[cesarb]

PostgreSQL uses 8K pages, so it won't read less than that. IIRC, it also has its own cache of recently read pages, so it won't have to read the same page over and over.

[pgaddict]

You can rebuild it with smaller pages, including 4kB, which may be beneficial for various reasons. The packages however stick to 8kB.

And yes, the database has it's own cache (aka shared buffers), on top of page cache (filesystem cache).