Hello @andatki,
On Chapter 8, page 145, in the Generating Data for Experiments section, one of the paragraphs stars with the following: "When does it flip from sequential scans to index scans?..”
I believe this should be "When does it flip from index scans to sequential scans?..” instead.
This makes more sense, since it then goes on to explain that a query that specifies many fields of data, using an index that does not cover all fields may trigger PostgreSQL to resort to a table sequential scan.