Andrei Lepikhov walks through an interesting scenario:
What was the impetus to begin this work? It was caused by many real cases that may be demonstrated clearly by the Join Order Benchmark. How much performance do you think Postgres loses if you change its preference of employing parallel workers from one to zero? Two times regression? What about 10 or 100 times slower?
The black line in the graph below shows the change in execution time of each query between two cases: with parallel workers disabled and with a single parallel worker per gather allowed. For details, see the test script and EXPLAINs, with and without parallel workers.
Click through for an overview of what Andrei wrote, including architectural notes. But stick around until the end to see just how difficult the challenge is to re-optimize without making performance worse in the end.