When working in simulation mode, you can divide the entire result set of the data source (as defined by a query in the source or staging component) into data blocks, which contains a subset of records. The number of records in each subset is related to the Read Block Size parameter, which appears in the Property window of the component. To enhance performance when stepping through the project, select a small number for the Read Block Size parameter.
Previewing transformation results
Step through the project, including the component you want to preview.
Right-click the component, port, or connecting link, and select Preview.
If a component has multiple ports, first select the port for which you are previewing transformation results.
