The planner weighs how to read a table, how to join, and how to aggregate by cost, and picks the cheapest candidate. Parallel query is one more decision layered on top of those candidates: whether one process handles a given scan or join alone, or several processes split the work, and if they sp...
Source: [Dev.to](https://dev.to/joonghyukshin/147-parallel-cost-when-to-go-parallel-and-how-many-workers-46o6)