What is Oracle Parallel Query?

Oracle Parallel Query (formerly Oracle Parallel Query Option or PQO) allows one
to break-up a given SQL statement so that its parts can run simultaneously on
different processors in a multi-processor machine. Typical operations that can
run in parallel are: full table scans, sorts, sub-queries, data loading etc.

Parallel Query can improve performance of certain types of operations dramatically
and is commonly used in Decision Support and Data Warehousing applications.