OS & Oracle

DFO means "Data Flow Operator". Actually, “queries” don’t run in parallel, it’s "data flow operations" (DFOs) that run in parallel, and a single query can be made up of several data flow operations. DFO tree is composite with DFOs, usually one query have one DFO tree. such as

select /*+ parallel(t1 4) parallel(t2 4) */ [...]

Two execution plans.
Disable optimizer_dynamic_sampling firstly.

alter session set optimizer_dynamic_sampling = 0;

Then, create two simple tables;

CREATE TABLE t1 ( c1 NUMBER, c2 NUMBER );
CREATE TABLE t2 ( c1 NUMBER, c2 NUMBER );

hr@ORCL> SELECT /*+ parallel(t1,2) */ t1.c2,t2.c2 FROM t1,t2 WHERE t1.c1 = t2.c1;

Execution Plan
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Plan hash value: 869483324

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| Id | Operation [...]