how to reach D5 in tuplesort.c 's polyphase merge algorithm?

  for studying polyphase merge algorithm of tuplesort.c, I use ddd and apend a table, which has a schema as follows:

CREATE TABLE Towns (
   id SERIAL UNIQUE NOT NULL,
   code VARCHAR(10) NOT NULL, -- Only unique inside a department
   article TEXT,
   name TEXT NOT NULL, -- Names are not really unique, for instance 'Sainte-Croix'
   department VARCHAR(4) NOT NULL REFERENCES Departments (code),
   UNIQUE (code, department)
   -- UNIQUE (name, department) -- Not perfectly unique but almost
  );