh.Dim_Client as MyTarget
   SELECT BusinessKey
   ,  Title
   ,  FirstName
   ,  LastName
   FROM staging.Clients
   ) as MySource
   ON MyTarget.BusinessKey = MySource.BusinessKey
    MySource.Title   = ISNULL(MyTarget.Title, '')
    AND MySource.FirstName = ISNULL(MyTarget.FirstName, '')
    AND MySource.LastName = ISNULL(MyTarget.LastName, '')
   THEN -- BusinessKey of client exists, but data is different
    Set MyTarget.Title  = MySource.Title
    , MyTarget.FirstName = MySource.FirstName
    , MyTarget.LastName = MySource.LastName
   THEN -- BusinessKey of client does not exist
    INSERT (BusinessKey, Title, FirstName, LastName)
    VALUES (MySource.BusinessKey, MySource.Title, MySource.FirstName, MySource.LastName)
OUTPUT $action INTO @tableVar;

-- Get the number of inserts and updates on one line
SELECT  SUM(Inserted) as Inserted
,   SUM(Updated) as Updated
   -- Count the number of inserts
   SELECT COUNT(*) as Inserted, 0 as Updated
   FROM @tableVar  
   WHERE MergeAction = 'INSERT'

   -- Count the number of updates
   SELECT 0 as Inserted, COUNT(*) as Updated
   FROM @tableVar  
   WHERE MergeAction = 'UPDATE'
   ) as CountTable;

5) Execute SQL Task - Result Set
Go to the Result Set pane and select the insert and update variables for storing the counts.
Result Set pane

6) The Result
For testing purposes I added a ridiculous number of clients in my text file and instead of logging to a table I added a MessageBox in a Script Task. My 4 year old, 4GB machine did quite well with the MERGE statement (around 30 second), but the SCD hadn't processed any records after two hours.
The result


  1. Hi Geplaatst,

    Is there any way to know which column has been updated by the Script task? Could you please post the code which you have used in the task.


    Arun Sasi

    1. Not sure to which Script Task you're referring, but you could create one WHEN MATCHED AND NOT (x=x) for each column, but could be a bit painful if there are a lot of columns.

    2. Unfortunately, T-SQL does not allow for more than one WHEN MATCHED-clauses in a MERGE-statement, unless you issue one UPDATE and one DELETE action, in which case you are allowed up to two WHEN MATCHED-clauses. See

    3. @Daniel: thanks for the info


Please use the SSIS MSDN forum for general SSIS questions that are not about this post. I'm a regular reader of that forum and will gladly answer those questions over there.

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