When you run a session that partitions relational or Application sources, the Integration Service creates a separate connection to the source database for each partition. It then creates an SQL query for each partition. You can customize the query for each source partition by entering filter conditions in the Transformation view on the Mapping tab. You can also override the SQL query for each source partition using the Transformations view on the Mapping tab.
When you create a custom SQL query to read database tables and you set database partitioning, the Integration Service reverts to pass-through partitioning and prints a message in the session log.