PowerCenter - The Source Qualifier transformation
1 - About
Every mapping that need to read data from relational or flat file sources, includes a Source Qualifier transformation, representing all data read from the source and temporarily stored by the integration service.
When the source is a database, you will find on this transformation the SQL query.
The SQL query and other attributes can be override by the session attributes when the mapping is run.
2 - Articles Related
3 - Performance
When you create a single Source Qualifier transformation, the PowerCenter - Integration Service increases performance with a single read on the source database instead of multiple reads.