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) */ [...]

After the parallel execution, this is the best chance to query the execution information. For Example, to analyze the execution of following parallel SQL statement.

select /*+ ordered use_hash(p c) parallel(p 4) parallel(c 4) pq_distribute(c hash, hash)*/
c.unit_price
from products p, costs c
where c.prod_id = p.prod_id;

After execution,

hlr@ORCL> select * from v$pq_sesstat;

STATISTIC [...]

The PQ_DISTRIBUTE hint instructs the optimizer how to distribute rows of joined tables among producer and consumer query servers. Such distribution can improve the performance of parallel join operations.
 
tablespec is the name or alias of a table to be used as the inner table of a join. [...]

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
———————————————————-
Plan hash value: 869483324

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| Id | Operation [...]