BIRT: Join too slow

Question

I have a large table in my DB from which I need a couple of rows for a report. The JOIN turned out to be too slow. So my solution was to read the values into a HashMap and simply join them manually in each row of the report.

How can I fill the map in the open script of a scripted data source?

My current solution was to pass the URL/user/password to the report and create a SQL connection manually. I would prefer to use the report's existing connection. Is that possible?