When certain SQL features get used in pre-12c versions that force non-parallel evaluation, like using ROWNUM or certain Analytic Functions like LAG/LEAD, then - depending on the overall plan shape - Oracle can start to decompose the parallel execution plan into several so called DFO trees (If you want learn more about DFO trees and DFOs I recommend watching my online tutorial on my Youtube channel).Now having multiple DFO trees in a single parallel execution plan comes with several side effects that are confusing and complicate matters unnecessarily, like each DFO tree allocates its own PX slave set(s), and so each one can potenially end up with a different DOP, which means you can have more than one DOP in a single parallel execution plan.Depending on the overall plan shape this might also mean that a DFO tree can get started multiple t
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