Some aspects of the new feature are explained in sparingly words only. Maybe, they will not get clear for anybody and need some more explanations.
1: What part of the system is parallelized by use of additional processes? Is it the I/O activity, or RAM access, or net activity, or anything else? The question gets clear if one reads the complete chapter. But it may be better to say some words about the basic idea as a very first statement, eg: "In order to reduce the elapsed time of a query PostgreSQL can create query plans which distribute the query execution across multiple concurrent running processes. Each one use its own CPU and deals with a different part of the shared buffers. All other components of PostgreSQL like disc access or replication are not directly involved. The feature is known as parallel query. ..."
2: In chapter 15.1 there is an example which says: "Workers Planned: 2". The fact that 3 processes will execute this part of the query is mentioned at the bottom of the chapter. My feeling is, that the number "3" shall be expressed more clearly and near to the given example. And: the last paragraph of 15.1 uses the two terms "leader" and "gather nodes". Are they equivalent terms or is there any difference?
3: In chapter 15.3: Do we miss the word "be" in the phrase "it must (be) constructed so that ..."?
Kind regards, Jürgen Purtz