Because each worker executes the parallel portion of the plan to completion, it is not possible to simply take an ordinary query plan and run it using multiple workers. Each

The planner classifies operations involved in a query as either parallel safe, parallel restricted, or parallel

There are several settings which can cause the query planner not to generate a parallel query plan under any circumstances. In order for any parallel query plans whatsoever

When the optimizer determines that parallel query is the fastest execution strategy for a particular query, it will create a query plan which includes a Gather