Limits on parallel operations associated with PDQ priority

The database server reduces the PDQ priority of queries that contain outer joins to LOW (if set to a higher value) for the duration of the query. If a subquery or a view contains outer joins, the database server lowers the PDQ priority only of that subquery or view, not of the parent query or of any other subquery.

The database server lowers the PDQ priority of queries that require access to a remote database (same or different database server instance) to LOW if you set it to a higher value. In that case, all local scans are parallel, but all local joins and remote accesses are nonparallel.