convert Image column is very slow - ssis

Question

I want to convert data from old database to new database with new structure.
in old database I have attachment table that must be convert to attachment table in new database.
old database attachment table structure is below:
Attachment (ID int, Image Image, ...)
and new database attachment table structure is below :
Attachment (ID int, Image Image, OldID Int, ...)
each time I execute convert package copy only not exists data (new data) from old database to new database.
I use below format for do it :
lookup between old table and new table (ID --> OldID) for check exists record.
When I run SSIS Packages; SSIS, first cache all lookups and source component data in memory then execute package. my source data in this package is very huge and when I run this package it will be run very slowly. I want to get Image column data from old database for each new record after lookup for check exists component. if I use new lookup component for get image column data from old database, SSIS cache this new lookup data and execution time of run this package not change. what must I do?
thanks in advance.

Answer 1

Are you sure you're thinking this through correctly? SSIS should not be slow even if the amount of data you are loading is huge.
Your LOOKUP component needs to make sure it's not doing anything it doesn't need to. If you are pointing it to the table in the new database, change it to a SQL Query at once. In this query you only need to SELECT OldId FROM tbl and point the incoming ID from old database to this. Your data flow should contain ID and Image from Old database, which is mapped ID -> OldIdand "Image -> Image` in your OLE DB Destination. No more is needed for "Insert new rows only" operation like you are doing here.
For this job, there is no need for any custom code or dynamic SQL. You -do- want to get the ID and Image from your source system in the data flow (unless you have major network bottlenecks to sort out) - doing a RBAR lookup to get the image data from the old system is a very backwards way of thinking your ETL.

Answer 2

Select only ID from source table
Do lookup in destination db with no change
For its no match output do lookup in source table, with Cache Mode set to No cache, which will append Image to the flow.
In this case each image will be fetched separately, which may affect performance.

Answer 3

You may also do it in two Data Flows.
In first:
Select only ID from source table
Do lookup in destination db with no change
Store new Ids in string variable IdListToBeFetched as comma separated list using Srcipt Component as destination witch code similar to:
using System.Text;
[Microsoft.SqlServer.Dts.Pipeline.SSISScriptComponentEntryPointAttribute]
public class ScriptMain : UserComponent
{
StringBuilder sb;
public override void PreExecute()
{
base.PreExecute();
sb = new StringBuilder();
}
public override void PostExecute()
{
base.PostExecute();
Variables.IdListToBeFetched = sb.ToString().TrimEnd(',');
}
public override void Input0_ProcessInputRow(Input0Buffer Row)
{
if (!Row.ID_IsNull)
{
sb.AppendFormat("{0},", Row.ID);
}
}
}
In second Data Flow set sql command of source to dynamic generated query from expression similar to "select ID, Image from Attachment where ID in (" + #[User::IdListToBeFetched] + ")" and set DelayValidation = True. It will take all Images in single select which should be faster.
To set dynamic generated query as SqlCommand in sources like ADO NET Source or ODBC Source:
select Expression property of Data Flow Task containing your source
find property [your source name].[SqlCommand] and set expression here
To set dynamic generated query as sql command in OLE DB Source (taken from Jamie Thomson blog):
Create a new variable called SourceSQL
Open up the properties pane for SourceSQL variable (by pressing F4)
Set EvaluateAsExpression=TRUE
Set Expression to "select ID, Image from Attachment where ID in (" + #[User::IdListToBeFetched] + ")"
For your OLE DB Source component, open up the editor
Set Data Access Mode="SQL Command from variable"
Set VariableName = "SourceSQL"