Oracle Database - The Query Coordinator (QC)
1 - About
The QC distributes the work to the PX servers and may have to perform a minimal – mostly logistical – portion of the work that cannot be executed in parallel.
For example a parallel query with a SUM() operation requires a final adding up of all individual sub-totals calculated by each PX server.
The QC is easily identified in a parallel execution plans as 'PX COORDINATOR'. The process acting as the QC of a parallel SQL operation is the actual user session process itself.